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

Abstract

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

Description

Technical field

[0001] The present disclosure relates to a high-strength steel and a manufacturing method therefor, in particular to a steel for gear shafts and a manufacturing method therefor.Background Art

[0002] With the development of the automobile industry, the requirements of automotive parts have been increased continuously. One of the important development directions is to produce gears with high strength-toughness, high fatigue life, high-temperature stability and economic efficiency.

[0003] Correspondingly, high strength-toughness steel for gear shafts can meet the high technical requirements of automobile light-weighting for materials. Furthermore, to ensure the quenching strength of gears, it is usually necessary to impose the requirement on the hardenability of a steel for gear shafts.

[0004] The main technical problem of high-hardenability MnCr-based carburized gear steel is how to avoid dimensional dispersion of gears caused by excessive hardenability bandwidth while improving the hardenability, and simultaneously ensure that the phenomenon of mixed crystal and coarse grains in gears does not appear after high-temperature carburizing.

[0005] For example, the Chinese patent document with publication number CN101096742A, published on January 2, 2008, and titled "A High-Strength Gear Steel for Automobiles", discloses a high-strength gear steel for automobiles. Alloy elements such as Nb, V, and Al are compositely added to the steel to refine original austenite grains. The steel comprises the following composition in percentage by mass: 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, and Al: 0.015-0.050, with a balance of Fe and unavoidable impurities. The grain size, hardenability, and hardenability bandwidth of the gear steel are improved by adding a trace amount of Nb and V.

[0006] For another example, the Chinese patent document with publication number CN103361559A, published on October 23, 2013, entitled "A Nb and Ti composite micro-alloyed high-temperature carburized gear steel", discloses a Nb and Ti composite micro-alloyed high-temperature carburized gear steel. The steel comprises the following composition: C: 0.17-0.22%, Si: 0.20-0.35%, Mn: 0.9-1.10%, P: ≤0.025%, S: 0.020-0.035%, Cr: 1.05-1.30%, Al: 0.015-0.035%, Ti: 0.02-0.06%, and Nb: 0.02-0.06%, with a balance of iron and unavoidable impurities. By controlling the content of micro-alloyed elements such as Nb, Ti, and Al, a carburizing temperature of a gear is increased or a carburizing time of a gear is reduced.

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

[0008] One of the objects of the present disclosure is to provide a high-strength, high-toughness and high-hardenability steel for gear shafts. By optimizing the composition of a steel for gear shafts, especially by reasonably controlling the content of micro-alloying elements and nitrogen in a gear steel, a high-strength steel for gear shafts with high hardenability, narrow hardenability bandwidth, and good high-temperature grain stability can be obtained at a relatively low cost.

[0009] To achieve the above object, the present disclosure provides a high-strength, high-toughness and high-hardenability steel for gear shafts, comprising Fe and unavoidable impurities, and further comprising the following chemical elements in 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%, and B: 0.0006-0.0034%; wherein a value of the micro-alloying element coefficient rM / N is 1.5-5.0, wherein rM / N = (10*[Nb] / 93+[Ti] / 480+[Al] / 27) / ([N] / 14), wherein each chemical element is substituted with the numerical value before the percent sign of its mass percentage content.

[0010] Furthermore, in the high-strength, high-toughness and high-hardenability steel for gear shafts of the present disclosure, the mass percentages of each chemical elements are: C: 0.16-0.22%, Si: 0.10-0.40%, Mn: 0.86-1.24%, Cr: 0.95-1.44%, Al: 0.02-0.05%, Ti: 0.015-0.039%, Nb: 0.001-0.034%, N: 0.006-0.015%, and B: 0.0006-0.0034%; with a balance of Fe and unavoidable impurities; wherein the value of micro-alloying element coefficient rM / N is 1.5-5.0, wherein rM / N = (10*[Nb] / 93+[Ti] / 480+[Al] / 27) / ([N]1 / 4), wherein each chemical element is substituted with the numerical value before the percent sign of its mass percentage content.

[0011] The design principles of the various chemical elements in the high-strength, high-toughness and high-hardenability steel for gear shafts of the present disclosure will be described in detail as follows: C: In the high-strength, high-toughness and high-hardenability steel for gear shafts of the present disclosure, C is an essential component in the steel and is also one of the main elements affecting the hardenability of the steel. Both surface strength and sufficient core impact toughness are required in the high-strength, high-toughness and high-hardenability steel for gear shafts. When the content of C in the steel is too low, which is less than 0.16%, the strength of the steel is insufficient, and the requirement for good hardenability of the steel may not be achieved. Correspondingly, the content of C in the steel should not be too high. When the content of C in the steel is too high, the requirement for the toughness of the gear core may not be satisfied, and excessive content of C is not conducive to the plasticity of the steel, especially for a carburized gear steel having a high content of Mn. When the content of C is higher than 0.22%, it is not conducive to the processability performance of the steel. Therefore, in the high-strength, high-toughness and high-hardenability steel for gear shafts of the present disclosure, the mass percentage of C is controlled at 0.16-0.22%.

[0012] Si: In the high-strength, high-toughness and high-hardenability steel for gear shafts of the present disclosure, Si element may not only better eliminate the adverse effects of iron oxide on the steel, but may also be dissolved in the ferrite, strengthening the ferrite and improving the strength, hardness, wear resistance, elasticity, and elastic limit of the steel. At the same time, it should be noted that, Si element helps to increase a temperature Ac 3 of the steel, and make the steel have a risk of cracking and a tendency of decarburization due to its poor thermal conductivity. As such, considering the beneficial effects and adverse effects of Si in combination, in the high-strength, high-toughness and high-hardenability steel for gear shafts of the present disclosure, the mass percentage of Si is controlled at 0.10-0.40%.

[0013] Mn: In the high-strength, high-toughness and high-hardenability steel for gear shafts of the present disclosure, Mn is one of the main elements affecting the hardenability of the steel. Mn element has a good deoxidizing ability, which may reduce the iron oxide in the steel, and effectively increase the yield of the steel. Mn may be dissolved into ferrite, improve the strength and hardness of the steel, and make the steel have pearlite with finer lamellae and higher strength when the steel is cooled after hot rolling. Furthermore, Mn may also form MnS with S in the steel, which may eliminate the harmful effects of S. Mn has an ability to form and stabilize austenite structures in the steel, and may strongly improve the hardenability of the steel, and may also reduce the high-temperature toughness of the steel. When the content of Mn in the steel is too low, the hardenability of the steel is insufficient; when the content of Mn in the steel is too high, the thermoplasticity of the steel may be deteriorated, the yield will be affected, and the steel is prone to cracking during water quenching. Therefore, in the high-strength, high-toughness and high-hardenability steel for gear shafts of the present disclosure, the mass percentage of Mn is controlled at 0.86-1.24%.

[0014] Cr: In the high-strength, high-toughness and high-hardenability steel for gear shafts of the present disclosure, Cr is one of the main alloy elements added in the steel of the present disclosure. Cr may significantly improve the hardenability, strength, wear resistance, and other properties of the steel. Additionally, Cr may also reduce the activity of C element in the steel, and prevent decarburization during heating, rolling, and heat treatment process. However, when the content of Cr is too high, the toughness of quenched and tempered steel will be reduced significantly, and coarse carbides distributed along grain boundaries are formed. Therefore, in the high-strength, high-toughness and high-hardenability steel for gear shafts of the present disclosure, the mass percentage of Cr is controlled at 0.95-1.44%.

[0015] Al: In the high-strength, high-toughness and high-hardenability steel for gear shafts of the present disclosure, Al is an element for refining grains. The combination of Al element and N element may further refine the grains and improve the toughness of the steel. Grain refinement plays an important role in improving the mechanical performance of steel, especially the strength and toughness. Meanwhile, grain refinement also helps to reduce the hydrogen embrittlement sensitivity of the steel. However, it should be noted that the content of Al in the steel should not be too high. when the content of Al is too high, it tends to promote the formation of inclusions in the steel. Therefore, in the high-strength, high-toughness and high-hardenability steel for gear shafts of the present disclosure, the mass percentage of Al is controlled at 0.02-0.05%.

[0016] Ti: The addition of Ti may form fine precipitates in the steel. But when the content of Ti in the steel is too high, coarse TiN particles with edges and corners will be formed during the smelting process, thereby reducing the impact toughness of the steel. Therefore, in the high-strength, high-toughness and high-hardenability steel for gear shafts of the present disclosure, the content of Ti is controlled at 0.015-0.039%.

[0017] Nb: In the high-strength, high-toughness and high-hardenability steel for gear shafts of the present disclosure, the addition of Nb element in the steel may help to form fine precipitates, thereby inhibiting the recrystallization of the steel, and effectively refining the grain size. It should be noted that the content of Nb 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 the smelting process, which will reduce the impact toughness of the steel instead. Therefore, in the high-strength, high-toughness and high-hardenability steel for gear shafts of the present disclosure, the mass percentage of Nb is controlled at 0.001-0.034%.

[0018] N: In the high-strength, high-toughness and high-hardenability steel for gear shafts of the present disclosure, N is an interstitial atom that may combine with microalloys in the steel to form MN-type precipitates, which may pin grain boundaries at a high temperature, thereby inhibiting the growth of austenite grains. When the content of N in the steel is too low, the amount of MN formed is less, and the pinning effect is not significant; when the content of N in the steel is too high, the N element tends to be enriched in steel-making, thereby reducing the toughness of the steel. Therefore, in the high-strength, high-toughness and high-hardenability steel for gear shafts of the present disclosure, the mass percentage of N is controlled at 0.006-0.015%.

[0019] B: Boron helps to greatly improving the hardenability of steel, and the required content is small. Its effect is hundreds or even thousands of times that of general alloy elements, and have significant economic effects. Moreover, boron steel may be water-quenched, which saves quenching oil and tends to obtain martensite structure, thus making boron-containing steel have good strength and hardness. As long as the content of boron is appropriate, the production process is proper, and complete quenching is ensured, plasticity and toughness will not be significantly reduced. However, the B element is prone to segregation, which can cause significant fluctuations of the hardenability of the steel. Therefore, in the high-strength, high-toughness and high-hardenability steel for gear shafts of the present disclosure, the B content is controlled at 0.0006-0.0034%.

[0020] Additionally, it is important that in the high-strength, high-toughness and high-hardenability steel for gear shafts of the present disclosure, it is necessary to control the value of the micro-alloying element coefficient r M / N to be 1.5-5.0, wherein r M / N = (10*[Nb] / 93+[Ti] / 480+[Al] / 27) / ([N] / 14), wherein each chemical element is substituted with a numerical value before the percent sign of its mass percentage content.

[0021] In the present disclosure, Al, Nb, Ti, and N are all main grain refining elements. By controlling the content of Al, Nb, Ti, N, and the micro-alloying element coefficient r M / N in the gear steel, the microalloying elements combine with excess N to form precipitates, thereby inhibiting the growth of austenite grain at a high temperature.

[0022] Furthermore, in the high-strength, high-toughness and high-hardenability steel for gear shafts of the present disclosure, among the unavoidable impurities, the mass percentage of each impurity elements satisfies at least one of the following: P ≤ 0.030%, O ≤ 0.0020%, H ≤ 0.0002%, and Ca ≤ 0.0034%.

[0023] In the above technical solution, P, O, H, and Ca are all impurity elements in the steel. In order to obtain the steel with better performance and higher quality, the content of impurity elements in the steel should be reduced as much as possible if the technical conditions allow. Wherein: P: P is prone to segregate at 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 disclosure, the content of P may be controlled at P ≤ 0.030%. O: O tends to form oxides and complex oxides with Al element in the steel, destroying the continuity of the steel, reducing organizational uniformity, low-temperature impact energy and fatigue performance. As such, in some embodiments of the present disclosure, the content of O may be controlled at O ≤ 0.0020%. H: H may accumulate at defects in the steel, especially in the steel with a tensile strength grade of greater than1000 MPa, so that hydrogen-induced delayed fracture may occur. Therefore, in some embodiments of the present disclosure, the content of H may be controlled at H ≤ 0.0002%. Ca: In the high-strength, high-toughness and high-hardenability steel for gear shafts of the present disclosure, Ca element tends to forms inclusions, thereby affecting the fatigue performance of a final product. Therefore, the content of Ca element may be controlled at Ca ≤ 0.0034%, for example, ≤ 0.003%.

[0024] Furthermore, the high-strength, high-toughness and high-hardenability steel for gear shafts of the present disclosure further comprises at least one of the following chemical elements: 0 < S ≤ 0.04%, 0 < Ni ≤ 0.25%, 0 < Mo ≤ 0.10%, 0 < Cu ≤ 0.20%, and 0 < V ≤ 0.03%.

[0025] Optionally, in the high-strength, high-toughness and high-hardenability steel for gear shafts of the present disclosure, at least one of S, Ni, Mo, Cu, and V elements may be further added to further improve the performance of the high-strength, high-toughness and high-hardenability steel for gear shafts of the present disclosure. Wherein: S: S is generally present as an impurity element in the steel, which will reduce the plasticity and toughness of the steel. However, in the high-hardenability steel for gear shafts of the present disclosure, a certain content of S element may form non-metallic inclusions with Mn to improve the machinability of the steel. As such, in the high-hardenability steel for gear shafts of the present disclosure, the mass percentage of S is controlled at 0 < S ≤ 0.04%, for example, 0.001% ≤ S ≤ 0.04%. Ni: In the high-strength, high-toughness and high-hardenability steel for gear shafts of the present disclosure, Ni exists in the form of solid solution in the steel, which may effectively improve the low-temperature impact performance of the steel. However, it should be noted that when the content of Ni is too high, the steel may have excessive retained austenite, thereby reducing the strength of the steel. Therefore, in the high-strength, high-toughness and high-hardenability steel for gear shafts of the present disclosure, preferably, the mass percentage of Ni may be controlled at 0 < Ni ≤ 0.25%, for example, 0.03% ≤ Ni ≤ 0.25%. Mo: In the high-strength, high-toughness and high-hardenability steel for gear shafts of the present disclosure, Mo may be solid-dissolved in the steel, which is conducive to improving the hardenability and strength of the steel. When the steel is tempered at a relatively high temperature, Mo will also be formed as fine carbides to further improve the strength of the steel. The combination action of molybdenum and manganese may also significantly increase the stability of austenite. As such, in the high-strength, high-toughness and high-hardenability steel for gear shafts of the present disclosure, preferably, the mass percentage of Mo may be controlled at 0 < Mo ≤ 0.10%, for example, 0.01% ≤ Mo ≤ 0.10%. Cu: In the high-strength, high-toughness and high-hardenability steel for gear shafts of the present disclosure, Cu may improve the strength of the steel and is conducive to improving the weather resistance and corrosion resistance of the steel. However, the content of Cu 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 boundaries during the heating process, resulting in weakening of grain boundary and even cracking. Therefore, in the high-strength, high-toughness and high-hardenability steel for gear shafts of the present disclosure, preferably, the mass percentage of Cu may be controlled at 0 < Cu ≤ 0.20%, for example, 0.03% ≤ Cu ≤ 0.20%. V: In the high-hardenability steel for gear shafts of the present disclosure, V may effectively improve the hardenability of the steel. V element in the steel may form precipitates with C element or N element, thereby further improving the strength of the steel. However, if the contents of C and V element are too high, coarse VC particles will be formed. As such, in the high-hardenability steel for gear shafts of the present disclosure, the mass percentage of V may be controlled at 0 < V ≤ 0.03%, for example, 0.005% ≤ V ≤ 0.03%.

[0026] Furthermore, the austenite grain size of the high-strength, high-toughness and high-hardenability steel for gear shafts of the present disclosure remains at grade 5-8 after high-temperature carburizing heat treatment. Wherein, the conditions of the simulated high-temperature carburizing heat treatment test for detecting the austenite grain size may be: first the steel is heated to 1200°C with a holding time of 40 min, water cooled, then heated at a heating rate of 500-800°C per 40-60 min to 1000°C with a holding time of 4 h, water cooled and quenched.

[0027] Furthermore, the high-strength, high-toughness and high-hardenability steel for gear shafts of the present disclosure has a J9mm hardenability of 34-42 HRC.

[0028] Furthermore, after high-temperature carburizing heat treatment, the high-strength, high-toughness and high-hardenability steel for gear shafts of the present disclosure has a tensile strength R m of ≥ 1100 MPa, a yield strength R p0.2 of ≥ 980 MPa, an elongation after fracture A of ≥ 12%, a reduction of area of ≥ 50%, and a Charpy impact energy A ku of ≥ 55 J. The conditions of the simulated high-temperature carburizing heat treatment test for detecting mechanical performance may be: heating at 880±10°C with a holding time of 90 min and oil quenching + heating at 870±10°C with a holding time of 90 min and oil quenching + heating at 200±10°C with a holding time of 150 min, tempering and air cooling.

[0029] Furthermore, after high-temperature carburizing heat treatment, the high-strength, high-toughness and high-hardenability steel for gear shafts of the present disclosure may have a tensile strength R m of 1100-1400 MPa.

[0030] Furthermore, after high-temperature carburizing heat treatment, the high-strength, high-toughness and high-hardenability steel for gear shafts of the present disclosure may have a yield strength R p0.2 of 980 MPa-1250 MPa.

[0031] Furthermore, after high-temperature carburizing heat treatment, the high-strength, high-toughness and high-hardenability steel for gear shafts of the present disclosure may have an elongation after fracture A of 12-15%.

[0032] Furthermore, after high-temperature carburizing heat treatment, the high-strength, high-toughness and high-hardenability steel for gear shafts of the present disclosure may have a reduction of area of 50-62%.

[0033] Furthermore, after high-temperature carburizing heat treatment, the high-strength, high-toughness and high-hardenability steel for gear shafts of the present disclosure may have a Charpy impact energy A ku of 55-130 J.

[0034] Accordingly, another object of the present disclosure is to provide a manufacturing method for the high-strength, high-toughness and high-hardenability steel for gear shafts. This manufacturing method is simple in production, and the high-strength, high-toughness and high-hardenability steel for gear shafts obtained has the characteristics of high hardenability and high-strength, high-toughness.

[0035] To achieve the above object, the present disclosure further provides a manufacturing method for the high-strength, high-toughness and high-hardenability steel for gear shafts as described above, comprising the steps: (1) Smelting; (2) Casting; (3) Heating: a steel slab is firstly heated to 700°C or less in a preheating stage, and then heated to 980°C or less in a first heating stage and held, then heated to 950-1200°C in a second heating stage and held, then fed into a soaking stage with a soaking temperature of 1050-1250°C; Or, a steel slab is slowly heated at a speed of 50-300°C / h, firstly heated to 700°C or less in a preheating stage, then heated to 980°C or less in a first heating stag, then heated to 950-1200°C in a second heating stage, then fed into a soaking stage with a soaking temperature of 1050-1250°C; (4) Forging or rolling.

[0036] Compared with the prior art, in the manufacturing method of the present disclosure, the heating step adopts a unique process, wherein a soaking temperature is relatively high. This is because a relatively high soaking temperature may improve the composition uniformity and structural uniformity of a continuous casting slab during a diffusion process of heating the steel slab. At the same time, the precipitates have a faster dissolution rate at this temperature. Therefore, a high heating temperature helps to have more dissolution of original undissolved precipitate particles in the steel, increase the concentration of micro-alloying elements in the matrix, have more and more dispersed particles precipitated during subsequent cooling. Furthermore, only after a heating temperature is increased, the final rolling or final forging temperature may be increased, thereby resulting in the recrystallization of austenite after rolling more sufficient and the distribution of precipitates more uniform.

[0037] In the manufacturing method of the present disclosure, in step (1), an electric furnace or converter may be used for smelting, and refining and vacuum treatment are carried out. Of course, in some other embodiments, a vacuum induction furnace may also be used for smelting.

[0038] Furthermore, in step (1), a furnace charge for electric furnace smelting may be selected from low P, S scrap steel, crop ends and high-quality pig iron. Alloys may be selected from ferrochrome, lowphosphorus ferromanganese, ferromolybdenum and so on. Reducing agents may include: calcium carbide, carbon powder, and aluminum powder. During the oxidation period: slagging frequently for removing P; the conditions of slagging may be controlled as follow: a slagging temperature is 1630-1660°C; and P ≤ 0.015%. The tapping conditions may be controlled as follow: a tapping temperature is 1630-1650°C; [P] ≤ 0.010%, and [C] ≥ 0.03%.

[0039] After the completion of electric furnace smelting or converter smelting, molten steel refining may be carried out on a ladle refining furnace to remove harmful gases and inclusions in the steel. A ladle seating is controlled, temperature measurement and analysis is performed, and an argon pressure may be adjusted as needed. Initial deoxidation of a Ladle Furnace (LF) may be achieved by feeding Al, then alloy blocks may be added and stirred for 5-10 minutes. When a temperature of molten steel is measured to be T = 1650-1670°C, vacuum degassing may be performed to ensure [0] ≤ 0.0020%, and [H] ≤ 0.00015%. In a specific embodiment, a vacuum degree for the vacuum degassing may be controlled at ≤ 66.7 Pa, and kept for 15 minutes or more.

[0040] Additionally, in step (1), a temperature of a crane ladle may be controlled at 1550-1570°C. Since a temperature of the crane ladle is reduced, element diffusion is accelerated, which is conductive to further reducing dendritic segregation.

[0041] Furthermore, in step (2), casting may be carried out by ingot casting or continuous casting. During continuous casting and pouring process, a high-temperature molten steel in the steel ladle is poured into a tundish through a protective sleeve; wherein a superheat degree of tundish may be controlled at 20-40°C. A tundish is completely cleaned before use, and the inner surface of the tundish is coated with a refractory coating and should not have cracks. The molten steel in a tundish passes through a continuous casting crystallizer with sufficient electromagnetic stirring, so that a qualified continuous casting slab having a cross-sectional dimension of 140 mm×140 mm ~ 320 mm×425 mm may be obtained.

[0042] In step (2), a pouring rate may be controlled at 0.6-2.1m / min according to different sizes of square slabs. Then, a continuous casting slab is slowly cooled in a slow cooling pit, wherein a slow cooling time may be 24 hours or more.

[0043] Furthermore, in step (3) of the manufacturing method of the present disclosure, a heating temperature of the preheating stage may be 600-700°C, and a temperature of the first heating stage may be 900-980°C.

[0044] Furthermore, in step (3) of the manufacturing method of the present disclosure, holding for a period of time in the soaking stage is required, and a holding time of the soaking stage may be 3-12 h.

[0045] In step (3) of the manufacturing method of the present disclosure, after heating is performed in the first heating stage, holding may or may not be performed, then heating is performed in the second heating stage. A holding time of the first heating stage may be 0-3h, for example, 0.5h, 1h, 2h. In step (3) of the manufacturing method of the present disclosure, after heating is performed in the second heating stage, holding may or may not be performed, then a soaking stage is performed. A holding time of the second heating stage may be 0-3h, for example, 0.5h, 1h and 2h.

[0046] In some other embodiments, in step (3) of the manufacturing method of the present disclosure, heating is slowly performed at a rate of 50-300°C / h. First heating is performed in the preheating stage to 700°C or less, then heating is performed to 980°C or less in the first heating stage without holding, then heating is performed to 950-1200°C in the second heating stage without holding, then the soaking stage is performed. This process may be carried out in a walking beam heating furnace.

[0047] Further, in step (4) of the manufacturing method of the present disclosure, a start forging temperature or a start rolling temperature may be controlled at 1050-1250°C, and a finish forging temperature or a finish rolling temperature may be controlled at ≥ 900°C. In some embodiments, in step (4) of the manufacturing method of the present disclosure, a finish forging temperature or a finish rolling temperature is 900-1000°C.

[0048] In this embodiment, a start forging temperature or a start rolling temperature is controlled at 1050-1250°C, and a finish forging temperature or a finish rolling temperature is controlled at ≥ 900°C. It is because: this process is further conducive for N to dissolve from the γ solid solution and combine with micro-alloying elements in the steel to form nitrides.

[0049] It should be noted that, N has less solubility in α-Fe than in γ-Fe, and due to the excitation of phase transformation, two peaks of the precipitation appear. When a finish forging or finish rolling temperature is low, peaks of precipitates will occur, resulting in uneven distribution of precipitates and insufficient recrystallization, and anisotropy in the microstructure. Therefore, a finish forging or finish rolling temperature is controlled at ≥ 900°C. Additionally, increasing a finish forging or finish rolling temperature will result in finer grains obtained. Fine grains increase the difference between an average grain diameter of ferrite after transformation of supercooled austenite and a band spacing of manganese-rich bands, reduce the tendency of pearlite formation in manganese-rich bands, thereby reducing banded structures.

[0050] In the above technical solution, in step (4) of the manufacturing method of the present disclosure, after a steel slab is discharged from a furnace, high-pressure water descaling may be carried out to remove the oxide scale.

[0051] Furthermore, in step (4) of the manufacturing method of the present disclosure, when forging is performed, a steel slab may be directly forged to a final finished product. When rolling is performed, a steel slab may be directly rolled to a final finished product, or a steel slab may be first rolled to a specified intermediate slab, then heated and rolled to a final finished product.

[0052] In some embodiments, in step (4) of the manufacturing method of the present disclosure, a steel slab is first rolled into an intermediate slab ( may having a size of 140mm×140mm to 260mm×260mm), a finish rolling temperature of the intermediate slab is controlled at 1000-1050°C; then the intermediate slab is heated according to the following process: the intermediate slab is firstly heated to 680-700°C in a preheating stage, then heated to 1050-1100°C in a first heating stage, then heated to 1200-1220°C in a second heating stage with a heating rate of 300-500°C / h; then enters a soaking stage with a soaking temperature of 1200-1250°C, and with a holding time of 3-5 h in the soaking stage; then the heated intermediate slab is rolled into a finished product, and a finish rolling temperature of the finished product is controlled at≥ 900°C (for example, 900-1000°C).

[0053] Compared with the prior art, the high-strength, high-toughness and high-hardenability steel for gear shafts and its manufacturing method of the present disclosure have the following advantages and beneficial effects: (1) Through reasonable chemical composition design combined with optimized process, a steel for gear shafts with high hardenability can be obtained in the present disclosure. The bars rolled or forged by the high-strength, high-toughness and high-hardenability steel for gear shafts can be effectively processed into gears, and may have excellent strength and toughness by subsequent high-temperature carburizing heat treatment. (2) The high-strength, high-toughness and high-hardenability steel for gear shafts of the present disclosure controls micro-alloying element coefficient and the content of nitrogen element, strictly controls the atomic molar ratio, and simultaneously adds an appropriate amount of Nb element, so that the abnormal growth of austenite grains is hindered, the austenite grain coarsening temperature of the gear steel is increased, the grain size of the gear steel remains stable at grade 5-8 even after high-temperature carburizing at up to 1000°C for 4 hours, and various performances achieve the usage performance indicators of the steel for gear shafts. (3) The composition and process design of the high-strength, high-toughness and high-hardenability steel for gear shafts of the present disclosure are reasonable. By controlling the content of micro-alloying elements in the steel, the appearance of large harmful inclusions in the steel is avoided, the stable production quality of the steel is ensured, the production cost of the steel is reduced, and mass production on a bar production line is achieved. (4) The hardenability, austenite grain size, and cost competitiveness of the high-strength, high-toughness and high-hardenability steel for gear shafts of the present disclosure are superior to those in the existing technology. The types and quantities of alloy elements in the steel can be controlled under the premise of ensuring the performance of high hardenability, narrow bandwidth and so on, and the applicability of the steel is improved. (5) When the high-strength, high-toughness and high-hardenability steel for gear shafts of the present disclosure is subsequently used to produce gear shafts, it can shorten the carburizing time, reduce the production cost of gear shafts, and has broad industrial application prospects. Detailed Description

[0054] The high-strength, high-toughness and high-hardenability steel for gear shafts and its manufacturing method of the present disclosure will be further explained and illustrated with reference to the specific examples. Nonetheless, the explanation and illustration are not intended to unduly limit the technical solution of the disclosure.

[0055] The high-strength, high-toughness and high-hardenability steel for gear shafts of Examples 1-8 were all made using the following steps: (1) Smelting and casting according to the chemical composition shown in Tables 1-1 and 1-2 below: Smelting can be carried out using a 50kg vacuum induction furnace, a 150kg vacuum induction furnace, or a 500kg vacuum induction furnace, or by electric furnace smelting + external refining + vacuum degassing, or by converter smelting + external refining + vacuum degassing. (2) Casting. (3) Heating: A steel slab was first heated to 700°C or less in a preheating stage, then heated to 980°C or less in a first heating stage and held, then heated to 950-1200°C in a second heating stage and held, then fed into a soaking stage with a soaking temperature of 1050-1250°C and held, then subsequently rolled or forged. (4) Forging or rolling: A start forging or start rolling temperature was controlled at 1050-1250°C, and a finish forging or finish rolling temperature was controlled at ≥ 900°C.

[0056] Furthermore, the specific process procedures for the high-hardenability steel for gear shafts in Examples 1-8 and Comparative Examples 1-4 were as follows: Example 1: Smelting was carried out on a 50kg vacuum induction furnace according to the chemical composition shown in Table 1-1 and 1-2 below. The molten steel was cast into a steel ingot, and then heated and breakdown forged. The steel ingot was first heated to 700°C in the preheating stage, then continued to be heated to 900°C in the first heating stage with a holding time of 0h, then continued to be heated to 950°C in the second heating stage with a holding time of 1h, then entered the soaking stage with a soaking temperature of 1050°C and a holding time of 3h, then subsequently forged, wherein a finish forging temperature was controlled to be 910°C, and it was finally forged into a bar with Φ60mm. Example 2: Smelting was carried out on a 150kg vacuum induction furnace according to the chemical composition shown in Tables 1-1 and 1-2 below. The molten steel was cast into a steel ingot, and then heated and breakdown forged. The steel ingot was first heated to 650°C in the preheating stage, then continued to be heated to 950°C in the first heating stage with a holding time of 0.5h, then continued to be heated to 1100°C in the second heating stage with a holding time of 0h, then entered the soaking stage with a soaking temperature of 1200°C and a holding time of 5h, then subsequently forged, wherein a finish forging temperature was controlled to be 1000°C, and it was finally forged into a bar with Φ90m. Example 3: Smelting was carried out on a 500kg vacuum induction furnace according to the chemical composition shown in Tables 1-1 and 1-2 below. The molten steel was cast into a steel ingot, and then heated and breakdown forged. The steel ingot was first heated to 600°C in the preheating stage, then continued to be heated to 980°C in the first heating stage with a holding time of 3 h, then continued to be heated to 1200°C in the second heating stage with a holding time of 3 h, then entered the soaking stage with a soaking temperature of 1250°C and a holding time of 12 h, then subsequently forged, wherein a finish forging temperature was controlled to be 1000°C, and it was finally forged into a bar with Φ120mm. Example 4: Electric furnace smelting was carried out according to the chemical composition shown in Tables 1-1 and 1-2, and refining and vacuum treatment were carried out, then a continuous casting slab of 280mm×280mm was cast. The continuous casting slab was heated slowly at a rate of 300°C / h in a walking beam heating furnace: first heated to 620°C in the preheating stage, then continued to be heated to 950°C in the first heating stage, continued to be heated to 1150°C in the second heating stage, then entered the soaking stage with a soaking temperature of 1200°C and a holding time of 4 h, then rolled. After discharged from the heating furnace, the slab was descaled with high-pressure water and then rolled, wherein a finish rolling temperature was controlled to be 970°C, and it was finally rolled into a bar with Φ80mm. Example 5: Electric furnace smelting was carried out according to the chemical composition shown in Tables 1-1 and 1-2, and refining and vacuum treatment were carried out, and then a continuous casting slab with 320mm×425mm was cast. The continuous casting slab was heated slowly at a rate of 150°C / h in a walking beam heating furnace, first heated to 600°C in the preheating stage, then continued to be heated to 950°C in the first heating stage, continued to be heated to 1200°C in the second heating stage, then entered the soaking stage with a soaking temperature of 1230°C and a holding time of 4.5 h, and then rolled. After discharged from the heating furnace, the casting slab was descaled with high-pressure water and then rolled into an intermediate slab, wherein a first finish rolling temperature (a finish rolling temperature of the intermediate slab) was controlled to be 1050°C, and the intermediate slab had a size of 220mm×220mm. Then the intermediate slab was heated slowly at a rate of 400°C / h in a walking beam heating furnace, first heated to 680°C in the preheating stage, then continued to be heated to 1050°C in the first heating stage, continued to be heated to 1200°C in the second heating stage, hold, then entered the soaking stage with a soaking temperature of 1220°C and a holding time of 3.5h. After discharged from the furnace, the slab was descaled with high-pressure water and then rolled, wherein a second finish rolling temperature (a finish rolling temperature of the finished product) is controlled to be 950°C, and the sab was finally rolled into a finished bar with Φ90mm. Example 6: Electric furnace smelting was carried out according to the chemical composition shown in Tables 1-1 and 1-2, and refining and vacuum treatment were carried out, then a continuous casting slab with 280mm×280mm was cast. The continuous casting slab was heated slowly at a rate of 300°C / h in a walking beam heating furnace, first heated to 680°C in the preheating stage, then continued to be heated to 900°C in the first heating stage, continued to be heated to 1180°C in the second heating stage, then entered the soaking stage with a soaking temperature of 1200°C and with a holding time of 4.5h, then subsequently rolled. After discharged from a heating furnace, the slab was descaled with high-pressure water and then rolled into an intermediate slab, wherein a first finish rolling temperature (a finish rolling temperature of the intermediate slab) was controlled to be 1000°C, and the intermediate slab had a size of 140mm×140mm. Then the intermediate slab was heated slowly at a rate of 500°C / h in a walking beam heating furnace, first heated to 700°C, then continued to be heated to 1100°C in the first heating stage, continued to be heated to 1200°C in the second heating stage, entered the soaking stage with a soaking temperature of 1220°C and a holding time of 3.5h. After discharged from the furnace, the slab was descaled with high-pressure water and then rolled, wherein a second finish rolling temperature (a finish rolling temperature of the finished product) is controlled to be 920°C, and it was finally rolled into a finished bar with Φ25mm. Example 7: Converter smelting was carried out according to the chemical composition shown in Tables 1-1 and 1-2, and refining and vacuum treatment were carried out, and then an ingot casting slab was cast. The casting slab was heated slowly at a rate of 50°C / h in a walking beam heating furnace, first heated to 620°C in the preheating stage, then continued to be heated to 950°C in the first heating stage, continued to be heated to 1150°C in the second heating stage, then entered the soaking stage with a soaking temperature of 1200°C and a holding time of 8 h, and then rolled. After discharged from the heating furnace, the slab was descaled with high-pressure water and then rolled, wherein a finish rolling temperature was controlled to be 970°C, and it was finally rolled into a bar with Φ90mm. Example 8: Converter smelting was carried out according to the chemical composition shown in Tables 1-1 and 1-2, and refining and vacuum treatment were carried out, and then an ingot casting slab was cast. The casting slab was heated slowly at a rate of 100°C / h in a walking beam heating furnace, first heated to 600°C in the preheating stage, then continued to be heated to 950°C in the first heating stage, continued to be heated to 1200°C in the second heating stage, then entered the soaking stage with a soaking temperature of 1230°C and a holding time of 7 h, and then rolled. After discharged from a heating furnace, the casting slab was descaled with high-pressure water and then rolled into an intermediate slab, wherein a first finish rolling temperature (a finish rolling temperature of the intermediate slab) was controlled to be 1050°C, and the intermediate slab had a size of 260mm×260mm. Then the intermediate slab was heated slowly at a rate of 300°C / h in a walking beam heating furnace, first heated to 680°C in the preheating stage, then continued to be heated to 1050°C in the first heating stage, continued to be heated to 1200°C in the second heating stage, then entered the soaking stage with a soaking temperature of 1220°C and a holding time of 5h, and then discharged from the furnace. After discharged from the furnace, the intermediate slab was descaled with high-pressure water and then rolled, wherein a second finish rolling temperature (a finish rolling temperature of the finished product) is controlled to be 950°C, and it was finally rolled into a finished bar with Φ60mm.

[0057] Steels in Comparative examples 1 and 2 are from commercial materials. These steels were made by electric furnace smelting and refining processes to ensure the purity of the commercial materials.

[0058] Comparative Example 3: The steel was smelted in a 50 kg vacuum induction furnace according to the chemical composition shown in Tables 1-1 and 1-2. The molten steel was cast into a steel ingot, and then heated and breakdown forged. The ingot is heated at a rate of 300°C / h to 1100°C in a box-type furnace with a holding time of 3h, and then subsequently forged, wherein a finish forging temperature was controlled to be 910°C, and it was finally forged into a bar with a diameter of Φ60 mm.

[0059] Comparative Example 4: The steel was smelted in an electric furnace according to the chemical composition shown in Tables 1-1 and 1-2, and refining and vacuum treatment were carried out, and then a continuous casting slab of 320mm×425mm was cast. The continuous casting slab was heated slowly at a rate of 150°C / h in a walking beam heating furnace, first heated to 600°C in the preheating stage, then heated to 950°C in the first heating stage, continued to be heated to 1200°C in the second heating stage, and then entered the soaking stage with a soaking temperature of 1230°C and a holding time of 4.5 hours; then subsequent rolling was carried out. After discharged from a heating furnace, the continuous casting slab was descaled with high-pressure water and then rolled into an intermediate slab, wherein the first finish rolling temperature (a finish rolling temperature for the intermediate slab) was controlled to be 1050°C, and the size of the intermediate billet slab was 220 mm × 220 mm. Then the intermediate slab was heated slowly at a rate of 400°C / h in a walking beam heating furnace: heated to 680°C in the preheating stage, then heated to 1050°C in the first heating stage, continued to be heated to 1200°C in the second heating stage, and entered the soaking stage with a soaking temperature of 1220°C and a holding time of 6h, and then rolled. After discharged from the heating furnace, the intermediate slab was descaled with high-pressure water and then rolled into a finished bar with a size of Φ90 mm, wherein the second finish rolling temperature (a finish rolling temperature for the finished product) is controlled to be 950°C.

[0060] Tables 1-1 and 1-2 list the mass percentages of the chemical elements for the high-hardenability steel for gear shafts of Examples 1-8 and the comparative steels of Comparative Examples 1-4. Table 1-1 (wt%, a balance of Fe and other unavoidable impurities except P, O, H and Ca)NoCSiMnCrAlTiNbNBEx.10.160.261.161.110.020.0330.0030.0060.0009Ex.20.220.100.991.260.0320.0340.0340.0150.0006Ex.30.190.400.931.440.0420.0150.0250.0120.0022Ex.40.170.291.040.950.0290.0230.0010.0110.0015Ex.50.190.180.861.020.0250.0340.0130.010.0026Ex.60.200.221.091.120.0350.0190.0070.0060.0034Ex.70.180.311.241.410.050.0270.0180.0130.0012Ex.80.210.230.971.340.0370.0390.0040.0120.0013Com.Ex.10.190.250.891.130.0240.013 0 0.0090.0037 Com.Ex.20.190.210.82 1.030.0250.0370.0070.0120.0007Com.Ex.30.200.230.871.110.0210.0080.0010.0080.0002Com.Ex.40.220.270.971.120.0230.043 0.0030.0120.0008 Table 1-2 (wt%, a balance of Fe and other unavoidable impurities except P, O, H and Ca) NoSNiMoCuVPOCaHEx.10.040.050.0100.0060.0180.0020.00130.00022.64Ex.20.0090.250.10.080.030.0070.00120.0020.00024.58Ex.30.0290.030.060.200.0180.0280.00150.00150.00024.99Ex.40.01200000.0030.00190.00310.00011.56Ex.50.0030.080.0200.0240.0050.00110.00130.00013.35Ex.60.034000.080.0160.0080.0020.00230.00014.87Ex.70.0180.0700.030.0050.0130.00080.00080.00014.14Ex.80.0010.160.030.1300.0090.00110.00340.00012.20Com.Ex.10.0130.210.070.040.0020.0070.00130.00130.00021.42 Com.Ex.20.0030.0200.030.0030.0060.00130.00130.00022.05Com.Ex.30.0020.020.020.0100.0070.00080.00140.00021.58Com.Ex.40.0210.060.010.050.0060.0080.00190.00110.00021.47 Note: r M / N =(10*[Nb] / 93+[Ti] / 480+[Al] / 27) / ([N] / 14), wherein each chemical element in the formula is substituted with a numerical value before the percent sign of its mass percentage content.

[0061] Table 2 lists the specific process parameters of the high-strength, high-toughness and high-hardenability steel for gear shafts of Examples 1-8 and Comparative steels of Comparative Examples 1-4. Table 2NoStep (1)Step (3)Step (4)Size of finished bar (mm)Method of SmeltingTemperature of preheating stage (°C)Temperature of First heating stage (°C)Temperature of Second heating stage (°C)Soaking temperature (°C)Holding time in soaking stage (h)Start For ging or start rolling temperature (°C)Finish forging or finish rolling temperature of finished product (°C)Interme diate slab size (mm)Heating process of Intermediate slabFinish forging or finish rolling temperature of Intermediate slabEx.1Smelting on a 50kg vacuum induction furnace700900950105031050910 / / / Φ60Ex.2Smelting on 150kg vacuum induction furnace65095011001200512001000 / / / Φ90Ex.3Smelting on 500kg vacuum induction furnace600980120012501212501000 / / / Φ120Ex.4Electric furnace smelting+ refining6209501150120041200970 / / / Φ80Ex.5Electric furnace smelting+ refining600950120012304.51230950220×22 0680°C in the preheating stage +1050°C in the first heating stage +1200°C in the second heating stage +1220°C in the soaking stage with a holding time of 3.5h1050Φ90Ex.6Electric furnace smelting+ refining680900118012004.51200920140x14 0700°C in the preheating stage +1100°C in the first heating stage +1200°C in the second heating stage +1220°C in the soaking stage with a holding time of 3.5h1000Φ25Ex.7Converter smelting+ refining6209501150120081200970 / / / Φ90Ex.8Converter smelting+ refining6009501200123071230950260×26 0680°C in the preheating stage +1050°C in the first heating stage +1200°C in the second heating stage +1220°C in the soaking stage with a holding time of 5h1050Φ60Com. Ex.1Electric furnace smelting+ refining / / / / / / / / / / Φ60Com. Ex.2Electric furnace smelting+ refining / / / / / / / / / / Φ90Com. Ex.3Smelting on a 50kg vacuum induction furnace / / / 110031100910 / / / Φ60Com. Ex.4Electric furnace Smelting+ refining600950120012304.51230950220×22 0680°C in the preheating stage +1050°C in the first heating stage +1200°C in the second heating stage +1220°C in the soaking stage with a holding time of 6h1050Φ90

[0062] As can be seen form Table 2 that, in Example 5, Example 6, Example 8 and Comparative Example 4, steel slabs were all initially rolled into an intermediate slab with a specified size, and then heated and rolled again to a final product.

[0063] To verify the performance of the high-strength, high-toughness and high-hardenability steel for gear shafts of the present disclosure, the high-strength, high-toughness and high-hardenability steels for gear shafts obtained in Examples 1-8 and comparative steels obtained in Comparative examples 1-4 were respectively sampled and subjected to a simulated high-temperature carburizing heat treatment testing for detecting the mechanical performance and austenite grain size. The test results were shown in Table 3. Wherein: A simulated high-temperature carburizing heat treatment testing for detecting austenite grain size: a sample was heated to 1200°C with a holding time of 40min, water cooled, and then heated to 1000°C at a rate of 600°C per 50min and with a holding time of 4h, and then water quenched.

[0064] Austenite grain size test: austenite grain size was evaluated according to Standard ASTM E112.

[0065] Mechanical performance test: samples were prepared according to GB / T 2975-2018 (Steel and steel products - Location and preparation of samples for mechanical performance testing)." According to GB / T 3077-2015, blank samples with Φ15 mm were made for the simulated high-temperature carburizing heat treatment testing for detecting the mechanical performance: blank samples were heated at 880°C with a holding time of 90min, and oil quenched, then heated at 870°C with a holding time of 90 min, and oil quenched, and then heated at 200°C with a holding time of 15 min, tempered and air cooled. Tensile testing was conducted according to GB / T 228.1-2010 "Metallic materials - Tensile testing - Part 1: Method of test at room temperature,", and a tensile strength (R m ), a yield strength (R p0.2 ), an elongation after fracture (A), and a reduction of area (Z) were tested. Additionally, the Charpy impact energy (A ku ) of each examples and comparative examples at room temperature was tested according to GB / T 229-2007 "Metallic materials - Charpy pendulum impact test method."

[0066] Furthermore, the high-strength, high-toughness and high-hardenability steel for gear shafts obtained in Examples 1-8 and comparative steels obtained in Comparative Examples 1-4 were sampled to test their hardenability and hardness. The test results are also listed in Table 3. Wherein: Hardenability Testing: each steel of examples and comparative example steels were sampled and prepared from the hot-rolled round bars in accordance with Chinese Standard GB / T 225. End-quench hardenability testing (Jominy test) was conducted in accordance with GB / T 5216, with a normalizing temperature controlled of 920 ± 10°C and a quenching temperature controlled of 870 ± 5°C. Rockwell hardness testing was conducted according to GB / T 230.2 to obtain a hardness value (HRC) at a specific position, such as a hardness at 9 mm from the quenched end, J9mm.

[0067] Table 3 lists the test results for the high-strength, high-toughness and high-hardenability steels for gear shafts of Examples 1-8 and the comparative steels of Comparative Examples 1-4. Table 3No.Yield strength R p0.2 , MPaTensile strength R m , MPaElongation after fracture A, %Reduction of area Z, %Impact energy A ku, JJ9mm hardenability, HRC1000°C*4h austenite grain size, gradeEx.11009115813.561116 / 123386.5Ex.212101364126059 / 72426Ex.31094125314.560115 / 124408Ex.4100511031459101 / 112345.5Ex.51015112613.561103 / 108357Ex.610431118145590 / 106408Ex.710181179125194 / 102367.5Ex.81199133912.55265 / 67416.5Com.Ex.110471210145992 / 80390Com.Ex.2963 1058 13.55890 / 9633 5Com.Ex.3105411701355113 / 10529 5 (0)Com.Ex.41235132313.56142 / 53 400 (4)Note: The impact energy values for each Example and Comparative Example, presented as two numerical values separated by a slash, represent the results of two separate measurements.

[0068] As can be seen from Table3, after the simulated high-temperature carburizing heat treatment at 1000°C, the austenite grain size of the high-strength, high-toughness and high-hardenability steel for gear shafts of Examples 1-8 of the present disclosure are maintained within the range of Grade 5~8. No phenomena such as mixed crystals or abnormal coarse grains was observed. Therefore, the steels have excellent high-temperature grain stability.

[0069] Furthermore, it can be seen from Table 3, the hardenability at the representative J9mm position for the high-strength, high-toughness and high-hardenability steels for gear shafts of Examples 1-8 is 34~42 HRC, having high hardenability and a narrow hardenability band. In addition, after the simulated high-temperature carburizing heat treatment, steels of each examples have a tensile strength R m of greater than 1100 MPa, a yield strength R p0.2 of greater than 980 MPa, an elongation after fracture A of ≥12%, a reduction of area of greater than 50%, and a Charpy impact energy A ku of greater than 55 J.

[0070] It can be concluded that, the bars rolled or forged by the high-strength, high-toughness and high-hardenability steel for gear shafts can be effectively processed into gear shafts. After undergoing subsequent high-temperature carburizing heat treatment, they have high strength and toughness, so that they are suitable for high-end parts such as automotive gearboxes, reducers for new energy vehicles, and industrial speed reducers, and have good application prospects and significant value.

[0071] In contrast to the various examples of the present disclosure, Comparative Example 1 has a grain size of Grade 0 after the simulated high-temperature carburizing testing, which means that the grains grow abnormally, and the service requirements cannot be met.

[0072] Although the comparative steel of Comparative Example 2 fails to have the phenomena of mixed crystals, it has fine grain size and low hardenability after the simulated high-temperature carburizing heat treatment testing, which does not satisfy the requirement for high hardenability. Furthermore, Comparative Example 2 has a relatively low strength.

[0073] For the comparative steel of Comparative Example 3, a mixed crystal is observed after the simulated high-temperature carburizing heat treatment at 1000°C, wherein "5 (0)" - representing an average grain size of Grade 5, and the grain size in some regions has coarsened to Grade 0.

[0074] For the comparative steel of Comparative Example 4, a mixed crystal (Grade 4) is also observed after the simulated high-temperature carburizing heat treatment at 1000°C, wherein "0 (4)" - representing an average grain size of Grade 0, and the grain size in some regions has coarsened to Grade 4. Additionally, Comparative Example 4 also has a relatively low impact energy.

[0075] In addition, the ways in which the various technical features of the present disclosure are combined are not limited to the ways recited in the claims of the present disclosure or the ways described in the specific examples. All the technical features recited in the present disclosure may be combined or integrated freely in any manner, unless contradictions are resulted.

[0076] It should also be noted that the Examples set forth above are only specific examples according to the present disclosure. Obviously, the present disclosure is not limited to the above Examples. Similar variations or modifications made thereto can be directly derived or easily contemplated from the present disclosure by those skilled in the art. They all fall in the protection scope of the present disclosure.

Examples

example 1

Smelting was carried out on a 50kg vacuum induction furnace according to the chemical composition shown in Table 1-1 and 1-2 below. The molten steel was cast into a steel ingot, and then heated and breakdown forged. The steel ingot was first heated to 700°C in the preheating stage, then continued to be heated to 900°C in the first heating stage with a holding time of 0h, then continued to be heated to 950°C in the second heating stage with a holding time of 1h, then entered the soaking stage with a soaking temperature of 1050°C and a holding time of 3h, then subsequently forged, wherein a finish forging temperature was controlled to be 910°C, and it was finally forged into a bar with Φ60mm. Example 2: Smelting was carried out on a 150kg vacuum induction furnace according to the chemical composition shown in Tables 1-1 and 1-2 below. The molten steel was cast into a steel ingot, and then heated and breakdown forged. The steel ingot was first heated to 650°C in the preheating stage, t...

Technical field

[0001] The present disclosure relates to a high-strength steel and a manufacturing method therefor, in particular to a steel for gear shafts and a manufacturing method therefor.Background Art

[0002] With the development of the automobile industry, the requirements of automotive parts have been increased continuously. One of the important development directions is to produce gears with high strength-toughness, high fatigue life, high-temperature stability and economic efficiency.

[0003] Correspondingly, high strength-toughness steel for gear shafts can meet the high technical requirements of automobile light-weighting for materials. Furthermore, to ensure the quenching strength of gears, it is usually necessary to impose the requirement on the hardenability of a steel for gear shafts.

[0004] The main technical problem of high-hardenability MnCr-based carburized gear steel is how to avoid dimensional dispersion of gears caused by excessive hardenability bandwidth while improving the hardenability, and simultaneously ensure that the phenomenon of mixed crystal and coarse grains in gears does not appear after high-temperature carburizing.

[0005] For example, the Chinese patent document with publication number CN101096742A, published on January 2, 2008, and titled "A High-Strength Gear Steel for Automobiles", discloses a high-strength gear steel for automobiles. Alloy elements such as Nb, V, and Al are compositely added to the steel to refine original austenite grains. The steel comprises the following composition in percentage by mass: 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, and Al: 0.015-0.050, with a balance of Fe and unavoidable impurities. The grain size, hardenability, and hardenability bandwidth of the gear steel are improved by adding a trace amount of Nb and V.

[0006] For another example, the Chinese patent document with publication number CN103361559A, published on October 23, 2013, entitled "A Nb and Ti composite micro-alloyed high-temperature carburized gear steel", discloses a Nb and Ti composite micro-alloyed high-temperature carburized gear steel. The steel comprises the following composition: C: 0.17-0.22%, Si: 0.20-0.35%, Mn: 0.9-1.10%, P: ≤0.025%, S: 0.020-0.035%, Cr: 1.05-1.30%, Al: 0.015-0.035%, Ti: 0.02-0.06%, and Nb: 0.02-0.06%, with a balance of iron and unavoidable impurities. By controlling the content of micro-alloyed elements such as Nb, Ti, and Al, a carburizing temperature of a gear is increased or a carburizing time of a gear is reduced.

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

[0008] One of the objects of the present disclosure is to provide a high-strength, high-toughness and high-hardenability steel for gear shafts. By optimizing the composition of a steel for gear shafts, especially by reasonably controlling the content of micro-alloying elements and nitrogen in a gear steel, a high-strength steel for gear shafts with high hardenability, narrow hardenability bandwidth, and good high-temperature grain stability can be obtained at a relatively low cost.

[0009] To achieve the above object, the present disclosure provides a high-strength, high-toughness and high-hardenability steel for gear shafts, comprising Fe and unavoidable impurities, and further comprising the following chemical elements in 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%, and B: 0.0006-0.0034%; wherein a value of the micro-alloying element coefficient rM / N is 1.5-5.0, wherein rM / N = (10*[Nb] / 93+[Ti] / 480+[Al] / 27) / ([N] / 14), wherein each chemical element is substituted with the numerical value before the percent sign of its mass percentage content.

[0010] Furthermore, in the high-strength, high-toughness and high-hardenability steel for gear shafts of the present disclosure, the mass percentages of each chemical elements are: C: 0.16-0.22%, Si: 0.10-0.40%, Mn: 0.86-1.24%, Cr: 0.95-1.44%, Al: 0.02-0.05%, Ti: 0.015-0.039%, Nb: 0.001-0.034%, N: 0.006-0.015%, and B: 0.0006-0.0034%; with a balance of Fe and unavoidable impurities; wherein the value of micro-alloying element coefficient rM / N is 1.5-5.0, wherein rM / N = (10*[Nb] / 93+[Ti] / 480+[Al] / 27) / ([N]1 / 4), wherein each chemical element is substituted with the numerical value before the percent sign of its mass percentage content.

[0011] The design principles of the various chemical elements in the high-strength, high-toughness and high-hardenability steel for gear shafts of the present disclosure will be described in detail as follows: C: In the high-strength, high-toughness and high-hardenability steel for gear shafts of the present disclosure, C is an essential component in the steel and is also one of the main elements affecting the hardenability of the steel. Both surface strength and sufficient core impact toughness are required in the high-strength, high-toughness and high-hardenability steel for gear shafts. When the content of C in the steel is too low, which is less than 0.16%, the strength of the steel is insufficient, and the requirement for good hardenability of the steel may not be achieved. Correspondingly, the content of C in the steel should not be too high. When the content of C in the steel is too high, the requirement for the toughness of the gear core may not be satisfied, and excessive content of C is not conducive to the plasticity of the steel, especially for a carburized gear steel having a high content of Mn. When the content of C is higher than 0.22%, it is not conducive to the processability performance of the steel. Therefore, in the high-strength, high-toughness and high-hardenability steel for gear shafts of the present disclosure, the mass percentage of C is controlled at 0.16-0.22%.

[0012] Si: In the high-strength, high-toughness and high-hardenability steel for gear shafts of the present disclosure, Si element may not only better eliminate the adverse effects of iron oxide on the steel, but may also be dissolved in the ferrite, strengthening the ferrite and improving the strength, hardness, wear resistance, elasticity, and elastic limit of the steel. At the same time, it should be noted that, Si element helps to increase a temperature Ac 3 of the steel, and make the steel have a risk of cracking and a tendency of decarburization due to its poor thermal conductivity. As such, considering the beneficial effects and adverse effects of Si in combination, in the high-strength, high-toughness and high-hardenability steel for gear shafts of the present disclosure, the mass percentage of Si is controlled at 0.10-0.40%.

[0013] Mn: In the high-strength, high-toughness and high-hardenability steel for gear shafts of the present disclosure, Mn is one of the main elements affecting the hardenability of the steel. Mn element has a good deoxidizing ability, which may reduce the iron oxide in the steel, and effectively increase the yield of the steel. Mn may be dissolved into ferrite, improve the strength and hardness of the steel, and make the steel have pearlite with finer lamellae and higher strength when the steel is cooled after hot rolling. Furthermore, Mn may also form MnS with S in the steel, which may eliminate the harmful effects of S. Mn has an ability to form and stabilize austenite structures in the steel, and may strongly improve the hardenability of the steel, and may also reduce the high-temperature toughness of the steel. When the content of Mn in the steel is too low, the hardenability of the steel is insufficient; when the content of Mn in the steel is too high, the thermoplasticity of the steel may be deteriorated, the yield will be affected, and the steel is prone to cracking during water quenching. Therefore, in the high-strength, high-toughness and high-hardenability steel for gear shafts of the present disclosure, the mass percentage of Mn is controlled at 0.86-1.24%.

[0014] Cr: In the high-strength, high-toughness and high-hardenability steel for gear shafts of the present disclosure, Cr is one of the main alloy elements added in the steel of the present disclosure. Cr may significantly improve the hardenability, strength, wear resistance, and other properties of the steel. Additionally, Cr may also reduce the activity of C element in the steel, and prevent decarburization during heating, rolling, and heat treatment process. However, when the content of Cr is too high, the toughness of quenched and tempered steel will be reduced significantly, and coarse carbides distributed along grain boundaries are formed. Therefore, in the high-strength, high-toughness and high-hardenability steel for gear shafts of the present disclosure, the mass percentage of Cr is controlled at 0.95-1.44%.

[0015] Al: In the high-strength, high-toughness and high-hardenability steel for gear shafts of the present disclosure, Al is an element for refining grains. The combination of Al element and N element may further refine the grains and improve the toughness of the steel. Grain refinement plays an important role in improving the mechanical performance of steel, especially the strength and toughness. Meanwhile, grain refinement also helps to reduce the hydrogen embrittlement sensitivity of the steel. However, it should be noted that the content of Al in the steel should not be too high. when the content of Al is too high, it tends to promote the formation of inclusions in the steel. Therefore, in the high-strength, high-toughness and high-hardenability steel for gear shafts of the present disclosure, the mass percentage of Al is controlled at 0.02-0.05%.

[0016] Ti: The addition of Ti may form fine precipitates in the steel. But when the content of Ti in the steel is too high, coarse TiN particles with edges and corners will be formed during the smelting process, thereby reducing the impact toughness of the steel. Therefore, in the high-strength, high-toughness and high-hardenability steel for gear shafts of the present disclosure, the content of Ti is controlled at 0.015-0.039%.

[0017] Nb: In the high-strength, high-toughness and high-hardenability steel for gear shafts of the present disclosure, the addition of Nb element in the steel may help to form fine precipitates, thereby inhibiting the recrystallization of the steel, and effectively refining the grain size. It should be noted that the content of Nb 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 the smelting process, which will reduce the impact toughness of the steel instead. Therefore, in the high-strength, high-toughness and high-hardenability steel for gear shafts of the present disclosure, the mass percentage of Nb is controlled at 0.001-0.034%.

[0018] N: In the high-strength, high-toughness and high-hardenability steel for gear shafts of the present disclosure, N is an interstitial atom that may combine with microalloys in the steel to form MN-type precipitates, which may pin grain boundaries at a high temperature, thereby inhibiting the growth of austenite grains. When the content of N in the steel is too low, the amount of MN formed is less, and the pinning effect is not significant; when the content of N in the steel is too high, the N element tends to be enriched in steel-making, thereby reducing the toughness of the steel. Therefore, in the high-strength, high-toughness and high-hardenability steel for gear shafts of the present disclosure, the mass percentage of N is controlled at 0.006-0.015%.

[0019] B: Boron helps to greatly improving the hardenability of steel, and the required content is small. Its effect is hundreds or even thousands of times that of general alloy elements, and have significant economic effects. Moreover, boron steel may be water-quenched, which saves quenching oil and tends to obtain martensite structure, thus making boron-containing steel have good strength and hardness. As long as the content of boron is appropriate, the production process is proper, and complete quenching is ensured, plasticity and toughness will not be significantly reduced. However, the B element is prone to segregation, which can cause significant fluctuations of the hardenability of the steel. Therefore, in the high-strength, high-toughness and high-hardenability steel for gear shafts of the present disclosure, the B content is controlled at 0.0006-0.0034%.

[0020] Additionally, it is important that in the high-strength, high-toughness and high-hardenability steel for gear shafts of the present disclosure, it is necessary to control the value of the micro-alloying element coefficient r M / N to be 1.5-5.0, wherein r M / N = (10*[Nb] / 93+[Ti] / 480+[Al] / 27) / ([N] / 14), wherein each chemical element is substituted with a numerical value before the percent sign of its mass percentage content.

[0021] In the present disclosure, Al, Nb, Ti, and N are all main grain refining elements. By controlling the content of Al, Nb, Ti, N, and the micro-alloying element coefficient r M / N in the gear steel, the microalloying elements combine with excess N to form precipitates, thereby inhibiting the growth of austenite grain at a high temperature.

[0022] Furthermore, in the high-strength, high-toughness and high-hardenability steel for gear shafts of the present disclosure, among the unavoidable impurities, the mass percentage of each impurity elements satisfies at least one of the following: P ≤ 0.030%, O ≤ 0.0020%, H ≤ 0.0002%, and Ca ≤ 0.0034%.

[0023] In the above technical solution, P, O, H, and Ca are all impurity elements in the steel. In order to obtain the steel with better performance and higher quality, the content of impurity elements in the steel should be reduced as much as possible if the technical conditions allow. Wherein: P: P is prone to segregate at 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 disclosure, the content of P may be controlled at P ≤ 0.030%. O: O tends to form oxides and complex oxides with Al element in the steel, destroying the continuity of the steel, reducing organizational uniformity, low-temperature impact energy and fatigue performance. As such, in some embodiments of the present disclosure, the content of O may be controlled at O ≤ 0.0020%. H: H may accumulate at defects in the steel, especially in the steel with a tensile strength grade of greater than1000 MPa, so that hydrogen-induced delayed fracture may occur. Therefore, in some embodiments of the present disclosure, the content of H may be controlled at H ≤ 0.0002%. Ca: In the high-strength, high-toughness and high-hardenability steel for gear shafts of the present disclosure, Ca element tends to forms inclusions, thereby affecting the fatigue performance of a final product. Therefore, the content of Ca element may be controlled at Ca ≤ 0.0034%, for example, ≤ 0.003%.

[0024] Furthermore, the high-strength, high-toughness and high-hardenability steel for gear shafts of the present disclosure further comprises at least one of the following chemical elements: 0 < S ≤ 0.04%, 0 < Ni ≤ 0.25%, 0 < Mo ≤ 0.10%, 0 < Cu ≤ 0.20%, and 0 < V ≤ 0.03%.

[0025] Optionally, in the high-strength, high-toughness and high-hardenability steel for gear shafts of the present disclosure, at least one of S, Ni, Mo, Cu, and V elements may be further added to further improve the performance of the high-strength, high-toughness and high-hardenability steel for gear shafts of the present disclosure. Wherein: S: S is generally present as an impurity element in the steel, which will reduce the plasticity and toughness of the steel. However, in the high-hardenability steel for gear shafts of the present disclosure, a certain content of S element may form non-metallic inclusions with Mn to improve the machinability of the steel. As such, in the high-hardenability steel for gear shafts of the present disclosure, the mass percentage of S is controlled at 0 < S ≤ 0.04%, for example, 0.001% ≤ S ≤ 0.04%. Ni: In the high-strength, high-toughness and high-hardenability steel for gear shafts of the present disclosure, Ni exists in the form of solid solution in the steel, which may effectively improve the low-temperature impact performance of the steel. However, it should be noted that when the content of Ni is too high, the steel may have excessive retained austenite, thereby reducing the strength of the steel. Therefore, in the high-strength, high-toughness and high-hardenability steel for gear shafts of the present disclosure, preferably, the mass percentage of Ni may be controlled at 0 < Ni ≤ 0.25%, for example, 0.03% ≤ Ni ≤ 0.25%. Mo: In the high-strength, high-toughness and high-hardenability steel for gear shafts of the present disclosure, Mo may be solid-dissolved in the steel, which is conducive to improving the hardenability and strength of the steel. When the steel is tempered at a relatively high temperature, Mo will also be formed as fine carbides to further improve the strength of the steel. The combination action of molybdenum and manganese may also significantly increase the stability of austenite. As such, in the high-strength, high-toughness and high-hardenability steel for gear shafts of the present disclosure, preferably, the mass percentage of Mo may be controlled at 0 < Mo ≤ 0.10%, for example, 0.01% ≤ Mo ≤ 0.10%. Cu: In the high-strength, high-toughness and high-hardenability steel for gear shafts of the present disclosure, Cu may improve the strength of the steel and is conducive to improving the weather resistance and corrosion resistance of the steel. However, the content of Cu 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 boundaries during the heating process, resulting in weakening of grain boundary and even cracking. Therefore, in the high-strength, high-toughness and high-hardenability steel for gear shafts of the present disclosure, preferably, the mass percentage of Cu may be controlled at 0 < Cu ≤ 0.20%, for example, 0.03% ≤ Cu ≤ 0.20%. V: In the high-hardenability steel for gear shafts of the present disclosure, V may effectively improve the hardenability of the steel. V element in the steel may form precipitates with C element or N element, thereby further improving the strength of the steel. However, if the contents of C and V element are too high, coarse VC particles will be formed. As such, in the high-hardenability steel for gear shafts of the present disclosure, the mass percentage of V may be controlled at 0 < V ≤ 0.03%, for example, 0.005% ≤ V ≤ 0.03%.

[0026] Furthermore, the austenite grain size of the high-strength, high-toughness and high-hardenability steel for gear shafts of the present disclosure remains at grade 5-8 after high-temperature carburizing heat treatment. Wherein, the conditions of the simulated high-temperature carburizing heat treatment test for detecting the austenite grain size may be: first the steel is heated to 1200°C with a holding time of 40 min, water cooled, then heated at a heating rate of 500-800°C per 40-60 min to 1000°C with a holding time of 4 h, water cooled and quenched.

[0027] Furthermore, the high-strength, high-toughness and high-hardenability steel for gear shafts of the present disclosure has a J9mm hardenability of 34-42 HRC.

[0028] Furthermore, after high-temperature carburizing heat treatment, the high-strength, high-toughness and high-hardenability steel for gear shafts of the present disclosure has a tensile strength R m of ≥ 1100 MPa, a yield strength R p0.2 of ≥ 980 MPa, an elongation after fracture A of ≥ 12%, a reduction of area of ≥ 50%, and a Charpy impact energy A ku of ≥ 55 J. The conditions of the simulated high-temperature carburizing heat treatment test for detecting mechanical performance may be: heating at 880±10°C with a holding time of 90 min and oil quenching + heating at 870±10°C with a holding time of 90 min and oil quenching + heating at 200±10°C with a holding time of 150 min, tempering and air cooling.

[0029] Furthermore, after high-temperature carburizing heat treatment, the high-strength, high-toughness and high-hardenability steel for gear shafts of the present disclosure may have a tensile strength R m of 1100-1400 MPa.

[0030] Furthermore, after high-temperature carburizing heat treatment, the high-strength, high-toughness and high-hardenability steel for gear shafts of the present disclosure may have a yield strength R p0.2 of 980 MPa-1250 MPa.

[0031] Furthermore, after high-temperature carburizing heat treatment, the high-strength, high-toughness and high-hardenability steel for gear shafts of the present disclosure may have an elongation after fracture A of 12-15%.

[0032] Furthermore, after high-temperature carburizing heat treatment, the high-strength, high-toughness and high-hardenability steel for gear shafts of the present disclosure may have a reduction of area of 50-62%.

[0033] Furthermore, after high-temperature carburizing heat treatment, the high-strength, high-toughness and high-hardenability steel for gear shafts of the present disclosure may have a Charpy impact energy A ku of 55-130 J.

[0034] Accordingly, another object of the present disclosure is to provide a manufacturing method for the high-strength, high-toughness and high-hardenability steel for gear shafts. This manufacturing method is simple in production, and the high-strength, high-toughness and high-hardenability steel for gear shafts obtained has the characteristics of high hardenability and high-strength, high-toughness.

[0035] To achieve the above object, the present disclosure further provides a manufacturing method for the high-strength, high-toughness and high-hardenability steel for gear shafts as described above, comprising the steps: (1) Smelting; (2) Casting; (3) Heating: a steel slab is firstly heated to 700°C or less in a preheating stage, and then heated to 980°C or less in a first heating stage and held, then heated to 950-1200°C in a second heating stage and held, then fed into a soaking stage with a soaking temperature of 1050-1250°C; Or, a steel slab is slowly heated at a speed of 50-300°C / h, firstly heated to 700°C or less in a preheating stage, then heated to 980°C or less in a first heating stag, then heated to 950-1200°C in a second heating stage, then fed into a soaking stage with a soaking temperature of 1050-1250°C; (4) Forging or rolling.

[0036] Compared with the prior art, in the manufacturing method of the present disclosure, the heating step adopts a unique process, wherein a soaking temperature is relatively high. This is because a relatively high soaking temperature may improve the composition uniformity and structural uniformity of a continuous casting slab during a diffusion process of heating the steel slab. At the same time, the precipitates have a faster dissolution rate at this temperature. Therefore, a high heating temperature helps to have more dissolution of original undissolved precipitate particles in the steel, increase the concentration of micro-alloying elements in the matrix, have more and more dispersed particles precipitated during subsequent cooling. Furthermore, only after a heating temperature is increased, the final rolling or final forging temperature may be increased, thereby resulting in the recrystallization of austenite after rolling more sufficient and the distribution of precipitates more uniform.

[0037] In the manufacturing method of the present disclosure, in step (1), an electric furnace or converter may be used for smelting, and refining and vacuum treatment are carried out. Of course, in some other embodiments, a vacuum induction furnace may also be used for smelting.

[0038] Furthermore, in step (1), a furnace charge for electric furnace smelting may be selected from low P, S scrap steel, crop ends and high-quality pig iron. Alloys may be selected from ferrochrome, lowphosphorus ferromanganese, ferromolybdenum and so on. Reducing agents may include: calcium carbide, carbon powder, and aluminum powder. During the oxidation period: slagging frequently for removing P; the conditions of slagging may be controlled as follow: a slagging temperature is 1630-1660°C; and P ≤ 0.015%. The tapping conditions may be controlled as follow: a tapping temperature is 1630-1650°C; [P] ≤ 0.010%, and [C] ≥ 0.03%.

[0039] After the completion of electric furnace smelting or converter smelting, molten steel refining may be carried out on a ladle refining furnace to remove harmful gases and inclusions in the steel. A ladle seating is controlled, temperature measurement and analysis is performed, and an argon pressure may be adjusted as needed. Initial deoxidation of a Ladle Furnace (LF) may be achieved by feeding Al, then alloy blocks may be added and stirred for 5-10 minutes. When a temperature of molten steel is measured to be T = 1650-1670°C, vacuum degassing may be performed to ensure [0] ≤ 0.0020%, and [H] ≤ 0.00015%. In a specific embodiment, a vacuum degree for the vacuum degassing may be controlled at ≤ 66.7 Pa, and kept for 15 minutes or more.

[0040] Additionally, in step (1), a temperature of a crane ladle may be controlled at 1550-1570°C. Since a temperature of the crane ladle is reduced, element diffusion is accelerated, which is conductive to further reducing dendritic segregation.

[0041] Furthermore, in step (2), casting may be carried out by ingot casting or continuous casting. During continuous casting and pouring process, a high-temperature molten steel in the steel ladle is poured into a tundish through a protective sleeve; wherein a superheat degree of tundish may be controlled at 20-40°C. A tundish is completely cleaned before use, and the inner surface of the tundish is coated with a refractory coating and should not have cracks. The molten steel in a tundish passes through a continuous casting crystallizer with sufficient electromagnetic stirring, so that a qualified continuous casting slab having a cross-sectional dimension of 140 mm×140 mm ~ 320 mm×425 mm may be obtained.

[0042] In step (2), a pouring rate may be controlled at 0.6-2.1m / min according to different sizes of square slabs. Then, a continuous casting slab is slowly cooled in a slow cooling pit, wherein a slow cooling time may be 24 hours or more.

[0043] Furthermore, in step (3) of the manufacturing method of the present disclosure, a heating temperature of the preheating stage may be 600-700°C, and a temperature of the first heating stage may be 900-980°C.

[0044] Furthermore, in step (3) of the manufacturing method of the present disclosure, holding for a period of time in the soaking stage is required, and a holding time of the soaking stage may be 3-12 h.

[0045] In step (3) of the manufacturing method of the present disclosure, after heating is performed in the first heating stage, holding may or may not be performed, then heating is performed in the second heating stage. A holding time of the first heating stage may be 0-3h, for example, 0.5h, 1h, 2h. In step (3) of the manufacturing method of the present disclosure, after heating is performed in the second heating stage, holding may or may not be performed, then a soaking stage is performed. A holding time of the second heating stage may be 0-3h, for example, 0.5h, 1h and 2h.

[0046] In some other embodiments, in step (3) of the manufacturing method of the present disclosure, heating is slowly performed at a rate of 50-300°C / h. First heating is performed in the preheating stage to 700°C or less, then heating is performed to 980°C or less in the first heating stage without holding, then heating is performed to 950-1200°C in the second heating stage without holding, then the soaking stage is performed. This process may be carried out in a walking beam heating furnace.

[0047] Further, in step (4) of the manufacturing method of the present disclosure, a start forging temperature or a start rolling temperature may be controlled at 1050-1250°C, and a finish forging temperature or a finish rolling temperature may be controlled at ≥ 900°C. In some embodiments, in step (4) of the manufacturing method of the present disclosure, a finish forging temperature or a finish rolling temperature is 900-1000°C.

[0048] In this embodiment, a start forging temperature or a start rolling temperature is controlled at 1050-1250°C, and a finish forging temperature or a finish rolling temperature is controlled at ≥ 900°C. It is because: this process is further conducive for N to dissolve from the γ solid solution and combine with micro-alloying elements in the steel to form nitrides.

[0049] It should be noted that, N has less solubility in α-Fe than in γ-Fe, and due to the excitation of phase transformation, two peaks of the precipitation appear. When a finish forging or finish rolling temperature is low, peaks of precipitates will occur, resulting in uneven distribution of precipitates and insufficient recrystallization, and anisotropy in the microstructure. Therefore, a finish forging or finish rolling temperature is controlled at ≥ 900°C. Additionally, increasing a finish forging or finish rolling temperature will result in finer grains obtained. Fine grains increase the difference between an average grain diameter of ferrite after transformation of supercooled austenite and a band spacing of manganese-rich bands, reduce the tendency of pearlite formation in manganese-rich bands, thereby reducing banded structures.

[0050] In the above technical solution, in step (4) of the manufacturing method of the present disclosure, after a steel slab is discharged from a furnace, high-pressure water descaling may be carried out to remove the oxide scale.

[0051] Furthermore, in step (4) of the manufacturing method of the present disclosure, when forging is performed, a steel slab may be directly forged to a final finished product. When rolling is performed, a steel slab may be directly rolled to a final finished product, or a steel slab may be first rolled to a specified intermediate slab, then heated and rolled to a final finished product.

[0052] In some embodiments, in step (4) of the manufacturing method of the present disclosure, a steel slab is first rolled into an intermediate slab ( may having a size of 140mm×140mm to 260mm×260mm), a finish rolling temperature of the intermediate slab is controlled at 1000-1050°C; then the intermediate slab is heated according to the following process: the intermediate slab is firstly heated to 680-700°C in a preheating stage, then heated to 1050-1100°C in a first heating stage, then heated to 1200-1220°C in a second heating stage with a heating rate of 300-500°C / h; then enters a soaking stage with a soaking temperature of 1200-1250°C, and with a holding time of 3-5 h in the soaking stage; then the heated intermediate slab is rolled into a finished product, and a finish rolling temperature of the finished product is controlled at≥ 900°C (for example, 900-1000°C).

[0053] Compared with the prior art, the high-strength, high-toughness and high-hardenability steel for gear shafts and its manufacturing method of the present disclosure have the following advantages and beneficial effects: (1) Through reasonable chemical composition design combined with optimized process, a steel for gear shafts with high hardenability can be obtained in the present disclosure. The bars rolled or forged by the high-strength, high-toughness and high-hardenability steel for gear shafts can be effectively processed into gears, and may have excellent strength and toughness by subsequent high-temperature carburizing heat treatment. (2) The high-strength, high-toughness and high-hardenability steel for gear shafts of the present disclosure controls micro-alloying element coefficient and the content of nitrogen element, strictly controls the atomic molar ratio, and simultaneously adds an appropriate amount of Nb element, so that the abnormal growth of austenite grains is hindered, the austenite grain coarsening temperature of the gear steel is increased, the grain size of the gear steel remains stable at grade 5-8 even after high-temperature carburizing at up to 1000°C for 4 hours, and various performances achieve the usage performance indicators of the steel for gear shafts. (3) The composition and process design of the high-strength, high-toughness and high-hardenability steel for gear shafts of the present disclosure are reasonable. By controlling the content of micro-alloying elements in the steel, the appearance of large harmful inclusions in the steel is avoided, the stable production quality of the steel is ensured, the production cost of the steel is reduced, and mass production on a bar production line is achieved. (4) The hardenability, austenite grain size, and cost competitiveness of the high-strength, high-toughness and high-hardenability steel for gear shafts of the present disclosure are superior to those in the existing technology. The types and quantities of alloy elements in the steel can be controlled under the premise of ensuring the performance of high hardenability, narrow bandwidth and so on, and the applicability of the steel is improved. (5) When the high-strength, high-toughness and high-hardenability steel for gear shafts of the present disclosure is subsequently used to produce gear shafts, it can shorten the carburizing time, reduce the production cost of gear shafts, and has broad industrial application prospects. Detailed Description

[0054] The high-strength, high-toughness and high-hardenability steel for gear shafts and its manufacturing method of the present disclosure will be further explained and illustrated with reference to the specific examples. Nonetheless, the explanation and illustration are not intended to unduly limit the technical solution of the disclosure.

[0055] The high-strength, high-toughness and high-hardenability steel for gear shafts of Examples 1-8 were all made using the following steps: (1) Smelting and casting according to the chemical composition shown in Tables 1-1 and 1-2 below: Smelting can be carried out using a 50kg vacuum induction furnace, a 150kg vacuum induction furnace, or a 500kg vacuum induction furnace, or by electric furnace smelting + external refining + vacuum degassing, or by converter smelting + external refining + vacuum degassing. (2) Casting. (3) Heating: A steel slab was first heated to 700°C or less in a preheating stage, then heated to 980°C or less in a first heating stage and held, then heated to 950-1200°C in a second heating stage and held, then fed into a soaking stage with a soaking temperature of 1050-1250°C and held, then subsequently rolled or forged. (4) Forging or rolling: A start forging or start rolling temperature was controlled at 1050-1250°C, and a finish forging or finish rolling temperature was controlled at ≥ 900°C.

[0056] Furthermore, the specific process procedures for the high-hardenability steel for gear shafts in Examples 1-8 and Comparative Examples 1-4 were as follows: Example 1: Smelting was carried out on a 50kg vacuum induction furnace according to the chemical composition shown in Table 1-1 and 1-2 below. The molten steel was cast into a steel ingot, and then heated and breakdown forged. The steel ingot was first heated to 700°C in the preheating stage, then continued to be heated to 900°C in the first heating stage with a holding time of 0h, then continued to be heated to 950°C in the second heating stage with a holding time of 1h, then entered the soaking stage with a soaking temperature of 1050°C and a holding time of 3h, then subsequently forged, wherein a finish forging temperature was controlled to be 910°C, and it was finally forged into a bar with Φ60mm. Example 2: Smelting was carried out on a 150kg vacuum induction furnace according to the chemical composition shown in Tables 1-1 and 1-2 below. The molten steel was cast into a steel ingot, and then heated and breakdown forged. The steel ingot was first heated to 650°C in the preheating stage, then continued to be heated to 950°C in the first heating stage with a holding time of 0.5h, then continued to be heated to 1100°C in the second heating stage with a holding time of 0h, then entered the soaking stage with a soaking temperature of 1200°C and a holding time of 5h, then subsequently forged, wherein a finish forging temperature was controlled to be 1000°C, and it was finally forged into a bar with Φ90m. Example 3: Smelting was carried out on a 500kg vacuum induction furnace according to the chemical composition shown in Tables 1-1 and 1-2 below. The molten steel was cast into a steel ingot, and then heated and breakdown forged. The steel ingot was first heated to 600°C in the preheating stage, then continued to be heated to 980°C in the first heating stage with a holding time of 3 h, then continued to be heated to 1200°C in the second heating stage with a holding time of 3 h, then entered the soaking stage with a soaking temperature of 1250°C and a holding time of 12 h, then subsequently forged, wherein a finish forging temperature was controlled to be 1000°C, and it was finally forged into a bar with Φ120mm. Example 4: Electric furnace smelting was carried out according to the chemical composition shown in Tables 1-1 and 1-2, and refining and vacuum treatment were carried out, then a continuous casting slab of 280mm×280mm was cast. The continuous casting slab was heated slowly at a rate of 300°C / h in a walking beam heating furnace: first heated to 620°C in the preheating stage, then continued to be heated to 950°C in the first heating stage, continued to be heated to 1150°C in the second heating stage, then entered the soaking stage with a soaking temperature of 1200°C and a holding time of 4 h, then rolled. After discharged from the heating furnace, the slab was descaled with high-pressure water and then rolled, wherein a finish rolling temperature was controlled to be 970°C, and it was finally rolled into a bar with Φ80mm. Example 5: Electric furnace smelting was carried out according to the chemical composition shown in Tables 1-1 and 1-2, and refining and vacuum treatment were carried out, and then a continuous casting slab with 320mm×425mm was cast. The continuous casting slab was heated slowly at a rate of 150°C / h in a walking beam heating furnace, first heated to 600°C in the preheating stage, then continued to be heated to 950°C in the first heating stage, continued to be heated to 1200°C in the second heating stage, then entered the soaking stage with a soaking temperature of 1230°C and a holding time of 4.5 h, and then rolled. After discharged from the heating furnace, the casting slab was descaled with high-pressure water and then rolled into an intermediate slab, wherein a first finish rolling temperature (a finish rolling temperature of the intermediate slab) was controlled to be 1050°C, and the intermediate slab had a size of 220mm×220mm. Then the intermediate slab was heated slowly at a rate of 400°C / h in a walking beam heating furnace, first heated to 680°C in the preheating stage, then continued to be heated to 1050°C in the first heating stage, continued to be heated to 1200°C in the second heating stage, hold, then entered the soaking stage with a soaking temperature of 1220°C and a holding time of 3.5h. After discharged from the furnace, the slab was descaled with high-pressure water and then rolled, wherein a second finish rolling temperature (a finish rolling temperature of the finished product) is controlled to be 950°C, and the sab was finally rolled into a finished bar with Φ90mm. Example 6: Electric furnace smelting was carried out according to the chemical composition shown in Tables 1-1 and 1-2, and refining and vacuum treatment were carried out, then a continuous casting slab with 280mm×280mm was cast. The continuous casting slab was heated slowly at a rate of 300°C / h in a walking beam heating furnace, first heated to 680°C in the preheating stage, then continued to be heated to 900°C in the first heating stage, continued to be heated to 1180°C in the second heating stage, then entered the soaking stage with a soaking temperature of 1200°C and with a holding time of 4.5h, then subsequently rolled. After discharged from a heating furnace, the slab was descaled with high-pressure water and then rolled into an intermediate slab, wherein a first finish rolling temperature (a finish rolling temperature of the intermediate slab) was controlled to be 1000°C, and the intermediate slab had a size of 140mm×140mm. Then the intermediate slab was heated slowly at a rate of 500°C / h in a walking beam heating furnace, first heated to 700°C, then continued to be heated to 1100°C in the first heating stage, continued to be heated to 1200°C in the second heating stage, entered the soaking stage with a soaking temperature of 1220°C and a holding time of 3.5h. After discharged from the furnace, the slab was descaled with high-pressure water and then rolled, wherein a second finish rolling temperature (a finish rolling temperature of the finished product) is controlled to be 920°C, and it was finally rolled into a finished bar with Φ25mm. Example 7: Converter smelting was carried out according to the chemical composition shown in Tables 1-1 and 1-2, and refining and vacuum treatment were carried out, and then an ingot casting slab was cast. The casting slab was heated slowly at a rate of 50°C / h in a walking beam heating furnace, first heated to 620°C in the preheating stage, then continued to be heated to 950°C in the first heating stage, continued to be heated to 1150°C in the second heating stage, then entered the soaking stage with a soaking temperature of 1200°C and a holding time of 8 h, and then rolled. After discharged from the heating furnace, the slab was descaled with high-pressure water and then rolled, wherein a finish rolling temperature was controlled to be 970°C, and it was finally rolled into a bar with Φ90mm. Example 8: Converter smelting was carried out according to the chemical composition shown in Tables 1-1 and 1-2, and refining and vacuum treatment were carried out, and then an ingot casting slab was cast. The casting slab was heated slowly at a rate of 100°C / h in a walking beam heating furnace, first heated to 600°C in the preheating stage, then continued to be heated to 950°C in the first heating stage, continued to be heated to 1200°C in the second heating stage, then entered the soaking stage with a soaking temperature of 1230°C and a holding time of 7 h, and then rolled. After discharged from a heating furnace, the casting slab was descaled with high-pressure water and then rolled into an intermediate slab, wherein a first finish rolling temperature (a finish rolling temperature of the intermediate slab) was controlled to be 1050°C, and the intermediate slab had a size of 260mm×260mm. Then the intermediate slab was heated slowly at a rate of 300°C / h in a walking beam heating furnace, first heated to 680°C in the preheating stage, then continued to be heated to 1050°C in the first heating stage, continued to be heated to 1200°C in the second heating stage, then entered the soaking stage with a soaking temperature of 1220°C and a holding time of 5h, and then discharged from the furnace. After discharged from the furnace, the intermediate slab was descaled with high-pressure water and then rolled, wherein a second finish rolling temperature (a finish rolling temperature of the finished product) is controlled to be 950°C, and it was finally rolled into a finished bar with Φ60mm.

[0057] Steels in Comparative examples 1 and 2 are from commercial materials. These steels were made by electric furnace smelting and refining processes to ensure the purity of the commercial materials.

[0058] Comparative Example 3: The steel was smelted in a 50 kg vacuum induction furnace according to the chemical composition shown in Tables 1-1 and 1-2. The molten steel was cast into a steel ingot, and then heated and breakdown forged. The ingot is heated at a rate of 300°C / h to 1100°C in a box-type furnace with a holding time of 3h, and then subsequently forged, wherein a finish forging temperature was controlled to be 910°C, and it was finally forged into a bar with a diameter of Φ60 mm.

[0059] Comparative Example 4: The steel was smelted in an electric furnace according to the chemical composition shown in Tables 1-1 and 1-2, and refining and vacuum treatment were carried out, and then a continuous casting slab of 320mm×425mm was cast. The continuous casting slab was heated slowly at a rate of 150°C / h in a walking beam heating furnace, first heated to 600°C in the preheating stage, then heated to 950°C in the first heating stage, continued to be heated to 1200°C in the second heating stage, and then entered the soaking stage with a soaking temperature of 1230°C and a holding time of 4.5 hours; then subsequent rolling was carried out. After discharged from a heating furnace, the continuous casting slab was descaled with high-pressure water and then rolled into an intermediate slab, wherein the first finish rolling temperature (a finish rolling temperature for the intermediate slab) was controlled to be 1050°C, and the size of the intermediate billet slab was 220 mm × 220 mm. Then the intermediate slab was heated slowly at a rate of 400°C / h in a walking beam heating furnace: heated to 680°C in the preheating stage, then heated to 1050°C in the first heating stage, continued to be heated to 1200°C in the second heating stage, and entered the soaking stage with a soaking temperature of 1220°C and a holding time of 6h, and then rolled. After discharged from the heating furnace, the intermediate slab was descaled with high-pressure water and then rolled into a finished bar with a size of Φ90 mm, wherein the second finish rolling temperature (a finish rolling temperature for the finished product) is controlled to be 950°C.

[0060] Tables 1-1 and 1-2 list the mass percentages of the chemical elements for the high-hardenability steel for gear shafts of Examples 1-8 and the comparative steels of Comparative Examples 1-4. Table 1-1 (wt%, a balance of Fe and other unavoidable impurities except P, O, H and Ca)NoCSiMnCrAlTiNbNBEx.10.160.261.161.110.020.0330.0030.0060.0009Ex.20.220.100.991.260.0320.0340.0340.0150.0006Ex.30.190.400.931.440.0420.0150.0250.0120.0022Ex.40.170.291.040.950.0290.0230.0010.0110.0015Ex.50.190.180.861.020.0250.0340.0130.010.0026Ex.60.200.221.091.120.0350.0190.0070.0060.0034Ex.70.180.311.241.410.050.0270.0180.0130.0012Ex.80.210.230.971.340.0370.0390.0040.0120.0013Com.Ex.10.190.250.891.130.0240.013 0 0.0090.0037 Com.Ex.20.190.210.82 1.030.0250.0370.0070.0120.0007Com.Ex.30.200.230.871.110.0210.0080.0010.0080.0002Com.Ex.40.220.270.971.120.0230.043 0.0030.0120.0008 Table 1-2 (wt%, a balance of Fe and other unavoidable impurities except P, O, H and Ca) NoSNiMoCuVPOCaHEx.10.040.050.0100.0060.0180.0020.00130.00022.64Ex.20.0090.250.10.080.030.0070.00120.0020.00024.58Ex.30.0290.030.060.200.0180.0280.00150.00150.00024.99Ex.40.01200000.0030.00190.00310.00011.56Ex.50.0030.080.0200.0240.0050.00110.00130.00013.35Ex.60.034000.080.0160.0080.0020.00230.00014.87Ex.70.0180.0700.030.0050.0130.00080.00080.00014.14Ex.80.0010.160.030.1300.0090.00110.00340.00012.20Com.Ex.10.0130.210.070.040.0020.0070.00130.00130.00021.42 Com.Ex.20.0030.0200.030.0030.0060.00130.00130.00022.05Com.Ex.30.0020.020.020.0100.0070.00080.00140.00021.58Com.Ex.40.0210.060.010.050.0060.0080.00190.00110.00021.47 Note: r M / N =(10*[Nb] / 93+[Ti] / 480+[Al] / 27) / ([N] / 14), wherein each chemical element in the formula is substituted with a numerical value before the percent sign of its mass percentage content.

[0061] Table 2 lists the specific process parameters of the high-strength, high-toughness and high-hardenability steel for gear shafts of Examples 1-8 and Comparative steels of Comparative Examples 1-4. Table 2NoStep (1)Step (3)Step (4)Size of finished bar (mm)Method of SmeltingTemperature of preheating stage (°C)Temperature of First heating stage (°C)Temperature of Second heating stage (°C)Soaking temperature (°C)Holding time in soaking stage (h)Start For ging or start rolling temperature (°C)Finish forging or finish rolling temperature of finished product (°C)Interme diate slab size (mm)Heating process of Intermediate slabFinish forging or finish rolling temperature of Intermediate slabEx.1Smelting on a 50kg vacuum induction furnace700900950105031050910 / / / Φ60Ex.2Smelting on 150kg vacuum induction furnace65095011001200512001000 / / / Φ90Ex.3Smelting on 500kg vacuum induction furnace600980120012501212501000 / / / Φ120Ex.4Electric furnace smelting+ refining6209501150120041200970 / / / Φ80Ex.5Electric furnace smelting+ refining600950120012304.51230950220×22 0680°C in the preheating stage +1050°C in the first heating stage +1200°C in the second heating stage +1220°C in the soaking stage with a holding time of 3.5h1050Φ90Ex.6Electric furnace smelting+ refining680900118012004.51200920140x14 0700°C in the preheating stage +1100°C in the first heating stage +1200°C in the second heating stage +1220°C in the soaking stage with a holding time of 3.5h1000Φ25Ex.7Converter smelting+ refining6209501150120081200970 / / / Φ90Ex.8Converter smelting+ refining6009501200123071230950260×26 0680°C in the preheating stage +1050°C in the first heating stage +1200°C in the second heating stage +1220°C in the soaking stage with a holding time of 5h1050Φ60Com. Ex.1Electric furnace smelting+ refining / / / / / / / / / / Φ60Com. Ex.2Electric furnace smelting+ refining / / / / / / / / / / Φ90Com. Ex.3Smelting on a 50kg vacuum induction furnace / / / 110031100910 / / / Φ60Com. Ex.4Electric furnace Smelting+ refining600950120012304.51230950220×22 0680°C in the preheating stage +1050°C in the first heating stage +1200°C in the second heating stage +1220°C in the soaking stage with a holding time of 6h1050Φ90

[0062] As can be seen form Table 2 that, in Example 5, Example 6, Example 8 and Comparative Example 4, steel slabs were all initially rolled into an intermediate slab with a specified size, and then heated and rolled again to a final product.

[0063] To verify the performance of the high-strength, high-toughness and high-hardenability steel for gear shafts of the present disclosure, the high-strength, high-toughness and high-hardenability steels for gear shafts obtained in Examples 1-8 and comparative steels obtained in Comparative examples 1-4 were respectively sampled and subjected to a simulated high-temperature carburizing heat treatment testing for detecting the mechanical performance and austenite grain size. The test results were shown in Table 3. Wherein: A simulated high-temperature carburizing heat treatment testing for detecting austenite grain size: a sample was heated to 1200°C with a holding time of 40min, water cooled, and then heated to 1000°C at a rate of 600°C per 50min and with a holding time of 4h, and then water quenched.

[0064] Austenite grain size test: austenite grain size was evaluated according to Standard ASTM E112.

[0065] Mechanical performance test: samples were prepared according to GB / T 2975-2018 (Steel and steel products - Location and preparation of samples for mechanical performance testing)." According to GB / T 3077-2015, blank samples with Φ15 mm were made for the simulated high-temperature carburizing heat treatment testing for detecting the mechanical performance: blank samples were heated at 880°C with a holding time of 90min, and oil quenched, then heated at 870°C with a holding time of 90 min, and oil quenched, and then heated at 200°C with a holding time of 15 min, tempered and air cooled. Tensile testing was conducted according to GB / T 228.1-2010 "Metallic materials - Tensile testing - Part 1: Method of test at room temperature,", and a tensile strength (R m ), a yield strength (R p0.2 ), an elongation after fracture (A), and a reduction of area (Z) were tested. Additionally, the Charpy impact energy (A ku ) of each examples and comparative examples at room temperature was tested according to GB / T 229-2007 "Metallic materials - Charpy pendulum impact test method."

[0066] Furthermore, the high-strength, high-toughness and high-hardenability steel for gear shafts obtained in Examples 1-8 and comparative steels obtained in Comparative Examples 1-4 were sampled to test their hardenability and hardness. The test results are also listed in Table 3. Wherein: Hardenability Testing: each steel of examples and comparative example steels were sampled and prepared from the hot-rolled round bars in accordance with Chinese Standard GB / T 225. End-quench hardenability testing (Jominy test) was conducted in accordance with GB / T 5216, with a normalizing temperature controlled of 920 ± 10°C and a quenching temperature controlled of 870 ± 5°C. Rockwell hardness testing was conducted according to GB / T 230.2 to obtain a hardness value (HRC) at a specific position, such as a hardness at 9 mm from the quenched end, J9mm.

[0067] Table 3 lists the test results for the high-strength, high-toughness and high-hardenability steels for gear shafts of Examples 1-8 and the comparative steels of Comparative Examples 1-4. Table 3No.Yield strength R p0.2 , MPaTensile strength R m , MPaElongation after fracture A, %Reduction of area Z, %Impact energy A ku, JJ9mm hardenability, HRC1000°C*4h austenite grain size, gradeEx.11009115813.561116 / 123386.5Ex.212101364126059 / 72426Ex.31094125314.560115 / 124408Ex.4100511031459101 / 112345.5Ex.51015112613.561103 / 108357Ex.610431118145590 / 106408Ex.710181179125194 / 102367.5Ex.81199133912.55265 / 67416.5Com.Ex.110471210145992 / 80390Com.Ex.2963 1058 13.55890 / 9633 5Com.Ex.3105411701355113 / 10529 5 (0)Com.Ex.41235132313.56142 / 53 400 (4)Note: The impact energy values for each Example and Comparative Example, presented as two numerical values separated by a slash, represent the results of two separate measurements.

[0068] As can be seen from Table3, after the simulated high-temperature carburizing heat treatment at 1000°C, the austenite grain size of the high-strength, high-toughness and high-hardenability steel for gear shafts of Examples 1-8 of the present disclosure are maintained within the range of Grade 5~8. No phenomena such as mixed crystals or abnormal coarse grains was observed. Therefore, the steels have excellent high-temperature grain stability.

[0069] Furthermore, it can be seen from Table 3, the hardenability at the representative J9mm position for the high-strength, high-toughness and high-hardenability steels for gear shafts of Examples 1-8 is 34~42 HRC, having high hardenability and a narrow hardenability band. In addition, after the simulated high-temperature carburizing heat treatment, steels of each examples have a tensile strength R m of greater than 1100 MPa, a yield strength R p0.2 of greater than 980 MPa, an elongation after fracture A of ≥12%, a reduction of area of greater than 50%, and a Charpy impact energy A ku of greater than 55 J.

[0070] It can be concluded that, the bars rolled or forged by the high-strength, high-toughness and high-hardenability steel for gear shafts can be effectively processed into gear shafts. After undergoing subsequent high-temperature carburizing heat treatment, they have high strength and toughness, so that they are suitable for high-end parts such as automotive gearboxes, reducers for new energy vehicles, and industrial speed reducers, and have good application prospects and significant value.

[0071] In contrast to the various examples of the present disclosure, Comparative Example 1 has a grain size of Grade 0 after the simulated high-temperature carburizing testing, which means that the grains grow abnormally, and the service requirements cannot be met.

[0072] Although the comparative steel of Comparative Example 2 fails to have the phenomena of mixed crystals, it has fine grain size and low hardenability after the simulated high-temperature carburizing heat treatment testing, which does not satisfy the requirement for high hardenability. Furthermore, Comparative Example 2 has a relatively low strength.

[0073] For the comparative steel of Comparative Example 3, a mixed crystal is observed after the simulated high-temperature carburizing heat treatment at 1000°C, wherein "5 (0)" - representing an average grain size of Grade 5, and the grain size in some regions has coarsened to Grade 0.

[0074] For the comparative steel of Comparative Example 4, a mixed crystal (Grade 4) is also observed after the simulated high-temperature carburizing heat treatment at 1000°C, wherein "0 (4)" - representing an average grain size of Grade 0, and the grain size in some regions has coarsened to Grade 4. Additionally, Comparative Example 4 also has a relatively low impact energy.

[0075] In addition, the ways in which the various technical features of the present disclosure are combined are not limited to the ways recited in the claims of the present disclosure or the ways described in the specific examples. All the technical features recited in the present disclosure may be combined or integrated freely in any manner, unless contradictions are resulted.

[0076] It should also be noted that the Examples set forth above are only specific examples according to the present disclosure. Obviously, the present disclosure is not limited to the above Examples. Similar variations or modifications made thereto can be directly derived or easily contemplated from the present disclosure by those skilled in the art. They all fall in the protection scope of the present disclosure.

Examples

example 1

Smelting was carried out on a 50kg vacuum induction furnace according to the chemical composition shown in Table 1-1 and 1-2 below. The molten steel was cast into a steel ingot, and then heated and breakdown forged. The steel ingot was first heated to 700°C in the preheating stage, then continued to be heated to 900°C in the first heating stage with a holding time of 0h, then continued to be heated to 950°C in the second heating stage with a holding time of 1h, then entered the soaking stage with a soaking temperature of 1050°C and a holding time of 3h, then subsequently forged, wherein a finish forging temperature was controlled to be 910°C, and it was finally forged into a bar with Φ60mm. Example 2: Smelting was carried out on a 150kg vacuum induction furnace according to the chemical composition shown in Tables 1-1 and 1-2 below. The molten steel was cast into a steel ingot, and then heated and breakdown forged. The steel ingot was first heated to 650°C in the preheating stage, t...

Claims

1. A high-strength, high-toughness and high-hardenability steel for gear shafts, comprising Fe and unavoidable impurities, wherein further comprising the following chemical elements in 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%, and B: 0.0006-0.0034%; wherein a value of micro-alloy element coefficient rMN is 1.5 - 5.0, wherein rM / N = (10*[Nb]193+[Ti] / 480+[Al] / 27) / ([N] / 14), wherein each chemical element is substituted with a numerical value before the percent sign of its mass percentage content.

2. The high-strength, high-toughness and high-hardenability steel for gear shafts according to claim 1, wherein the mass percentage of each chemical elements 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%; with a balance of Fe and other unavoidable impurities; wherein the value of micro-alloy element coefficient is 1.5 - 5.0, wherein rM / N=(10*[Nb] / 93+[Ti] / 480+[Al] / 27) / ([N] / 14), wherein each chemical element is substituted with a numerical value before the percent sign of its mass percentage content.

3. The high-strength, high-toughness and high-hardenability steel for gear shafts according to claim 1 or 2, wherein among the other unavoidable impurities: P≤0.030%, 0≤0.002%, H≤0.0002%, and Ca≤0.0034%.

4. The high-strength, high-toughness and high-hardenability steel for gear shafts according to claim 1 or 2, wherein the high-strength, high-toughness and high-hardenability steel for gear shafts further comprises at least one of the following chemical elements: 0 < S ≤ 0.04%, 0 < Ni ≤ 0.25%, 0 < Mo ≤ 0.10%, 0 < Cu ≤ 0.20%, and 0 < V ≤ 0.03%.

5. The high-strength, high-toughness and high-hardenability steel for gear shafts according to claim 1 or 2, wherein the high-strength, high-toughness and high-hardenability steel for gear shafts maintains an austenite grain size of grade 5-8 after high-temperature carburizing heat treatment.

6. The high-strength, high-toughness and high-hardenability steel for gear shafts according to claim 1 or 2, wherein the high-strength, high-toughness and high-hardenability steel for gear shafts has a J9mm hardenability of 34-42 HRC.

7. The high-strength, high-toughness and high-hardenability steel for gear shafts according to claim 1 or 2, wherein after high-temperature carburizing heat treatment, the high-strength, high-toughness and high-hardenability steel for gear shafts has a tensile strength Rm of ≥ 1100 MPa, a yield strength Rp0.2 of ≥ 980 MPa, an elongation after fracture A of ≥ 12%, a reduction of area of ≥ 50%, a charpy impact energy Aku of ≥ 55 J.

8. A manufacturing method for the high-strength, high-toughness and high-hardenability steel for gear shafts according to any one of claims 1-7, comprising the following steps: (1) Smelting; (2) Casting; (3) Heating: a steel slab is firstly heated to 700°C or less in a preheating stage, and then heated to 980°C or less in a first heating stage and held, then heated to 950-1200°C in a second heating stage and held, then fed into a soaking stage with a soaking temperature of 1050-1250°C; (4) Forging or rolling.

9. The manufacturing method according to claim 8, wherein in step (4), a start-forging or start-rolling temperature is controlled at 1050-1250°C, and a finish rolling or finish forging temperature is controlled to be ≥900°C.

10. The manufacturing method according to claim 8, wherein in step (4), the steel slab is rolled or forged directly into a finished product.

11. The manufacturing method according to claim 8, wherein in step (4), the steel slab is rolled into an intermediate slab first, then heated, and then rolled into a final finished product.

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