High-toughness high-temperature carburizing steel for gear shafts and method for manufacturing same

Abstract

This invention provides a high-strength, high-toughness, high-temperature carburized gear shaft steel, which contains Fe and unavoidable impurities, and also contains the following chemical elements in the following mass percentages: C: 0.16–0.23%, Si: 0.10–0.40%, Mn: 0.86–1.24%, Cr: 0.95–1.44%, Al: 0.025–0.050%, N: 0.008–0.015%, Nb: 0.003–0.030%, Ti: 0.02–0.08%; its microalloying element coefficient r M / N The range is 1.5 to 6.0, where r M / N = (10*[Nb] / 93+[Ti] / 480+[Al] / 27) / ([N] / 14). Furthermore, this invention also provides a method for manufacturing high-temperature carburized gear shaft steel. This high-strength and high-toughness high-temperature carburized gear shaft steel exhibits good high-temperature grain stability, maintaining an austenite grain size of 5-9 after simulated carburizing at up to 1050℃, and possesses a narrow hardenability bandwidth and excellent strength and toughness.

Description

TECHNICAL FIELD

[0001] The present application relates to a high-strength steel material and a manufacturing method thereof, and in particular to a steel for a pinion shaft and a manufacturing method thereof. BACKGROUND

[0002] The development of the automobile industry has put forward higher and higher requirements for automobile parts. Among them, high-strength, high-toughness, high-fatigue life and high-temperature stability and economical and efficient gears are important development directions.

[0003] High-performance gear surfaces generally undergo carburizing and quenching + tempering treatments to obtain a higher-hardness surface and a better-toughness core, and ultimately obtain excellent fatigue life and wear resistance.

[0004] In addition, the gear obtained by high-temperature vacuum carburizing has little or even no intergranular oxidation, which can significantly improve the impact fracture resistance.

[0005] The main technical problem of CrMnTi carburizing gear steel is that when the carburizing temperature is increased, the gear does not appear mixed crystal and grain coarsening phenomenon; once the grain abnormally grows, it is easy to cause heat treatment deformation and early fatigue fracture. Moreover, in order to cope with the quenching and tempering of complex-shaped gears, gas quenching with high-temperature vacuum carburizing is increasingly widely used, and higher requirements are put forward for the hardenability of the gear steel. The gear bears instantaneous high speed and large torque, and higher requirements are put forward for the strength, plasticity and toughness of the gear steel.

[0006] For example: CN100473746C, published on April 1, 2009, entitled "High-strength automobile steel", a high-strength automobile gear steel is disclosed, which is compounded with Nb, V, Al and other alloy elements to refine the original austenite grain, and the composition 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. After adding trace amounts of Nb and V, the grain size, hardenability and bandwidth of the gear steel are obviously optimized.

[0007] For example, the patent document CN103361559A, published on October 23, 2013, entitled "Nb, Ti composite micro-alloyed high-temperature carburizing gear steel", discloses a 20CrMnTi easy-to-cut gear steel with Nb, Ti composite micro-alloying, the composition of the steel 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%, the balance being iron and inevitable impurities. By controlling the content of micro-alloying elements such as Nb, Ti and Al, the carburizing temperature of the gear is increased or the carburizing time is shortened, such as 1050℃ for 1h, or 1000℃ for 6h. The patent adds 0.02-0.06% of Ti and Nb, which can increase the carburizing temperature to 1000℃.

[0008] However, the above patent documents cannot solve the technical problems to be solved by the present application. SUMMARY

[0009] One of the purposes of the present application is to provide a high-strength and high-toughness high-temperature carburizing gear shaft steel, which has good high-temperature grain stability, a relatively narrow hardenability band width, and good strength and toughness.

[0010] In order to achieve the above-mentioned purpose, the present application proposes a high-strength and high-toughness high-temperature carburizing gear shaft steel, which contains the following chemical elements in mass percentage in addition to Fe and inevitable impurities:

[0011] C: 0.16-0.23%,

[0012] Si: 0.10-0.40%,

[0013] Mn: 0.86-1.24%,

[0014] Cr: 0.95-1.44%,

[0015] Al: 0.025-0.050%,

[0016] N: 0.008-0.015%,

[0017] Nb: 0.003-0.030%,

[0018] Ti: 0.02-0.08%;

[0019] The micro-alloying element coefficient r M / N is in the range of 1.5-6.0, wherein r M / N= (10*[Nb] / 93 + [Ti] / 480 + [Al] / 27) / ([N] / 14), wherein each chemical element is substituted into the value before the percentage sign of the mass percentage content of the chemical element.

[0020] Further, in the high-strength and high-toughness high-temperature carburized gear shaft steel according to the present application, the mass percentage content of each chemical element is as follows:

[0021] C: 0.16-0.23%,

[0022] Si: 0.10-0.40%,

[0023] Mn: 0.86-1.24%,

[0024] Cr: 0.95-1.44%,

[0025] Al: 0.025-0.050%,

[0026] N: 0.008-0.015%,

[0027] Nb: 0.003-0.030%,

[0028] Ti: 0.02-0.08%;

[0029] the balance being Fe and other unavoidable impurities;

[0030] the micro-alloying element coefficient r M / N is in the range of 1.5-6.0, wherein r M / N = (10*[Nb] / 93 + [Ti] / 480 + [Al] / 27) / ([N] / 14), wherein each chemical element is substituted into the value before the percentage sign of the mass percentage content of the chemical element.

[0031] In the high-strength and high-toughness high-temperature carburized gear shaft steel according to the present application, the design principles of each chemical element are as follows:

[0032] C: In the high strength and toughness high temperature carburizing steel shaft steel described in the application, C is an essential component in the steel, and it is also one of the main elements affecting the hardenability of the steel. Carburizing gear steel needs high surface strength and sufficient core impact toughness. When the content of C element in the steel is too low, lower than 0.16%, the strength of the steel is insufficient, and the good hardenability requirement cannot be guaranteed. Correspondingly, the content of C element in the steel should not be too high. When the content of C element in the steel is too high, the toughness requirement of the gear core cannot be met, and the high content of C is not conducive to the plasticity of the steel, especially for the carburizing gear steel with high Mn content, the content of C greater than 0.23% is not conducive to the processability of the steel. Therefore, in the high strength and toughness high temperature carburizing steel shaft steel described in the application, the mass percentage of C is controlled between 0.16-0.23%.

[0033] Si: In the high strength and toughness high temperature carburizing steel shaft steel described in the application, 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 elastic limit of the steel. At the same time, it should be noted that Si element can increase the Ac3 temperature of the steel, and has the risk of cracking and decarburization tendency due to poor thermal conductivity. Based on this, considering the beneficial effects and adverse effects of Si, in the high strength and toughness high temperature carburizing steel shaft steel described in the application, the mass percentage of Si is controlled between 0.10-0.40%.

[0034] Mn: In the high strength and toughness high temperature carburizing steel shaft steel described in the application, Mn is one of the main elements affecting the hardenability of the steel. Mn element has good deoxidizing ability, which can reduce the iron oxide in the steel and effectively improve the yield of the steel. Mn can dissolve into ferrite to improve the strength and hardness of the steel, and the steel can 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, can strongly increase the hardenability of the steel, and can reduce the red toughness of the steel. When the content of Mn element in the steel is less than 0.86%, 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 high strength and toughness high temperature carburizing steel shaft steel described in the application, the mass percentage of Mn is controlled between 0.86-1.24%.

[0035] Cr: In the high strength and toughness high temperature carburizing steel shaft steel described in the present application, Cr is one of the main alloying elements added in the steel of the present application, Cr can significantly improve the hardenability of the steel and the strength, wear resistance and other properties. In addition, Cr 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 the quenched and tempered steel, forming coarse carbides distributed along the grain boundary. Therefore, in the high strength and toughness high temperature carburizing steel shaft steel described in the present application, the mass percentage of Cr element is controlled between 0.95-1.44%.

[0036] Al: In the high strength and toughness high temperature carburizing steel shaft steel described in the present application, Al belongs to grain refining element. Al element 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 embrittlement sensitivity of the steel. However, it should be noted that the content of Al element in the steel should not be too high, and too high content of Al can easily increase the opportunity of inclusion in the steel. Therefore, in the high strength and toughness high temperature carburizing steel shaft steel described in the present application, the mass percentage of Al element is controlled between 0.025-0.050%.

[0037] N: In the high strength and toughness high temperature carburizing steel shaft steel described in the present application, N is an interstitial atom, which can form MN type precipitates with micro-alloy in the steel ("M" refers to alloying element), which 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, which reduces the toughness of the steel. Therefore, in the high strength and toughness high temperature carburizing steel shaft steel described in the present application, the mass percentage of N element is controlled between 0.008-0.015%.

[0038] Nb: In the high strength and toughness high temperature carburizing steel shaft steel described in the present application, Nb element is added to the steel, which can form fine precipitates, thereby inhibiting the recrystallization of the steel, which can effectively refine the grain. It should be noted that the content of Nb element in the steel should not be too high, when the content of Nb in the steel is too high, coarse NbC particles will be formed during smelting, which will reduce the impact toughness of the steel. Therefore, in the high strength and toughness high temperature carburizing steel shaft steel described in the present application, the mass percentage of Nb element is controlled between 0.003-0.030%.

[0039] Ti: In the high strength and toughness high temperature carburizing gear shaft steel described in the present application, although Ti can form fine precipitates in the steel, when the content of Ti element in the steel is too high, coarse TiN particles with edges and corners will be formed during smelting, reducing the impact toughness of the steel. Therefore, in the high strength and toughness high temperature carburizing gear shaft steel described in the present application, the content of Ti element is controlled to be between 0.02-0.08%.

[0040] In addition, it is important that, in the high strength and toughness high temperature carburizing gear shaft steel described in the present application, the microalloy element coefficient r M / N of the steel is in the range of 1.5-6.0, where r M / N =(10*[Nb] / 93+[Ti] / 480+[Al] / 27) / ([N] / 14), and in the formula, the numerical values before the percentage signs of the mass percentage contents of the chemical elements are substituted.

[0041] One of the technical cores of the present application is to control the contents of Ti, Al, Nb and N in the gear steel and the microalloy element coefficient, to form precipitates with the excess N element and carbon element by adding appropriate amounts of Al and Nb, thereby inhibiting the growth of austenite grains at high temperature.

[0042] Further, in the inevitable impurities of the high strength and toughness high temperature carburizing gear shaft steel described in the present application, the content of each impurity element satisfies at least one of the following:

[0043] P≤0.030%;

[0044] O≤0.002%;

[0045] H≤0.0002%;

[0046] B≤0.0005%;

[0047] Ca≤0.004%.

[0048] In the high strength and toughness high temperature carburizing gear shaft steel described in the present application, P, O, H, B and Ca are all impurity elements in the steel, and under the condition that the technical conditions permit, in order to obtain steel with better performance and quality, the content of impurity elements in the steel should be reduced as much as possible. Among them:

[0049] P: P is easy to segregate at the grain boundaries of the steel, which can reduce the grain boundary binding energy and deteriorate the impact toughness of the steel. Therefore, in some embodiments of the present application, the content of P can be controlled to be P≤0.030%.

[0050] O: O can form oxides and complex oxides with Al elements in the steel, in order to ensure the uniformity of the steel structure and the low temperature impact energy and fatigue performance. Therefore, in some embodiments of the present application, the content of O element can be controlled to be O≤0.0020%.

[0051] H: H will gather at defects in steel, especially in steel with tensile strength level exceeding 1000 MPa, hydrogen-induced delayed fracture will occur. Therefore, in some embodiments of the present application, the content of H element can be controlled as H≤0.0002%.

[0052] B: B is an element more sensitive to hardenability, due to the easy segregation of B element, small changes in the content of B element will cause large fluctuations in the hardenability of steel. Therefore, B element is not conducive to the narrow control of the hardenability band width of gear steel, in some embodiments of the present application, the content of B element can be controlled as B≤0.0005%.

[0053] Ca: Ca element is easy to form inclusions, which will affect the fatigue performance of the final product. Therefore, in some embodiments of the present application, the content of Ca element can be controlled as Ca≤0.004%.

[0054] Further, in the high strength and toughness high temperature carburizing steel for gear shafts described in the present application, it also contains at least one of the following chemical elements:

[0055] 0<S≤0.035%,

[0056] 0<Ni≤0.25%,

[0057] 0<Mo≤0.10%,

[0058] 0<Cu≤0.20%,

[0059] 0<V≤0.01%

[0060] In some embodiments of the present application, at least one of the above elements can be optionally contained to further improve the performance of the high strength and toughness high temperature carburizing steel for gear shafts. Specifically:

[0061] S: S generally exists as an impurity element in steel, which will reduce the plasticity and toughness of steel, however, in the high strength and toughness high temperature carburizing steel for gear shafts described in the present application, a certain content of S element can form non-metallic inclusions with Mn, and appropriate amount of S can improve the cutting performance of steel. Based on this, in the high strength and toughness high temperature carburizing steel for gear shafts described in the present application, the mass percentage of S can be preferably controlled as 0<S≤0.035%.

[0062] Ni: In the high strength and toughness high temperature carburizing steel for gear shafts described in the present application, Ni exists in the form of solid solution in steel, which can effectively improve the low temperature impact performance of steel. However, it needs to be noted that too high Ni content will lead to too high content of retained austenite in steel, which will reduce the strength of steel. Therefore, in the high strength and toughness high temperature carburizing steel for gear shafts described in the present application, the mass percentage of Ni can be preferably controlled as 0<Ni≤0.25%.

[0063] Mo: In the high strength and toughness high temperature carburizing steel shaft steel described in the present application, Mo can be solid-solved in the steel, which is beneficial to improve the hardenability of the steel and improve the strength of the steel. At a higher temperature tempering, Mo can also form fine carbides to further improve the strength of the steel; the combined action of Mo and Mn can also significantly improve the stability of austenite. Therefore, in the high strength and toughness high temperature carburizing steel shaft steel described in the present application, the mass percentage of Mo can be preferably controlled as 0 < Mo ≤ 0.10%.

[0064] Cu: In the high strength and toughness high temperature carburizing steel shaft steel described in 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. However, the content of Cu element in the steel should not be too high. If the content of Cu in the steel is too high, it will be enriched in the grain boundary during heating, resulting in weakening of the grain boundary and cracking. Therefore, in the high strength and toughness high temperature carburizing steel shaft steel described in the present application, the mass percentage of Cu can be preferably controlled as 0 < Cu ≤ 0.20%.

[0065] V: In the high temperature carburizing steel shaft steel described in the present application, V can effectively improve the hardenability of the steel. V element in the steel can form precipitates with C element or N element, thereby further improving the strength of the steel. If the content of C element and V element is too high, coarse VC particles will be formed. Therefore, in the high temperature carburizing steel shaft steel described in the present application, the mass percentage of V element is controlled as 0 < V ≤ 0.01%.

[0066] Further, the high strength and toughness high temperature carburizing steel shaft steel described in the present application has an austenite grain size of 5-9 levels after high temperature carburizing heat treatment.

[0067] Further, in the high strength and toughness high temperature carburizing steel shaft steel described in the present application, the critical ideal diameter DI value is 1.5-3.5 in., wherein: DI = 0.54[C] × (3.333[Mn] + 1) × (0.70[Si] + 1) × (0.363[Ni] + 1) × (2.16[Cr] + 1) × (3.00[Mo] + 1) × (0.365[Cu] + 1) × (1.73[V] + 1), wherein each element symbol is substituted into the value before the mass percentage of the corresponding element.

[0068] The steel of the present application can preferably control the value of the critical ideal diameter DI to be between 1.5-3.5 in. while controlling the mass percentage of each chemical element. This is because when the DI value is low, the hardenability of the steel is insufficient and the core structure cannot obtain sufficient strength; and when the DI value is high, the manufacturing is difficult and the cost is high.

[0069] Further, the high-toughness high-temperature carburized gear shaft steel according to the present application has a typical position hardenability satisfying: J9mm is 30-38HRC, and J15mm is 22-31HRC.

[0070] Further, the high-toughness high-temperature carburized gear shaft steel according to the present application has a tensile strength R m ≥1100MPa, a yield strength R p0.2 ≥980MPa, an elongation A ≥12%, a reduction of area ≥50%, and a Charpy impact energy A ku ≥60J.

[0071] Another object of the present application is to provide a manufacturing method of the high-toughness high-temperature carburized gear shaft steel, and the high-toughness high-temperature carburized gear shaft steel manufactured by the method has refined grains and good high-temperature grain stability, for example, the austenite grain size before and after high-temperature (vacuum) carburization at a maximum temperature of 1050℃ is 5-9 levels, and the high-toughness high-temperature carburized gear shaft steel has a narrow hardenability band, is easy to cut, and has high fatigue performance.

[0072] In order to achieve the above object, the present application further provides a manufacturing method of the high-toughness high-temperature carburized gear shaft steel as described above, which comprises the following steps:

[0073] (1) smelting;

[0074] (2) casting;

[0075] (3) heating: slowly heating to a heating temperature of 1100-1250℃ at a speed of not more than 500℃ / h, and holding for 3-12h;

[0076] (4) forging or rolling.

[0077] In the manufacturing method according to the present application, in the smelting and casting steps, electric furnace smelting or converter smelting can be used, and refining and vacuum treatment can be performed. In some other embodiments, vacuum induction furnace can also be used for smelting.

[0078] In some specific embodiments, in the smelting and casting steps, the furnace charge of the 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; in the oxidation period: P is removed by slagging; the slagging conditions can be controlled as follows: the slagging 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%.

[0079] After the electric furnace smelting or converter smelting is completed, the molten steel can be refined in a ladle refining furnace to remove harmful gases and inclusions in the steel. By controlling the ladle seating, temperature measurement and analysis, the argon pressure can be adjusted according to the situation. LF initial deoxidation can add Al, and then the alloy block can be added and stirred for 5-10 min. When the temperature of the molten steel is T=1650-1670℃, vacuum degassing can be performed to ensure that [O]≤0.0020% and [H]≤0.00015%. In some specific embodiments, the vacuum degree of vacuum degassing can be controlled to be 66.7 Pa and maintained for not less than 15 min.

[0080] In addition, in some embodiments of the manufacturing method described in the present application, the temperature of the ladle when hoisted can be controlled to be 1550-1570℃, thereby reducing the ladle temperature and accelerating the element diffusion, which is beneficial to further reduce the dendritic segregation.

[0081] In addition, in some embodiments, in the smelting and casting steps, casting can be performed by mold casting or continuous casting. During the continuous casting pouring process, the high-temperature molten steel in the ladle is poured into the tundish through the protection sleeve, and the superheat of the tundish can be controlled to be 20-40℃. The tundish is completely cleaned before use, the inner surface is a refractory coating and must not have cracks; the molten steel in the tundish is fully stirred by the electromagnetic stirring through the continuous casting crystallizer, and qualified continuous casting billets with a section size of 140mm×140mm-320mm×425mm can be poured.

[0082] In some embodiments of the manufacturing method of the high-strength and high-toughness high-temperature carburizing gear shaft steel described in the present application, the pouring speed can be controlled to be 0.6-1.3 m / min according to different bloom sizes. Then, the continuous casting billet is slowly cooled in the slow cooling pit, and the slow cooling time is not less than 24 hours.

[0083] Further, in step (3) of the manufacturing method described in the present application, the slow heating to a heating temperature of 1100-1250℃ at a speed of not more than 500℃ / h specifically includes: first heating to not higher than 700℃ in the preheating section, then continuing to heat to not higher than 1100℃ in the first heating section, after holding, continuing to heat to 950-1200℃ in the second heating section, entering the soaking section for holding for 3-12 h, and the soaking section temperature is 1100-1250℃.

[0084] In addition, in step (3) of the manufacturing method described in the present application, a box-type furnace, a bogie furnace or a walking beam type heating furnace can be used.

[0085] In the manufacturing method of the high-strength and high-toughness high-temperature carburizing gear shaft steel provided by the present application, compared with the prior art, the soaking section temperature is higher, and the higher soaking temperature is beneficial to improve the composition uniformity and microstructure uniformity of the continuous casting billet during the diffusion process of the billet heating.

[0086] Further, in the step (4) of the manufacturing method of the high strength and toughness high temperature carburized gear shaft steel, the open forging or open rolling temperature is controlled to be 1100-1250°C, and the finish rolling or finish forging temperature is controlled to be ≥900°C.

[0087] In the step (4) of the manufacturing method of the high strength and toughness high temperature carburized gear shaft steel, the finish forging or finish rolling temperature is controlled to be ≥900°C, after the 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 1100-1250°C, and the finish forging or finish rolling temperature is controlled to be ≥900°C. Through this process, the N can be beneficially precipitated from the γ solid solution and combined with the micro-alloying elements in the steel to form nitrides. The solubility of N in α-Fe is less than that in γ-Fe, and due to the excitation of phase transformation, two peaks of precipitated amount are caused. If the finish forging or finish rolling temperature is low, the peak of the precipitated phase is precipitated, which causes uneven distribution of the precipitated phase and insufficient recovery and recrystallization to cause anisotropy in the structure. Therefore, the finish forging or finish rolling temperature is controlled to be ≥900°C. In addition, increasing the finish forging or finish rolling temperature can obtain finer grains. The fine grains increase the difference between the average grain diameter of the ferrite after the transformation of the supercooled austenite and the spacing between the Mn-rich bands, reduce the tendency of the Mn-rich bands to form pearlite, and thus reduce the banded structure.

[0088] In addition, at a higher soaking temperature, the precipitated phase has a faster solid solution speed. Therefore, controlling a higher rolling heating temperature can promote the dissolution of the original unsolved precipitated phase particles in the steel, increase the micro-alloying element concentration in the matrix, and precipitate more and more dispersed particles during subsequent cooling. In addition, increasing the rolling heating temperature can increase the finish rolling temperature, so that the recovery and recrystallization of the austenite after rolling is more sufficient, and the distribution of the precipitated phase is more uniform.

[0089] Further, in the step (4) of the manufacturing method of the high strength and toughness high temperature carburized gear shaft steel, direct rolling or forging is performed to the finished product size.

[0090] Further, in the step (4) of the manufacturing method of the high strength and toughness high temperature carburized gear shaft steel, the steel is first rolled to an intermediate billet size, then intermediate heating is performed, and then the steel is rolled to the final finished product size; wherein the intermediate heating is slowly heated to an intermediate heating temperature of 1100-1250°C at a speed of not more than 500°C / h, and the holding time is 3-12h.

[0091] Further, in the step (4) of the manufacturing method of the high-toughness high-temperature carburizing gear shaft steel, the intermediate heating is slowly heated to an intermediate heating temperature of 1100-1250 DEG C at a speed of not more than 500 DEG C / h, and the holding time is 3-12 h, which specifically comprises: firstly heated to not higher than 700 DEG C in a preheating section, then continuously heated to not higher than 1100 DEG C in a first heating section, after holding, continuously heated to 950-1200 DEG C in a second heating section, and then held for 3-12 h in a soaking section at a temperature of 1100-1250 DEG C.

[0092] The high-toughness high-temperature carburizing gear shaft steel and the manufacturing method thereof have the following beneficial effects:

[0093] The high-toughness high-temperature carburizing gear shaft steel obtained by reasonable chemical composition design has stable high-temperature austenite grains, and the rod material rolled or forged by using the high-toughness high-temperature carburizing gear shaft steel can be processed into a gear shaft, which has good hardenability, toughness, wear resistance and fatigue resistance after high-temperature carburizing heat treatment.

[0094] The application controls the contents of micro-alloying elements, nitrogen and carbon elements, strictly controls the atomic molar ratio, adds appropriate amounts of Ti, Al and Nb elements, and combines the heating process to hinder the abnormal growth of high-temperature austenite grains, thereby improving the austenite grain coarsening temperature of the gear steel, so that the grain size of the gear steel is still stably maintained at 5-9 levels after high-temperature carburizing at a temperature as high as 1050 DEG C, and various performances reach the performance indexes of the gear shaft steel.

[0095] The high-toughness high-temperature carburizing gear shaft steel has reasonable component and process design, does not need to add a large amount of Ni, Mo, Cu and V alloy elements, and controls the content of micro-alloying elements in the steel, thereby avoiding the presence of large harmful inclusions in the steel.

[0096] The high-toughness high-temperature carburizing gear shaft steel controls the types and amounts of alloy elements in the steel under the premise of ensuring high-temperature carburizing, high hardenability and narrow bandwidth, improves the applicability of the steel, and reduces the production cost of the steel.

[0097] The high-toughness high-temperature carburizing gear shaft steel can greatly shorten the carburizing time of downstream users in the production of downstream gear shaft products. DETAILED DESCRIPTION

[0098] The high-toughness high-temperature carburizing gear shaft steel and the manufacturing method thereof will be further explained in combination with specific examples, but the explanation and description do not constitute undue limitation on the technical solutions of the application.

[0099] The high-toughness high-temperature carburizing gear shaft steel of examples 1-8 is obtained by the following steps:

[0100] (1) Smelt and cast according to the chemical composition shown in Table 1 below: wherein smelting can be performed 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.

[0101] (2) Heating: slowly heat to 1100-1250℃ at a speed of not more than 500℃ / h, and the holding time is 3-12h.

[0102] In some embodiments, a walking beam heating furnace can be used, first heated to not higher than 700℃ in the preheating section, then continue to heat to not higher than 1100℃ in the first heating section, after holding, continue to heat to 950-1200℃ in the second heating section, enter the soaking section for 3-12h, the soaking section temperature is 1100-1250℃, after holding, proceed to subsequent forging or rolling.

[0103] In some other embodiments, a box furnace or a bogie furnace can also be used for heating.

[0104] (3) Forging or rolling: control the open forging or open rolling temperature to be 1100-1250℃, and control the finish rolling temperature or finish forging temperature to be ≥900℃.

[0105] Specifically:

[0106] Example 1: Smelt according to the chemical composition shown in Table 1-1 and 1-2 below in a 50kg vacuum induction furnace. Cast the molten steel into ingots, heat in a box furnace and perform open forging, control the ingots to be heated to 1100℃ at a speed of 500℃ / h, hold for 3h, after holding, proceed to subsequent forging, control the finish forging temperature to be 910℃, and finally forge into Φ60mm bars.

[0107] Example 2: Smelt according to the chemical composition shown in Table 1-1 and 1-2 below in a 150kg vacuum induction furnace. Cast the molten steel into ingots, heat in a bogie furnace and perform open forging, control the ingots to be heated to 1200℃ at a speed of 250℃ / h, hold for 5h, then proceed to subsequent forging, control the finish forging temperature to be 1000℃, and finally forge into Φ90mm bars.

[0108] Example 3: Smelt according to the chemical composition shown in Table 1-1 and 1-2 below in a 500kg vacuum induction furnace. Cast the molten steel into ingots, heat in a bogie furnace and perform open forging, control the ingots to be heated to 1220℃ at a speed of 200℃ / h, hold for 12h, then proceed to subsequent forging, control the finish forging temperature to be 1000℃, and finally forge into Φ120mm bars.

[0109] Example 4: The steel is smelted in an electric furnace according to the chemical composition shown in Tables 1-1 and 1-2, and then refined and vacuum treated, and then cast into 280 mm x 280 mm continuous casting billets. The continuous casting billets are heated in a walking beam furnace, and slowly heated at a rate of 300°C / h, first heated to 620°C in the preheating section, then continuously heated to 950°C in the first heating section, and continuously heated to 1150°C in the second heating section, and then enters the soaking section, the soaking section temperature is 1200°C, and after 4 hours of soaking, the rolling is carried out. After the billets exit the heating furnace, high-pressure water descaling is carried out, and then rolling is started, and the finish rolling temperature is controlled to be 970°C, and finally rolled into Φ80 mm bars.

[0110] Example 5: The steel is smelted in an electric furnace according to the chemical composition shown in Tables 1-1 and 1-2, and then refined and vacuum treated, and then cast into 320 mm x 425 mm continuous casting billets. The continuous casting billets are heated in a walking beam furnace, and slowly heated at a rate of 150°C / h, first heated to 600°C in the preheating section, then continuously heated to 1100°C in the first heating section, and continuously heated to 1200°C in the second heating section, and then enters the soaking section, the soaking section temperature is 1230°C, and after 4.5 hours of soaking, the subsequent rolling is carried out. After the billets exit the heating furnace, high-pressure water descaling is carried out, and then rolling is started, and the intermediate billets are rolled, and the first finish rolling temperature is controlled to be 1000°C, and the intermediate billet size is 220 mm x 220 mm. Then the intermediate billets are placed in a walking beam furnace and slowly heated at a rate of 400°C / h, heated to 680°C in the preheating section, heated to 1050°C in the first heating section, and heated to 1100°C in the second heating section, and then enters the soaking section after soaking, the soaking temperature is 1120°C, and after 12 hours of soaking, the furnace is discharged, high-pressure water descaling is carried out, and then rolling is started, and the second finish rolling temperature is controlled to be 950°C, and the finished bar size is Φ90 mm.

[0111] Example 6: The chemical composition shown in Tables 1-1 and 1-2 is smelted in an electric furnace, and then refined and vacuum treated, and then cast into a 280 mm x 280 mm continuous casting billet. The billet is heated in a walking beam furnace at a rate of 300°C / h, and first heated to 700°C in a preheating section, then continuously heated to 1000°C in a first heating section, and continuously heated to 1180°C in a second heating section, and then enters a soaking section at a temperature of 1200°C, and after 4.5 h of soaking, subsequent rolling is performed. After the billet exits the furnace, it is descaled by high-pressure water, and then rolling is started, and an intermediate billet is formed, with the first finish rolling temperature controlled to be 950°C, and the intermediate billet having a size of 140 mm x 140 mm. Then the intermediate billet is again placed in a walking beam furnace and heated at a rate of 500°C / h, and first preheated to 700°C, then heated to 1100°C in a first heating section, and heated to 1150°C in a second heating section, and then enters a soaking section at a temperature of 1200°C, and after 3 h of soaking, the billet is removed from the furnace, descaled by high-pressure water, and then rolling is started, with the second finish rolling temperature controlled to be 900°C, and the finished product being a Φ25 mm bar.

[0112] Example 7: The chemical composition shown in Tables 1-1 and 1-2 is smelted in a converter, and then refined and vacuum treated, and then cast into a mold casting billet. The billet is heated in a walking beam furnace at a rate of 50°C / h, and first heated to 620°C in a preheating section, then continuously heated to 900°C in a first heating section, and continuously heated to 950°C in a second heating section, and then enters a soaking section at a temperature of 1150°C, and after 8 h of soaking, rolling is performed. After the billet exits the furnace, it is descaled by high-pressure water, and then rolling is started, with the finish rolling temperature controlled to be 970°C, and the final product being a Φ90 mm bar.

[0113] Example 8: The chemical composition shown in Tables 1-1 and 1-2 is smelted in a converter, and then refined and vacuum treated, and then cast into a mold casting billet. The billet is heated in a walking beam furnace at a rate of 100°C / h, and first heated to 600°C in a preheating section, then continuously heated to 950°C in a first heating section, and continuously heated to 1200°C in a second heating section, and then enters a soaking section at a temperature of 1250°C, and after 7 h of soaking, subsequent rolling is performed. After the billet exits the furnace, it is descaled by high-pressure water, and then rolling is started, and an intermediate billet is formed, with the first finish rolling temperature controlled to be 1050°C, and the intermediate billet having a size of 260 mm x 260 mm. Then the intermediate billet is again placed in a walking beam furnace and heated at a rate of 300°C / h, preheated to 650°C in a preheating section, then heated to 1050°C in a first heating section, and heated to 1200°C in a second heating section, and then enters a soaking section at a temperature of 1250°C, and after 5 h of soaking, the billet is removed from the furnace, descaled by high-pressure water, and then rolling is started, with the second finish rolling temperature controlled to be 950°C, and the finished product being a Φ60 mm bar.

[0114] The detailed process of the steels of Comparative Examples 1-4 is described as follows:

[0115] Comparative Example 1 and Comparative Example 2 are from commercial materials, which are subjected to electric furnace smelting and refining treatment to ensure the purity of the commercial materials.

[0116] Comparative Example 3: Smelting is performed in a 50 kg vacuum induction furnace according to the chemical compositions shown in Tables 1-1 and 1-2. The molten steel is cast into an ingot, heated and subjected to open die forging, heated in a box furnace, and the ingot is controlled to be heated to 1100°C at a rate of 300°C / h, and after holding for 3 h, subsequent forging is performed, and the final forging temperature is controlled to be 910°C, and finally forged into a Φ60 mm bar.

[0117] Comparative Example 4: Electric furnace smelting is performed according to the chemical compositions shown in Tables 1-1 and 1-2, and refining and vacuum treatment are performed, and then cast into a 320 mm x 425 mm continuous casting billet, and the continuous casting billet is controlled to be slowly heated in a walking beam heating furnace at a rate of 150°C / h, first heated to 600°C in a preheating section, then continuously heated to 950°C in a first heating section, and then continuously heated to 1200°C in a second heating section, and then enters a soaking section, and the soaking section temperature is 1230°C, and after holding for 4.5 h, subsequent rolling is performed. After the billet is discharged from the heating furnace and subjected to high-pressure water descaling, rolling is started, and a bloom is formed, and the first final rolling temperature is controlled to be 1050°C, and the bloom size is 220 mm x 220 mm. Then the bloom is placed in a walking beam heating furnace and slowly heated at a rate of 400°C / h, heated to 680°C in a preheating section, heated to 1050°C in a first heating section, and heated to 1200°C in a second heating section, and then enters a soaking section, and the soaking temperature is 1220°C, and holding for 6 h, and after discharging from the furnace and subjected to high-pressure water descaling, rolling is started, and the second final rolling temperature is controlled to be 950°C, and the finished bar size is Φ90 mm.

[0118] Tables 1-1 and 1-2 list the mass percent composition of each chemical element of the high-strength and high-toughness high-temperature carburizing steel for axle shafts of Examples 1-8 and the comparative steels of Comparative Examples 1-4.

[0119] Table 1-1 (wt.%, balance being Fe and other unavoidable impurities except P, B, O, H and Ca)

[0120]

[0121]

[0122] Table 1-2. (wt.%, balance being Fe and other unavoidable impurities except P, O, H, B and Ca)

[0123] No. P O H B Ca S Ni Mo Cu V r M / N ]]> DI Example 1 0.012 0.0017 0.0002 0.0002 0.0024 0.035 0.09 0 0.04 0.002 4.5 2.00 Example 2 0.03 0.002 0.0002 0.0005 0.0027 0.002 0.02 0.05 0 0.003 5.2 2.17 Example 3 0.022 0.006 0.0002 0.0003 0.0034 0.023 0.05 0.03 0.2 0.005 4.0 2.73 Example 4 0.015 0.0016 0.0001 0.0003 0.0014 0.012 0.03 0.05 0 0.010 2.4 1.76 Example 5 0.007 0.0015 0.0001 0.0002 0.0009 0.030 0 0 0 0 3.4 1.81 Example 6 0.009 0.0018 0.0001 0 0.0013 0 0.25 0.03 0.02 0.009 3.7 2.22 Example 7 0.005 0.0016 0.0001 0.0001 0.0015 0.017 0.03 0.1 0.07 0 1.7 2.29 Example 8 0.011 0.007 0.0001 0.0004 0.004 0.025 0.06 0.01 0.06 0.003 4.9 2.17 Comparative Example 1 0.013 0.0018 0.0002 0.0002 0.001 0.011 0 0.01 0.03 0.002 2.9 1.80 Comparative Example 2 0.007 0.0014 0.0002 0.0002 0.0015 0.013 0.03 0.01 0.03 0.003 2.6 1.45 Comparative Example 3 0.005 0.002 0.0002 0.0005 0.0022 0.003 0.01 0.05 0.02 0.009 1.2 1.78 Comparative Example 4 0.006 0.0014 0.0002 0.0003 0.0014 0.003 0.02 0.006 0.01 0.002 1.4 1.73

[0124] Note: r M / N= (10*[Nb] / 93 + [Ti] / 480 + [Al] / 27) / ([N] / 14), where each chemical element in the formula is substituted with the value in front of the percentage sign of the mass percentage content of the chemical element; DI = 0.54[C] x (3.333[Mn] + 1) x (0.70[Si] + 1) x (0.363[Ni] + 1) x (2.16[Cr] + 1) x (3.00[Mo] + 1) x (0.365[Cu] + 1) x (1.73[V] + 1), where each element symbol represents the value in front of the percentage sign of the mass percentage content of the corresponding element.

[0125] Table 2-1 and 2-2 list the specific process parameters of the high-strength and high-toughness high-temperature carburizing steel axle steels of Examples 1-8 and the steels of Comparative Examples 1-4 in the above process steps.

[0126] Table 2-1

[0127]

[0128]

[0129] Note: In Table 2-1, the preheating temperature, the first heating temperature and the second heating temperature of Example 1-3 are all represented by " / ", which means that the heating of these three examples is not by the way of segmented heating.

[0130] Table 2-2

[0131]

[0132]

[0133] Note: In the above Table 2-2, since the process of intermediate billet rolling is adopted in Example 5, Example 6 and Example 8 and Comparative Example 4, there are two columns of parameters in Step (2) and Step (3) in the above process of the present application, which are the first finish rolling temperature and the second finish rolling temperature, respectively.

[0134] The obtained high-strength and high-toughness high-temperature carburizing steel axle steels of Examples 1-8 and the comparative steels of Comparative Examples 1-4 are sampled respectively, and are respectively held at 960℃, 990℃ and 1020℃ for 4 hours; 1050℃ for 3 hours, and then are water quenched to simulate the high-temperature carburizing process, and the austenite grain size is evaluated according to the standard ASTM E112. Table 3 lists the test results of the austenite grain size of the high-strength and high-toughness high-temperature carburizing steel axle steels of Examples 1-8 and the comparative steels of Comparative Examples 1-4.

[0135] In addition, the obtained high-strength and high-toughness high-temperature carburizing steel axle steels of Examples 1-8 and the comparative steels of Comparative Examples 1-4 are sampled respectively, and are subjected to hardenability test and hardness test, and the test results are listed in Table 4.

[0136] wherein:

[0137] Quenching test: The steel of each example and the steel of the comparative example are sampled and sampled from the hot-rolled round steel according to the national standard GB / T 225, the end quenching test (Jominy test) is carried out according to GB / T 5216, the normalizing temperature is controlled at 920±10℃, the quenching temperature is controlled at 870±5℃, the Rockwell hardness test is carried out according to GB / T 230.2, the hardness value (HRC) at a specific position is obtained, such as the hardness at 9mm from the quenching end, that is, J9mm; and the hardness at 15mm from the quenching end, that is, J15mm.

[0138] Table 3

[0139]

[0140]

[0141] Table 4

[0142]

[0143] Note: The impact energy of each example and the comparative example in Table 3 has multiple numerical values representing multiple measurement results.

[0144] As can be seen from Table 3, the high-strength and high-toughness high-temperature carburizing gear shaft steel of the examples 1-8 of the present application has an austenite grain size maintained in the range of 5-9 levels after being simulated carburized at the highest temperature of 1050℃ in the simulation carburizing and quenching test, and no mixed crystal, abnormal coarse grain and other phenomena are observed, so it has good high-temperature grain stability.

[0145] In addition, as can be seen from Table 3, the high-strength and high-toughness high-temperature carburizing gear shaft steel of each example 1-8 of the present application has a representative position J9mm quenching hardness of 31-38HRC, and J15mm of 23-31HRC, and has higher quenching and narrower quenching width.

[0146] In addition, the tensile strength R m of each example after simulated high-temperature carburizing heat treatment is greater than 1100MPa, the yield strength R p0.2 is greater than 980MPa, the elongation A after fracture is greater than or equal to 12%, the reduction of area is greater than 50%, and the Charpy impact energy A ku is greater than 60J.

[0147] Therefore, the high-strength and high-toughness high-temperature carburizing gear shaft steel rod rolled or forged can be effectively processed into a gear shaft, has excellent mechanical properties after high-temperature carburizing heat treatment, and has good use prospect and value.

[0148] The contrast steel of Comparative Example 2 has a lower hardenability (29HRC at J9mm, 21HRC at J15mm), which does not reach the requirement of 20CrMnTiH guaranteed hardening gear steel in GB / T5216-2014 (30-42HRC at J9mm, 22-35HRC at J15mm), and also has a lower strength (R m = 1073MPa, R p0.2 = 951MPa).

[0149] The contrast steel of Comparative Example 3 observed a mixed crystal phenomenon (1 level) after simulated carburizing quenching at a temperature of 960℃, wherein 6(1) indicates that the average grain size is 6 level, and the local area is coarsened to 1 level. After continuing to increase the simulated carburizing temperature of Comparative Example 1, Comparative Example 2 and Comparative Example 4 to 990℃, the austenite grains abnormally grow seriously (1 level), wherein 5.5(1) indicates that the average grain size is 5.5 level, and the local area is coarsened to 1 level. Among them, according to the standard ASTM E112, the grain size (00) level is coarser than the (0) level.

[0150] In addition, the combination manner of each technical feature in the present case is not limited to the combination manner recorded in the claims of the present case or the combination manner recorded in the specific embodiments. All technical features recorded in the present case can be freely combined or combined in any manner, unless contradictory to each other.

[0151] It should also be noted that the above-mentioned examples are only specific embodiments of the present application. Obviously, the present application is not limited to the above examples, and similar changes or modifications made from the disclosure of the present application are directly derived or easily conceived by those skilled in the art, which should belong to the protection scope of the present application.

Claims

1. A high-toughness high-temperature carburized gear shaft steel, characterized in that, The mass percentage of each chemical element is as follows: C: 0.16-0.23%, Si: 0.10-0.40%, Mn: 0.86-1.24%, Cr: 0.95-1.44%, Al: 0.025-0.050%, N: 0.008-0.015%, Nb: 0.003-0.030%, Ti: 0.02-0.08%; the balance is Fe and other inevitable impurities; a microalloying element coefficient r M / N r = (10*[Nb] / 93 + [Ti] / 480 + [Al] / 27) / ([N] / 14) wherein the chemical elements are entered in the formula with the values before the percentage sign of the mass percentage content of the chemical element; and M / N r is in the range of 1.5 to 6.

0. The typical position hardenability of the high-strength and high-toughness high-temperature carburizing steel tooth shaft meets the following requirements: J9mm is 30-38HRC, and J15mm is 22-31HRC. its tensile strength Rm after high temperature carburizing heat treatment m ≥ 1100 MPa, yield strength R p0.2 ≥ 980 MPa, elongation A ≥ 12%, reduction of area ≥ 50%, charpy impact energy A ku ≥ 60 J.

2. The high tough high temperature carburized gear shaft steel according to claim 1, characterized in that, In the inevitable impurities, P≤0.030%, O≤0.002%, H≤0.0002%, B≤0.0005%, Ca≤0.004%.

3. The high-toughness high temperature carburized gear shaft steel of claim 1, wherein, It also contains at least one of the following chemical elements: 0 4. The high-toughness high temperature carburized gear shaft steel of claim 1, wherein, The austenite grain size of the steel after high-temperature carburizing heat treatment is maintained at 5-9 levels.

5. The high-toughness high temperature carburized gear shaft steel of claim 1, wherein, The critical ideal diameter DI value is 1.5-3.5in., wherein: DI=0.54[C]×(3.333[Mn]+1)×(0.70[Si]+1)×(0.363[Ni]+1)×(2.16[Cr]+1)×(3.00[Mo]+1)×(0.365[Cu]+1)×(1.73[V]+1), wherein each element symbol is substituted into the value before the mass percentage of the corresponding element.

6. The method of producing a high-toughness high-temperature carburizing steel for a gear shaft according to any one of claims 1 to 5, characterized in that, It comprises the steps of: (1) smelting; (2) casting; (3) heating: slowly heating at a speed not more than 500℃ / h to a heating temperature of 1100-1250℃, and maintaining for 3-12h; (4) forging or rolling.

7. The production method according to claim 6, wherein In step (3), the slow heating at a speed not more than 500℃ / h to a heating temperature of 1100-1250℃ specifically includes: first heating to not higher than 700℃ in a preheating section, then continuing to heat to not higher than 1100℃ in a first heating section, maintaining and then continuing to heat to 950-1200℃ in a second heating section, entering a soaking section for 3-12h, and the soaking section temperature is 1100-1250℃.

8. The production method according to claim 6, wherein In step (4), the open forging or open rolling temperature is controlled to be 1100-1250℃, and the finish rolling temperature or finish forging temperature is controlled to be ≥900℃.

9. The production method according to claim 6, wherein In step (4), directly rolling or forging to the finished product size.

10. The production method according to claim 6, wherein In step (4), first rolling to an intermediate blank size, then intermediate heating, and then rolling to the final finished product size; wherein the intermediate heating is slowly heated to an intermediate heating temperature of 1100-1250℃ at a speed not more than 500℃ / h, and maintaining for 3-12h.

11. The production method according to claim 9, wherein In step (4), the intermediate heating is slowly heated to an intermediate heating temperature of 1100-1250℃ at a speed of not more than 500℃ / h, and the holding time is 3-12h, specifically including: first heating to not higher than 700℃ in a preheating section, then continuing to heat to not higher than 1100℃ in a first heating section, after holding, continuing to heat to 950-1200℃ in a second heating section, and entering a soaking section for holding for 3-12h, and the soaking section temperature is 1100-1250℃.

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