High-temperature carburizing gear steel for electric vehicles and method for manufacturing same

Abstract

This invention discloses a high-temperature carburized gear steel for electric vehicles, which contains Fe and unavoidable impurities, and also contains the following chemical elements in the following mass percentages: C: 0.18%–0.25%, Si: 0.15%–0.25%, Mn: 1.30%–1.40%, S: 0.015%–0.025%, Ni: 0.10%–0.20%, Cr: 1.20%–1.30%, Nb: 0.006%–0.02%, Al: 0.030%–0.045%, Ti: 0.006%–0.025%, N: 0.01%–0.015%, Cu≤0.1%, Mo≤0.020%; the high-temperature carburized gear steel does not contain V. Accordingly, the present invention also discloses a method for manufacturing the above-mentioned high-temperature carburized gear steel, which includes the following steps: (1) electric furnace smelting; (2) LF refining; (3) VD vacuum degassing treatment; (4) continuous casting; (5) heating: hot charging of steel billets into a heating furnace, wherein the temperature of the preheating section in the heating furnace is 850-900℃, the temperature of the heating section is controlled at 1140-1170℃, the soaking temperature in the heating furnace is 1160-1240℃, and the heating time in the heating furnace is not less than 4h; (6) rolling: controlling the initial rolling temperature at 1100-1170℃ and the final rolling temperature at 980-1020℃.

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, which has promoted the rapid progress of gear steel on a global scale to a certain extent. Countries around the world have gradually begun to strengthen their research and development of gear steel.

[0003] In the current era of carbon peaking and carbon neutrality, the automotive industry faces various challenges. The concept of carbon emissions throughout the entire lifecycle of automobiles has led to increased attention from automakers to both raw material supply and user manufacturing processes. High-temperature carburizing, which can improve production efficiency and reduce costs, is considered a green technology and is increasingly valued by users. It is well known that in the actual production of gear steel, carburizing is typically carried out at around 930℃, and the carburizing time is generally controlled to around 10 hours. However, 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, to shorten the carburizing time, many researchers have conducted extensive research on high-temperature carburizing processes at 960℃ and above.

[0004] However, research has found that when the carburizing temperature reaches 960℃ or above, the austenite grains in gear steel tend to coarsen, which can lead to a decrease in gear fatigue performance and even mixed grain phenomenon.

[0005] Currently, in order to suppress the abnormal growth of austenite grains after high-temperature carburizing of gear steel, microalloying or composite microalloying with Ti, Nb, V and Al is mainly used to form precipitates such as NbC, TiN, AlN, (Nb,Ti)(C,N) and V(C,N) to pin grain boundaries, thereby effectively suppressing the growth of austenite grains.

[0006] 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 element 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.

[0007] For example, Chinese patent document CN103361559A, published on October 23, 2013, entitled "A Nb, Ti Composite Microalloyed High-Temperature Carburizing Gear Steel," discloses a Nb, Ti composite microalloyed high-temperature carburizing gear steel. This technical solution, by adding 0.02%–0.06% Ti, 0.02%–0.06% Nb, and 0.015%–0.035% Al, can form Ti(C,N) and Nb(C,N) precipitates pinning grain boundaries, thereby ensuring that the austenite grain size of the gear steel after carburizing at 1000℃ is not less than grade 8.0. However, this technical solution adds a large amount of Nb, which easily leads to cracks in the cast billet and increases production costs, making it difficult to achieve industrial-scale production.

[0008] Therefore, in order to improve carburizing efficiency and overcome the limitation of carburizing temperature caused by austenite grain coarsening, current technologies mainly use the addition of large amounts of Nb, Ti, and V for alloying when designing high-temperature carburizing gear steel. This undoubtedly increases production costs and does not achieve the best effect of composite microalloying on austenite grain refinement.

[0009] Furthermore, with the rapid development of new energy electric vehicles and the continuous improvement of vehicle performance, higher requirements have been put forward for their non-metallic inclusions, grain size and other indicators. The traditional Mn-Cr series gear steel has the following requirements for inclusion grades: sulfide A≤2, alumina B≤2, silicate C≤1, spheroidal oxide D≤1, and [O] in the steel≤0.0020%. The current traditional gear steel can no longer meet the requirements of new energy electric vehicles for purity and performance quality.

[0010] Therefore, in order to meet market demands, in addition to requiring high-temperature carburizing performance of gear steel, the prepared gear steel also needs to have excellent purity. To this end, this invention, through rational chemical composition design, aims to obtain a new high-temperature carburizing gear steel and its manufacturing method while reducing alloy costs. Summary of the Invention

[0011] One of the objectives of this invention is to provide a high-temperature carburizing gear steel for electric vehicles. This high-temperature carburizing gear steel incorporates Nb, Ti, and Al elements. Through a reasonable chemical composition design, it can ensure that the austenite grain size is not less than grade 8.0 under high-temperature carburizing conditions of 1080℃, and that there is no mixed crystal phenomenon, thereby improving the carburizing efficiency of gear steel, reducing production costs, and reducing carbon emissions.

[0012] This high-temperature carburized gear steel not only has good austenite grain size stability, but also has the advantage of high purity. Its inclusion rating meets A≤1.5, B≤1.5, C≤1, and D≤1. It can meet the performance requirements of gear steel materials in application scenarios such as new energy electric vehicles, and improve the stability and safety of steel performance in new energy electric vehicles. It has good prospects for promotion and application value.

[0013] To achieve the above objectives, this invention proposes a high-temperature carburizing gear steel, which contains Fe and unavoidable impurities, and also contains the following chemical elements in the following mass percentages:

[0014] C: 0.18% ~ 0.25%, Si: 0.15% ~ 0.25%, Mn: 1.30% ~ 1.40%, S: 0.015% ~ 0.025%, Ni: 0.10%~0.20%, Cr: 1.20%~1.30%, Nb: 0.006%~0.02%, Al: 0.030%~0.045%, Ti: 0.006%~0.025%, N: 0.01%~0.015%, Cu≤0.1%, Mo≤0.020%;

[0015] The high-temperature carburized gear steel does not contain V.

[0016] Furthermore, in the high-temperature carburizing gear steel described in this invention, the mass percentage content of each chemical element is as follows:

[0017] C: 0.18%–0.25%, Si: 0.15%–0.25%, Mn: 1.30%–1.40%, S: 0.015%–0.025%, Ni: 0.10%–0.20%, Cr: 1.20%–1.30%, Nb: 0.006%–0.02%, Al: 0.030%–0.045%, Ti: 0.006%–0.025%, N: 0.01%–0.015%, Cu≤0.1%, Mo≤0.020%; balance Fe and other unavoidable impurities.

[0018] The design principles of each chemical element in the high-temperature carburizing gear steel described in this invention are as follows:

[0019] C: In the high-temperature carburizing gear steel described in this invention, carbon (C) is the most important element affecting the hardenability of MnCr series gear steel and is also the most basic and effective strengthening element in steel. However, it should be noted that the C content in the steel should not be too high. Since carburizing gear steel needs to ensure the toughness of the core, the C content in the steel cannot exceed 0.25%. Based on this, considering the influence of C content on the properties 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%.

[0020] 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 0.18% and 0.22%.

[0021] Si: In the high-temperature carburizing gear steel described in this invention, Si has a strong solid solution strengthening effect. Si can substitute Fe atoms in the steel through substitution, thereby hindering dislocation movement and significantly improving the yield strength of the steel. Furthermore, adding an appropriate amount of Si can also improve the 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 maximize the beneficial effects of Si, the mass percentage of Si in the high-temperature carburizing gear steel described in this invention is controlled between 0.15% and 0.25%.

[0022] 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 1.30% and 1.40%.

[0023] S: In the high-temperature carburizing gear steel described in this invention, sulfur (S) can combine with manganese (Mn) to form MnS inclusions, which can improve the machinability of the gear steel and significantly improve 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. Due to the increased number of ferrite nuclei, the ferrite-pearlite microstructure is refined. It should be noted that excessive S should not be added to the steel. When the S content in the steel is too high, it will lead to an excessively high inclusion content and a decrease in the fatigue performance of the steel. Therefore, considering the influence of S on the steel properties, the mass percentage of S in the high-temperature carburizing gear steel described in this invention is controlled between 0.015% and 0.025%.

[0024] Ni: In the high-temperature carburizing gear steel described in this invention, Ni can effectively improve the core toughness of the gear steel, lower the ductile-brittle transition temperature, improve low-temperature impact performance, and refine the microstructure, thus achieving a strengthening effect. However, the Ni content in the steel should not be too high, as excessive Ni content will reduce the machinability of the gear steel after hot working. Therefore, in the high-temperature carburizing gear steel described in this invention, the mass percentage of Ni is controlled between 0.10% and 0.20%.

[0025] Cr: In the high-temperature carburizing gear steel of this invention, Cr can increase the nucleation work and transformation activation energy of pearlite, thereby reducing the nucleation rate and growth rate of pearlite and increasing the stability of supercooled austenite. Adding an appropriate amount of Cr to the steel can achieve a good strengthening effect, improving the hardenability of the steel and having a positive impact on its impact toughness. Therefore, to maximize the beneficial effects of Cr, the mass percentage of Cr in the high-temperature carburizing gear steel of this invention is controlled between 1.20% and 1.30%.

[0026] 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) precipitates formed by Nb can effectively pin grain boundaries, hinder austenite grain growth, and reduce carburizing and quenching deformation. However, it should be noted that the Nb content in the steel should not be too high. When the Nb content exceeds a certain value, 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 these coarse precipitates cannot prevent austenite grain growth during high-temperature carburizing. Therefore, to maximize the beneficial effects of Nb, the mass percentage of Nb in the high-temperature carburizing gear steel described in this invention is controlled between 0.006% and 0.02%.

[0027] Of course, in some preferred embodiments, in order to obtain better implementation results, the mass percentage content of Nb element can be further preferably controlled between 0.01% and 0.015%.

[0028] Al: In the high-temperature carburizing gear steel described in this invention, Al can combine with N to form AlN. The AlN formed can effectively inhibit austenite grain growth and refine austenite grains during rolling heating and high-temperature carburizing. However, AlN will aggregate and dissolve at around 940℃, and as the temperature increases, Al's inhibition of austenite grain size has a detrimental effect. At the same time, the Al content should not be too high, as excessive Al content will lead to the formation of a large number of brittle Al2O3 inclusions and worsen the fatigue life of the gear steel. Therefore, in the high-temperature carburizing gear steel described in this invention, the mass percentage of Al is controlled between 0.030% and 0.045%.

[0029] Ti: In the high-temperature carburized gear steel described in this invention, Ti has a strong affinity for C, O, and N. The TiN and TiC phases precipitated by the combination of Ti and C and N can effectively hinder the growth of austenite grains and refine the grain size. Furthermore, the Ti(C,N) precipitated in the steel can act as nucleation sites for NbC, thereby promoting the precipitation of Nb-containing carbides and increasing the volume fraction of the precipitated phases. In addition, the Ti(C,N) precipitated in the steel also exhibits high-temperature stability, maintaining a relatively fine size even at 1100–1200℃, which is beneficial for hindering grain growth in gear steel at high temperatures. It should be noted that the Ti content in the steel should not be too high. When the Ti content is too high, coarse rhomboid TiN particles will form in the steel, which will affect the fatigue performance of the steel. Therefore, in the high-temperature carburized gear steel described in this invention, the mass percentage of Ti is controlled between 0.006% and 0.025%.

[0030] Of course, in some preferred embodiments, in order to obtain better implementation results, the mass percentage content of Ti element can be further preferably controlled between 0.015% and 0.02%.

[0031] N: In the high-temperature carburized gear steel described in this invention, the main role of N is to form AlN, TiN, and Nb(C,N) with elements such as Al, Ti, and Nb. Under high-temperature conditions, AlN, TiN, and Nb(C,N) can be dispersed at the austenite grain boundaries, hindering the migration of austenite grain boundaries 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. Based on this, in the high-temperature carburized gear steel described in this invention, the mass percentage of N is controlled between 0.01% and 0.015%.

[0032] Cu: In the high-temperature carburizing gear steel described in this invention, Cu can improve the strength of the steel and enhance its corrosion resistance. It is important to note that the Cu content in the steel should not be too high. If the Cu content is too high, it will accumulate at the grain boundaries during heating, leading to grain boundary weakening and even cracking. Therefore, to maximize the beneficial effects of Cu, an appropriate amount of Cu can be added to the high-temperature carburizing gear steel described in this invention, and the mass percentage of Cu should be controlled to meet the following requirement: Cu ≤ 0.1%.

[0033] Mo: In the high-temperature carburizing gear steel described in this invention, the proper combination of Mo and Cr can significantly improve the hardenability and tempering resistance of the steel, and Mo can refine the grain size. However, the content of Mo in the steel should not be too high. When the content of Mo in the steel is too high, it will lead to the formation of a ferrite film at the grain boundaries, which is not conducive to the hot plasticity of the steel. It will not only increase the tendency of the steel to reheat cracking, but also increase the 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 as: Mo ≤ 0.020%.

[0034] Furthermore, it is particularly important to note that in the high-temperature carburizing gear steel designed in this invention, the element V is not required. In this invention, the inventors discovered through prior laboratory studies that adding the strong carbide element V to 20MnCr gear steel did not result in the precipitation of fine MC carbides, but rather in the formation of larger MC-type carbides, which are predominantly M3C-type carbides in the equilibrium phase. Therefore, in the design of the chemical element composition, V is not added to the high-temperature carburizing gear steel designed in this invention; even if a very small amount of V is present, it is unavoidably introduced, and its V content is no greater than 0.005%.

[0035] Furthermore, in the high-temperature carburized gear steel described in this invention, among the unavoidable impurities, P≤0.015%, O≤0.0015%, As≤0.01%, Sn≤0.01%, Sb≤0.01%, Pb≤0.01%, and Bi≤0.01%.

[0036] In the above technical solution, P, O, As, Sn, Sb, Pb, and Bi are all impurity elements in steel. Where technical conditions permit, to obtain steel with better performance and superior quality, the content of these impurity elements in the material should be reduced as much as possible. Specifically, the mass percentage of As, Sn, Sb, Pb, and Bi, which are harmful impurity elements, should each be ≤0.01%, and the total mass percentage of these five elements should be controlled to ≤0.05%.

[0037] P: In this invention, phosphorus (P) tends to segregate at grain boundaries in steel, which reduces the grain boundary bonding energy and deteriorates the impact toughness of the steel. Therefore, in the high-temperature carburizing gear steel described in this invention, the mass percentage content of phosphorus is controlled to be P ≤ 0.015%.

[0038] O: In this invention, the element O can form oxides and composite oxides with Al in steel. In order to ensure the uniformity of steel structure and low-temperature impact energy and fatigue performance, the mass percentage content of the element O in the high-temperature carburizing gear steel of this invention is controlled to be: O≤0.0015%.

[0039] Furthermore, in the high-temperature carburizing gear steel described in this invention, the content of each chemical element also satisfies at least one of the following conditions:

[0040] C: 0.18%–0.22%;

[0041] Nb: 0.01%–0.015%;

[0042] Ti: 0.015%–0.02%.

[0043] Furthermore, in the high-temperature carburized gear steel described in this invention, its microstructure contains NbC, TiN, and (Nb,Ti)(C,N) precipitates.

[0044] Furthermore, in the high-temperature carburized gear steel described in this invention, the inclusion ratings satisfy A≤1.5, B≤1.5, C≤1, and D≤1.

[0045] In the above-described technical solution of this invention, the high-temperature carburized gear steel prepared also has the advantage of high purity, with its inclusion ratings satisfying A≤1.5, B≤1.5, C≤1, and D≤1. Wherein, A represents sulfide inclusions, B represents alumina inclusions, C represents silicate inclusions, and D represents spherical oxide inclusions.

[0046] Furthermore, in the high-temperature carburized gear steel described in this invention, the austenite grain size under high-temperature carburizing conditions at 1080℃ is not less than grade 8.0, and there is no mixed crystal phenomenon.

[0047] Accordingly, another objective of the present invention is to provide a manufacturing method for the above-mentioned high-temperature carburized gear steel. This manufacturing method is simple to produce and has a reasonable process design. The high-temperature carburized gear steel obtained has the advantage of high purity while having good austenite grain size stability. It can meet the performance requirements of gear steel materials for application scenarios such as new energy electric vehicles and has good promotion prospects and application value.

[0048] To achieve the above objectives, the present invention provides a method for manufacturing the aforementioned high-temperature carburized gear steel, comprising the following steps:

[0049] (1) Electric furnace smelting;

[0050] (2) LF refining;

[0051] (3) VD vacuum degassing treatment;

[0052] (4) Continuous casting;

[0053] (5) Heating: The steel billet is hot-charged into the heating furnace, wherein the temperature of the preheating section in the heating furnace is 850-900℃, the temperature of the heating section is controlled at 1140-1170℃, the temperature of the soaking in the heating furnace is 1160-1240℃, and the heating time in the heating furnace is not less than 4 hours.

[0054] (6) Rolling: The initial rolling temperature is controlled at 1100~1170℃ and the final rolling temperature is controlled at 980~1020℃.

[0055] In the heating process of step (5) of the present invention, when the steel billet is hot-charged into the heating furnace, the heating furnace can be specifically controlled as a regenerative walking beam type. The preheating section temperature in the heating furnace is controlled at 850–900°C, the heating section temperature is controlled at 1140–1170°C, and the homogenization temperature in the heating furnace is controlled at 1160–1240°C to ensure that it is above the complete dissolution temperature of the Nb-containing precipitate (1150°C), allowing Nb to fully dissolve back into the matrix and diffuse uniformly, thereby helping to avoid the formation of coarse precipitates during rolling. To ensure the final quality and performance of the steel, the heating time must be controlled to be no less than 4 hours.

[0056] 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 980-1020°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.

[0057] Furthermore, in the manufacturing method described in this invention, in step (1), the C at the end of the steel tapping is controlled to be ≥0.03%, and the steel tapping temperature is controlled to be 1650℃~1680℃.

[0058] 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≤0.010%, and the steel tapping temperature can be controlled between 1650℃ and 1680℃.

[0059] Furthermore, in the manufacturing method described in this invention, in step (2), the final temperature of LF refining is controlled to be ≥1650℃.

[0060] In some embodiments of the above technical solution of the present invention, the LF refining process in step (2) can be carried out by precipitation deoxidation + diffusion deoxidation, and then the molten steel is controlled to enter the station to make white slag, and the white slag holding time is controlled to be ≥20min, the smelting time is ≥40min, and the final temperature of LF refining is ≥1650℃.

[0061] Furthermore, in the manufacturing method described in this invention, in step (3), the vacuum degree of the VD vacuum degassing treatment is controlled to be ≤66.7Pa, the high vacuum time of the VD vacuum degassing treatment is controlled to be ≥15min, the sedation time is controlled to be ≥15min, and the final temperature of VD is ≥1550℃. In this step, nitrogen is added by bottom blowing throughout the entire process.

[0062] In the above technical solution of the present invention, the entire process of VD vacuum degassing in step (3) can be specifically carried out by bottom blowing nitrogen to prevent secondary oxidation of molten steel and improve the purity of molten steel.

[0063] Furthermore, in the manufacturing method described in this invention, in step (4), the frequency of the electromagnetic stirring in the crystallizer is controlled to be 2.0-3.0 Hz and the current is 300-305 A, the frequency of the electromagnetic stirring at the end is 8.0-8.5 Hz and the current is 590-610 A; the superheat of the molten steel is controlled to be 22-38°C and the continuous casting speed is controlled to be between 0.62-0.68 m / min.

[0064] In some embodiments of the above-mentioned technical solution of the present invention, in the continuous casting process of step (4), the frequency of the electromagnetic stirring in the crystallizer can be specifically controlled to be 2.0-3.0 Hz and the current to be 300-305 A, while the frequency of the electromagnetic stirring at the end is 8.0-8.5 Hz and the current is 590-610 A. In the continuous casting process, electromagnetic stirring can promote the rapid floating of larger inclusions, improve the purity of the molten steel, prevent inclusions from agglomerating, reduce the size of inclusions, and reduce the harm of inclusions to the properties of the steel.

[0065] Accordingly, in actual continuous casting, low superheat casting can be used, and the superheat of molten steel can be controlled between 22 and 38°C, with a casting cross-section of 320mm × 425mm, and the continuous casting speed can be controlled between 0.62 and 0.68m / min.

[0066] Compared with the prior art, the high-temperature carburizing gear steel and its manufacturing method described in this invention have the following advantages and beneficial effects:

[0067] (1) This invention promotes the precipitation of MC and MN type precipitates by adding Nb, Ti, and Al elements in combination. Furthermore, the increase in Ti content will be beneficial to the precipitation of MN type carbides. In actual design, the addition of Nb, Ti, and Al elements can form a composite alloy element precipitate phase (Nb, Ti)(C, N), which greatly improves the Ti / N ratio, reduces the influence of TiN on the fatigue performance of gear steel, and at the same time, the austenite grain size is significantly refined, which can effectively prevent grain growth and reduce the occurrence of mixed crystal phenomena.

[0068] (2) In this invention, by adding Ti to Nb-containing gear steel and optimizing the addition content of Nb, Ti and Al elements, the solid solution content of Nb can be effectively reduced, thereby increasing the precipitation and enhancing the pinning effect of grain boundaries. At the same time, the amount of Nb added is reduced, thereby effectively reducing the cost.

[0069] (3) When the high-temperature carburizing experiment was carried out on the composite high-temperature carburizing gear steel with Nb, Ti and Al elements designed in this invention, when the steel was controlled to be carburized at a temperature below 1000°C, the precipitation of the AlN precipitate phase formed could keep the grains small; while when the steel was controlled to be carburized at a higher temperature, its NbC and TiN could also exist stably at 1100 to 1200°C.

[0070] (4) This invention designs a novel high-temperature carburizing gear steel through reasonable chemical composition design and optimized process. Under high-temperature carburizing conditions at 1080℃, the austenite grain size is not less than grade 8.0, and there is no mixed crystal phenomenon, thereby improving the carburizing efficiency of gear steel, reducing production costs, and reducing carbon emissions. Simultaneously, this high-temperature carburizing gear steel not only has good austenite grain size stability but also the advantage of high purity. Its inclusion rating meets A≤1.5, B≤1.5, C≤1, and D≤1, and can meet the performance requirements of gear steel materials for applications such as new energy electric vehicles, improving the stability and safety of steel performance in new energy electric vehicles. It has good prospects for promotion and application value. Attached Figure Description

[0071] Figure 1 The image shows the austenite grain size of the high-temperature carburized gear steel from Example 1 after water quenching at 1080℃ for 4 hours.

[0072] Figure 2 The image shows the austenite grain size of the high-temperature carburized gear steel from Example 2 after water quenching at 1080℃ for 4 hours.

[0073] Figure 3 and Figure 4 The image shows the morphology of inclusions in the high-temperature carburized gear steel of Example 3.

[0074] Figure 5 for Figure 3 The energy dispersive spectroscopy (EDS) diagram of the inclusions is shown.

[0075] Figure 6 for Figure 4 The energy dispersive spectroscopy (EDS) diagram of the inclusions is shown.

[0076] Figure 7 The image shows the austenite grain size of the carburized gear steel after water quenching at 1080℃ for 4 hours, as a comparison of Example 1.

[0077] Figure 8 The image shows the austenite grain size of the carburized gear steel after water quenching at 1080℃ for 4 hours, as a comparison of Example 3. Detailed Implementation

[0078] The high-temperature carburizing gear steel and its manufacturing method described in this invention will be further explained and described below with reference to the accompanying drawings and specific embodiments. However, this explanation and description do not constitute an undue limitation on the technical solution of this invention.

[0079] Examples 1-6 and Comparative Examples 1-3

[0080] The high-temperature carburized gear steels in Examples 1-6 were all prepared using the following steps:

[0081] (1) Electric furnace smelting is carried out according to the chemical composition shown in Table 1 below, and the C at the end of the tapping of the electric furnace smelting is controlled to be ≥0.03% and P ≤0.010%, and the tapping temperature is controlled to be 1650℃~1680℃.

[0082] (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 ≥20min, the smelting time is ≥40min, and the final temperature of LF refining is controlled to be ≥1650℃.

[0083] (3) VD vacuum degassing treatment: control the vacuum degree of VD vacuum degassing treatment ≤66.7Pa, control the high vacuum time of VD vacuum degassing treatment ≥15min, the calming time ≥15min, and control the final temperature of VD ≥1550℃. In this step, bottom blowing nitrogen is used to increase nitrogen throughout the process to prevent secondary oxidation of molten steel.

[0084] (4) Continuous casting: The frequency of electromagnetic stirring in the crystallizer is controlled at 2.0-3.0 Hz and the current is 300-305 A. The frequency of electromagnetic stirring at the end is controlled at 8.0-8.5 Hz and the current is 590-610 A. Low superheat casting is adopted, and the superheat of molten steel is controlled between 22 and 38 ℃. The casting cross section is 320 mm × 425 mm, and the continuous casting speed is controlled between 0.62 and 0.68 m / min.

[0085] (5) Heating: The steel billet is hot-charged into the heating furnace, where the preheating section temperature is 850-900℃, the heating section temperature is controlled at 1140-1170℃, the soaking temperature in the heating furnace is 1160-1240℃, and the heating time in the heating furnace is not less than 4 hours.

[0086] (6) Rolling: The initial rolling temperature is controlled at 1100~1170℃ and the final rolling temperature is controlled at 980~1020℃.

[0087] In this invention, the high-temperature carburized gear steels of Examples 1-6 are all prepared using the above steps, and their chemical composition and related process parameters meet the design specifications and control requirements of this invention. However, the comparative carburized gear steels of Comparative Examples 1-3 are also prepared using the above-mentioned process flow of steps (1)-(6), but in their specific chemical composition design and related manufacturing processes, there are parameters that do not meet the design specifications of this invention.

[0088] Table 1 lists the mass percentage of each chemical element in the high-temperature carburized gear steels of Examples 1-6 and the comparative carburized gear steels of Comparative Examples 1-3.

[0089] Table 1. (wt.%, balance Fe and other unavoidable impurities other than P, O, As, Sn, Sb, Pb and Bi)

[0090]

[0091] As can be seen from Table 1 above, when designing the chemical element composition, Nb and Ti elements were not added in Comparative Example 1, Nb element was not added in Comparative Example 2, and Ti element was not added in Comparative Example 3.

[0092] Tables 2-1 and 2-2 list the specific process parameters of the high-temperature carburized gear steels of Examples 1-6 and the comparative carburized gear steels of Comparative Examples 1-3 in the above process steps (1)-(6).

[0093] Table 2-1.

[0094]

[0095] Table 2-2.

[0096]

[0097] To verify that the high-temperature carburized gear steels of Examples 1-6 of this invention can still maintain fine grains under high-temperature carburizing conditions of 1080℃, samples of the obtained finished high-temperature carburized gear steels of Examples 1-6 and the comparative carburized gear steels of Comparative Examples 1-3 were taken respectively. Simulated carburizing heat treatment was then performed on the sample gear steels of Examples 1-6 and Comparative Examples 1-3, i.e., the samples of each example and comparative example were heated to 1080℃, held at that temperature for 4 hours, and then water-quenched. Then, according to the cutoff point method of GB / T 6394-2017, the austenite grain size of the treated steel plates of the examples and comparative examples was rated. The relevant rating results are listed in Table 3 below.

[0098] Table 3 lists the austenite grain size ratings obtained after the corresponding high-temperature carburizing tests for the high-temperature carburizing gear steels of Examples 1-6 and the comparative carburizing gear steels of Comparative Examples 1-3.

[0099] Table 3.

[0100]

[0101] As can be seen from Table 3 above, after being subjected to high-temperature carburizing conditions of 1080℃ for 4 hours and then water quenched, the high-temperature carburized gear steels of Examples 1-6 all have excellent austenite grain size, with austenite grain size of not less than grade 8.0 and no mixed grain phenomenon.

[0102] Accordingly, the operators also observed the microstructure of the high-temperature carburized gear steels of Examples 1-6. The observation revealed that the microstructure of the high-temperature carburized gear steels of Examples 1-6 all had NbC, TiN and (Nb,Ti)(C,N) precipitates.

[0103] Further reference to Table 3 shows that, unlike Examples 1-6, the comparative carburized gear steels of Comparative Examples 1-3 prepared in this invention exhibited coarsening or mixed grain phenomena of austenite grains after water quenching at 1080℃ for 4 hours.

[0104] In Comparative Example 1, no Nb or Ti elements were added to the designed steel. Only AlN precipitates were generated, which hindered the growth of austenite grains during high-temperature carburizing. However, the AlN precipitates began to dissolve and aggregate at 940℃. As the carburizing temperature increased, AlN could not effectively hinder the growth of austenite grains, and some grains would grow preferentially, resulting in mixed crystals. After holding at 1080℃, the austenite grains all coarsened, thus finally obtaining an austenite grain size of grade 2.0.

[0105] In Comparative Example 2, Ti was added to the designed steel, which could further refine the austenite grains and inhibit the growth of austenite grains during high-temperature carburizing. However, the coarsening rate of Ti(C,N) precipitates was greater than that of Nb(C,N) and Nb,Ti composite precipitates. Therefore, adding Ti alone could not prevent the coarsening of austenite grains in gear steel at high temperatures, thus ultimately obtaining an austenite grain size of grade 4.0.

[0106] In Comparative Example 3, Nb was added to the designed steel. The NbC precipitate phase exhibits high-temperature stability and is small and spherical, effectively suppressing grain growth. However, when Nb exists in a solid solution state, it has no effect on pinning grain boundaries. Simultaneously, NbN precipitates preferentially over NbC, and NbN cannot effectively suppress grain growth. Therefore, during carburizing at 1080℃ in Comparative Example 3, some austenite grains grew abnormally, but fine austenite grains also existed, resulting in mixed grains. The austenite grain size of the comparative carburized gear steel in Comparative Example 3 after the above high-temperature carburizing treatment was 8.0 (0.0), where 8.0 represents a fine grain size level and 0.0 represents a coarse grain size level.

[0107] Accordingly, after completing the above observations and analyses, in order to further verify whether the inclusions in the high-temperature carburized gear steel of Examples 1-6 meet the design objectives, the inventors further sampled the high-temperature carburized gear steel of Examples 1-6 and analyzed it according to GB / T 10561-2005 "Determination of Non-metallic Inclusion Content in Steel". The inclusions in the samples of each example were rated, and the relevant rating results are listed in Table 4 below.

[0108] Table 4 lists the inclusion rating results for the high-temperature carburized gear steels of Examples 1-6.

[0109] Table 4.

[0110] serial number A B C D Example 1 1.0 1.0 0.5 0.5 Example 2 0.5 1.0 1.0 0.5 Example 3 1.0 1.0 1.0 0.5 Example 4 1.0 1.0 0.5 0.5 Example 5 0.5 1.0 1.0 0.5 Example 6 0.5 1.0 0.5 0.5

[0111] Note: In Table 4 above, A represents sulfide inclusions, B represents alumina inclusions, C represents silicate inclusions, and D represents spherical oxide inclusions.

[0112] As can be seen from Table 4 above, the high-temperature carburized gear steels of Examples 1-6 prepared by the chemical composition and optimized production process designed in this invention all have high purity. The inclusions in the high-temperature carburized gear steels of Examples 1-6 are all fine-grained. The ratings of sulfide inclusions A are all ≤1.5, the ratings of alumina inclusions B are all ≤1.5, the ratings of silicate inclusions C are all ≤1, and the ratings of spherical oxide inclusions D are all ≤1. They can meet the high purity requirements of steel for new energy electric vehicles.

[0113] Figure 1 The image shows the austenite grain size of the high-temperature carburized gear steel from Example 1 after water quenching at 1080℃ for 4 hours.

[0114] Figure 2 The image shows the austenite grain size of the high-temperature carburized gear steel from Example 2 after water quenching at 1080℃ for 4 hours.

[0115] In this invention, the austenite grain size of the high-temperature carburized gear steel prepared in Examples 1 and 2 after water quenching at 1080℃ for 4 hours is shown in the following photographs. Figure 1 and Figure 2 As shown.

[0116] The austenite grain size of the high-temperature carburized gear steels of Examples 1 and 2 after the above-mentioned high-temperature carburizing treatment was not less than grade 8.0, and no mixed grain phenomenon was found. This is because: Nb, Ti, and Al elements were added in combination in Examples 1-6. The combined addition of Nb, Ti, and Al is conducive to the precipitation of MC and MN type precipitates. When the steel is subjected to high-temperature carburizing below 1000℃, the precipitation of AlN precipitate phase keeps the grains small. Ti is a strong nitride element, which is more likely to combine with N element. It reduces the occurrence of NbN precipitate phase and allows more NbC precipitate phase to precipitate. At the same time, the combined addition of Nb, Ti, and Al forms a composite alloy element precipitate phase (Nb, Ti)(C, N), which has a higher solid solution temperature and greatly improves the Ti / N ratio, which can have a good effect on improving the austenite grain size.

[0117] In summary, under this design concept, based on the combined effect of NbC, TiN and (Nb, Ti)(C, N), it can be ensured that the high-temperature carburized gear steels prepared in Examples 1-6 of this invention can still obtain fine austenitic grains during high-temperature carburizing at 1080℃, and no mixed crystal phenomenon will occur.

[0118] Figure 3 and Figure 4 The image shows the morphology of inclusions in the high-temperature carburized gear steel of Example 3.

[0119] like Figure 3 and Figure 4 As shown, in this embodiment, the inclusions in the high-temperature carburized gear steel of Example 3 exhibit two different morphologies: one is elliptical (i.e., Figure 3 One type is spherical (i.e., its long side is about 8 μm, and its short side is about 2 μm); the other type is spherical (i.e., Figure 4 The sphere has a diameter of approximately 1.5 μm. Furthermore, based on its color, the spherical inclusions can be identified as composite inclusions.

[0120] Figure 5 for Figure 3 The energy dispersive spectroscopy (EDS) diagram of the inclusions is shown.

[0121] like Figure 5 As shown, energy dispersive spectroscopy (EDS) analysis was performed on the elliptical inclusions in this invention. Based on the elemental composition and morphology of the inclusions displayed by the EDS, it can be determined that... Figure 3 The elliptical inclusion shown is MnS, which belongs to type A inclusions.

[0122] Figure 6 for Figure 4 The energy dispersive spectroscopy (EDS) diagram of the inclusions is shown.

[0123] like Figure 6As shown, energy dispersive spectroscopy (EDS) analysis was performed on the circular inclusions in this invention. Based on the elements displayed in the EDS, it can be determined that... Figure 4 The spherical inclusion shown is composed of MnS and Al2O3.

[0124] Figure 7 The image shows the austenite grain size of the carburized gear steel after water quenching at 1080℃ for 4 hours, as a comparison of Example 1.

[0125] like Figure 7 As shown, the comparative carburized gear steel in Comparative Example 1 exhibits austenite grain coarsening, with an austenite grain size of 2.0 grade. This is because: in Comparative Example 1, only the AlN precipitate phase hinders the growth of austenite grains during high-temperature carburizing. However, the AlN precipitate phase begins to dissolve and aggregate at 940℃. As the carburizing temperature increases, AlN cannot effectively hinder austenite grain growth, and some grains preferentially grow, resulting in mixed grains. After holding at 1080℃, all austenite grains coarsen, such as... Figure 7 As shown.

[0126] Figure 8 The image shows the austenite grain size of the carburized gear steel after water quenching at 1080℃ for 4 hours, as a comparison of Example 3.

[0127] like Figure 8 As shown, the comparative carburized gear steel of Comparative Example 3 exhibits a mixed austenite grain phenomenon, with an austenite grain size of 8.0 (0.0), where 8.0 represents the fine grain size level and 0.0 represents the coarse grain size level.

[0128] Comparative Example 3, with the addition of Nb in its chemical composition design, resulted in NbC precipitates that exhibited high-temperature stability and small, spherical size, effectively suppressing grain growth. However, when Nb existed in a solid solution state, it had no effect on pinning grain boundaries. Furthermore, NbN preferentially precipitated over NbC, and NbN could not effectively inhibit grain growth. Therefore, in Comparative Example 3, the carburized gear steel exhibited abnormal growth of some austenite grains during carburizing at 1080℃, although fine austenite grains also remained, such as... Figure 8 As shown, mixed crystals appeared.

[0129] In summary, this invention, through reasonable chemical composition design and optimized production process, yields a high-temperature carburizing gear steel with composite additions of Nb, Ti, and Al elements. Under high-temperature carburizing conditions of 1080℃, the austenite grain size is not less than grade 8.0, and all inclusions are fine-grained, with a rating of A ≤1.5, B ≤1.5, C ≤1, and D ≤1, which can effectively meet the high purity requirements of steel for new energy vehicles.

[0130] The high-temperature carburizing gear steel designed in this invention can effectively improve the carburizing efficiency of users and reduce production costs. It is a green and low-carbon product with good prospects for promotion and application value.

[0131] 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.

[0132] 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 high temperature carburizing gear steel for electric vehicles, characterized by, Its mass percentage content of each chemical element is as follows: C: 0.18%~0.25%, Si: 0.15%~0.25%, Mn: 1.30%~1.40%, S: 0.015%~0.025%, Ni: 0.10%~0.20%, Cr: 1.20%~1.30%, Nb: 0.006%~0.02%, Al: 0.030%~0.045%, Ti: 0.006%~0.025%, N: 0.01%~0.015%, Cu≤0.1%, Mo≤0.020%; the balance is Fe and other unavoidable impurities. The inclusion rating of the high-temperature carburized gear steel meets the following criteria: A≤1.5, B≤1.5, C≤1, D≤1. The microstructure of the high-temperature carburized gear steel has NbC, TiN and (Nb,Ti)(C,N) precipitates; The high-temperature carburized gear steel has an austenite grain size of not less than grade 8.0 under high-temperature carburizing conditions of 1080℃, and there is no mixed crystal phenomenon.

2. The high temperature carburizing gear steel of claim 1 wherein, Among unavoidable impurities, P ≤ 0.015%, O ≤ 0.0015%, As ≤ 0.01%, Sn ≤ 0.01%, Sb ≤ 0.01%, Pb ≤ 0.01%, and Bi ≤ 0.01%.

3. The high temperature carburizing gear steel of claim 1 wherein, Its chemical element content also satisfies at least one of the following conditions: C：0.18%~0.22%； Nb: 0.01%~0.015%; Ti: 0.015%~0.02%.

4. The method of producing a high temperature carburizing gear steel according to any one of claims 1 to 3, characterized by, It includes the following steps: (1) Electric furnace smelting; (2) LF refining; (3) VD vacuum degassing treatment; (4) Continuous casting: The frequency of electromagnetic stirring in the crystallizer is controlled at 2.0-3.0 Hz and the current is 300-305 A. The frequency of electromagnetic stirring at the end is controlled at 8.0-8.5 Hz and the current is 590-610 A. The superheat of molten steel is controlled at 22~38℃ and the continuous casting billet pulling speed is controlled between 0.62~0.68 m / min. (5) Heating: The steel billet is hot-charged into the heating furnace, wherein the temperature of the preheating section in the heating furnace is 850~900℃, the temperature of the heating section is controlled at 1140~1170℃, the temperature of the homogenization in the heating furnace is 1160~1240℃, and the heating time in the heating furnace is not less than 4 hours. (6) Rolling: The initial rolling temperature is controlled at 1100~1170℃ and the final rolling temperature is controlled at 980~1020℃.

5. The production method according to claim 4, wherein In step (1), the C at the end of the tapping is controlled to be ≥0.03%, and the tapping temperature is controlled to be 1650℃~1680℃.

6. The production method according to claim 4, wherein In step (2), the final temperature of LF refining is controlled to be ≥1650℃.

7. The production method according to claim 4, wherein In step (3), the vacuum degree of VD vacuum degassing is controlled to be ≤66.7 Pa, the high vacuum time of VD vacuum degassing is controlled to be ≥15 min, the sedation time is controlled to be ≥15 min, and the final temperature of VD is ≥1550℃. In this step, nitrogen is added by bottom blowing throughout the process.

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