Thick-walled seamless steel pipe with yield strength of 355 MPa and its preparation method

Abstract

The present application relates to the technical field of seamless steel pipe manufacturing, and particularly relates to a yield strength 355 MPa thick-walled seamless steel pipe and a preparation method thereof. The preparation method of the yield strength 355 MPa thick-walled seamless steel pipe comprises the following steps: (a) heating a continuous casting round billet, perforating, continuous rolling, and cooling the rough pipe after the continuous rolling to 900-930 DEG C at a cooling speed of 20-30 DEG C / s; (b) reducing the diameter of the rough pipe cooled to 900-930 DEG C, and then cooling to 600-650 DEG C at a cooling speed of 5-10 DEG C / s, and naturally cooling to room temperature to obtain a seamless steel pipe product. The preparation method of the seamless steel pipe significantly improves the strength and toughness of the seamless steel pipe under the premise of no subsequent heat treatment through the synergistic effect of fine-grain strengthening and precipitation strengthening, and simultaneously simplifies the process and reduces energy consumption.

Description

Technical Field

[0001] This invention relates to the field of seamless steel pipe manufacturing technology, and in particular to a thick-walled seamless steel pipe with a yield strength of 355 MPa and its preparation method. Background Technology

[0002] Seamless steel pipes with a yield strength of 355 MPa have been widely used in industries such as industrial fluid transportation and space frame structures. In the field of industrial fluid transportation, the demand for this type of steel pipe from equipment such as hydraulic machinery is showing a continuous growth trend. As hydraulic machinery continues to develop towards higher pressure and higher power, the industry's requirements for the wall thickness of seamless steel pipes are also increasing.

[0003] Chinese patent application CN112981256A discloses a low-cost hot-rolling production method for Q345D seamless steel pipes. Through composition design and production process control, the method ensures the strength and toughness of the seamless steel pipes. However, this method is only applicable to seamless steel pipes with a wall thickness not exceeding 16 mm. Furthermore, applying the composition to seamless pipes with a wall thickness exceeding 16 mm would pose welding risks due to the higher carbon equivalent. To address these issues in the production of thick-walled seamless steel pipes, existing technologies have attempted to optimize the microstructure using heat treatment processes such as quenching and tempering or normalizing. However, these methods lead to prolonged production processes and increased energy consumption. Additionally, increasing the carbon content or alloy additives can improve strength, but this results in excessively high carbon equivalents, affecting the weldability of the seamless steel pipes and increasing alloy costs. Therefore, achieving a short-process production of 355 MPa-grade seamless steel pipes with a wall thickness >16 mm and possessing both high strength and toughness as well as excellent weldability is of great significance.

[0004] In view of this, the present invention is hereby proposed. Summary of the Invention

[0005] The purpose of this invention is to provide a thick-walled seamless steel pipe with a yield strength of 355 MPa and its preparation method. This invention, through reasonable alloy composition design and appropriate rolling process, enables the production of thick-walled seamless steel pipes with satisfactory strength and toughness without the need for additional heat treatment after rolling.

[0006] To achieve the above-mentioned objectives of the present invention, a first aspect of the present invention provides a method for preparing a thick-walled seamless steel pipe with a yield strength of 355 MPa, comprising the following steps: (a) Heating, piercing, and rolling the continuously cast round billet, and cooling the rolled rough tube to 900-930°C at a cooling rate of 20-30°C / s; (b) Reduce the diameter of the rough tube cooled to 900~930℃, then cool it to 600~650℃ at a cooling rate of 5~10℃ / s, and then let it cool naturally to room temperature to obtain the finished seamless steel pipe. The chemical composition of the continuously cast round billet, by mass percentage, includes: C 0.14%~0.16%, Si 0.30%~0.40%, Mn 1.25%~1.40%, Al 0.02%~0.04%, Nb 0.03%~0.05%, V 0.03%~0.05%, 0.001% < Ti < 0.010%, N < 0.008%, P ≤ 0.015%, S ≤ 0.008%, with the balance being Fe and unavoidable impurities; the carbon equivalent of the continuously cast round billet is ≤ 0.40%. The wall thickness of the finished seamless steel pipe is greater than 16 mm.

[0007] In a specific embodiment of the present invention, the wall thickness of the finished seamless steel pipe is 17~30 mm.

[0008] In a specific embodiment of the present invention, the carbon equivalent of the continuously cast round billet is 0.35%~0.40%.

[0009] In a specific embodiment of the present invention, in the continuous rolling process, the initial rolling temperature is ≥1100℃ and the final rolling temperature is 1000~1030℃.

[0010] In a specific embodiment of the present invention, in step (a), the heating temperature is 1200~1250℃. Further, in step (a), the heating holding time is 1.5~2 h.

[0011] In a specific embodiment of the present invention, the temperature of the perforation is 1200~1230℃.

[0012] In a specific embodiment of the present invention, the rolling elongation in the continuous rolling process is 2 to 3.

[0013] In a specific embodiment of the present invention, the diameter reduction is 20% to 30%.

[0014] The second aspect of the present invention provides a thick-walled seamless steel pipe with a yield strength of 355 MPa, which is prepared by the preparation method provided in the first aspect of the present invention.

[0015] In a specific embodiment of the present invention, the thick-walled seamless steel pipe with a yield strength of 355 MPa satisfies the following characteristics: (1) Room temperature yield strength ≥ 355 MPa; (2) Tensile strength at room temperature ≥ 480 MPa; (3) Elongation at room temperature after fracture ≥22%; (4) Charpy V-notch low-temperature impact energy at -20℃ ≥80 J; (5) Grain size ≥ 7; (6) Wall thickness > 16 mm.

[0016] Compared with the prior art, the beneficial effects of the present invention are as follows: This invention rationally controls the alloy composition based on the precipitation sequence of microalloying elements. It utilizes the preferential precipitation of a suitable amount of TiN to pin grain boundaries, suppressing high-temperature austenite coarsening while avoiding excessive and large TiN particles that could impair toughness. The solute dragging effect of Nb suppresses austenite recrystallization during continuous rolling, and combined with rapid cooling after continuous rolling to precipitate Nb(C,N), effectively refining the austenite grains. Controlled cooling after sizing promotes the full precipitation of most fine Nb(C,N) to refine the microstructure, further refining the austenite. The remaining Nb(C,N) and V(C,N) disperse during natural cooling, improving strength and toughness. This invention's seamless steel pipe preparation method, through the synergistic effect of grain refinement strengthening and precipitation strengthening, significantly improves the strength and toughness of seamless steel pipes without the need for subsequent heat treatment, while simplifying the process and reducing energy consumption. Attached Figure Description

[0017] To more clearly illustrate the specific embodiments of the present invention or the technical solutions in the prior art, the drawings used in the description of the specific embodiments or the prior art will be briefly introduced below. Obviously, the drawings described below are some embodiments of the present invention. For those skilled in the art, other drawings can be obtained from these drawings without creative effort.

[0018] Figure 1 This is a metallographic diagram of the seamless steel pipe product provided in Embodiment 2 of the present invention. Detailed Implementation

[0019] The technical solution of the present invention will be clearly and completely described below with reference to the accompanying drawings and specific embodiments. However, those skilled in the art will understand that the embodiments described below are some embodiments of the present invention, but not all embodiments, and are only used to illustrate the present invention, and should not be regarded as limiting the scope of the present invention. Based on the embodiments of the present invention, all other embodiments obtained by those skilled in the art without creative effort are within the scope of protection of the present invention. Where specific conditions are not specified in the embodiments, conventional conditions or conditions recommended by the manufacturer shall be followed. Where the manufacturers of reagents or instruments are not specified, they are all conventional products that can be purchased commercially.

[0020] In this invention, a yield strength of 355 MPa means a yield strength ≥ 355 MPa.

[0021] The first aspect of this invention provides a method for preparing thick-walled seamless steel pipes with a yield strength of 355 MPa, comprising the following steps: (a) Heating, piercing, and rolling the continuously cast round billet, and cooling the rolled rough tube to 900-930°C at a cooling rate of 20-30°C / s; (b) Reduce the diameter of the rough tube cooled to 900~930℃, then cool it to 600~650℃ at a cooling rate of 5~10℃ / s, and then let it cool naturally to room temperature to obtain the finished seamless steel pipe. The chemical composition of the continuously cast round billet, by mass percentage, includes: C 0.14%~0.16%, Si 0.30%~0.40%, Mn 1.25%~1.40%, Al 0.02%~0.04%, Nb 0.03%~0.05%, V 0.03%~0.05%, 0.001% < Ti < 0.010%, N < 0.008%, P ≤ 0.015%, S ≤ 0.008%, with the balance being Fe and unavoidable impurities; the carbon equivalent of the continuously cast round billet is ≤ 0.40%. The wall thickness of the finished seamless steel pipe is greater than 16 mm.

[0022] This invention rationally controls the alloy composition based on the precipitation sequence of microalloying elements. It utilizes the preferential precipitation of a suitable amount of TiN to pin grain boundaries, suppressing high-temperature austenite coarsening while avoiding excessively large TiN particles that could impair toughness. The solute dragging effect of Nb suppresses austenite recrystallization during continuous rolling, and, combined with rapid cooling after continuous rolling to precipitate Nb(C,N), effectively refines the austenite grains. Controlled cooling after sizing promotes the full precipitation of most fine Nb(C,N) to refine the microstructure, while the remaining Nb(C,N) and V(C,N) disperse during natural cooling, improving strength and toughness. The seamless steel pipe manufacturing method of this invention, through the synergistic effect of grain refinement strengthening and precipitation strengthening, significantly improves the strength and toughness of seamless steel pipes without the need for subsequent heat treatment, while simplifying the process and reducing energy consumption.

[0023] In some embodiments, the carbon (C) content in the continuously cast round billet is 0.14% to 0.16%, specifically within the range of 0.14%, 0.145%, 0.15%, 0.155%, 0.16%, or any combination thereof. Carbon is one of the effective and inexpensive elements for improving the strength of steel pipes. This invention controls the C content within the above range, which is beneficial for balancing the strength, toughness, and weldability of the steel pipe. If the C content is too high, it is detrimental to the toughness and weldability of the steel pipe.

[0024] In some embodiments, the Si content in the continuously cast round billet is 0.30% to 0.40%, specifically within the range of 0.30%, 0.32%, 0.35%, 0.38%, 0.40%, or any combination thereof. Si has a solid solution strengthening effect. This invention controls the Si content within the above range, which is beneficial for balancing the strength, toughness, ductility, and weldability of the steel pipe. If the Si content is too high, it will reduce the toughness, plasticity, and ductility of the steel pipe and is detrimental to welding.

[0025] In some embodiments, the Mn content in the continuously cast round billet is 1.25% to 1.40%, specifically within the range of 1.25%, 1.28%, 1.3%, 1.32%, 1.35%, 1.38%, 1.40%, or any combination thereof. If the Mn content is too low, it is detrimental to the strength of the steel pipe; if the Mn content is too high, it will exacerbate segregation in the steel pipe, impairing its toughness and making it prone to cracking during welding.

[0026] In some embodiments, the Al content in the continuously cast round billet is 0.02% to 0.04%, specifically within the range of 0.02%, 0.025%, 0.03%, 0.035%, 0.04%, or any combination thereof. Excessive Al content can lead to an increase in Al oxide inclusions, reducing the purity of the steel and negatively impacting its toughness. The introduction of an appropriate amount of Al can inhibit grain growth by forming AlN. In this invention, controlling the Al content in the continuously cast round billet to meet the above-mentioned range achieves a good balance between effective grain refinement and controlled inclusions.

[0027] In some embodiments, the Nb content in the continuously cast round billet is 0.03% to 0.05%, specifically within the range of 0.03%, 0.035%, 0.04%, 0.045%, 0.05%, or any combination thereof. Nb is a strong carbonitride forming element; Nb in solid-solution austenite can exert a solute dragging effect, effectively inhibiting austenite growth and delaying austenite recrystallization. Simultaneously, in conjunction with the cooling steps after continuous rolling and sizing in this invention, Nb can play a role in grain refinement and precipitation strengthening. Through strain-induced precipitation of Nb(C,N), austenite grains are pinned, refining the austenite grains and thus improving strength and low-temperature toughness. This invention, by controlling the Nb content within the above range, facilitates the formation of sufficient, fine, and dispersed Nb(C,N) precipitates during continuous rolling and sizing cooling processes. If the Nb content is too low, the solute dragging and precipitation strengthening effects will not be obvious; if the Nb content is too high, it will not only damage the toughness, but also easily form melting point copolymers with elements such as Fe and C, increasing the tendency to crack during continuous casting.

[0028] In some embodiments, the content of V in the continuously cast round billet is 0.03% to 0.05%, specifically within the range of 0.03%, 0.035%, 0.04%, 0.045%, 0.05%, or any combination thereof. V carbonitrides have high solubility in austenite and mainly precipitate in the ferrite after austenite transformation, thus improving strength. Controlling the V content within the above range ensures sufficient and fine V (C, N) dispersion after sizing, significantly improving strength. If the V content is too low, the precipitation strengthening effect is insufficient; if the V content is too high, it easily impairs toughness.

[0029] In some embodiments, the Ti content in the continuously cast round billet satisfies 0.001% < Ti < 0.010%, specifically, the Ti content can be 0.0015%, 0.002%, 0.003%, 0.004%, 0.005%, 0.006%, 0.007%, 0.008%, 0.009%, 0.0095%, or any combination thereof, preferably 0.002% to 0.009%. Ti has a stronger affinity for N than Nb and V, and the TiN precipitates formed can effectively pin austenite grain boundaries and inhibit grain coarsening. However, since Ti preferentially precipitates in molten steel, if the amount of Ti added is too high, large-sized liquid-precipitated TiN particles are easily generated, significantly reducing the low-temperature toughness of the steel pipe. Therefore, by controlling the Ti content within the above range, this invention ensures the precipitation of sufficient fine TiN particles to exert the grain boundary pinning effect, while effectively avoiding the formation of coarse liquid-precipitated TiN, thus achieving a balance between austenite grain control and low-temperature toughness.

[0030] In some embodiments, the nitrogen (N) content in the continuously cast round billet is less than 0.008%, specifically within the range of 0.007%, 0.0065%, 0.006%, 0.0055%, 0.005%, 0.0045%, 0.004%, or any combination thereof, preferably 0.005% to 0.007%. An appropriate amount of N ensures the formation of TiN, Nb(C,N), and V(C,N) precipitates, thereby improving the toughness and strength of the steel pipe. However, excessive N promotes the formation of coarse, liquid-precipitated TiN, deteriorating the low-temperature impact toughness and weldability of the steel pipe.

[0031] In some embodiments, the phosphorus (P) content in the continuously cast round billet does not exceed 0.015%, specifically within the range of 0.015%, 0.014%, 0.012%, 0.01%, 0.008%, or any combination thereof, such as 0.01% to 0.015%. Increasing the P content exacerbates the cold brittleness tendency of steel and easily leads to segregation in the core of the continuously cast billet, adversely affecting subsequent processes such as continuous rolling. However, excessively low P content requirements significantly increase smelting difficulty and cost. In the steel pipe system of this invention, controlling the P content to no more than 0.015% balances both processability and performance requirements.

[0032] In some embodiments, the sulfur (S) content in the continuously cast round billet does not exceed 0.008%, specifically within the range of 0.008%, 0.006%, 0.005%, 0.004%, 0.003%, or any combination thereof, such as 0.003% to 0.005%. An increase in S content leads to a decrease in the mechanical properties and toughness of the steel pipe. Similarly, excessively low S content requirements increase smelting difficulty and cost. In the steel pipe system of this invention, controlling the S content to not exceed 0.008% balances both processability and performance requirements.

[0033] In some embodiments, the carbon equivalent of the continuously cast round billet is ≤0.40%, specifically within the range of 0.40%, 0.39%, 0.38%, 0.37%, 0.36%, 0.35%, or any combination thereof, such as 0.35%~0.40%. The carbon equivalent (CEV) is calculated as: CEV = C + Mn / 6 + (Cr + Mo + V) / 5 + (Ni + Cu) / 15. By controlling the carbon equivalent of the continuously cast round billet within the above range, this invention ensures excellent weldability of the steel pipe. Simultaneously, this carbon equivalent range is well-suited to the process of this invention, effectively avoiding the deterioration of low-temperature toughness while ensuring a yield strength ≥355 MPa, achieving a good match between strength and toughness.

[0034] In some embodiments, the wall thickness of the finished seamless steel pipe is >16 mm, specifically within the range of 17 mm, 18 mm, 20 mm, 23 mm, 25 mm, 28 mm, 30 mm, or any combination thereof, preferably 17-30 mm. This invention, through the synergistic combination of alloy composition design and manufacturing process, enables steel pipes within the aforementioned wall thickness range to achieve excellent high strength and toughness without the need for additional heat treatment.

[0035] In some embodiments, in step (a), the heating temperature is 1200~1250℃, specifically a range of 1200℃, 1210℃, 1220℃, 1230℃, 1240℃, 1250℃, or any combination thereof. Further, in step (a), the holding time for heating is 1.5~2 hours. In actual operation, the heating in step (a) can be carried out in a ring furnace.

[0036] In some embodiments, in step (a), the perforation temperature is 1200~1230°C, specifically a range of 1200, 1210, 1220, 1230°C, or any combination thereof. In actual operation, perforation is performed in a conventional perforation machine; during perforation, the perforation mandrel is preheated to above 300°C to avoid excessively rapid local temperature drop.

[0037] In some embodiments, during continuous rolling, the initial rolling temperature is ≥1100℃ and the final rolling temperature is 1000~1030℃, specifically a range of 1000℃, 1010℃, 1020℃, 1030℃ or any combination thereof.

[0038] In some embodiments, the rolling elongation during continuous rolling is 2 to 3, specifically a range of 2, 2.2, 2.5, 2.8, 3, or any combination thereof. Controlling the rolling elongation within this range ensures sufficient wall thickness reduction, promotes austenite grain refinement, and, in conjunction with post-rolling cooling, provides fine-grained, uniformly structured rough tubes for subsequent sizing and reducing processes.

[0039] In some embodiments, the continuously rolled rough tube is cooled to 900-930°C at a cooling rate of 20-30°C / s. Specifically, the cooling rate can be within the range of 20°C / s, 22°C / s, 24°C / s, 26°C / s, 28°C / s, 30°C / s, or any combination thereof; and then cooled to 900°C, 905°C, 910°C, 915°C, 920°C, 925°C, 930°C, or any combination thereof. Controlling the cooling rate and cooling termination temperature after continuous rolling within the above range can effectively suppress austenite recrystallization, promote the precipitation of an appropriate amount of Nb(C,N), and effectively refine the austenite grains.

[0040] In some embodiments, the reduction in diameter is 20% to 30%, specifically 20%, 22%, 25%, 28%, 30%, or any combination thereof. Controlling the reduction in diameter within the above range, in conjunction with the cooling process after the reduction in diameter, helps to promote the precipitation of fine Nb(C,N) and refine the austenite.

[0041] In some embodiments, the reduced-diameter pipe is cooled to 600-650°C at a cooling rate of 5-10°C / s. Specifically, the cooling rate can be a range of 5°C / s, 6°C / s, 7°C / s, 8°C / s, 9°C / s, 10°C / s, or any combination thereof; and cooled to 600°C, 610°C, 620°C, 630°C, 640°C, 650°C, or any combination thereof at this cooling rate.

[0042] In some embodiments, the continuously cast round billet can be obtained through conventional smelting and continuous casting. This invention provides an optional embodiment, but is not limited thereto, which specifically includes the following steps: batching materials according to the composition of the continuously cast round billet, smelting in a converter or electric furnace, then refining in an LF furnace to fine-tune the composition to the target continuously cast round billet composition, and then continuously casting using a protective casting method.

[0043] In some implementations, when using converters and electric furnaces for smelting, the final carbon content is controlled at 0.05% to 0.08%, the final temperature is 1620 to 1650°C, the FeO+MnO content in the slag is controlled at 14% to 18%, and the steel is tapped with slag blocked and pre-alloyed.

[0044] In some implementations, during LF furnace refining, the white slag is desulfurized to less than 0.005%, and the composition is finely adjusted to the target composition after the treatment is completed.

[0045] The second aspect of the present invention provides a thick-walled seamless steel pipe with a yield strength of 355 MPa, which is prepared by the preparation method provided in the first aspect of the present invention.

[0046] In some embodiments, the thick-walled seamless steel pipe with a yield strength of 355 MPa satisfies the following characteristics: (1) The room temperature yield strength is ≥355 MPa, specifically 355 MPa, 358 MPa, 360 MPa, 363 MPa, 366 MPa, 367 MPa, 370 MPa, 373 MPa or any combination thereof. (2) The room temperature tensile strength is ≥480 MPa, specifically 480 MPa, 500 MPa, 515 MPa, 530 MPa, 536 MPa, 541 MPa, 548 MPa or any combination thereof; (3) The elongation at room temperature after fracture is ≥22%, specifically 22%, 24%, 25%, 26%, 27%, 28%, 29% or any combination thereof; (4) The Charpy V-notch low-temperature impact energy at -20℃ is ≥80 J, specifically 80 J, 85 J, 90 J, 94 J, 98 J, 100 J, 104 J, 106 J, 109 J or any two of them; (5) Grain size ≥ 7; (6) Wall thickness > 16 mm, specifically 17 mm, 18 mm, 20 mm, 23 mm, 25 mm, 28 mm, 30 mm or any combination thereof.

[0047] Example 1 This embodiment provides a method for preparing thick-walled seamless steel pipes with a yield strength of 355 MPa, including the following steps: (1) The raw materials are batched according to the composition of the continuously cast round billet, and the converter is used for smelting. The final carbon content is controlled at 0.05%~0.08% (e.g., 0.06%), the final temperature is 1620~1650℃ (e.g., 1635℃), and the FeO+MnO in the slag is controlled at 14%~18% (e.g., 16%). The steel is tapped and pre-alloyed. Then it is refined in an LF furnace to produce white slag and desulfurize it to within 0.005%. After the treatment, the composition is slightly adjusted to the target composition. Continuous casting is carried out by protective casting at a casting temperature of 1530~1540℃ (e.g., 1535℃) to obtain a φ250 mm continuously cast round billet. The target composition of the continuously cast round billet includes the following components by mass percentage: C 0.14%, Si 0.30%, Mn 1.25%, Al 0.02%, Nb 0.03%, V 0.03%, Ti 0.002%, N 0.005%, P 0.012%, S 0.004%, with the balance being Fe and unavoidable impurities; CEV = 0.35%.

[0048] (2) The continuously cast round billet is heated in the ring furnace at a temperature of 1230℃ and held for 1.5 h. Then it is sent to the piercing machine for piercing to obtain the tube. The piercing temperature is 1210℃ and the piercing mandrel is preheated to above 300℃.

[0049] (3) The temperature of the tube obtained in step (2) is above 1100℃. It is directly sent to the continuous rolling mill for continuous rolling. The rolling elongation rate is 3 and the final rolling temperature is 1030℃ to obtain the rough tube. Then the rough tube is cooled to 930℃ at a cooling rate of 20℃ / s.

[0050] (4) The rough tube cooled to 930℃ is reduced in diameter by 30%, and then cooled to 650℃ at a cooling rate of 5℃ / s, and then naturally air-cooled to room temperature to obtain a seamless steel pipe product with φ210×17 mm.

[0051] Example 2 This embodiment provides a method for preparing thick-walled seamless steel pipes with a yield strength of 355 MPa, including the following steps: (1) The preparation of the continuous casting round billet is the same as in Example 1, except that the target composition of the continuous casting round billet is different. In this example, the target composition of the continuous casting round billet includes the following components by mass percentage: C 0.15%, Si 0.35%, Mn 1.33%, Al 0.03%, Nb 0.04%, V 0.04%, Ti 0.005%, N 0.006%, P 0.015%, S 0.005%, with the balance being Fe and unavoidable impurities; CEV=0.38%.

[0052] (2) The continuously cast round billet is heated in the ring furnace at a temperature of 1240℃ for 1.8 h; then it is sent to the piercing machine for piercing to obtain the tube; the piercing temperature is 1220℃, and the piercing mandrel is preheated to above 300℃.

[0053] (3) The temperature of the tube obtained in step (2) is above 1100℃. It is directly sent to the continuous rolling mill for continuous rolling. The rolling elongation rate in continuous rolling is 2.5, and the final rolling temperature is 1020℃ to obtain the rough tube. Then the rough tube is cooled to 920℃ at a cooling rate of 25℃ / s.

[0054] (4) The rough tube cooled to 920℃ is reduced in diameter by 25%, and then cooled to 620℃ at a cooling rate of 8℃ / s, and then naturally air-cooled to room temperature to obtain a seamless steel pipe product with φ200×23 mm.

[0055] Example 3 This embodiment provides a method for preparing thick-walled seamless steel pipes with a yield strength of 355 MPa, including the following steps: (1) The preparation of the continuous casting round billet is the same as in Example 1, except that the target composition of the continuous casting round billet is different. In this example, the target composition of the continuous casting round billet includes the following components by mass percentage: C 0.16%, Si 0.40%, Mn 1.40%, Al 0.04%, Nb 0.05%, V 0.05%, Ti 0.009%, N 0.007%, P 0.010%, S 0.003%, with the balance being Fe and unavoidable impurities; CEV=0.40%.

[0056] (2) The continuously cast round billet is heated in the ring furnace at a temperature of 1250℃ for 2 hours; then it is sent to the piercing machine for piercing to obtain the tube; the piercing temperature is 1230℃, and the piercing mandrel is preheated to above 300℃.

[0057] (3) The temperature of the tube obtained in step (2) is above 1100℃. It is directly sent to the continuous rolling mill for continuous rolling. The rolling elongation rate is 2 and the final rolling temperature is 1000℃ to obtain the rough tube. Then the rough tube is cooled to 900℃ at a cooling rate of 30℃ / s.

[0058] (4) The rough tube cooled to 900℃ is reduced in diameter by 20%, and then cooled to 600℃ at a cooling rate of 10℃ / s, and then naturally air-cooled to room temperature to obtain a seamless steel pipe product with φ180×30 mm.

[0059] Comparative Example 1 Comparative Example 1 follows the same preparation method as Example 2, except that the cooling rate of the rough tube obtained after continuous rolling is different in step (3), while the rest is the same as Example 2.

[0060] In step (3) of Comparative Example 1, the raw tube was cooled to 920°C at a cooling rate of 35°C / s.

[0061] Comparative Example 2 Comparative Example 2 follows the same preparation method as Example 2, except that the cooling rate of the rough tube obtained after continuous rolling is different in step (3), while the rest is the same as Example 2.

[0062] In step (3) of Comparative Example 2, the raw tube was cooled to 920°C at a cooling rate of 15°C / s.

[0063] Comparative Example 3 Comparative Example 3 follows the same preparation method as Example 2, except that the cooling rate after sizing in step (4) is different, while the rest is the same as Example 2.

[0064] In step (4) of Comparative Example 3, after the sizing is reduced, the temperature is cooled to 620°C at a cooling rate of 3°C / s.

[0065] Comparative Example 4 Comparative Example 4 follows the same preparation method as Example 2, except that the cooling rate after sizing in step (4) is different, while the rest is the same as Example 2.

[0066] In step (4) of Comparative Example 4, after the sizing is reduced, the temperature is cooled to 620°C at a cooling rate of 15°C / s.

[0067] Comparative Example 5 Comparative Example 5 follows the same preparation method as Example 2, except that in step (4), the final temperature of cooling at a cooling rate of 8°C / s after sizing is different, while the rest is the same as Example 2.

[0068] In step (4) of Comparative Example 5, after the sizing is reduced, the temperature is cooled to 700°C at a cooling rate of 8°C / s, and then naturally cooled to room temperature.

[0069] Comparative Example 6 Comparative Example 6 follows the same preparation method as Example 2, except that in step (4), the final temperature of cooling at a cooling rate of 8°C / s after sizing is different, while the rest is the same as Example 2.

[0070] In step (4) of Comparative Example 6, after the sizing is reduced, the temperature is cooled to 550°C at a cooling rate of 8°C / s, and then naturally cooled to room temperature.

[0071] Experimental Example Sampling was performed on seamless steel pipes prepared in different embodiments and comparative examples, and their microstructure and grain size were observed and measured (referencing GB / T 6394-2017 "Method for Determination of Average Grain Size of Metals"). Figure 1 The image shows the metallographic structure of the seamless steel pipe product provided in Embodiment 2 of the present invention. Table 1 below lists the microstructure results of the seamless steel pipe products obtained in different embodiments and comparative examples.

[0072] Table 1. Microstructure of different seamless steel pipe products

[0073] As can be seen from the above test results, the microstructure of the seamless steel pipe product of the present invention is a ferrite + pearlite composite structure with a grain size level ≥ 7.

[0074] Seamless steel pipe products from different embodiments and comparative examples were sampled and their mechanical properties were tested. The test results are shown in Table 2. Tensile properties were tested according to standard GB / T 228.1-2021 "Metallic materials, tensile testing—Part 1: Tensile testing at room temperature," and impact energy at -20℃ was tested according to standard GB / T 229-2020 "Metallic materials, Charpy impact test."

[0075] Table 2 Performance test results of different seamless steel pipe products

[0076] The test results above show that the method of this invention can produce thick-walled seamless steel pipes with a wall thickness of 17-30 mm, achieving a yield strength ≥355 MPa and a Charpy V-notch low-temperature impact energy ≥80 J at -20℃ without additional heat treatment, ensuring excellent strength and toughness. Furthermore, the carbon equivalent (CEV) of the steel pipe is ≤0.40%, guaranteeing excellent field welding performance, low cold cracking sensitivity, and compatibility with conventional welding processes, thus improving the product's practicality and ease of construction.

[0077] Finally, it should be noted that the above embodiments are only used to illustrate the technical solutions of the present invention, and not to limit them; although the present invention has been described in detail with reference to the foregoing embodiments, those skilled in the art should understand that modifications can still be made to the technical solutions described in the foregoing embodiments, or equivalent substitutions can be made to some or all of the technical features; and these modifications or substitutions do not cause the essence of the corresponding technical solutions to deviate from the scope of the technical solutions of the embodiments of the present invention.

Claims

1. A method for preparing thick-walled seamless steel pipes with a yield strength of 355 MPa, characterized in that, Includes the following steps: (a) The continuously cast round billet is heated, pierced, and continuously rolled, and the rolled rough tube is cooled to 900-930°C at a cooling rate of 20-30°C / s; (b) Reduce the diameter of the rough tube cooled to 900~930℃, then cool it to 600~650℃ at a cooling rate of 5~10℃ / s, and then let it cool naturally to room temperature to obtain the finished seamless steel pipe. The chemical composition of the continuously cast round billet, by mass percentage, includes: C 0.14%~0.16%, Si 0.30%~0.40%, Mn 1.25%~1.40%, Al 0.02%~0.04%, Nb 0.03%~0.05%, V 0.03%~0.05%, 0.001% < Ti < 0.010%, N < 0.008%, P ≤ 0.015%, S ≤ 0.008%, with the balance being Fe and unavoidable impurities; the carbon equivalent of the continuously cast round billet is ≤ 0.40%. The wall thickness of the finished seamless steel pipe is >16 mm.

2. The method for preparing a thick-walled seamless steel pipe with a yield strength of 355 MPa according to claim 1, characterized in that, The wall thickness of the finished seamless steel pipe is 17~30 mm.

3. The method for preparing thick-walled seamless steel pipes with a yield strength of 355 MPa according to claim 1, characterized in that, The carbon equivalent of the continuously cast round billet is 0.35%~0.40%.

4. The method for preparing a thick-walled seamless steel pipe with a yield strength of 355 MPa according to claim 1, characterized in that, In the continuous rolling process, the initial rolling temperature is ≥1100℃, and the final rolling temperature is 1000~1030℃.

5. The method for preparing a thick-walled seamless steel pipe with a yield strength of 355 MPa according to claim 1, characterized in that, In step (a), the heating temperature is 1200~1250℃; Preferably, in step (a), the heating and heat preservation time is 1.5 to 2 hours.

6. The method for preparing a thick-walled seamless steel pipe with a yield strength of 355 MPa according to claim 1, characterized in that, The temperature for perforation is 1200~1230℃.

7. The method for preparing a thick-walled seamless steel pipe with a yield strength of 355 MPa according to claim 1, characterized in that, In the continuous rolling process, the rolling elongation is 2 to 3.

8. The method for preparing a thick-walled seamless steel pipe with a yield strength of 355 MPa according to claim 1, characterized in that, In the reduction of diameter, the reduction amount is 20%~30%.

9. A thick-walled seamless steel pipe with a yield strength of 355 MPa, characterized in that, It is prepared by the preparation method according to any one of claims 1 to 8.

10. The thick-walled seamless steel pipe with a yield strength of 355 MPa according to claim 9, characterized in that, The seamless steel pipe meets the following characteristics: (1) Room temperature yield strength ≥ 355 MPa; (2) Tensile strength at room temperature ≥ 480 MPa; (3) Elongation at room temperature after fracture ≥22%; (4) Charpy V-notch low-temperature impact energy at -20℃ ≥80 J; (5) Grain size ≥ 7; (6) Wall thickness > 16 mm.

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