Low-deformation gas-quenched bainite gear steel and method for producing same

Abstract

This invention discloses a low-deformation gas-quenched bainitic gear steel and its production method, belonging to the technical field of gear steel. The gear steel of this invention comprises the following chemical composition by weight percentage: C: 0.16–0.20%, Si: 1.00–1.30%, Mn: 0.75–1.00%, P ≤ 0.010%, S: ≤ 0.010%, Cr: 0.25–0.55%, Mo: 0.10–0.30%, V: 0.15–0.25%, Al: 0.030–0.050%, B: 0.0010–0.0015%, P: ≤ 0.010%, [N]: 90–160 ppm, with the remainder being Fe and unavoidable impurities. The steel is produced by electric arc furnace smelting, LF refining, RH vacuum treatment, continuous casting, and rolling (finishing). The resulting steel has end hardenability that meets the requirements of J9: 40~47HRC, J15: 40~45HRC, and J25: 35~44HRC. After carburizing and gas quenching at 900~930℃ followed by low-temperature tempering, the product has a grain size ≥8.0, a residual austenite content ≤15%, and a C-notch deformation ≤0.1mm. This method can fundamentally solve the problem of excessive deformation of high hardenability gear steel after carburizing heat treatment, as well as the problem of excessive deformation caused by the continuous improvement of gear steel performance.

Description

Technical Field

[0001] This invention belongs to the field of gear steel technology, and relates to a low-deformation gas-quenched 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 steels typically undergo surface strengthening treatment via carburizing heat treatment to ensure a certain level of surface hardness while maintaining a certain level of toughness in the core, thus achieving a good balance between strength and toughness. However, most gear steels currently undergoing carburizing heat treatment exhibit a martensitic surface and a pearlite + ferrite + small amount of bainite core, especially pronounced in large gears. In recent years, new high-performance gear steels such as bainitic gear steel have emerged. These steels consist of slender bainitic ferrite and retained austenite, with high-density dislocations and internal carbides intertwining and significantly improving strength. However, the coarseness of the carbides affects the steel's strength and toughness. Meanwhile, new energy vehicles require higher gear meshing precision, making low carburizing deformation a technical bottleneck. Therefore, it is necessary to develop a gear material with high hardenability and low deformation to meet the automotive industry's demand for high meshing and low vibration in gears used in new energy vehicles.

[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. The 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, the invention patent with international patent publication number WO2015197007A discloses a high hardenability CrNiMo gear steel. This invention patent effectively improves the hardenability of the material by increasing the content of elements such as Mn and Ni to a higher range. Its hardenability can reach J9: 40~46HRC, J15: 39~44HRC, and J25: 38~44HRC. Furthermore, by controlling the content of Al and N, it ensures that the austenite grains do not grow significantly during the carburizing process, thereby improving the fatigue resistance and comprehensive mechanical properties of the carburized alloy steel. However, the torsional strength of the material in this patent solution is insufficient, and the deformation after heat treatment is relatively large. Summary of the Invention

[0006] 1. The problem to be solved The purpose of this invention is to fundamentally solve the problem of excessive deformation of high hardenability gear steel after carburizing heat treatment, as well as the problem of excessive deformation caused by the continuous improvement of gear steel performance, and to provide a low deformation gas-quenched bainitic gear steel and its production method.

[0007] 2. Technical Solution To solve the above problems, the technical solution adopted by the present invention is as follows: Firstly, this invention provides a low-deformation gas-quenchable bainitic gear steel, which comprises the following chemical composition by weight percentage: C: 0.16~0.20%, Si: 1.00~1.30%, Mn: 0.75~1.00%, P≤0.010%, S:≤0.010%, Cr: 0.25~0.55%, Mo: 0.10~0.30%, V: 0.15~0.25%, Al: 0.030~0.050%, B: 0.0010~0.0015%, P:≤0.010%, [N]: 90~160ppm, with the remainder being Fe and unavoidable impurity elements. It should be noted that the function and content control of each component in the gear steel composition provided by this invention are as follows: 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 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%.

[0008] 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. It refines the carbide size and reduces the content of retained austenite. Therefore, the Si content should not be less than 1.0%. However, excessive silicon increases the activity of C, promotes the decarburization and graphitization of steel during rolling and heat treatment, and makes the carburized layer easy to oxidize. Therefore, the Si content is controlled at 1.00~1.30%.

[0009] 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 phase transformation regions, increase the bainite phase transformation region, and improve hardenability. However, excessive Mn will reduce the plasticity of the steel, causing a deterioration in toughness during hot rolling. The Mn content is controlled between 0.75% and 1.00%.

[0010] Cr: Cr can improve the hardenability and strength of steel. Cr combines with carbon in steel to form carbides. Because gear steel is tempered at low temperatures after quenching, no large carbides precipitate; instead, fine carbides precipitate. These precipitated carbides accumulate between the martensite laths, inhibiting lath movement under stress. Dislocations in the martensite can become entangled, improving strength and fatigue resistance. However, excessive Cr can form a carbide film, affecting the carburizing effect and reducing the performance of the carburized layer. The Cr content should be controlled between 0.25% and 0.55%.

[0011] 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%.

[0012] B: Boron can significantly improve hardenability, and its addition can improve the density and hot rolling properties of steel, increase strength, and stabilize the toughness of gear steel after quenching and low-temperature tempering heat treatment. However, excessive boron content can lead to the formation of acicular ferrite in the core, affecting mechanical properties. Therefore, the boron content should be controlled at 0.0010~0.0015%.

[0013] 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 have a significant effect on grain refinement or strength increase, but it increases additional costs. Therefore, the V content should be controlled at 0.15~0.25%.

[0014] 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, which deteriorates the properties of the steel. 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%.

[0015] P and S: Sulfur readily forms MnS inclusions with manganese in steel, causing hot brittleness; P is an element with a strong tendency to segregate, increasing cold brittleness, reducing plasticity, and detrimental to the uniformity of product microstructure and properties. Control P ≤ 0.010%, S ≤ 0.010%.

[0016] 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.

[0017] [N]: It can form compounds with Nb, B, and Al, refining the 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 90-130 ppm.

[0018] To further improve the performance of the obtained gear steel, the composition of the gear steel is controlled to satisfy the following formula: 1.8≤K=(Mn+2.2*Si+1.3*C-0.2*V)*A f ≤2.3 Among them, A f For material parameters, A f The content of Mn, Si, C and V is 0.6~0.7, where Mn, Si, C and V represent the mass percentage of the elements. By combining the above components, bainite structure can be generated by air cooling in the presence of medium Mn, ensuring that the rolled material has suitable hardness for sawing and good hardenability.

[0019] 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.

[0020] Secondly, this invention provides a production process for the aforementioned gear steel, including electric arc furnace smelting, LF refining, RH vacuum treatment, continuous casting, and rolling (finishing) to form the finished product. During the refining process, the experimental steel is thoroughly deoxidized to ensure a low oxygen content. Aluminum wire is added in the later stages of vacuum treatment to adjust the Al content, thus ensuring the aluminum content while preventing excessive inclusions in the steel. During continuous casting, the superheat is maintained at 10~20℃, using low superheat casting. Low superheat casting effectively reduces microsegregation and avoids fluctuations in hardenability and the occurrence of mixed crystals. Specifically, the process includes the following steps: 1) Heating: The uniform heating temperature of the steel billet in the heating furnace is controlled at 1230~1280℃, and the total time for preheating, heating and uniform heating is controlled at 5.0h~10.0h.

[0021] 2) Rolling: Initial rolling temperature 1120~1200℃, final rolling temperature 930~970℃.

[0022] 3) Slow cooling: After rolling, the surface is cooled to 600~650℃ on a cooling bed and then put into the pit for slow cooling. The temperature when it comes out of the pit shall not be higher than 200℃. The slow cooling time shall be ≥7h. After it comes out of the pit, it is ground and peeled to ensure that the surface is free of decarburization and has zero defects.

[0023] 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 rolling, is enriched at the original austenite grain boundaries and does not change with the grain boundary changes after heat treatment.

[0024] The low-cost, high-torque output gear steel produced using the above technical solution, when tested for end-hardenability according to GB / T 225, shows a significant improvement in end-hardenability control (J9, J15, and J25) compared to the CiNiMo system, while the cost is significantly reduced compared to 18CrNiMo7-6. Its end-hardenability meets the requirements of J9: 40~47HRC, J15: 40~45HRC, and J25: 35~44HRC. Simultaneously, it ensures that after carburizing and gas quenching at 900~930℃ followed by low-temperature tempering, the product's grain size is ≥8.0 grade, the retained austenite content is ≤15%, and the C-notch deformation is ≤0.1mm. This fundamentally solves the problem of excessive deformation in high-hardenability gear steel after carburizing heat treatment, as well as the problem of excessive deformation due to the continuous improvement in gear steel performance. Attached Figure Description

[0025] The technical solution of the present invention will be further described in detail below with reference to the accompanying drawings and embodiments. However, it should be understood that these drawings are designed for illustrative purposes only and are not intended to limit the scope of the present invention. Furthermore, unless specifically indicated, these drawings are intended only to conceptually illustrate the structural construction described herein and are not necessarily drawn to scale.

[0026] Figure 1 The grain size after carburizing in Example 1 of this invention; Detailed Implementation 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.

[0027] 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 rolling (finishing). The continuously cast billet is heated to 1230-1280℃ and held for ≥5 hours before being rolled into round steel. The superheat during continuous casting is 10-20℃, the initial rolling temperature is 1120-1200℃, and the final rolling temperature is 930-970℃. After rolling, the steel is cooled to ≥650℃ on a cooling bed and then slowly cooled in a pit. The temperature upon exiting the pit must not exceed 200℃, and the slow cooling time is ≥7 hours.

[0028] Comparative Examples 1 and 2 were produced according to the requirements of EN 10084, consisting of two heats of 18CrNiMo7-6 steel (centerline). The process involved electric arc furnace smelting, LF refining, RH vacuum treatment, continuous casting, and rolling (finishing). The continuously cast billets were heated to 1200~1250℃ and held for ≥4h before being rolled into round bars. The initial rolling temperature was 1100~1150℃, and the final rolling temperature was 900~950℃. After rolling, the steel was cooled to 600~650℃ on a cooling bed and then slowly cooled in a pit. The temperature at the bottom of the pit should not exceed 200℃, and the slow cooling time was ≥7h.

[0029] Specifically, the chemical composition of the steel in Examples 1-5 and Comparative Examples 1-2 is shown in Table 1: Table 1. Chemical composition of steel in the embodiments and comparative examples of the present invention (unit: [N] is ppm, others are wt%)

[0030] The steel rolling production process parameters of Examples 1-5 and Comparative Examples 1-2 of this invention are shown in Table 2: Table 2. Rolling process parameters of the embodiments and comparative examples of the present invention.

[0031] 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 to 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.

[0032] Table 3. End hardenability values ​​(HRC) of gear steels obtained in the embodiments and comparative examples of the present invention.

[0033] Table 4 shows the austenite grain size and grade after carburizing and gas quenching at 930℃ and low-temperature tempering in the embodiments of the present invention. As can be seen from Table 4, after carburizing and gas quenching at 930℃ and low-temperature tempering, the gear steel described in embodiments 1 to 5 of the present invention has a grain size ≥ 8.0 grade, the C-notch deformation amount is ≤ 0.1 mm, and the surface residual austenite content is significantly reduced.

[0034] Table 4. Deformation and grain size of gear steel obtained by the embodiments and comparative examples of the present invention after carburizing heat treatment.

Claims

1. A low-deformation gas-quenched bainitic gear steel, characterized in that: The chemical composition includes the following weight percentages: C: 0.16~0.20%, Si: 1.00~1.30%, Mn: 0.75~1.00%, S: ≤0.010%, Cr: 0.25~0.55%, Mo: 0.10~0.30%, V: 0.15~0.25%, Al: 0.030~0.050%, B: 0.0010~0.0015%, P: ≤0.010%, [N]: 90~160ppm, with the remainder being Fe and unavoidable impurity elements; Its composition satisfies the following formula: 1.8≤K=(Mn+2.2*Si+1.3*C-0.2*V)*A f ≤2.3, where A f For material parameters, A f Take a value of 0.6 to 0.

7.

2. The method for producing low-deformation gas-quenched bainitic gear steel according to claim 1, characterized in that: The process includes the following steps: electric arc furnace smelting - LF refining - RH vacuum treatment - continuous casting - rolling into finished products.

3. The method for producing low-deformation gas-quenched bainitic gear steel according to claim 2, characterized in that, The rolling process includes heating, rolling, and slow cooling.

4. The method for producing low-deformation gas-quenched bainitic gear steel according to claim 2, characterized in that, Fully deoxidize during LF refining, add aluminum wire in the later stage of RH vacuum treatment, and ensure superheating at 10~20℃ during continuous casting.

5. The method for producing low-deformation gas-quenched 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 1230~1280℃, and the total time for preheating, heating and uniform heating is controlled at 5.0h~10.0h.

6. The method for producing a low-deformation gas-quenched bainitic gear steel according to claim 3, characterized in that, During the rolling stage, high-temperature rolling is carried out at an initial rolling temperature of 1120~1200℃ and a final rolling temperature of 930~970℃.

7. The method for producing low-deformation gas-quenched bainitic gear steel according to claim 3, characterized in that, After rolling, the product is cooled to 600~650℃ on a cooling bed and then slowly cooled in a pit. The temperature when it comes out of the pit should not exceed 200℃, and the slow cooling time should be ≥7h.

8. A method for producing low-deformation gas-quenched bainitic gear steel according to any one of claims 2-7, characterized in that, The end hardenability of gear steel meets the following requirements: J9: 40~47HRC, J15: 40~45HRC, J25: 35~44HRC.

9. A method for producing low-deformation gas-quenched bainitic gear steel according to any one of claims 2-7, characterized in that, Including carburizing heat treatment, the grain size of the gear steel obtained after carburizing and gas quenching at 900~930℃ and low-temperature tempering is ≥8.0 grade, the residual austenite content is ≤15%, and the C-notch deformation is ≤0.1mm.

Images