A production method for high-strength shaft tube steel

Abstract

The present invention relates to the field of metallurgy, and in particular to a method for producing high-strength steel for shaft tubes. The chemical composition and mass percentage of the high-strength steel for shaft tubes are C: 0.06%-0.10%, Si: ≤0.20%, Mn: 1.70%-1.90%, P≤0.015%, S≤0.004%, Nb: 0.050%-0.070%, Ti: 0.09%-0.13%, Als: 0.015%-0.050%, N≤0.0035%, and the balance is Fe and other impurities. The production method includes refining, continuous casting, slab heating, rough rolling, finish rolling, cooling, and coiling. The cooling process adopts four-stage cooling to reduce the temperature of the steel plate to the coiling temperature of 540-580°C. The production method of the high-strength steel for shaft tubes provided by the present invention significantly improves the strength and toughness of the steel for shaft tubes.

Description

Technical Field

[0001] The present invention relates to the technical field of metallurgy, and in particular to a production method of high-strength steel for shaft tubes. Background Art

[0002] Vehicle lightweighting is the process of reducing vehicle weight as much as possible while ensuring vehicle strength and safety, thereby improving vehicle power, reducing energy consumption, and lowering exhaust pollution. In recent years, the rapid development of the automotive transportation industry has led to increasingly significant energy consumption and air pollution issues. Energy conservation and emission reduction have become urgent needs for the industry, and vehicle lightweighting is one of the primary approaches to achieving this goal.

[0003] During operation, automotive axle tubes must withstand both randomly changing alternating loads and impact loads caused by starting, braking, and changing road conditions in different climates. Therefore, to improve vehicle safety, axle tube steel must possess high strength and toughness, including high low-temperature toughness. Currently, the primary approach to achieving vehicle lightweighting is to use lightweight materials to reduce vehicle weight. Axle tube steel is a key component of this material selection process. To reduce the weight of axle tube steel, it is typically necessary to simultaneously increase its strength while reducing its thickness. However, as the strength of axle tube steel increases from 440MPa to 700MPa, its toughness decreases, hindering vehicle safety.

[0004] The existing technology has disclosed the use of vanadium or titanium elements to strengthen the strength and toughness of shaft tube steel. Although adding vanadium to shaft tube steel can improve the toughness of shaft tube steel, it is difficult to increase the strength of shaft tube steel to a higher level, and the cost is high, which cannot meet the requirements of automobile lightweighting for steel strength and toughness; adding titanium to shaft tube steel can significantly improve the strength of shaft tube steel, but titanium needs to be heated at high temperature to fully exert its precipitation strengthening effect, and high-temperature heating consumes a lot of energy. When the precipitation strengthening effect of titanium is more obvious, it will cause the toughness of shaft tube steel to deteriorate and the yield strength ratio to increase. In addition, this method has strict requirements on the coiling temperature, a narrow coiling process window, and greater processing difficulty. It can be seen that this method cannot solve the problem of improving the toughness while improving the strength of shaft tube steel. Summary of the Invention

[0005] In view of the problem of mismatch between high strength and high toughness of shaft tube steel in the prior art, the present invention provides a production method for high-strength shaft tube steel, which significantly improves the strength and toughness of the shaft tube steel.

[0006] The invention provides a production method of high-strength steel for axle tubes. The chemical composition and mass percentage of the high-strength steel for axle tubes are respectively C: 0.06%-0.10%, Si: ≤0.20%, Mn: 1.70%-1.90%, P≤0.015%, S≤0.004%, Nb: 0.050%-0.070%, Ti: 0.09%-0.13%, Als: 0.015%-0.050%, N≤0.0035%, and the balance is Fe and other impurities.

[0007] The production method includes the following steps: refining, continuous casting, slab heating, rough rolling, finishing rolling, cooling, and coiling. During the slab heating process, the heating time is ≥200 minutes, the soaking period is controlled to be ≥35 minutes, the soaking period temperature is controlled to be 1170-1270°C, and the furnace discharge temperature is 1210-1250°C. The final rough rolling pass temperature is controlled at 1050-1090°C, and the finishing rolling process temperature is 850-870°C.

[0008] The cooling process adopts four-stage cooling: the first stage cooling, the cooling rate is 60-70℃ / s, the steel plate is cooled to 660-720℃; the second stage cooling, the cooling method is air cooling, the air cooling time is 3-5s; the third stage cooling, the cooling rate is 15-30℃ / s, the steel plate is cooled to 580-630℃; the fourth stage cooling is cooling water fine adjustment, after the steel plate temperature is cooled to 580-630℃, cooling fine adjustment water is used to reduce the steel plate temperature to the coiling temperature of 540-580℃.

[0009] The reasons for limiting the chemical composition and mass percentage of the high-strength shaft tube steel in the present invention to the above ranges are as follows:

[0010] Carbon (C) has a significant strengthening effect, significantly increasing the strength and hardenability of steel. However, excessive C content can significantly reduce the steel's toughness, cold forming, and weldability. The present invention controls the C content to 0.06%-0.10%, which helps improve the strength of axle tube steel to a certain extent while reducing its adverse effects on toughness, cold forming, and weldability.

[0011] Manganese: Mn is the most effective element for improving strength and toughness, and is also a good desulfurizer and deoxidizer. Mn is an austenite stabilizing element that can expand the austenite phase region. Mn is a carbide-forming element. Since the atomic diffusion coefficient of carbide-forming elements in austenite is much slower than that of C atoms, the nucleation and growth of alloy carbides or alloy cementite are significantly slowed down, thereby delaying the pearlite transformation, playing a role in refining ferrite grains and improving the hardenability of steel. However, excessive Mn content can easily cause segregation and form banded structures. In severe cases, it can cause stratification of the steel and reduce the toughness of the steel. The present invention controls the Mn content to 1.70%-1.90%, which can reduce the content of harmful elements such as S and O in the shaft tube steel, improve the purity of the shaft tube steel, and at the same time improve the strength and toughness of the shaft tube steel.

[0012] Silicon: Silicon dissolves in ferrite, acting as a solid solution strengthening agent and significantly improving the strength and hardness of steel. However, excessive silicon content can make surface oxide scale difficult to remove. The present invention controls the silicon content to ≤ 0.20%, making it easier to descale the axle tube steel.

[0013] Phosphorus: Phosphorus has a strong solid solution strengthening effect, significantly increasing the strength and hardness of steel, but it can also significantly reduce the steel's low-temperature toughness. Phosphorus also has a tendency to segregate, affecting the overall performance of the steel. This invention controls the phosphorus content to below 0.015%, reducing the adverse effects of phosphorus on the low-temperature toughness of axle tube steel.

[0014] Sulfur: S is a harmful element in steel and affects the formability of steel. Sulfide inclusions significantly affect the impact toughness and fatigue properties of steel. Therefore, the sulfur content should be as low as possible. In the present invention, the sulfur content is controlled to be ≤0.004%.

[0015] Niobium: Nb has a significant refining effect in steel, effectively refining the original austenite grain size and expanding the unrecrystallized zone during rolling. The Nb (NC) formed during rolling provides a favorable location for ferrite nucleation, pinning grain boundaries and preventing grain boundary movement, thereby increasing steel strength. This simultaneously refines grains and improves strength and ductility. The present invention controls the niobium content to 0.050%-0.070%.

[0016] Titanium: Ti is a strong carbide-forming element. It has a strong affinity with carbon and nitrogen, and can refine the structure of steel, improving toughness and strength. Since Ti and N can form titanium nitride, titanium nitride is brittle and does not deform during pressure processing. It is mostly dispersed and has a significant adverse effect on the toughness of steel. The present invention strictly controls the N content to ≤0.0035%. Adding a trace amount of Ti to steel, especially adding an appropriate amount of Ti to Nb-containing steel, can reduce the crack sensitivity of steel by reasonably controlling the Nb / Ti ratio, effectively reducing transverse cracks in continuous slabs and improving the strength and toughness of steel. The titanium content of the present invention is controlled at 0.09%-0.13%.

[0017] Aluminum: Al acts as a deoxidizer in steel and also refines grain size, increasing both strength and toughness. Al oxides are brittle inclusions. Larger inclusions can significantly negatively impact steel toughness. Therefore, to ensure steel cleanliness, Al content should be kept to a minimum. In this invention, the Al content is controlled within a range of 0.015% to 0.050%.

[0018] Furthermore, refining includes LF refining and RH refining, wherein the treatment cycle of LF refining is ≥15min, the soft blowing time is ≥13min, and the soft blowing gas is argon. LF refining can remove most of the sulfur and oxygen in the molten steel, and the use of argon soft blowing is beneficial to removing inclusions in the molten steel. RH refining can achieve dehydrogenation and denitrification of molten steel, and the present invention strictly controls the nitrogen content through RH refining. The LF+RH dual refining process fully guarantees the purity of the molten steel, controls the content of harmful elements such as P, S, O, N, and H at a low level, reduces the adverse effects of harmful elements on the strength and toughness of shaft tube steel, improves the crack resistance of shaft tube steel, and thus ensures the safety of shaft tube steel.

[0019] Furthermore, the entire continuous casting process is carried out under argon protection, the ladle superheat is controlled at 15-35°C, and the crystallizer liquid level fluctuation is controlled to be ≤±3mm. During the continuous casting process, molten steel is easily in contact with air, generating secondary oxide inclusions and / or nitride inclusions, which destroy the continuity of the steel. The present invention uses argon to protect the entire continuous casting process, isolates the molten steel from the air, avoids the generation of new inclusions during the continuous casting process, and improves the purity of the molten steel. Controlling the ladle superheat at 15-35°C can improve the internal quality of the slab, avoid steel leakage, and prevent the water outlet from being blocked; controlling the crystallizer liquid level fluctuation at ≤±3mm is beneficial to promote the infiltration of the protective slag liquid slag film, reduce the generation of inclusions, improve the purity of the molten steel, and thus improve the quality of the slab.

[0020] Furthermore, the slab heating time is 200-240 minutes, and the soaking period is 35-45 minutes. By extending the slab heating time, the present invention ensures that the slab is fully heated, allowing easily segregated elements such as C, P, S, Mn, and N to gain diffusion momentum and become homogenized, thereby reducing the element segregation degree in the slab and minimizing the increase in slab brittleness caused by localized enrichment of elements such as C, P, S, Mn, and N, thereby improving the strength and toughness of the slab.

[0021] Furthermore, a full-pass descaling process is performed before rough rolling, with a descaling pressure of ≥25 MPa. The present invention utilizes a descaling pressure of ≥25 MPa for full-pass descaling, which more comprehensively and thoroughly removes the iron oxide scale generated during the slab heating process, improving the slab surface quality and ensuring the accuracy of temperature measurement. Iron oxide scale is relatively small and easy to remove before rough rolling, and the present invention achieves higher descaling efficiency by descaling before rough rolling.

[0022] Furthermore, the rough rolling adopts five passes and the finish rolling adopts seven passes.

[0023] Furthermore, stacking and slow cooling are carried out after finishing rolling, and the stacking and slow cooling time is ≥72h, which helps to release the internal stress of the steel for the shaft tube. At the same time, stacking and slow cooling can improve the temperature uniformity of the steel coil and reduce the performance fluctuation of the coil.

[0024] Furthermore, the yield strength of the steel for the shaft tube with a thickness of 4-8 mm is ≥700 MPa, the tensile strength is ≥820 MPa, the elongation is ≥22%, and the -20°C impact energy of the half-size sample of the steel for the shaft tube with a thickness greater than 5 mm is ≥50 J.

[0025] Furthermore, the metallographic structure of the shaft tube steel includes ferrite and a small amount of pearlite.

[0026] The beneficial effects of the present invention are:

[0027] The present invention increases the stability of austenite by regulating the contents of C and Mn. It utilizes the interaction between Ti, Nb, Al, C, and N to promote grain refinement, increase the grain boundary area of ​​the steel, and improve the strength and toughness of the shaft tube steel. It utilizes the solid solution strengthening effect of Si and P to create a certain degree of lattice distortion, thereby improving the strength of the shaft tube steel. By controlling the S content to a low level, the effect of sulfide inclusions on the impact toughness of the shaft tube steel is reduced. The present invention also utilizes the deoxidation and / or desulfurization effects of Mn, Si, and Al to reduce the precipitation of oxides and / or sulfides. By regulating the chemical composition and mass percentage of the shaft tube steel, the present invention improves the strength and toughness of the shaft tube steel from a structural perspective.

[0028] The present invention provides a production method for high-strength steel for shaft tubes. By controlling the slab heating time to be ≥200min and the soaking period time to be ≥35min, the slab is ensured to be fully heated in the heating furnace, element segregation is reduced, and the elements easily segregated are promoted to fully diffuse, thereby improving the strength and toughness of the slab. The soaking period temperature is controlled to be 1170-1270°C and the furnace discharge temperature is controlled to be 1210-1250°C, providing power for the diffusion of elements easily segregated. This application controls the temperature of the final rough rolling pass to be 1050-1090°C. The rough rolling process is rolled above the recrystallization temperature. Uniform and fine austenite grains are obtained through repeated deformation and recrystallization of austenite. The finishing rolling process temperature is 850-870°C. Flat austenite is obtained through rolling in the non-recrystallization zone and is transformed into uniform and fine ferrite during the subsequent cooling process. This final rolling temperature setting can ensure the stability of rolling and give full play to the precipitation strengthening effect of alloy elements.

[0029] The present invention employs a four-stage cooling process: the first stage, at a cooling rate of 60-70°C / s, cools the steel plate to 660-720°C. Increasing the cooling rate refines the precipitated carbide particles, thereby increasing the strength of the axle tube steel. The second stage, air cooling for 3-5 seconds, controls the ferrite transformation and precipitation. The third stage, at a cooling rate of 15-30°C / s, cools the steel plate to 580-630°C. The fourth stage, water-cooling, involves fine-tuning the steel plate temperature to 580-630°C, which is then lowered to the coiling temperature of 540-580°C using fine-tuning cooling water. Because the present invention incorporates the element Ti, the coiling temperature must be kept within a certain range to ensure precipitation strengthening. Lower coiling temperatures can compromise the precipitation strengthening effect, while higher coiling temperatures can reduce the toughness of the axle tube steel. The present invention optimizes the process parameters of continuous casting, slab heating, rough rolling and finish rolling in the production process of shaft tube steel, and adopts a four-stage cooling process to improve the purity of shaft tube steel, promote the dispersion of easily segregated elements, promote grain refinement, and further improve the strength and toughness of shaft tube steel. BRIEF DESCRIPTION OF THE DRAWINGS

[0030] In order to more clearly illustrate the embodiments of the present invention or the technical solutions in the prior art, the following briefly introduces the drawings required for use in the embodiments or the description of the prior art. Obviously, for ordinary technicians in this field, other drawings can be obtained based on these drawings without any creative work.

[0031] Figure 1 This is the metallographic structure diagram of the steel for the shaft tube according to Example 2 of the present invention.

[0032] Figure 2 1 is the metallographic structure diagram of the steel for the shaft tube according to comparative example 2 of the present invention. DETAILED DESCRIPTION

[0033] In order to enable those skilled in the art to better understand the technical solutions of the present invention, the technical solutions in the embodiments of the present invention will be clearly and completely described below in conjunction with the accompanying drawings of the embodiments of the present invention. Obviously, the described embodiments are only part of the embodiments of the present invention, not all of the embodiments. Based on the embodiments of the present invention, all other embodiments obtained by ordinary technicians in this field without making creative efforts should fall within the scope of protection of the present invention.

[0034] Example 1

[0035] A high-strength steel for axle tube with a thickness of 5 mm has the following chemical compositions and mass percentages: C: 0.075%, Si: 0.20%, Mn: 1.71%, P: 0.012%, S: 0.003%, Nb: 0.050%, Ti: 0.095%, Als: 0.030%, N: 0.0023%, and the balance being Fe and other impurities.

[0036] The yield strength of the above-mentioned steel for the shaft tube is 734 MPa, the tensile strength is 858 MPa, the elongation is 24.0%, and the yield strength ratio is 0.86.

[0037] The production method of the high-strength shaft tube steel includes the following steps: converter smelting, refining, continuous casting, slab heating, rough rolling, finish rolling, cooling and coiling.

[0038] The refining process includes LF refining and RH refining, wherein the processing cycle of LF refining is 17 minutes, the soft blowing time is 15 minutes, and the soft blowing gas is argon.

[0039] During the continuous casting process, the entire continuous casting process is carried out under argon protection, the ladle superheat is controlled at 23°C, and the crystallizer liquid level fluctuation is controlled to be ≤±3mm. After continuous casting, a slab with a thickness of 230mm is obtained.

[0040] In the slab heating process, the heating time is 230 minutes, the soaking period time is controlled to be 40 minutes, and the furnace temperature is 1234℃.

[0041] Before rough rolling, full-pass descaling was performed at a descaling pressure of 25 MPa. Rough rolling was performed in five passes, with the final rough rolling pass temperature controlled at 1086°C. After rough rolling, an intermediate billet with a thickness of 36.7 mm was obtained.

[0042] Finish rolling is carried out in seven passes, with the final rolling temperature at 867°C. After finishing rolling, the steel is stacked and slowly cooled for 72 hours.

[0043] The cooling process adopts four-stage cooling: the first stage cooling, the cooling rate is 64℃ / s, the steel plate is cooled to 709℃; the second stage cooling, the cooling method is air cooling, the air cooling time is 3s, the steel plate is cooled to 593℃; the third stage cooling, the cooling rate is 24℃ / s, the steel plate is cooled to 580-630℃; the fourth stage cooling is cooling water fine adjustment, after the steel plate temperature is cooled to 580-630℃, cooling fine adjustment water is used to reduce the steel plate temperature to the coiling temperature of 574℃ to obtain steel for shaft tubes.

[0044] Example 2

[0045] A high-strength steel for axle tube with a thickness of 6 mm has the following chemical compositions and mass percentages: C: 0.085%, Si: 0.18%, Mn: 1.86%, P: 0.011%, S: 0.001%, Nb: 0.065%, Ti: 0.12%, Als: 0.030%, N: 0.0030%, and the balance being Fe and other impurities.

[0046] The yield strength of the above-mentioned shaft tube steel is 721 MPa, the tensile strength is 842 MPa, the elongation is 24.5%, the impact energy of a half-size sample with a specification of 5mm×10mm×55mm at -20°C is 67J, and the yield strength ratio is 0.86.

[0047] The metallographic structure of the above-mentioned shaft tube steel is as follows: Figure 1 shown.

[0048] The production method of the high-strength shaft tube steel includes the following steps: converter smelting, refining, continuous casting, slab heating, rough rolling, finish rolling, cooling and coiling.

[0049] The refining process includes LF refining and RH refining, wherein the processing cycle of LF refining is 16 minutes, the soft blowing time is 15 minutes, and the soft blowing gas is argon.

[0050] During the continuous casting process, the entire continuous casting process is carried out under argon protection, the superheat of the ladle is controlled at 25°C, and the fluctuation of the crystallizer liquid level is controlled to be ≤±3mm. After continuous casting, a slab with a thickness of 230mm is obtained.

[0051] In the slab heating process, the heating time is 232 minutes, the soaking period time is controlled to be 36 minutes, and the furnace temperature is 1228℃.

[0052] Before rough rolling, full-pass descaling was performed at a descaling pressure of 25 MPa. Rough rolling was performed in five passes, with the final rough rolling pass temperature controlled at 1062°C. After rough rolling, an intermediate billet with a thickness of 36.7 mm was obtained.

[0053] Finish rolling is carried out in seven passes, with the final temperature of the finishing rolling being 852°C. After finishing rolling, the steel is stacked and slowly cooled for 72 hours.

[0054] The cooling process adopts four-stage cooling: the first stage cooling, the cooling rate is 56℃ / s, the steel plate is cooled to 692℃; the second stage cooling, the cooling method is air cooling, the air cooling time is 5s, the steel plate is cooled to 585℃; the third stage cooling, the cooling rate is 28℃ / s, the steel plate is cooled to 580-630℃; the fourth stage cooling is cooling water fine adjustment, after the steel plate temperature is cooled to 580-630℃, cooling fine adjustment water is used to reduce the steel plate temperature to the coiling temperature of 562℃ to obtain steel for shaft tubes.

[0055] Comparative Example 1

[0056] A high-strength steel for axle tube with a thickness of 5 mm has the following chemical compositions and mass percentages: C: 0.080%, Si: 0.15%, Mn: 1.62%, P: 0.012%, S: 0.002%, Nb: 0.046%, Ti: 0.082%, Als: 0.030%, N: 0.0024%, and the balance being Fe and other impurities.

[0057] The yield strength of the above-mentioned steel for the shaft tube is 675 MPa, the tensile strength is 794 MPa, the elongation is 25.0%, and the yield strength ratio is 0.85.

[0058] The production method of the high-strength shaft tube steel includes the following steps: converter smelting, refining, continuous casting, slab heating, rough rolling, finish rolling, cooling and coiling.

[0059] The refining process includes LF refining and RH refining, wherein the processing cycle of LF refining is 17 minutes, the soft blowing time is 15 minutes, and the soft blowing gas is argon.

[0060] During the continuous casting process, the entire continuous casting process is carried out under argon protection, the ladle superheat is controlled at 21°C, and the crystallizer liquid level fluctuation is controlled to be ≤±3mm. After continuous casting, a slab with a thickness of 230mm is obtained.

[0061] In the slab heating process, the heating time is 226 minutes, the soaking time is controlled to be 37 minutes, and the furnace temperature is 1241℃.

[0062] Before rough rolling, full-pass descaling was performed at a descaling pressure of 25 MPa. Rough rolling was performed in five passes, with the final rough rolling pass temperature controlled at 1105°C. After rough rolling, an intermediate billet with a thickness of 36.7 mm was obtained.

[0063] Finish rolling is carried out in seven passes, with the final temperature of the finishing rolling being 859°C. After finishing rolling, the steel is stacked and slowly cooled for 72 hours.

[0064] The cooling process adopts four-stage cooling: the first stage cooling does not adopt the cooling rate disclosed in the present invention, and the cooling rate is 43°C / s, cooling the steel plate to 715°C; the second stage cooling adopts air cooling, the air cooling time is 4s, and cooling the steel plate to 614°C; the third stage cooling adopts a cooling rate of 26°C / s, and cooling the steel plate to 580-630°C; the fourth stage cooling is cooling water fine adjustment, after the steel plate temperature is cooled to 580-630°C, cooling fine adjustment water is used to reduce the steel plate temperature to the coiling temperature of 578°C to obtain steel for shaft tubes.

[0065] In Comparative Example 1, since the cooling rate of the present invention was not adopted in the first cooling process, the yield strength of the shaft tube steel obtained was 675 MPa, the tensile strength was 794 MPa, the elongation was 25.0%, and the yield strength ratio was 0.85; the yield strength of the shaft tube steel obtained in Example 1 was 734 MPa, the tensile strength was 858 MPa, the elongation was 24.0%, and the yield strength ratio was 0.86; the yield strength and tensile strength of the shaft tube steel obtained in Comparative Example 1 were much lower than those of the shaft tube steel obtained in Example 1, indicating that the first cooling process of the present invention helps to improve the yield strength and tensile strength of the shaft tube steel; the elongation and yield strength ratio of the shaft tube steel obtained in Comparative Example 1 were not much different from those in Example 1, indicating that the first cooling process of the present invention would not have a significant adverse effect on the elongation and yield strength ratio of the shaft tube steel.

[0066] Comparative Example 2

[0067] A high-strength steel for axle tube with a thickness of 6 mm has the following chemical compositions and mass percentages: C: 0.090%, Si: 0.20%, Mn: 1.76%, P: 0.010%, S: 0.003%, Nb: 0.058%, Ti: 0.10%, Als: 0.030%, N: 0.0028%, and the balance being Fe and other impurities.

[0068] The yield strength of the above-mentioned shaft tube steel is 785 MPa, the tensile strength is 863 MPa, the elongation is 23.4%, the impact energy of a half-size sample with a specification of 5 mm × 10 mm × 55 mm at -20°C is 71 J, and the yield strength ratio is 0.91.

[0069] The metallographic structure of the above-mentioned shaft tube steel is as follows: Figure 2 shown.

[0070] The production method of the high-strength shaft tube steel includes the following steps: converter smelting, refining, continuous casting, slab heating, rough rolling, finish rolling, cooling and coiling.

[0071] The refining process includes LF refining and RH refining, wherein the processing cycle of LF refining is 18 minutes, the soft blowing time is 15 minutes, and the soft blowing gas is argon.

[0072] During the continuous casting process, the entire continuous casting process is carried out under argon protection, the ladle superheat is controlled at 23°C, and the crystallizer liquid level fluctuation is controlled to be ≤±3mm. After continuous casting, a slab with a thickness of 230mm is obtained.

[0073] In the slab heating process, the heating time is 223 minutes, the soaking period time is controlled to be 36 minutes, and the furnace temperature is 1238°C.

[0074] Before rough rolling, full-pass descaling was performed at a descaling pressure of 25 MPa. Rough rolling was performed in five passes, with the final rough rolling pass temperature controlled at 1078°C. After rough rolling, an intermediate billet with a thickness of 36.7 mm was obtained.

[0075] Finish rolling is carried out in seven passes, with the final rolling temperature at 862°C. After finishing rolling, the steel is stacked and slowly cooled for 72 hours.

[0076] The cooling process only uses the first stage cooling with a cooling rate of 82℃ / s to cool the steel plate to 670℃; water cooling is used to reduce the steel plate temperature to the coiling temperature of 559℃ to obtain steel for shaft tubes.

[0077] In Comparative Example 2, since the second and third cooling stages of the present invention were not used in the cooling process, the yield strength of the obtained shaft tube steel was 785MPa, the tensile strength was 863MPa, the elongation was 23.4%, the -20°C impact energy of the half-size sample with a specification of 5mm×10mm×55mm was 71J, and the yield strength ratio was 0.91. The yield strength of the shaft tube steel obtained in Example 2 was 721MPa, the tensile strength was 842MPa, and the elongation was 24.5%. The -20°C impact energy of the half-size sample with a specification of 5mm×10mm×55mm was 67J, and the yield strength ratio was 0.86. Although the yield strength of the shaft tube steel obtained in Comparative Example 2 exceeded 700Mpa and the tensile strength exceeded 820MPa, since the second and third cooling stages of the present invention were not used, the elongation decreased and the yield strength ratio increased. It can be seen that the second and third cooling stages of the present invention are more conducive to improving the toughness of the shaft tube steel. In addition, Figure 1 and Figure 2 By comparison, it can be seen that Example 2 adopts the four-stage cooling of the present invention, so the metallographic structure is more uniform and the grains are finer than those of Comparative Example 2, indicating that the second and third stage cooling can refine the grain size, thereby improving the toughness of the shaft tube steel.

[0078] Although the present invention has been described in detail by way of preferred embodiments, the present invention is not limited thereto. Without departing from the spirit and substance of the present invention, persons of ordinary skill in the art may make various equivalent modifications or substitutions to the embodiments of the present invention, and such modifications or substitutions shall be within the scope of the present invention. Any changes or substitutions that can be readily conceived by persons skilled in the art within the technical scope disclosed in the present invention shall be within the scope of protection of the present invention.

Claims

1. A method for producing high-strength steel for shaft tubes, characterized in that: The chemical composition and mass percentage of the high-strength shaft tube steel are as follows: C: 0.06%-0.10%, Si: ≤0.20%, Mn: 1.70%-1.90%, P≤0.015%, S≤0.004%, Nb: 0.050%-0.070%, Ti: 0.09%-0.13%, Als: 0.015%-0.050%, N≤0.0035%, and the balance is Fe and other impurities; The production method includes the following steps: refining, continuous casting, slab heating, rough rolling, finishing rolling, cooling and coiling; in the slab heating step, the heating time is ≥200 minutes, the soaking time is controlled to be ≥35 minutes, and the furnace temperature is 1210-1250°C; the temperature of the final rough rolling pass is controlled at 1050-1090°C; and the finishing rolling process temperature is 850-870°C; The cooling process adopts four-stage cooling: the first stage cooling, the cooling rate is 60-70℃ / s, the steel plate is cooled to 660-720℃; the second stage cooling, the cooling method is air cooling, the air cooling time is 3-5s; the third stage cooling, the cooling rate is 15-30℃ / s, the steel plate is cooled to 580-630℃; the fourth stage cooling is cooling water fine adjustment, after the steel plate temperature is cooled to 580-630℃, cooling fine adjustment water is used to reduce the steel plate temperature to the coiling temperature of 540-580℃.

2. The method for producing high-strength steel for shaft tubes according to claim 1, wherein: Refining includes LF refining and RH refining, wherein the processing cycle of LF refining is ≥15min, the soft blowing time is ≥13min, and the soft blowing gas is argon.

3. The method for producing high-strength steel for shaft tubes according to claim 1, wherein: The entire continuous casting process is carried out under argon protection, the ladle superheat is controlled at 15-35℃, and the crystallizer liquid level fluctuation is controlled to ≤±3mm.

4. The method for producing high-strength steel for shaft tubes according to claim 1, wherein: The heating time for slab heating is 200-240 minutes, and the soaking time is 35-45 minutes.

5. The method for producing high-strength steel for shaft tubes according to claim 1, wherein: Before rough rolling, full-pass descaling is carried out, and the descaling pressure is ≥25MPa.

6. The method for producing high-strength steel for shaft tubes according to claim 1, wherein: Rough rolling uses five passes and finishing rolling uses seven passes.

7. A method for producing high-strength steel for shaft tubes according to claim 1 or 6, characterized in that: After finishing rolling, the steel is stacked and slowly cooled, and the stacking and slow cooling time is ≥72h.

8. The method for producing high-strength steel for shaft tubes according to claim 1, wherein: The yield strength of steel for shaft tubes with a thickness of 4-8mm shall be ≥700MPa, the tensile strength shall be ≥820MPa, and the elongation shall be ≥22%. The impact energy of steel for shaft tubes with a thickness greater than 5mm shall be ≥50J at -20℃.

9. A method for producing high-strength steel for shaft tubes according to claim 1 or 8, characterized in that: The metallographic structure of the steel for the shaft tube includes ferrite and pearlite.

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