High-toughness and high-hardenability gear shaft steel and manufacturing method thereof

Abstract

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

Description

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 automobile industry has put forward higher and higher requirements for automobile parts. Among them, gears with high strength, high fatigue life, high temperature stability and economic efficiency are important development directions.

[0003] Accordingly, high-strength and tough gear shaft steel can meet the high technical requirements of automobile lightweight materials. In addition, in order to ensure the quenching strength of the gear, it is usually 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 the hardenability while avoiding the gear size dispersion caused by excessive hardenability bandwidth, and at the same time ensuring that the gear does not have mixed crystals and coarse grains after high-temperature carburizing.

[0005] For example, a Chinese patent document with the publication number CN101096742A, the publication date of January 2, 2008, and the name "High Strength Automotive Gear Steel" discloses a high-strength automotive gear steel. The steel contains alloy elements such as Nb, V, and Al, which refine the original austenite grains. The mass percentage of the components 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, and the rest is Fe and unavoidable impurities. By adding trace amounts of Nb and V, the grain size, hardenability and bandwidth of the gear steel are optimized.

[0006] Another example: the Chinese patent document with publication number CN103361559A, publication date October 23, 2013, and titled "A Nb, Ti composite microalloyed 20CrMnTi free-cutting gear steel", discloses a Nb, Ti composite microalloyed 20CrMnTi free-cutting gear steel, the steel composition 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 is iron and unavoidable impurities. It increases the gear carburizing temperature or shortens the carburizing time by controlling the content of microalloying elements such as Nb, Ti and Al.

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

[0008] One of the purposes of the present invention is to provide a high-strength, tough and high-hardenability gear shaft steel, which can obtain high-strength gear shaft steel with high hardenability, narrow hardenability bandwidth and good high-temperature grain stability at a low cost by optimizing the component system of the gear shaft steel, especially reasonably controlling the content of microalloy elements and nitrogen in the gear steel.

[0009] In order to achieve the above object, the present invention provides a high-strength, high-toughness and high-hardenability steel for gear shafts, which contains the following chemical elements in the following mass percentages in addition to Fe and inevitable impurities:

[0010] C: 0.16~0.22%,

[0011] Si: 0.10~0.40%,

[0012] Mn: 0.86~1.24%,

[0013] Cr: 0.95~1.44%,

[0014] Al: 0.02~0.05%,

[0015] Ti: 0.015~0.039%,

[0016] Nb: 0.001~0.034%,

[0017] N: 0.006~0.015%,

[0018] B: 0.0006~0.0034%;

[0019] 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.

[0020] Furthermore, in the high-strength, toughness and hardenability gear shaft steel of the present invention, the mass percentage of each chemical element is:

[0021] C: 0.16~0.22%,

[0022] Si: 0.10~0.40%,

[0023] Mn: 0.86~1.24%,

[0024] Cr: 0.95~1.44%,

[0025] Al: 0.02~0.05%,

[0026] Ti: 0.015~0.039%,

[0027] Nb: 0.001~0.034%,

[0028] N: 0.006~0.015%,

[0029] B: 0.0006~0.0034%;

[0030] The balance is Fe and unavoidable impurities;

[0031] 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.

[0032] In the high-strength, toughness and hardenability gear shaft steel of the present invention, the design principles of each chemical element are specifically described as follows:

[0033] C: In the high-strength, toughness and high hardenability gear shaft steel 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 gear shaft steel requires surface strength as well as sufficient core impact toughness. When the C content in the steel is too low, less than 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 toughness requirements of the gear core cannot be met, and too high a 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 gear shaft steel described in the present invention, the mass percentage of C is controlled between 0.16 and 0.22%.

[0034] 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 to strengthen the ferrite, thereby 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 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%.

[0035] Mn: In the high-strength, toughness and hardenability gear shaft steel described in the present invention, Mn is one of the main elements that affect the hardenability of steel. The deoxidation ability of the Mn element is very good. It can reduce the iron oxide in the steel and effectively increase the output of the steel. Mn can dissolve into ferrite, improve the strength and hardness of the steel, and make the steel obtain pearlite with finer layers and higher strength when it is cooled after hot rolling. In addition, Mn can also form MnS with S in the steel, which can eliminate the harmful effects of S. It has the ability to form and stabilize the austenite structure of the steel, can strongly increase the hardenability of the steel, and can also reduce the red toughness of the steel. When the Mn content in the steel is too small, the hardenability of the steel is insufficient; and when the Mn content in the steel is too high, the thermoplasticity of the steel will deteriorate, affecting production, and the steel is prone to cracking during water quenching. Therefore, in the high-strength, toughness and hardenability gear shaft steel described in the present invention, the mass percentage of Mn is controlled between 0.86 and 1.24%.

[0036] Cr: In the high-strength, toughness and hardenability gear shaft steel described in the present invention, Cr is one of the main alloying elements added to the steel of the present invention. Cr can significantly improve the hardenability, strength, wear resistance and other properties of the steel. In addition, Cr can also reduce the activity of the C element in the steel, which can prevent decarburization during heating, rolling and heat treatment. However, too high Cr will significantly reduce the toughness of the quenched and tempered steel, and form coarse carbides distributed along the grain boundaries. Therefore, in the high-strength, toughness and hardenability gear shaft steel described in the present invention, the mass percentage of the Cr element is controlled between 0.95 and 1.44%.

[0037] Al: In the high-strength, toughness and hardenability steel for gear shafts described in the present invention, Al is a grain refining element. The combination of Al and 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 strength and toughness. At the same time, grain refinement also helps to reduce the hydrogen embrittlement sensitivity of steel. However, it should be noted that the Al content in the steel should not be too high. Too high Al content tends to increase the chance of inclusions 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%.

[0038] Ti: Although Ti can form fine precipitates when added to steel, when the Ti content in the steel is too high, coarse and angular TiN particles will be formed during the smelting process, reducing the impact toughness of the steel. Therefore, in the high-strength, high-toughness and high-hardenability gear shaft steel of the present invention, the Ti content is controlled to be 0.015-0.039%.

[0039] Nb: In the high-strength, toughness and hardenability gear shaft steel described in the present invention, Nb element is added to the steel to form a fine precipitate phase, thereby inhibiting the recrystallization of the steel and effectively refining the grains. It should be noted that the Nb content in the steel should not be too high. When the Nb content in the steel is too high, coarse NbC particles will be formed during the smelting process, which will reduce the impact toughness of the steel. Therefore, in the high-strength, toughness and hardenability gear shaft steel described in the present invention, the mass percentage of the Nb element is controlled to be 0.001-0.034%.

[0040] N: In the high-strength, toughness and hardenability gear shaft steel described in the present invention, N is an interstitial atom, which can combine with the microalloy in the steel to form MN-type precipitates, which can pin the grain boundaries at high temperatures, thereby inhibiting the growth of austenite grains. When the N content in the steel is low, less MN is formed, and the pinning effect is not obvious; when the N content in the steel is too high, it is easy to be enriched in steelmaking, reducing the toughness of the steel. Therefore, in the high-strength, toughness and hardenability gear shaft steel described in the present invention, the mass percentage of the N element is controlled between 0.006 and 0.015%.

[0041] B: Boron can greatly improve the hardenability of steel, and the required content is relatively small. Its effect is hundreds or even thousands of times that of general alloying elements, which has a significant economic effect. Moreover, boron steel can be quenched by water, which not only saves quenching oil but also makes it easy to obtain martensitic structure, thus making boron-containing steel have good strength and hardness. As long as the boron content is appropriate, the production process is appropriate, and complete hardening is guaranteed, the plasticity and toughness will not be significantly reduced. However, the B element is easy to segregate, which will cause large 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%.

[0042] In addition, it is important to control the microalloying element coefficient r in the high-strength, high-toughness and high-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 with the numerical value before the percentage sign of the mass percentage content of the chemical element.

[0043] 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 micro-alloy elements and excess N elements form precipitates, thereby inhibiting the growth of austenite grains in the high temperature stage.

[0044] Furthermore, among the inevitable impurities in the high-strength, high-toughness and high-hardenability gear shaft steel of the present invention, the content of each impurity element satisfies at least one of the following items:

[0045] P≤0.030%,

[0046] O≤0.0020%,

[0047] H≤0.0002%,

[0048] Ca≤0.0034%.

[0049] In the above technical solution, P, O, H and Ca are all impurity elements in steel. Under the condition that 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:

[0050] P: P is easily segregated at the grain boundaries in the steel, which will reduce the grain boundary binding energy and deteriorate the impact toughness of the steel. Therefore, in some embodiments of the present invention, the P content can be controlled to P≤0.030%.

[0051] O: O easily forms oxides and composite oxides with Al element in steel, destroying the continuity of steel, reducing the uniformity of the structure and the low-temperature impact energy and fatigue performance. Based on this, in some embodiments of the present invention, the O element content can be controlled to O≤0.0020%.

[0052] H: H will accumulate at defects in steel, especially in steel with a tensile strength level exceeding 1000 MPa, and hydrogen-induced delayed fracture will occur. Therefore, in some embodiments of the present invention, the H element content can be controlled to H≤0.0002%.

[0053] Ca: In the high-strength, high-toughness and high-hardenability gear shaft steel of the present invention, Ca element easily forms inclusions, thereby affecting the fatigue performance of the final product. Therefore, the Ca element content can be controlled to Ca≤0.0034%.

[0054] Furthermore, the high-strength, toughness and hardenability gear shaft steel of the present invention further contains at least one of the following chemical elements:

[0055] 0<S≤0.04%, 0<Ni≤0.25%, 0<Mo≤0.10%, 0<Cu≤0.20%, 0<V≤0.03%.

[0056] Optionally, in the high-strength, toughness and hardenability gear shaft steel of the present invention, at least one of the elements S, Ni, Mo, Cu and V may be further added to further improve the performance of the high-strength, toughness and hardenability gear shaft steel of the present invention.

[0057] S: S generally exists as an impurity element in steel, which reduces the plasticity and toughness of the steel. However, in the high hardenability gear shaft steel described in 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 gear shaft steel described in the present invention, the mass percentage of S is controlled to be 0<S≤0.04%.

[0058] Ni: In the high-strength, toughness and hardenability gear shaft steel described in the present invention, Ni exists in the steel in the form of solid solution, which can effectively improve the low-temperature impact resistance of the steel. However, it should be noted that too high a Ni content will lead to an excessively high content of retained austenite in the steel, thereby reducing the strength of the steel. Therefore, in the high-strength, toughness and hardenability gear shaft steel described in the present invention, the mass percentage of Ni can be preferably controlled to 0<Ni≤0.25%.

[0059] Mo: In the high-strength, toughness and hardenability gear shaft steel described in the present invention, Mo can be dissolved in the steel, which is beneficial to improving the hardenability of the steel and the strength of the steel. Under high temperature tempering, Mo will also form fine carbides to further improve the strength of the steel, and the combined effect of molybdenum and manganese can significantly improve the stability of austenite. Based on this, in the high-strength, toughness and hardenability gear shaft steel described in the present invention, the mass percentage of Mo can be preferably controlled to 0<Mo≤0.10%.

[0060] Cu: In the high-strength, toughness and hardenability gear shaft steel of the present invention, Cu can improve the strength of the steel and is beneficial to improving the weather resistance and corrosion resistance of the steel. However, the Cu content in the steel should not be too high. If the Cu content in the steel is too high, it will be enriched in the grain boundary during the heating process, resulting in weakening of the grain boundary and cracking. Therefore, in the high-strength, toughness and hardenability gear shaft steel of the present invention, the mass percentage of Cu can be preferably controlled to 0<Cu≤0.20%.

[0061] V: In the high hardenability gear shaft steel 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 the N element, thereby further improving the strength of the steel. However, if the content of the C element and the V element is too high, coarse VC particles will be formed. Based on this, in the high hardenability gear shaft steel of the present invention, the mass percentage of the V element can be controlled to 0<V≤0.03%.

[0062] Furthermore, in the high-strength, toughness and hardenability steel for gear shafts described in the present invention, the austenite grain size is maintained at level 5 to 8 after high-temperature carburizing heat treatment.

[0063] Furthermore, the high-strength, toughness and hardenability steel for gear shafts described in the present invention has a J9mm hardenability of 34 to 42 HRC.

[0064] Furthermore, in the high-strength, toughness and hardenability gear shaft steel of the present invention, the tensile strength R m ≥1100MPa, yield strength R p0.2 ≥980MPa, elongation after fracture A≥12%, section shrinkage ≥50%, Charpy impact energy A ku ≥55J.

[0065] Correspondingly, another object of the present invention is to provide a method for manufacturing high-strength, toughness and high hardenability steel for gear shafts. The manufacturing method 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.

[0066] 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:

[0067] (1) Smelting;

[0068] (2) Casting;

[0069] (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;

[0070] (4) Forging or rolling.

[0071] In the manufacturing method described in the present invention, compared with the prior art, the heating step adopts a unique process, wherein the soaking section temperature is relatively high, because the relatively high soaking section temperature can improve the composition uniformity and organizational 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 upwards, can the final rolling or final forging temperature be increased, so that the austenite recovery recrystallization after rolling is more sufficient and the precipitation phase distribution is more uniform.

[0072] 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.

[0073] In addition, in step (1), the charge for electric furnace smelting can be low P, S scrap steel, offcuts 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 slag flow; the slag tapping conditions can be controlled as: the slag tapping temperature is 1630-1660°C; P≤0.015%; the steel tapping conditions can be controlled as: the steel tapping temperature is 1630-1650°C; [P]≤0.010%, [C]≥0.03%.

[0074] After 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. Control the seating of the ladle, measure the temperature and analyze it, and adjust the argon pressure according to the situation. 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,

[0075] In addition, in step (1), the bag temperature can be controlled to be 1550-1570° C., thereby lowering the bag temperature and accelerating element diffusion, which is beneficial to further reduce dendrite segregation.

[0076] In addition, in step (2), 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 to 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 a cross-sectional size of 140mm×140mm to 320mm×425mm can be cast.

[0077] 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.

[0078] 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 final rolling temperature is controlled to be ≥900°C.

[0079] 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 N to dissolve from the γ solid solution and combine with microalloying elements in the steel to form nitrides.

[0080] It should be noted that the solubility of N in α-Fe is less than that in γ-Fe, and due to the two peaks of precipitation caused by the phase transformation, if the final forging or final rolling temperature is low, the peak of the precipitation phase will precipitate, which will cause uneven distribution of the precipitation phase and insufficient recovery recrystallization, resulting in anisotropy in the structure, so the final forging or final rolling temperature is controlled to be ≥900°C. In addition, increasing the final forging or final rolling temperature will result in 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 manganese-rich bands, reduce the tendency of manganese-rich bands to form pearlite, and thus reduce the banded structure.

[0081] 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 oxide skin.

[0082] In addition, in step (3) 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.

[0083] The high-strength, high-toughness, high-hardenability gear shaft steel and the manufacturing method thereof described in the present invention have the following advantages and beneficial effects compared with the prior art:

[0084] (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 can have excellent strength and toughness after subsequent downstream high-temperature carburizing heat treatment.

[0085] (2) The high-strength, toughness and hardenability gear shaft steel of the present invention controls the content of microalloying elements and nitrogen, 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 remain stable at level 5 to 8 after high-temperature carburizing at 1000°C for 4 hours, and various performances meet the performance indicators of gear shaft steel.

[0086] (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 micro-alloy elements in the steel, large particles of harmful inclusions in the steel are avoided to ensure stable production quality of the steel, reduce the production cost of the steel, and realize mass production on the bar production line.

[0087] (4) The high-strength, high-toughness and high-hardenability gear shaft steel of the present invention is superior to the prior art in terms of hardenability, austenite grain size and cost competitiveness. The type and amount of alloy elements in the steel can be controlled while ensuring high hardenability and narrow bandwidth, thereby improving the applicability of the steel.

[0088] (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 it is subsequently used in the production of gear shafts, and has broad industrial application prospects. DETAILED DESCRIPTION

[0089] The high-strength, toughness and hardenability gear shaft steel and the manufacturing method thereof 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 on the technical solution of the present invention.

[0090] The high-strength, toughness and hardenability gear shaft steels of Examples 1-8 are all prepared by the following steps:

[0091] (1) Smelting and casting are carried out according to the chemical composition shown in the following Tables 1-1 and 1-2: 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, or in a converter smelting + refining outside the furnace + vacuum degassing.

[0092] (2) Casting.

[0093] (3) Heating: The steel billet is first heated to no higher than 700°C in the preheating section, and then further heated to no higher than 980°C in the first heating section. After insulation, it is further heated to 950-1200°C in the second heating section. After insulation, it enters the soaking section, and the temperature of the soaking section is 1050-1250°C. After insulation, subsequent rolling or forging is carried out.

[0094] (4) Forging or rolling: Control the start forging or rolling temperature to 1050-1250°C, and control the final forging or rolling temperature to ≥900°C.

[0095] 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:

[0096] Example 1: According to the chemical composition shown in Tables 1-1 and 1-2 below, smelting was carried out in a 50kg vacuum induction furnace. Molten steel was cast into an ingot, heated and forged to form a blank. The ingot was first heated to 700°C in the preheating section, then continued to be heated to 900°C in the first heating section, and then continued to be heated to 950°C in the second heating section after insulation. After insulation, it entered the soaking section, the temperature of the soaking section was 1050°C, and subsequent forging was carried out after insulation. The final forging temperature was controlled to be 910°C, and finally forged into a Φ60mm bar.

[0097] Example 2: According to the chemical composition shown in Tables 1-1 and 1-2 below, smelting was carried out in a 150kg vacuum induction furnace. Molten steel was cast into an ingot, heated and forged into a blank, and the ingot was first heated to 650°C in the preheating section, and then continued to be heated to 950°C in the first heating section, and then continued to be heated to 1100°C in the second heating section after insulation, and then entered the soaking section after insulation, the soaking section temperature was 1200°C, and then subsequent forging was carried out after insulation, and the final forging temperature was controlled to be 1000°C, and finally forged into a Φ90mm bar.

[0098] Example 3: According to the chemical composition shown in Tables 1-1 and 1-2 below, smelting was carried out in a 500kg vacuum induction furnace. Molten steel was cast into an ingot, heated and forged into a blank, and the ingot was first heated to 600°C in the preheating section, then continued to be heated to 980°C in the first heating section, and then continued to be heated to 1200°C in the second heating section after insulation, and then entered the soaking section after insulation, the soaking section temperature was 1250°C, and then subsequent forging was carried out after insulation, and the final forging temperature was controlled to be 1000°C, and finally forged into a Φ120mm bar.

[0099] Example 4: According to the chemical composition shown in Tables 1-1 and 1-2, the steel is smelted in an electric furnace, refined and vacuum treated, and then cast into a 280mm×280mm continuous casting billet. The continuous casting billet is heated in a walking beam heating furnace at a rate of 300℃ / h. It is first heated to 620℃ in the preheating section, then continued to be heated to 950℃ in the first heating section, and continued to be heated to 1150℃ in the second heating section. It enters the soaking section with a temperature of 1200℃. After being kept warm for 4 hours, it is rolled. The billet is removed from the heating furnace and then rolled. The final rolling temperature is controlled to be 970℃, and finally rolled into a Φ80mm bar.

[0100] Example 5: According to the chemical composition shown in Tables 1-1 and 1-2, the steel is smelted in an electric furnace, refined and vacuum treated, and then cast into a 320mm×425mm continuous casting billet. The continuous casting billet is heated in a walking beam furnace and slowly heated at a rate of 150℃ / h. First, it is heated to 600℃ in the preheating section, then continued to be heated to 950℃ in the first heating section, and continued to be heated to 1200℃ in the second heating section. It enters the soaking section, and the soaking section temperature is 1230℃. After keeping warm for 4.5 hours, subsequent rolling is carried out. The billet is rolled after being descaled by high-pressure water after leaving the heating furnace, and rolled into an intermediate billet. The first final rolling temperature is controlled to be 1050℃, and the size of the intermediate billet is 220mm×220mm. Then the intermediate billet is placed in a walking beam furnace for heating, slowly heated at a speed 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 equalizing section with an equalizing temperature of 1220℃. It is taken out of the furnace after keeping warm for 3.5h, and rolling begins after descaling with high-pressure water. The second final rolling temperature is controlled at 950℃, and the specification of the finished bar is Φ90mm.

[0101] Example 6: According to the chemical composition shown in Tables 1-1 and 1-2, the steel is smelted in an electric furnace, refined and vacuum treated, and then cast into a 280mm×280mm continuous casting billet. The continuous casting billet is heated in a walking beam heating furnace and slowly heated at a rate of 300℃ / h. First, it is heated to 680℃ in the preheating section, then continued to be heated to 900℃ in the first heating section, and continued to be heated to 1180℃ in the second heating section. It enters the soaking section, and the soaking section temperature is 1200℃. After keeping warm for 4.5 hours, subsequent rolling is carried out. The billet is rolled after being descaled by high-pressure water after leaving the heating furnace, and rolled into an intermediate billet. The first final rolling temperature is controlled to be 1000℃, and the size of the intermediate billet is 140mm×140mm. Then the intermediate billet is placed in a walking beam heating furnace for heating, slowly heated at a speed of 500℃ / h, first preheated to 700℃, heated to 1100℃ in the first heating section, heated to 1220℃ in the second heating section, and entered the equalizing section with an equalizing temperature of 1220℃. It is taken out of the furnace after being kept warm for 3.5 hours, and rolled after descaling with high-pressure water. The second final rolling temperature is controlled at 920℃, and the specification of the finished bar is Φ25mm.

[0102] Example 7: According to the chemical composition shown in Tables 1-1 and 1-2, the steel is smelted in a converter, refined and vacuum treated, and then cast into a molded billet. The billet is heated in a walking beam heating furnace at a rate of 50°C / h. It is first heated to 620°C in the preheating section, then continued to be heated to 950°C in the first heating section, and continued to be heated to 1150°C in the second heating section. It enters the soaking section with a temperature of 1200°C. After being kept warm for 8 hours, it is rolled. The billet is removed from the heating furnace and then rolled. The final rolling temperature is controlled to be 970°C, and finally rolled into a Φ90mm bar.

[0103] Example 8: According to the chemical composition shown in Tables 1-1 and 1-2, the steel is smelted in a converter, refined and vacuum treated, and then cast into a molded billet. The billet is heated in a walking beam heating furnace at a rate of 100°C / h. It is first heated to 600°C in the preheating section, then continued to be heated to 950°C in the first heating section, and continued to be heated to 1200°C in the second heating section. It enters the soaking section with a temperature of 1230°C. After keeping the temperature for 7 hours, subsequent rolling is performed. The billet is rolled after being descaled by high-pressure water after leaving the heating furnace, and rolled into an intermediate billet. The first final rolling temperature is controlled to be 1050°C, and the size of the intermediate billet is 260mm×260mm. Then place the intermediate billet in a walking beam heating furnace and heat it slowly 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 equalizing section with an equalizing temperature of 1220℃. After keeping warm for 5 hours, it is taken out of the furnace and rolled after descaling with high-pressure water. The second final rolling temperature is controlled at 950℃. The specification of the finished bar is Φ60mm.

[0104] Comparative Examples 1 and 2 are commercially available materials, which have been smelted and refined in an electric furnace to ensure the purity of the commercial materials.

[0105] Comparative Example 3: According to the chemical composition shown in Tables 1-1 and 1-2, smelting was carried out in a 50kg vacuum induction furnace. The molten steel was cast into an ingot, heated and forged, and heated in a box furnace. The ingot was heated to 1100°C at a rate of 300°C / h, and then forged after 3 hours of heat preservation. The final forging temperature was controlled to be 910°C, and finally forged into a Φ60mm bar.

[0106] Comparative Example 4: According to the chemical composition shown in Tables 1-1 and 1-2, the steel was smelted in an electric furnace, refined and vacuum treated, and then cast into a 320mm×425mm continuous casting billet. The continuous casting billet was heated in a walking beam heating furnace at a rate of 150℃ / h. It was first heated to 600℃ in the preheating section, then continued to be heated to 950℃ in the first heating section, and continued to be heated to 1200℃ in the second heating section. It entered the soaking section with a soaking section temperature of 1230℃. After keeping the temperature for 4.5h, it was rolled. The billet was descaled by high-pressure water after leaving the heating furnace and then rolled into an intermediate billet. The first final rolling temperature was controlled to be 1050℃, and the size of the intermediate billet was 220mm×220mm. Then place the intermediate billet in a walking beam heating furnace and slowly heat it at a speed of 400℃ / h. Heat it to 680℃ in the preheating section, 1050℃ in the first heating section, and 1200℃ in the second heating section. Then enter the equalizing section with an equalizing temperature of 1220℃ and keep it warm for 6 hours. After being taken out of the furnace and descaled by high-pressure water, rolling begins. The second final rolling temperature is controlled at 950℃, and the specification of the finished bar is Φ90mm.

[0107] Table 1-1 and Table 1-2 list the mass percentages of the chemical elements of the high hardenability gear shaft steels of Examples 1-8 and the comparative steels of Comparative Examples 1-4.

[0108] Table 1-1 (wt.%, the balance is Fe and other unavoidable impurities except P, O, H and Ca)

[0109]

[0110]

[0111] Table 1-2. (wt.%, the balance is Fe and other inevitable impurities except P, O, H and Ca)

[0112] serial number S Ni Mo Cu V P O Ca H <![CDATA[r M / N ]]> Example 1 0.04 0.05 0.01 0 0.006 0.018 0.002 0.0013 0.0002 2.64 Example 2 0.009 0.25 0.1 0.08 0.03 0.007 0.0012 0.002 0.0002 4.58 Example 3 0.029 0.03 0.06 0.20 0.018 0.028 0.0015 0.0015 0.0002 4.99 Example 4 0.012 0 0 0 0 0.003 0.0019 0.0031 0.0001 1.56 Example 5 0.003 0.08 0.02 0 0.024 0.005 0.0011 0.0013 0.0001 3.35 Example 6 0.034 0 0 0.08 0.016 0.008 0.002 0.0023 0.0001 4.87 Example 7 0.018 0.07 0 0.03 0.005 0.013 0.0008 0.0008 0.0001 4.14 Example 8 0.001 0.16 0.03 0.13 0 0.009 0.0011 0.0034 0.0001 2.20 Comparative Example 1 0.013 0.21 0.07 0.04 0.002 0.007 0.0013 0.0013 0.0002 1.42 Comparative Example 2 0.003 0.02 0 0.03 0.003 0.006 0.0013 0.0013 0.0002 2.05 Comparative Example 3 0.002 0.02 0.02 0.01 0 0.007 0.0008 0.0014 0.0002 1.58 Comparative Example 4 0.021 0.06 0.01 0.05 0.006 0.008 0.0019 0.0011 0.0002 1.47

[0113] Note: M / N =(10*[Nb] / 93+[Ti] / 480+[Al] / 27) / ([N] / 14), where each chemical element in the formula is substituted with the numerical value before the percentage sign of the mass percentage content of the chemical element.

[0114] 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.

[0115] Table 2.

[0116]

[0117]

[0118]

[0119] As can be seen from Table 2, in Example 5, Example 6, Example 8 and Comparative Example 4, the steel billet is first rolled to a specified intermediate billet size during rolling, and then intermediately heated and rolled again to a final finished product size.

[0120] In order to verify the performance of the high-strength, toughness and hardenability gear shaft steel of the present invention, the high-strength, toughness and hardenability gear shaft steels of Examples 1-8 and the comparative steels of Comparative Examples 1-4 were sampled, and subjected to simulated high-temperature carburizing heat treatment tests to perform mechanical property tests and austenite grain size tests, and the test results are listed in Table 3. Among them:

[0121] Simulated high-temperature carburizing heat treatment test: the sample is heated to 1200℃, kept at this temperature for 40min, water-cooled, then heated to 1000℃ at a rate of 500-800℃ in 40-60min, kept at this temperature for 4h, and quenched by water cooling.

[0122] Austenite grain size test: Austenite grain size is evaluated according to standard ASTM E112.

[0123] Mechanical properties test: Samples were prepared according to GB / T 2975-2018 "Steel and Steel Products Mechanical Properties Test Sampling Location and Sample Preparation", and GB / T 3077-2015 was used to prepare Φ15 mm blanks, heated at 880°C for 90 min, oil quenched + heated at 870°C for 90 min, oil quenched + heated at 200°C for 150 min, tempered and air cooled, and tensile tests were performed according to GB / T 228.1-2010 "Metallic Materials Tensile Test Part 1: Room Temperature Test Method", and GB / T 229-2007 "Metallic Materials Charpy Pendulum Impact Test Method" was used to test the Charpy impact energy A of each embodiment and comparative example at room temperature. ku .

[0124] In addition, the high-strength and high-hardenability gear shaft steels obtained in Examples 1-8 and the comparative steels in Comparative Examples 1-4 were sampled and subjected to hardenability and hardness tests, and the test results are also listed in Table 3. Among them:

[0125] Hardenability test: The steels of the examples and comparative examples were sampled and prepared from hot-rolled round steel according to the national standard GB / T 225, and the end hardenability test (Jominy test) was performed with reference to GB / T 5216, with the normalizing temperature at 920±10°C and the quenching temperature at 870±5°C. The Rockwell hardness test was performed according to GB / T 230.2 to obtain the hardness value (HRC) at a specific position, such as the hardness at 9mm from the quenching end, i.e. J9mm.

[0126] 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.

[0127] Table 3.

[0128]

[0129] Note: The impact energy of each embodiment and comparative example in Table 3 has two values ​​representing two measurement results.

[0130] It can be seen from Table 3 that after the high-strength, toughness and hardenability gear shaft steels of Examples 1-8 of the present invention are subjected to 1000°C simulated high-temperature carburizing heat treatment, the austenite grain size is maintained in the range of 5 to 8, and no mixed crystals or abnormally coarse grains are observed, so they have good high-temperature grain stability.

[0131] In addition, it can be seen from Table 3 that the high-strength and high-hardenability gear shaft steels of Examples 1-8 of the present invention have a hardenability of 34-42 HRC at a representative position J9mm, which has a 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, the elongation after fracture A is ≥12%, the section shrinkage is greater than 50%, and the Charpy impact energy A ku All greater than 55J.

[0132] 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.

[0133] 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.

[0134] Although the comparative steel of Comparative Example 2 does not have the mixed crystal phenomenon, its grains are fine and hardenability is low after the simulated high-temperature carburizing heat treatment, and it does not meet the requirement of high hardenability. In addition, the strength of Comparative Example 2 is low.

[0135] Mixed crystal phenomenon was observed in the comparative steel of Comparative Example 3 after simulated high temperature carburizing heat treatment at 1000°C, where 5(0) means the average grain size is level 5 and some areas are coarsened to level 0.

[0136] The mixed crystal phenomenon (level 4) was also observed in the comparative steel of comparative example 4 after the 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 relatively low.

[0137] 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.

[0138] It should also be noted that the above-listed embodiments are only specific embodiments of the present invention. Obviously, the present invention is not limited to the above embodiments, and similar changes or modifications made therewith can be directly derived or easily associated with the contents disclosed by those skilled in the art from the present invention, and all should belong to the protection scope 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 numerical 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≤0.003%.

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: 0<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, high-toughness and high-hardenability gear shaft steel 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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