High hardenability gear steel having excellent impact properties and method for manufacturing the same

Abstract

This invention discloses a high hardenability gear steel with excellent impact performance, which contains Fe and unavoidable impurities, and also contains the following chemical elements in the following mass percentages: C: 0.14-0.24%, Si: 0.10-0.40%, Mn: 0.40-1.00%, S: 0.01-0.03%, Cr: 0.3-0.80%, Ni: 1.6-2.5%, Mo: 0.25-0.65%, Cu: 0.02-0.20%, Al: 0.015-0.030%, N: 0.006-0.015%, Nb: 0.005-0.02%. Accordingly, the present invention also discloses a method for manufacturing the above-mentioned high hardenability gear steel, which includes the following steps: (1) smelting; (2) continuous casting; (3) heating: heating is carried out in a walking beam furnace, wherein the temperature of the preheating section is controlled to be below 800°C, the temperature of the first heating section is 1050-1120°C, the temperature of the second heating section is 1190-1240°C, and the temperature of the soaking section is 1150-1200°C; the total heating time is not less than 3 hours; (4) rolling: the initial rolling temperature of the billet is controlled to be ≥1050°C, and the final rolling temperature is controlled to be ≥900°C; after rolling, air cooling or slow cooling to room temperature is performed.

Description

Technical Field

[0001] This invention relates to a carburized gear steel and its manufacturing method, and more particularly to a high hardenability gear steel and its manufacturing method. Background Technology

[0002] In recent years, with the advancement of manufacturing and material production technologies in industries such as automobiles and high-speed rail, automobiles have gradually begun to develop towards higher speeds, higher load capacities, lower noise levels, and lighter weights. The market has also placed higher demands on special steels for automotive gears.

[0003] As is well known, heavy-duty gears have higher requirements for mechanical properties such as strength, wear resistance, fatigue strength, and impact toughness than ordinary gears. To meet the requirement of high bending strength, the steel must have high hardenability to ensure that the carburized layer achieves a fully martensitic structure after carburizing and quenching. Furthermore, the hardenability design of the gear steel should ensure minimal heat treatment deformation to guarantee good meshing between gears. If the hardenability of the steel is insufficient, the bending strength of the gear after heat treatment will be insufficient, leading to pitting and spalling on the tooth surface during service, and increasing the risk of tooth breakage. Conversely, if the deformation during carburizing and quenching heat treatment is excessive, poor tooth meshing will occur, not only generating noise and affecting user experience but also causing abnormal wear and reducing the gear's service life.

[0004] Currently, the most widely used heavy-duty gear steels in existing technologies are CrMo, CrNi, and CrNiMo series gear steels. Among them, CrMo series gear steel has the greatest advantage of low heat treatment deformation and is the most widely used grade of gear steel. It is suitable for rear axle drive gears and engine gears in medium and heavy-duty trucks. CrNi series gear steel is mainly used for gearbox gears and rear axle drive gears in heavy-duty trucks and large buses. CrNiMo series gear steel is used for gearbox gears in medium and heavy-duty trucks and large buses. NiCrMo series gear steel reduces the Cr content and increases the Ni content, giving it better impact toughness, making it particularly suitable for gears in transmission systems of heavy-duty trucks.

[0005] With the development of new energy vehicles, new energy heavy-duty trucks have also begun to be put into the market. Compared with traditional fuel trucks, the motors of new energy heavy-duty trucks can output greater torque, and at the same time, the requirements for transmission gears are also higher. Therefore, there is an urgent need for a high hardenability gear steel with better impact resistance.

[0006] In the current technology, some researchers have developed some high hardenability gear steels, but these steels all have certain defects to varying degrees.

[0007] For example, Chinese patent document CN108866439A, published on November 23, 2018, entitled "A Nb, Ti Composite Microalloyed High-Temperature Vacuum Carburizing Heavy-Duty Gear Steel," discloses a Nb, Ti composite microalloyed high-temperature vacuum carburizing heavy-duty gear steel with the following composition by mass percentage: C: 0.15–0.23%, Si: 0.10–0.40%, Mn: 0.45–0.90%, Cr: 1.5%. The alloy composition is as follows: Ni: 1.40–1.70%, Mo: 0.15–0.55%, Nb: 0.02–0.08%, Ti: 0.015–0.08%, P≤0.020%, S≤0.020%. By adding Nb and Ti microalloying elements and controlling their content, the precipitated phases pinning grain boundaries can suppress the coarsening and growth of austenite grains during high-temperature vacuum carburizing, thereby increasing the carburizing temperature of heavy-duty gear steel to 1000℃ and above. However, in this technical solution, excessively high Nb and Ti content in the steel can easily lead to the precipitation and aggregation of precipitated phases at high temperatures, weakening the effect of pinning grain boundaries and adversely affecting hardenability.

[0008] For example, Chinese patent document CN105316592A, published on February 10, 2016, entitled "A High-Strength and High-Toughness Gear Steel and Its Manufacturing Method," discloses a high-strength and high-toughness gear steel and its manufacturing method. The composition by mass percentage is as follows: C: 0.14-0.22%, Si: 0.15-0.35%, Mn: 0.45-0.75%, Cr: 1.00-1.35%, Ni: 2.80-3.20%, Mo: 0.15-0.35%, V: 0.08-0.15%, Nb: 0.08-0.15%, N≤0.002%, O≤0.0015%. By adding a high content of Nb and V elements, the strength is improved, resulting in a tensile strength Rm≥1280MPa and an impact energy Akv≥98J. This technical solution employs vacuum induction melting and vacuum consumable metallurgy processes, resulting in extremely low nitrogen content in the steel, making it difficult to produce economically using traditional continuous casting processes.

[0009] For example, Chinese patent document CN108342640A, published on July 31, 2018, entitled "A High Hardenability Gear Steel and Its Manufacturing Method," discloses a high hardenability gear steel and its manufacturing method, with the following composition by mass percentage: C: 0.19%-0.25%, Si: 0.15%-0.35%, Mn: 0.70%-1.05%, Cr: 0.70%-0.80%, Ni: 0.4%. The composition is 0%-0.65%, 0 < Cu ≤ 0.20%, Mo: 0.33-0.40%, Al: 0.020-0.050%, Ti: 0.003-0.010%, b: 0.05-0.08%, B: 0.0001-0.0003%, N: 0.0080%-0.0150%. This method refines the grain size by controlling the content of Al, Ti, Nb, and N in the gear steel, as well as the (Al, Ti) / N ratio. However, in this technical solution, when the content of microalloying elements Nb and Ti is high, large, sharp-shaped Nb and Ti carbonitrides are easily precipitated during high-temperature carburizing. For NiMo-based heavy-duty gears, these sharp-shaped large-particle precipitates are particularly sensitive, severely affecting the gear's impact toughness and fatigue life. Summary of the Invention

[0010] One of the objectives of this invention is to provide a high hardenability gear steel with excellent impact performance. This high hardenability gear steel has excellent hardenability, high strength, and good toughness. It can meet the requirements of heavy-duty gear materials, is suitable for manufacturing gears for automotive transmission systems, and can also be used to manufacture other large-size gear parts.

[0011] To achieve the above objectives, this invention proposes a high hardenability gear steel with excellent impact performance, which contains Fe and unavoidable impurities, as well as the following chemical elements in the following mass percentages:

[0012] C: 0.14~0.24%, Si: 0.10~0.40%, Mn: 0.40~1.00%, S: 0.01~0.03%, Cr: 0.3~0.80%, Ni: 1.6~2. 5%, Mo: 0.25~0.65%, Cu: 0.02~0.20%, Al: 0.015~0.030%, N: 0.006~0.015%, Nb: 0.005~0.02%.

[0013] Furthermore, in the high hardenability gear steel described in this invention, the mass percentage content of each chemical element is as follows:

[0014] C: 0.14–0.24%, Si: 0.10–0.40%, Mn: 0.40–1.00%, S: 0.01–0.03%, Cr: 0.3–0.80%, Ni: 1.6–2.5%, Mo: 0.25–0.65%, Cu: 0.02–0.20%, Al: 0.015–0.030%, N: 0.006–0.015%, Nb: 0.005–0.02%; balance Fe and unavoidable impurities.

[0015] In this invention, the high hardenability gear steel designed is a NiCrMo series gear steel, and the design principles of its various chemical elements are as follows:

[0016] C: In the high hardenability gear steel described in this invention, carbon (C) is an essential component of the steel and one of the most important elements affecting its hardenability. The gear steel designed in this invention requires both high surface strength and sufficient core impact toughness. When the C content in the steel is too low, below 0.14%, the steel's strength is insufficient, and good hardenability cannot be guaranteed. Correspondingly, the C content in the steel should not be too high either; if the C content is too high, the core toughness requirement of the gear cannot be met. Therefore, in the high hardenability gear steel described in this invention, the mass percentage of C is controlled between 0.14% and 0.24%.

[0017] Si: In the high hardenability gear steel described in this invention, Si can dissolve into ferrite, strengthening it and improving the steel's strength, hardness, wear resistance, elasticity, and elastic limit. However, the Si content in the steel should not be too high, as excessive Si content increases the steel's brittleness. Therefore, in the high hardenability gear steel described in this invention, the mass percentage of Si is controlled between 0.10% and 0.40%.

[0018] Mn: In the high hardenability gear steel described in this invention, Mn is one of the main elements affecting the hardenability of the steel. Adding an appropriate amount of Mn to the steel can not only significantly increase its hardenability but also reduce its toughness. Furthermore, Mn can dissolve into ferrite, improving the strength and hardness of the steel, and resulting in finer, stronger pearlite lamellars during cooling after hot rolling. However, it should be noted that the Mn content in the steel should not be too high, as excessive Mn content can lead to poor hot plasticity. Therefore, in the high hardenability gear steel described in this invention, the mass percentage of Mn is controlled between 0.40% and 1.00%.

[0019] S: In the high hardenability gear steel described in this invention, sulfur (S) segregates severely, which deteriorates the quality of the steel and reduces its plasticity, making it a harmful element. However, S is also one of the main free-machining elements; as the S content in the steel increases, the machinability index of the steel will significantly improve. However, MnS tends to elongate along the rolling direction during hot rolling, reducing the transverse mechanical properties of the steel. Therefore, considering both the beneficial and adverse effects of S, the mass percentage of S in the high hardenability gear steel described in this invention is controlled between 0.01% and 0.03%.

[0020] Cr: In the high hardenability gear steel described in this invention, adding an appropriate amount of Cr can significantly improve the hardenability, strength, and wear resistance of the steel. Simultaneously, Cr can prevent decarburization during heating, rolling, and heat treatment. However, excessive Cr content will significantly reduce the toughness of quenched and tempered steel, forming coarse carbides distributed along grain boundaries. Therefore, in the high hardenability gear steel described in this invention, the mass percentage of Cr is controlled between 0.3% and 0.80%.

[0021] Ni: In the high hardenability gear steel described in this invention, Ni is one of the main alloying elements added to the steel. It exists in the steel in solid solution form, which can effectively improve the low-temperature impact performance of the steel. However, it should be noted that Ni is also a valuable alloying element. Considering the cost of alloys, excessive Ni should not be added to the steel. Therefore, in the high hardenability gear steel described in this invention, the mass percentage of Ni is controlled between 1.6% and 2.5%.

[0022] Mo: In the high hardenability gear steel described in this invention, Mo, as a medium-strong carbide-forming element, can strongly inhibit the nucleation and growth of carbides, while also effectively improving the hardenability of the steel. In this technical solution, Mo can work in combination with Mn to significantly improve the stability of austenite and enhance the hardenability of the steel. Furthermore, Mo can delay the pearlite transformation and promote the formation of acicular ferrite; adding an appropriate amount of Mo can effectively inhibit the formation of banded structures. However, it should be noted that the Mo content in the steel should not be too high, as excessive Mo content will increase smelting costs. Therefore, considering production costs and the beneficial effects of adding Mo, the mass percentage of Mo in the high hardenability gear steel described in this invention is controlled between 0.25% and 0.65%.

[0023] Cu: In the high hardenability gear steel described in this invention, adding an appropriate amount of Cu can improve the strength of the steel and is beneficial to improving its weather resistance and corrosion resistance. However, it should be noted that the Cu content in the steel should not be too high. When the Cu content in the steel is too high, it will accumulate at the grain boundaries during heating, leading to grain boundary weakening and cracking. Therefore, to maximize the beneficial effects of Cu, the mass percentage of Cu in the high hardenability gear steel described in this invention is controlled between 0.02% and 0.20%.

[0024] Al: In the high hardenability gear steel described in this invention, Al is a grain-refining element. It can combine with N to form AlN, thereby further refining the grains and improving the strength and toughness of the steel. However, it should be noted that the Al content in the steel should not be too high. Excessive Al content not only easily increases the chance of inclusion formation in the steel, affecting the casting performance, but also impairs the toughness of the steel. Therefore, in the high hardenability gear steel described in this invention, the mass percentage of Al is controlled between 0.015% and 0.030%.

[0025] N: In the high hardenability gear steel described in this invention, N is an interstitial atom that can combine with Al and Nb microalloying in the steel to form MN-type precipitates. These precipitates pin grain boundaries at high temperatures, thereby inhibiting austenite grain growth. When the N content in the steel is low, fewer MN-type precipitates are formed, and the pinning effect is not significant. Conversely, when the N content is too high, it will prematurely precipitate and accumulate in the steel, reducing the effect of inhibiting grain growth and decreasing the toughness of the steel. Therefore, in the high hardenability gear steel described in this invention, the mass percentage of N is controlled between 0.006% and 0.015%.

[0026] Nb: In the high hardenability gear steel described in this invention, Nb can form fine precipitates in the steel, which can inhibit recrystallization and effectively refine the grains. However, it should be noted that the Nb content in the steel should not be too high. When the Nb content is too high, coarse NbC particles will form during high-temperature tempering, which will reduce the impact toughness of the steel. Therefore, to maximize the beneficial effects of Nb, the mass percentage of Nb in this invention is controlled between 0.005% and 0.02%.

[0027] Furthermore, in the high hardenability gear steel described in this invention, the hardenability DI value is controlled to be 2.5–4.5 inches, wherein:

[0028] DI=0.54×[C]×(3.33[Mn]+1)×(0.70[Si]+1)×(0.331+1.45×[Ni]-0.612×[Ni] 2+0.125×[Ni] 3 )×(2.16[Cr]+1)×(3.00[Mo]+1)×(0.365[Cu]+1)×(1.73[V]+1)

[0029] In the above-mentioned technical solution of the present invention, the present invention can also control the hardenability DI value of high hardenability gear steel. When the DI value is lower than 2.5 inches, the hardenability of the steel is insufficient; while when the DI value is higher than 4.5 inches, the production and manufacturing are difficult and the overall cost is high.

[0030] Furthermore, the chemical composition of the high hardenability gear steel described in this invention also satisfies the following requirements:

[0031] 2.0≤(1.2Ni+0.9Mo+0.8Cu) / (Cr+0.8Mn)≤3.2;

[0032] 1.8≤(Al+0.15Nb) / N≤2.8;

[0033] In the formula, each element is substituted with its corresponding chemical element mass percentage.

[0034] In the above technical solution of the present invention, the high hardenability gear steel of the present invention controls the mass percentage content of the single chemical elements Al, Nb and N, while also specifying the ratio of the three elements and controlling their mass percentage content to satisfy: 1.8≤(Al+0.15Nb) / N≤2.8, which can obtain the optimal element ratio for refining grains. The size and quantity of precipitates are controlled by the element ratio and content, which promotes the formation of AlN and Nb(C,N) precipitates in the steel and inhibits the growth of austenite grains.

[0035] Accordingly, in this invention, the mass percentage ratio of the main elements Ni, Mo, Mn, Cu, and Cr in the steel can be further controlled to satisfy the following relationship: 2.0≤(1.2Ni+0.9Mo+0.8Cu) / (Cr+0.8Mn)≤3.2.

[0036] Ni, as the main element in the steel of this invention, can improve both the strength and impact toughness of the steel, achieving a good balance between strength and impact toughness. It can also improve the hardenability of the gear steel core and enhance the bending resistance of the gear. Mo can significantly improve the stability of austenite, delay the pearlite transformation, promote the formation of acicular ferrite, refine the microstructure, improve the impact toughness of the steel, and also improve the hardenability of the gear steel core. Cu can also improve the hardenability of the gear steel core. Mn and Cr in the steel of this invention can improve the overall hardenability curve of the gear steel, but at the same time, they will reduce the impact performance. By controlling the above composition range and formula, not only the impact performance of the gear steel can be improved, but also the hardenability of the gear steel core can be improved, resulting in a high-hardenability gear steel with excellent impact performance.

[0037] Furthermore, in the high hardenability gear steel described in this invention, among the unavoidable impurities, P≤0.015%, O≤0.0020%, H≤0.0002%, B≤0.0010%, V≤0.02%, and Ti≤0.01%.

[0038] In the above technical solution, P, B, O, H, V and Ti are all impurity elements in steel. Under the condition that the technical conditions permit, in order to obtain high hardenability gear steel with stable performance, the content of impurity elements in the material should be reduced as much as possible.

[0039] Furthermore, in the high hardenability gear steel described in this invention, the austenite grain size after high-temperature (e.g., 950°C) carburizing treatment is grade 7-8.

[0040] Furthermore, in the high hardenability gear steel described in this invention, after carburizing heat treatment, its room temperature impact energy Aku≥130J and its -20℃ impact energy Aku≥100J.

[0041] Furthermore, in the high hardenability gear steel described in this invention, the diameter of the rolled round steel is Φ30~200mm.

[0042] Accordingly, another objective of the present invention is to provide a method for manufacturing the above-mentioned high hardenability gear steel. This method is simple to produce, and the high hardenability gear steel obtained has an austenite grain size of 7 to 8 after high-temperature carburizing. Its room temperature impact energy Aku≥130J and -20℃ impact energy Aku≥100J after carburizing heat treatment have high room temperature and low temperature impact toughness, and can be widely used in engineering machinery and other applications that require high strength and toughness steel.

[0043] To achieve the above objectives, the present invention provides a method for manufacturing the aforementioned high hardenability gear steel, comprising the following steps:

[0044] (1) Smelting;

[0045] (2) Continuous casting;

[0046] (3) Heating: Heating is carried out in a walking beam furnace, wherein the temperature of the preheating section is controlled to be below 800℃, the temperature of the first heating section is 1050-1120℃, the temperature of the second heating section is 1190-1240℃, and the temperature of the soaking section is 1150-1200℃; the total heating time is not less than 3 hours.

[0047] (4) Rolling: Control the initial rolling temperature of the billet to ≥1050℃ and the final rolling temperature to ≥900℃; after rolling, air cool or slow cool to room temperature.

[0048] In the smelting process of step (1) of the present invention, smelting can be carried out in an electric furnace or converter, followed by LF refining, VD or RH vacuum treatment, and steel is tapped after the composition meets the requirements. Among them, electric furnace smelting can be carried out using scrap steel with low P and S content and molten iron.

[0049] In the manufacturing process described above in this invention, the inventors have optimized the heating and rolling processes. During the heating process, the ingot obtained from continuous casting needs to be heated in a walking beam furnace, and the temperature of the preheating section is controlled to be below 800°C, the temperature of the first heating section is 1050-1120°C, the temperature of the second heating section is 1190-1240°C, the temperature of the soaking section is 1150-1200°C, and the total heating time is not less than 3 hours, so as to ensure that AlN, Nb (C, N) and MC-type carbides in the steel can be completely dissolved into the steel, making the internal composition of the ingot uniform.

[0050] When the billet is removed from the heating furnace and cooled to about 1100℃, AlN and Nb(C,N) precipitates begin to precipitate from the steel. Controlling the initial rolling temperature ensures that a certain number of AlN and Nb(C,N) particles have already begun to precipitate in the steel when the ingot is rolled. After dynamic recrystallization occurs during the rolling process, the precipitated AlN and Nb(C,N) particles can inhibit the growth of recrystallized grains and play a role in refining the microstructure.

[0051] In this invention, the initial rolling temperature of the cast billet is controlled to be ≥1050℃, and the final rolling temperature is controlled to be ≥900℃. If the initial rolling temperature is too low, the final rolling temperature will be too low, making dynamic recrystallization difficult to occur during rolling, resulting in uneven steel deformation and an uneven microstructure after rolling. Conversely, if the initial rolling temperature is too high, the grains will grow rapidly after dynamic recrystallization, which is detrimental to grain refinement.

[0052] After rolling, the round steel can be air-cooled or placed in an insulated pit to slowly cool to room temperature. Finally, depending on the specific needs, conventional treatment processes such as post-rolling straightening, post-rolling annealing, or peeling can be selected.

[0053] Furthermore, in the manufacturing method described in this invention, in step (2), the superheat of the molten steel in the tundish is controlled to be 15-35°C during continuous casting.

[0054] Furthermore, the manufacturing method of the present invention further includes step (5) post-rolling treatment, which includes at least one of the following: straightening, heat treatment, and peeling.

[0055] Furthermore, in the manufacturing method described in this invention, a carburizing heat treatment is also included after step (4) or (5).

[0056] Compared with the prior art, the high hardenability gear steel with excellent impact performance and its manufacturing method described in this invention have the following advantages and beneficial effects:

[0057] The high hardenability NiCrMo gear steel of this invention optimizes the design of strengthening elements such as Ni, Mo, Cu, Cr, and Mn in the steel. While controlling the mass percentage content of a single chemical element, it can also control: 2.0≤(1.2Ni+0.9Mo+0.8Cu) / (Cr+0.8Mn)≤3.2. By controlling the above formula, the core hardenability of the gear steel can be improved, resulting in excellent impact performance. The high hardenability gear steel obtained after carburizing heat treatment has a room temperature impact energy Akv≥130J and an impact energy Akv≥100J at -20℃.

[0058] Furthermore, while controlling the content range of Al, Nb, and N elements, this invention also stipulates that the above three elements must simultaneously satisfy: 1.8≤(Al+0.15Nb) / N≤2.8. Based on the conventional AlN grain refinement, the use of the microalloying element Nb further refines the grains, controls the size and quantity of precipitates, and promotes the formation of AlN and Nb(C,N) precipitates in the steel to inhibit the growth of austenite grains. This results in the NiCrMo gear steel obtained by this invention having a fine-grain strengthening effect and excellent impact toughness, meeting the requirements of heavy-duty gear materials, and can also be used to manufacture other large-size gear parts.

[0059] Accordingly, during the manufacturing process, the present invention has also optimized the heating and rolling processes. The process design is reasonable, the casting performance is good, and the production process window is wide, which can realize mass commercial production.

[0060] In actual manufacturing, this invention, based on chemical composition design, controls heating temperature and holding time to ensure that AlN, Nb(C, N), and MC-type carbides in the steel can be completely dissolved into the steel, while preventing grains from becoming too coarse due to overheating. Simultaneously, during the rolling process, by controlling the initial and final rolling temperatures, this invention can achieve a more reasonable precipitation of AlN and Nb(C, N) particles in the ingot, inhibiting the growth of recrystallized grains and thus refining the microstructure. Detailed Implementation

[0061] The high hardenability gear steel and its manufacturing method described in this invention will be further explained and described below with reference to specific embodiments. However, such explanation and description do not constitute an improper limitation on the technical solution of this invention.

[0062] Examples 1-6 and Comparative Examples 1-2

[0063] The high hardenability gear steels in Examples 1-6 were all prepared using the following steps:

[0064] (1) Smelting shall be carried out in accordance with the chemical composition shown in Tables 1-1 and 1-2 below: Smelting may be carried out in an electric furnace or converter, and the steel shall be tapped after LF refining and VD or RH vacuum treatment, and the composition shall meet the design target requirements. Among them, electric furnace smelting may be carried out using scrap steel and molten iron with low P and S content.

[0065] (2) Continuous casting: Continuous casting is used to cast molten steel that meets the composition design requirements into ingots. During continuous casting, the superheat of molten steel in the tundish is controlled between 15-35℃.

[0066] (3) Heating: Heating is carried out in a walking beam furnace, wherein the temperature of the preheating section is controlled to be below 800℃, the temperature of the first heating section is 1050-1120℃, the temperature of the second heating section is 1190-1240℃, and the temperature of the soaking section is 1150-1200℃; the total heating time is not less than 3 hours.

[0067] (4) Rolling: Before rolling the billet, high pressure water is used to remove the surface oxide scale, and then rolling is carried out. The initial rolling temperature of the billet is controlled to be ≥1050℃ and the final rolling temperature is ≥900℃. After rolling, it is air-cooled or slowly cooled to room temperature. The finished product after rolling is a bar. The size specification range of the rolled product is Φ30~200mm.

[0068] Of course, in the actual manufacturing process of gear steel, the round steel obtained after rolling can be further subjected to conventional treatments such as straightening, heat treatment, and peeling after cooling to room temperature, which will not be elaborated here.

[0069] In this invention, the chemical composition design and related processes of the high hardenability gear steels in Examples 1-6 all meet the design specifications of this invention. Although Comparative Examples 1-2 were also prepared using the above-mentioned process of "smelting, continuous casting, heating, and rolling", their chemical composition design, DI value control, and related processes all have parameters that do not meet the design specifications of this invention. Their chemical composition design can be found in Tables 1-1 and 1-2 below.

[0070] Tables 1-1 and 1-2 list the mass percentage of each chemical element in the high hardenability gear steel of Examples 1-6 and the comparative steel of Comparative Examples 1-2.

[0071] Table 1-1. (wt.%, balance Fe and other unavoidable impurities other than P, O, H, B, V and Ti)

[0072]

[0073] Table 1-2.

[0074]

[0075] Note: In the above table,

[0076] DI=0.54×[C]×(3.33[Mn]+1)×(0.70[Si]+1)×(0.331+1.45×[Ni]-0.612×[Ni] 2 +0.125×[Ni] 3 )×(2.16[Cr]+1)×(3.00[Mo]+1)×(0.365[Cu]+1)×(1.73[V]+1)

[0077] "(Al+0.15Nb) / N", "(1.2Ni+0.9Mo+0.8Cu) / (Cr+0.8Mn)"

[0078] Each chemical element in the formula is substituted with its mass percentage.

[0079] Table 2 lists the specific process parameters for the high hardenability gear steels of Examples 1-6 and the comparative steels of Comparative Examples 1-2 in the above process steps.

[0080] Table 2.

[0081]

[0082]

[0083] Samples were taken from the high hardenability gear steel of Examples 1-6 and the comparative steel of Comparative Examples 1-2 in the rolled state, and simulated carburizing and quenching tests, end hardenability and mechanical property tests were conducted using the samples of each example and comparative example.

[0084] In the simulated carburizing and quenching test, different samples were kept at 950℃ for 4 hours, then removed and water-quenched. Metallographic samples were then prepared and etched to observe the microstructure of each example and comparative example. The austenite grain size was evaluated according to standard GB / T6394-2017. The measured austenite grain size is listed in Table 3 below.

[0085] Table 3 lists the austenite grain size test results after simulated carburizing and quenching tests of the high hardenability gear steels of Examples 1-6 and the comparative steels of Comparative Examples 1-2 in the rolled state.

[0086] Table 3.

[0087]

[0088] Accordingly, after observing the austenite grain size of the steels in Examples 1-6 and Comparative Examples 1-2, end-hardenability tests were also conducted on the rolled steels of each example and comparative example.

[0089] When conducting the end-hardenability test, the end-hardenability test was carried out in accordance with the GB / T225-2006 standard to test the hardness of the J13 position 13mm from the end of the sample. The end-hardenability of each embodiment and comparative steel in the rolled state was obtained and listed in Table 4 below.

[0090] Table 4 lists the end J13mm hardenability results of the high hardenability gear steels of Examples 1-6 and the comparative steels of Comparative Examples 1-2 in the rolled state.

[0091] Table 4.

[0092]

[0093] As can be seen from Table 4 above, in this embodiment, the hardenability of the high hardenability gear steel end J13mm in Examples 1-6 is between 36.4 and 39.2 HRC.

[0094] Accordingly, in this invention, based on the rolled steels of Examples 1-6 and Comparative Examples 1-2, Φ25mm blanks were prepared for simulated carburizing heat treatment. During the simulated carburizing heat treatment, different samples were held at 950℃ for 4 hours, then removed and oil-quenched, followed by tempering at 200℃. Impact test specimens were prepared from the steels of Examples 1-6 and Comparative Examples 1-2 after simulated carburizing heat treatment for impact performance testing. The impact performance test was conducted according to GB / T 229-2007 to obtain the impact energy AKu of each example and comparative example steel after simulated carburizing heat treatment at room temperature and -20℃. The test results are listed in Table 5 below.

[0095] Table 5 lists the impact performance test results of the high hardenability gear steels of Examples 1-6 and the comparative steels of Comparative Examples 1-2 after carburizing heat treatment.

[0096] Table 5.

[0097]

[0098] As can be seen from Table 3 above, the austenite grain size of the high hardenability gear steels in Examples 1-6 of the present invention is between grade 7 and 8 after simulated carburizing and holding at 950℃ for 4 hours. In contrast, the austenite grain size of the comparative steels in Comparative Examples 1-2 is grade 5.5-6 after carburizing and holding at 950℃ for 4 hours. Therefore, compared to Comparative Examples 1-2, the steel of the present invention has a better effect on inhibiting austenite grain growth under high-temperature carburizing conditions.

[0099] Accordingly, as can be seen from Table 4 above, the end hardenability J13 of the high hardenability gear steels of Examples 1-6 is between 37.1 and 39.5 HRC, while the end hardenability J13 of Comparative Examples 1-2 is 29.9 and 42.5 HRC, respectively, indicating that the present technical solution has higher hardenability than Example 1.

[0100] Furthermore, as can be seen from Table 5, compared with Comparative Examples 1 and 2, the high hardenability gear steels of Examples 1-6 prepared in the rolled state according to the present invention still have good impact performance after further carburizing heat treatment. Their room temperature impact energy Aku after carburizing heat treatment is between 135-189J, and their impact energy Aku at -20℃ is between 105-165J.

[0101] Although the hardenability value of Comparative Example 2 is higher than that of Examples 1-6, the DI value and the proportion of the main alloying elements in this composition system are not within the composition ratio range of the present invention. After carburizing heat treatment, its impact performance is significantly lower than that of Examples 1-6, and its austenite grain size is also significantly lower than that of Examples 1-6. This indicates that the composition ratio of this patent can be used to obtain a high hardenability gear steel with better overall performance.

[0102] In summary, this invention, through rational chemical composition design and optimized processes, can obtain high-hardenability gear steel with excellent performance. The high-hardenability NiCrMo gear steel obtained by this invention possesses fine-grain strengthening effect and excellent impact toughness. After carburizing treatment, it exhibits high room temperature and low temperature impact toughness, meeting the requirements of heavy-duty gear materials. It is suitable for manufacturing automotive transmission system gears and can also be used to manufacture other large-size gear parts.

[0103] Furthermore, the high hardenability gear steel designed in this invention has a wide production process window, making it very easy to achieve mass commercial production and showing good application prospects.

[0104] It should be noted that the combination of the technical features in this case is not limited to the combination methods described in the claims of this case or the combination methods described in the specific embodiments. All technical features described in this case can be freely combined or combined in any way, unless they contradict each other.

[0105] It should also be noted that the embodiments listed above are merely specific embodiments of the present invention. Obviously, the present invention is not limited to the above embodiments, and similar changes or modifications made thereto are those that can be directly derived or easily conceived by those skilled in the art from the content disclosed in the present invention, and should all fall within the protection scope of the present invention.

Claims

1. A high-hardenability gear steel with excellent impact performance, characterized in that, Its mass percentage content of each chemical element is as follows: C: 0.14~0.24%, Si: 0.10~0.40%, Mn: 0.40~1.00%, S: 0.01~0.03%, Cr: 0.3~0.80%, Ni: 1.6~2.5%, Mo: 0.25~0.65%, Cu: 0.02~0.20%, Al: 0.015~0.030%, N: 0.006~0.015%, Nb: 0.005~0.02%; the balance is Fe and unavoidable impurities. The content of each chemical element also satisfies the following condition: 2.0 ≤ (1.2Ni + 0.9Mo + 0.8Cu) / (Cr + 0.8Mn) ≤ 3.2; The hardenability DI value of the high hardenability gear steel is 2.5 to 4.5 inches; in Each chemical element in the formula should be substituted with its mass percentage. The high hardenability gear steel, after carburizing heat treatment, has an impact energy Aku≥130J at room temperature and an impact energy Aku≥100J at -20℃. The high hardenability gear steel is prepared by the following steps: Smelting; Continuous casting; Heating: Heating is carried out in a walking beam furnace, with the preheating section temperature controlled below 800℃, the first heating section temperature at 1050-1120℃, the second heating section temperature at 1190-1240℃, and the soaking section temperature at 1150-1200℃; the total heating time is not less than 3 hours. Rolling: Control the initial rolling temperature of the billet to ≥1050℃ and the final rolling temperature to ≥900℃; after rolling, air cool or slow cool to room temperature.

2. The high hardenability gear steel as described in claim 1, characterized in that, It satisfies: 1.8≤(Al+0.15Nb) / N≤2.

8.

3. The high hardenability gear steel as described in claim 1, characterized in that, In unavoidable impurities, P ≤ 0.015%, O ≤ 0.0020%, H ≤ 0.0002%, B ≤ 0.0010%, V ≤ 0.02%, and Ti ≤ 0.01%.

4. The high hardenability gear steel as described in claim 1 or 2, characterized in that, After carburizing heat treatment, its austenite grain size is grade 7-8.

5. The high hardenability gear steel as described in claim 1, characterized in that, The diameter of the rolled round steel is Φ30~200mm.

6. The method for manufacturing high hardenability gear steel according to any one of claims 1-5, characterized in that, It includes the following steps: (1) Smelting; (2) Continuous casting; (3) Heating: Heating is carried out in a walking beam furnace, wherein the temperature of the preheating section is controlled to be below 800℃, the temperature of the first heating section is 1050-1120℃, the temperature of the second heating section is 1190-1240℃, and the temperature of the soaking section is 1150-1200℃; the total heating time is not less than 3 hours. (4) Rolling: Control the initial rolling temperature of the billet to ≥1050℃ and the final rolling temperature to ≥900℃; air cool or slow cool to room temperature after rolling.

7. The manufacturing method as described in claim 6, characterized in that, In step (2), the superheat of the molten steel in the tundish is controlled at 15-35℃ during continuous casting.

8. The manufacturing method as described in claim 6, characterized in that, It also includes step (5) post-rolling treatment, which includes at least one of the following: straightening, heat treatment, peeling.

9. The manufacturing method according to any one of claims 6-7, characterized in that, The process also includes carburizing heat treatment after step (4).

10. The manufacturing method as described in claim 8, characterized in that, The process also includes carburizing heat treatment after step (5).

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