A manufacturing method of a rare earth microalloyed large-diameter chromium-molybdenum seamless steel pipe

Abstract

The application discloses a manufacturing method of a rare earth micro-alloyed large-diameter chromium-molybdenum seamless steel pipe and belongs to the technical field of metallurgical materials. The steel pipe has low cost and is easy to produce through reasonable selection of alloy elements and appropriate element content ratio, and the steel pipe has high strength, excellent impact toughness and high size precision through reasonable smelting and continuous casting processes and rolling processes. The application is the main raw material of gas cylinders and can be widely applied in industrial production and other fields, is the special raw material for various high-pressure gas cylinders, accumulators and other energy industries, and has the characteristics of easy production, low cost and excellent performance.

Description

Technical Field

[0001] This invention belongs to the field of metallurgical materials technology, and in particular relates to a method for manufacturing large-diameter chromium-molybdenum seamless steel pipes with rare earth microalloying. Background Technology

[0002] With the development of the gas and automotive industries, the gas cylinder industry has also developed rapidly, and the demand for gas cylinders is increasing. The consumption of industrial and civilian gas cylinders such as oxygen, nitrogen, argon, acetylene, and liquefied petroleum gas remains stable year-round, with continuous consumption in the mining, metallurgy, chemical, construction, and medical industries, leading to a continuously expanding market. New energy sources are driving explosive growth, with the rapid popularization of vehicle-mounted hydrogen cylinders, hydrogen storage cylinders, and high-pressure cylinders for hydrogen refueling stations. The development of the hydrogen energy industry is significantly increasing the demand for high-pressure seamless steel pipes. The large-scale market entry of new energy vehicles has led to a year-on-year increase in the consumption of vehicle-mounted natural gas (CNG) cylinders and vehicle-mounted gas storage cylinders. The surge in demand for high-purity gases in energy storage and specialty industries such as semiconductors, photovoltaics, and electronics is driving the demand for high-end gas cylinder steel pipes. The demand for fire-fighting cylinders, fire extinguishers, diving cylinders, medical cylinders, and military and aviation specialty cylinders is steadily increasing.

[0003] Table 1 shows the statistics of domestic production units capable of producing seamless steel pipes for gas cylinders.

[0004] Table 1 Statistics of Seamless Steel Pipe Units in China

[0005]

[0006] Table 2 Development Status of Seamless Steel Pipes for High-Pressure Gas Cylinders by Major Domestic Steel Enterprises

[0007]

[0008] In recent years, with the rapid growth in the use of industrial gases, especially natural gas, and the rapid development of the modern automotive industry, natural gas has become a trend to replace gasoline and diesel as vehicle fuel in order to reduce exhaust pollutant emissions. Therefore, how to achieve rapid, efficient, and convenient gas transportation has become a focus of attention in the industrial gas industry, bringing huge business opportunities to the production of large-diameter, high-pressure gas cylinders.

[0009] The research and development of seamless steel pipes for large-diameter gas cylinders is of great significance. The rare-earth microalloyed large-diameter chromium-molybdenum seamless steel pipe developed in this patent, as an important raw material for high-pressure gas cylinders, can enrich the variety of seamless pipe products available to enterprises in the high-pressure gas cylinder field, enhance product quality, increase market competitiveness, and bring significant economic and social benefits to enterprises. Summary of the Invention

[0010] The purpose of this invention is to provide a method for manufacturing large-diameter chromium-molybdenum seamless steel pipes with rare earth microalloying. Considering the usage conditions of chromium-molybdenum products, this invention uses medium C and low Mn as a base, adding microalloying elements such as Cr and Mo. Through steelmaking and pipe rolling processes, it produces hot-rolled seamless steel pipes for machining with a target tensile strength (Rm) of 890 MPa (measured value 995 MPa–1050 MPa), a target yield strength (Rp0.2) greater than 755 MPa (measured value 840 MPa–920 MPa), a target elongation (A) greater than 14% (measured value 16%), and a target V-shaped full-size longitudinal impact energy greater than 60 J (measured value reaching 110 J) under a -50℃ experimental environment.

[0011] To solve the above-mentioned technical problems, the present invention adopts the following technical solution:

[0012] This invention discloses a manufacturing method for large-diameter chromium-molybdenum seamless steel pipes with rare earth microalloying. The production process includes: blast furnace hot metal → hot metal pretreatment → converter smelting → LF ladle refining → VD vacuum treatment → continuous casting → billet slow cooling → warehousing → continuous casting billet → sawing → ring furnace heating → piercing → continuous rolling → tube removal → sizing (tension reduction) → cooling bed → sawing → straightening → magnetic flux leakage flaw detection → ultrasonic flaw detection → inspection → heat treatment → length measurement, weighing, marking → packaging and warehousing → shipment; characterized in that:

[0013] The steelmaking process employs "LF+VD" to ensure the cleanliness of the cast billet. The final slag basicity of the converter smelting is ≥3.0, and the final carbon content must be carefully controlled to prevent over-oxidation and ultra-low carbon tapping. The converter final control targets are: C≥0.06%, P≤0.010%, and tapping temperature 1600~1620℃. Argon blowing is carried out throughout the process according to the requirements of LF refining, and the argon blowing intensity is controlled to maintain the molten steel's creep and prevent it from being exposed. In the later stage of refining, the composition is finely adjusted, and rare earth alloys are added in proportion. After vacuum treatment, the soft blowing time is ≥13min, and attention should be paid to the effect of weak stirring to prevent the molten steel from being exposed. Non-metallic inclusions in the molten steel must be removed.

[0014] The superheat of the continuously cast steel is 15℃~35℃; electromagnetic stirring in the crystallizer and electromagnetic stirring at the end of solidification are used to suppress the flow of impurity-rich molten steel toward the center at the end of solidification, thereby reducing center segregation.

[0015] Hot rolling process:

[0016] The billet exit temperature is controlled at 1260–1280℃; during piercing, the stability of pushing steel into the hole, piercing, clamping the roll and the throwing steel is strictly controlled; rough rolling is carried out in the austenite recrystallization zone, and in order to fully refine the original austenite grains, the rolling reduction is maximized, and the total deformation in the rough rolling stage is increased within the limits of equipment parameters; high-precision, high-quality tubes of different sizes are obtained by combining the PQF of the three-roll limited-movement mandrel continuous rolling mill with high-end hot rolling control technology;

[0017] Heat treatment process:

[0018] The heat treatment is quenching and tempering. Quenching is carried out at 870±5℃, water quenching, and holding time is 20-40 minutes. Tempering is carried out at 600±5℃, holding time is 50-70 minutes, and air cooling is performed.

[0019] Its chemical element mass percentages are as follows: C 0.30-0.37%; Si 0.20-0.35%; Mn 0.60-0.80%; P≤0.015%; S≤0.005%; Cr 0.95-1.20%; Mo 0.15-0.25%; Re added 0.002%; the remainder is Fe and unavoidable impurities, with a total mass fraction of 100%.

[0020] Furthermore, the heat treatment involves quenching and tempering. The quenching temperature is 870℃, water quenching, and holding time is 30 minutes. The tempering temperature is 600℃, and holding time is 60 minutes. The product is then air cooled.

[0021] Furthermore, its chemical element mass percentage content is as follows: C 0.34%; Si 0.27%; Mn 0.62%; P 0.008%; S 0.002%; Cr 1.00%; Mo 0.22%; Re added 0.002%; the remainder is Fe and unavoidable impurities, with a total mass fraction of 100%.

[0022] Furthermore, its chemical element mass percentage content is as follows: C 0.32%; Si 0.26%; Mn 0.61%; P 0.007%; S 0.002%; Cr 0.97%; Mo 0.21%; Re added amount 0.002%; the remainder is Fe and unavoidable impurities, with a total mass fraction of 100%.

[0023] Furthermore, its chemical element mass percentage content is as follows: C 0.32%; Si 0.29%; Mn 0.69%; P 0.007%; S 0.003%; Cr 0.96%; Mo 0.22%; Re added amount 0.002%; the remainder is Fe and unavoidable impurities, with a total mass fraction of 100%.

[0024] Furthermore, its chemical element mass percentage content is as follows: C 0.33%; Si 0.28%; Mn 0.66%; P 0.007%; S 0.003%; Cr 0.97%; Mo 0.21%; Re added amount 0.002%; the remainder is Fe and unavoidable impurities, with a total mass fraction of 100%.

[0025] Furthermore, its chemical element mass percentage content is as follows: C 0.32%; Si 0.30%; Mn 0.65%; P 0.008%; S 0.005%; Cr 0.96%; Mo 0.23%; Re added 0.002%; the remainder is Fe and unavoidable impurities, with a total mass fraction of 100%.

[0026] Carbon (C): Carbon is the most effective strengthening element in steel. Appropriately increasing the carbon content in steel improves the material's strength, but reduces toughness. Carbon strengthens solid solutions and, more importantly, forms different types of carbides with various alloying elements, improving the steel's thermal stability and wear resistance. It is also a crucial element affecting carbon equivalent, thus influencing the weldability of engineering machinery. This invention appropriately controls the carbon content, improving the steel's toughness while ensuring strength. For this invention, the C content is 0.30-0.37%.

[0027] Silicon (Si): In steel, it mainly exists in a solid solution state, primarily for solid solution strengthening. It does not form carbonitrides and can significantly increase the volume fraction of ferrite and strengthen the ferrite microstructure. For this invention, the Si content is 0.20–0.35%.

[0028] Manganese (Mn): An important alloying element that can remove oxygen and sulfur, increase tensile strength and toughness, and improve the strength and toughness of non-quenched and tempered steel. Increasing the Mn content will reduce the eutectoid carbon content of the steel, leading to an increase in the amount of pearlite and refining the pearlite clusters and lamellar spacing. It can also refine ferrite grains, thereby improving the strength and toughness of the steel. However, increasing the Mn content increases the volume fraction of pearlite, which reduces toughness, creating a contradiction between strength and toughness. For this invention, the Mn content ranges from 0.60% to 0.80%.

[0029] Chromium (Cr): Chromium forms a continuous solid solution with iron and various carbides with carbon. Its affinity for carbon is greater than that for iron and manganese but less than that for tungsten and molybdenum. Cr can improve the thermodynamic stability, strength, and hardness of steel. The effect is more significant when added with other alloying elements, significantly increasing the brittle transition temperature of steel, slowing down the decomposition rate of austenite, and significantly improving the hardenability of steel, but also increasing the tendency of steel to temper brittleness. Therefore, the Cr content in this invention is 0.95-1.20%.

[0030] Molybdenum (Mo): Mo can shrink the γ-region, strongly inhibit the precipitation and growth of proeutectoid ferrite, and promote the formation of acicular ferrite with high-density dislocation substructures. It plays an important role in controlling phase transformation microstructure, further refining the microstructure, and improving the strength of steel through phase transformation strengthening. In quenched and tempered steel, Mo can also improve hardenability, prevent or reduce temper brittleness, and improve tempering stability. Furthermore, because Mo is a large atomic radius element, adding a small amount can effectively increase the steel's resistance to localized corrosion and chloride ion corrosion, and also improve its resistance to hydrogen sulfide corrosion. Therefore, the Mo content in this invention is 0.15-0.25%.

[0031] Rare earth elements (Re): Adding appropriate amounts of rare earth elements to steel helps purify the molten steel, refine the grains, increase the resistance to grain growth, inhibit the growth of columnar crystals, and change the properties, morphology, and distribution of inclusions, thereby improving the various properties of the steel. Therefore, the amount of Re added in this invention is 0.002%.

[0032] Compared with the prior art, the beneficial technical effects of the present invention are as follows:

[0033] This invention provides a large-diameter chromium-molybdenum seamless steel pipe and its manufacturing method. The rational selection of alloying elements and appropriate element content ratios result in a lower cost and easier production of the steel. The optimized smelting, continuous casting, and rolling processes give the steel pipe high strength, excellent impact toughness, and high dimensional accuracy. This invention provides a primary raw material for gas cylinders and can be widely used in industrial production and other fields. It is a dedicated raw material for various high-pressure gas cylinders, accumulators, and other energy industries, characterized by ease of production, low cost, and excellent performance. Attached Figure Description

[0034] Figure 1 Metallographic structure of steel pipes for industrial production. Detailed Implementation

[0035] A method for manufacturing a large-diameter chromium-molybdenum seamless steel pipe with rare earth microalloying includes:

[0036] Production process: Blast furnace molten iron → Molten iron pretreatment → Converter smelting → LF ladle refining → VD vacuum treatment → Continuous casting → Slow cooling of billet → Warehousing → Continuous casting billet → Sawing → Ring furnace heating → Piercing → Continuous rolling → Tube removal → Sizing (tension reduction) → Cooling bed → Sawing → Straightening → Magnetic flux leakage testing → Ultrasonic testing → Inspection → Heat treatment → Length measurement, weighing, marking → Packaging and warehousing → Shipment.

[0037] The steelmaking process employs "LF+VD" to ensure the cleanliness of the cast billets. The final slag basicity of the converter smelting is ≥3.0. Strict control of the final carbon content is essential to prevent over-oxidation and ultra-low carbon tapping. The converter's final control targets are: C ≥0.06%, P ≤0.010%, and tapping temperature 1600–1620℃. Argon blowing is performed throughout the process according to LF refining requirements, with controlled argon blowing intensity to maintain steel creep and prevent exposed molten steel. Fine-tuning of the composition is performed in the later stages of refining, with rare earth alloys added proportionally. After vacuum treatment, the soft blowing time is ≥13 minutes, while paying attention to weak stirring to prevent exposed molten steel and ensure proper removal of non-metallic inclusions.

[0038] During continuous casting, full-process protective pouring is employed, and the casting speed is strictly controlled according to the corresponding specifications to ensure constant casting speed. The superheat of the continuously cast steel is controlled within the range of 15℃ to 35℃ to increase the equiaxed crystal ratio. Electromagnetic stirring in the crystallizer and at the end of the solidification process are used to suppress the flow of impurity-rich molten steel towards the center at the end of solidification, thereby reducing center segregation.

[0039] Hot rolling process:

[0040] During the heating process of the billet in the furnace, the atmosphere inside the furnace is strictly controlled, and heating is carried out according to the prescribed heating regime. The temperature and time of each section of the furnace are monitored during the heating process to reduce the propagation of thermal stress to defects in the core or outer surface of the billet, preventing cracks in subsequent production. Uniform heating of the billet in the furnace is ensured to prevent uneven heating, incomplete heating, overheating, or burning. Based on the alloy composition design and the actual billet shape specifications, the billet exit temperature is controlled at 1260–1280℃. During piercing, the smoothness of the pushing of steel, piercing, roll clamping, and steel throwing is strictly controlled. Rough rolling is performed in the austenite recrystallization zone. To fully refine the original austenite grains, the rolling reduction is maximized, and the total deformation in the rough rolling stage is increased within the limits of equipment parameters. High-precision, high-quality tubes of different sizes are obtained through a three-roll limited-movement mandrel continuous rolling mill (PQF) combined with high-end hot rolling control technology. These tubes not only possess high strength and good plasticity but also excellent toughness.

[0041] Heat treatment process:

[0042] The heat treatment process involves quenching and tempering. The quenching temperature is 870℃, water quenching, and holding time is 30 minutes. The tempering temperature is 600℃, holding time is 60 minutes, and air cooling is performed.

[0043] The chemical composition of the steel pipe obtained according to the present invention, taking the production specification Φ405×15mm as an example, is shown in Table 3:

[0044] Table 3. Statistical results of the chemical composition (wt.%) of the steel pipe of the present invention.

[0045]

[0046] As can be seen from Table 3, the chemical composition of the steel pipe fully meets the requirements of this invention for the steel grade. The composition content of each furnace has very small deviation. The stable composition content is conducive to temperature control during heat treatment, thus providing the prerequisite for the pipe to have good structure and performance.

[0047] Steelmaking process: Steel pipes with a specification of Φ405×15mm are produced using continuously cast round billets with a cross-section of Φ390mm. Ferrosilicon and ferromanganese are used for deoxidation and alloying, with final deoxidation employing an aluminized deoxidation process. During tapping, an appropriate amount of quicklime is added. Slag formation, desulfurization, composition adjustment, and temperature increases are performed according to the converter steel composition and temperature. Argon blowing is carried out normally throughout the process according to LF refining requirements. Before the end of refining, an appropriate amount of high-calcium wire is fed. Ferrocene alloy is added at the VD (Vacuum Deposition) stage. During continuous casting, full-process protective pouring is used, with strict control of the casting speed to ensure constant pouring speed. The tundish liquid level should be constantly monitored during continuous casting, with an initial pouring level of not less than 500mm. Full-ladle pouring is used during the process, employing electromagnetic stirring in the crystallizer and at the end of the process. The round billets are then slowly cooled in a slow cooling pit at a temperature greater than 600℃ to eliminate structural and thermal stress. Through these measures, high-quality round billets are obtained.

[0048] Round billets are heated in the annular furnace strictly according to the prescribed heating regime. During heating, the temperature and time of each section of the furnace are monitored to reduce the propagation of thermal stress to defects in the billet's core or outer surface, ensuring uniform heating of the billet within the furnace. The billet heating temperature deviation is within ±10℃, and excessive furnace time is prevented. During piercing, the mandrel extension is strictly controlled according to the rolling schedule requirements. The mandrel and roll clamps are in good condition, and there is no shaking of the mandrel during piercing, ensuring the stability of pushing the steel into the roll, piercing, clamping the tube, and throwing the steel. The temperature entering the continuous rolling mill is 1100℃. The initial temperature and cooling of the mandrel are controlled to ensure that the mandrel does not bubble or flow down in strands after being coated with graphite emulsion. After sizing, the temperature is 950℃. The rolling rhythm is controlled to ensure continuous stepping and rotation of the tube after it enters the cooling bed, avoiding bending caused by waiting. During straightening, the straightening force is strictly controlled to ensure the required bending degree of the steel tube and prevent straightening defects such as concavity, ellipticity, and cracking.

[0049] To achieve excellent surface quality, the production process of this invention employs a series of control measures. To prevent surface defects such as pitting, the oxidizing atmosphere within the ring furnace is strictly controlled during heating to prevent overheating. The temperature and time of each section of the heating furnace are monitored to reduce the propagation of thermal stress to defects on the core or outer surface of the billet. During piercing, the smoothness of the pushing, piercing, roller clamping, and throwing processes is strictly controlled, and severely worn rollers are replaced promptly. The high-pressure water descaling pressure after continuous rolling and sizing is required to be ≥8MPa. The measured out-of-roundness and wall thickness unevenness of the steel pipe do not exceed 80% of the outer diameter and wall thickness tolerances. The outer diameter (D) tolerance is ±1% D, and the wall thickness (S) unevenness is ≤±8% S. The steel pipe of this invention exhibits good surface quality and high dimensional accuracy, meeting the requirements for high-precision dimensions.

[0050] Table 4 shows the mechanical properties of the steel pipe. It can be seen that the steel pipe with specification Φ405×15mm has high strength and excellent impact toughness. The longitudinal impact at -60℃ is greater than 110J, and the mechanical properties of the product meet the design requirements of the steel pipe.

[0051] Table 4. Longitudinal mechanical properties of the steel pipe of the present invention

[0052]

[0053] The inclusions and microstructure of the steel pipe of the present invention are shown in Table 5.

[0054] Table 5. Inclusions and microstructure of the steel pipe of the present invention

[0055]

[0056] Note: F represents ferrite, and S represents tempered sorbite.

[0057] Laboratory analysis was conducted on samples of industrial steel pipes. After grinding and polishing, the samples were etched with 4% nitric acid alcohol, and the microstructure was observed under an optical microscope. Grain size was graded according to GB / T 6394 standard requirements. The results are shown in Table 5. The microstructure of the steel pipes is ferrite + tempered sorbite, as follows: Figure 1 Under a 500x microscope, the heat-treated metallographic structure contains a large amount of ferrite + tempered sorbite, which is evenly distributed and fine.

[0058] Through meticulous production organization and strict adherence to process requirements at each stage, the final product, measuring Φ405×15mm, exhibits high dimensional accuracy, excellent internal and external surface quality, high strength, and uniform and stable overall performance. Particularly noteworthy is its superior low-temperature impact toughness, meeting design requirements. Furthermore, the adoption of low-cost components will bring significant economic and social benefits to the enterprise. This invention's product is a crucial raw material for gas cylinders. Gas cylinder steel pipes are core materials for high-pressure gas cylinders and storage and transportation equipment. With the development of new energy, industrial gases, hydrogen energy, and special equipment, the overall outlook is positive in the long term, demand continues to expand, and the trend towards high-end products is evident. This invention is characterized by ease of production, low cost, and excellent performance.

[0059] The embodiments described above are merely preferred embodiments of the present invention and are not intended to limit the scope of the present invention. Various modifications and improvements made by those skilled in the art to the technical solutions of the present invention without departing from the spirit of the present invention should fall within the protection scope defined by the claims of the present invention.

Claims

1. A method for manufacturing a large-diameter chromium-molybdenum seamless steel pipe with rare earth microalloying, comprising the following production process: blast furnace hot metal → hot metal pretreatment → converter smelting → LF ladle refining → VD vacuum treatment → continuous casting → billet slow cooling → warehousing → continuous casting billet → sawing → annular furnace heating → piercing → continuous rolling → tube removal → sizing → cooling bed → sawing → straightening → magnetic flux leakage testing → ultrasonic testing → inspection → heat treatment → length measurement, weighing, and marking → packaging and warehousing → shipment; characterized in that: The steelmaking process employs "LF+VD" to ensure the cleanliness of the cast billet. The final slag basicity of the converter smelting is ≥3.0, and the final carbon content must be carefully controlled to prevent over-oxidation and ultra-low carbon tapping. The converter final control targets are: C≥0.06%, P≤0.010%, and tapping temperature 1600~1620℃. Argon blowing is carried out throughout the process according to the requirements of LF refining, and the argon blowing intensity is controlled to maintain the creep of the molten steel and prevent it from being exposed. In the later stage of refining, the composition is finely adjusted, and rare earth alloys are added in proportion. After vacuum treatment, the soft blowing time is ≥13min, and attention should be paid to the effect of weak stirring to prevent the molten steel from being exposed. Non-metallic inclusions in the molten steel must be removed. The superheat of the continuously cast steel is 15℃~35℃; electromagnetic stirring in the crystallizer and electromagnetic stirring at the end of solidification are used to suppress the flow of impurity-rich molten steel toward the center at the end of solidification, thereby reducing center segregation. Hot rolling process: The billet exit temperature is controlled at 1260–1280℃; during piercing, the stability of pushing steel into the hole, piercing, clamping the roll and the throwing steel is strictly controlled; rough rolling is carried out in the austenite recrystallization zone, and in order to fully refine the original austenite grains, the rolling reduction is maximized, and the total deformation in the rough rolling stage is increased within the limits of equipment parameters; high-precision, high-quality tubes of different sizes are obtained by combining the PQF of the three-roll limited-movement mandrel continuous rolling mill with high-end hot rolling control technology; Heat treatment process: The heat treatment is quenching and tempering. Quenching is carried out at 870±5℃, water quenching, and holding time is 20-40 minutes. Tempering is carried out at 600±5℃, holding time is 50-70 minutes, and air cooling is performed. Its chemical element mass percentages are as follows: C 0.30-0.37%; Si 0.20-0.35%; Mn 0.60-0.80%; P≤0.015%; S≤0.005%; Cr 0.95-1.20%; Mo 0.15-0.25%; Re added 0.002%; the remainder is Fe and unavoidable impurities, with a total mass fraction of 100%.

2. The method for manufacturing rare earth microalloyed large-diameter chromium-molybdenum seamless steel pipe according to claim 1, characterized in that: The heat treatment process involves quenching and tempering. The quenching temperature is 870℃, water quenching, and holding time is 30 minutes. The tempering temperature is 600℃, holding time is 60 minutes, and air cooling is performed.

3. The method for manufacturing rare earth microalloyed large-diameter chromium-molybdenum seamless steel pipe according to claim 1, characterized in that: Its chemical element mass percentages are as follows: C 0.34%; Si 0.27%; Mn 0.62%; P 0.008%; S 0.002%; Cr 1.00%; Mo 0.22%; Re added 0.002%; the remainder is Fe and unavoidable impurities, with a total mass fraction of 100%.

4. The method for manufacturing large-diameter chromium-molybdenum seamless steel pipes with rare earth microalloying according to claim 1, characterized in that: Its chemical element mass percentages are as follows: C 0.32%; Si 0.26%; Mn 0.61%; P 0.007%; S 0.002%; Cr 0.97%; Mo 0.21%; Re added 0.002%; the remainder is Fe and unavoidable impurities, with a total mass fraction of 100%.

5. The method for manufacturing rare earth microalloyed large-diameter chromium-molybdenum seamless steel pipe according to claim 1, characterized in that: Its chemical element mass percentages are as follows: C 0.32%; Si 0.29%; Mn 0.69%; P 0.007%; S 0.003%; Cr 0.96%; Mo 0.22%; Re added 0.002%; the remainder is Fe and unavoidable impurities, with a total mass fraction of 100%.

6. The method for manufacturing large-diameter chromium-molybdenum seamless steel pipes with rare earth microalloying according to claim 1, characterized in that: Its chemical element mass percentages are as follows: C 0.33%; Si 0.28%; Mn 0.66%; P 0.007%; S 0.003%; Cr 0.97%; Mo 0.21%; Re added 0.002%; the remainder is Fe and unavoidable impurities, with a total mass fraction of 100%.

7. The method for manufacturing large-diameter chromium-molybdenum seamless steel pipes with rare earth microalloying according to claim 1, characterized in that: Its chemical element mass percentages are as follows: C 0.32%; Si 0.30%; Mn 0.65%; P 0.008%; S 0.005%; Cr 0.96%; Mo 0.23%; Re added 0.002%; the remainder is Fe and unavoidable impurities, with a total mass fraction of 100%.

Images