Low-carbon low-alloy high-strength seamless steel pipe and method for manufacturing the same

Abstract

The present application belongs to the technical field of seamless steel pipe preparation and heat treatment, and discloses a low-carbon low-alloy high-strength seamless steel pipe and a preparation method thereof. The chemical composition of the steel pipe comprises C, Si, Mn, Ni, Cr, Mo, V and Nb, and the total content of (Ni+Cr+Mo+V+Nb) is 0.90%-1.00%, and the balance is Fe and impurities. The preparation method comprises hot rolling forming, pretreatment, intercritical quenching and high-temperature tempering. Through the synergistic cooperation of ultra-low carbon+low-alloy composition and special intercritical quenching+high-temperature tempering process, the alloy cost is significantly reduced, the tensile strength of the steel pipe is greater than or equal to 900 MPa, and the low-temperature impact energy at-40 DEG C is greater than or equal to 65 J, realizing high strength and excellent low-temperature toughness, and the process window is wide, the stability is good, and the process is suitable for harsh working conditions such as high pressure, heavy load and low temperature.

Description

Technical Field

[0001] This invention relates to the field of seamless steel pipe preparation and heat treatment technology, specifically to a low-carbon, low-alloy, high-strength seamless steel pipe and its preparation method, which is suitable for the preparation of high-strength seamless steel pipes used in harsh working conditions such as engineering machinery, oil and gas transportation, high-pressure pressure vessels, and heavy equipment. Background Technology

[0002] High-strength seamless steel pipes are core basic materials in high-end equipment manufacturing, energy engineering, and infrastructure construction. The working environment places stringent requirements on the strength, toughness, fatigue resistance, low-temperature impact performance, and weldability of the steel pipes. Currently, the mainstream high-strength seamless steel pipes in the industry generally adopt a high carbon and high alloy ratio design, achieving high strength indicators by significantly increasing the content of alloys such as Cr, Mo, and Ni. However, this design has many inherent defects.

[0003] On the one hand, a high carbon and high alloy ratio will significantly increase the material production cost, and excessive carbon content will easily lead to increased low-temperature brittleness of steel pipes, poor welding performance, and subsequent processing and forming problems such as cracks and brittle fracture. On the other hand, existing heat treatment processes mostly adopt a fully austenitizing quenching + low-temperature tempering process, which can improve the strength of steel pipes, but will greatly sacrifice the toughness of the material, resulting in an imbalance between strength and toughness. In addition, the process temperature window is narrow and the stability is poor, resulting in large fluctuations in product performance and low pass rate during mass production.

[0004] In recent years, in order to reduce alloy costs, some technological explorations have emerged that use sub-temperature heat treatment to replace full quenching. For example, CN115838904A discloses a seamless steel pipe for rotary drilling rods and its preparation method, which uses a sub-temperature quenching process at 830-850℃ followed by a low-temperature tempering process at 473-493℃. However, its carbon content is still as high as 0.18%-0.23%, and the low-temperature tempering cannot fully eliminate the internal stress of quenching. The low-temperature impact toughness and welding performance still have significant shortcomings. Patent CN102277479A discloses a sub-temperature heat treatment process for pipeline pipes used in low-temperature environments, but its process type is normalizing, and the strength is less than 850MPa, which cannot meet the requirements of high-pressure and heavy-load conditions. Although the sub-temperature quenching and high-temperature tempering test of X56QS pipeline steel uses a low-carbon composition, the tensile strength is only about 560MPa, which is far from the 900MPa strength index required for industrial applications. None of the aforementioned existing technologies have been able to achieve a synergistic match between 900MPa strength and excellent low-temperature toughness under ultra-low carbon conditions with carbon content below 0.10% and total alloy addition below 1.00%.

[0005] In existing technologies, low-carbon, low-alloy seamless steel pipes generally suffer from insufficient strength, failing to meet the demands of heavy-duty, high-pressure, and fatigue-resistant applications in high-end equipment. Conversely, high-strength seamless steel pipes present a contradiction: high alloy costs, complex manufacturing processes, and significant shortcomings in overall performance. Furthermore, conventional heat treatment processes cannot precisely control phase transformation microstructure, making it difficult to achieve a synergistic balance of high strength and high toughness with low alloy proportions. Moreover, existing sub-critical quenching processes primarily target medium-carbon compositions or forgings, lacking a dedicated sub-critical quenching + high-temperature tempering process for ultra-low carbon (0.06%-0.08%) and low alloy content (≤1.00%) seamless steel pipes. This has become a critical technical bottleneck that urgently needs to be addressed in this field. Therefore, developing a low-carbon, low-alloy, low-cost seamless steel pipe with excellent strength, toughness, and strong process stability, along with a dedicated heat treatment method, has significant engineering application value and market potential. Summary of the Invention

[0006] To address the shortcomings of existing high-strength seamless steel pipes, such as high alloy content, high cost, poor welding and low-temperature performance, unbalanced strength and toughness, and poor stability of heat treatment processes, especially the technical bottlenecks of existing sub-critical quenching schemes with still relatively high carbon content (≥0.18%), residual internal stress during low-temperature tempering, and mismatched process windows with ultra-low carbon and low alloy composition systems, this invention provides a low-carbon and low-alloy high-strength seamless steel pipe and its heat treatment method. By optimizing the chemical composition ratio and sub-critical heat treatment process, under the premise of ultra-low carbon and low alloy content ≤0.08% and total alloy addition ≤1.00%, the invention achieves a tensile strength ≥900MPa and low-temperature toughness at -40℃ in seamless steel pipes, thereby improving the strength and toughness balance, reducing alloy costs, and enhancing process stability.

[0007] The technical solution of this invention is: This invention provides a low-carbon, low-alloy, high-strength seamless steel pipe with the following chemical composition by mass percentage: C: 0.06%~0.08%, Si: 0.10%~0.50%, Mn: 0.80%~1.20%, Ni: 0.10%~0.40%, Cr: 0.10%~0.50%, Mo: 0.10%~0.50%, V: 0.01%~0.06%, Nb: 0.01%~0.06%, P≤0.010%, S≤0.005%, and the total content of (Ni+Cr+Mo+V+Nb) is 0.90%~1.00%, with the balance being Fe and unavoidable impurities.

[0008] Preferably, the metallographic structure of the seamless steel pipe is tempered sorbite plus dispersed undissolved ferrite, wherein the undissolved ferrite is granular or short strips uniformly distributed on the tempered sorbite matrix.

[0009] Preferably, the seamless steel pipe has a tensile strength ≥900MPa and a low-temperature impact energy of -40℃ ≥65J.

[0010] Another aspect of the present invention provides a method for preparing the above-mentioned seamless steel pipe, comprising the following steps: (1) Hot rolling: The material is prepared by dispensing the chemical composition according to the mass percentage, and then producing rolled tubes by electric furnace / converter steelmaking, vacuum treatment, continuous casting, piercing, continuous rolling, sizing, sawing and straightening. (2) Pretreatment: The hot-rolled tube is shot-blasted to remove oxide scale and mechanically straightened to obtain pretreated steel tubes. This removes surface oxidation defects and impact damage, eliminates residual rolling stress, and provides a uniform microstructure for subsequent heat treatment, thus avoiding deformation and uneven microstructure during heat treatment. (3) Sub-temperature quenching: The pretreated steel pipe is sent into the heat treatment furnace and heated at a uniform rate to Ac3-(15-20)℃ (that is, 15-20℃ lower than the Ac3 temperature, the specific Ac3 temperature is determined according to the material). The wall thickness ratio is maintained at 2-3 min / mm to ensure that the overall temperature of the steel pipe is uniform and the partial austenitization is sufficient. Then, water quenching medium is used for rapid cooling to obtain a steel pipe with a composite structure containing martensite + a small amount of fine-grained ferrite. (4) High-temperature tempering: The quenched steel pipe is heated to the high-temperature tempering range of 580-650℃ and held for 2-4 hours to fully decompose the residual austenite, refine the quenched structure and eliminate internal stress. Then, it is air-cooled to room temperature at a rate of 8-10℃ / min to finally obtain a low-carbon, low-alloy, high-strength seamless steel pipe.

[0011] The innovative aspects of this invention are as follows: 1. Pipe chemical composition design This invention relates to a seamless steel pipe with an ultra-low carbon design. The carbon (C) content is strictly controlled at 0.06%-0.08% by mass, representing a reduction of over 60% compared to the existing medium-carbon low-alloy sub-temperature quenching scheme (CN115838904A, C: 0.18%-0.23%). This fundamentally improves the material's low-temperature brittleness and weldability. The composite strengthening alloying elements selected are Ni, Cr, Mo, V, and Nb, with controlled contents of Ni: 0.10-0.40%, Cr: 0.10-0.50%, Mo: 0.10-0.50%, V: 0.01-0.06%, and Nb: 0.01-0.06%. The total alloy content is 0.90% ≤ (Ni+Cr+Mo+V+Nb) ≤ 1.00%, belonging to a low-alloy system. The total alloy content is approximately 0.5%- Compared to 0.6% (strength only 850MPa level) or the total amount of traditional high-alloy steel solutions ≥2.0%, this invention precisely limits the total alloy amount to a critical equilibrium point of 0.90%-1.00%, combined with the dispersion strengthening effect of V and Nb microalloying, replacing the traditional single strengthening mode of high-alloy steel through a multi-element micro-synergistic strengthening mechanism. Simultaneously controlling Si: 0.10-0.50% and Mn: 0.80-1.20%, this invention minimizes alloy costs while ensuring strengthening effects. This invention strictly controls harmful impurity elements, P≤0.010% and S≤0.005%, avoiding material mechanical property degradation and microstructure inhomogeneity caused by impurity segregation, thus ensuring the purity and performance stability of the steel pipe matrix.

[0012] 2. Dedicated heat treatment process design This invention abandons the traditional fully austenitizing quenching process and the low-temperature tempering process of existing medium-carbon low-alloy materials (such as the 473-493℃ tempering in CN115838904A), and creatively proposes a "sub-temperature quenching + high-temperature tempering" heat treatment process that is deeply integrated with the aforementioned ultra-low carbon and extremely low-alloy composition system. The core innovation is that the sub-temperature quenching temperature is precisely limited to 15-20℃ below the Ac3 phase transformation critical point. This temperature range allows the material to undergo partial austenitization, retaining a small amount of uniformly dispersed fine-grained ferrite structure. Ferrite has excellent toughness, which can effectively offset the brittleness increase caused by quench hardening. Combined with the subsequent high-temperature tempering process, the martensitic structure is refined, the quenching internal stress is completely eliminated, and the microstructure composition is homogenized, achieving a synergistic improvement in strength and toughness. Compared with the sub-temperature normalizing of CN102277479A (strength less than 850MPa) and the high-temperature tempering of X56QS (strength only 560MPa) in the prior art, this invention compensates for the strength loss caused by ultra-low carbon by strengthening the dispersion precipitation of V and Nb microalloys during the high-temperature tempering process. For the first time, it achieves a synergistic match between 900MPa strength and low-temperature impact energy above 65J under ultra-low carbon (0.06%-0.08%) conditions through high-temperature tempering.

[0013] The advantages and beneficial effects of this invention are: 1. This invention employs an ultra-low carbon design of 0.06%-0.08%, significantly improving the low-temperature impact toughness and weldability of steel pipes while avoiding the brittle fracture defects of high-carbon materials. Simultaneously, the total content of the five core alloys is controlled to 0.90% ≤ (Ni+Cr+Mo+V+Nb) ≤ 1.00%, far lower than the alloy addition amounts in existing high-strength low-alloy steel pipes, significantly reducing raw material costs and making it suitable for large-scale mass production. Through multi-element trace-level synergistic strengthening, replacing a single high-alloy strengthening scheme, a technological breakthrough of low-ratio, high-performance is achieved.

[0014] 2. Traditional sub-critical quenching processes have an ambiguous temperature range, leading to significant performance fluctuations in mass production. This invention precisely defines the sub-critical quenching temperature as Ac3 - (15-20)℃ (i.e., 15-20℃ below Ac3 temperature, the specific Ac3 temperature being determined based on the material). The temperature window is precisely controllable, enabling the stable acquisition of a martensitic + uniform fine-grained ferrite composite structure, thus addressing the industry pain point of insufficient toughness in high-strength steel pipes at the microscopic level. Combined with a high-temperature tempering process, it completely eliminates heat treatment internal stress, resulting in high dimensional accuracy of the steel pipe, no risk of cracking or deformation, and excellent process repeatability and stability.

[0015] 3. The seamless steel pipe prepared by this invention has high tensile strength, high yield strength, excellent elongation, and low-temperature impact performance under the premise of low carbon and low alloy. It overcomes the technical bottleneck of "increasing strength but decreasing toughness" in traditional processes, and is suitable for harsh working conditions such as high pressure, heavy load, low temperature, and alternating load, with a wide range of applicable scenarios.

[0016] 4. Sub-temperature quenching involves heating temperatures more than 50°C lower than full quenching, shortening heating time and reducing energy consumption. In large-scale industrial production, this results in significant energy savings, reduced production costs, and compliance with the national "dual-carbon" strategy and green manufacturing requirements. Furthermore, the lower heating temperature extends the service life of heating furnaces, heat-resistant tooling, and other equipment, reducing equipment maintenance costs. Attached Figure Description

[0017] Figure 1 The metallographic structure of the seamless steel pipe prepared in Example 1. Detailed Implementation

[0018] The present invention will be further described in detail below with reference to specific embodiments. However, the present invention is not limited to the following embodiments. All equivalent substitutions and improvements made based on the technical solutions of the present invention shall fall within the protection scope of the present invention.

[0019] Example 1 A low-carbon, low-alloy, high-strength seamless steel pipe has the following chemical composition by mass percentage: C: 0.06%, Si: 0.14%, Mn: 1.16%, Ni: 0.15%, Cr: 0.23%, Mo: 0.42%, V: 0.06%, Nb: 0.04%, P: 0.009%, S: 0.004%, with a total content of Ni+Cr+Mo+V+Nb of 0.90%, and the balance being Fe and unavoidable impurities. The seamless steel pipe produced has a specification of Φ114×6mm.

[0020] The above-mentioned method for preparing seamless steel pipes: 1. Hot rolling forming: The materials are prepared according to the chemical composition by mass percentage, and rolled into rolled tubes by electric furnace / converter steelmaking, vacuum treatment, continuous casting, piercing, continuous rolling, sizing, sawing and straightening. 2. Pretreatment: The Φ114×6mm hot-rolled seamless steel pipe is shot-blasted to remove oxide scale and straightened to remove surface defects and residual rolling stress, thus obtaining the pretreated steel pipe. 3. Sub-temperature quenching: The Ac3 phase transformation temperature of the pipe is detected to be 875℃. The pretreated steel pipe is heated to 855℃ (Ac3-20℃), held for 12 minutes, and then rapidly cooled using water quenching medium. 4. High-temperature tempering: Heat the quenched steel pipe to 630℃, hold for 3 hours, and then air cool to room temperature at a rate of 8℃ / min.

[0021] The metallographic structure of Example 1 is as follows Figure 1 As shown, sub-temperature quenching in the two-phase region results in incomplete austenitization, leaving a small amount of fine, dispersed undissolved ferrite in the steel. After quenching, the microstructure consists of martensite and undissolved ferrite. Upon tempering, the martensite transforms into tempered sorbite, while the undissolved ferrite remains stable, ultimately forming a composite microstructure of tempered sorbite and dispersed undissolved ferrite. In this microstructure, the undissolved ferrite is granular or short-ribbed, uniformly distributed on the sorbite matrix, with a significantly finer grain size compared to fully quenched steel.

[0022] Example 2 A low-carbon, low-alloy, high-strength seamless steel pipe has the following chemical composition by mass percentage: C: 0.07%, Si: 0.16%, Mn: 1.18%, Ni: 0.12%, Cr: 0.30%, Mo: 0.44%, V: 0.04%, Nb: 0.05%, P: 0.008%, S: 0.003%, with a total content of Ni+Cr+Mo+V+Nb of 0.95%, and the balance being Fe and unavoidable impurities. The seamless steel pipe produced has a specification of Φ168×8mm.

[0023] The above-mentioned method for preparing seamless steel pipes: 1. Hot rolling forming: The materials are prepared according to the chemical composition by mass percentage, and rolled into rolled tubes by electric furnace / converter steelmaking, vacuum treatment, continuous casting, piercing, continuous rolling, sizing, sawing and straightening. 2. Pretreatment: The Φ168×8mm hot-rolled seamless steel pipe is shot-blasted to remove oxide scale and straightened to remove surface defects and residual rolling stress, thus obtaining the pretreated steel pipe. 3. Sub-temperature quenching: The Ac3 phase transformation temperature of the pipe is 880℃. The pretreated steel pipe is heated to 865℃ (Ac3-15℃), held for 20 minutes, and then rapidly cooled by water quenching. 4. High-temperature tempering: Heat the quenched steel pipe to 620℃, hold for 3.5 hours, and then air cool to room temperature at a rate of 9℃ / min.

[0024] Example 3 A low-carbon, low-alloy, high-strength seamless steel pipe has the following chemical composition by mass percentage: C: 0.08%, Si: 0.17%, Mn: 1.17%, Ni: 0.12%, Cr: 0.33%, Mo: 0.45%, V: 0.05%, Nb: 0.05%, P: 0.008%, S: 0.004%, with a total content of Ni+Cr+Mo+V+Nb of 1.00%, and the balance being Fe and unavoidable impurities. The seamless steel pipe produced has a specification of Φ273×12mm.

[0025] The above-mentioned method for preparing seamless steel pipes: 1. Hot rolling forming: The materials are prepared according to the chemical composition by mass percentage, and rolled into rolled tubes by electric furnace / converter steelmaking, vacuum treatment, continuous casting, piercing, continuous rolling, sizing, sawing and straightening. 2. Pretreatment: The Φ273×12mm hot-rolled seamless steel pipe is shot-blasted to remove oxide scale and straightened to remove surface defects and residual rolling stress, thus obtaining the pretreated steel pipe. 3. Sub-temperature quenching: The Ac3 phase transformation temperature of the pipe is 885℃. The pretreated steel pipe is heated to 870℃ (Ac3-15℃), held for 36 minutes, and then rapidly cooled by water quenching. 4. High-temperature tempering: Heat the quenched steel pipe to 610℃, hold for 4 hours, and then air cool to room temperature at a rate of 10℃ / min.

[0026] Comparative Example 1 (Ingredient Comparison) Table 1 shows a comparison of the main components of the steel grade of this invention with those of the traditional components used by a certain factory: Table 1. Comparison of components (%)

[0027] As can be seen from Table 1, the steel grade of the present invention has a lower C content and a lower total alloy content. Therefore, seamless steel pipes produced using the steel grade of the present invention have higher weldability and lower cost.

[0028] Comparative Example 2 The only difference from Example 1 is that the heating temperature in step 3 is 835°C (Ac3-40°C).

[0029] Comparative Example 3 The only difference from Example 1 is that the heating temperature in step 3 is 925°C (Ac3+50°C).

[0030] Comparative Example 4 The only difference from Example 1 is that the heating temperature in step 3 is 845℃ (Ac3-30℃).

[0031] Comparative Example 5 The only difference from Example 1 is that the heating temperature in step 3 is 870°C (Ac3-5°C).

[0032] The performance of the embodiments and comparative examples was tested, and the results are shown in Table 2.

[0033] Table 2 Performance Testing

[0034] As can be seen from Table 2, for the same sub-temperature quenching, the steel pipe obtained by the sub-temperature quenching temperature determined by the present invention has a much higher strength than the steel pipe obtained by the lower sub-temperature quenching temperature while maintaining the same toughness. Compared with full quenching, the steel pipe obtained by the heat treatment process of the present invention has a higher toughness than the steel pipe obtained by the full quenching process while maintaining the same strength.

[0035] The comparison of examples and comparative examples confirms that the present invention, by matching a low-carbon, low-alloy composition with a dedicated sub-temperature quenching + high-temperature tempering process, significantly improves the strength, toughness, low-temperature performance, and weldability of seamless steel pipes while reducing alloy costs. The sub-temperature quenching heating temperature determined by the present invention is more than 50°C lower than that of complete quenching, which shortens the heating time, reduces energy consumption, and at the same time, the lower heating temperature can extend the service life of heating furnaces, heat-resistant tooling, and other equipment, thereby reducing equipment maintenance costs.

[0036] The above description is merely a specific embodiment of the present invention. However, those skilled in the art should understand that various changes and substitutions can be made to the specific embodiments without departing from the spirit and scope of the present invention. All such changes and substitutions fall within the protection scope of the claims of the present invention.

Claims

1. A low-carbon, low-alloy, high-strength seamless steel pipe, characterized in that, The chemical composition by mass percentage is as follows: C: 0.06%~0.08%, Si: 0.10%~0.50%, Mn: 0.80%~1.20%, Ni: 0.10%~0.40%, Cr: 0.10%~0.50%, Mo: 0.10%~0.50%, V: 0.01%~0.06%, Nb: 0.01%~0.06%, P≤0.010%, S≤0.005%, and the total content of (Ni+Cr+Mo+V+Nb) is 0.90%~1.00%, with the balance being Fe and unavoidable impurities.

2. The low-carbon, low-alloy, high-strength seamless steel pipe according to claim 1, characterized in that, The metallographic structure of the seamless steel pipe consists of tempered sorbite and dispersed undissolved ferrite, wherein the undissolved ferrite is granular or short strip-shaped and uniformly distributed on the tempered sorbite matrix.

3. The low-carbon, low-alloy, high-strength seamless steel pipe according to claim 1 or 2, characterized in that, The seamless steel pipe has a tensile strength ≥900MPa and a low-temperature impact energy of -40℃ ≥65J.

4. A method for preparing a low-carbon, low-alloy, high-strength seamless steel pipe as described in any one of claims 1-3, characterized in that, Includes the following steps: (1) Hot rolling: The material is prepared by dispensing the chemical composition according to the mass percentage, and then producing rolled tubes by electric furnace / converter steelmaking, vacuum treatment, continuous casting, piercing, continuous rolling, sizing, sawing and straightening. (2) Pretreatment: The hot-rolled tube is shot-blasted to remove oxide scale and mechanically straightened to obtain the pretreated steel tube; (3) Sub-temperature quenching: The pretreated steel pipe is heated to a temperature range 15-20°C below its Ac3 phase transformation temperature, held at a wall thickness ratio of 2-3 min / mm, and then rapidly cooled by water quenching; (4) High-temperature tempering: The quenched steel pipe is heated and kept at a constant temperature, and then air-cooled to room temperature.

5. The preparation method according to claim 4, characterized in that, The sub-temperature quenching heating temperature in step (3) is determined based on the Ac3 phase transformation temperature of the seamless steel pipe.

6. The preparation method according to claim 4, characterized in that, The high-temperature tempering in step (4) is at a temperature of 580-650℃ and is held for 2-4 hours.

7. The preparation method according to claim 4, characterized in that, In step (4), the air cooling rate is 8-10℃ / min.

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