A niobium microalloyed high temperature carburized gear steel and a method of manufacturing the same

Abstract

The application discloses a niobium micro-alloyed high-temperature carburizing gear steel, which contains Fe and inevitable impurities, and further contains the following chemical elements in the mass percentage: C: 0.14%-0.18%, Si: 0.05%-0.20%, Mn: 1.00%-1.20%, S: 0.020%-0.035%, Al: 0.020%-0.040%, Cr: 1.00%-1.10%, Ni: 0.10%-0.30%, N: 0.0080%-0.0160%, Nb: 0.006%-0.03%, and Mo≤0.050%; and the niobium micro-alloyed high-temperature carburizing gear steel does not contain Ti. Accordingly, the application further discloses a manufacturing method of the niobium micro-alloyed high-temperature carburizing gear steel, which comprises the following steps: (1) electric furnace smelting; (2) LF refining; (3) VD vacuum degassing treatment; (4) continuous casting; (5) heating; and (6) rolling. The niobium micro-alloyed high-temperature carburizing gear steel has an austenite grain size not less than 8.0 levels under the condition of 1000 DEG C high-temperature carburizing, and has the low quenching hardenability characteristics of J5≤36HRC and J9≤32HRC, so that the deformation of gear heat treatment can be effectively reduced.

Description

Technical Field

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

[0002] In recent years, with the rapid development of industries such as automobiles, high-speed rail, and wind power, the application of gear steel has become increasingly widespread. This has, to a certain extent, promoted the rapid progress of gear steel globally, and countries have gradually begun to strengthen their research and development of gear steel. Among them, Mn-Cr series gear steel is a commonly used gear steel. This series of steels has many advantages such as good hardenability, small heat treatment deformation, good low-temperature toughness, and good machinability, and can be used as carburized parts and quenched and tempered parts with large cross-sections.

[0003] However, with the further intensification of market competition, manufacturers are striving to reduce costs while ensuring the service life of components. Gear steel is typically carburized at around 930℃, with a carburizing time of about 10 hours. When the carburizing temperature is increased to 960–1000℃, the entire carburizing heat treatment time is shortened by 50% compared to the conventional 930℃ temperature. Therefore, high-temperature carburizing processes at 960℃ and above have received widespread attention in recent years. However, when the carburizing temperature reaches 960℃ and above, austenite grains tend to coarsen, leading to reduced gear fatigue performance and even mixed grain phenomena.

[0004] Currently, in order to suppress the abnormal growth of austenite grains after high-temperature carburizing of gear steel, the main methods are to use microalloying or composite microalloying of Ti, Nb, V and Al to form precipitates such as NbC, TiN, AlN, (Nb,Ti)(C,N) and V(C,N) to pin grain boundaries and suppress austenite grain growth.

[0005] For example, Chinese patent document CN104928588A, published on September 23, 2015, entitled "A High-Temperature Carburizing Steel and Its Smelting Method," discloses a high-temperature carburizing steel and its smelting method. This method involves adding a large amount of Nb (0.085%–0.23%), Al (0.01%–0.21%), and Ti (0.03%–0.25%) elements to form precipitates such as TiN, AlN, and Nb(C, N) in the steel, thereby inhibiting austenite grain growth. However, in this technical solution, the large amount of Ti in the steel easily generates coarse rhombic TiN particles, affecting the fatigue performance of the steel. Furthermore, the high Al content worsens the continuous casting castability of the molten steel; simultaneously, the excessively high Nb content significantly increases the cost.

[0006] For example, Chinese patent document CN101319294A, published on December 10, 2008, entitled "A Fine-Grained Carburized Gear Steel and Its Manufacturing Method," discloses a fine-grained carburized gear steel and its manufacturing method. This technical solution mainly utilizes the micro-alloying of Nb and Al to suppress austenite grain growth. However, due to the high Al and N content in the steel, incomplete removal of Al2O3 can easily cause blockage of the tundish nozzle during continuous casting, making molten steel pouring difficult. Furthermore, this patent document does not mention the impact of quenching deformation or the role of Nb in carburizing at higher temperatures.

[0007] Besides the requirement for high-temperature carburizing performance of gear steel, hardenability has always been one of the key performance indicators for gear steel. The size of the hardenability bandwidth of gear steel depends on the content and distribution of microstructure at various locations. The differences in microstructure and distribution largely determine the amount of deformation after gear heat treatment. The narrower the hardenability bandwidth, the smaller the hardness fluctuation, which is more beneficial for controlling the deformation after gear machining.

[0008] For example, Chinese patent document CN106967925A, published on July 21, 2017, entitled "A High-Temperature Carburizing Gear Steel with Fine Grain Size and Narrow Hardenability Bandwidth," describes a high-temperature carburizing gear steel with fine grain size and narrow hardenability bandwidth. This steel achieves this by adding Al and N elements, utilizing AlN to inhibit the growth of austenite grains during high-temperature carburizing. However, it should be noted that the hardness of this gear steel with a J15 mm diameter is already no less than 37 HRC. Furthermore, this AlN compound aggregates and dissolves at around 940℃, reducing its pinning effect on grain boundaries, thus failing to meet the requirements for high-temperature carburizing at 960℃ and above.

[0009] In summary, there is currently no publicly available 16MnCrS5 gear steel that is low-cost, high-purity, has a narrow hardenability range, and simultaneously meets the requirements for high-temperature carburizing at 960℃ and above. Therefore, to meet the material demands of technological advancements in the automotive gear industry, this invention designs and proposes a novel niobium microalloyed high-temperature carburizing gear steel and its manufacturing method. Summary of the Invention

[0010] One of the objectives of this invention is to provide a niobium microalloyed high-temperature carburizing gear steel. This niobium microalloyed high-temperature carburizing gear steel has low production cost, high purity, and is easy to process. It has a narrow low hardenability bandwidth (J5≤36HRC, J9≤32HRC), and the austenite grain size under high-temperature carburizing conditions at 1000℃ is not less than grade 8.0, with no mixed crystal phenomenon. It can effectively meet the performance requirements of gear steel materials in automotive and engineering machinery applications, and has good prospects for promotion and application value.

[0011] To achieve the above objectives, this invention proposes a niobium microalloyed high-temperature carburizing gear steel, which contains Fe and unavoidable impurities, and also contains the following chemical elements in the following mass percentages:

[0012] C: 0.14% to 0.18%, Si: 0.05% to 0.20%, Mn: 1.00% to 1.20%, S: 0.020% to 0.035%, Al: 0.020% to 0.040 %, Cr: 1.00% ~ 1.10%, Ni: 0.10% ~ 0.30%, N: 0.0080% ~ 0.0160%, Nb: 0.006% ~ 0.03%, Mo ≤ 0.050%;

[0013] The niobium microalloyed high-temperature carburized gear steel does not contain Ti.

[0014] Furthermore, in the niobium microalloyed high-temperature carburizing gear steel described in this invention, the mass percentage content of each chemical element is as follows:

[0015] C: 0.14%–0.18%, Si: 0.05%–0.20%, Mn: 1.00%–1.20%, S: 0.020%–0.035%, Al: 0.020%–0.040%, Cr: 1.00%–1.10%, Ni: 0.10%–0.30%, N: 0.0080%–0.0160%, Nb: 0.006%–0.03%, Mo ≤0.050%; the balance is Fe and other unavoidable impurities.

[0016] The design principles of each chemical element in the niobium microalloyed high-temperature carburizing gear steel described in this invention are as follows:

[0017] C: In the niobium microalloyed high-temperature carburizing gear steel described in this invention, carbon (C) is the most basic 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 niobium microalloyed high-temperature carburizing gear steel described in this invention, the mass percentage of C is controlled between 0.14% and 0.18%.

[0018] Of course, in some preferred embodiments, in order to obtain better implementation results, the mass percentage content of element C can be further preferably controlled between 0.14% and 0.17%.

[0019] Si: In the niobium microalloyed 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. Excessive Si will promote intergranular oxidation during the carburizing process and significantly affect the J5 value. Therefore, to ensure the toughness of the material, the mass percentage of Si in the niobium microalloyed high-temperature carburizing gear steel described in this invention is controlled between 0.05% and 0.20%.

[0020] Of course, in some preferred embodiments, in order to obtain better implementation results, the mass percentage content of Si element can be further preferably controlled between 0.05% and 0.18%.

[0021] Mn: In the niobium microalloyed high-temperature carburizing 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 niobium microalloyed high-temperature carburizing gear steel described in this invention, the mass percentage of Mn element is controlled between 1.00% and 1.20%.

[0022] Of course, in some preferred embodiments, in order to obtain better implementation results, the mass percentage content of Mn element can be further preferably controlled between 1.10% and 1.20%.

[0023] S: In the niobium microalloyed high-temperature carburizing 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 austenitic transformation of the gear steel, 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 niobium microalloyed high-temperature carburizing gear steel described in this invention is controlled between 0.020% and 0.035%.

[0024] Al: In the niobium microalloyed 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 not conducive to the fatigue life of gear steel. Therefore, in the niobium microalloyed high-temperature carburizing gear steel described in this invention, the mass percentage of Al is controlled between 0.020% and 0.040%.

[0025] Cr: In the niobium microalloyed 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 niobium microalloyed high-temperature carburizing gear steel described in this invention is controlled between 1.00% and 1.10%.

[0026] Ni: In the niobium microalloyed 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 niobium microalloyed high-temperature carburizing gear steel described in this invention, the mass percentage of Ni is controlled between 0.10% and 0.30%.

[0027] Of course, in some preferred embodiments, in order to obtain better implementation results, the mass percentage content of Ni element can be further controlled between 0.10% and 0.20%.

[0028] Nitrogen (N): In the niobium microalloyed high-temperature carburizing gear steel described in this invention, the main role of nitrogen (N) is to form nitrides with elements such as 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 nitrogen content in the steel should not be too high. When the nitrogen content is too high, coarse nitride particles will form, affecting the fatigue performance of the steel. Therefore, in order to maximize the beneficial effects of nitrogen, the mass percentage of nitrogen in the niobium microalloyed high-temperature carburizing gear steel described in this invention is controlled between 0.0080% and 0.0160%.

[0029] Nb: In the niobium microalloyed high-temperature carburizing gear steel described in this invention, Nb is a very effective grain-refining alloying element. 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 niobium microalloyed high-temperature carburizing gear steel described in this invention, the mass percentage of Nb is controlled between 0.006% and 0.03%.

[0030] Mo: In the niobium microalloyed high-temperature carburizing gear steel described in this invention, the proper combination of molybdenum and chromium 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 grain boundary ferrite film, 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 niobium microalloyed high-temperature carburizing gear steel described in this invention is controlled to be: Mo ≤ 0.050%.

[0031] Of course, in some preferred embodiments, in order to obtain better implementation results, the mass percentage content of Mo element can be further preferably controlled between 0.006% and 0.045%.

[0032] Furthermore, in the niobium microalloyed high-temperature carburized gear steel described in this invention, among the unavoidable impurities, P≤0.02%, As≤0.02%, Sn≤0.02%, Sb≤0.010%, Ca≤0.004%, and O≤0.0020%.

[0033] In the above technical solution, P, As, Sn, Sb, Ca, and O are all impurity elements in steel. Under the condition that 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.

[0034] Furthermore, in the niobium microalloyed high-temperature carburized gear steel described in this invention, the content of each chemical element also satisfies at least one of the following conditions:

[0035] C: 0.14%–0.17%;

[0036] Si: 0.05%–0.18%;

[0037] Mn: 1.10%–1.20%;

[0038] Mo: 0.006%–0.045%;

[0039] Ni: 0.10%–0.20%.

[0040] Furthermore, in the niobium microalloyed high-temperature carburized gear steel described in this invention, its microstructure has spherical or ellipsoidal NbC and Nb(C,N) precipitates.

[0041] Furthermore, in the niobium microalloyed high-temperature carburizing gear steel described in this invention, its properties meet the following requirements: the austenite grain size is not less than grade 8.0 under high-temperature carburizing conditions at 1000℃, and there is no mixed crystal phenomenon; and J5≤36HRC, J9≤32HRC.

[0042] In this invention, the designed gear steel has a narrow low hardenability bandwidth. The inventors discovered, based on literature research, that J5 and J9 of gear steel significantly affect the deformation during heat treatment. The gear steel designed in this invention, with a low hardenability bandwidth satisfying J5≤36HRC and J9≤32HRC, exhibits significantly reduced deformation during heat treatment. Here, J5 represents the hardness at 5mm from the end, and J9 represents the hardness at 9mm from the end.

[0043] Accordingly, another objective of the present invention is to provide a method for manufacturing the above-mentioned niobium microalloyed high-temperature carburized gear steel. This method is simple to produce and has a reasonable process design. The obtained niobium microalloyed high-temperature carburized gear steel has a narrow low hardenability bandwidth (J5≤36HRC, J9≤32HRC) and the austenite grain size under high-temperature carburizing conditions at 1000℃ is not less than grade 8.0, with no mixed crystal phenomenon.

[0044] To achieve the above objectives, the present invention proposes a method for manufacturing the aforementioned niobium microalloyed high-temperature carburized gear steel, comprising the following steps:

[0045] (1) Electric furnace smelting;

[0046] (2) LF refining;

[0047] (3) VD vacuum degassing treatment;

[0048] (4) Continuous casting;

[0049] (5) Heating: The steel billet is hot-charged into the heating furnace, wherein the temperature of the preheating section in the heating furnace is less than 900℃, the temperature of the heating section is controlled at 1140~1180℃, the soaking temperature in the heating furnace is 1160~1245℃, and the heating time in the heating furnace is not less than 3.5h.

[0050] (6) Rolling: The initial rolling temperature is controlled at 1100~1170℃, and the final rolling temperature is greater than 970℃.

[0051] In the above technical solution of the present invention, in the heating process of step (5), when the steel billet is hot-charged into the heating furnace for heating, the heating furnace can be specifically controlled as a regenerative walking beam type, wherein the preheating section temperature in the heating furnace is less than 900°C, the heating section temperature is controlled at 1140-1180°C, and the homogenization temperature in the heating furnace is controlled at 1160-1245°C to ensure that it is greater than the complete dissolution temperature of the Nb-containing precipitate phase of 1150°C, so that Nb can be fully dissolved back into the matrix and diffused uniformly, thereby helping to avoid the formation of coarse precipitates during the rolling process. Among them, in order to ensure the final quality and performance of the steel, the heating time needs to be controlled to be not less than 3.5 hours.

[0052] Accordingly, in the rolling process of step (6), the initial rolling temperature is specifically controlled to be 1100-1170°C and the final rolling temperature is greater than 970°C, so as to avoid rolling in the nose temperature range of 900-950°C where Nb precipitates and reduce the dwell time in this temperature range.

[0053] Furthermore, in the manufacturing method described in this invention, in step (1), the tapping temperature is controlled to be 1650℃~1680℃.

[0054] In some embodiments of the above technical solutions of the present invention, when smelting is actually carried out using an electric furnace, the C at the end point of steel tapping in the electric furnace smelting can be further controlled to be ≥0.03% and P≤0.012%, and the tapping temperature can be controlled to be 1650℃~1680℃.

[0055] Furthermore, in the manufacturing method described in this invention, in step (2), the final temperature of LF refining is controlled to be ≥1660℃.

[0056] In some embodiments of the above technical solution of the present invention, the LF refining process in step (2) can specifically adopt precipitation deoxidation + diffusion deoxidation, and then control the molten steel to enter the station to form white slag, and control the white slag holding time to be ≥15min and the smelting time to be ≥40min. Among them, at the end of the LF refining process, it is necessary to ensure that Cr≤1.05% and the final temperature of LF refining is ≥1660℃.

[0057] Furthermore, in the manufacturing method described in this invention, in step (3), the high vacuum time of the VD vacuum degassing treatment is controlled to be ≥20 min, the sedation time is controlled to be ≥20 min, and the VD final temperature is controlled to be ≥1550℃.

[0058] It should be noted that, in some embodiments, when actually performing the VD vacuum degassing process in step (3), the high vacuum time of the VD vacuum degassing process can be specifically controlled to be ≥20 min, the calming time can be controlled to be ≥20 min, and the aluminum wire is initially fed to 0.020% Al in the VD process, requiring Al / N ≥2, and the final temperature of the VD process can be controlled to be ≥1550℃.

[0059] Furthermore, in the manufacturing method described in this invention, in step (4), the superheat of the molten steel is controlled to be 18-35°C, and the continuous casting speed is 0.62-0.68 m / min.

[0060] Compared with existing technologies, the niobium microalloyed high-temperature carburizing gear steel and its manufacturing method described in this invention have the following advantages and beneficial effects:

[0061] In this invention, by rationally controlling the alloy content of C, Si, Mn, Cr, etc., and combining it with optimized processes, a low hardenability gear steel with a narrow hardenability bandwidth has been developed. It has low production cost, high purity, and is easy to process. The niobium microalloyed high-temperature carburizing gear steel prepared has excellent performance, with J5≤36HRC and J9≤32HRC.

[0062] In designing the chemical composition, the inventors further added 0.006%–0.03% Nb to the existing traditional 16MnCrS5 gear steel composition. Through Nb microalloying, the high-temperature stability of the Nb precipitates NbC and Nb(C,N) effectively improved the performance of the gear steel. Furthermore, the small size of the Nb precipitates, which are spherical or ellipsoidal, provides excellent pinning effect on grain boundaries, effectively inhibiting grain growth.

[0063] Accordingly, this invention also incorporates an optimized manufacturing process to ensure that the prepared niobium microalloyed high-temperature carburized gear steel has an austenite grain size ≥ 8.0 grade under high-temperature carburizing conditions at 1000℃, without exhibiting mixed grain phenomenon. This reduces gear heat treatment deformation and effectively meets the performance requirements of gear steel materials for applications such as automobiles and engineering machinery, demonstrating promising prospects and application value. In contrast, traditional 16MnCrS5 gear steel exhibits mixed grain phenomenon after carburizing at 960℃. Attached Figure Description

[0064] Figure 1 The image shows the austenite grain size of the niobium microalloyed high-temperature carburized gear steel from Example 1 after water quenching at 1000℃ for 4 hours.

[0065] Figure 2 The image shows the austenite grain size of the niobium microalloyed high-temperature carburized gear steel from Example 2 after water quenching at 1000℃ for 4 hours.

[0066] Figure 3 The image shows the austenite grain size of the niobium microalloyed high-temperature carburized gear steel of Example 3 after water quenching at 1000℃ for 4 hours.

[0067] Figure 4 Photographs showing the austenite grain size of conventional carburized gear steel 16MnCrS5 (Comparative Example 1) after water quenching at 960℃ for 4 hours.

[0068] Figure 5 Photograph of the austenite grain size of conventional carburized gear steel 16MnCrS5 in Comparative Example 1 after water quenching at 1000℃ for 4 hours. Detailed Implementation

[0069] The following will provide further explanation and description of the niobium microalloyed high-temperature carburizing gear steel and its manufacturing method according to the present invention, with reference to specific embodiments. However, such explanation and description do not constitute an undue limitation on the technical solution of the present invention.

[0070] Examples 1-6 and Comparative Example 1

[0071] The niobium microalloyed high-temperature carburized gear steels in Examples 1-6 were all prepared using the following steps:

[0072] (1) Electric furnace smelting is carried out according to the chemical composition shown in Table 1 below, and the C at the end point of tapping of the electric furnace smelting is controlled to be ≥0.03% and P ≤0.012%, and the tapping temperature is controlled to be 1650℃~1680℃.

[0073] (2) LF refining: precipitation deoxidation + diffusion deoxidation is adopted. White slag is generated when molten steel enters the station. The white slag holding time is controlled to be ≥15min, the smelting time is controlled to be ≥40min, the Cr at the LF endpoint is controlled to be ≤1.05%, and the final temperature of LF refining is controlled to be ≥1660℃.

[0074] (3) VD vacuum degassing treatment: control the high vacuum time of VD vacuum degassing treatment to ≥20min, control the calming time to ≥20min, feed aluminum wire to 0.020%Al in VD initial feeding, and require Al / N ≥2, while controlling the final temperature of VD to ≥1550℃.

[0075] (4) Continuous casting: Low superheat casting is adopted, the superheat of molten steel is controlled at 18 to 35℃, the casting cross section is 320×425mm, and the continuous casting speed is controlled at 0.62 to 0.68m / min.

[0076] (5) Heating: The steel billet is hot-loaded into the heating furnace. The heating furnace is a regenerative walking beam type. The temperature of the preheating section in the heating furnace is less than 900℃, the temperature of the heating section is controlled at 1140~1180℃, the temperature of the homogenization in the heating furnace is 1160~1245℃, and the heating time in the heating furnace is not less than 3.5h.

[0077] (6) Rolling: The initial rolling temperature is controlled at 1100~1170℃, and the final rolling temperature is greater than 970℃.

[0078] In this invention, the chemical composition design and related processes of the niobium microalloyed high-temperature carburizing gear steels in Examples 1-6 all meet the design specifications of this invention. Correspondingly, Comparative Example 1 is a conventional 16MnCrS5 gear steel prepared according to standard EN10084-2008, and its chemical composition design is also listed in Table 1 below.

[0079] Table 1 lists the mass percentage of each chemical element in the niobium microalloyed high-temperature carburized gear steel of Examples 1-6 and the comparative steel of Comparative Example 1.

[0080] Table 1. (wt.%, balance Fe and other unavoidable impurities other than P, As, Sn, Sb, Ca and O)

[0081]

[0082]

[0083] Tables 2-1 and 2-2 list the specific process parameters of the niobium microalloyed high-temperature carburized gear steel of Examples 1-6 and the comparative steel of Comparative Example 1 in the above process steps (1)-(6).

[0084] Table 2-1.

[0085]

[0086] Table 2-2.

[0087]

[0088] Samples of the niobium microalloyed high-temperature carburizing gear steels of Examples 1-6 and the comparative steel of Comparative Example 1 were taken respectively. High-temperature carburizing tests were performed on the gear steels of Examples 1-6, i.e., water quenching at 1000℃ for 4 hours. Then, the austenite grain size of the treated steel plates of the examples and comparative examples was rated according to the cutoff point method of GB / T 6394-2017. The relevant rating results are listed in Table 3 below.

[0089] Accordingly, for the conventional gear steel 16MnCrS5 prepared in Comparative Example 1, two sets of high-temperature carburizing tests were conducted on the prepared samples, namely, water quenching at 960℃ and 1000℃ for 4 hours respectively. Then, the austenite grain size of the two sets of steel plates of Comparative Example 1 after treatment was rated according to the cutoff point method of GB / T 6394-2017. The relevant rating results are also listed in Table 3 below.

[0090] Table 3 lists the austenitic grain size rating results obtained after the corresponding high-temperature carburizing tests for the gear steels of Examples 1-6 and Comparative Example 1.

[0091] Table 3.

[0092]

[0093] As can be seen from Table 3 above, after being subjected to high-temperature carburizing conditions of 1000℃ for 4 hours and water quenching, Examples 1-6 all exhibit excellent austenite grain size, with an austenite grain size of not less than grade 8.0, and no mixed crystal phenomenon.

[0094] The traditional gear steel 16MnCrS5 prepared in Comparative Example 1, after high-temperature carburizing conditions of holding at 960℃ for 4 hours and then water quenching, had an austenite grain size grade of 9.0 (0.0), where 9.0 represents a fine grain size and 0.0 represents a coarse grain size. However, after high-temperature carburizing conditions of holding at 1000℃ for 4 hours and then water quenching, its austenite grain size grade was 8.0 (4.0), where 8.0 represents a fine grain size and 4.0 represents a coarse grain size. Mixed grains were observed in both processes, indicating that the steel was not pure enough.

[0095] Accordingly, after completing the above observations and analyses, further samples were taken from the steels prepared in Examples 1-6 and Comparative Example 1, and J5 and J9 of each example and comparative example were calculated using the SEP1664 model. The relevant calculation results are listed in Table 4 below. J5 represents the hardness at a distance of 5 mm from the end, and J9 represents the hardness at a distance of 9 mm from the end.

[0096] Table 4 lists the calculated values ​​of J5 and J9 for the gear steels of each embodiment and comparative example.

[0097] Table 4.

[0098]

[0099] As can be seen from Table 4 above, the niobium microalloyed high-temperature carburized gear steels of Examples 1-6 prepared by the present invention have a narrow hardenability bandwidth, with J5 between 35.3-36.0 HRC and J9 between 25.5-27.9 HRC. In contrast, the hardenability bandwidth of Comparative Example 1 is higher, with J5 = 39.4 HRC and J9 = 32.1 HRC.

[0100] Figure 1 The image shows the austenite grain size of the niobium microalloyed high-temperature carburized gear steel from Example 1 after water quenching at 1000℃ for 4 hours.

[0101] In this invention, the austenite grain size of the niobium microalloyed high-temperature carburized gear steel prepared in Example 1 after water quenching at 1000℃ for 4 hours is as follows: Figure 1 As shown. The inventors rated the austenite grain size as grade 9.0 according to the cutoff point method in GB / T 6394-2017, and no mixed crystal phenomenon was found. This is because: in the 16MnCrS5 gear steel designed in this invention, Nb element is added to the steel, which can precipitate Nb(C,N) and NbC precipitates in the steel. Nb(C,N) and NbC have high melting points and can still maintain a relatively small size even after holding at 1100℃ for 5 hours. Therefore, the Nb precipitates can still effectively pin the austenite grains and inhibit austenite grain growth at 1000℃.

[0102] Figure 2 The image shows the austenite grain size of the niobium microalloyed high-temperature carburized gear steel from Example 2 after water quenching at 1000℃ for 4 hours.

[0103] Figure 3 The image shows the austenite grain size of the niobium microalloyed high-temperature carburized gear steel of Example 3 after water quenching at 1000℃ for 4 hours.

[0104] Accordingly, the austenite grain size of the niobium microalloyed high-temperature carburized gear steel prepared in Example 2 after water quenching at 1000℃ for 4 hours is as follows: Figure 2 As shown, its austenite grain size rating is 8.5, and there is no mixed grain phenomenon. The austenite grain size of the niobium microalloyed high-temperature carburized gear steel prepared in Example 3 after water quenching at 1000℃ for 4 hours is as follows. Figure 3 As shown, its austenite grain size rating is 9.0, and there is also no mixed crystal phenomenon.

[0105] Figure 4 Photographs showing the austenite grain size of conventional carburized gear steel 16MnCrS5 (Comparative Example 1) after water quenching at 960℃ for 4 hours.

[0106] Figure 5 Photograph of the austenite grain size of conventional carburized gear steel 16MnCrS5 in Comparative Example 1 after water quenching at 1000℃ for 4 hours.

[0107] like Figure 4 and Figure 5 As shown, the austenite grain size of the conventional gear steel 16MnCrS5 prepared in Comparative Example 1, after being water-quenched at 960℃ and 1000℃ for 4 hours each, is as follows: Figure 4 and Figure 5 As shown, the austenite grains grow abnormally.

[0108] According to the intercept point method in GB / T 6394-2017, Figure 4The steel plate processed using this method exhibits an austenite grain size of 9.0 (0.0) grade, showing mixed grain characteristics. This is because, at high temperatures, the precipitate phase that can inhibit austenite grain growth in Comparative Example 1 is mainly the AlN second phase. However, AlN undergoes dissolution at temperatures above 940℃, thus failing to effectively inhibit austenite grain growth. Furthermore, as the temperature increases, the AlN precipitate actually has a detrimental effect on inhibiting austenite grain growth, such as… Figure 5 As shown, the austenite grain size rating at this time is 8.0 (4.0).

[0109] It should be noted that 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.

[0110] It should also be noted that the embodiments listed above are merely specific embodiments of the present invention. Obviously, the present invention is not limited to the above embodiments, and similar changes or modifications made thereto are those that can be directly derived or easily conceived by those skilled in the art from the content disclosed in the present invention, and should all fall within the protection scope of the present invention.

Claims

1. A niobium microalloyed high temperature carburized gear steel, characterized in that, The mass percentage of each chemical element is: C: 0.14%~0.18%, Si: 0.05%~0.20%, Mn: 1.00%~1.20%, S: 0.020%~0.035%, Al: 0.020%~0.040%, Cr: 1.00%~1.10%, Ni: 0.10%~0.30%, N: 0.0080%~0.0160%, Nb: 0.006%~0.03%, Mo≤0.050%; the balance is Fe and other inevitable impurities; The performance meets: the austenite grain size is not less than 8.0 level under the high-temperature carburizing condition at 1000℃, without mixed crystal phenomenon; and J5≤36HRC, J9≤32HRC.

2. The niobium microalloyed high temperature carburized gear steel of claim 1 wherein, In the inevitable impurities, P≤0.02%, As≤0.02%, Sn≤0.02%, Sb≤0.010%, Ca≤0.004%, O≤0.0020%.

3. The niobium microalloyed high temperature carburized gear steel of claim 1 wherein, The content of each chemical element also meets at least one of the following: C：0.14％～0.17％； Si: 0.05%~0.18%; Mn: 1.10%~1.20%; Mo: 0.006%~0.045%; Ni: 0.10%~0.20%.

4. The niobium microalloyed high temperature carburized gear steel of claim 1 wherein, The microstructure has NbC and Nb(C,N) precipitated phases in the form of spherical or ellipsoidal.

5. The method of manufacturing a niobium microalloyed high-temperature carburized gear steel according to any one of claims 1 to 4, characterized in that, The method comprises the steps of: (1) electric furnace smelting; (2) LF refining; (3) VD vacuum degassing treatment; (4) continuous casting; (5) heating: the billet is hot charged into a heating furnace, the preheating temperature in the heating furnace is less than 900℃, the heating temperature is controlled at 1140~1180℃, the soaking temperature in the heating furnace is 1160~1245℃, and the heating time in the heating furnace is not less than 3.5h; (6) rolling: the open rolling temperature is controlled at 1100~1170℃, and the finish rolling temperature is greater than 970℃.

6. The production method according to claim 5, wherein In step (1), the tapping temperature is controlled at 1650℃~1680℃.

7. The production method according to claim 5, wherein In step (2), the final temperature of LF refining is controlled at ≥1660℃.

8. The production method according to claim 5, wherein In step (3), the high vacuum time of VD vacuum degassing treatment is controlled at ≥20min, the settling time is controlled at ≥20min, and the VD final temperature is controlled at ≥1550℃.

9. The production method according to claim 5, wherein In step (4), the superheat of the molten steel is controlled at 18~35℃, and the continuous casting withdrawal speed is controlled at 0.62~0.68m / min.

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