A free-cutting high temperature carburizing gear steel and a method of manufacturing the same

Abstract

The present application discloses a kind of free-cutting high-temperature carburizing gear steel, it contains Fe and inevitable impurities, also contains the following chemical elements with mass percentage as follows: C: 0.225~0.275%, Si: 0.05~0.40%, Mn: 0.55~0.85%, S: 0.016~0.040%, Cr: 0.85~1.25%, Ni: 0.02~0.25%, Mo: 0.15~0.30%, Al: 0.030~0.050%, N: 0.008~0.020%, Nb: 0.003~0.019%, Ti: 0.003~0.019%. Correspondingly, the present application also discloses the manufacturing method of the above-mentioned free-cutting high-temperature carburizing gear steel, which comprises the following steps: (1) smelting; (2) casting; (3) heating: first heating to not higher than 700 DEG C in preheating section, then continue to heat to not higher than 980 DEG C in first heating section, after holding, continue to heat to 950~1200 DEG C in second heating section, after holding, enter soaking section, the temperature of soaking section is 1050~1250 DEG C and holding, then continue rolling or forging; (4) forging or rolling.

Description

TECHNICAL FIELD

[0001] The present application relates to a kind of steel and its manufacturing method, especially a kind of gear steel and its manufacturing method. BACKGROUND

[0002] It is known that the surface of high-performance gears is generally subjected to carburizing and quenching + tempering treatment to obtain a higher hardness surface and a more ductile core, thereby achieving excellent fatigue life and wear resistance, etc. In recent years, in the face of high technical requirements for gears of automobile, especially new energy vehicle reducer and differential, the application of high-temperature carburizing technology has also become increasingly widespread. Using this high-temperature nitriding process not only can obtain surface-hardened gears with excellent performance, but also can greatly improve production efficiency, effectively save energy, reduce gas emissions, and thus protect the environment.

[0003] Currently, the commonly used gas carburizing temperature is generally not higher than 930℃, while the high-temperature vacuum carburizing can reach as high as 960℃, or even higher, due to its oxygen-free treatment environment. According to the calculation of carburizing principle, it is found that if the carburizing temperature is increased by about 50℃, the carburizing time for obtaining the same thickness of hardened layer can be shortened by about 50%. Therefore, if the carburizing temperature can be increased from 930℃ to 980℃, the carburizing time can be reduced to 50% of the original, significantly improving the production efficiency. In addition, the gears obtained by high-temperature vacuum carburizing have little or even no intergranular oxidation on the surface, which can significantly improve the impact fracture resistance.

[0004] Therefore, high-temperature vacuum carburizing technology has gradually become an inevitable choice to replace gas carburizing technology due to its own advantages.

[0005] In the current prior art, CrMo carburizing gear steel is widely used in new energy vehicle reducers and differentials due to its excellent comprehensive cost performance. A typical CrMo carburizing gear steel, such as 20CrMoH, is a structural steel widely used to ensure hardenability, with stable smelting process, good hardenability, and mature carburizing or carbonitriding process, which can be used to prepare medium and small modulus gears, shafts and other parts.

[0006] The main technical problem of CrMo high-temperature carburizing gear steel is how to increase the carburizing temperature while ensuring that the gear does not appear mixed crystal and grain coarsening. Once the grain abnormally grows, it is easy to cause heat treatment deformation and early fatigue fracture, which may affect the transmission efficiency and cause traffic accidents. Moreover, in order to cope with the quenching and tempering of complex shape gears, the application of gas quenching after high-temperature vacuum carburizing is becoming more and more widespread, which puts higher requirements on the hardenability of gear steel.

[0007] For example: the announcement number is CN101096742A, the publication date is January 2, 2008, and the name is "high strength automobile gear steel" of Chinese patent literature discloses a kind of high strength automobile gear steel, and its component mass percentage is: C:0.20~0.40%, Si:0.20~0.50%, Mn:0.50~1.00%, Cr:0.80~1.30%, Nb:0.015~0.080%, V:0.030~0.090%, Mo:0.15~0.55%, Al:0.015~0.050%, the rest is Fe and inevitable impurities.In this technical solution, after adding trace Nb, V element, the grain size, hardenability and bandwidth of gear steel can be obviously optimized;At the same time, the comprehensive mechanical properties of gear steel are increased, and the service life is extended.But the patent does not specify the specific carburizing temperature, adds Al, Nb and V and other micro-alloy elements, and only can meet the temperature requirement of conventional gas carburizing.

[0008] For example: the announcement number is CN103361559A, the publication date is October 23, 2013, and the name is "a kind of Nb, Ti compound micro-alloyed high-temperature carburizing gear steel" of Chinese patent, discloses a kind of Nb, Ti compound micro-alloyed 20CrMnTi easy cutting gear steel, and the component of steel material is: C:0.17~0.22%, Si:0.20-0.35%, Mn:0.9~1.10%, P:≤0.025%, S:0.020~0.035%, Cr:1.05~1.30%, Al:0.015~0.035%, Ti:0.02~0.06%, Nb:0.02~0.06%, balance is iron and inevitable impurities.In this technical solution, by controlling Nb, Ti and Al and other micro-alloy element content, gear carburizing temperature is improved or carburizing time is shortened, such as 1050℃*1h, or 1000℃*6h.The patent adds 0.02~0.06% of Ti and Nb, and can increase carburizing temperature to 1000℃.

[0009] For another example: the application number CN106967925A, the publication date is July 21, 2017, and the name is "a kind of high temperature carburizing gear steel with fine grain narrow hardenability bandwidth", a kind of high temperature carburizing gear steel is disclosed, its chemical composition includes C:0.19~0.21%, Si:0.20~0.30%, Mn:0.70~0.80%, P≤0.010%, S≤0.005%, Cr:1.10~1.20%, Mo:0.35~0.38%, Al:0.025~0.055%, Ca:0.015~0.0025%, N:0.0080~0.020%, [O]≤0.0015%, the rest is Fe and inevitable impurities.The steel material designed by the technical scheme still keeps in the range of 15~20μm after 980-1000℃ high temperature carburizing treatment, and the grain size is controlled in 7-8 level.

[0010] Theoretical analysis and experimental research show that adding Al, Nb, V, Ti and N elements in CrMo carburizing gear steel can prevent grain coarsening during high temperature carburizing by using carbonitride, but there are still problems such as abnormal grain growth of gear at high temperature, unstable grain size of gear steel obtained in mass production.

[0011] With the rapid development of new energy vehicles, in order to meet the increasingly high technical requirements of carburizing gear steel for variable speed system, the research and manufacture of CrMo high temperature (vacuum) carburizing gear steel suitable for high hardenability and easy cutting is imminent.

[0012] Therefore, in order to overcome the problems in the prior art, the present application economically obtains a high temperature carburizing gear steel which not only has good high temperature grain stability, but also has high and narrow hardenability, and is easy to process, can be effectively applied to high-end parts such as gearbox for automobile or reducer and differential for new energy vehicle, has good use prospect and value. SUMMARY

[0013] One of the purposes of the present application is to provide an easy-to-cut high temperature carburizing gear steel, which can obtain a high temperature carburizing gear steel not only having good high temperature grain stability, but also having narrow hardenability bandwidth and being easy to process by optimizing the chemical element composition design of gear steel and reasonably controlling the content of micro-alloying elements and nitrogen elements in gear steel.

[0014] In order to achieve the above purpose, the present application provides an easy-to-cut high temperature carburizing gear steel, which contains Fe and inevitable impurities, and also contains the following chemical elements with mass percentage as follows:

[0015] C: 0.225-0.275%, Si: 0.05-0.40%, Mn: 0.55-0.85%, S: 0.016-0.040%, Cr: 0.85-1.25%, Ni: 0.02-0.25%, Mo: 0.15-0.30%, Al: 0.030-0.050%, N: 0.008-0.020%, Nb: 0.003-0.019%, Ti: 0.003-0.019%.

[0016] Further, in the easy-to-cut high-temperature carburizing gear steel described in the present application, the mass percentage of each chemical element is as follows:

[0017] C: 0.225-0.275%, Si: 0.05-0.40%, Mn: 0.55-0.85%, S: 0.016-0.040%, Cr: 0.85-1.25%, Ni: 0.02-0.25%, Mo: 0.15-0.30%, Al: 0.030-0.050%, N: 0.008-0.020%, Nb: 0.003-0.019%, Ti: 0.003-0.019%; the balance being Fe and other unavoidable impurities.

[0018] In the easy-to-cut high-temperature carburizing gear steel described in the present application, the design principles of each chemical element are as follows:

[0019] C: In the easy-to-cut high-temperature carburizing gear steel described in the present application, C is an essential element in steel, and it is also one of the most important elements affecting the hardenability of steel. Carburizing gear steel needs to have sufficient core impact toughness while obtaining high surface strength. When the C content in steel is too low, the strength of the steel is insufficient, and good hardenability requirements cannot be guaranteed. Correspondingly, the C content in steel should not be too high. When the C content in steel is too high, the toughness of the gear core cannot be met, and high C content is not conducive to the plasticity of steel, especially for carburizing gear steel with high Mn content. When the C content is greater than 0.275%, it will have an adverse effect on the machinability of the steel. Therefore, in the easy-to-cut high-temperature carburizing gear steel described in the present application, the mass percentage of C is controlled between 0.225-0.275%.

[0020] Si: In the easy-to-cut high temperature carburizing gear steel described in the present application, the Si element can not only better eliminate the adverse effects of iron oxide on the steel, but also can dissolve into ferrite to strengthen the ferrite, improve the strength, hardness, wear resistance and elasticity and elastic limit of the steel. At the same time, it needs to be noted that the Si element can increase the Ac3 temperature of the steel, and has the risk of cracking and decarburization due to poor thermal conductivity. Considering the beneficial effects and adverse effects of Si, in the easy-to-cut high temperature carburizing gear steel described in the present application, the mass percentage content of the Si element is controlled between 0.05-0.40%.

[0021] Mn: In the easy-to-cut high temperature carburizing gear steel described in the present application, Mn is one of the main elements affecting the hardenability of the steel. The deoxidizing ability of Mn element is very good, which can reduce the iron oxide in the steel and effectively improve the yield of the steel. At the same time, Mn can dissolve into ferrite to improve the strength and hardness of the steel, and make the steel obtain fine lamellar and high strength pearlite after hot rolling and cooling. In addition, Mn can form MnS with S in the steel, which can eliminate the harmful effect of S, has the ability to form and stabilize austenite structure of the steel, can strongly increase the hardenability of the steel, and reduce the red toughness of the steel. When the content of Mn element in the steel is less than 0.55%, the hardenability of the steel is insufficient; and when the content of Mn element in the steel is too high, the hot plasticity of the steel will be poor, which will affect the production, and the steel is prone to crack when water quenching. Therefore, in the easy-to-cut high temperature carburizing gear steel described in the present application, the mass percentage content of the Mn element is controlled between 0.55-0.85%.

[0022] S: In the easy-to-cut high temperature carburizing gear steel described in the present application, S generally exists as an impurity element in the steel, which can significantly reduce the plasticity and toughness of the steel, and a certain content of S element can form non-metallic inclusions with Mn, and appropriate amount of S can improve the cutting performance of the steel. Therefore, considering the beneficial effects of S element, in the easy-to-cut high temperature carburizing gear steel described in the present application, the mass percentage content of the S element is controlled between 0.016-0.040%.

[0023] Cr: In the easy-to-cut high temperature carburizing gear steel described in the present application, Cr is one of the main alloying elements added in the steel, which can significantly improve the hardenability and properties such as strength and wear resistance of the steel. In addition, the Cr element can also reduce the activity of C element in the steel, which can prevent decarburization during heating, rolling and heat treatment, but too high Cr will significantly reduce the toughness of quenched and tempered steel, and form coarse carbides distributed along the grain boundary. Therefore, in the easy-to-cut high temperature carburizing gear steel described in the present application, the mass percentage content of the Cr element is controlled between 0.85-1.25%.

[0024] Ni: In the high temperature carburizing gear steel with easy cutting according to the present application, the element Ni exists in the steel in solid solution form, which can effectively improve the low temperature impact performance of the steel. However, it should be noted that too high content of Ni will result in too high content of retained austenite in the steel, thereby reducing the strength of the steel. Therefore, considering the production cost and competitiveness, the mass percentage content of the element Ni in the high temperature carburizing gear steel with easy cutting according to the present application is controlled between 0.02-0.25%.

[0025] Mo: In the high temperature carburizing gear steel with easy cutting according to the present application, the element Mo can exist in the steel in solid solution form, which is beneficial to improve the hardenability of the steel and improve the strength of the steel. At a higher temperature tempering, Mo will form fine carbides, thereby further improving the strength of the steel; in addition, the combined action of Mo and Mn can also significantly improve the stability of austenite. Considering that Mo is a precious metal element and its cost is relatively high, in order to control the production cost, the mass percentage content of the element Mo in the high temperature carburizing gear steel with easy cutting according to the present application is controlled between 0.15-0.30%.

[0026] Al: In the high temperature carburizing gear steel with easy cutting according to the present application, Al is a grain refining element. The element Al cooperates with N to further refine the grain and improve the toughness of the steel. Grain refinement plays an important role in improving the mechanical properties of the steel, especially the strength and toughness, and grain refinement also helps to reduce the hydrogen brittleness sensitivity of the steel. However, it should be noted that the content of Al in the steel should also not be too high, too high content of Al can easily increase the opportunity of inclusion in the steel. Therefore, the mass percentage content of the element Al in the high temperature carburizing gear steel with easy cutting according to the present application is controlled between 0.030-0.050%.

[0027] N: In the high temperature carburizing gear steel with easy cutting according to the present application, N is an interstitial atom, which can form MN type precipitates ("M" refers to alloying elements) with micro-alloying in the steel, and can pin the grain boundary at high temperature, thereby inhibiting the growth of austenite grains. When the content of N element in the steel is low, the MN formed is less, and the pinning effect is not obvious; while when the content of N element in the steel is too high, it is easy to enrich in steelmaking, thereby reducing the toughness of the steel. Therefore, the mass percentage content of the element N in the high temperature carburizing gear steel with easy cutting according to the present application is controlled between 0.008-0.020%.

[0028] Nb: In the high temperature carburizing gear steel with easy cutting according to the present application, Nb element is added into the steel, which can form fine precipitates, so as to play a role in inhibiting the recrystallization of the steel and effectively refining the grains. It should be noted that the content of Nb element in the steel should also not be too high, and when the content of Nb in the steel is too high, coarse NbC particles will be formed in the smelting process, which will reduce the impact toughness of the steel. Therefore, in the high temperature carburizing gear steel with easy cutting according to the present application, the mass percentage of Nb element is controlled to be between 0.003-0.019%.

[0029] Ti: In the high temperature carburizing gear steel with easy cutting according to the present application, although Ti can form fine precipitates when added into the steel, when the content of Ti element in the steel is too high, coarse TiN particles with edges will be formed in the smelting process, which will reduce the impact toughness of the steel. Therefore, in order to exert the beneficial effects of Ti element, the mass percentage of Ti element in the high temperature carburizing gear steel with easy cutting according to the present application is controlled to be between 0.003-0.019%.

[0030] Further, in the high temperature carburizing gear steel with easy cutting according to the present application, it also contains 0

[0031] In the above technical solution of the present application, in order to obtain a more optimal implementation effect, an appropriate amount of Cu element and V element can be further added into the high temperature carburizing gear steel with easy cutting, and both Cu and V elements can further improve the performance of the high temperature carburizing gear steel with easy cutting according to the present application.

[0032] Cu: In the high temperature carburizing gear steel with easy cutting according to the present application, Cu can improve the strength of the steel and is beneficial to improve the weather resistance and corrosion resistance of the steel. Of course, the content of Cu element in the steel should also not be too high, and if the content of Cu in the steel is too high, it will be enriched in the grain boundary during the heating process, which will lead to the weakening of the grain boundary and cracking. Therefore, in the high temperature carburizing gear steel with easy cutting according to the present application, the mass percentage of Cu can be preferably controlled to be 0

[0033] V: In the high temperature carburizing gear steel with easy cutting according to the present application, V can effectively improve the hardenability of the steel. V element can form precipitates with C element or N element in the steel, so as to further improve the strength of the steel. Of course, if the content of C element and V element is too high, coarse VC particles will be formed. Therefore, considering the production cost and competitiveness, in the high temperature carburizing gear steel with easy cutting according to the present application, the mass percentage of V element is controlled to be 0

[0034] Further, in the free-cutting high-temperature carburizing gear steel, among the inevitable impurities, P≤0.015%, O≤0.0020%, and B≤0.0005%.

[0035] In the above technical solution, P, B, and O are all impurity elements in the steel, and in the case where the technical conditions permit, the content of the impurity elements in the steel should be reduced as much as possible in order to obtain a steel material with better performance and higher quality.

[0036] P: In the present application, P elements tend to segregate at the grain boundaries in the steel, which will reduce the grain boundary binding energy and deteriorate the impact toughness of the steel. Therefore, in the free-cutting high-temperature carburizing gear steel, the mass percentage content of P elements is controlled to be P≤0.015%.

[0037] O: In the present application, O elements can form oxides and complex oxides with Al elements in the steel, and in order to ensure the uniformity of the steel structure and the low-temperature impact energy and fatigue performance, the mass percentage content of O elements in the free-cutting high-temperature carburizing gear steel is controlled to be O≤0.0020%.

[0038] B: In the present application, the impurity element B is an element that is relatively sensitive to hardenability, and since B elements tend to segregate, a slight change in the content of B will cause a large fluctuation in the hardenability of the steel material. Adding B elements to the gear steel is not conducive to the narrow control of the hardenability band width of the gear steel. Therefore, in the free-cutting high-temperature carburizing gear steel, the content of B elements in the steel must be strictly controlled, and the content of the impurity element B is controlled to be B≤0.0005%.

[0039] Further, in the free-cutting high-temperature carburizing gear steel, the value of the critical ideal diameter DI of the hardenability is 2.5-4.0; wherein:

[0040] DI=0.54xCx(3.33Mn+1)x(0.70Si+1)x(0.36Ni+1)x(2.16Cr+1)x(3.00Mo+1)x(0.36Cu+1)x(1.73V+1);

[0041] In the formula, each chemical element is substituted into the numerical value before the percentage sign of the mass percentage content of the chemical element.

[0042] In the above technical solution of the present application, the free-cutting high-temperature carburizing gear steel can also preferably control the value of the critical ideal diameter DI to be between 2.5-4.0 inches. This is because: when the DI value is low, the hardenability of the steel material will be insufficient; and when the DI value is high, manufacturing difficulties will occur, and the cost will be high.

[0043] Further, in the easy-to-cut high-temperature carburizing gear steel provided by the application, the micro-alloy element coefficient r M / X ranges from 1 to 2; wherein:

[0044] r M / X =(20*[Nb] / 93+[Ti] / 48-[V] / 510+[Al] / 27-[Mo] / 1920) / ([N] / 14+[C] / 120);

[0045] In the formula, each chemical element is substituted into the numerical value in front of the percentage symbol of the mass percentage content of the chemical element.

[0046] In the above technical solution of the application, the inventor optimizes the range of the micro-alloy element coefficient r M / X . Al, Nb and Ti are main grain-refining micro-alloy elements. The positive effect of the application is to control the content of Mo, Al, Nb, Ti, V and N in the gear steel and the micro-alloy element coefficient r M / X . By adding appropriate amounts of Mo and Al, Nb, Ti and V, nano-scale precipitates are formed with N and C elements in the steel, thereby inhibiting the growth of austenite grains at a high temperature.

[0047] Further, in the easy-to-cut high-temperature carburizing gear steel provided by the application, the austenite grain size temperature before and after high-temperature vacuum carburization at 940-1020 DEG C is kept at 5-8 levels.

[0048] The easy-to-cut high-temperature carburizing gear steel prepared by the application keeps the austenite grain size at 5-8 levels before and after high-temperature vacuum carburization at 940-1020 DEG C, and can be used to prepare high-end parts such as gearboxes for automobiles, reducers and differentials for new energy vehicles, etc., and has good application prospect and use value.

[0049] Further, in the easy-to-cut high-temperature carburizing gear steel provided by the application, the hardenability at J9mm is 35-40HRC.

[0050] Correspondingly, another object of the application is to provide a manufacturing method of the above-mentioned easy-to-cut high-temperature carburizing gear steel, which is simple to produce, and the obtained high-temperature carburizing gear steel has the characteristics of high-temperature austenite stability, narrow hardenability band, high strength and toughness, easy cutting, high dimensional accuracy and high fatigue performance, and can be used to prepare high-end parts such as gearboxes for automobiles, reducers and differentials for new energy vehicles, etc., and has good popularization prospect and application value.

[0051] In order to achieve the above-mentioned object, the application provides a manufacturing method of the above-mentioned easy-to-cut high-temperature carburizing gear steel, which comprises the following steps:

[0052] (1) smelting;

[0053] (2) casting;

[0054] (3) heating: firstly heated to not higher than 700℃ in a preheating section, then continued to be heated to not higher than 980℃ in a first heating section, after holding, continued to be heated to 950-1200℃ in a second heating section, after holding, entered into a soaking section, the temperature of the soaking section is 1050-1250℃, and holding for enough time to make the temperature of the core of the billet same as the temperature of the surface, then subsequent rolling or forging is carried out;

[0055] (4) forging or rolling.

[0056] In the above technical solution of the present application, the inventors have optimized the heating process, in the heating process of step (3), compared with the prior art, the technical solution adopted has a higher temperature of the soaking section, the higher temperature of the soaking section can help to improve the composition uniformity and the organization uniformity of the continuous casting billet in the diffusion process of the heating of the billet. Meanwhile, at this temperature, the precipitated phase has a faster solid solution speed, therefore, the higher temperature of the soaking section will make more of the original undissolved precipitated phase particles in the steel dissolve, increase the concentration of the micro-alloying elements in the matrix, and precipitate more dispersed particles when cooled later.

[0057] In addition, only after the heating temperature is increased, the finish rolling temperature can be increased, the austenite recovery recrystallization after rolling can be more sufficient, and the distribution of the precipitated phase can be more uniform.

[0058] Correspondingly, in the technical solution of the present application, the smelting in step (1) of the manufacturing method of the present application can adopt electric furnace smelting or converter smelting, and can be subjected to refining and vacuum treatment. Of course, in some other embodiments, vacuum induction furnace can also be used for smelting.

[0059] In step (1), when electric furnace smelting is adopted, the furnace charge of the electric furnace smelting can select low P, S scrap steel, cut head and high-quality pig iron; the alloy can prepare chromium iron, low-phosphorus manganese iron, molybdenum iron, etc.; the reducing agent can include: calcium carbide, carbon powder and aluminum powder; in the oxidation period: frequently flow slag to remove P; the out-slag condition can be controlled as: the out-slag temperature is 1630-1660℃; P≤0.015%; the out-steel condition can be controlled as: the out-steel temperature is 1630-1650℃; [P]≤0.010%, [C]≥0.03%.

[0060] Accordingly, in step (2), the casting can be performed by mold casting or continuous casting. In the continuous casting process, the high-temperature molten steel in the ladle can be poured into the tundish through the protection sleeve. The molten steel in the tundish can be fully stirred by the electromagnetic stirring through the continuous casting crystallizer, and then a qualified continuous casting billet with a section size of 140mmx140mm to 320mmx425mm can be poured. In the technical solution, the pouring speed can be controlled to be 0.6-1.9m / min according to different square billet sizes, and then the continuous casting billet is slowly cooled in the slow cooling pit, and the slow cooling time is not less than 24 hours.

[0061] Further, in the manufacturing method, in step (4), direct forging or rolling to the final product size.

[0062] In the present application, when forging is performed, direct forging to the final product size can be performed; and when rolling is performed, the steel billet can be directly rolled to the final product size, or the steel billet can be first rolled to a specified intermediate billet size, and then heated and rolled to the final product size.

[0063] Further, in the manufacturing method, in step (4), the intermediate billet is first rolled to an intermediate billet size, then the intermediate billet is segmented heated, and then rolled to the final product size; wherein, when the intermediate billet is segmented heated, the intermediate billet is heated to not higher than 700℃ in the preheating section, then continuously heated to not higher than 1050℃ in the first heating section, after holding, continuously heated to 950-1200℃ in the second heating section, after holding, enters the soaking section, the soaking section temperature is 1100-1250℃, and holding for enough time to make the core temperature of the steel billet the same as the surface temperature, then discharged for rolling.

[0064] Further, in the manufacturing method, in step (4), the open forging or open rolling temperature is controlled to be between 1050-1250℃, and the final forging or final rolling temperature is controlled to be ≥900℃.

[0065] In the above technical solution, in step (3) of the manufacturing method, after the steel billet is discharged, the high-pressure water descaling is used to remove the oxide skin, the open forging or open rolling temperature is controlled to be between 1050-1250℃, and the final forging or final rolling temperature is controlled to be ≥900℃. This is because: under this process, the N element is beneficial to be desorbed from the austenite solid solution and combined with the micro-alloying elements in the steel to form nitrides.

[0066] Compared with the prior art, the easy-to-cut high-temperature carburizing gear steel and the manufacturing method thereof have the following advantages and beneficial effects:

[0067] (1) The present application can develop high-temperature austenite grain stable gear steel by reasonable chemical composition design and combined with optimized process, and the easy cutting high-temperature carburizing gear steel rolled or forged rod can be effectively processed into gears, which still has suitable hardenability, strength and toughness, and wear resistance and fatigue resistance after high-temperature carburizing heat treatment.

[0068] (2) In the present application, the inventors control the content of micro-alloying elements and nitrogen and carbon elements, and strictly control the atomic molar ratio, add appropriate Mo, Al, Nb, Ti and V elements to hinder the abnormal growth of high-temperature austenite grains, increase the austenite grain coarsening temperature of the gear steel, so that the grain size of the easy cutting high-temperature carburizing gear steel is still stable at 5-8 levels before and after high-temperature vacuum carburizing up to 940-1020℃, and the performance reaches the performance index of gear steel.

[0069] (3) The composition and process design of the easy cutting high-temperature carburizing gear steel designed in the present application is reasonable, which controls the content of micro-alloying elements in the steel to avoid the presence of large particle harmful inclusions in the steel, which not only ensures the stable production quality of the steel, but also reduces the production cost of the steel, and realizes batch production on the rod production line.

[0070] As can be seen from the above, the austenite grain size, hardenability and cost competitiveness of the easy cutting high-temperature carburizing gear steel of the present application are superior to those of the prior art, which can control the types and amounts of alloying elements in the steel under the premise of ensuring high-temperature carburizing, high hardenability and narrow bandwidth, and improve the applicability of the steel.

[0071] In practical application, the application of the easy cutting high-temperature carburizing gear steel of the present application can greatly shorten the carburizing time of the gear, reduce the production cost of the gear, reduce CO2 emission, meet the requirements of environmental protection and energy saving, and has broad industrial application prospect. DETAILED DESCRIPTION

[0072] The easy cutting high-temperature carburizing gear steel and the manufacturing method thereof of the present application will be further explained and described below in combination with specific examples, however, the explanation and description do not constitute undue limitation on the technical solutions of the present application.

[0073] Examples 1-8 and Comparative Examples 1-4

[0074] The easy cutting high-temperature carburizing gear steel of Examples 1-8 is prepared by the following steps:

[0075] (1) Smelting according to the chemical composition shown in Table 1-1 and Table 1-2: wherein smelting can be carried out by using a 50kg vacuum induction furnace, a 150kg vacuum induction furnace or a 500kg vacuum induction furnace, or by using an electric furnace smelting + secondary refining + vacuum degassing, or by using a converter smelting + secondary refining + vacuum degassing.

[0076] Of course, when smelting by using an electric furnace, the furnace charge of electric furnace smelting can be selected from low P, S scrap steel, cut head and high-quality pig iron; the alloy can be prepared from ferrochrome, low-phosphorus ferromanganese, ferromolybdenum, etc.; the reducing agent can include: calcium carbide, carbon powder and aluminum powder; during the oxidation period: frequently flow slag to remove P; and the tapping conditions can be controlled as follows: the tapping temperature is 1630-1660℃; P≤0.015%; the tapping conditions can be controlled as follows: the tapping temperature is 1630-1650℃; [P]≤0.010%, [C]≥0.03%.

[0077] (2) Casting: casting by using mold casting or continuous casting to obtain ingots.

[0078] When continuous casting is used for casting, during the continuous casting pouring process, the high-temperature molten steel in the ladle is poured into the tundish through the protection sleeve. The molten steel in the tundish passes through the continuous casting crystallizer and electromagnetic stirring to fully fill the 140mm×140mm-320mm×425mm section size of qualified continuous casting billets; in the present technical solution, the pouring speed can be controlled to be 0.6-1.9m / min according to different square billet sizes, and then the continuous casting billets are put into the slow cooling pit for slow cooling, and the slow cooling time is not less than 24 hours.

[0079] (3) Heating: the billet is first heated to not higher than 700℃ in the preheating section, then continuously heated to not higher than 980℃ in the first heating section, after holding, continuously heated to 950-1200℃ in the second heating section, after holding, enters the soaking section, controls the soaking section temperature to be 1050-1250℃, and holds for enough time to make the core temperature of the billet and the surface temperature basically the same, and then carries out subsequent rolling or forging.

[0080] (4) Forging or rolling: control the open forging or open rolling temperature to be between 1050-1250℃, control the final forging or final rolling temperature to be ≥900℃;

[0081] When forging, it can be directly forged to the final product size; when rolling, it can be directly rolled to the final product size, or it can be rolled to a specified intermediate billet size first, then heated and rolled to the final product size; wherein when heating the intermediate billet, the intermediate billet is heated to not higher than 700℃ in a preheating section, then continuously heated to not higher than 1050℃ in a first heating section, after holding, continuously heated to 950-1200℃ in a second heating section, after holding, enters a soaking section, the soaking section temperature is 1100-1250℃, and holding for enough time to make the core temperature of the billet the same as the surface temperature, then discharged for rolling.

[0082] In the present application, the chemical composition design and related process of the easy-to-cut high-temperature carburizing gear steel of Examples 1-8 meet the design specification requirements of the present application. The chemical composition design and related process of Comparative Examples 1-4 all have parameters that do not meet the design specification requirements of the present application, and the chemical composition design can be seen from Table 1-1 and Table 1-2 below.

[0083] Table 1-1 lists the mass percentage of each chemical element of the easy-to-cut high-temperature carburizing gear steel of Examples 1-8 and the comparative steel of Comparative Examples 1-4.

[0084] Table 1-1. (wt.%, the balance is Fe and other unavoidable impurities except P, B, O)

[0085]

[0086]

[0087] Table 1-2 lists the hardenability critical ideal diameter DI value and microalloy element coefficient r calculated from the mass percentage of each chemical element of the easy-to-cut high-temperature carburizing gear steel of Examples 1-8 and the comparative steel of Comparative Examples 1-4. M / X .

[0088] Table 1-2.

[0089] No. DI r M / X ]]> Example 1 3.56 1.90 Example 2 2.99 1.63 Example 3 3.27 1.54 Example 4 3.42 1.37 Example 5 3.51 1.52 Example 6 3.33 1.01 Example 7 2.61 1.66 Example 8 2.67 1.61 Comparative Example 1 2.89 2.05 Comparative Example 2 3.05 1.63 Comparative Example 3 4.17 1.43 Comparative Example 4 2.51 0.70

[0090] Note: In the above table, DI = 0.54xCx(3.33Mn+1)x(0.70Si+1)x(0.36Ni+1)x(2.16Cr+1)x(3.00Mo+1)x(0.36Cu+1)x(1.73V+1); r M / X = (20*[Nb] / 93 + [Ti] / 48 - [V] / 510 + [Al] / 27 - [Mo] / 1920) / ([N] / 14 + [C] / 120); in the formula, each chemical element is substituted into the numerical value before the percentage sign of the mass percentage of the chemical element.

[0091] The detailed production process operation of the free-cutting high temperature carburizing gear steel of Examples 1-8 and the comparative steel materials of Comparative Examples 1-4 is described as follows:

[0092] Example 1

[0093] The chemical composition shown in Table 1-1 and Table 1-2 was smelted on a 50kg vacuum induction furnace. The molten steel was molded into an ingot, heated and subjected to open die forging, and the ingot was first heated to 700°C in a preheating section, then continuously heated to 900°C in a first heating section, and after holding, continuously heated to 1000°C in a second heating section, and after holding, entered an soaking section at a temperature of 1150°C, and after holding, subjected to subsequent forging, and the open forging temperature was controlled at 1100°C, and the finish forging temperature was 910°C, and finally forged into a Φ60mm bar.

[0094] Example 2

[0095] The chemical composition shown in Table 1-1 and Table 1-2 was smelted on a 150kg vacuum induction furnace. The molten steel was molded into an ingot, heated and subjected to open die forging, and the ingot was first heated to 650°C in a preheating section, then continuously heated to 950°C in a first heating section, and after holding, continuously heated to 1100°C in a second heating section, and after holding, entered an soaking section at a temperature of 1200°C, and after holding, subjected to subsequent forging, and the open forging temperature was controlled at 1200°C, and the finish forging temperature was 1000°C, and finally forged into a Φ75mm bar.

[0096] Example 3

[0097] The chemical composition shown in Table 1-1 and Table 1-2 was smelted on a 150kg vacuum induction furnace. The molten steel was molded into an ingot, heated and subjected to open die forging, and the ingot was first heated to 650°C in a preheating section, then continuously heated to 950°C in a first heating section, and after holding, continuously heated to 1100°C in a second heating section, and after holding, entered an soaking section at a temperature of 1200°C, and after holding, subjected to subsequent forging, and the open forging temperature was controlled at 1200°C, and the finish forging temperature was 1000°C, and finally forged into a Φ75mm bar.

[0098] Example 4

[0099] The chemical composition shown in Table 1-1 and Table 1-2 is used for electric furnace smelting, and refining and vacuum treatment are performed, and then 280mm x 280mm continuous casting billets are cast, the continuous casting billets are controlled to be heated to 620°C in the preheating section first, then heated to 950°C in the first heating section, and after holding, heated to 1150°C in the second heating section, and after holding, enter the soaking section, the soaking section temperature is 1150°C, and after holding, subsequent rolling is performed. After the billet exits the heating furnace, it begins to be rolled after high-pressure water descaling, and is directly rolled into finished size, the rolling temperature is controlled to be 1150°C, the finishing temperature is 970°C, and finally Φ80mm rods are rolled.

[0100] Example 5

[0101] The chemical composition shown in Table 1-1 and Table 1-2 is used for electric furnace smelting, and refining and vacuum treatment are performed, and then 320mm x 425mm continuous casting billets are cast, the continuous casting billets are controlled to be heated to 600°C in the preheating section first, then heated to 950°C in the first heating section, and after holding, heated to 1200°C in the second heating section, and after holding, enter the soaking section, the soaking section temperature is 1230°C, and after holding, subsequent rolling is performed. After the billet exits the heating furnace, it begins to be rolled after high-pressure water descaling, and is rolled into an intermediate billet, the rolling temperature is controlled to be 1230°C, the finishing temperature is 1050°C, and the intermediate billet size is 220mm x 220mm. Then the intermediate billet is heated to 680°C in the preheating section, heated to 1000°C in the first heating section, and heated to 1200°C in the second heating section, and after holding, enters the soaking section, the soaking temperature is 1220°C, and after exiting the furnace after high-pressure water descaling, it begins to be rolled, the intermediate billet rolling temperature is controlled to be 1220°C, the intermediate billet finishing temperature is 950°C, and the finished rod size is Φ50mm.

[0102] Example 6

[0103] The chemical composition shown in Table 1-1 and Table 1-2 is used for electric furnace smelting, and refining and vacuum treatment are performed, and then 280mm x 280mm continuous casting billets are cast, the continuous casting billets are controlled to be heated to 680°C in the preheating section first, then heated to 900°C in the first heating section, and after holding, heated to 1180°C in the second heating section, and after holding, enter the soaking section, the soaking section temperature is 1200°C, and after holding, subsequent rolling is performed. After the billet exits the heating furnace, it begins to be rolled after high-pressure water descaling, and is rolled into an intermediate billet, the rolling temperature is controlled to be 1200°C, the finishing temperature is 1000°C, and the intermediate billet size is 140mm x 140mm. Then the intermediate billet is heated to 700°C in the preheating section, heated to 1050°C in the first heating section, and heated to 1180°C in the second heating section, and after holding, enters the soaking section, the soaking temperature is 1200°C, and after exiting the furnace after high-pressure water descaling, it begins to be rolled, the intermediate billet rolling temperature is controlled to be 1200°C, the intermediate billet finishing temperature is 920°C, and the finished rod size is Φ20mm.

[0104] Example 7

[0105] The chemical compositions shown in Table 1-1 and Table 1-2 were subjected to converter smelting, and then to refining and vacuum treatment, and then cast into a mold casting billet. The billet was first heated to 620°C in a preheating section, then continuously heated to 950°C in a first heating section, and then continuously heated to 1150°C in a second heating section after holding, and then entered into a soaking section with a temperature of 1200°C, and then subjected to subsequent rolling after holding. The billet started rolling after descaling by high-pressure water after exiting the heating furnace, and the rolling was controlled to have a starting rolling temperature of 1200°C and a finishing rolling temperature of 970°C, and finally rolled into a Φ90 mm bar.

[0106] Example 8

[0107] The chemical compositions shown in Table 1-1 and Table 1-2 were subjected to converter smelting, and then to refining and vacuum treatment, and then cast into a mold casting billet. The billet was first heated to 620°C in a preheating section, then continuously heated to 950°C in a first heating section, and then continuously heated to 1150°C in a second heating section after holding, and then entered into a soaking section with a temperature of 1200°C, and then subjected to subsequent rolling after holding. The billet started rolling after descaling by high-pressure water after exiting the heating furnace, and the rolling was controlled to have a starting rolling temperature of 1200°C and a finishing rolling temperature of 970°C, and finally rolled into a Φ90 mm bar.

[0108] Comparative Example 1 and Comparative Example 2 were from commercial materials, and the preparation process thereof will not be described here.

[0109] Comparative Example 3

[0110] The chemical compositions shown in Table 1-1 and Table 1-2 were subjected to smelting in a 50 kg vacuum induction furnace. The molten steel was mold-cast into an ingot, heated and subjected to open-die forging, and the ingot was first heated to 700°C in a preheating section, then continuously heated to 900°C in a first heating section, and then continuously heated to 1000°C in a second heating section after holding, and then entered into a soaking section with a temperature of 1100°C, and then subjected to subsequent forging after holding. The open-die forging was controlled to have a starting forging temperature of 1100°C and a finishing forging temperature of 910°C, and finally forged into a Φ60 mm bar.

[0111] Comparative Example 4

[0112] The steel is smelted in an electric furnace with the chemical composition shown in Table 1-1 and Table 1-2, and then refined and vacuum treated, and then cast into a 320 mm x 425 mm continuous casting billet. The billet is heated to 600°C in the preheating section, then heated to 950°C in the first heating section, and then heated to 1200°C in the second heating section after holding. After holding, it enters the soaking section at a temperature of 1230°C, and then is subjected to subsequent rolling. The billet is rolled into an intermediate billet after being descaled by high-pressure water after exiting the heating furnace. The rolling temperature is controlled to be 1230°C, and the final rolling temperature is 1050°C. The intermediate billet has a size of 220 mm x 220 mm. Then the intermediate billet is heated to 680°C in the preheating section, to 1000°C in the first heating section, and to 1200°C in the second heating section. After holding, it enters the soaking section at a temperature of 1220°C. After exiting the furnace and being descaled by high-pressure water, rolling is started. The intermediate billet is controlled to have a rolling temperature of 1220°C, and a final rolling temperature of 950°C. The finished rod has a size of Φ50 mm.

[0113] Table 2-1 and Table 2-2 list the specific process parameters of the free-cutting high-temperature carburizing gear steel of Examples 1-8 and the comparative steel of Comparative Examples 1-4 in the above process steps (1)-(4).

[0114] Table 2-1.

[0115]

[0116]

[0117] Table 2-2.

[0118]

[0119] In the above Table 2, Examples 5, 6 and 8 and Comparative Example 4 are rolled to the specified intermediate billet size first, and then heated and rolled to the final product size, i.e. the so-called “two-fire material”.

[0120] The free-cutting high-temperature carburizing gear steel of Examples 1-8 and the comparative steel of Comparative Examples 1-4 are sampled respectively, and subjected to simulated carburizing and quenching tests, hardenability tests, hardness tests and cutting performance evaluations. The test results of each example and comparative example are listed in Table 3.

[0121] The relevant simulated carburizing and quenching tests, hardenability tests, hardness tests and cutting performance evaluation methods are as follows:

[0122] (a) Simulated carburizing and quenching test: The samples were respectively held at 940°C for 6 hours, 980°C and 1020°C for 4 hours, and then quenched in water. The microstructure of each example and comparative example was observed, and the austenite grain size was evaluated according to the standard ASTM E112.

[0123] (b) Hardenability test: The steels of each example and comparative example were sampled and sampled from the hot-rolled round steel according to the national standard GB / T 225, and the end hardenability test (Jominy test) was carried out according to GB / T 5216. The normalizing temperature was controlled at 920±10°C, the quenching temperature was controlled at 870±5°C, and the Rockwell hardness test was carried out according to GB / T 230.2 to obtain the hardness value (HRC) at a specific position, such as the hardness at 9mm from the quenched end, i.e. J9mm.

[0124] (c) Evaluation of cutting performance: The steels of each example and comparative example were cut by using a general lathe, and the cutting performance of the steels was evaluated by collecting the chips: the chips were evaluated as "good" if they were easily broken into granular shape, as "poor" if they were continuous and not easily broken into spiral shape, and as "medium" if they were between the two, in the shape of "C".

[0125] Table 3 lists the test results of the easy-to-cut high-temperature carburizing gear steels of examples 1-8 and the comparative steels of comparative examples 1-4.

[0126] Table 3.

[0127]

[0128] As can be seen from Table 3, the austenite grain size of the easy-to-cut high-temperature carburizing gear steels of examples 1-8 of the present application maintained in the range of 5-8 levels after simulated carburizing at three temperatures not more than 1020°C in the simulated carburizing and quenching test, and no mixed crystal or abnormal grain growth was observed. Moreover, the hardenability of the easy-to-cut high-temperature carburizing gear steels of examples 1-8 at J9mm was between 35-40HRC, and all had good cutting performance.

[0129] However, the comparative steels of comparative examples 1-3 observed mixed crystal phenomenon after simulated carburizing and quenching at 980°C, wherein 5(1) means that the average grain size is 5 levels, and the local area is coarsened to 1 level. After further increasing the simulated carburizing temperature to 1020°C, the austenite grains of each comparative steel abnormally grew seriously, wherein 5(00) means that the average grain size is 5 levels, and the local area is coarsened to 00 level.

[0130] In addition, the comparative steels of comparative examples 1 and 4 had low hardenability, which was difficult to meet the use requirements. The cutting performance of comparative examples 2 and 3 was poor, which was not conducive to the machining and manufacturing of gears or shafts.

[0131] In summary, it can be seen that the easy-to-cut high-temperature carburizing gear steel can obtain higher temperature austenite grain stability and higher hardenability by reasonable chemical composition design and combined optimization process, is easy to cut, is suitable for high-temperature carburizing, and the representative position J9mm hardenability is between 35-40HRC, and the austenite grain size temperature before and after high-temperature vacuum carburizing at 940-1020℃ is maintained at 5-8 levels.

[0132] The rolled or forged rod of the easy-to-cut high-temperature carburizing gear steel can be effectively machined into gears or tooth shafts, and after high-temperature carburizing and other heat treatment, has suitable strength and toughness, and can be effectively applied to high-end parts such as automobile gearbox or new energy vehicle reducer and differential, and has good use effect.

[0133] It should be noted that the combination of various technical features in the case is not limited to the combination mode described in the claims of the case or the combination mode described in the specific embodiments, and all the technical features described in the case can be freely combined or combined in any way, unless contradictory to each other.

[0134] It should also be noted that the above-mentioned embodiments are only specific embodiments of the present application. Obviously, the present application is not limited to the above-mentioned embodiments, and similar changes or modifications made directly from the disclosure of the present application or easily thought of by those skilled in the art should all belong to the protection scope of the present application.

Claims

1. A machinable high temperature carburized gear steel, characterized in that, The mass percentage of each chemical element is as follows: C: 0.225~0.275%, Si: 0.05~0.40%, Mn: 0.55~0.85%, S: 0.016~0.040%, Cr: 0.85~1.25%, Ni: 0.02~0.25%, Mo: 0.15~0.30%, Al: 0.030~0.050%, N: 0.008~0.020%, Nb: 0.003~0.019%, Ti: 0.003~0.019%; the balance is Fe and other inevitable impurities, The coefficient r of micro-alloying elements of the high-temperature carburizing free-cutting gear steel M / X is in the range of 1-2; wherein: ； The value of the critical ideal diameter DI of the hardenability is 2.5~4.0; wherein: DI=0.54×C×(3.33Mn+1) ×(0.70Si+1) ×(0.36Ni+1) ×(2.16Cr+1) ×(3.00Mo+1) ×(0.36Cu+1) ×(1.73V+1); In the formula, each chemical element is substituted into the numerical value before the percentage sign of the mass percentage of the chemical element.

2. The free machining high temperature carburized gear steel of claim 1 wherein, It also contains 0 3. The free machining high temperature carburized gear steel of claim 1 wherein, In the inevitable impurities, P≤0.015%, O≤0.0020%, B≤0.0005%.

4. The free machining high temperature carburized gear steel of claim 1 wherein, The austenite grain size temperature is kept at 5~8 levels before and after high-temperature vacuum carburizing at 940~1020℃.

5. The free machining high temperature carburized gear steel of claim 1 wherein, The hardenability at J9mm is 35~40HRC.

6. The method of producing a free-machining high temperature carburized gear steel according to any one of claims 1 to 5, characterized by, It includes the steps of: Smelting; Casting; (3) Heating: first heating to not higher than 700℃ in the preheating section, then continuing to heat to not higher than 980℃ in the first heating section, after holding, continuing to heat to 950-1200℃ in the second heating section, after holding, entering the soaking section, the temperature of the soaking section is 1050-1250℃ and holding, so that the core temperature of the billet is the same as the surface temperature, and then subsequent rolling or forging is carried out; (4) Forging or rolling.

7. The production method according to claim 6, wherein In step (4), direct forging or rolling to the final product size.

8. The production method according to claim 6, wherein In step (4), first rolling to the intermediate billet size, then segmentally heating the intermediate billet, and then rolling to the final product size; wherein when the intermediate billet is segmentally heated, the intermediate billet is heated to not higher than 700℃ in the preheating section, then continues to heat to not higher than 1050℃ in the first heating section, after holding, continues to heat to 950-1200℃ in the second heating section, after holding, enters the soaking section, the temperature of the soaking section is 1100-1250℃ and holding for a sufficient time, so that the core temperature of the billet is the same as the surface temperature.

9. The production method according to claim 6, wherein In step (4), the open forging or open rolling temperature is controlled to be between 1050~1250℃, and the final forging or final rolling temperature is controlled to be ≥900℃.

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