High-strength, high-toughness, and highly hardenable steel for gear shafts and method for manufacturing the same

Abstract

This invention discloses a high-strength, high-toughness, and highly hardenable steel for gear shafts, which contains Fe and unavoidable impurities, and further contains the following chemical elements in the following mass percentages: C: 0.16~0.22%, Si: 0.10~0.40%, Mn: 0.86~1.24%, Cr: 0.95~1.44%, Al: 0.02~0.05%, Ti: 0.015~0.039%, Nb: 0.001~0.034%, N: 0.006~0.015%, B: 0.0006~0.0034%; its trace alloying element coefficient r M / N The range is 1.5 to 5.0, however, [Math 1] TIFF2026521573000011.tif8170 Furthermore, the present invention also discloses a method for manufacturing the high-strength, high-toughness, and high-hardenability gear shaft steel described above. The high-strength, high-toughness, and high-hardenability gear shaft steel according to the present invention has higher hardenability, a narrower hardenability band width, and good high-temperature grain stability.

Description

[Technical Field]

[0001] This invention relates to high-strength steel materials and methods for manufacturing the same, and more particularly to steel for gear shafts and methods for manufacturing the same. [Background technology]

[0002] With the development of the automotive industry, the demands on automotive parts are increasing. In particular, gears that are economically efficient while possessing high strength, high toughness, high fatigue life, and high temperature stability are important development directions.

[0003] Accordingly, high-strength, high-toughness gear shaft steel can meet the advanced technical requirements for materials used in lightweight automobiles. Furthermore, to ensure the hardenability of the gears, requirements are usually also imposed on the hardenability of the gear shaft steel.

[0004] The main technical challenge for highly hardenable MnCr-based carburized gear steels is how to improve hardenability while simultaneously avoiding gear dimensional variations due to excessive hardenability band width, and ensuring that phenomena such as mixed crystals and grain coarsening do not occur in the gears after high-temperature carburizing.

[0005] For example, a Chinese patent document with publication number CN101096742A, publication date January 2, 2008, and title "High-Strength Automotive Gear Steel" discloses high-strength automotive gear steel. This steel has refined prior austenite grains due to the addition of alloying elements such as Nb, V, and Al. Its composition, by mass percentage, is C: 0.20-0.40, Si: 0.20-0.50, Mn: 0.50-1.00, Cr: 0.80-1.30, Nb: 0.015-0.080, V: 0.030-0.090, Mo: 0.15-0.55, Al: 0.015-0.050, with the remainder being Fe and unavoidable impurities. The addition of trace amounts of Nb and V optimizes the grain size, hardenability, and band width of the gear steel.

[0006] Furthermore, for example, in a Chinese patent document with publication number CN103361559A, publication date October 23, 2013, and title "Nb, Ti composite trace alloy high-temperature carburizing gear steel," Nb, Ti composite trace alloy high-temperature carburizing gear steel is disclosed. The composition of the steel is C: 0.17~0.22%, Si: 0.20~0.35%, Mn: 0.9~1.10%, P: ≤0.025%, S: 0.020~0.035%, Cr: 1.05~1.30%, Al: 0.015~0.035%, Ti: 0.02~0.06%, Nb: 0.02~0.06%, with the remainder being iron and unavoidable impurities. By controlling the content of trace alloying elements such as Nb, Ti, and Al, the carburizing temperature of the gear can be increased or the carburizing time can be shortened.

[0007] However, the above-mentioned patent documents do not completely solve the problems in controlling the hardenability and band width of steel for high-strength gear shafts. [Overview of the project] [Problems that the invention aims to solve]

[0008] One objective of the present invention is to provide a high-strength, high-toughness, and high-hardenability gear shaft steel that can be obtained at a lower cost, with higher hardenability, a narrower hardenability band width, and good high-temperature grain stability, by optimizing the composition system of the gear shaft steel, and in particular by rationally controlling the content of trace alloying elements and nitrogen elements in the gear shaft steel. [Means for solving the problem]

[0009] To achieve the above objective, the present invention provides a high-strength, high-toughness, and high-hardenability steel for gear shafts, which contains, in addition to Fe and unavoidable impurities, the following chemical elements in the following mass percentage content: 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%, containing; its micro-alloying element coefficient r M / N is in the range of 1.5~5.0, provided that

[0010]

Number

[0011] In the formula, for each chemical element, the numerical value before the percentage symbol of the mass percentage content of the chemical element is substituted.

[0012] Furthermore, in the high-strength, high-toughness, and high-hardening gear shaft steel according to the present invention, the mass percentage content of each chemical element is: C: 0.16~0.22%, Si: 0.10~0.40%, Mn: 0.86~1.24%, Cr: 0.95~1.44%, Al: 0.02~0.05%, Ti: 0.015~0.039%, Nb: 0.001~0.034%, N: 0.006~0.015%, B: 0.0006~0.0034%; the balance is Fe and inevitable impurities; its micro-alloying element coefficient r M / N is in the range of 1.5~5.0, provided that

[0013]

Number

[0014] In the formula, for each chemical element, the numerical value before the percentage symbol of the mass percentage content of the chemical element is substituted.

[0015] In the high-strength, high-toughness, and highly hardenable gear shaft steel according to the present invention, the design principles for each chemical element are as follows.

[0016] C: In the high-strength, high-toughness, and high-hardenability gear shaft steel according to the present invention, carbon (C) is an essential component of steel and also one of the main elements that affect the hardenability of the steel. High-strength, high-toughness, and high-hardenability gear shaft steel requires both surface strength and sufficient core impact toughness. However, if the carbon (C) content in the steel is too low, less than 0.16%, the strength of the steel will be insufficient, and the requirement for good hardenability will not be met. Conversely, the carbon (C) content in the steel should not be too high either, as it will not meet the requirement for toughness in the gear core, and excessively high carbon content is detrimental to the plasticity of the steel, especially carburized gear steel with a high Mn content. If the carbon content exceeds 0.22%, the machinability of the steel is detrimental. Therefore, in the high-strength, high-toughness, and high-hardenability gear shaft steel according to the present invention, the mass percentage of carbon (C) is controlled to 0.16-0.22%.

[0017] Si: In the high-strength, high-toughness, and high-hardenability gear shaft steel according to the present invention, the Si element can not only more effectively eliminate the adverse effects of iron oxide on the steel, but also dissolve into ferrite to strengthen it, thereby improving the strength, hardness, wear resistance, elasticity, and elastic limit of the steel. At the same time, it is important to note that while the Si element raises the Ac3 temperature of the steel, its low thermal conductivity may lead to a risk of cracking and a tendency towards decarburization. Therefore, considering the beneficial and adverse effects of Si, the mass percentage of Si in the high-strength, high-toughness, and high-hardenability gear shaft steel according to the present invention is controlled to 0.10-0.40%.

[0018] Mn: In the high-strength, high-toughness, and high-hardenability gear shaft steel according to the present invention, Mn is one of the main elements that affect the hardenability of the steel. The element Mn has excellent deoxidizing ability and can reduce iron oxide in the steel, thereby effectively improving the production volume of steel. Mn dissolves into ferrite to improve the strength and hardness of the steel, and as a result, when the steel material is hot-rolled and then cooled, pearlite with a fine lamellar structure and high strength can be obtained. In addition, Mn can eliminate the harmful effects of sulfur by forming MnS with sulfur in the steel, which has the ability to stabilize the steel by forming an austenite structure, significantly improving the hardenability of the steel, and can also reduce the high-temperature toughness of the steel. If the element Mn content in the steel is too low, the hardenability of the steel material will be insufficient, but if the element Mn content in the steel is too high, the thermoplasticity of the steel material will deteriorate, affecting production, and the steel material will be prone to cracking when water granulation is performed. Therefore, in the high-strength, high-toughness, and highly hardenable gear shaft steel according to the present invention, the mass percentage of Mn is controlled to 0.86 to 1.24%.

[0019] Cr: In the high-strength, high-toughness, and high-hardenability gear shaft steel according to the present invention, Cr is one of the main alloying elements added to the steel, and Cr can significantly improve the hardenability, strength, and wear resistance of the steel. Furthermore, Cr can reduce the activity of C element in the steel and prevent decarburization during heating, rolling, and heat treatment processes. However, if the Cr content is too high, the toughness of the hardened and tempered steel material will be significantly reduced, and coarse carbides distributed along the grain boundaries will be formed. Therefore, in the high-strength, high-toughness, and high-hardenability gear shaft steel according to the present invention, the mass percentage of Cr element is controlled to 0.95 to 1.44%.

[0020] Al: In the high-strength, high-toughness, and high-hardenability gear shaft steel according to the present invention, Al is a grain-refining element. By working in conjunction with N, the element Al can further refine the crystal grains and improve the toughness of the steel. Grain refinement plays an important role in improving the mechanical properties of steel, especially strength and toughness, and also contributes to reducing the steel's susceptibility to hydrogen embrittlement. However, it is important to note that the Al content in the steel should not be too high, as excessively high Al content increases the risk of inclusion formation in the steel. Therefore, in the high-strength, high-toughness, and high-hardenability gear shaft steel according to the present invention, the mass percentage of the element Al is controlled to 0.02-0.05%.

[0021] Ti: Adding Ti to steel can form a fine precipitate phase, but if the Ti content in the steel is too high, coarse, angular TiN particles are formed during the smelting process, reducing the impact toughness of the steel. Therefore, in the high-strength, high-toughness, and high-hardenability gear shaft steel according to the present invention, the Ti content is controlled to 0.015 to 0.039%.

[0022] Nb: In the high-strength, high-toughness, and high-hardenability gear shaft steel according to the present invention, the addition of Nb element to the steel allows for the formation of a fine precipitate phase, which inhibits the recrystallization of the steel and effectively refines the crystal grains. However, it is important to note that the Nb element content in the steel should not be too high. If the Nb content in the steel is too high, coarse NbC particles will be formed during the smelting process, which will conversely reduce the impact toughness of the steel. Therefore, in the high-strength, high-toughness, and high-hardenability gear shaft steel according to the present invention, the mass percentage of Nb element is controlled to 0.001 to 0.034%.

[0023] N: In the high-strength, high-toughness, and high-hardenability gear shaft steel according to the present invention, N is an interstitial atom that combines with trace amounts of alloy in the steel to form MN-type precipitates, which pin the grain boundaries at high temperatures, thereby suppressing the growth of austenite grains. If the N element content in the steel is low, the amount of MN formed will be small, and the pinning effect will not be as pronounced. However, if the N element content in the steel is too high, it tends to localize during steelmaking, reducing the toughness of the steel. Therefore, in the high-strength, high-toughness, and high-hardenability gear shaft steel according to the present invention, the mass percentage of N element is controlled to 0.006 to 0.015%.

[0024] Boron (B) can significantly improve the hardenability of steel, and even with a small required content, its effect is hundreds to thousands of times greater than that of common alloying elements, resulting in remarkable economic benefits. Furthermore, boron steel can be water-quenched, saving on quenching oil, and easily acquiring a martensitic structure, thus providing boron steel with excellent strength and hardness. As long as the boron content is appropriate, the production process is appropriate, and complete quenching is guaranteed, plasticity and toughness will not be significantly reduced. However, element B tends to be unevenly distributed, causing large fluctuations in the hardenability of the steel material. Therefore, in the high-strength, high-toughness, and high-hardenability gear shaft steel according to the present invention, the content of element B is controlled to 0.0006 to 0.0034%.

[0025] Furthermore, and importantly, in the high-strength, high-toughness, and highly hardenable gear shaft steel according to the present invention, the trace alloy element coefficient r M / N The range needs to be controlled to 1.5 to 5.0, however,

[0026]

number

[0027] In the formula, the numerical value before the percentage sign of the mass percentage content of each chemical element is substituted for each chemical element.

[0028] In this invention, Al, Nb, Ti, and N are all major grain-refining elements, and in this invention, the content of Al, Nb, Ti, and N in gear steel and the trace alloying element coefficient r M / N By controlling this, trace alloying elements form precipitates with excess N elements, thereby suppressing the growth of austenite crystal grains at high temperatures.

[0029] Furthermore, among the unavoidable impurities in the high-strength, high-toughness, and high-hardenability gear shaft steel according to the present invention, the mass percentage content of each impurity element satisfies at least one of the following conditions: P ≤ 0.030%, O ≤ 0.0020%, H ≤ 0.0002%, Ca ≤ 0.0034%.

[0030] In the above technical plan, P, O, H, and Ca are all impurity elements in steel, and to the extent technically permissible, the content of these impurity elements in the steel should be reduced as much as possible in order to obtain steel with better performance and superior quality. However, P:P tends to be concentrated at grain boundaries in steel, reducing grain boundary bond energy and worsening the impact toughness of steel. Therefore, in some embodiments of the present invention, the P content may be controlled to P ≤ 0.030%.

[0031] Since O:O readily forms oxides and complex oxides with Al elements in steel, impairing the continuity of the steel material and reducing the uniformity of the structure, low-temperature impact energy, and fatigue properties, in some embodiments of the present invention, the O element content may be controlled to O ≤ 0.0020%.

[0032] H:H accumulates in defective areas in steel, and hydrogen-induced delayed cracking occurs, especially in steel with a tensile strength level exceeding 1000 MPa. Therefore, in some embodiments of the present invention, the H element content may be controlled to H ≤ 0.0002%.

[0033] Ca: In the high-strength, high-toughness, and high-hardening gear shaft steel according to the present invention, since the Ca element is likely to form inclusions and thus affects the fatigue characteristics of the final product, the Ca element content may be controlled such that Ca ≤ 0.0034%, for example, ≤ 0.003%.

[0034] Furthermore, in the high-strength, high-toughness, and high-hardening gear shaft steel according to the present invention, it further contains at least one of the following chemical elements: 0 < S ≤ 0.04%, 0 < Ni ≤ 0.25%, 0 < Mo ≤ 0.10%, 0 < Cu ≤ 0.20%, 0 < V ≤ 0.03%.

[0035] Optionally, in the high-strength, high-toughness, and high-hardening gear shaft steel according to the present invention, in order to further improve the performance of the high-strength, high-toughness, and high-hardening gear shaft steel according to the present invention, at least one of the S, Ni, Mo, Cu, and V elements may be further added. However, S: S usually exists as an impurity element in steel and reduces the plasticity and toughness of steel. However, in the high-strength, high-toughness, and high-hardening gear shaft steel according to the present invention, a certain amount of S element can form Mn and non-metallic inclusions and improve the machinability of the steel. Therefore, in the high-strength, high-toughness, and high-hardening gear shaft steel according to the present invention, the mass percentage of S is controlled such that 0 < S ≤ 0.04%, for example, 0.001% ≤ S ≤ 0.04%.

[0036] Ni: In the high-strength, high-toughness, and high-hardening gear shaft steel according to the present invention, Ni exists in the form of a solid solution in the steel and can effectively improve the low-temperature impact performance of the steel. However, it should be noted that if the Ni content is too high, the retained austenite content in the steel will be too high and reduce the strength of the steel. Therefore, in the high-strength, high-toughness, and high-hardening gear shaft steel according to the present invention, preferably, the mass percentage of Ni is controlled such that 0 < Ni ≤ 0.25%, for example, 0.03% ≤ Ni ≤ 0.25%.

[0037] [[ID=第十九]] Mo: In the high-strength, high-toughness, and high hardenability steel for gear shafts according to the present invention, Mo can be dissolved in the steel, contributing to the improvement of the hardenability of the steel and the improvement of the strength of the steel material. In high-temperature tempering, Mo can form fine carbides to further improve the strength of the steel, and also, due to the synergistic effect of molybdenum and manganese, the stability of austenite can be significantly improved. Therefore, in the high-strength, high-toughness, and high hardenability steel for gear shafts according to the present invention, preferably, the mass percentage of Mo is controlled to be 0 < Mo ≤ 0.10%, for example, 0.01% ≤ Mo ≤ 0.10%.

[0038] Cu: In the high-strength, high-toughness, and high hardenability steel for gear shafts according to the present invention, Cu can improve the strength of the steel material and also contribute to the improvement of the weather resistance and corrosion resistance of the steel material. However, the content of the Cu element in the steel should not be too high. If the content of Cu in the steel is too high, it will localize at the grain boundaries during the heating process, causing the weakening of the grain boundaries and eventually leading to cracking. Therefore, in the high-strength, high-toughness, and high hardenability steel for gear shafts according to the present invention, preferably, the mass percentage of Cu is controlled to be 0 < Cu ≤ 0.20%, for example, 0.03% ≤ Cu ≤ 0.20%.

[0039] V: In the high-strength, high-toughness, and high hardenability steel for gear shafts according to the present invention, V can effectively improve the hardenability of the steel. In the steel, the V element can further improve the strength of the steel by forming precipitates with the C element or the N element. However, if the contents of the C element and the V element are too high, coarse VC particles will be formed. Therefore, in the high-strength, high-toughness, and high hardenability steel for gear shafts according to the present invention, the mass percentage of the V element can be controlled to be 0 < V ≤ 0.03%, for example, 0.005% ≤ V ≤ 0.03%.

[0040] Furthermore, the high-strength, high-toughness, and high-hardening steel for gear shafts according to the present invention maintains an austenite crystal grain size of level 5 to 8 after high-temperature carburizing heat treatment. However, the conditions for the simulated high-temperature carburizing heat treatment test for measuring the austenite crystal grain size may be as follows: first, heat to 1200°C, hold for 40 minutes, water-cool, and then heat to 1000°C at a heating rate of 500 to 800°C in 40 to 60 minutes, hold for 4 hours, and perform hardening by water-cooling.

[0041] Furthermore, the high-strength, high-toughness, and high-hardening steel for gear shafts according to the present invention has a J9mm hardening property of 34 to 42 HRC.

[0042] Furthermore, the high-strength, high-toughness, and high-hardening steel for gear shafts according to the present invention, after high-temperature carburizing heat treatment, has a tensile strength R m ≥1100 MPa, a yield strength R p0.2 ≥980 MPa, an elongation at break A≥12%, a reduction of area≥50%, and a Charpy impact energy A ku ≥55 J. However, the conditions for the simulated high-temperature carburizing heat treatment test for measuring mechanical properties may be as follows: heat at 880 ± 10°C for 90 minutes and perform oil hardening + heat at 870 ± 10°C for 90 minutes and perform oil hardening + heat at 200 ± 10°C for 150 minutes, temper, and air-cool.

[0043] Furthermore, the high-strength, high-toughness, and high-hardening steel for gear shafts according to the present invention may have a tensile strength R m of 1100 to 1400 MPa after high-temperature carburizing heat treatment.

[0044] Furthermore, the high-strength, high-toughness, and high-hardening steel for gear shafts according to the present invention may have a yield strength R p0.2 of 980 MPa to 1250 MPa after high-temperature carburizing heat treatment.

[0045] Furthermore, the high-strength, high-toughness, and high-hardening steel for gear shafts according to the present invention may have an elongation at break A of 12 to 15% after high-temperature carburizing heat treatment.

[0046] Furthermore, the high-strength, high-toughness, and high-hardenability gear shaft steel according to the present invention may have a cross-sectional shrinkage rate of 50-62% after high-temperature carburizing heat treatment.

[0047] Furthermore, the high-strength, high-toughness, and high-hardenability gear shaft steel according to the present invention has a Charpy impact energy A after high-temperature carburizing heat treatment. ku It may be 55-130J.

[0048] Accordingly, another objective of the present invention is to provide a method for producing high-strength, high-toughness, and high-hardenability gear shaft steel that is easy to produce and in which the resulting high-strength, high-toughness, and high-hardenability gear shaft steel possesses the characteristics of high hardenability, high strength, and high toughness.

[0049] To achieve the above objectives, the method for manufacturing the high-strength, high-toughness, and highly hardenable gear shaft steel provided by the present invention includes the following steps: (1) Smelting; (2) Casting; (3) Heating: The steel billet is first heated to below 700°C in the preheating section, then continued to be heated to below 980°C in the first heating section, maintained at that temperature, then continued to be heated to 950-1200°C in the second heating section, maintained at that temperature, and then moved to the soaking section, where the temperature is set to 1050-1250°C; Alternatively, the steel billet is slowly heated at a rate of 50-300°C / h, first in a preheating section to below 700°C, then in a first heating section to continue heating to below 980°C, then in a second heating section to continue heating to 950-1200°C, before entering a soaking section where the temperature is set to 1050-1250°C; (4) Blacksmithing or rolling.

[0050] The manufacturing method according to the present invention employs a unique process in its heating step compared to the prior art. However, a high temperature in the soaking section is used because it can improve the compositional uniformity and microstructure uniformity of the continuously cast billet during the diffusion process of heating the steel billet. At the same time, the precipitated phase has a faster solid solution rate at this temperature. Therefore, a higher heating temperature causes more undissolved precipitated phase particles in the steel to dissolve, increasing the concentration of trace alloying elements in the matrix, and resulting in the precipitation of more dispersed particles during subsequent cooling. Furthermore, only by increasing the heating temperature can the rolling or forging end temperature be raised, thereby allowing for more sufficient recovery and recrystallization of austenite after rolling and a more uniform distribution of the precipitated phase.

[0051] In the manufacturing method according to the present invention, the smelting in step (1) may be performed using electric furnace smelting or converter smelting, and may also be followed by refining and vacuum treatment. Of course, in some other embodiments, smelting may be performed using a vacuum induction furnace.

[0052] Furthermore, in process (1), low P·S scrap, offcuts, and high-quality pig iron may be selected as the furnace material for electric furnace smelting; ferrochrome, low phosphorus ferromanganese, ferromolybdenum, etc. may be prepared as alloys; calcium carbide, carbon powder, and aluminum powder may be included as reducing agents; dephosphorization with slag is performed frequently during the oxidation period, and slag discharge conditions may be controlled as follows: slag discharge temperature 1630~1660℃, P≦0.015%; tapping conditions may be controlled as follows: tapping temperature 1630~1650℃, [P]≦0.010%, [C]≧0.03%.

[0053] After electric furnace smelting or converter smelting is completed, the molten steel may be refined in a ladle furnace to remove harmful gases and inclusions from the steel. The ladle may be controlled to enter the base, the temperature may be measured and analyzed, and the argon gas pressure may be adjusted as needed. In the initial deoxidation of the ladle furnace (LF), aluminum may be added, and then alloy ingots may be added and stirred for 5 to 10 minutes. If the temperature of the molten steel is measured to be T=1650 to 1670°C, vacuum degassing may be performed to ensure that [O]≦0.0020% and [H]≦0.00015%. In one specific example, the vacuum level for vacuum degassing may be controlled to ≦66.7 Pa and maintained for 15 minutes or more.

[0054] Furthermore, in step (1), the temperature of the suspended ladle may be controlled to 1550-1570°C, which lowers the temperature of the suspended ladle, accelerating the diffusion of elements and contributing to a further reduction of dendritic segregation.

[0055] Furthermore, in step (2), casting may be performed by die casting or continuous casting. In the continuous casting process, the hot molten steel in the ladle is poured through a protective sleeve into an intermediate ladle, and the superheating of the intermediate ladle may be controlled to 20-40°C. The intermediate ladle must be thoroughly cleaned before use, have a refractory coating on its interior surface, and be free from cracks; the molten steel in the intermediate ladle must pass through a continuous casting crystallizer and be thoroughly stirred by electromagnetic stirring, and a suitable continuous casting billet with cross-sectional dimensions of 140mm x 140mm to 320mm x 425mm may be cast.

[0056] In step (2), the casting speed may be controlled to 0.6 to 2.1 m / min depending on the different billet dimensions. After that, the continuously cast billet may be placed in a slow-cooling pit and cooled slowly for 24 hours or more.

[0057] Furthermore, in step (3) of the manufacturing method according to the present invention, the heating temperature of the preheating section may be set to 600 to 700°C, and the temperature of the first heating section may be set to 900 to 980°C.

[0058] Furthermore, in step (3) of the manufacturing method according to the present invention, it is necessary to maintain the temperature in the soaking section for a predetermined time, and the temperature maintenance time in the soaking section may be 3 to 12 hours.

[0059] In step (3) of the manufacturing method according to the present invention, heating is performed in the first heating section, and then the product may or may not be kept warm, after which it is heated in the second heating section, and the warming time in the first heating section may be 0 to 3 hours, for example 0.5 hours, 1 hour, or 2 hours. In step (3) of the manufacturing method according to the present invention, heating is performed in the second heating section, and then the product may or may not be kept warm, after which it is heated in the soaking section, and the warming time in the second heating section may be 0 to 3 hours, for example 0.5 hours, 1 hour, or 2 hours.

[0060] In some other embodiments, step (3) of the manufacturing method according to the present invention involves slowly heating at a rate of 50 to 300°C / h, first heating to below 700°C in a preheating section, then continuing to heat to below 980°C in a first heating section without holding, then continuing to heat to 950 to 1200°C in a second heating section without holding, and entering a soaking section without holding, and this process may be carried out in a walking beam heating furnace.

[0061] Furthermore, in step (4) of the manufacturing method according to the present invention, the forging start temperature or rolling start temperature is controlled to 1050 to 1250°C, and the forging end temperature or rolling end temperature is controlled to ≥ 900°C. In some embodiments, in step (4) of the manufacturing method according to the present invention, the forging end temperature or rolling end temperature is set to 900 to 1000°C.

[0062] In this embodiment, the reason for controlling the forging start temperature or rolling start temperature to 1050-1250°C and the forging end temperature or rolling end temperature to ≥900°C is that this process further contributes to the dissolution of N from the γ solid solution and its combination with trace alloying elements in the steel to form nitrides.

[0063] It needs to be explained that the solubility of N in α-Fe is lower than that of N in γ-Fe, and furthermore, the acceleration of phase transformation results in two peaks in the amount of precipitation. However, if the forging or rolling end temperature is low, the precipitated phase precipitates at the peak, leading to anisotropy in the microstructure due to the uneven distribution of the precipitated phase and insufficient recovery recrystallization. Therefore, the forging or rolling end temperature is controlled to ≥900°C. In addition, by increasing the forging or rolling end temperature, finer crystal grains can be obtained. However, as the crystal grains become finer, the difference between the average ferrite grain size after supercooled austenite transformation and the spacing of the manganese-rich bands increases, and the tendency to form pearlite in the manganese-rich bands weakens, thus reducing the banded structure.

[0064] In the above technical solution, in step (4) of the manufacturing method according to the present invention, after removing the steel billet from the furnace, scale may be removed with high-pressure water to eliminate oxide scale.

[0065] Furthermore, in step (4) of the manufacturing method according to the present invention, the steel billet may be directly forged to the final finished product dimensions during forging. During rolling, the steel billet may be directly rolled to the final finished product dimensions, or the steel billet may be first rolled to a specified intermediate billet dimension, then heated and rolled to the final finished product dimensions.

[0066] In some embodiments, step (4) of the manufacturing method according to the present invention is to first roll a steel billet into an intermediate billet (which may have dimensions of 140 mm × 140 mm to 260 mm × 260 mm), and control the intermediate billet rolling completion temperature to 1000 to 1050°C; next, the intermediate billet is heated in the following process: first, the intermediate billet is heated to 680 to 700°C in a preheating section, then to 1050 to 1100°C in a first heating section, and further to 1200 to 1220°C in a second heating section, with a heating rate of 300 to 500°C / h; then, it is placed in a soaking section, the temperature of the soaking section is set to 1200 to 1250°C, and the soaking section is kept warm for 3 to 5 hours; then, the heated intermediate billet is rolled into a finished product, and the finished product rolling completion temperature is controlled to ≥ 900°C (for example, 900 to 1000°C). [Effects of the Invention]

[0067] The high-strength, high-toughness, and highly hardenable steel for gear shafts according to the present invention and its manufacturing method have the following advantages and beneficial effects compared to the prior art: (1) The present invention enables the development of gear shaft steel with high hardenability through a rational chemical composition design combined with an optimized manufacturing process. Bar stock rolled or forged using this high-strength, high-toughness, and high-hardenability gear shaft steel can be efficiently processed into gears and, after subsequent downstream high-temperature carburizing heat treatment, can possess excellent strength and toughness.

[0068] (2) The high-strength, high-toughness, and high-hardenability gear shaft steel according to the present invention controls the coefficient of trace alloying elements and the nitrogen element content, and strictly controls the atomic molar ratio, while simultaneously adding an appropriate amount of Nb element to suppress the abnormal growth of austenite crystal grains and improve the austenite crystal grain coarsening temperature of the gear steel. As a result, even after carburizing treatment at a high temperature of 1000°C for 4 hours, the crystal grain size is stably maintained at level 5 to 8, and various performance characteristics can be achieved to meet the practical performance indicators for gear shaft steel.

[0069] (3) The high-strength, high-toughness, and highly hardenable gear shaft steel according to the present invention has a rational composition and manufacturing process design, and by controlling the content of trace alloying elements in the steel, it prevents the generation of large harmful inclusions in the steel material, ensures stable production quality of the steel material, reduces the production cost of the steel material, and enables mass production on a bar stock production line.

[0070] (4) The high-strength, high-toughness, and highly hardenable gear shaft steel according to the present invention is superior to conventional technology in terms of hardenability, austenitic grain size, and cost competitiveness. On the premise of ensuring high hardenability and a narrow band width, the type and amount of alloying elements in the steel can be controlled to improve the applicability of the steel.

[0071] (5) The high-strength, high-toughness, and high-hardenability gear shaft steel according to the present invention has the potential for a wide range of industrial applications, as it can shorten the carburizing time and reduce the production cost of gear shafts when applied to the production and manufacture of subsequent gear shafts. [Modes for carrying out the invention]

[0072] The high-strength, high-toughness, and highly hardenable gear shaft steel and its manufacturing method according to the present invention will be further interpreted and explained below based on specific examples, but this interpretation and explanation will not unduly limit the technical solutions of the present invention.

[0073] The high-strength, high-toughness, and high-hardenability gear shaft steels used in Examples 1-8 were all manufactured using the following process: (1) The smelting and casting were carried out according to the chemical compositions shown in Tables 1-1 and 1-2 below. However, smelting may be carried out using a 50 kg vacuum induction furnace, a 150 kg vacuum induction furnace, or a 500 kg vacuum induction furnace, or by an electric furnace smelting + off-furnace refining + vacuum degassing method, or by a converter smelting + off-furnace refining + vacuum degassing method. (2) Casting. (3) Heating: The steel billet was first heated to below 700°C in the preheating section, then continued to be heated to below 980°C in the first heating section and kept warm, then continued to be heated to 950-1200°C in the second heating section and kept warm, before being moved to the soaking section, where the temperature was set to 1050-1250°C and kept warm before the subsequent rolling or forging was carried out; (4) Forging or rolling: The forging start temperature or rolling start temperature was controlled to 1050-1250°C, and the forging end temperature or rolling end temperature was controlled to ≥900°C.

[0074] Furthermore, the specific processes for the high-strength, high-toughness, and high-hardenability gear shaft steels in Examples 1-8 and the steels in Comparative Examples 1-4 were as follows: [Examples]

[0075] The molten steel was smelted in a 50 kg vacuum induction furnace according to the chemical compositions shown in Tables 1-1 and 1-2 below. The molten steel was cast into a steel billet, heated, and then forged and divided. The steel billet was first heated to 700°C in the preheating section, then continued to be heated to 900°C in the first heating section and held for 0 hours. After that, it was heated to 950°C in the second heating section and held for 1 hour before entering the soaking section, where the temperature was set to 1050°C and held for 3 hours. Subsequent forging was then performed, with the forging completion temperature controlled to 910°C, and finally forged into a Φ60 mm bar. [Examples]

[0076] The molten steel was smelted in a 150 kg vacuum induction furnace according to the chemical compositions shown in Tables 1-1 and 1-2 below. The molten steel was cast into a steel billet, heated, and then forged and divided. The steel billet was first heated to 650°C in the preheating section, then continued to be heated to 950°C in the first heating section and held for 0.5 hours. After that, it was heated to 1100°C in the second heating section and held for 0 hours before entering the soaking section, where the temperature was set to 1200°C and held for 5 hours. Subsequent forging was then performed, with the forging completion temperature controlled to 1000°C, and finally forged into a Φ90 mm bar. [Examples]

[0077] The molten steel was smelted in a 500 kg vacuum induction furnace according to the chemical compositions shown in Tables 1-1 and 1-2 below. The molten steel was cast into a steel billet, heated, and then forged and divided. The steel billet was first heated to 600°C in the preheating section, then continued to be heated to 980°C in the first heating section and held for 3 hours. After that, it was heated to 1200°C in the second heating section and held for 3 hours before entering the soaking section, where the temperature was set to 1250°C and held for 12 hours. Subsequent forging was then performed, with the forging completion temperature controlled to 1000°C, and finally forged into a Φ120 mm bar. [Examples]

[0078] The steel was smelted in an electric furnace according to the chemical compositions shown in Tables 1-1 and 1-2, followed by refining and vacuum treatment. The resulting material was then cast into a 280mm x 280mm continuous casting billet. The continuous casting billet was heated in a walking beam type heating furnace, controlled slowly at a rate of 300°C / h. First, it was heated to 620°C in the preheating section, then to 950°C in the first heating section, then to 1150°C in the second heating section, and finally into the soaking section, where the temperature was set to 1200°C and held for 4 hours before rolling. After removing the steel billet from the heating furnace, the scale was removed with high-pressure water before rolling began. The rolling end temperature was controlled to 970°C, and the billet was ultimately rolled into a Φ80mm bar. [Examples]

[0079] The steel was smelted in an electric furnace according to the chemical compositions shown in Tables 1-1 and 1-2, followed by refining and vacuum treatment. The resulting material was then cast into a 320mm x 425mm continuous casting billet. The heating of the continuous casting billet in a walking beam furnace was controlled, slowly heating at a rate of 150°C / h. First, it was heated to 600°C in the preheating section, then to 950°C in the first heating section, then to 1200°C in the second heating section, before entering the soaking section. The temperature in the soaking section was set to 1230°C and maintained for 4.5 hours before subsequent rolling. After removing the steel billet from the heating furnace, the scale was removed with high-pressure water before rolling began. The billet was rolled into an intermediate billet, with the first rolling completion temperature (i.e., the intermediate billet rolling completion temperature) controlled to 1050°C, resulting in an intermediate billet size of 220mm x 220mm. Then, the intermediate billet was heated in a walking beam furnace at a rate of 400°C / h, heated to 680°C in the preheating section, heated to 1050°C in the first heating section, heated to 1200°C in the second heating section, held at a constant temperature, and then moved to the soaking section, where the temperature was set to 1220°C. After being held at this temperature for 3.5 hours, it was removed from the furnace, scale was removed with high-pressure water, and then rolling was started. The second rolling completion temperature (i.e., the finished product rolling completion temperature) was controlled to 950°C, and the finished bar material was specified to be Φ90mm. [Examples]

[0080] The steel was smelted in an electric furnace according to the chemical compositions shown in Tables 1-1 and 1-2, followed by refining and vacuum treatment. The resulting material was then cast into a 280 mm x 280 mm continuous casting billet. The heating of the continuous casting billet in a walking beam heating furnace was controlled, slowly heating at a rate of 300°C / h. First, it was heated to 680°C in the preheating section, then to 900°C in the first heating section, then to 1180°C in the second heating section, before entering the soaking section where the temperature was set to 1200°C and held for 4.5 hours before subsequent rolling. After removing the steel billet from the heating furnace, the scale was removed with high-pressure water before rolling began to create an intermediate billet. The first rolling completion temperature (i.e., the intermediate billet rolling completion temperature) was controlled to 1000°C, resulting in an intermediate billet with dimensions of 140 mm x 140 mm. Then, the intermediate billet was heated in a walking beam type heating furnace, slowly heating at a rate of 500°C / h. First, it was heated to 700°C in the preheating section, then to 1100°C in the first heating section, then to 1200°C in the second heating section, and then into the soaking section, where the temperature was set to 1220°C. After being kept warm for 3.5 hours, it was removed from the furnace, scale was removed with high-pressure water, and then rolling was started. The second rolling end temperature (i.e., the finished product rolling end temperature) was controlled to 920°C, and the finished product bar size was set to Φ25mm. [Examples]

[0081] According to the chemical compositions shown in Tables 1-1 and 1-2, converter smelting was performed, followed by refining and vacuum treatment. The resulting material was then cast into die-cast billets. The billets were heated in a walking-beam heating furnace, controlled slowly at a rate of 50°C / h. First, they were heated to 620°C in the preheating section, then to 950°C in the first heating section, then to 1150°C in the second heating section, before entering the soaking section. The temperature in the soaking section was set to 1200°C and held for 8 hours before rolling. After removing the steel billets from the heating furnace, scale was removed with high-pressure water before rolling began. The rolling end temperature was controlled to 970°C, and the billets were finally rolled into Φ90mm bars. [Examples]

[0082] According to the chemical compositions shown in Tables 1-1 and 1-2, converter smelting was performed, followed by refining and vacuum treatment. The resulting material was then cast into die-cast billets. The billets were heated in a walking-beam type heating furnace, controlled slowly at a rate of 100°C / h. First, they were heated to 600°C in the preheating section, then to 950°C in the first heating section, then to 1200°C in the second heating section, before entering the soaking section. The temperature in the soaking section was set to 1230°C and maintained for 7 hours before subsequent rolling. After removing the steel billets from the heating furnace, scale was removed using high-pressure water before rolling began to create intermediate billets. The first rolling completion temperature (i.e., the intermediate billet rolling completion temperature) was controlled to 1050°C, resulting in intermediate billet dimensions of 260mm x 260mm. Then, the intermediate billet was heated in a walking beam type heating furnace, slowly heated at a rate of 300°C / h, heated to 680°C in the preheating section, heated to 1050°C in the first heating section, heated to 1200°C in the second heating section, then entered the soaking section, where the temperature was set to 1220°C and maintained for 5 hours. After being removed from the furnace, the scale was removed with high-pressure water, and then rolling was started. The second rolling end temperature (i.e., the finished product rolling end temperature) was controlled to 950°C, and the finished bar material was specified to be Φ60mm.

[0083] Comparative Example 1 and Comparative Example 2 Comparative Examples 1 and 2 were derived from commercially available materials, and underwent electric furnace smelting and refining processes to guarantee the purity of the materials. Comparative Example 3 Smelting was carried out in a 50 kg vacuum induction furnace according to the chemical compositions shown in Tables 1-1 and 1-2. Molten steel was cast into steel billets, heated, forged, and divided. The steel billets were heated in a box furnace at a rate of 300 °C / h to 1100 °C, held for 3 hours, and then the subsequent forging was carried out, with the forging completion temperature controlled to 910 °C, finally forging into Φ60 mm rods. Comparative Example 4 The steel was smelted in an electric furnace according to the chemical compositions shown in Tables 1-1 and 1-2, and then refined and vacuum-treated. After that, it was cast into a 320 mm × 425 mm continuous casting billet. The continuous casting billet was heated in a walking beam heating furnace at a rate of 150 °C / h, first to 600 °C in the preheating section, then to 950 °C in the first heating section, then to 1200 °C in the second heating section, and then into the soaking section, where the temperature was set to 1230 °C and held for 4.5 hours before subsequent rolling. The steel billet removed from the heating furnace was descaled with high-pressure water, and then rolling was started to roll it into an intermediate billet. The first rolling completion temperature (i.e., the intermediate billet rolling completion temperature) was controlled to 1050 °C, and the dimensions of the intermediate billet were 220 mm × 220 mm. Then, the intermediate billet was heated in a walking beam type heating furnace, slowly heated at a rate of 400°C / h, heated to 680°C in the preheating section, heated to 1050°C in the first heating section, heated to 1200°C in the second heating section, then entered the soaking section, where the temperature was set to 1220°C and maintained for 6 hours. After being removed from the furnace, the scale was removed with high-pressure water, and then rolling was started. The second rolling end temperature (i.e., the finished product rolling end temperature) was controlled to 950°C, and the finished product bar size was set to Φ90mm.

[0084] The mass percentage ratios of each chemical element in the high-strength, high-toughness, and high-hardenability gear shaft steels of Examples 1 to 8 and the comparative steels of Comparative Examples 1 to 4 are shown in Tables 1-1 and 1-2.

[0085] [Table 1-1]

[0086] [Table 1-2]

[0087] Table 2 shows the specific process parameters for the high-strength, high-toughness, and high-hardenability gear shaft steels used in Examples 1-8 and the comparative steels used in Comparative Examples 1-4.

[0088] [Table 2-1]

[0089] [Table 2-2]

[0090] As can be seen from Table 2, in Examples 5, 6, 8, and Comparative Example 4, during the rolling process, the steel billet was first rolled to the specified intermediate billet dimensions, and then intermediate heating was performed again to secondary rolling to the final finished product dimensions.

[0091] To verify the performance of the high-strength, high-toughness, and high-hardenability gear shaft steel according to the present invention, samples were taken from the high-strength, high-toughness, and high-hardenability gear shaft steels obtained in Examples 1 to 8 and the comparative steels obtained in Comparative Examples 1 to 4, and simulated high-temperature carburizing heat treatment tests were performed to measure the mechanical performance and austenite grain size. The measurement results are shown in Table 3. However, For the simulated high-temperature carburizing heat treatment test to measure the austenite grain size, the sample was first heated to 1200°C and held for 40 minutes, then cooled with water. After that, it was heated to 1000°C at a rate of 600°C over 50 minutes, held for 4 hours, and then quenched by water cooling.

[0092] For measuring austenite grain size, the austenite grain size was evaluated in accordance with the ASTM E112 standard.

[0093] For the measurement of mechanical performance, samples were prepared in accordance with GB / T 2975-2018 "Sampling locations and sample preparation for mechanical performance testing of steel and steel products," and a Φ15 mm blank was prepared referring to GB / T 3077-2015; for the simulated high-temperature carburizing heat treatment test to measure mechanical performance, the samples were heated at 880°C for 90 mins and oil-quenched, then heated at 870°C for 90 mins and oil-quenched, then heated at 200°C for 150 mins and tempered, followed by air cooling. A tensile test was then performed in accordance with GB / T 228.1-2010 "Tensile tests for metallic materials, Part 1: Room temperature test methods," and the tensile strength R was measured. m , yield strength R p0.2 Simultaneously measuring the elongation at break A and the sectional shrinkage Z, the room temperature Charpy impact energy A of each example and comparative example was measured using GB / T 229-2007 "Metallic Materials Charpy Pendulum Impact Test Method". ku We measured it.

[0094] Furthermore, samples were taken from the high-strength, high-toughness, and high-hardenability gear shaft steels obtained in Examples 1-8 and the comparative steels obtained in Comparative Examples 1-4, and hardenability and hardness measurements were performed. The results of the measurement tests are also shown in Table 3. However, For the measurement of hardenability, samples were taken from hot-rolled round steel in accordance with national standard GB / T 225 for each example and the comparative example. Samples were prepared, and a tip hardenability test (Jominy test) was performed referring to GB / T 5216. The normalizing temperature was controlled to 920±10℃, and the quenching temperature was controlled to 870±5℃. Rockwell hardness was measured in accordance with GB / T 230.2, and the hardness value (HRC) at a specific location was obtained, for example, the hardness at a position 9 mm away from the quenched end (i.e., J9 mm).

[0095] The results of the measurement tests for the high-strength, high-toughness, and high-hardenability gear shaft steels used in Examples 1-8 and the comparative steels used in Comparative Examples 1-4 are shown in Table 3.

[0096] [Table 3]

[0097] As can be seen from Table 3, the high-strength, high-toughness, and high-hardenability gear shaft steels according to Examples 1 to 8 of the present invention maintain their austenite grain size within the range of levels 5 to 8 even after undergoing simulated high-temperature carburizing heat treatment at 1000°C, and no phenomena such as mixed crystals or abnormal grain coarsening were observed, thus demonstrating excellent high-temperature grain stability.

[0098] Furthermore, as can be seen from Table 3, the high-strength, high-toughness, and high-hardenability gear shaft steels according to each of Examples 1 to 8 of the present invention all have a J9mm hardenability of 34 to 42 HRC at a typical position, exhibiting higher hardenability and a narrower hardenability band width. In addition, each example is subjected to simulated high-temperature carburizing heat treatment, and then the tensile strength R m All of them exceed 1100 MPa, yield strength R p0.2 All of the MPa values ​​exceeded 980 MPa, the elongation at break A was ≥ 12% in all cases, the sectional shrinkage rate exceeded 50% in all cases, and the Charpy impact energy A ku All of them exceeded 55J.

[0099] Therefore, bar stock rolled or forged using this high-strength, high-toughness, and high-hardenability gear shaft steel can be efficiently processed into gear shafts. After high-temperature carburizing heat treatment by downstream users, it acquires high strength and toughness, making it efficiently applicable to high-end components such as automotive transmissions, retarders for new energy vehicles, and industrial retarders, thus possessing good prospects for practical application and value.

[0100] Unlike the embodiments of the present invention, Comparative Example 1 had a crystal grain size of level 0 after simulated high-temperature carburizing, meaning the crystal grains grew abnormally, and thus failed to meet practical requirements.

[0101] Although the comparative steel in Comparative Example 2 did not exhibit mixed crystallization, its crystal grains were fine after simulated high-temperature carburizing heat treatment, resulting in low hardenability and failing to meet the requirement for high hardenability. Furthermore, Comparative Example 2 had low strength.

[0102] The comparative steel in Comparative Example 3 exhibited mixed crystal phenomena after being subjected to simulated high-temperature carburizing heat treatment at a temperature of 1000°C. However, 5(0) indicates that the average grain size was level 5, but in some areas it became coarser, resulting in level 0.

[0103] On the other hand, the comparative steel in Comparative Example 4 showed mixed crystal phenomena (level 4) even after simulated high-temperature carburizing heat treatment at a temperature of 1000°C. However, 0(4) indicates that the average grain size was level 0, and in some areas it became coarser, resulting in level 4. In addition, Comparative Example 4 had a low impact energy.

[0104] Furthermore, the combinations of technical features in this application are not limited to the combinations described in the claims or the specific embodiments, and all technical features described in this application can be freely combined or combined in any form, as long as they do not contradict each other.

[0105] Furthermore, it should be noted that the embodiments described above are merely specific examples of the present invention. The present invention is not limited to the above embodiments, and it is clear that any similar changes or modifications that a person skilled in the art can directly derive from the disclosure of the present invention or readily conceive are also covered within the scope of the present invention.

Claims

1. It contains Fe and unavoidable impurities, and further contains the following chemical elements in the following mass percentages: C: 0.16-0.22%, Si: 0.10-0.40%, Mn: 0.86-1.24%, Cr: 0.95-1.44%, Al: 0.02-0.05% , Ti: 0.015 to 0.039%, Nb: 0.001 to 0.034%, N: 0.006 to 0.015%, B: 0.0006 to 0.0034%; The trace alloy element coefficient r Ｍ／Ｎ The range is 1.5 to 5.0, however, [Math 1] In the formula, each chemical element is replaced with the number before the percentage sign representing the mass percentage content of that chemical element. A high-strength, high-toughness, and highly hardenable steel for gear shafts, characterized by the following properties.

2. The mass percentage content of each chemical element is: The composition is as follows: C: 0.16–0.22%, Si: 0.10–0.40%, Mn: 0.86–1.24%, Cr: 0.95–1.44%, Al: 0.02–0.05%, Ti: 0.015–0.039%, Nb: 0.001–0.034%, N: 0.006–0.015%, B: 0.0006–0.0034%, with the remainder being Fe and unavoidable impurities; The trace alloy element coefficient r Ｍ／Ｎ The range is 1.5 to 5.0, however, [Math 2] In the formula, each chemical element is replaced with the number before the percentage sign representing the mass percentage content of that chemical element. A high-strength, high-toughness, and highly hardenable steel for gear shafts according to claim 1, characterized in that it is a high-strength, high-toughness, and highly hardenable steel.

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

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

5. The high-strength, high-toughness, and highly hardenable steel for gear shafts according to claim 1 or 2, characterized in that the austenite grain size after high-temperature carburizing heat treatment is maintained at level 5 to 8.

6. The high-strength, high-toughness, and high-hardenability gear shaft steel according to claim 1 or 2, characterized in that its J9mm hardenability is 34 to 42 HRC.

7. After high-temperature carburizing heat treatment, the tensile strength R ｍ At ≥1100 MPa, yield strength R ｐ０．２ At pressure ≥ 980 MPa, with elongation at break A ≥ 12%, and sectional shrinkage ≥ 50%, the Charpy impact energy A is... ｋｕ A high-strength, high-toughness, and highly hardenable steel for gear shafts according to claim 1 or 2, characterized in that it is ≥ 55 J.

8. A method for manufacturing high-strength, high-toughness, and highly hardenable steel for gear shafts according to any one of claims 1 to 7, characterized by comprising the following steps. (1) Smelting; (2) Casting; (3) Heating: The steel billet is first heated to below 700°C in the preheating section, then continued to be heated to below 980°C in the first heating section, maintained at that temperature, then continued to be heated to 950-1200°C in the second heating section, maintained at that temperature, and then moved to the soaking section, where the temperature is set to 1050-1250°C; (4) Blacksmithing or rolling.

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

10. The manufacturing method according to claim 8, characterized in that step (4) involves directly rolling or forging the material to the dimensions of the finished product.

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

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