Fine-grained low-temperature bainite gear steel and method for producing same

Abstract

The application discloses a fine-grained low-temperature bainite gear steel and a production method thereof, and belongs to the technical field of gear steel. The gear steel comprises the following chemical components in percentage by weight: C: 0.16-0.20%, Si: 1.60-2.0%, Mn: 1.50-2.00%, S: ≤0.010%, Cr: 1.00-1.40%, Mo: 0.10-0.30%, Ni: 1.40-1.70%, Nb: 0.020-0.030%, V: 0.10-0.20%, Al: 0.030-0.050%, P: ≤0.010%, [N]: 90-160ppm, and the rest is Fe and inevitable impurity elements. The gear steel is produced by adopting an electric arc furnace smelting-LF refining-RH vacuum treatment-continuous casting-forging (finishing) process. The end quenching property of the obtained steel satisfies 43-47HRC, J15: 42-46HRC, and J25: 36-44HRC. After 900-930 DEG C carburizing gas quenching and low-temperature tempering treatment, the impact energy of the product at -40 DEG C is greater than or equal to 50J, the impact energy at -80 DEG C is greater than or equal to 30J, and the austenite grain is greater than or equal to 8.0 grade. The gear steel obtained can effectively improve the high quenching property and low-temperature resistance, thereby meeting the service requirements of new energy vehicles in different environments.

Description

Technical Field

[0001] This invention belongs to the field of gear steel technology, and relates to a fine-grained, low-temperature resistant bainitic gear steel and its production method, which is suitable for gears used in automobiles and engineering machinery. Background Technology

[0002] With the continuous development of China's automotive industry, automobile production and sales have increased year by year, and the domestic automotive industry's international influence has grown significantly. Key automotive components are also experiencing rapid development, and my country has become an indispensable backbone of the global automotive parts industry. Automobiles consist of chassis systems, transmission systems, engine systems, etc. Among these, gears, as key components in power transmission, require high strength, toughness, wear resistance, and torsional resistance. Especially in recent years, the explosive growth of new energy vehicles has led to a year-on-year increase in demand for gears. Furthermore, due to the high instantaneous acceleration and torque of new energy vehicles, the performance requirements for gears are becoming increasingly stringent.

[0003] Commonly used gear steel materials typically undergo surface strengthening treatment via carburizing heat treatment to ensure a certain level of surface hardness and core toughness, thus achieving a good balance of strength and toughness. However, most gear steels currently undergoing carburizing heat treatment exhibit a martensitic surface and a core of pearlite, ferrite, and a small amount of bainite, especially pronounced in large gears. Traditional gear steels, after carburizing heat treatment, form a martensitic structure, resulting in significant deformation and insufficient low-temperature resistance, failing to meet the performance requirements of automobiles in cold weather. Bainitic gears, achieved by controlling the cooling rate, lack sufficient strength and toughness and have poor low-temperature performance. Therefore, designing a gear steel that satisfies the requirements of strength, toughness, and low-temperature resistance through composition and processing is urgently needed.

[0004] A search revealed a Chinese patent publication number CN108866439B, which discloses a Nb-Ti composite microalloyed high-temperature vacuum carburizing heavy-duty gear steel. Its composition by mass percentage is: C: 0.15-0.23%, Si: 0.10-0.40%, Mn: 0.45-0.90%, Cr: 1.50-1.80%, 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%, with the remainder being Fe and unavoidable impurities. This patent employs a composite microalloying method, adding Nb and Ti microalloying elements and controlling their content to increase the carburizing temperature and refine the grain size of the heavy-duty gear steel. However, it does not strictly control the hardenability of the gear steel.

[0005] For example, Chinese invention patent CN111471938B, published on June 4, 2021, discloses a carbide-free bainitic steel for electric vehicle gears and its production method. By overcoming the problems of deformation and process difficulty in the quenching and tempering process of existing gear steels, the hot properties reach tensile strength of 1700-18500MPa, yield strength of 1500-1650MPa, and impact energy of 45-65J. However, the impact energy is relatively low, and it is difficult to guarantee the low-temperature performance of the gear steel.

[0006] In summary, current patent literature on gear steel focuses on improvements to existing products or the development of processes, without fundamentally addressing the problem of insufficient low-temperature performance in gear steel. There is an urgent need for a low-temperature resistant, fine-grained gear steel to solve the aforementioned issues. Summary of the Invention

[0007] 1. The problem to be solved

[0008] One of the objectives of this invention is to provide a fine-grained, low-temperature resistant bainitic gear steel with end hardenability that meets the following requirements: J9: 43-47 HRC, J15: 42-46 HRC, J25: 36-44 HRC, impact energy at -40℃ reaches 50 J, impact energy at -80℃ reaches 30 J, and austenite grain size ≥ 8.0.

[0009] Another object of the present invention is to provide a method for producing the above-mentioned gear steel, which adopts the process of electric arc furnace smelting - LF refining - RH vacuum treatment - round billet continuous casting - forging (finishing) into finished products.

[0010] The purpose of this invention is to fundamentally solve the problem of insufficient low-temperature performance of gear steel and to address the issue of insufficient service life of gear steel in low-temperature environments.

[0011] 2. Technical Solution

[0012] To solve the above problems, the technical solution adopted by the present invention is as follows:

[0013] Firstly, this invention provides a fine-grained, low-temperature resistant bainitic gear steel, which comprises the following chemical composition by weight percentage:

[0014] C: 0.16–0.20%, Si: 1.60–2.0%, Mn: 1.50–2.00%, S: ≤0.010%, Cr: 1.00–1.40%, Mo: 0.10–0.30%, Ni: 1.40–1.70%, Nb: 0.020–0.030%, V: 0.10–0.20%, Al: 0.030–0.050%, P: ≤0.010%, [N]: 90–160 ppm, with the remainder being Fe and unavoidable impurity elements.

[0015] It should be noted that the function of each component and the content control of each component in the gear steel composition provided by this invention are as follows:

[0016] C: C is the most basic and effective strengthening element in steel, and it is the most effective element affecting hardenability. Moreover, it is relatively inexpensive. In order to ensure that the gear steel has sufficient strength and hardenability, it must contain a certain amount of carbon. This invention adopts a low carbon content, and at the same time, in order to ensure that the core has sufficient strength and toughness, the carbon content is controlled at 0.16-0.20%.

[0017] Si: Si is a deoxidizer and also improves the hardness of steel through solid solution strengthening. It can also improve the hardenability of gear steel. In this invention, Si plays a role in solid solution strengthening and promotes the formation of bainite while inhibiting the formation of carbides, especially carbides in the carburized layer, refining the carbide size and reducing the content of retained austenite. Therefore, the Si content should not be less than 1.6%. However, excessive silicon increases the activity of C, promoting the decarburization and graphitization tendency of steel during forging and heat treatment, making the carburized layer easy to oxidize. Therefore, the Si content is controlled between 1.60% and 2.0%.

[0018] Mn: Mn can expand the austenite phase region and stabilize the austenite structure, improving the hardenability of steel. However, excessive Mn can dissolve in ferrite, increasing the hardness and strength of ferrite and austenite in the steel. Simultaneously, Mn can improve the stability of the austenite structure, significantly enhancing the hardenability of the steel. In this invention, Mn is mainly used to reduce the pearlite and ferrite transformation regions, increase the bainite transformation region, and improve hardenability. However, excessive Mn will reduce the plasticity of the steel, causing a deterioration in toughness during hot forging. The Mn content is controlled between 1.50% and 2.00%.

[0019] Cr: Cr can improve the hardenability and strength of steel. Cr combines with carbon in steel to form carbides. Because gear steel undergoes low-temperature tempering after quenching, no large carbide blocks precipitate; instead, fine carbides precipitate. These precipitated carbides accumulate between martensite laths, inhibiting lath movement under stress. Dislocations in the martensite can become entangled, improving strength and fatigue resistance. However, excessively high Cr content can form a carbide film, affecting the carburizing effect and reducing the performance of the carburized layer. The Cr content should be controlled between 1.00-1.40%.

[0020] Mo (Mo) significantly improves the hardenability of steel and prevents temper brittleness and overheating tendency. Furthermore, the appropriate combination of Mo and Cr elements in this invention significantly improves hardenability and tempering resistance, and Mo also refines grain size. However, too low a Mo content limits these effects, while too high a Mo content promotes the formation of grain boundary ferrite films, which is detrimental to the hot plasticity of steel, increases the tendency for reheat cracking, and increases costs. Therefore, the Mo content is controlled at 0.10–0.30%.

[0021] Ni: Ni can effectively improve the core toughness of steel, lower the ductile-brittle transition temperature, improve low-temperature impact performance, and enhance the fatigue strength of steel materials. Another role of Ni in this project is to increase stacking fault energy, thereby increasing the dislocation's ability to cross the potential barrier and improving torsional resistance. However, Ni is relatively expensive, and excessively high Ni content can reduce machinability after hot working. Therefore, the Ni content is controlled between 1.40% and 1.70%.

[0022] V: V has a strong affinity for carbon and oxygen, which can refine grains and microstructure, and also produce solid solution strengthening. After heat treatment, it improves the strength and heat sensitivity of steel. However, excessively high V content does not significantly improve grain refinement or strength, but increases costs. Therefore, the V content should be controlled at 0.10–0.20%.

[0023] Nitrogen (Nb): Nitrogen has a strong affinity for carbon and oxygen, which can refine grains and microstructure. It also produces solid solution strengthening, improving the strength and heat sensitivity of steel after heat treatment. However, excessively high Nb content does not significantly improve grain refinement or strength, but increases costs. Therefore, the Nb content should be controlled at 0.020–0.030%.

[0024] Al: Al is an effective deoxidizer and can form AlN to refine grains. When the Al content is below 0.030%, its effect is not obvious, and when it is above 0.050%, it easily forms coarse inclusions, deteriorating the steel's properties. Another role of Al in this invention is to reduce the decrease in austenite coarsening temperature caused by the addition of B. Therefore, the timing of Al addition during the steelmaking process needs to be carefully adjusted to ensure that the Al content is controlled between 0.030% and 0.050%.

[0025] P and S: Sulfur readily combines with manganese in steel to form MnS inclusions, causing hot brittleness; P is an element with a strong tendency to segregate, increasing cold brittleness, reducing plasticity, and negatively impacting the uniformity of product microstructure and properties. Control P ≤ 0.010%, S ≤ 0.010%.

[0026] TO and [H]: TO forms oxide inclusions in steel, so TO should be controlled to ≤10ppm; [H] forms white spots in steel, which seriously affects product performance, so [H] should be controlled to ≤1.0ppm.

[0027] [N] can form compounds with Nb, B, and Al, refining grains. A reasonable Al / [N] ratio has a significant effect on grain refinement, while excessive [N] can lead to continuous casting defects such as bubbles. Therefore, the [N] content should be controlled between 90 and 130 ppm. Furthermore, in this invention, the Al to [N] ratio satisfies the following condition: 2 ≤ Al / [N] ≤ 5.

[0028] The hardenability of steel mainly depends on the stability of supercooled austenite. The more stable the supercooled austenite, the lower the critical cooling rate of the steel, and the greater its hardenability. Factors affecting the stability of supercooled austenite mainly include the steel's chemical composition, austenite homogeneity, austenite grain size, and austenitization state. Studies have found that alloying elements such as Cr, Mn, and Mo can increase the hardenability of materials; reducing the content of residual elements that embrittle grain boundaries, such as P and Sn; and repeated quenching to refine the crystal grains also help improve the hardenability of materials. However, due to their scarcity and high cost, it is desirable to avoid using alloying elements such as Cr and Mo. Generally, adding Al can act as a deoxidizer, and AlN also has a grain boundary pinning effect, which can also be added to prevent grain coarsening. Previous studies have also shown that free Al in austenite can delay the transformation of austenite to ferrite, which is believed to be related to the distribution of Al near the ferrite-austenite transformation interface. By employing a high Si content, solid solution strengthening is achieved, hardenability is increased, bainitic phase transformation is promoted, and the content of residual austenite on the surface is reduced.

[0029] This invention provides a production process for the aforementioned gear steel, including electric arc furnace smelting, LF refining, RH vacuum treatment, continuous casting, and forging (finishing) to form the finished product. The experimental steel undergoes thorough deoxidation during refining to ensure a low oxygen content, and aluminum wire is added in the later stages of vacuum treatment to adjust the Al content. This ensures sufficient aluminum content while preventing excessive inclusions in the steel. Specifically, the process includes the following steps:

[0030] 1) Heating: The uniform heating temperature of the steel billet in the heating furnace is controlled at 1200~1230℃, and the total time for preheating, heating and uniform heating is controlled at 5.0h~10.0h.

[0031] 2) Forging: initial forging temperature 1000~1100℃, final forging temperature 750~800℃.

[0032] 3) Slow cooling: After forging, the surface is cooled to 600-650℃ on a cooling bed and then put into the pit for slow cooling. The slow cooling time is ≥5h. After being taken out of the pit, it is ground and peeled to ensure that there is no decarburization and zero defects on the surface.

[0033] Gear steel processing technology: During high-temperature heating at 1200℃, both Al and N are dissolved in austenite. In the subsequent slow cooling stage, they accumulate at or around the austenite grain boundaries. Later, although AlN precipitation and diffusion of residual dissolved Al occur during quenching, the Al segregated near the original coarse austenite grain boundaries moves within the lattice and is difficult to migrate in large quantities to the vicinity of the newly formed austenite grain boundaries. This results in a relative reduction in the amount of dissolved Al near the grains, which is insufficient to improve hardenability. Therefore, the free aluminum enriched in the dendritic interstitial spaces, after forging, is enriched at the original austenite grain boundaries and does not change with the grain boundary changes after heat treatment.

[0034] The low-cost, high-torque output gear steel produced using the above technical solution was tested for end-hardenability according to GB / T 225. The end-hardenability control of J9, J15, and J25 was significantly improved compared with the CiNiMo system, and the cost was significantly reduced compared with 18CrNiMo7-6. Its end-hardenability meets the following requirements: 43~47HRC, J15: 42~46HRC, J25: 36~44HRC, impact energy at -40℃ ≥50J, impact energy at -80℃ ≥30J, and austenite grain size ≥8.0 grade. Attached Figure Description

[0035] Figure 1 The grain size after carburizing in Example 1 of this invention; Detailed Implementation

[0036] The following detailed description of exemplary embodiments of the invention is taken with reference to the accompanying drawings, which form part of the description and illustrate exemplary embodiments in which the invention may be practiced. While these exemplary embodiments have been described in sufficient detail to enable those skilled in the art to practice the invention, it should be understood that other embodiments may be implemented and various changes may be made to the invention without departing from the spirit and scope thereof. The more detailed description of embodiments of the invention below is not intended to limit the scope of the claimed invention, but is merely illustrative and not restrictive of the description of the features and characteristics of the invention, to suggest the best mode for carrying out the invention, and is sufficient to enable those skilled in the art to practice the invention. Therefore, the scope of the invention is defined only by the appended claims.

[0037] Examples 1-5 of this invention are five heats of steel produced using specific components and specific smelting processes described in this invention. The process involves electric arc furnace smelting, LF refining, RH vacuum treatment, continuous casting, and forging (finishing). The continuously cast billet is heated to 1200-1230℃ and held for ≥5 hours before being forged into round steel. The initial forging temperature is 1000-1100℃, and the final forging temperature is 750-8000℃. After forging, the steel is cooled to ≥650℃ on a cooling bed and then slowly cooled in a pit for 48 hours.

[0038] Comparative Examples 1 and 2 are two heats of 18CrNiMo7-6 steel (center line) produced according to GB / T 3077 requirements. The process involved electric arc furnace smelting, LF refining, RH vacuum treatment, continuous casting, and forging (finishing). The continuously cast billets were heated to 1200-1250℃ and held for ≥4h before being forged into round bars. The initial forging temperature was 1000-1100℃, and the final forging temperature was 750-800℃. After forging, the steel was cooled to 600-650℃ on a cooling bed and then placed in a pit for slow cooling for 48h.

[0039] Specifically, the chemical composition of the steel in Examples 1-5 and Comparative Examples 1-2 is shown in Table 1:

[0040] Table 1. Chemical composition of steel in the embodiments and comparative examples of the present invention (unit: [N], [H] is ppm, others are wt%)

[0041] Example 1 0.17 1.62 1.55 0.007 0.005 1.15 0.15 0.030 1.45 0.021 0.11 97 Example 2 0.17 1.70 1.65 0.008 0.006 1.20 0.16 0.034 1.50 0.022 0.15 95 Example 3 0.18 1.75 1.75 0.006 0.007 1.28 0.21 0.040 1.57 0.023 0.12 100 Example 4 0.19 1.83 1.82 0.005 0.008 1.32 0.25 0.037 1.62 0.025 0.13 105 Example 5 0.20 1.95 1.93 0.007 0.007 1.36 0.27 0.040 1.68 0.027 0.17 110 Comparative Example 1 0.16 0.22 0.66 0.007 0.005 1.62 0.030 0.030 1.57 / / 95 Comparative Example 2 0.16 0.23 0.72 0.008 0.007 1.70 0.032 0.034 1.64 / / 100

[0042] The forging steel production process parameters of Examples 1-5 and Comparative Examples 1-2 of this invention are shown in Table 2:

[0043] Table 2 Forging process parameters of the embodiments and comparative examples of the present invention

[0044] Example 1 1210 6.5 1050 755 639 6.5 Example 2 1205 6.5 1030 760 633 6.2 Example 3 1211 6.5 1060 780 636 7.1 Example 4 1222 6.5 1065 782 641 6.8 Example 5 1220 6.5 1070 776 639 6.5 Comparative Example 1 1230 6.5 1130 921 638 3.2 Comparative Example 2 1230 6.5 1130 927 639 4.5

[0045] Table 3 shows the end hardenability values ​​of the gear steels in the embodiments and comparative examples of the present invention. As can be seen from Table 3, the hardenability control values ​​J9, J15, and J25 of the gear steels described in Embodiments 1-5 of the present invention are all within the range required for gear steels used in automobiles and engineering machinery. Compared with the comparative examples, the hardenability is comparable, but the cost is lower. Simultaneously, they exhibit high bainitic strength and toughness, small deformation, and high low-temperature impact resistance. J9: 43-47, J15: 42-46 HRC, J25: 36-44 HRC, impact energy at -40℃ ≥ 50 J, impact energy at -80℃ ≥ 30 J, austenite grain size ≥ 8.0 grade.

[0046] Table 3. End hardenability values ​​(HRC) of the gear steels obtained in the embodiments and comparative examples of the present invention.

[0047] Require 44～48 42～47 38～44 Example 1 45.5 45.5 43.2 Example 2 45.7 44.8 43.2 Example 3 45.7 44.7 43.0 Example 4 45.2 44.5 43.5 Example 5 45.7 45.0 43.7 Comparative Example 1 43.5 41.3 35.9 Comparative Example 2 42.5 40.8 34.6

[0048] Table 4. Microstructure and notch deformation of the gear steel obtained in the embodiments and comparative examples of the present invention.

[0049] Example 1 8.5 0.05 12 55 42 Example 2 8.5 0.07 12 57 42 Example 3 8.5 0.06 13 54 40 Example 4 8.5 0.08 14 52 39 Example 5 9.0 0.07 12 57 45 Comparative Example 1 8.0 0.30 25 32 22 Comparative Example 2 8.5 0.40 30 35 21

[0050] As shown in Table 4, this invention utilizes high Si and high Mn to promote bainite formation. Simultaneously, high Si enables solid solution strengthening, improving the strength and toughness of the bainitic gear steel. High Ni content enhances hardenability and low-temperature resistance, while Nb+V composite microalloying refines the grain size. This invention provides a carburizing process followed by gas quenching and low-temperature tempering. Under this process, the steel microstructure is bainitic, and the high Si content inhibits the formation of retained austenite, resulting in a reduced retained austenite content. Traditional gear steels form martensitic microstructures after carburizing heat treatment, resulting in significant deformation and insufficient low-temperature resistance, failing to meet the performance requirements of automobiles in low-temperature conditions. This invention not only solves the problems of large deformation and high vibration but also provides high hardenability and low-temperature resistance, especially with an impact energy ≥50J at -40℃ and ≥30J at -80℃, meeting the service requirements of new energy vehicles in different environments.

Claims

1. A fine-grained, low-temperature resistant bainitic gear steel, characterized in that: The chemical composition includes the following weight percentages: C: 0.16~0.20%, Si: 1.60~2.0%, Mn: 1.50~2.00%, S: ≤0.010%, Cr: 1.00~1.40%, Mo: 0.10~0.30%, Ni: 1.40~1.70%, Nb: 0.020~0.030%, V: 0.10~0.20%, Al: 0.030~0.050%, P: ≤0.010%, [N]: 90~160ppm, with the remainder being Fe and unavoidable impurity elements; The ratio of Al to [N] content satisfies: 2≤Al / [N]≤5; After carburizing, gas quenching and low-temperature tempering, the gear steel has an impact energy of ≥50J at -40℃, an impact energy of ≥30J at -80℃, and an austenite grain size of ≥8.0 grade; the end hardenability of the gear steel meets the requirements of J9: 43~47 HRC, J15: 42~46 HRC, and J25: 36~44 HRC.

2. The method for producing a fine-grained, low-temperature resistant bainitic gear steel according to claim 1, characterized in that: The process includes: electric arc furnace smelting - LF refining - RH vacuum treatment - continuous casting - forging into finished products - carburizing and gas quenching + low-temperature tempering.

3. The method for producing a fine-grained, low-temperature resistant bainitic gear steel according to claim 2, characterized in that, The forging process includes heating, forging, and slow cooling.

4. The method for producing a fine-grained, low-temperature resistant bainitic gear steel according to claim 2, characterized in that, The aluminum wire is fully deoxidized during the LF refining process and added in the later stage of RH vacuum treatment.

5. The method for producing a fine-grained, low-temperature resistant bainitic gear steel according to claim 3, characterized in that, During heating, the uniform heating temperature of the steel billet in the heating furnace is controlled at 1200~1230℃, and the total time for preheating, heating and uniform heating is controlled at 5.0h~10.0h.

6. The method for producing a fine-grained, low-temperature resistant bainitic gear steel according to claim 3, characterized in that, During the forging stage, the initial forging temperature is 1000~1100℃ and the final forging temperature is 750~800℃.

7. The method for producing a fine-grained, low-temperature resistant bainitic gear steel according to claim 3, characterized in that, After forging, the product is cooled to 600~650℃ on a cooling bed and then placed in a pit for slow cooling for ≥5 hours.

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