Ultra-fine grained bainite gear steel and method for producing the same

Abstract

This invention discloses an ultrafine-grained 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: 0.18–0.22%, Si: 1.30–1.60%, Mn: 1.00–1.25%, P: ≤0.010%, S: ≤0.010%, Cr: 0.25–0.55%, Mo: 0.30–0.50%, Nb: 0.015–0.25%, Al: 0.030–0.050%, V: 0.10–0.20%, B: 0. 0.0030~0.0050%, [N]: 90~160ppm, the remainder being Fe and unavoidable impurity elements. The steel is produced by electric arc furnace smelting-LF refining-RH vacuum treatment-continuous casting-rolling (finishing). The hardenability of the resulting steel meets the following requirements: J9: 40~47HRC, J15: 40~46HRC, J25: 35~44HRC. The steel is produced by carburizing followed by gas quenching and low-temperature tempering. The carburized layer grain size is ≥11.0 grade, and the surface hardness is ≥650HV.

Description

Technical Field

[0001] This invention belongs to the field of gear steel technology, and relates to an ultrafine grain bainitic gear steel and its production method, which is suitable for gears used in automobiles and engineering machinery. Background Technology

[0002] With advancements in automotive manufacturing and materials production technologies, automobiles are evolving towards higher speeds, higher load capacities, lower noise levels, and lighter weights, placing higher demands on special steels used for automotive gears. Carburized gear steel is the dominant type of gear steel used in automobiles. Carburizing requires prolonged high-temperature holding, consuming significant energy and time. An effective way to reduce carburizing costs is to shorten the carburizing time. Experiments show that increasing the carburizing temperature from the commonly used 930℃ to 1000℃ can significantly shorten the carburizing time. In recent years, mature high-temperature carburizing equipment for large-scale industrial production can meet the conditions for carburizing at 960–980℃. Although high-temperature carburizing technology can improve carburizing efficiency and save energy, it also places higher demands on carburized gear steel. Austenite grains easily grow during high-temperature carburizing, therefore, controlling austenite grain size is particularly important for carburized gear steel. Currently, both domestically and internationally, fine grain control of gear steel is mainly achieved through microalloying with Al, N, and Nb.

[0003] A search revealed that Chinese patent CN103361559A discloses a high-temperature carburizing gear steel using Nb and Ti composite microalloying. The method involves adding 0.02–0.06% Ti and 0.02–0.06% Nb to form precipitates and control grain size. However, it does not mention controlling the N content in the steel. Ti-containing steel requires extremely low N content to avoid the formation of large-sized TiN precipitates that could damage the fatigue performance of the gear steel. Furthermore, the addition of Ti can reduce hardenability, indicating that this patent does not comprehensively consider the overall performance of the gear steel.

[0004] For example, Chinese patent CN102560255A invented a high-temperature vacuum carburizing gear steel. It refines the grain size by adding 0.033% to 0.055% Al. However, when Al > 0.033%, it causes blockage of the tundish nozzle, making continuous casting impossible. This patent does not mention a technical solution to this problem.

[0005] Currently, patent literature on gear steel focuses on improvements to existing products or the development of processes. The deformation and surface hardness after carburizing during production are relatively low. There is no focused research and development on the high strength and toughness required for new energy vehicles, and the problems of high strength and toughness, high hardenability, low deformation, and cost pressure currently faced by gear steel have not been fundamentally solved. Summary of the Invention

[0006] 1. The problem to be solved

[0007] The purpose of this invention is to fundamentally solve the problem of insufficient surface hardness in gear steel, while also addressing issues of deformation and toughness, in order to adapt to the continuous improvement in the comprehensive performance of gear steel. This invention provides an ultrafine-grained bainitic gear steel and its production method. The gear steel of this invention has end hardenability that meets the requirements of J9: 40–47 HRC, J15: 40–46 HRC, and J25: 35–44 HRC. It is produced by carburizing followed by gas quenching and low-temperature tempering. After carburizing, the grain size of the carburized layer is ≥11.0 grade, the surface hardness is ≥650 HV, the surface microstructure is acicular martensite, and the core microstructure is bainitic.

[0008] 2. Technical Solution

[0009] To solve the above problems, the technical solution adopted by the present invention is as follows:

[0010] Firstly, this invention provides an ultrafine-grained bainitic gear steel, which comprises the following chemical composition by weight percentage:

[0011] C: 0.18–0.22%, Si: 1.30–1.60%, Mn: 1.00–1.25%, S: ≤0.010%, Cr: 0.25–0.55%, Mo: 0.30–0.50%, Nb: 0.015–0.25%, Al: 0.030–0.050%, V: 0.10–0.20%, P: ≤0.010%, B: 0.0030–0.0050%, [N]: 90–160 ppm, with the remainder being Fe and unavoidable impurity elements.

[0012] 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:

[0013] 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 between 0.18% and 0.22%.

[0014] 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 on the one hand, and promotes the formation of bainite on the other hand, while inhibiting the formation of carbides, especially carbides in the carburized layer, refining the carbide size and reducing the content of retained austenite. In order to further improve the solid solution strengthening effect, the Si content cannot be lower than 1.0%. However, excessive silicon increases the activity of C, promotes the decarburization and graphitization tendency of steel during rolling and heat treatment, and makes the carburized layer easy to oxidize. Therefore, the Si content is controlled at 1.30-1.60%.

[0015] 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, leading to decreased toughness during hot rolling. The Mn content is controlled between 1.00% and 1.25%.

[0016] 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, 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%.

[0017] 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 0.30–0.50%.

[0018] 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 between 0.015% and 0.25%.

[0019] 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%.

[0020] 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%.

[0021] 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 detrimental to the uniformity of product microstructure and properties. Control P ≤ 0.010%, S ≤ 0.010%.

[0022] 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.

[0023] [N]: It can form compounds with Nb, B and Al, which refine 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 and 130 ppm, and 3.0 ≤ Al / [N] ≤ 5.0.

[0024] Furthermore, the gear steel of the present invention has a composition that meets the following requirements:

[0025] 1.8≤K=(Mn+1.5*Si+0.8*C-0.2*(Nb+V)*A f ≤2.5

[0026] Where A f The material parameters are taken as 0.6 to 0.7, where Mn, Si, C, Nb, and V represent the mass percentage of each element. Through the combination of these components, the resulting gear steel exhibits good hardenability and suitable sawing hardness. Furthermore, controlling the content of Nb+V to be 0.012 ≤ Nb+V ≤ 0.23 refines the grain size. The grain refinement effect is most pronounced when Nb+V ≥ 0.012. When it exceeds 0.23, grain refinement ceases and becomes insignificant; further increasing its content increases costs. The applicant's research found that controlling the content of Nb+V to be 0.12 ≤ Nb+V ≤ 0.2 is the optimal range, resulting in gear steel products that simultaneously possess good grain size, mechanical properties, and cost-effectiveness.

[0027] It is worth further explaining that 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. Mn is an austenite-forming element that can stabilize austenite, increase the strength of steel, and improve its hardenability. Increased hardenability promotes the formation of martensite, but Mn is also a bainite-forming element. To avoid the formation of martensite, the content of Si is increased to promote bainite formation and inhibit martensitic transformation. This further increases hardenability and inhibits austenite formation while achieving solid solution strengthening. By using (Nb, V)(C, N) and AlN to refine the grain size of the carburized layer, and by using Cr and Mo to improve hardenability and increase strength and toughness, the final product has excellent comprehensive performance.

[0028] 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 casting, a constant casting speed is maintained to avoid coarse dendrites that could lead to severe microsegregation and mixed crystals. Specifically, the process includes the following steps:

[0029] 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.

[0030] 2) Rolling: Initial rolling temperature 1120~1200℃, final rolling temperature 930~970℃.

[0031] 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 slow cooling time is ≥24h. After leaving the pit, the surface is ground and peeled to ensure that there is no decarburization and zero defects.

[0032] 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.

[0033] 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 following requirements: J9: 40–47, J15: 40–45 HRC, J25: 35–44 HRC. Furthermore, after carburizing and gas quenching at 930℃ followed by low-temperature tempering, the product exhibits a grain size ≥11.0 grade, a surface carburized layer hardness ≥650 HV, and a surface microstructure of acicular martensite with a core of bainite after carburizing and gas quenching, and a notch deformation ≤0.05. Attached Figure Description

[0034] Figure 1 The grain size after carburizing in Example 1 of this invention is austenite grain size of 11.0.

[0035] Figure 2 The grain size of the austenite grain after carburizing in Comparative Example 1 of this invention is at the 7.0 level.

[0036] Figure 3 The matrix structure of Embodiment 1 of the present invention is bainitic.

[0037] Figure 4 The matrix structure of Comparative Example 1 of the present invention is martensitic. Detailed Implementation

[0038] 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 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 for 48 hours.

[0039] Comparative Examples 1 and 2 were produced according to the requirements of GB / T 3077, consisting of two heats of 18CrNiMo7-6 steel (center line). 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 ≥4 hours 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 placed in a slow cooling pit for 48 hours.

[0040] Specifically, the chemical composition of the steel in Examples 1-5 and Comparative Examples 1-2 is shown in Table 1:

[0041] Table 1. Chemical composition of steel in the embodiments and comparative examples of the present invention (unit: [N] is ppm, others are wt%)

[0042] Example 1 0.18 1.32 1.00 0.007 0.005 0.28 0.30 0.030 0.010 / 0.11 97 3.1 1.86 0.12 Example 2 0.19 1.40 1.05 0.008 0.006 0.32 0.35 0.034 0.011 / 0.13 95 3.6 1.96 0.141 Example 3 0.18 1.42 1.10 0.006 0.007 0.37 0.40 0.040 0.015 / 0.14 100 4.0 2.0 0.155 Example 4 0.20 1.50 1.15 0.005 0.008 0.45 0.42 0.037 0.020 / 0.12 110 3.3 2.12 0.14 Example 5 0.22 1.55 1.20 0.007 0.007 0.52 0.47 0.047 0.023 / 0.13 120 3.9 2.20 0.153 Comparative Example 1 0.16 0.22 0.65 0.007 0.005 1.65 0.025 0.032 / 1.55 / 95 3.4 0.66 0 Comparative Example 2 0.18 0.27 0.70 0.008 0.007 1.63 0.030 0.034 / 1.60 / 100 3.4 0.75 0

[0043] The steel rolling production process parameters of Examples 1-5 and Comparative Examples 1-2 of this invention are shown in Table 2:

[0044] Table 2. Rolling process parameters of the embodiments and comparative examples of the present invention.

[0045] Example 1 1250 6.5 1130 935 639 48 Example 2 1250 6.5 1130 948 633 48 Example 3 1250 6.5 1130 939 636 48 Example 4 1250 6.5 1130 941 641 48 Example 5 1250 6.5 1130 941 639 48 Comparative Example 1 1230 6.5 1130 921 638 48 Comparative Example 2 1230 6.5 1130 927 639 48

[0046] 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.

[0047] Table 3. End hardenability values ​​(HRC) of the gear steels obtained in the embodiments and comparative examples of the present invention.

[0048] Require 40～47 40～45 39～44 Example 1 44.2 44.5 43.2 Example 2 44.5 44.2 43.5 Example 3 45.2 44.8 43.2 Example 4 44.9 45.2 44.5 Example 5 46.2 45.3 44.7 Comparative Example 1 43.5 41.3 35.9 Comparative Example 2 42.5 40.8 34.6

[0049] Table 4 shows the austenitic grain size of φ120mm round steel after carburizing and quenching at 930℃ and then tempering at low temperature in the embodiments of the present invention. As can be seen from Table 4, the gear steel described in embodiments 1 to 5 of the present invention, after carburizing and quenching at 930℃ and then tempering at low temperature, has a grain size ≥11.0 grade and a surface carburized layer hardness ≥650HV.

[0050] Table 4. Relevant properties of the gear steel obtained from the embodiments and comparative examples of the present invention after carburizing heat treatment.

[0051] Example 1 11.0 670 0.02 Example 2 11.0 660 0.03 Example 3 11.5 685 0.05 Example 4 11.0 690 0.04 Example 5 12.0 700 0.03 Comparative Example 1 7.0 540 0.3 Comparative Example 2 7.5 560 0.4

[0052] More specifically, although exemplary embodiments of the invention have been described herein, the invention is not limited to these embodiments, but includes any and all embodiments modified, omitted, such as combinations between various embodiments, adaptive changes, and / or substitutions, as would be apparent to those skilled in the art from the foregoing detailed description. The limitations in the claims are to be interpreted broadly as used in the language of the claims and are not limited to the examples described in the foregoing detailed description or during the implementation of this application, which should be considered non-exclusive. Any step listed in any method or process claim may be performed in any order and is not limited to the order set forth in the claims. Therefore, the scope of the invention should be determined solely by the appended claims and their legal equivalents, and not by the description and examples given above.

[0053] Unless otherwise specified, all technical and scientific terms used herein have the same meaning as commonly understood by one of ordinary skill in the art to which this invention pertains. In case of conflict, the definitions in this specification shall prevail. When a rate, pressure, temperature, time, or other value or parameter is expressed as a range, preferred range, or a range defined by a series of upper and lower preferred values, this shall be understood to specifically disclose all ranges formed by any pair of any upper or preferred value with any lower or preferred value, regardless of whether such range is disclosed individually. For example, the range 1-50 should be understood to include any number, combination of numbers, or subrange selected from 1, 2, 3, 4, 5, 6, 7, 8, 9, 10, 11, 12, 13, 14, 15, 16, 17, 18, 19, 20, 21, 22, 23, 24, 25, 26, 27, 28, 29, 30, 31, 32, 33, 34, 35, 36, 37, 38, 39, 40, 41, 42, 43, 44, 45, 46, 47, 48, 49, or 50, as well as all decimal values ​​between the integers mentioned above, such as 1.1, 1.2, 1.3, 1.4, 1.5, 1.6, 1.7, 1.8, and 1.9. Regarding subranges, specifically consider "nested subranges" extending from any endpoint of the range. For example, nested sub-ranges of the exemplary range 1-50 may include 1-10, 1-20, 1-30 and 1-40 in one direction, or 50-40, 50-30, 50-20 and 50-10 in another direction.

Claims

1. A type of ultrafine-grained bainitic gear steel, characterized in that: The chemical composition includes the following weight percentages: C: 0.18~0.22%, Si: 1.30~1.60%, Mn: 1.00~1.25%, P≤0.010%, S: ≤0.010%, Cr: 0.25~0.55%, Mo: 0.30~0.50%, Nb: 0.015~0.025%, Al: 0.030~0.050%, V: 0.10~0.20%, B: 0.0030~0.0050%, [N]: 90~160ppm, with the remainder being Fe and unavoidable impurity elements; Its composition satisfies the following formula: 1.8≤K=(Mn+1.5*Si+0.8*C-0.2*(Nb+V))*A f ≤2.5, where A f For material parameters, take values ​​of 0.6 to 0.7, while satisfying 0.12 ≤ Nb + V ≤ 0.22; The end hardenability of the gear steel meets the following requirements: J9: 40~47HRC, J15: 40~46HRC, J25: 35~44HRC.

2. The method for producing an ultrafine-grained 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, where rolling into finished products includes heating, rolling and slow cooling.

3. The method for producing ultrafine-grained bainitic gear steel according to claim 2, characterized in that, During the LF refining process, the oxygen content is fully deoxidized and reduced to below 10ppm; aluminum wire is added in the later stage of RH vacuum treatment and adjusted to the target value; during the casting process, a constant casting speed is maintained.

4. The method for producing an ultrafine-grained bainitic gear steel according to claim 2, 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.

5. The method for producing ultrafine-grained bainitic gear steel according to claim 2, characterized in that, During the rolling stage, the initial rolling temperature is 1120~1200℃, and the final rolling temperature is 930~970℃.

6. The method for producing an ultrafine-grained bainitic gear steel according to claim 2, characterized in that, After rolling, the product is cooled to 600~650℃ on a cooling bed and then slowly cooled in a pit for ≥24 hours.

7. A method for producing ultrafine-grained bainitic gear steel according to any one of claims 2-6, characterized in that, Including carburizing heat treatment, the grain size of the gear steel obtained after carburizing and gas quenching at 930℃ and low-temperature tempering is ≥11.0 grade, the surface hardness is ≥650HV, the surface structure after carburizing and gas quenching is acicular martensite, and the core is bainite.

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