High-temperature carburizing gear steel and its preparation method

Abstract

The present invention relates to the technical field of steel smelting. Specifically, it relates to a high-temperature carburizing gear steel and a preparation method thereof. The components of the high-temperature carburizing gear steel can be used to predict the hardenability at 15 mm from the end-quenched surface through the hardenability at 9 mm from the end-quenched surface by Formula 1 and / or Formula 2. Formula 1 = 110.99×C + 9.62×Si + 13.88×Mn + 10.24×Cu + 6.35×Ni + 13.49×Cr - 22.71; Formula 2 = 105.03×C + 10.12×Si + 13.82×Mn + 8.11×Cu + 11.91×Ni + 13.97×Cr - 27.58. The high-temperature carburizing gear steel of the present invention can effectively inhibit grain growth, effectively control the grain size, improve the accuracy and precision of hardenability control, ensure that the gear steel has good hardness and quality, has good fatigue strength and casting performance, and reduces the occurrence of cracks.

Description

Technical Field

[0001] The present invention relates to the technical field of steel smelting, and more particularly to a high-temperature carburizing gear steel and a preparation method thereof. Background Art

[0002] High-quality gears not only need to have good strength and toughness, wear resistance, and be able to well withstand impact, bending, and contact stresses, but also require small deformation, high precision, and low noise. For this reason, it is required that the carburizing hardenability bandwidth range of the gear steel for producing gears is narrow, the cleanliness is high, the composition is uniform, and the grains are fine and uniform.

[0003] Related technical research shows that when the carburizing temperature of gear steel is increased from 920 - 930 °C to 950 °C, the carburizing cycle can be reduced by about 30%, and when the carburizing temperature is increased to 1000 °C, the carburizing cycle can be reduced by about 55%. Although the high-temperature carburizing technology can improve production efficiency, save energy, and reduce production costs, when the carburizing temperature is increased, abnormal grain growth or mixed grain phenomenon is likely to occur, thus affecting the accuracy and stability of the gears and having a greater adverse impact on the reliability of the gear materials.

[0004] In order to ensure the performance of the gears after carburizing, it is generally required that the grain size of the gear steel is above grade 5 and there are no grains coarser than grade 3 after carburizing heat treatment. In related technologies, in order to prevent the austenite grains of gear steel from abnormally growing during high-temperature carburizing, mainly by adding Al, Nb, Ti alloys, etc., so as to pin the grain boundaries by forming particles such as AlN, Nb(C, N), and Ti(C, N), thereby inhibiting the growth of austenite grains.

[0005] In related technologies, although it is possible to adjust the composition of the gear steel to achieve the purpose of controlling the austenite grain size; however, it is still difficult to simultaneously ensure that the gear steel has good fatigue strength, good casting performance, and the quality is not good and cracks are likely to occur. Summary of the Invention

[0006] The purpose of the present invention is to provide a high-temperature carburizing gear steel and a preparation method thereof. The high-temperature carburizing gear steel of the present invention can effectively inhibit grain growth, so as to effectively control the high-temperature carburizing grain size, and can also improve the accuracy and precision of the control of the hardenability of the gear steel, thereby ensuring that the gear steel has good hardness and quality, ensuring that the gear steel has good fatigue strength and casting performance, and reducing the problem of crack occurrence.

[0007] The present invention is implemented as follows:

[0008] In the first aspect, the present invention provides a high-temperature carburizing gear steel. The components of the high-temperature carburizing gear steel, calculated by mass percentage, include:

[0009] C: 0.17 - 0.23%, Si ≤ 0.40%, Mn: 1.10 - 1.40%, P ≤ 0.020%, S ≤ 0.030%, Cu: ≤ 0.20%, Cr: 1.00 - 1.30%, Ni ≤ 0.05%, V ≤ 0.01%, Ti ≤ 0.01%, Mo ≤ 0.01%, B ≤ 0.0005%, O ≤ 0.0010%, Nb: 0.0075 - 0.0095%, Al: 0.020 - 0.040%, N: 0.0080 - 0.0160%, with the balance being Fe and impurities; wherein,

[0010] The hardenability of the high-temperature carburized gear steel satisfies Formula 1 and / or Formula 2,

[0011] Formula 1:

[0012] J9 = 110.99×C + 9.62×Si + 13.88×Mn + 10.24×Cu + 6.35×Ni + 13.49×Cr - 22.71;

[0013] Formula 2:

[0014] J15 = 105.03×C + 10.12×Si + 13.82×Mn + 8.11×Cu + 11.91×Ni + 13.97×Cr - 27.58;

[0015] Wherein, Formula 1 represents the Rockwell hardness at 9 mm from the quenched end face, Formula 2 represents the Rockwell hardness at 15 mm from the quenched end face, Formula 1 satisfies the first preset value, Formula 2 satisfies the second preset value, the value range of the first preset value is: 25 - 45, and the value range of the second preset value is: 19 - 39.

[0016] In a second aspect, the present invention provides a method for preparing a high-temperature carburized gear steel, including:

[0017] Controlling the components of the high-temperature carburized gear steel according to the following mass percentages:

[0018] C: 0.17 - 0.23%, Si ≤ 0.40%, Mn: 1.10 - 1.40%, P ≤ 0.020%, S ≤ 0.030%, Cu: ≤ 0.20%, Cr: 1.00 - 1.30%, Ni ≤ 0.05%, V ≤ 0.01%, Ti ≤ 0.01%, Mo ≤ 0.01%, B ≤ 0.0005%, O ≤ 0.0010%, Nb: 0.0075 - 0.0095%, Al: 0.020 - 0.040%, N: 0.0080 - 0.0160%, with the balance being Fe and impurities; wherein,

[0019] Using Formula 1 and / or Formula 2 to predict the hardenability of the high-temperature carburized gear steel,

[0020] Formula 1:

[0021] J9 = 110.99×C + 9.62×Si + 13.88×Mn + 10.24×Cu + 6.35×Ni + 13.49×Cr - 22.71;

[0022] Formula 2:

[0023] J15 = 105.03×C + 10.12×Si + 13.82×Mn + 8.11×Cu + 11.91×Ni + 13.97×Cr - 27.58;

[0024] When the hardenability value predicted by Formula 1 does not meet the first preset value and / or the hardenability value predicted by Formula 2 does not meet the second preset value, adjust the mass percentage of at least one of C, Si, Mn, Cu, Cr, and Ni so that the hardenability value predicted by Formula 1 meets the first preset value and / or the hardenability value predicted by Formula 2 meets the second preset value;

[0025] Among them, Formula 1 is the Rockwell hardness at 9 mm from the quenched end face, Formula 2 is the Rockwell hardness at 15 mm from the quenched end face, the value range of the first preset value is: 25 - 45; the value range of the second preset value is: 19 - 39.

[0026] In an alternative embodiment, when the hardenability value predicted by Formula 1 does not meet the first preset value and / or the hardenability value predicted by Formula 2 does not meet the second preset value, adjust the mass percentage of at least one of C, Si, Mn, and Cr.

[0027] In an alternative embodiment, when the hardenability value predicted by Formula 1 does not meet the first preset value and / or the hardenability value predicted by Formula 2 does not meet the second preset value, adjust the mass percentage of at least one of C, Si, Mn, and Cr; among them, C, Si, Mn, and Cr are all adjusted by ±0.01%.

[0028] In an alternative embodiment, the preparation method of the high-temperature carburized gear steel further includes:

[0029] Converter smelting;

[0030] LF refining;

[0031] RH vacuum degassing;

[0032] Continuous casting;

[0033] Hot rolling.

[0034] In an alternative embodiment, in the continuous casting step, it includes controlling the secondary cooling water cooling intensity to be 0.27 ± 0.02 L / kg.

[0035] In an alternative embodiment, in the BOF steelmaking step, the end tapping control target is: C ≥ 0.08%.

[0036] In an alternative embodiment, in the RH vacuum degassing step, it is maintained for ≥ 18 min under the condition that the vacuum degree ≤ 0.266 kPa.

[0037] In an alternative embodiment, in the continuous casting step, it further includes: controlling the casting superheat of the tundish to be 10 - 35°C; performing pouring at a constant drawing speed of 0.90 ± 0.02 m / min and argon blowing at the nozzle; using mold electromagnetic stirring and controlling the parameters to be 300 ± 30 A / 2.5 Hz, and final electromagnetic stirring and controlling the parameters to be 600 ± 30 A / 4 Hz;

[0038] The mold liquid level fluctuation curve and the stopper rod curve are monitored in real time. When the mold liquid level fluctuation is greater than ±5 mm, the billet with a length of 1 m before and after the liquid level fluctuation position is cut off. When the stopper rod curve rises by more than 5 mm, the billet is judged as an abnormal billet;

[0039] After the billet is cut, it is slowly cooled at a high temperature. The surface temperature of the billet entering the pit is ≥ 600°C, and the slow cooling time is ≥ 72 h.

[0040] In an alternative embodiment, the billet heating in the hot rolling step includes: a preheating section, a first heating section, a second heating section, and a soaking section. Among them, the upper and lower surface temperature difference of the billet in the preheating section, the first heating section, the second heating section, and the soaking section is ≤ 30°C;

[0041] The temperature of the second heating section and the soaking section is 1200 - 1260°C, and the time is 90 - 230 min.

[0042] The present invention has the following beneficial effects:

[0043] The high-temperature carburizing gear steel provided by the present invention contains Nb with a mass percentage of 0.0075 - 0.095%, Al with a mass percentage of 0.020 - 0.040%, and N with a mass percentage of 0.0080 - 0.0160%, ensuring that there are sufficient Nb(C, N) and AlN precipitation phases in the gear steel. By using the Nb(C, N) precipitation phase that is relatively stable at high temperatures and the AlN precipitation phase to pin the grain boundaries, the grain growth is inhibited, thereby ensuring the control effect of the high-temperature carburizing grain size.

[0044] Moreover, the preparation method of the high-temperature carburizing gear steel of the present invention can also predict the hardenability of the gear steel by using Formula 1 and Formula 2, and adjust the composition content of the gear steel according to the predicted results, so that the finally prepared gear steel can improve the accuracy and precision of hardenability control while effectively inhibiting grain growth, ensure that the gear steel has good fatigue resistance and pouring performance, and reduce the problem of crack occurrence. Brief Description of the Drawings

[0045] To more clearly illustrate the technical solutions of the embodiments of the present invention, the drawings required for use in the embodiments will be briefly introduced below. It should be understood that the following drawings only show some embodiments of the present invention and should not be regarded as limiting the scope. For those of ordinary skill in the art, other relevant drawings can be obtained based on these drawings without creative efforts.

[0046] Figure 1 It is the austenite grain size diagram of the gear steel in Example 1 of the present invention after holding at 1000°C for 4 h and water cooling;

[0047] Figure 2 It is the austenite grain size diagram of the gear steel in Example 2 of the present invention after holding at 1000°C for 4 h and water cooling;

[0048] Figure 3 It is the austenite grain size diagram of the gear steel in Example 3 of the present invention after holding at 1000°C for 4 h and water cooling;

[0049] Figure 4 It is the austenite grain size diagram of the gear steel in Example 4 of the present invention after holding at 1000°C for 4 h and water cooling;

[0050] Figure 5 It is the austenite grain size diagram of the gear steel in Comparative Example 1 of the present invention after holding at 1000°C for 4 h and water cooling;

[0051] Figure 6 It is the austenite grain size diagram of the gear steel in Comparative Example 2 of the present invention after holding at 1000°C for 4 h and water cooling;

[0052] Figure 7 It is the austenite grain size diagram of the gear steel in Comparative Example 3 of the present invention after holding at 1000°C for 4 h and water cooling;

[0053] Figure 8 It is the austenite grain size diagram of the gear steel in Comparative Example 4 of the present invention after holding at 1000°C for 4 h and water cooling;

[0054] Figure 9 It is the austenite grain size diagram of the gear steel in Comparative Example 5 of the present invention after holding at 1000°C for 4 h and water cooling;

[0055] Figure 10 It is the austenite grain size diagram of the gear steel in Comparative Example 6 of the present invention after holding at 1000°C for 4 h and water cooling;

[0056] Figure 11 It is the austenite grain size diagram of the gear steel in Comparative Example 7 of the present invention after holding at 1000°C for 4 h and water cooling. Detailed implementation mode

[0057] To make the objectives, technical solutions, and advantages of the embodiments of the present invention clearer, the technical solutions in the embodiments of the present invention will be clearly and completely described below. Those not specified in the embodiments are carried out according to conventional conditions or conditions recommended by the manufacturer. Those reagents or instruments not specified by the manufacturer can all be obtained as conventional products through commercial purchase.

[0058] The related technical patent CN106967925A discloses a high-temperature carburized gear steel with fine grains and a narrow hardenability bandwidth; by adding Al with a mass percentage of 0.025-0.055% and N with a mass percentage of 0.0080-0.0200%, sufficient precipitation of AlN is ensured, and the grain size can still be controlled within grades 7-8 after high-temperature carburization at 980-1000°C. However, relevant research has found that when only Al and N are added, local mixed grains are likely to occur during high-temperature carburization above 960°C, and a large amount of Al added to the steel is likely to generate a large number of brittle inclusions, which is not conducive to improving the fatigue strength of the gear steel. At the same time, adding a large amount of Al deteriorates the pouring performance of the molten steel, resulting in difficult pouring.

[0059] The related technical patent CN103361559A discloses a Nb, Ti composite microalloyed high-temperature carburized gear steel; by adding Ti with a mass percentage of 0.02-0.06% and Nb with a mass percentage of 0.02-0.06%, Ti(C, N) and Nb(C, N) precipitation phases are formed to pin the grain boundaries. When this steel grade is held at 1000°C for 1 hour and 6 hours, its austenite grain size can be maintained above grade 8.0, and no mixed grain phenomenon is found. However, adding a certain amount of Ti to the steel is likely to generate coarse rhombic TiN particles, which affects the fatigue performance of the gear steel, while adding a large amount of Nb is likely to cause cracks in the billet.

[0060] The related technical patent CN114855079A discloses a steel for cold-extruded gear shafts and its preparation method; the mass percentages of its chemical components are as follows: C: 0.19%-0.21%, Si: 0.15%-0.25%, Mn: 1.11%-1.20%, P≤0.015%, S: 0.015%-0.025%, Cr: 1.03%-1.10%, Al: 0.035%-0.050%, Cu≤0.20%, Ti≤0.010%, V: 0.01%-0.03%, Nb≤0.010%, Sn≤0.010%, B≤0.0010%, [H]≤2.0 ppm, [N] 110 ppm-140 ppm. After cold pressing and forming by users, this gear steel has high dimensional accuracy, excellent internal and external surface quality, uniform and dense structure, and a fatigue life higher than that of ordinary hot-forged gear shafts. However, its Nb content only has an upper limit requirement and no lower limit requirement. Related research has found that after adding Al, V, and N, when the Nb content is less than 0.005%, local mixed crystals are still likely to occur during high-temperature carburizing above 980°C, and adding V has a high cost and is likely to cause cracking of the casting blank.

[0061] In order to overcome the deficiencies of the prior art, the present disclosure provides a high-temperature carburizing gear steel and its preparation method to ensure a small hardenability bandwidth, low inclusion content, and good surface quality of the gear steel on the premise of maintaining the austenite grain size from growing during high-temperature carburizing.

[0062] The components of the high-temperature carburizing gear steel of the present invention, calculated by mass percentage, include:

[0063] C: 0.17 - 0.23% (e.g., 0.17%, 0.19%, 0.20%, 0.23%, etc.), Si ≤ 0.40% (e.g., 0.40%, 0.38%, 0.36%, etc.), Mn: 1.10 - 1.40% (e.g., 1.10%, 1.15%, 1.20%, 1.25%, 1.30%, 1.35%, 1.40%, etc.), P ≤ 0.020% (e.g., 0.020%, 0.018%, 0.015%, etc.), S ≤ 0.030% (e.g., 0.030%, 0.028%, 0.025%, 0.020%, etc.), Cu: ≤ 0.20% (e.g., 0.20%, 0.10%, 0.12%, 0.15%, 0.080%, 0.020%, 0.030%, 0.040%, 0.050%, etc.), Cr: 1.00 - 1.30% (e.g., 1.00%, 1.10%, 1.20%, 1.30%, etc.), Ni ≤ 0.05% (e.g., 0.05%, 0.04%, 0.03%, 0.02%, etc.), V ≤ 0.01% (e.g., 0.01%, 0.008%, 0.005%, etc.), Ti ≤ 0.01% (e.g., 0.01%, 0.008%, 0.005%, etc.), Mo ≤ 0.01% (e.g., 0.01%, 0.008%, 0.005%, etc.), B ≤ 0.0005% (e.g., 0.0005%, 0.0003%, etc.), O ≤ 0.0010% (e.g., 0.0010%, 0.0008%, 0.0005%, etc.), Nb: 0.0075 - 0.0095% (e.g., 0.0075%, 0.0080%, 0.0085%, 0.0090%, 0.0095%, etc.), Al: 0.020 - 0.040% (e.g., 0.020%, 0.025%, 0.030%, 0.035%, 0.040%, etc.), N: 0.0080 - 0.0160% (e.g., 0.0080%, 0.0090%, 0.0100%, 0.0120%, 0.0140%, 0.0160%, etc.), the balance being Fe and impurities; wherein,

[0064] The hardenability of the high-temperature carburized gear steel needs to satisfy Formula 1 and / or Formula 2,

[0065] Formula 1:

[0066] J9 = 110.99×C + 9.62×Si + 13.88×Mn + 10.24×Cu + 6.35×Ni + 13.49×Cr - 22.71;

[0067] Formula 2:

[0068] J15 = 105.03×C + 10.12×Si + 13.82×Mn + 8.11×Cu + 11.91×Ni + 13.97×Cr - 27.58;

[0069] Formula 1 is the Rockwell hardness at 9 mm from the quenched end face, and formula 2 is the Rockwell hardness at 15 mm from the quenched end face. Formula 1 meets the first preset value, and formula 2 meets the second preset value. The value range of the first preset value is: 25 - 45, and the value range of the second preset value is: 19 - 39.

[0070] The present invention also provides a preparation method of the high-temperature carburized gear steel as described above, which includes:

[0071] Controlling the components of the high-temperature carburized gear steel according to the above mass percentages; using formula 1 and / or formula 2 to predict the hardenability of the high-temperature carburized gear steel; when the hardenability value predicted by formula 1 does not meet the first preset value and / or the hardenability value predicted by formula 2 does not meet the second preset value, adjusting the mass percentage of at least one of C, Si, Mn, Cu, Cr, and Ni so that the hardenability value predicted by formula 1 meets the first preset value and / or the hardenability value predicted by formula 2 meets the second preset value.

[0072] The high-temperature carburized gear steel provided by the present invention contains Nb with a mass percentage of 0.0075 - 0.095%, Al with a mass percentage of 0.020 - 0.040%, and N with a mass percentage of 0.0080 - 0.0160%, ensuring that there are sufficient Nb(C, N) and AlN precipitation phases in the gear steel. Utilizing the relatively stable Nb(C, N) precipitation phase at high temperatures and the AlN precipitation phase pinning grain boundaries to inhibit grain growth, thereby ensuring the control effect of the high-temperature carburized grain size.

[0073] Moreover, the preparation method of the high-temperature carburized gear steel of the present invention can also use formula 1 and formula 2 to predict the hardenability of the gear steel, and adjust the composition content of the gear steel according to the predicted results, so that the finally prepared gear steel can improve the accuracy and precision of hardenability control while effectively inhibiting grain growth, ensuring that the gear steel has good fatigue resistance and casting performance, and reducing the problem of crack occurrence.

[0074] In a preferred embodiment, when the hardenability value predicted by formula 1 does not meet the first preset value and / or the hardenability value predicted by formula 2 does not meet the second preset value, adjust the mass percentage of at least one of C, Si, Mn, and Cr.

[0075] Furthermore, C, Si, Mn, and Cr can all be adjusted by ±0.01%. In this way, the melting composition can be controlled in a narrow band according to the calculation results of formula 1 and formula 2, thereby improving the accuracy and precision of hardenability control.

[0076] In the present invention, the components of the high-temperature carburizing gear steel are all melting components. To reduce the inclusion content, deep deoxidation and protective casting are carried out during the steelmaking process to reduce the oxygen content in the steel. By blowing argon through the tundish nozzle and real-time monitoring of the liquid level fluctuation curve and stopper rod curve of the mold, the billets with large liquid level fluctuations are cut off, thereby reducing the inclusion content.

[0077] The preparation method of the high-temperature carburizing gear steel of the present invention specifically further includes: converter smelting; LF refining; RH vacuum degassing; continuous casting; hot rolling.

[0078] Among them, the gear steel of the present invention belongs to low-C, high-Al, and high-N steel, and its surface quality is generally poor, mainly because its crack sensitivity is relatively high and surface cracks are likely to occur. In the present invention, by reducing the cooling intensity of the secondary cooling water for continuous casting, the straightening of the billet in the temperature range with low plasticity can be avoided. Specifically, the cooling intensity of the secondary cooling water is controlled at 0.27±0.02 L / kg (for example: 0.25 L / kg, 0.27 L / kg, 0.29 L / kg, etc.), the billet is slowly cooled at high temperature, and the upper and lower temperature differences are strictly controlled during the heating of the billet (to reduce the stress concentration of the billet), thereby reducing the occurrence of cracks.

[0079] In a preferred embodiment, the preparation method of the present invention further includes:

[0080] The first step: Converter smelting, the control target for tapping at the end point: C≥0.08% (for example: 0.08%, 0.10%, 0.15%, 0.20%, etc.), and a slide gate slag blocking operation is carried out during the tapping process to reduce the slag entrainment.

[0081] The second step: LF refining, control the Ar gas flow rate to prevent excessive rolling of the molten steel and slag entrainment; adopt comprehensive deoxidation with a slag surface composite deoxidizer and silicon carbide diffusion deoxidation.

[0082] The third step: RH vacuum degassing, the vacuum degree is ≤0.266 kPa (for example: 0.266 kPa, 0.250 kPa, 0.240 kPa, etc.), and it is maintained for ≥18 min (for example: 18 min, 20 min, 22 min, 25 min, etc.); in order to ensure the stability and controllability of nitrogen, nitrogen gas is blown throughout the vacuum treatment to increase nitrogen content, and nitrogen-chromium wire is fed after the vacuum treatment to increase nitrogen content; after the vacuum treatment, calcium treatment is prohibited to reduce the formation of large-size calcium aluminate composite inclusions; after the vacuum treatment, keep blowing argon for more than 20 min (for example: 20 min, 21 min, 25 min, 30 min, etc.), and the soft blowing argon flow rate is based on not breaking the slag surface and the slag surface fluctuating slightly, and it is strictly prohibited to expose the molten steel.

[0083] Step 4: Continuous casting. For the casting from the tundish to the intermediate ladle, long nozzles with Ar gas protection are used for pouring, and at the same time, submerged nozzles are used for protection pouring in the intermediate ladle to prevent secondary oxidation of the molten steel. The superheat of pouring in the intermediate ladle is controlled at 10 - 35 °C (e.g., 10 °C, 15 °C, 20 °C, 25 °C, 30 °C, 35 °C, etc.). The constant casting speed of 0.90 ± 0.02 m / min (e.g., 0.88 m / min, 0.90 m / min, 0.92 m / min, etc.) is adopted for pouring and argon blowing through the nozzle. At the same time, the mold electromagnetic stirring of 300 ± 30 A / 2.5 Hz (e.g., 270 A / 2.5 Hz, 280 A / 2.5 Hz, 290 A / 2.5 Hz, 300 A / 2.5 Hz, 310 A / 2.5 Hz, 320 A / 2.5 Hz, 330 A / 2.5 Hz, etc.) and the final electromagnetic stirring of 600 ± 30 A / 4 Hz (e.g., 570 A / 4 Hz, 580 A / 4 Hz, 590 A / 4 Hz, 600 A / 4 Hz, 610 A / 4 Hz, 620 A / 4 Hz, 630 A / 4 Hz, etc.) are adopted as process measures to improve the internal quality of the billet. During the continuous casting process, the liquid level fluctuation curve and stopper rod curve of the mold are monitored in real time. When the liquid level fluctuation in the mold is greater than ±5 mm, the billets with a length of 2 m (1 m before and after including the specific position of the liquid level fluctuation) are cut off. When the stopper rod curve rises by more than 5 mm, the billets are all classified as abnormal billets, and these abnormal billets are not allowed to be rolled into steel with high requirements for inclusions and can be removed. To prevent stress cracks in the billets, after cutting, the billets are lifted into a slow cooling pit for high-temperature slow cooling. It is necessary to ensure that the surface temperature of the billets entering the pit is ≥600 °C (e.g., 600 °C, 620 °C, 650 °C, etc.), and the slow cooling time is ≥72 h (e.g., 72 h, 75 h, 80 h, etc.).

[0084] Step 5: Heating and rolling. When the billets are heated, the temperature difference between the upper and lower surfaces in the preheating section, the first heating section, the second heating section, and the soaking section is ≤30 °C (e.g., 30 °C, 28 °C, 25 °C, etc.). The temperature in the second heating section and the soaking section is controlled at 1200 - 1260 °C (e.g., 1200 °C, 1220 °C, 1240 °C, 1260 °C, etc.), and the holding time is controlled at 90 - 230 min (e.g., 90 min, 120 min, 150 min, 180 min, 200 min, 230 min, etc.). Then the billets are rolled into round steel.

[0085] Of course, after rolling into round steel, the round steel samples can be heated to 960 - 1000 °C (e.g., 960 °C, 970 °C, 980 °C, 990 °C, 1000 °C, etc.) by means of simulated high-temperature carburization, held for 4 - 6 h (e.g., 4 h, 5 h, 6 h, etc.), and the austenite grain size is detected after water cooling.

[0086] The present invention will be further described in detail below in conjunction with embodiments.

[0087] The components of the examples and comparative examples are shown in Tables 1-1 and 1-2 respectively.

[0088] Table 1-1 Chemical composition of gear steel in examples (wt, %)

[0089] Component Example 1 Example 2 Example 3 Example 4 C 0.2 0.2 0.19 0.2 Si 0.25 0.27 0.04 0.05 Mn 1.34 1.35 1.37 1.25 P 0.01 0.014 0.008 0.009 S 0.013 0.002 0.027 0.026 Cu 0.025 0.024 0.044 0.025 Ni 0.018 0.017 0.022 0.023 Cr 1.25 1.26 1.24 1.24 Mo 0.006 0.006 0.006 0.008 Ti 0.002 0.001 0.002 0.001 Nb 0.0095 0.009 0.009 0.0075 V 0.005 0.005 0.006 0.006 Al 0.033 0.03 0.027 0.028 B 0.0003 0.0001 0.0003 0.0003 N 0.012 0.016 0.008 0.013 O 0.0008 0.0007 0.0009 0.0007

[0090] Table 1-2 Chemical composition of gear steel in comparative examples (wt, %)

[0091] Component Comparative Example 1 Comparative Example 2 Comparative Example 3 Comparative Example 4 Comparative Example 5 Comparative Example 6 Comparative Example 8 C 0.2 0.21 0.2 0.19 0.2 0.2 0.23 Si 0.31 0.27 0.08 0.25 0.25 0.25 0.39 Mn 1.34 1.35 1.35 1.33 1.34 1.34 1.40 P 0.012 0.014 0.017 0.013 0.01 0.01 0.011 S 0.018 0.014 0.002 0.002 0.013 0.013 0.004 Cu 0.028 0.029 0.028 0.044 0.025 0.025 0.19 Ni 0.017 0.018 0.018 0.03 0.018 0.018 0.017 Cr 1.2 1.27 1.25 1.26 1.25 1.25 1.30 Mo 0.004 0.006 0.007 0.004 0.006 0.006 0.009 Ti 0.003 0.001 0.002 0.003 0.002 0.002 0.002 Nb 0.028 0.005 0.001 0.0095 0.0210 0.0095 0.009 V 0.006 0.004 0.013 0.002 0.005 0.005 0.005 Al 0.009 0.041 0.042 0.032 0.033 0.033 0.031 B 0.0003 0.0002 0.0002 0.0001 0.0003 0.0003 0.0003 N 0.015 0.018 0.014 0.013 0.012 0.0050 0.010 O 0.002 0.0015 0.0016 0.0015 0.0008 0.0008 0.0009

[0092] The preparation methods of the examples and comparative examples are shown in Table 2.

[0093] Table 2 Preparation methods of examples and comparative examples

[0094]

[0095] The round steel of the examples and comparative examples was chamfered and straightened, and then magnetic flux leakage testing was carried out. The rolled round steel was subjected to a simulated high-temperature carburizing test, that is, water cooling was carried out after holding at 1000 °C for 6 h and 1030 °C for 4 h. The austenite grain size of the gear steel was evaluated according to GB / T 6394, and the results are shown in Table 3. 8 specimens were taken from each furnace for inclusion detection according to GB / T 10561, and the results are shown in Table 4. 2 specimens were taken from each furnace for hardenability detection according to GB / T 225, and the results are shown in Table 5.

[0096] Table 3 Austenite grain size and magnetic flux leakage testing of examples and comparative examples Table 4 Inclusion situation (8 specimens were taken from each furnace) Table 5 Hardenability situation (2 specimens were taken from each furnace)

[0097]

[0098]

[0099] Combined with Example 1 and Comparative Examples 1-3, Nb, Al, and N were added in Example 1, only Nb and N were added in Comparative Example 1, a large amount of Al, N, and 0.005% Nb were added in Comparative Example 2, and Al, N, 0.013% V were added in Comparative Example 3, and Nb was not added (there was 0.001% Nb in the molten steel itself). The austenite grain size of Example 1 was 8.0 grades after holding at 1000 °C for 4 h and water cooling, and there was no mixed crystal, while obvious mixed crystal phenomena existed in Comparative Examples 1-3. This shows that adding appropriate amounts of Nb, Al, and N simultaneously can ensure the high-temperature grain size stability of carburized gear steel.

[0100] Combined with Example 1 and Comparative Example 4, the components of the two are basically the same. The temperatures of the second heating section and soaking section of the continuous casting billet in Example 1 are 1233 °C, while those in Comparative Example 4 are 1135 °C. After holding at 1000 °C for 4 h and then water cooling in Example 1, the austenite grain size is Grade 8.0 without mixed grains, while obvious mixed grain phenomenon exists in Comparative Example 4. This shows that when the heating temperature of the continuous casting billet is high, it is helpful for Nb(C, N) and AlN in the continuous casting billet to be fully dissolved in the steel, so that fine and uniform Nb(C, N) and AlN can precipitate and pin the grain boundaries after subsequent rolling, improving the stability of the high-temperature grain size.

[0101] Combined with Example 1 and Comparative Examples 1-4, argon blowing through the tundish is carried out in Example 1, and the oxygen content is within 0.0010%, while argon blowing through the tundish is carried out in Comparative Examples 1-4, and the oxygen content is above 0.0015% in all of them. The inclusions of types B, D, and DS in Example 1 can all be controlled within Grade 1, while the inclusions of types B, D, and DS in Comparative Examples 1-4 often exceed Grade 1. That is, by reducing the oxygen content and carrying out argon blowing through the tundish, oxides inclusions such as B, D, and DS can be effectively reduced.

[0102] Combined with Example 1 and Comparative Example 5, 0.021% of Nb is added in Comparative Example 5, and there is no mixed grain phenomenon in the high-temperature grain size of both. However, the initial inspection pass rate of magnetic flux leakage inspection in Comparative Example 5 is only 83.6%, that is, adding more Nb will cause the surface quality of the round steel to deteriorate, resulting in a lower initial inspection pass rate of magnetic flux leakage inspection.

[0103] Combined with Example 1 and Comparative Example 6, only 0.005% of N is added in Comparative Example 6. After holding at 1000 °C for 4 h and then water cooling in Example 1, the austenite grain size is Grade 8.0 without mixed grains, while obvious mixed grain phenomenon exists in Comparative Example 6.

[0104] Combined with Example 1 and Comparative Example 7, the secondary cooling water cooling intensity in Comparative Example 7 is set to a smaller cooling intensity of 0.21 L / kg, and the initial inspection pass rate of magnetic flux leakage inspection in Comparative Example 7 is only 86.4%, that is, if the secondary cooling water cooling intensity is set too low, the initial inspection pass rate of magnetic flux leakage inspection will also become low.

[0105] It can be seen from Table 3 that on the premise of adopting the same heat treatment process for pretreatment, based on the optimization of the composition of the gear steel provided by the present invention, the round steel in the examples is heated to 960-1000 °C and held for 4-6 h, and the austenite grain size can still remain at Grade 7.0-8.5 without mixed grain phenomenon, while there are serious mixed grains in the comparative examples, or even if there is no mixed grain, the initial inspection pass rate of magnetic flux leakage inspection is low (please refer to Figures 1 to 11 ). At the same time, through the optimization of the secondary cooling water cooling intensity of continuous casting, slow cooling of the continuous casting billet and heating process, the initial inspection pass rate of magnetic flux leakage inspection can reach more than 90%.

[0106] As can be seen from Table 4, during the steelmaking process, through deep deoxidation, argon blowing at the nozzle, strictly controlling the liquid level fluctuation in the mold and the rising of the stopper rod, the generation of large particle oxide inclusions and the entry of foreign large-sized inclusions into the molten steel are prevented, and inclusions of types B, D, and DS can all be controlled within Grade 1.

[0107] As can be seen from Table 5, the prediction models of Formula 1 and Formula 2 can accurately predict the actual hardenability of steel, and the prediction error is within ±2HRC. And by adopting methods such as narrow-band control of melting composition and electromagnetic stirring, the control accuracy of the hardenability bandwidth is significantly improved. As can be seen from Comparative Example 8, the material composition is within the control range requirements, but Formula 1 does not meet the first preset value and Formula 2 does not meet the second preset value, and at this time the prediction error exceeds ±2HRC. Using the technical solution of the present invention, the proportions of the hardenability bandwidth of Formula 1 and Formula 2 not greater than 4HRC reach 90.45% and 96.63% respectively, and the proportions of the hardenability bandwidth of Formula 1 and Formula 2 not greater than 6HRC both reach 100%. The technical solution of the present invention can improve the surface and internal quality and increase the yield of round steel on the basis of ensuring the stability of the high-temperature grain size and narrow-band hardenability of carburizing gear steel.

[0108] In summary, the high-temperature carburizing gear steel of the present invention adopts Nb microalloying design. In the composition design, to ensure the formation of a sufficient amount of Nb(C, N) precipitation phase to pin the grain boundaries and inhibit grain growth, effectively ensuring the high-temperature grain stability of the carburizing gear steel. Since the addition of Nb content is small, only 0.0075 - 0.095%, the smelting cost does not increase significantly. By controlling the Al and N contents, reducing the cooling intensity of the secondary cooling water for continuous casting, slowly cooling the casting blank in the high-temperature pit, and controlling the temperature difference between the upper and lower surfaces in the preheating section, the first heating section, the second heating section, and the soaking section, stress cracks in the casting blank and round steel can be prevented, and the yield of round steel can be increased. The high-temperature carburizing gear steel of the present invention is heated to 960 - 1000 °C and held for 4 - 6 h, and the austenite grain size can still be maintained at Grade 7.0 - 8.5 without mixed grain phenomenon. Using the high-temperature carburizing gear steel of the present invention, high-temperature short-time carburizing can be carried out during gear processing, thereby greatly shortening the production cycle, improving production efficiency, and reducing production costs.

[0109] The above are only the preferred embodiments of the present invention and are not used to limit the present invention. For those skilled in the art, the present invention can have various changes and modifications. Any modification, equivalent replacement, improvement, etc. made within the spirit and principle of the present invention shall be included within the protection scope of the present invention.

Claims

1. A preparation method of a high-temperature carburized gear steel, characterized in that, Including: Controlling the components of the high-temperature carburized gear steel according to the following mass percentages: C: 0.17 - 0.23%, Si ≤ 0.40%, Mn: 1.10 - 1.40%, P ≤ 0.020%, S ≤ 0.030%, Cu: ≤ 0.20%, Cr: 1.00 - 1.30%, Ni ≤ 0.05%, V ≤ 0.01%, Ti ≤ 0.01%, Mo ≤ 0.01%, B ≤ 0.0005%, O ≤ 0.0010%, Nb: 0.0075 - 0.0095%, Al: 0.020 - 0.040%, N: 0.0080 - 0.0160%, with the balance being Fe and impurities; where Using Formula 1 and / or Formula 2 to predict the hardenability of the high-temperature carburized gear steel, Formula 1: J9 = 110.99×C + 9.62×Si + 13.88×Mn + 10.24×Cu + 6.35×Ni + 13.49×Cr - 22.71; Formula 2: J15 = 105.03×C + 10.12×Si + 13.82×Mn + 8.11×Cu + 11.91×Ni + 13.97×Cr - 27.58; When the hardenability value predicted by Formula 1 does not meet the first preset value and / or the hardenability value predicted by Formula 2 does not meet the second preset value, adjusting the mass percentage of at least one of C, Si, Mn, Cu, Cr, and Ni so that the hardenability value predicted by Formula 1 meets the first preset value and / or the hardenability value predicted by Formula 2 meets the second preset value; Wherein, Formula 1 is the Rockwell hardness at 9 mm from the quenched end face, Formula 2 is the Rockwell hardness at 15 mm from the quenched end face, the value range of the first preset value is: 25 - 45, and the value range of the second preset value is: 19 - 39.

2. The preparation method of the high-temperature carburizing gear steel according to claim 1, characterized in that, When the hardenability value predicted by Formula 1 does not meet the first preset value and / or the hardenability value predicted by Formula 2 does not meet the second preset value, adjusting the mass percentage of at least one of C, Si, Mn, and Cr.

3. The preparation method of the high-temperature carburizing gear steel according to claim 2, characterized in that, When the hardenability value predicted by Formula 1 does not meet the first preset value and / or the hardenability value predicted by Formula 2 does not meet the second preset value, adjusting the mass percentage of at least one of C, Si, Mn, and Cr; where C, Si, Mn, and Cr are all adjusted by ±0.01%.

4. The preparation method of the high-temperature carburizing gear steel according to any one of claims 1-3, characterized in that, The preparation method of the high-temperature carburized gear steel further includes: Converter smelting; LF refining; RH vacuum degassing; Continuous casting; Hot rolling.

5. The preparation method of the high-temperature carburizing gear steel according to claim 4, characterized in that, In the step of continuous casting, controlling the secondary cooling water cooling intensity to be 0.27 ± 0.02 L / kg.

6. The preparation method of the high-temperature carburizing gear steel according to claim 4, characterized in that, In the step of converter smelting, the end-point tapping control target: C ≥ 0.08%.

7. The preparation method of the high-temperature carburizing gear steel according to claim 4, characterized in that In the step of RH vacuum degassing, maintaining for ≥ 18 min under the condition of a vacuum degree ≤ 0.266 kPa.

8. The preparation method of the high-temperature carburized gear steel according to claim 4, characterized in that, In the continuous casting step, it further includes: controlling the casting superheat of the tundish to be 10 - 35 °C; pouring at a constant drawing speed of 0.90 ± 0.02 m / min and blowing argon through the nozzle; using mold electromagnetic stirring and controlling the parameters to be 300 ± 30 A / 2.5 Hz, and using final electromagnetic stirring and controlling the parameters to be 600 ± 30 A / 4 Hz; Real-time monitoring is carried out on the mold liquid level fluctuation curve and the stopper rod curve. When the mold liquid level fluctuation is greater than ±5 mm, the billet with a length of 1 m before and after the liquid level fluctuation position is cut off. When the stopper rod curve rises by more than 5 mm, the billet is judged as an abnormal billet; After the billet is cut, it is slowly cooled at high temperature. The surface temperature of the billet entering the pit is ≥600 °C, and the slow cooling time is ≥72 h.

9. The preparation method of the high-temperature carburizing gear steel according to claim 4, characterized in that, The billet heating in the heating and rolling step includes: a preheating section, a first heating section, a second heating section, and a soaking section. Among them, the temperature difference between the upper and lower surfaces of the billet in the preheating section, the first heating section, the second heating section, and the soaking section is ≤30 °C; The temperature of the second heating section and the soaking section is 1200 - 1260 °C, and the time is 90 - 230 min.

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