High-toughness high-hardenability steel for toothed shafts and method for manufacturing same

Abstract

This invention discloses a high-strength, high-toughness, and high-hardenability gear shaft steel, which contains Fe and unavoidable impurities, as well as 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 to 5.0, where r M / N = (10*[Nb] / 93+[Ti] / 480+[Al] / 27) / ([N] / 14). Furthermore, this invention also discloses a method for manufacturing the above-mentioned high-strength, high-toughness, and high-hardenability gear shaft steel. The high-strength, high-toughness, and high-hardenability gear shaft steel described in this invention possesses high hardenability, a narrow hardenability bandwidth, and good high-temperature grain stability.

Description

TECHNICAL FIELD

[0001] The present application relates to a kind of high-strength steel and its manufacturing method, and more particularly to a kind of gear shaft steel and its manufacturing method. BACKGROUND

[0002] The development of automobile industry puts forward higher and higher requirements for automobile parts. Among them, high strength, high toughness, high fatigue life and high temperature stability and economic and efficient gear are important development direction.

[0003] Correspondingly, high strength and toughness gear shaft steel can meet the high technical requirements of material for automobile lightweight. In addition, in order to ensure the quenching strength of gear, the hardenability of gear shaft steel is usually required.

[0004] The main technical problem of high hardenability MnCr carburizing gear steel is how to avoid gear size dispersion caused by too large hardenability bandwidth while improving hardenability, and at the same time meet the phenomenon that gear does not appear mixed crystal and grain coarsening after high temperature carburizing.

[0005] For example: the Chinese patent document with publication number CN101096742A, publication date January 2, 2008, and name "high strength automobile gear steel" discloses a kind of high strength automobile gear steel. Nb, V, Al and other alloy elements are added in the steel, which refines the original austenite grain, and the composition mass percentage is: C: 0.20~0.40, Si: 0.20~0.50, Mn: 0.50~1.00, Cr: 0.80~1.30, Nb: 0.015~0.080, V: 0.030~0.090, Mo: 0.15~0.55, Al: 0.015~0.050, the rest is Fe and inevitable impurities. By adding trace amounts of Nb, V, the grain size, hardenability and bandwidth of gear steel are optimized.

[0006] For example: the Chinese patent document with publication number CN103361559A, publication date October 23, 2013, and name "Nb, Ti compound micro-alloyed 20CrMnTi easy cutting gear steel" discloses a kind of Nb, Ti compound micro-alloyed 20CrMnTi easy cutting gear steel, and the composition of the steel is: C: 0.17~0.22%, Si: 0.20-0.35%, Mn: 0.9~1.10%, P: ≤0.025%, S: 0.020~0.035%, Cr: 1.05~1.30%, Al: 0.015~0.035%, Ti: 0.02~0.06%, Nb: 0.02~0.06%, the balance is iron and inevitable impurities. By controlling the content of Nb, Ti and Al micro-alloying elements, the gear carburizing temperature is increased or the carburizing time is shortened.

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

[0008] One of the purposes of the present application is to provide a high-strength gear shaft steel with high toughness and high hardenability, which can be obtained at a low cost by optimizing the component system of the gear shaft steel, especially by reasonably controlling the content of micro-alloying elements and nitrogen in the gear steel, and has high hardenability, narrow hardenability band width and good high-temperature grain stability.

[0009] To achieve the above purpose, the present application proposes a high-strength gear shaft steel with high toughness and high hardenability, which contains the following chemical elements in mass percentage, 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] The micro-alloying element coefficient r M / N of the present application is in the range of 1.5-5.0, where r M / N =(10*[Nb] / 93+[Ti] / 480+[Al] / 27) / ([N] / 14), and in the formula, each chemical element is substituted by the numerical value before the percentage sign of the mass percentage of the chemical element.

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

[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 being Fe and inevitable impurities;

[0031] a microalloying element coefficient r M / N ranging from 1.5 to 5.0, wherein r M / N =(10*[Nb] / 93 + [Ti] / 480 + [Al] / 27) / ([N] / 14), wherein 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 and high-toughness high-hardening high-penetration gear shaft steel described in the present application, the design principles of each chemical element are specifically described as follows:

[0033] C: In the high-strength and high-toughness high-hardening high-penetration gear shaft steel described in the present application, C is an essential component in steel, and it is also one of the most important elements affecting the hardenability of steel. High-strength and high-toughness high-hardening high-penetration gear shaft steel requires surface strength as well as 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. Correspondingly, the C content in the steel should not be too high. When the C content in the steel is too high, the toughness of the gear core cannot be met, and a high C content is not conducive to the plasticity of the steel, especially for carburized gear steel with high Mn content. Therefore, in the high-strength and high-toughness high-hardening high-penetration gear shaft steel described in the present application, the mass percentage of C is controlled between 0.16% and 0.22%.

[0034] Si: In the high-strength and high-toughness high-hardening high-penetration gear shaft steel described in the present application, Si element not only can better eliminate the adverse effects of iron oxide on steel, but also can dissolve into ferrite, strengthen ferrite, and improve the strength, hardness, wear resistance and elastic limit of steel. At the same time, it needs to be noted that Si element can increase the Ac3 temperature of steel, and has the risk of cracking and decarburization tendency due to poor thermal conductivity. Based on this, considering the beneficial effects and adverse effects of Si, in the high-strength and high-toughness high-hardening high-penetration gear shaft steel described in the present application, the mass percentage of Si is controlled between 0.10% and 0.40%.

[0035] Mn: In the high strength and toughness high hardenability steel for toothed shafts described in the present application, Mn is one of the main elements affecting the hardenability of the steel. The deoxidizing ability of Mn element is very good, which can reduce the iron oxide in the steel, and can effectively improve the yield of the steel. Mn can dissolve into ferrite, increase the strength and hardness of the steel, and make the steel obtain fine lamellar and high strength pearlite when cooling after hot rolling. In addition, Mn can form MnS with S in the steel, which can eliminate the harmful effect of S, has the ability to form and stabilize austenite structure, can strongly increase the hardenability of the steel, and can also reduce the red toughness of the steel. When the content of Mn element in the steel is too low, the hardenability of the steel is insufficient; and when the content of Mn element in the steel is too high, the hot plasticity of the steel will be poor, which will affect the production, and the steel is prone to crack when water quenching. Therefore, in the high strength and toughness high hardenability steel for toothed shafts described in the present application, the mass percentage of Mn is controlled to be between 0.86% and 1.24%.

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

[0037] Al: In the high strength and toughness high hardenability steel for toothed shafts described in the present application, Al belongs to grain refining element. Al element cooperates with N to further refine the grain, and improve the toughness of the steel. Grain refinement plays an important role in improving the mechanical properties of the steel, especially the strength and toughness, and grain refinement also helps to reduce the hydrogen embrittlement sensitivity of the steel. However, it should be noted that the content of Al element in the steel should not be too high, and too high content of Al can easily increase the opportunity of inclusion in the steel. Therefore, in the high strength and toughness high hardenability steel for toothed shafts described in the present application, the mass percentage of Al element is controlled to be between 0.02% and 0.05%.

[0038] Ti: Although Ti can form fine precipitates when added to the steel, when the content of Ti element in the steel is too high, coarse and angular TiN particles will be formed during smelting, which will reduce the impact toughness of the steel. Therefore, in the high strength and toughness high hardenability steel for toothed shafts described in the present application, the content of Ti element is controlled to be between 0.015% and 0.039%.

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

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

[0041] B: Boron can greatly improve the hardenability of the steel, and the required content is less, which is several hundred times or even thousands of times of the general alloying elements, and has significant economic effect. Moreover, boron steel can be water quenched, which not only saves quenching oil but also easily obtains martensite structure, so that the boron steel has good strength and hardness. As long as the boron content is appropriate, the production process is appropriate, and the complete hardenability is ensured, the plasticity and toughness will not be significantly reduced. However, B element is easy to segregate, which will cause large fluctuation of the hardenability of the steel. Therefore, in the high strength and toughness high hardenability gear shaft steel, the content of B element is controlled to be 0.0006-0.0034%.

[0042] In addition, it is important that in the high strength and toughness high hardenability gear shaft steel, the micro-alloying element coefficient r M / N is controlled to be 1.5-5.0, wherein r M / N =(10*[Nb] / 93+[Ti] / 480+[Al] / 27) / ([N] / 14), and the numerical value before the percentage of the mass percentage of each chemical element in the formula is substituted.

[0043] In the present application, Al, Nb, Ti and N are all main grain refining elements, and the content of Al, Nb, Ti and N in the gear steel and the micro-alloying element coefficient r M / N are controlled, so that the micro-alloying elements and the excess N elements form precipitates, thereby inhibiting the growth of austenite grains at high temperature.

[0044] Further, in the inevitable impurities of the high-toughness high-hardenability gear shaft steel according to the present application, the content of each impurity element satisfies at least one of the following:

[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 the steel, and in the case where the technical conditions permit, in order to obtain a steel material with better performance and higher quality, the content of the impurity elements in the steel should be reduced as much as possible. Among them:

[0050] P: P is easy to segregate at the grain boundaries of the steel, which can reduce the grain boundary binding energy and deteriorate the impact toughness of the steel, so in some embodiments of the present application, the content of P can be controlled to be P≤0.030%.

[0051] O: O is easy to form oxides and complex oxides with Al elements in the steel, which can destroy the continuity of the steel material and reduce the uniformity of the structure, low-temperature impact energy and fatigue performance. Based on this, in some embodiments of the present application, the content of O element can be controlled to be O≤0.0020%.

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

[0053] Ca: In the high-toughness high-hardenability gear shaft steel according to the present application, Ca element is easy to form inclusions, which can affect the fatigue performance of the final product, so the content of Ca element can be controlled to be Ca≤0.0034%.

[0054] Further, in the high-toughness high-hardenability gear shaft steel according to the present application, it further contains at least one of the following chemical elements:

[0055] 0

[0056] Optionally, in the high-toughness high-hardenability gear shaft steel according to the present application, at least one of S, Ni, Mo, Cu and V elements can be further added to further improve the performance of the high-toughness high-hardenability gear shaft steel according to the present application. Among them:

[0057] S: S generally exists as an impurity element in steel, which can reduce the plasticity and toughness of the steel. However, in the high hardenability gear shaft steel provided by the present application, a certain content of S element can form non-metallic inclusions with Mn to improve the cutting performance of the steel. Therefore, in the high hardenability gear shaft steel provided by the present application, the mass percentage of S is controlled as 0 < S ≤ 0.04%.

[0058] Ni: In the high strength and toughness high hardenability gear shaft steel provided by the present application, Ni exists in the steel in a solid solution form, which can effectively improve the low temperature impact performance of the steel. However, it should be noted that too high content of Ni can result in too high content of retained austenite in the steel, thereby reducing the strength of the steel. Therefore, in the high strength and toughness high hardenability gear shaft steel provided by the present application, the mass percentage of Ni is preferably controlled as 0 < Ni ≤ 0.25%.

[0059] Mo: In the high strength and toughness high hardenability gear shaft steel provided by the present application, Mo can be solid-solved in the steel, which is beneficial to improve the hardenability of the steel and the strength of the steel. Mo can also form fine carbides at a higher temperature tempering to further improve the strength of the steel, and the combined action of Mo and Mn can significantly improve the stability of austenite. Therefore, in the high strength and toughness high hardenability gear shaft steel provided by the present application, the mass percentage of Mo is preferably controlled as 0 < Mo ≤ 0.10%.

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

[0061] V: In the high hardenability gear shaft steel provided by the present application, V can effectively improve the hardenability of the steel. V element in the steel can form precipitates with C element or N element, thereby further improving the strength of the steel. However, if the content of C element and V element is too high, coarse VC particles will be formed. Therefore, in the high hardenability gear shaft steel provided by the present application, the mass percentage of V element is controlled as 0 < V ≤ 0.03%.

[0062] Further, the high strength and toughness high hardenability gear shaft steel provided by the present application has an austenite grain size of 5-8 levels after high temperature carburizing heat treatment.

[0063] Further, the high strength and toughness high hardenability gear shaft steel provided by the present application has a J9mm hardenability of 34-42HRC.

[0064] Further, the high-toughness high-hardenability gear shaft steel has a tensile strength R m ≥1100MPa, a yield strength R p0.2 ≥980MPa, an elongation A≥12%, a reduction of area≥50%, and a Charpy impact energy A ku ≥55J.

[0065] Correspondingly, another object of the present application is to provide a manufacturing method of the high-toughness high-hardenability gear shaft steel, which is simple in production and has the characteristics of high hardenability and high toughness.

[0066] In order to achieve the above-mentioned objects, the present application further provides a manufacturing method of the high-toughness high-hardenability gear shaft steel as described above, which comprises the following steps:

[0067] (1) smelting;

[0068] (2) casting;

[0069] (3) heating: the billet is firstly heated to not higher than 700℃ in a preheating section, then continuously heated to not higher than 980℃ in a first heating section, after holding, continuously heated to 950-1200℃ in a second heating section, after holding, enters a soaking section, and the temperature of the soaking section is 1050-1250℃;

[0070] (4) forging or rolling.

[0071] In the manufacturing method of the present application, compared with the prior art, the heating step adopts a unique process, in which the temperature of the soaking section is higher, because the higher temperature of the soaking section can improve the composition uniformity and the structure uniformity of the continuous casting billet in the diffusion process of the heating of the billet. Meanwhile, at this temperature, the precipitated phase has a faster solid solution speed. Therefore, the higher heating temperature will make more of the original undissolved precipitated phase particles in the steel dissolve, increase the concentration of the micro-alloying elements in the matrix, and precipitate more and more dispersed particles during the subsequent cooling. In addition, only when the heating temperature is increased, the finish rolling or finish forging temperature can be increased, so that the austenite recovery recrystallization after rolling is more sufficient, and the distribution of the precipitated phase is more uniform.

[0072] In the manufacturing method of the present application, the smelting in step (1) can adopt electric furnace smelting or converter smelting, and can be subjected to refining and vacuum treatment. Of course, in some other embodiments, vacuum induction furnace can also be used for smelting.

[0073] In addition, in step (1), the furnace charge of the electric furnace smelting can be selected from low-P, S scrap steel, cut head and high-quality pig iron; the alloy can be prepared from ferrochrome, low-phosphorus ferromanganese, ferromolybdenum and the like; the reducing agent can include: calcium carbide, carbon powder and aluminum powder; in the oxidation period: frequently flow slag to remove P; the conditions for tapping can be controlled as follows: the tapping temperature is 1630-1660℃; P≤0.015%; the conditions for tapping can be controlled as follows: the tapping temperature is 1630-1650℃; [P]≤0.010%, [C]≥0.03%.

[0074] After the electric furnace smelting or the converter smelting is completed, the molten steel can be refined in a ladle refining furnace to remove harmful gases and inclusions in the steel. By controlling the ladle seating, temperature measurement and analysis, the argon pressure can be adjusted according to the situation. The LF primary deoxidation can feed Al, and then the alloy block can be added and stirred for 5-10 minutes. When the temperature of the molten steel T=1650-1670℃, vacuum degassing can be carried out to ensure that [O]≤0.0020%, [H]≤0.00015%. In a specific example, the vacuum degree of vacuum degassing can be controlled to be ≤66.7Pa, and maintained for not less than 15 minutes,

[0075] In addition, in step (1), the ladle temperature can be controlled to be 1550-1570℃, thereby reducing the ladle temperature, accelerating the diffusion of elements, and being beneficial to further reducing dendritic segregation.

[0076] In addition, in step (2), casting can adopt mold casting or continuous casting. During the continuous casting pouring process, the high-temperature molten steel in the ladle is poured into the tundish through the protection sleeve, and the superheat of the tundish can be controlled to be 20-40℃. The tundish is completely cleaned before use, the inner surface is a refractory coating and must not have cracks; the molten steel in the tundish passes through the continuous casting crystallizer and is fully stirred by electromagnetic stirring, and a qualified continuous casting billet with a section size of 140mm×140mm-320mm×425mm can be poured.

[0077] In step (2), the pouring speed can be controlled to be 0.6-2.1m / min according to different billet sizes. Then, the continuous casting billet is put into the slow cooling pit for slow cooling, and the slow cooling time can be not less than 24 hours.

[0078] Further, in step (4) of the manufacturing method, the open forging or open rolling temperature is controlled to be 1050-1250℃, and the final forging or final rolling temperature is controlled to be ≥900℃.

[0079] In this embodiment, the open forging or open rolling temperature is controlled to be between 1050-1250℃, and the final forging or final rolling temperature is controlled to be ≥900℃, because: this process is further beneficial to the desorption of N from the γ solid solution and the combination of the micro-alloying elements in the steel into nitrides.

[0080] It should be noted that the solubility of N in alpha-Fe is less than that in gamma-Fe, and due to the excitation of phase transition, two peaks of precipitation amount are caused, if the final forging or final rolling temperature is low, the peak precipitation of the precipitated phase causes uneven distribution of the precipitated phase and insufficient recovery and recrystallization to produce anisotropy in the structure, therefore, the final forging or final rolling temperature is controlled to be greater than or equal to 900 DEG C. In addition, increasing the final forging or final rolling temperature can obtain finer grains, the difference between the average grain diameter of ferrite after transformation of supercooled austenite and the spacing between Mn-rich bands is increased due to the small grain size, the tendency of the Mn-rich bands to form pearlite is reduced, thereby reducing the banded structure.

[0081] In the above technical solution, in step (4) of the manufacturing method, after the billet is discharged, the high-pressure water can be used to remove the scale and remove the oxide scale.

[0082] In addition, in step (3) of the manufacturing method, when forging, it can be directly forged to the final product size. When rolling, the billet can be directly rolled to the final product size, or the 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 and high-toughness high-hardness gear shaft steel and the manufacturing method thereof have the following advantages and beneficial effects compared with the prior art:

[0084] (1) The high-strength and high-toughness high-hardness gear shaft steel can be developed by reasonable chemical composition design and combined with optimized process, and the rod material rolled or forged by the high-strength and high-toughness high-hardness gear shaft steel can be effectively processed into a gear, and after subsequent high-temperature carburizing heat treatment, the gear can have excellent strength and toughness.

[0085] (2) The high-strength and high-toughness high-hardness gear shaft steel controls the content of micro-alloying element coefficient and nitrogen element, and strictly controls the atomic molar ratio, and adds appropriate Nb element to hinder the abnormal growth of austenite grains, so that the austenite grain coarsening temperature of the gear steel is improved, the grain size is still stable at 5-8 levels after high-temperature carburizing at 1000 DEG C for 4 hours, and the performance reaches the use performance index of the gear shaft steel.

[0086] (3) The composition and process design of the high-strength and high-toughness high-hardness gear shaft steel are reasonable, the content of micro-alloying elements in the steel is controlled, so that large harmful inclusions in the steel are avoided, the stable production quality of the steel is ensured, the production cost of the steel is reduced, and batch production on the rod production line is realized.

[0087] (4) The high-strength and high-toughness high-hardening high-tooth-axle steel has high hardening, high austenite grain size and cost competitiveness, and can control the types and amounts of alloying elements in the steel material and improve the applicability of the steel material under the premise of ensuring high hardening and narrow bandwidth.

[0088] (5) The high-strength and high-toughness high-hardening high-tooth-axle steel can shorten the carburizing time and reduce the production cost of the tooth axle when used for subsequent production and manufacturing of the tooth axle, and has a broad industrial application prospect. DETAILED DESCRIPTION

[0089] The high-strength and high-toughness high-hardening high-tooth-axle steel and the manufacturing method thereof will be further explained in combination with specific examples, but the explanation and description do not constitute undue limitation on the technical solutions of the present application.

[0090] The high-strength and high-toughness high-hardening high-tooth-axle steel of Examples 1-8 is prepared by the following steps:

[0091] (1) Smelting and casting are performed according to the chemical compositions shown in Tables 1-1 and 1-2 below: The smelting can be performed by using a 50 kg vacuum induction furnace, a 150 kg vacuum induction furnace or a 500 kg vacuum induction furnace, or by using an electric furnace smelting + external refining + vacuum degassing method, or by using a converter smelting + external refining + vacuum degassing method.

[0092] (2) Casting.

[0093] (3) Heating: The billet is first heated to not higher than 700 DEG C in the preheating section, then continuously heated to not higher than 980 DEG C in the first heating section, after holding, continuously heated to 950-1200 DEG C in the second heating section, after holding, enters the soaking section, the soaking section temperature is 1050-1250 DEG C, after holding, subsequent rolling or forging is performed.

[0094] (4) Forging or rolling: The open forging or open rolling temperature is controlled to be 1050-1250 DEG C, and the final forging or final rolling temperature is controlled to be greater than or equal to 900 DEG C.

[0095] Further, the specific process of the high-hardening high-tooth-axle steel of Examples 1-8 and the steel of Comparative Examples 1-4 is as follows:

[0096] Example 1: Smelting is performed according to the chemical compositions shown in Tables 1-1 and 1-2 below on a 50 kg vacuum induction furnace. The molten steel is cast into an ingot, heated and subjected to open forging, the ingot is first heated to 700 DEG C in the preheating section, then continuously heated to 900 DEG C in the first heating section, after holding, continuously heated to 950 DEG C in the second heating section, after holding, enters the soaking section, the soaking section temperature is 1050 DEG C, after holding, subsequent forging is performed, the final forging temperature is controlled to be 910 DEG C, and finally forged into a Φ60 mm bar.

[0097] Example 2: The steel was smelted according to the chemical composition shown in Table 1-1 and 1-2 below in a 150kg vacuum induction furnace. The molten steel was cast into an ingot, heated and subjected to open die forging. The ingot was first heated to 650℃ in a preheating section, then continuously heated to 950℃ in a first heating section, and after holding, continuously heated to 1100℃ in a second heating section. After holding, the ingot was subjected to a soaking section at a temperature of 1200℃, and after holding, subsequent forging was performed. The final forging temperature was controlled to be 1000℃, and the ingot was finally forged into a Φ90mm bar.

[0098] Example 3: The steel was smelted according to the chemical composition shown in Table 1-1 and 1-2 below in a 500kg vacuum induction furnace. The molten steel was cast into an ingot, heated and subjected to open die forging. The ingot was first heated to 600℃ in a preheating section, then continuously heated to 980℃ in a first heating section, and after holding, continuously heated to 1200℃ in a second heating section. After holding, the ingot was subjected to a soaking section at a temperature of 1250℃, and after holding, subsequent forging was performed. The final forging temperature was controlled to be 1000℃, and the ingot was finally forged into a Φ120mm bar.

[0099] Example 4: The steel was smelted according to the chemical composition shown in Table 1-1 and 1-2 in an electric furnace, and subjected to refining and vacuum treatment, and then cast into a 280mm x 280mm continuous casting billet. The continuous casting billet was heated in a walking beam heating furnace at a speed of 300℃ / h, and first heated to 620℃ in a preheating section, then continuously heated to 950℃ in a first heating section, and continuously heated to 1150℃ in a second heating section. After holding, the billet was subjected to a soaking section at a temperature of 1200℃, and after holding for 4h, the billet was subjected to rolling. After the billet exited the heating furnace, the billet was subjected to high-pressure water descaling and then started to be rolled. The final rolling temperature was controlled to be 970℃, and the billet was finally rolled into a Φ80mm bar.

[0100] Example 5: The steel is smelted in an electric furnace according to the chemical composition shown in Tables 1-1 and 1-2, and then refined and vacuum treated, and then cast into a 320 mm x 425 mm continuous casting billet. The continuous casting billet is heated in a walking beam furnace at a speed of 150°C / h, first heated to 600°C in the preheating section, then heated to 950°C in the first heating section, and then heated to 1200°C in the second heating section, and then enters the soaking section at a temperature of 1230°C, and then heated for 4.5 hours before subsequent rolling. After the billet is discharged from the heating furnace and descaled by high-pressure water, rolling begins, and the billet is rolled into an intermediate billet, with the first finish rolling temperature controlled at 1050°C, and the intermediate billet size being 220 mm x 220 mm. Then the intermediate billet is placed in a walking beam furnace and slowly heated at a speed of 400°C / h, heated to 680°C in the preheating section, heated to 1050°C in the first heating section, and heated to 1200°C in the second heating section, and then enters the soaking section after holding, with the soaking temperature being 1220°C, and then heated for 3.5 hours before discharging, and then descaled by high-pressure water before rolling begins, with the second finish rolling temperature controlled at 950°C, and the finished rod size being Φ90 mm.

[0101] Example 6: The steel is smelted in an electric furnace according to the chemical composition shown in Tables 1-1 and 1-2, and then refined and vacuum treated, and then cast into a 280 mm x 280 mm continuous casting billet. The continuous casting billet is heated in a walking beam furnace at a speed of 300°C / h, first heated to 680°C in the preheating section, then heated to 900°C in the first heating section, and then heated to 1180°C in the second heating section, and then enters the soaking section at a temperature of 1200°C, and then heated for 4.5 hours before subsequent rolling. After the billet is discharged from the heating furnace and descaled by high-pressure water, rolling begins, and the billet is rolled into an intermediate billet, with the first finish rolling temperature controlled at 1000°C, and the intermediate billet size being 140 mm x 140 mm. Then the intermediate billet is placed in a walking beam furnace and slowly heated at a speed of 500°C / h, first preheated to 700°C, then heated to 1100°C in the first heating section, and then heated to 1220°C in the second heating section, and then enters the soaking section at a temperature of 1220°C, and then heated for 3.5 hours before discharging, and then descaled by high-pressure water before rolling begins, with the second finish rolling temperature controlled at 920°C, and the finished rod size being Φ25 mm.

[0102] Example 7: The steel is smelted in a converter according to the chemical composition shown in Tables 1-1 and 1-2, and then refined and vacuum treated, and then cast into a mold casting billet. The casting billet is heated in a walking beam furnace at a speed of 50°C / h, first heated to 620°C in the preheating section, then heated to 950°C in the first heating section, and then heated to 1150°C in the second heating section, and then enters the soaking section at a temperature of 1200°C, and then heated for 8 hours before rolling. After the billet is discharged from the heating furnace and descaled by high-pressure water, rolling begins, with the finish rolling temperature controlled at 970°C, and the final rolling is Φ90 mm rod.

[0103] Example 8: The steel according to the chemical composition shown in Table 1-1 and 1-2 was smelted in a 50 kg vacuum induction furnace. The molten steel was cast into a 320 mm x 425 mm billet, and the billet was heated in a walking beam furnace at a rate of 150 °C / h, first heated to 600 °C in the preheating section, then heated to 950 °C in the first heating section, and then heated to 1200 °C in the second heating section, and then entered the soaking section at a temperature of 1230 °C. After soaking for 4.5 h, the billet was rolled into an intermediate billet, and the first finishing temperature was controlled at 1050 °C. The intermediate billet had a size of 220 mm x 220 mm. Then the intermediate billet was placed in a walking beam furnace and slowly heated at a rate of 400 °C / h, heated to 680 °C in the preheating section, heated to 1050 °C in the first heating section, and then heated to 1200 °C in the second heating section, and then entered the soaking section at a temperature of 1220 °C. After soaking for 6 h, the billet was rolled after descaling by high-pressure water, and the second finishing temperature was controlled at 950 °C. The finished product had a size of Φ 90 mm.

[0104] Comparative Example 1 and Comparative Example 2 were commercial materials that were smelted in an electric furnace and subjected to refining treatment to ensure the purity of the commercial materials.

[0105] Comparative Example 3: The steel according to the chemical composition shown in Table 1-1 and 1-2 was smelted in a 50 kg vacuum induction furnace. The molten steel was cast into an ingot, heated and subjected to open die forging, and then heated in a box furnace at a rate of 300 °C / h to 1100 °C. After soaking for 3 h, the ingot was subjected to subsequent forging, and the finish forging temperature was controlled at 910 °C. The final forging was performed to produce a Φ 60 mm bar.

[0106] Comparative Example 4: The steel according to the chemical composition shown in Table 1-1 and 1-2 was smelted in an electric furnace and subjected to refining and vacuum treatment, and then cast into a 320 mm x 425 mm continuous casting billet. The continuous casting billet was heated in a walking beam furnace at a rate of 150 °C / h, first heated to 600 °C in the preheating section, then heated to 950 °C in the first heating section, and then heated to 1200 °C in the second heating section, and then entered the soaking section at a temperature of 1230 °C. After soaking for 4.5 h, the billet was rolled after descaling by high-pressure water, and the first finishing temperature was controlled at 1050 °C. The intermediate billet had a size of 220 mm x 220 mm. Then the intermediate billet was placed in a walking beam furnace and slowly heated at a rate of 400 °C / h, heated to 680 °C in the preheating section, heated to 1050 °C in the first heating section, and then heated to 1200 °C in the second heating section, and then entered the soaking section at a temperature of 1220 °C. After soaking for 6 h, the billet was rolled after descaling by high-pressure water, and the second finishing temperature was controlled at 950 °C. The finished product had a size of Φ 90 mm.

[0107] Table 1-1 and Table 1-2 list the mass percentage of each chemical element 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.%, balance being Fe and other unavoidable impurities other than P, O, H and Ca)

[0109]

[0110]

[0111] Table 1-2 (wt.%, balance being Fe and other unavoidable impurities other than P, O, H and Ca)

[0112] No. S Ni Mo Cu V P O Ca H 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: r M / N = (10*[Nb] / 93 + [Ti] / 480 + [Al] / 27) / ([N] / 14), where each chemical element in the formula is substituted with the value before the percentage of the mass percentage of the chemical element.

[0114] Table 2 lists the specific process parameters of the high strength and toughness high 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, the steel blanks of Examples 5, 6, 8 and Comparative Example 4 are first rolled to the specified intermediate bloom size, and then reheated and rolled to the final product size.

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

[0121] Simulated high temperature carburizing heat treatment test: the sample was heated to 1200°C, held for 40 min, water cooled, then heated to 1000°C at a rate of 40-60 min to 500-800°C, held for 4 h, and water quenched.

[0122] Austenite grain size detection: the austenite grain size was evaluated according to the standard ASTM E112.

[0123] Mechanical property test: the sample was prepared according to GB / T 2975-2018 Steel and Steel Products - Location and Preparation of Test Pieces for Mechanical Testing, reference GB / T 3077-2015, Φ15mm blank was prepared, 880℃ heating for 90min oil quenching + 870℃ heating for 90min oil quenching + 200℃ heating for 150min tempering air cooling, and tensile test was carried out according to GB / T 228.1-2010 Metallic Materials - Tensile Test - Part 1: Method of Test at Room Temperature, and Charpy impact test was carried out according to GB / T 229-2007 Metallic Materials - Charpy Pendulum Impact Test Method to test the room temperature Charpy impact energy A ku .

[0124] In addition, the high strength and toughness high hardenability gear shaft steel of obtained examples 1-8 and the comparative steel of comparative examples 1-4 were sampled respectively, and quenching test and hardness test were carried out, and the test results were also listed in table 3. Among them:

[0125] Quenching test: the steel of each example and the steel of comparative example were sampled and prepared from hot rolled round steel according to national standard GB / T 225, end quenching test (Jominy test) was carried out according to GB / T 5216, normalizing temperature was controlled at 920±10℃, and quenching temperature was controlled at 870±5℃. Rockwell hardness test was carried out according to GB / T 230.2, and hardness value (HRC) at specific position was obtained, such as hardness at 9mm from quenching end, that is, J9mm.

[0126] Table 3 lists the test results of the high strength and toughness high hardenability gear shaft steel of examples 1-8 and the comparative steel of comparative examples 1-4.

[0127] Table 3.

[0128]

[0129] Note: the impact energy of each example and comparative example in table 3 has two values representing two measurement results.

[0130] As can be seen from table 3, the high strength and toughness high hardenability gear shaft steel of examples 1-8 of the present application maintains the austenite grain size in the range of 5-8 after 1000℃ simulated high temperature carburizing heat treatment, and no mixed grain, abnormal coarse grain and other phenomena are observed, so it has good high temperature grain stability.

[0131] In addition, it can be seen from Table 3 that the high-toughness and high-hardening high-axle steel of each embodiment 1-8 of the present application has a representative position J9mm hardenability of 34-42HRC, and has high hardenability and a narrow hardenability bandwidth. In addition, the tensile strength R m of each embodiment after simulated high-temperature carburizing heat treatment is greater than 1100MPa, the yield strength R p0.2 is greater than 980MPa, the elongation A is greater than 12%, the reduction of area is greater than 50%, and the Charpy impact energy A ku is greater than 55J.

[0132] Therefore, the rolled or forged rod of the high-toughness and high-hardening high-axle steel can be effectively processed into a gear shaft, and after high-temperature carburizing heat treatment by a downstream user, it has high toughness, so it can be effectively applied to high-end parts such as a gearbox for a car or a reducer for a new energy vehicle and an industrial reducer, and has good use prospects and value.

[0133] Unlike each embodiment of the present application, the grain size of the comparative example 1 after simulated high-temperature carburizing is 0, that is, the grains abnormally grow, which cannot meet the use requirements.

[0134] Although the comparative steel of comparative example 2 does not have a mixed grain phenomenon, it has small grains and low hardenability after simulated high-temperature carburizing heat treatment, which cannot meet the requirement of high hardenability. In addition, the strength of comparative example 2 is low.

[0135] The comparative steel of comparative example 3 observed a mixed grain phenomenon after simulated high-temperature carburizing heat treatment at a temperature of 1000℃, wherein 5(0) indicates that the average grain size is 5, and part of the area is coarsened to 0.

[0136] And the comparative steel of comparative example 4 also observed a mixed grain phenomenon (4) after simulated high-temperature carburizing heat treatment at a temperature of 1000℃, wherein 0(4) indicates that the average grain size is 0, and part of the area is coarsened to 4. In addition, the impact energy of comparative example 4 is also low.

[0137] In addition, the combination of each technical feature in the case is not limited to the combination mode or the combination mode of the specific embodiments described in the claims of the case, and all technical features described in the case can be freely combined or combined in any way, unless contradictory to each other.

[0138] It should also be noted that the above-mentioned embodiments are only specific embodiments of the present application. Obviously, the present application is not limited to the above-mentioned embodiments, and similar changes or modifications made by the skilled in the art from the disclosure of the present application or easily thought of should be within the scope of protection of the present application.

Claims

1. A high-strength, high-toughness, and high-hardenability steel for gear shafts, characterized in that, Its mass percentage content of each chemical element is as follows: 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%; balance Fe and other unavoidable impurities. Its microalloying element coefficient r M / N The range is 1.5 to 5.0, where r M / N = (10*[Nb] / 93+[Ti] / 480+[Al] / 27) / ([N] / 14), where each chemical element is represented by the value preceding the percentage sign of its mass percentage content; The high-strength, high-toughness, and high-hardenability gear shaft steel has a hardenability of 34–42 HRC for its J9mm diameter, and its tensile strength R after high-temperature carburizing heat treatment is... m ≥1100MPa, yield strength R p0.2 ≥980MPa, elongation at break A≥12%, reduction of area ≥50%, Charpy impact energy A ku ≥55J.

2. The high-strength, high-toughness, and high-hardenability gear shaft steel as described in claim 1, characterized in that, Among unavoidable impurities, P ≤ 0.030%, O ≤ 0.002%, H ≤ 0.0002%, and Ca ≤ 0.003%.

3. The high-strength, high-toughness, and high-hardenability gear shaft steel as described in claim 1, 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%.

4. The high-strength, high-toughness, and high-hardenability gear shaft steel as described in claim 1, characterized in that, Its austenite grain size remains at level 5 to 8 after high-temperature carburizing heat treatment.

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

6. The manufacturing method as described in claim 5, characterized in that, In step (4), the initial forging or rolling temperature is controlled at 1050-1250℃, and the final rolling or forging temperature is controlled at ≥900℃.

7. The manufacturing method as described in claim 5, characterized in that, In step (4), the product is directly rolled or forged to the finished size.

8. The manufacturing method as described in claim 5, characterized in that, In step (4), the billet is first rolled to the intermediate billet size, then heated in the middle, and then rolled to the final finished product size.

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