High-strength, high-toughness and high-hardenability gear shaft steel, and manufacturing method therefor

Abstract

Disclosed in the present invention is a high-strength, high-toughness and high-hardenability gear shaft steel. The steel contains Fe and inevitable impurities, and the following chemical elements in percentage by mass: C: 0.16%-0.22%, Si: 0.10%-0.40%, Mn: 0.86%-1.24%, Cr: 0.95%-1.44%, Al: 0.02%-0.05%, Ti: 0.015%-0.039%, Nb: 0.001%-0.034%, N: 0.006%-0.015%, and B: 0.0006%-0.0034%, wherein a microalloying element coefficient rM / N of the steel ranges from 1.5 to 5.0, and rM / N=(10*[Nb] / 93+[Ti] / 480+[Al] / 27) / ([N] / 14). In addition, further disclosed in the present invention is a manufacturing method for the high-strength, high-toughness and high-hardenability gear shaft steel. The high-strength, high-toughness and high-hardenability gear shaft steel of the present invention has high hardenability, narrow hardenability bandwidth, and good high-temperature grain stability.

Description

A high-strength, high-toughness, high-hardenability steel for gear shaft and its manufacturing method Technical Field

[0001] The present invention relates to a high-strength steel material and a manufacturing method thereof, and in particular to a steel material for a gear shaft and a manufacturing method thereof. Background Art

[0002] The development of the automotive industry has placed increasingly higher demands on automotive components. Among them, gears with high strength, high fatigue life, high temperature stability, and cost-effectiveness are an important development direction.

[0003] Accordingly, high-strength and tough gear shaft steel can meet the high technical requirements of lightweight vehicles for materials. In addition, in order to ensure the quenching strength of the gear, it is usually also necessary to put forward requirements for the hardenability of the gear shaft steel.

[0004] The main technical difficulty of high hardenability MnCr carburized gear steel is how to improve hardenability while avoiding gear size dispersion caused by excessive hardenability bandwidth, and at the same time ensure that the gears do not have mixed crystals and coarse grains after high-temperature carburizing.

[0005] For example, Chinese patent publication CN101096742A, published on January 2, 2008, and titled "High-Strength Automotive Gear Steel," discloses a high-strength automotive gear steel. The steel incorporates alloying elements such as Nb, V, and Al to refine the original austenite grains. The steel's composition by mass is as follows: 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, with the remainder being Fe and unavoidable impurities. By adding trace amounts of Nb and V, the steel's grain size, hardenability, and bandwidth are optimized.

[0006] Another example is Chinese patent publication CN103361559A, published on October 23, 2013, and titled "A Nb and Ti Composite Microalloyed High-Temperature Carburizing Gear Steel." The patent discloses a Nb and Ti composite microalloyed high-temperature carburizing gear steel with the following composition: 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%, with the balance being iron and unavoidable impurities. By controlling the contents of microalloying elements such as Nb, Ti, and Al, the gear carburizing temperature can be increased or the carburizing time shortened.

[0007] However, the above patent documents do not completely solve the problems of hardenability and bandwidth control of high-strength gear shaft steel.

[0008] Summary of the Invention

[0009] One of the objects of the present invention is to provide a high-strength, tough, and high-hardenability gear shaft steel. By optimizing the component system of the gear shaft steel, especially by reasonably controlling the contents of microalloying elements and nitrogen in the gear steel, a high-strength gear shaft steel with high hardenability, a narrow hardenability bandwidth, and good high-temperature grain stability can be obtained at a low cost.

[0010] To achieve the above-mentioned object, the present invention provides a high-strength, toughness, and hardenability steel for a gear shaft, which, in addition to Fe and unavoidable impurities, further contains the following chemical elements in the following mass percentages:

[0011] C: 0.16~0.22%,

[0012] Si: 0.10~0.40%,

[0013] Mn: 0.86~1.24%,

[0014] Cr: 0.95~1.44%,

[0015] Al: 0.02~0.05%,

[0016] Ti: 0.015~0.039%,

[0017] Nb: 0.001~0.034%,

[0018] N: 0.006~0.015%,

[0019] B: 0.0006~0.0034%;

[0020] Its microalloying element coefficient r M / N The range is 1.5~5.0, where r M / N =(10*[Nb] / 93+[Ti] / 480+[Al] / 27) / ([N] / 14), where each chemical element is substituted into the value before the percentage sign of the mass percentage content of the chemical element.

[0021] Furthermore, in the high-strength, toughness, and hardenability steel for gear shafts of the present invention, the mass percentages of the chemical elements are as follows:

[0022] C: 0.16~0.22%,

[0023] Si: 0.10~0.40%,

[0024] Mn: 0.86~1.24%,

[0025] Cr: 0.95~1.44%,

[0026] Al: 0.02~0.05%,

[0027] Ti: 0.015~0.039%,

[0028] Nb: 0.001~0.034%,

[0029] N: 0.006~0.015%,

[0030] B: 0.0006~0.0034%;

[0031] The balance is Fe and unavoidable impurities;

[0032] Its microalloying element coefficient r M / N The range is 1.5~5.0, where r M / N =(10*[Nb] / 93+[Ti] / 480+[Al] / 27) / ([N] / 14), where each chemical element is substituted into the value before the percentage sign of the mass percentage content of the chemical element.

[0033] In the high-strength, toughness, and hardenability gear shaft steel of the present invention, the design principles of the chemical elements are specifically described as follows:

[0034] C: In the high-strength, toughness, and high-hardenability steel for gear shafts described in the present invention, C is a necessary component in the steel, and it is also one of the most important elements affecting the hardenability of the steel. High-strength, toughness, and high-hardenability steel for gear shafts requires surface strength while also requiring sufficient core impact toughness. When the C content in the steel is too low, below 0.16%, the strength of the steel is insufficient, and good hardenability requirements cannot be guaranteed; accordingly, the C content in the steel should not be too high. When the C content in the steel is too high, the gear core toughness requirements cannot be met, and excessively high C content is detrimental to the plasticity of the steel, especially for carburized gear steel with a high Mn content. When the C content is greater than 0.22%, it is not conducive to the processing performance of the steel. Therefore, in the high-strength, toughness, and high-hardenability steel for gear shafts described in the present invention, the mass percentage of C is controlled between 0.16 and 0.22%.

[0035] Si: In the high-strength, toughness, and hardenability steel for gear shafts described in the present invention, the Si element can not only better eliminate the adverse effects of iron oxide on steel, but it can also dissolve into ferrite, strengthening the ferrite and improving the strength, hardness, wear resistance, elasticity, and elastic limit of the steel. At the same time, it should be noted that the Si element will increase the Ac3 temperature of the steel, and due to its poor thermal conductivity, there is a risk of cracking and a tendency to decarburization. Based on this, considering both the beneficial effects and adverse effects of Si, in the high-strength, toughness, and hardenability steel for gear shafts described in the present invention, the mass percentage of Si is controlled between 0.10 and 0.40%.

[0036] Mn: In the high-strength, toughness, and hardenability gear shaft steel described herein, Mn is one of the primary elements affecting its hardenability. Mn has excellent deoxidizing properties, reducing iron oxide in the steel and effectively increasing steel production. Mn dissolves into ferrite, increasing the steel's strength and hardness. It also produces finer, higher-strength pearlite during cooling after hot rolling. Furthermore, Mn reacts with sulfur in the steel to form MnS, counteracting the harmful effects of sulfur. Mn stabilizes the austenite structure, significantly increasing the steel's hardenability while also reducing its high-temperature toughness. Too little Mn in the steel results in insufficient hardenability. Excessive Mn content impairs the steel's thermoplastic properties, hindering production and increasing the likelihood of cracking during water quenching. Therefore, in the high-strength, toughness, and hardenability gear shaft steel described herein, the mass percentage of Mn is controlled between 0.86% and 1.24%.

[0037] Cr: Cr is one of the primary alloying elements added to the high-strength, toughness, and hardenability gear shaft steel described herein. Cr significantly improves the steel's hardenability, strength, wear resistance, and other properties. Cr also reduces the activity of carbon in the steel, preventing decarburization during heating, rolling, and heat treatment. However, excessive Cr content significantly reduces the toughness of the steel during quenching and tempering, forming coarse carbides distributed along grain boundaries. Therefore, the mass percentage of Cr in the high-strength, toughness, and hardenability gear shaft steel described herein is controlled between 0.95% and 1.44%.

[0038] Al: In the high-strength, toughness, and hardenability steel for gear shafts described in the present invention, Al is a grain-refining element. Al, in combination with N, can further refine the grains and improve the toughness of the steel. Grain refinement plays an important role in improving the mechanical properties of steel, especially its strength and toughness. Grain refinement also helps reduce the steel's sensitivity to hydrogen embrittlement. However, it should be noted that the Al content in the steel should not be too high. Excessive Al content tends to increase the chances of inclusions forming in the steel. Therefore, in the high-strength, toughness, and hardenability steel for gear shafts described in the present invention, the mass percentage of Al is controlled between 0.02 and 0.05%.

[0039] Ti: While adding Ti to steel can form fine precipitates, excessive Ti content can lead to the formation of coarse, angular TiN particles during smelting, reducing the steel's impact toughness. Therefore, in the high-strength, toughness, and hardenability gear shaft steel described herein, the Ti content is controlled between 0.015% and 0.039%.

[0040] Nb: In the high-strength, toughness, and hardenability gear shaft steel described herein, Nb is added to form fine precipitates, thereby inhibiting recrystallization and effectively refining the grain size. However, it is important to note that the Nb content in the steel should not be too high. Excessive Nb content can lead to the formation of coarse NbC particles during the smelting process, which in turn reduces the steel's impact toughness. Therefore, in the high-strength, toughness, and hardenability gear shaft steel described herein, the mass percentage of Nb is controlled to be between 0.001% and 0.034%.

[0041] N: In the high-strength, toughness, and hardenability gear shaft steel described herein, nitrogen (N) acts as an interstitial atom. It combines with microalloys in the steel to form MN-type precipitates, which pin grain boundaries at high temperatures and inhibit austenite grain growth. Low N content in the steel results in less MN formation, and the pinning effect is less pronounced. However, excessive N content can lead to accumulation during steelmaking, reducing toughness. Therefore, in the high-strength, toughness, and hardenability gear shaft steel described herein, the mass percentage of N is controlled between 0.006% and 0.015%.

[0042] Boron (B): Boron greatly improves the hardenability of steel, and the required content is relatively low. Its effect is hundreds or even thousands of times that of general alloying elements, resulting in significant economic benefits. Boron steel can also be water-quenched, saving quenching oil and easily obtaining a martensitic structure, thus imparting excellent strength and hardness to boron-containing steel. As long as the boron content is appropriate, the production process is appropriate, and complete hardening is ensured, there will be no significant reduction in plasticity and toughness. However, the B element tends to segregate, causing significant fluctuations in the hardenability of the steel. Therefore, in the high-strength, toughness, and hardenability gear shaft steel described in the present invention, the B element content is controlled between 0.0006% and 0.0034%.

[0043] In addition, it is important to control the microalloying element coefficient r in the high strength, toughness and hardenability gear shaft steel of the present invention. M / N The range is 1.5~5.0, where r M / N =(10*[Nb] / 93+[Ti] / 480+[Al] / 27) / ([N] / 14), where each chemical element in the formula is substituted into the value before the percentage sign of the mass percentage content of the chemical element.

[0044] In the present invention, Al, Nb, Ti and N are the main grain refining elements. The present invention controls the content of Al, Nb, Ti and N in the gear steel and the microalloying element coefficient r M / N , so that the microalloying elements and excess N elements form precipitates, thereby inhibiting the growth of austenite grains in the high temperature stage.

[0045] Furthermore, among the inevitable impurities in the high-strength, toughness, and hardenability gear shaft steel of the present invention, the mass percentage of each impurity element satisfies at least one of the following conditions:

[0046] P≤0.030%,

[0047] O≤0.0020%,

[0048] H≤0.0002%,

[0049] Ca≤0.0034%.

[0050] In the above technical solution, P, O, H and Ca are all impurity elements in steel. If technical conditions permit, in order to obtain steel with better performance and better quality, the content of impurity elements in steel should be reduced as much as possible. Among them:

[0051] P: P is easily segregated at the grain boundaries in steel, which reduces the grain boundary bonding energy and deteriorates the impact toughness of the steel. Therefore, in some embodiments of the present invention, the P content can be controlled to P≤0.030%.

[0052] O: O easily forms oxides and composite oxides with the Al element in steel, destroying the continuity of the steel, reducing the structural uniformity and low-temperature impact energy and fatigue properties. Based on this, in some embodiments of the present invention, the O element content can be controlled to O≤0.0020%.

[0053] H: H will accumulate at defects in steel, especially in steel with a tensile strength level exceeding 1000 MPa, causing hydrogen-induced delayed fracture. Therefore, in some embodiments of the present invention, the H element content can be controlled to H≤0.0002%.

[0054] Ca: In the high-strength, toughness, and hardenability gear shaft steel of the present invention, Ca easily forms inclusions, thereby affecting the fatigue properties of the final product. Therefore, the Ca content can be controlled to Ca≤0.0034%, for example, ≤0.003%.

[0055] Furthermore, the high-strength, toughness, and hardenability steel for gear shafts of the present invention further contains at least one of the following chemical elements:

[0056] 0 < S ≤ 0.04%, 0 < Ni ≤ 0.25%, 0 < Mo ≤ 0.10%, 0 < Cu ≤ 0.20%, 0 < V ≤ 0.03%.

[0057] Optionally, in the high-strength, high-toughness, and high hardenability steel for gear shafts of the present invention, at least one of the elements S, Ni, Mo, Cu, and V can be further added to further improve the properties of the high-strength, high-toughness, and high hardenability steel for gear shafts of the present invention. Among them:

[0058] S: S generally exists as an impurity element in steel, which will reduce the plasticity and toughness of the steel. However, in the high hardenability steel for gear shafts of the present invention, a certain content of S element can form non-metallic inclusions with Mn to improve the cutting performance of the steel. Based on this, in the high hardenability steel for gear shafts of the present invention, the mass percentage of S is controlled at 0 < S ≤ 0.04%, for example, 0.001% ≤ S ≤ 0.04%.

[0059] Ni: In the high-strength, high-toughness, and high hardenability steel for gear shafts of the present invention, Ni exists in a solid solution form in the steel, which can effectively improve the low-temperature impact performance of the steel. However, it should be noted that too high Ni content will lead to too high retained austenite content in the steel, reducing the strength of the steel. Therefore, in the high-strength, high-toughness, and high hardenability steel for gear shafts of the present invention, the mass percentage of Ni can preferably be controlled at 0 < Ni ≤ 0.25%, for example, 0.03% ≤ Ni ≤ 0.25%.

[0060] Mo: In the high-strength, high-toughness, and high hardenability steel for gear shafts of the present invention, Mo can be dissolved in the steel, which is beneficial to improving the hardenability of the steel and increasing the strength of the steel. At higher temperature tempering, Mo will also form fine carbides to further increase the strength of the steel. The combined action of molybdenum and manganese can significantly improve the stability of austenite. Based on this, in the high-strength, high-toughness, and high hardenability steel for gear shafts of the present invention, the mass percentage of Mo can preferably be controlled at 0 < Mo ≤ 0.10%, for example, 0.01% ≤ Mo ≤ 0.10%.

[0061] Cu: In the high-strength, high-toughness, and high hardenability steel for gear shafts of the present invention, Cu can increase the strength of the steel and is beneficial to improving 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 Cu content in the steel is too high, it will accumulate at the grain boundaries during the heating process, resulting in grain boundary weakening and cracking. Therefore, in the high-strength, high-toughness, and high hardenability steel for gear shafts of the present invention, the mass percentage of Cu can preferably be controlled at 0 < Cu ≤ 0.20%, for example, 0.03% ≤ Cu ≤ 0.20%.

[0062] V: In the steel for high hardenability gear shafts of the present invention, V can effectively improve the hardenability of the steel. In the steel, the V element can form precipitates with the C element or N element, thereby further increasing the strength of the steel. However, if the contents of the C element and the V element are too high, coarse VC particles will be formed. Based on this, in the steel for high hardenability gear shafts of the present invention, the mass percentage of the V element can be controlled at 0 < V ≤ 0.03%, for example, 0.005% ≤ V ≤ 0.03%.

[0063] Further, the austenite grain size of the high-strength, high-toughness, high-hardenability steel for gear shafts of the present invention after high-temperature carburizing heat treatment remains at levels 5 to 8. Among them, the conditions for the simulated high-temperature carburizing heat treatment test for detecting the austenite grain size can be: first heating to 1200 °C, holding for 40 min, water cooling, and then heating to 1000 °C at a rate of heating 500 - 800 °C in 40 - 60 min, holding for 4 h, and water quenching.

[0064] Further, the J9mm hardenability of the high-strength, high-toughness, high-hardenability steel for gear shafts of the present invention is 34 - 42 HRC.

[0065] Further, the tensile strength R of the high-strength, high-toughness, high-hardenability steel for gear shafts of the present invention after high-temperature carburizing heat treatment m ≥1100 MPa, the yield strength R p0.2 ≥980 MPa, the elongation after fracture A ≥ 12%, the reduction of area ≥ 50%, and the Charpy impact energy A ku ≥55 J. Among them, the conditions for the simulated high-temperature carburizing heat treatment for detecting the mechanical properties can be: heating at 880 ± 10 °C for 90 min and oil quenching + heating at 870 ± 10 °C for 90 min and oil quenching + heating at 200 ± 10 °C for 150 min and tempering with air cooling empty.

[0066] Further, the tensile strength R of the high-strength, high-toughness, high-hardenability steel for gear shafts of the present invention after high-temperature carburizing heat treatment m can be 1100 - 1400 MPa.

[0067] Further, the yield strength R of the high-strength, high-toughness, high-hardenability steel for gear shafts of the present invention after high-temperature carburizing heat treatment p0.2 can be 980 MPa - 1250 MPa.

[0068] Further, the elongation after fracture A of the high-strength, high-toughness, high-hardenability steel for gear shafts of the present invention after high-temperature carburizing heat treatment can be 12 - 15%.

[0069] Further, the reduction of area of the high-strength, high-toughness, high-hardenability steel for gear shafts of the present invention after high-temperature carburizing heat treatment can be 50 - 62%.

[0070] Furthermore, the Charpy impact energy A of the high-strength, high-toughness, high-hardenability gear shaft steel of the present invention after high-temperature carburizing heat treatment is ku It can be 55~130J.

[0071] Accordingly, another object of the present invention is to provide a method for manufacturing high-strength, toughness, and high hardenability steel for gear shafts, which is simple to produce and the obtained high-strength, toughness, and high hardenability steel for gear shafts has the characteristics of high hardenability and high strength and toughness.

[0072] In order to achieve the above object, the present invention also proposes a method for manufacturing the high-strength, toughness and hardenability gear shaft steel as described above, which comprises the steps of:

[0073] (1) smelting;

[0074] (2) Casting;

[0075] (3) Heating: The steel billet is first heated to no more than 700°C in the preheating section, then continued to be heated to no more than 980°C in the first heating section, and then continued to be heated to 950-1200°C in the second heating section after holding. After holding, it enters the soaking section, and the temperature in the soaking section is 1050-1250°C;

[0076] Alternatively, the steel billet is slowly heated at a rate of 50-300°C / h, first heated to no higher than 700°C in the preheating section, then continued to heat to no higher than 980°C in the first heating section, and continued to heat to 950-1200°C in the second heating section, and then entered the soaking section, where the temperature is 1050-1250°C;

[0077] (4) Forging or rolling.

[0078] In the manufacturing method described in the present invention, compared with the prior art, its heating step adopts a unique process, wherein the soaking section temperature is relatively high. This is because the higher soaking section temperature can improve the composition uniformity and structural uniformity of the continuous casting billet during the diffusion process of billet heating. At the same time, at this temperature, the precipitation phase has a relatively fast solid solution rate. Therefore, the high heating temperature will cause more dissolution of the original undissolved precipitation phase particles in the steel, increase the concentration of microalloying elements in the matrix, and precipitate more and more dispersed particles during subsequent cooling. In addition, only after the heating temperature is increased can the final rolling or final forging temperature be increased, thereby making the recovery recrystallization of the austenite after rolling more sufficient and the distribution of the precipitation phase more uniform.

[0079] In the manufacturing method of the present invention, the smelting in step (1) can be performed by electric furnace smelting or converter smelting, and then refined and vacuum treated. Of course, in some other embodiments, a vacuum induction furnace can also be used for smelting.

[0080] In addition, in step (1), the charge for electric furnace smelting can be low-P, S scrap steel, off-cuts and high-quality pig iron; the alloy can be prepared as ferrochrome, low-phosphorus ferromanganese, ferromolybdenum, etc.; the reducing agent can include: calcium carbide, carbon powder and aluminum powder; during the oxidation period: slag is frequently removed by flowing; the slag tapping conditions can be controlled to: slag tapping temperature is 1630~1660℃; P≤0.015%; the steel tapping conditions can be controlled to: steel tapping temperature is 1630~1650℃; [P]≤0.010%, [C]≥0.03%.

[0081] After the electric furnace smelting or converter smelting is completed, the molten steel can be refined in the ladle refining furnace to remove harmful gases and inclusions in the steel. Control the seating of the ladle, measure the temperature and analyze it, and adjust the argon pressure according to the situation. The ladle refining furnace (LF) can be fed with Al for initial deoxidation, and then alloy blocks can be added and stirred for 5 to 10 minutes. When the temperature of the molten steel is measured at T = 1650 to 1670 ° C, vacuum degassing can be performed to ensure that [O] ≤ 0.0020% and [H] ≤ 0.00015%. In a specific example, the vacuum degree of vacuum degassing can be controlled to ≤ 66.7 Pa and maintained for not less than 15 minutes,

[0082] In addition, in step (1), the hanging bag temperature can be controlled to be 1550-1570° C., thereby lowering the hanging bag temperature and accelerating element diffusion, which is beneficial to further reduce dendrite segregation.

[0083] In addition, in step (2), the casting can be performed by die casting or continuous casting. During the continuous casting process, the high-temperature molten steel in the ladle passes through the protective sleeve and is poured into the tundish, and the superheat of the tundish can be controlled to be 20-40°C. The tundish is completely cleaned before use, and the inner surface is refractory coated and free of cracks; the molten steel in the tundish passes through the continuous casting crystallizer and is fully electromagnetically stirred, and qualified continuous casting billets with cross-sectional dimensions of 140mm×140mm to 320mm×425mm can be cast.

[0084] In step (2), the pouring speed can be controlled to be 0.6-2.1 m / min according to different billet sizes. Then, the continuous casting billet is put into a slow cooling pit for slow cooling, and the slow cooling time can be no less than 24 hours.

[0085] Furthermore, in step (3) of the manufacturing method of the present invention, the heating temperature of the preheating section can be 600-700°C, and the temperature of the first heating section can be 900-980°C.

[0086] Furthermore, in step (3) of the manufacturing method of the present invention, the heat-sparing section needs to be kept warm for a period of time, and the heat-sparing section heat-sparing time can be 3 to 12 hours.

[0087] In step (3) of the manufacturing method of the present invention, after heating in the first heating section, insulation may or may not be performed, and then the second heating section is performed. The insulation time of the first heating section can be 0 to 3 hours, for example, 0.5 hours, 1 hour, or 2 hours. In step (3) of the manufacturing method of the present invention, after heating in the second heating section, insulation may or may not be performed, and then the soaking section is entered. The insulation time of the second heating section can be 0 to 3 hours, for example, 0.5 hours, 1 hour, or 2 hours.

[0088] In other embodiments, in step (3) of the manufacturing method described in the present invention, slow heating is performed at a rate of 50 to 300°C / h, first heated to no higher than 700°C in the preheating section, then continued to heat to no higher than 980°C in the first heating section without insulation, and continued to heat to 950 to 1200°C in the second heating section without insulation, and entered the equalizing section. This process can be carried out in a walking beam heating furnace.

[0089] Furthermore, in step (4) of the manufacturing method of the present invention, the start forging or rolling temperature is controlled to be 1050-1250° C., and the final forging or rolling temperature is controlled to be ≥ 900° C. In some embodiments, in step (4) of the manufacturing method of the present invention, the final forging or rolling temperature is 900-1000° C.

[0090] In this embodiment, the start forging or rolling temperature is controlled between 1050 and 1250°C, and the final forging or rolling temperature is controlled to be ≥900°C because: this process further facilitates nitrogen to be desolvated from the γ solid solution and combined with microalloying elements in the steel to form nitrides.

[0091] It should be noted that the solubility of N in α-Fe is lower than that in γ-Fe, and due to the two peaks of precipitation caused by the stimulation of phase transformation, if the final forging or final rolling temperature is low, the peak of the precipitation phase will precipitate, resulting in uneven distribution of the precipitation phase and insufficient recovery recrystallization, resulting in anisotropy in the structure. Therefore, the final forging or final rolling temperature is controlled to be ≥ 900°C. In addition, increasing the final forging or final rolling temperature will obtain finer grains. The fine grains increase the difference between the average grain diameter of ferrite after the transformation of supercooled austenite and the spacing between the manganese-rich bands, reducing the tendency of the manganese-rich bands to form pearlite, thereby reducing the banded structure.

[0092] In the above technical solution, in step (4) of the manufacturing method of the present invention, after the steel billet is taken out of the furnace, high-pressure water can be used to descale and remove the oxide skin.

[0093] Furthermore, in step (4) of the manufacturing method of the present invention, when forging is performed, the steel can be directly forged to the final product size. 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.

[0094] In some embodiments, in step (4) of the manufacturing method of the present invention, the steel billet is first rolled into an intermediate billet (the size can be 140mm×140mm~260mm×260mm), and the final rolling temperature of the intermediate billet is controlled to be 1000~1050℃; then the intermediate billet is heated according to the following process: the intermediate billet is first heated to 680~700℃ in the preheating section, then heated to 1050~1100℃ in the first heating section, and then heated to 1200~1220℃ in the second heating section, and the heating rate can be 300~500℃ / h; then enters the soaking section, the soaking section temperature is 1200~1250℃, and the soaking section holding time can be 3~5h; then the heated intermediate billet is rolled into a finished product, and the final rolling temperature of the finished product is controlled to be ≥900℃ (for example, 900~1000℃).

[0095] The high-strength, toughness, and hardenability gear shaft steel and its manufacturing method described in the present invention have the following advantages and beneficial effects compared to the prior art:

[0096] (1) The present invention can develop a gear shaft steel with high hardenability through reasonable chemical composition design combined with optimized process. The bars rolled or forged with the high-strength and high-hardenability gear shaft steel can be effectively processed into gears, and after subsequent downstream high-temperature carburizing heat treatment, they can have excellent strength and toughness.

[0097] (2) The high-strength, toughness and hardenability gear shaft steel of the present invention controls the microalloying element coefficient and the nitrogen content, and strictly controls the atomic molar ratio, while adding an appropriate amount of Nb element to hinder the abnormal growth of austenite grains, thereby increasing the austenite grain coarsening temperature of the gear steel, so that the grain size of the gear steel can be stably maintained at level 5 to 8 after carburizing at a high temperature of up to 1000°C for 4 hours, and various performances meet the performance indicators of gear shaft steel.

[0098] (3) The composition and process design of the high-strength, toughness and hardenability gear shaft steel described in the present invention are reasonable. By controlling the content of microalloying elements in the steel, the occurrence of large-particle harmful inclusions in the steel is avoided, thereby ensuring the stable production quality of the steel, reducing the production cost of the steel, and realizing mass production on the bar production line.

[0099] (4) The high-strength, toughness, and hardenability steel for gear shafts of the present invention are superior to the prior art in terms of hardenability, austenite grain size, and cost competitiveness. While ensuring high hardenability and narrow bandwidth, the types and quantities of alloy elements in the steel can be controlled, thereby improving the applicability of the steel.

[0100] (5) The high-strength, toughness, and hardenability gear shaft steel of the present invention can shorten the carburizing time and reduce the gear shaft production cost when subsequently used in the production of gear shafts, and has broad industrial application prospects. DETAILED DESCRIPTION

[0101] The high-strength, toughness, and hardenability gear shaft steel and its manufacturing method described in the present invention will be further explained below in conjunction with specific embodiments. However, such explanation and description do not constitute an undue limitation to the technical solution of the present invention.

[0102] The high-strength, toughness, and hardenability gear shaft steels of Examples 1-8 were all prepared by the following steps:

[0103] (1) Smelting and casting are carried out according to the chemical composition shown in Tables 1-1 and 1-2 below: the smelting can be carried out in a 50kg vacuum induction furnace, a 150kg vacuum induction furnace or a 500kg vacuum induction furnace, or in an electric furnace smelting + refining outside the furnace + vacuum degassing method, or in a converter smelting + refining outside the furnace + vacuum degassing method.

[0104] (2) Casting.

[0105] (3) Heating: The steel billet is first heated to no higher than 700°C in the preheating section, then continued to be heated to no higher than 980°C in the first heating section, and then continued to be heated to 950-1200°C in the second heating section after holding. After holding, it enters the soaking section, and the temperature of the soaking section is 1050-1250°C. After holding, subsequent rolling or forging is carried out.

[0106] (4) Forging or rolling: Control the start forging or rolling temperature to 1050-1250℃, and control the final forging or rolling temperature to ≥900℃.

[0107] Furthermore, the specific process of the high hardenability gear shaft steel of Examples 1-8 and the steel of Comparative Examples 1-4 is as follows:

[0108] Example 1: According to the chemical compositions shown in Tables 1-1 and 1-2 below, smelting was carried out in a 50kg vacuum induction furnace. Molten steel was cast into ingots, heated, and forged. The ingots were first heated to 700°C in a preheating section, then to 900°C in a first heating section. After holding for 0 hours, they were heated to 950°C in a second heating section. After holding for 1 hour, they were placed in a soaking section at 1050°C. After holding for 3 hours, subsequent forging was performed. The final forging temperature was controlled at 910°C, and the ingots were finally forged into Φ60mm bars.

[0109] Example 2: According to the chemical compositions shown in Tables 1-1 and 1-2 below, smelting was carried out in a 150kg vacuum induction furnace. Molten steel was cast into ingots, heated, and forged. The ingots were first heated to 650°C in a preheating section, then to 950°C in a first heating section. After holding at this temperature for 0.5 hours, they were heated to 1100°C in a second heating section. After holding at this temperature for 0 hours, they were then placed in a soaking section at 1200°C. After holding at this temperature for 5 hours, subsequent forging was performed. The final forging temperature was controlled at 1000°C, and the ingots were finally forged into Φ90mm bars.

[0110] Example 3: According to the chemical compositions shown in Tables 1-1 and 1-2 below, smelting was carried out in a 500kg vacuum induction furnace. Molten steel was cast into ingots, heated, and forged. The ingots were first heated to 600°C in a preheating section, then heated to 980°C in a first heating section, held at that temperature for 3 hours, and then heated to 1200°C in a second heating section. After holding that temperature for 3 hours, they were then placed in a soaking section at 1250°C. After holding that temperature for 12 hours, subsequent forging was performed. The final forging temperature was controlled at 1000°C, and the ingots were finally forged into 120mm Φ bars.

[0111] Example 4: Steel was smelted in an electric furnace according to the chemical compositions shown in Tables 1-1 and 1-2, refined, and vacuum-treated, then cast into 280 mm × 280 mm continuous casting billets. The billets were then heated in a walking beam furnace at a controlled rate of 300°C / h, first to 620°C in the preheating section, then to 950°C in the first heating section, and then to 1150°C in the second heating section. The billets then entered the soaking section at 1200°C and were held there for 4 hours before rolling. After exiting the furnace and undergoing high-pressure water descaling, rolling commenced, with the final rolling temperature controlled at 970°C, ultimately resulting in Φ80 mm bars.

[0112] Example 5: The chemical compositions shown in Tables 1-1 and 1-2 were smelted in an electric furnace, refined, and vacuum-treated, and then cast into 320 mm × 425 mm continuous casting billets. The continuous casting billets were heated in a walking beam furnace at a rate of 150°C / h, first to 600°C in the preheating section, then to 950°C in the first heating section, and then to 1200°C in the second heating section. The billets were then heated to a soaking temperature of 1230°C and held for 4.5 hours before subsequent rolling. After exiting the heating furnace and being descaled with high-pressure water, rolling began, and the billets were rolled into intermediate billets. The first finishing rolling temperature (i.e., the intermediate billet finishing rolling temperature) was controlled at 1050°C, and the intermediate billet size was 220 mm × 220 mm. Then the intermediate billet is placed in a walking beam furnace for heating, slowly heated at a rate of 400℃ / h, heated to 680℃ in the preheating section, 1050℃ in the first heating section, and 1200℃ in the second heating section. After keeping warm, it enters the soaking section with a soaking temperature of 1220℃. After keeping warm for 3.5 hours, it is taken out of the furnace and descaled with high-pressure water before rolling. The second final rolling temperature (i.e. the final rolling temperature of the finished product) is controlled at 950℃, and the specification of the finished bar is Φ90mm.

[0113] Example 6: According to the chemical composition shown in Tables 1-1 and 1-2, electric furnace smelting, refining and vacuum treatment were carried out, and then cast into 280mm×280mm continuous casting billets. The continuous casting billets were controlled to be heated in a walking beam heating furnace at a rate of 300℃ / h. They were first heated to 680℃ in the preheating section, then continued to heat to 900℃ in the first heating section, and continued to heat to 1180℃ in the second heating section. The billets entered the soaking section with a temperature of 1200℃ and were kept at this temperature for 4.5 hours before subsequent rolling. After the billets were removed from the heating furnace and descaled with high-pressure water, they were rolled into intermediate billets. The first finishing rolling temperature (i.e., the intermediate billet finishing rolling temperature) was controlled to 1000℃. The intermediate billet size was 140mm×140mm. Then the intermediate billet is placed in a walking beam heating furnace and heated slowly at a rate of 500℃ / h. First, it is preheated to 700℃, heated to 1100℃ in the first heating section, and heated to 1220℃ in the second heating section. It enters the soaking section with a soaking temperature of 1220℃ and is kept warm for 3.5 hours before being taken out of the furnace. After descaling with high-pressure water, rolling begins. The second final rolling temperature (i.e., the final rolling temperature of the finished product) is controlled at 920℃, and the specification of the finished bar is Φ25mm.

[0114] Example 7: Steel was smelted in a converter according to the chemical composition shown in Tables 1-1 and 1-2, refined, and vacuum-treated, then cast into a molded billet. The billet was then heated in a walking beam furnace at a controlled rate of 50°C / h, first to 620°C in the preheating section, then to 950°C in the first heating section, and then to 1150°C in the second heating section. The billet then entered a soaking section at 1200°C, where it was held for 8 hours before rolling. After exiting the furnace, the billet was descaled with high-pressure water and then rolled. The final rolling temperature was controlled at 970°C, ultimately resulting in Φ90 mm bars.

[0115] Example 8: Steel was smelted in a converter according to the chemical composition shown in Tables 1-1 and 1-2, refined, and vacuum-treated, and then cast into a molded billet. The billet was heated slowly in a walking beam furnace at a rate of 100°C / h, first to 600°C in the preheating section, then further heated to 950°C in the first heating section, and then to 1200°C in the second heating section. The billet then entered the soaking section at a temperature of 1230°C, was held there for 7 hours, and then subsequently rolled. After exiting the heating furnace and being descaled with high-pressure water, the billet was rolled into an intermediate billet. The first finishing rolling temperature (i.e., the intermediate billet finishing rolling temperature) was controlled at 1050°C, and the intermediate billet size was 260 mm × 260 mm. Then the intermediate billet is placed in a walking beam heating furnace and slowly heated at a speed of 300℃ / h. The preheating section is heated to 680℃, the first heating section is heated to 1050℃, and the second heating section is heated to 1200℃. It enters the soaking section with a soaking temperature of 1220℃. After keeping warm for 5 hours, it is taken out of the furnace and descaled with high-pressure water before rolling. The second final rolling temperature (i.e. the final rolling temperature of the finished product) is controlled at 950℃, and the specification of the finished bar is Φ60mm.

[0116] Comparative Examples 1 and 2 are commercially available materials that have been smelted and refined in an electric furnace to ensure the purity of the commercial materials.

[0117] Comparative Example 3: Smelting was performed in a 50kg vacuum induction furnace according to the chemical compositions shown in Tables 1-1 and 1-2. The molten steel was cast into ingots, heated, and forged to form blanks. The ingots were then heated in a box furnace at a controlled rate of 300°C / h to 1100°C. After holding at this temperature for 3 hours, they were subsequently forged with a final forging temperature of 910°C to produce Φ60mm bars.

[0118] Comparative Example 4: The chemical compositions shown in Tables 1-1 and 1-2 were smelted in an electric furnace, refined, and vacuum-treated, and then cast into 320 mm × 425 mm continuous casting billets. The continuous casting billets were heated in a walking beam heating furnace at a rate of 150°C / h, first to 600°C in the preheating section, then to 950°C in the first heating section, and then to 1200°C in the second heating section. The billets then entered the soaking section at a temperature of 1230°C, where they were held for 4.5 hours before subsequent rolling. After exiting the heating furnace and undergoing high-pressure water descaling, rolling began, resulting in intermediate billets with a first finishing temperature (i.e., the intermediate billet finishing temperature) controlled at 1050°C. The intermediate billets had a size of 220 mm × 220 mm. Then the intermediate billet is placed in a walking beam heating furnace and slowly heated at a speed of 400℃ / h. It is heated to 680℃ in the preheating section, 1050℃ in the first heating section, and 1200℃ in the second heating section. It enters the soaking section with a soaking temperature of 1220℃ and is kept warm for 6 hours. After being taken out of the furnace and descaled with high-pressure water, it begins rolling. The second final rolling temperature (i.e. the final rolling temperature of the finished product) is controlled at 950℃, and the specification of the finished bar is Φ90mm.

[0119] Table 1-1 and Table 1-2 list the mass percentages of the chemical elements in the high hardenability gear shaft steels of Examples 1-8 and the comparative steels of Comparative Examples 1-4.

[0120] Table 1-1 (wt.%, the balance is Fe and other inevitable impurities except P, O, H and Ca)

[0121] Table 1-2. (wt.%, the balance is Fe and other inevitable impurities except P, O, H and Ca) Note: M / N =(10*[Nb] / 93+[Ti] / 480+[Al] / 27) / ([N] / 14), where each chemical element in the formula is substituted into the value before the percentage sign of the mass percentage content of the chemical element.

[0122] Table 2 lists the specific process parameters of the high-strength, toughness, and hardenability gear shaft steels of Examples 1-8 and the comparative steels of Comparative Examples 1-4 in the above process steps.

[0123] Table 2.

[0124] As can be seen from Table 2, in Example 5, Example 6, Example 8 and Comparative Example 4, the steel billets are first rolled to a specified intermediate billet size during rolling, and then intermediately heated and rolled again to the final finished product size.

[0125] In order to verify the performance of the high-strength, toughness, and hardenability gear shaft steel of the present invention, samples of the high-strength, toughness, and hardenability gear shaft steels of Examples 1-8 and the comparative steels of Comparative Examples 1-4 were taken and subjected to simulated high-temperature carburizing heat treatment tests, followed by mechanical property tests and austenite grain size measurements. The test results are listed in Table 3.

[0126] Simulated high-temperature carburizing heat treatment test for detecting austenite grain size: the sample is heated to 1200℃, kept at this temperature for 40 minutes, water-cooled, then heated to 1000℃ at a rate of 600℃ in 50 minutes, kept at this temperature for 4 hours, and quenched in water.

[0127] Austenite grain size test: Evaluate the austenite grain size according to standard ASTM E112.

[0128] Mechanical properties test: Samples were prepared in accordance with GB / T 2975-2018 "Steel and Steel Products Mechanical Properties Test Sampling Location and Specimen Preparation" and GB / T 3077-2015. Φ15mm blanks were prepared for the simulated high-temperature carburizing heat treatment test for mechanical properties testing: heating at 880℃ for 90min oil quenching + heating at 870℃ for 90min oil quenching + heating at 200℃ for 150min tempering and air cooling. Tensile tests were performed in accordance with GB / T 228.1-2010 "Metallic Materials Tensile Tests Part 1: Room Temperature Test Methods" to measure the tensile strength R m , yield strength R p0.2 , elongation after fracture A and cross-sectional shrinkage Z, and GB / T 229-2007 "Charpy pendulum impact test method for metal materials" was used to test the Charpy impact energy A of each embodiment and comparative example at room temperature. ku .

[0129] In addition, samples of the high-strength, toughness, and hardenability gear shaft steels obtained in Examples 1-8 and the comparative steels of Comparative Examples 1-4 were taken for hardenability and hardness tests, and the test results are also listed in Table 3.

[0130] Hardenability Test: Samples of each example steel and comparative example steel were taken and prepared from hot-rolled round steel in accordance with national standard GB / T 225. End hardenability testing (Jominy test) was performed with reference to GB / T 5216, with normalizing temperatures at 920±10°C and quenching temperatures at 870±5°C. Rockwell hardness testing was performed according to GB / T 230.2 to obtain the hardness value (HRC) at a specific location, such as the hardness at 9 mm from the quenched end, i.e., J9mm.

[0131] Table 3 lists the test results of the high-strength, toughness, and hardenability gear shaft steels of Examples 1-8 and the comparative steels of Comparative Examples 1-4.

[0132] Table 3. Note: The two values ​​before and after the slash of the impact energy of each embodiment and comparative example in Table 3 represent two measurement results.

[0133] As can be seen from Table 3, after the high-strength, toughness, and hardenability gear shaft steels of Examples 1-8 of the present invention were subjected to a simulated high-temperature carburizing heat treatment at 1000°C, the austenite grain size was maintained in the range of 5 to 8, and no mixed crystals or abnormally coarse grains were observed. Therefore, they have good high-temperature grain stability.

[0134] In addition, it can be seen from Table 3 that the hardenability of the high-strength, toughness and hardenability gear shaft steels of Examples 1-8 of the present invention at a representative position J9mm is 34-42HRC, which means that they have high hardenability and a narrow hardenability bandwidth. In addition, the tensile strength R m All are greater than 1100MPa, yield strength R p0.2 All are greater than 980MPa, elongation after fracture A are ≥12%, section shrinkage is greater than 50%, Charpy impact energy A ku All greater than 55J.

[0135] It can be seen that the bars rolled or forged using this high-strength, toughness and hardenability gear shaft steel can be effectively processed into gear shafts. After high-temperature carburizing heat treatment by downstream users, they have high strength and toughness, and can be effectively used in high-end components such as automotive gearboxes or reducers for new energy vehicles and industrial reducers, and have good use prospects and value.

[0136] Different from the embodiments of the present invention, the grain size of Comparative Example 1 after simulated high-temperature carburizing is level 0, that is, the grains grow abnormally and cannot meet the use requirements.

[0137] Although the comparative steel of Comparative Example 2 did not exhibit mixed crystal phenomenon, its grain size was small after the simulated high-temperature carburizing heat treatment, and its hardenability was low, failing to meet the high hardenability requirement.

[0138] Mixed crystal phenomenon was observed in the comparative steel of Comparative Example 3 after simulated high-temperature carburizing heat treatment at a temperature of 1000°C, where 5(0) means that the average grain size is level 5 and some areas are coarsened to level 0.

[0139] The mixed crystal phenomenon (level 4) was also observed in the comparative steel of Comparative Example 4 after simulated high-temperature carburizing heat treatment at a temperature of 1000°C, where 0(4) means that the average grain size is level 0, while some areas are coarsened to level 4. In addition, the impact energy of Comparative Example 4 is also low.

[0140] In addition, the combination of the various technical features in this case is not limited to the combination described in the claims of this case or the combination described in the specific embodiments. All technical features recorded in this case can be freely combined or combined in any way unless there is a contradiction between them.

[0141] It should also be noted that the embodiments listed above are merely specific embodiments of the present invention. Obviously, the present invention is not limited to the above embodiments, and similar changes or modifications made therefrom that can be directly derived from or easily conceived by those skilled in the art based on the disclosure of the present invention are intended to fall within the scope of protection of the present invention.

Claims

1. A high-strength, high-toughness and high-hardenability steel for gear shafts, containing Fe and unavoidable impurities, characterized in that: It also contains the following chemical elements in the following mass percentages: C: 0.16-0.22%, Si: 0.10-0.40%, Mn: 0.86-1.24%, Cr: 0.95-1.44%, Al: 0.02-0.05%, Ti: 0.015-0.039%, Nb: 0.001-0.034%, N: 0.006-0.015%, B: 0.0006-0.0034%; Its microalloying element coefficient r M / N The range is 1.5~5.0, where r M / N =(10*[Nb] / 93+[Ti] / 480+[Al] / 27) / ([N] / 14), where each chemical element is substituted into the value before the percentage sign of the mass percentage content of the chemical element.

2. The high-strength, toughness and hardenability steel for gear shafts according to claim 1, characterized in that: The mass percentage of each chemical element is: C: 0.16-0.22%, Si: 0.10-0.40%, Mn: 0.86-1.24%, Cr: 0.95-1.44%, Al: 0.02-0.05%, Ti: 0.015-0.039%, Nb: 0.001-0.034%, N: 0.006-0.015%, B: 0.0006-0.0034%; the balance is Fe and other inevitable impurities; Its microalloying element coefficient r M / N The range is 1.5~5.0, where r M / N =(10*[Nb] / 93+[Ti] / 480+[Al] / 27) / ([N] / 14), where each chemical element is substituted into the numerical value before the percentage sign of the mass percentage content of the chemical element.

3. The high-strength, high-toughness and high-hardenability steel for gear shafts according to claim 1 or 2, characterized in that: Among the inevitable impurities, P≤0.030%, O≤0.002%, H≤0.0002%, and Ca≤0.0034%.

4. The high-strength, toughness and hardenability steel for gear shafts according to claim 1 or 2, characterized in that: It also contains at least one of the following chemical elements: <S≤0.04%,0<Ni≤0.25%,0<Mo≤0.10%,0<Cu≤0.20%,0<V≤0.03%。 5. The high-strength, toughness and hardenability steel for gear shafts according to claim 1 or 2, characterized in that: The austenite grain size is maintained at level 5 to 8 after high temperature carburizing heat treatment.

6. The high-strength, toughness and hardenability steel for gear shafts according to claim 1 or 2, characterized in that: Its J9mm hardenability is 34~42HRC.

7. The high-strength, toughness and hardenability steel for gear shafts according to claim 1 or 2, characterized in that: Its tensile strength R after high temperature carburizing heat treatment m ≥1100MPa, yield strength R p0.2 ≥980MPa, elongation after fracture A≥12%, section shrinkage ≥50%, Charpy impact energy A ku ≥55J.

8. The method for manufacturing a high-strength, toughness and hardenability steel for a gear shaft according to any one of claims 1 to 7, characterized in that: It includes the steps of: (1) Smelting; (2) Casting; (3) Heating: The steel billet is first heated to no higher than 700°C in the preheating section, then further heated to no higher than 980°C in the first heating section, and then heated to 950-1200°C in the second heating section after insulation. After insulation, it enters the soaking section, and the temperature in the soaking section is 1050-1250°C; (4) Forging or rolling.

9. The manufacturing method according to claim 8, characterized in that: In step (4), the start forging or rolling temperature is controlled to be 1050-1250°C, and the final rolling temperature or final forging temperature is controlled to be ≥900°C.

10. The manufacturing method according to claim 8, characterized in that: In step (4), it is directly rolled or forged to the finished size.

11. The manufacturing method according to claim 8, characterized in that: In step (4), the steel is first rolled to an intermediate billet size, then intermediately heated, and then rolled to a final finished product size.

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