A process for producing a t250 seamless steel pipe

Abstract

This invention discloses a production process for T250 seamless steel pipes. The steel composition for T250 seamless steel pipes, by mass percentage, is: C: ≤0.007%, Si: ≤0.12%, Mn: ≤0.13%, P: ≤0.01%, S: ≤0.007%, Ni: 17.8~20.2%, Cr: ≤0.60%, Co: ≤0.60%, Mo: 2.65~3.35%, Ti: 1.1~1.7%, Al: 0.03~0.17%, Cu: ≤0.60%, O: ≤0.0 The steel pipe has the following specifications: 0.25% N, ≤0.0025% N, hardness ≤34HRC, grain size ≥5, tensile strength ≥1760MPa, yield strength ≥1660MPa (Rp0.2), elongation ≥6.0%, reduction of area ≥45.0%, average full-size impact energy at room temperature longitudinal Charpy V-notch ≥14J, outer diameter φ114~φ325mm, and wall thickness 15mm~55mm. The manufacturing process includes billet preparation, forging, solution treatment of the forged bar, forging processing, extrusion molding, solution heat treatment, and aging heat treatment. This invention improves metal purity, refines bar grains, reduces component segregation, enhances material density, and stabilizes mechanical properties; eliminates shrinkage cavities and cracks, improving finished product quality; and enables seamless steel to maintain stable performance in high-temperature and corrosive environments (such as marine engineering), suitable for extreme working conditions, and applicable to the production of high-performance seamless steel pipes.

Description

Technical Field

[0001] This invention belongs to the field of seamless steel pipe forming technology, specifically relating to a production process for T250 seamless steel pipe. Background Technology

[0002] T250 steel uses carbon-free / micro-carbon martensite as its matrix and is strengthened through the dispersion precipitation of intermetallic compounds of elements such as titanium and molybdenum. It possesses ultra-high strength and toughness, a simple heat treatment process, and minimal deformation. It maintains stable performance even in high-temperature and corrosive environments (such as marine engineering), making it suitable for extreme working conditions. Furthermore, it is 20%-30% cheaper than traditional cobalt-containing martensitic aging steels (such as C250), making it a core material in cutting-edge fields such as aerospace and military equipment. Currently, key components have been domestically produced, and its application in emerging fields such as nuclear fusion devices and hypersonic vehicles will be further expanded.

[0003] In existing technologies, T250 steel is manufactured using an electric arc furnace + ladle refining + vacuum degassing process. This process inevitably introduces or generates a large number of inclusions (oxides, sulfides, silicates, etc.) due to the presence of oxide slag and reducing slag during the electric arc furnace smelting process. Complex ladle refining (such as LF, VD) is required to improve purity, but the effect is still limited. Secondly, the overall smelting environment is carried out under non-vacuum conditions, and the gases in the environment inevitably react chemically with the alloy, resulting in defects such as porosity and looseness in the produced bars. Furthermore, it is difficult to control impurity elements during the smelting process, resulting in reduced purity and greater component segregation, leading to poor bar density, reduced mechanical properties, and failure to meet the material requirements. Summary of the Invention

[0004] The purpose of this invention is to provide a production process for T250 seamless steel pipes, so as to reduce component segregation by improving metal purity, optimize mechanical properties, and improve product quality by reducing defects such as porosity and looseness.

[0005] To achieve the above objectives, the technical solution adopted by the present invention is as follows:

[0006] A production process for T250 seamless steel pipes, wherein the steel composition of the seamless steel pipes, by mass percentage, is: C: ≤0.007%, Si: ≤0.12%, Mn: ≤0.13%, P: ≤0.01%, S: ≤0.007%, Ni: 17.8~20.2%, Cr: ≤0.60%, Co: ≤0.60%, Mo: 2.65~3.35%, Ti: 1.1~1.7%, Al: 0.03~0.17%, Cu: ≤0.60%, O: ≤0.0025%, N: ≤0.0025%, with the remainder being Fe;

[0007] The properties of seamless steel pipes are as follows: hardness ≤34HRC, grain size ≥5, tensile strength ≥1760MPa, yield strength ≥1660MPa (Rp0.2), elongation ≥6.0%, reduction of area ≥45.0%, and average full-size impact energy of longitudinal Charpy V-notch at room temperature ≥14J.

[0008] The outer diameter of the seamless steel pipe is φ114~φ325mm, and the wall thickness is 15mm~55mm;

[0009] The production process includes the following steps:

[0010] S1. Billet preparation: Raw materials are formed into steel ingots through a double vacuum smelting process, VIM+VAR.

[0011] S2, Forging Bar: The steel ingot in S1 is heated and forged into a forging bar. The forging compression ratio is 2.0 to 5.0, the initial forging temperature is 1140℃ to 1180℃, the final forging temperature is 900℃ to 950℃, the compression amount per pass is 30mm to 50mm, and the feed rate is 50mm to 70mm.

[0012] S3, Solution treatment of forged bars: The forged bars obtained in S2 are subjected to solution heat treatment to reduce hardness;

[0013] S4. Forging bar processing: Machining 6mm to 10mm on one side of the outer diameter of the forged bar after S3 solution treatment, machining the center hole with a diameter of 45mm to 80mm, and removing the oxide scale on the outer surface.

[0014] S5. Extrusion molding: The forged bar obtained in S4 is heated to 1100℃~1150℃, descaled, lubricated with glass powder, and extruded into steel pipe using a horizontal extrusion press.

[0015] S6. Solution heat treatment: Heat the steel pipe obtained from S5 to 830±10℃, hold for 60min~120min, remove from the furnace and air cool, and then straighten under pressure.

[0016] S7. Aging heat treatment: The steel pipe obtained in S6 is heated to 500±10℃, held for 240min~360min, and then air-cooled after being taken out of the furnace to obtain the production process of T250 seamless steel pipe.

[0017] As a limitation of the present invention, 0.05% Ca, 0.02% Zr, and 0.003% B are added to the molten steel before casting.

[0018] As a further limitation of the present invention, in S2, when the steel ingot is heated to a furnace temperature of 500℃~550℃, it is held for 4 hours; then the furnace temperature is raised to 850℃~900℃ at a heating rate of 80℃~100℃ / h, and held for 3 hours; then the furnace temperature is raised to 1140℃~1180℃ at a heating rate of 100℃~120℃ / h, and held for 4.5 hours.

[0019] As a further limitation of the present invention, in S3, the solution heat treatment temperature of the forged bar is 810℃~850℃, and the holding time is 6h~12h, after which it is taken out of the furnace and air-cooled to room temperature.

[0020] As a further limitation of the present invention, in S5, during the extrusion molding stage, the cold forged bar is heated twice in an electromagnetic induction furnace; the first heating power is 350KW, which heats the outer surface temperature of the forged bar to 1000℃~1050℃; the second heating power is 700KW, which heats the outer surface temperature of the forged bar to 1100℃~1150℃.

[0021] As another limitation of the present invention, in S5, the process of extruding steel pipe includes: expanding the hole with a hole expander at a speed of 150mm to 250mm / s and an expansion ratio of 1.1 to 1.3; and extruding with a hole extruder at a speed of 150mm to 250mm / s and an extrusion ratio of 2 to 5.

[0022] By adopting the above technical solution, the technical progress achieved by this invention compared with the prior art is as follows:

[0023] (1) The present invention adopts a process combining double vacuum smelting of steel ingots, steel ingot forging, hot extrusion molding and solution heat treatment to produce T250 seamless steel pipes with high density, fine grain size, excellent structural performance and excellent mechanical properties.

[0024] (2) The double vacuum smelting process of the present invention can reduce the reaction between molten steel and air, reduce the influence of impurity elements in the air on the composition of seamless steel pipe, make its composition purer, reduce segregation of finished product, refine the grain size of steel ingot, and make T250 steel pipe exhibit excellent mechanical properties such as high hardness, high tensile strength, high yield strength, and large grain size.

[0025] (3) The steel ingot forging process of the present invention can make the forged bar material more compact and the compaction effect better, effectively eliminating defects such as porosity and looseness, so as to give it excellent structural performance; through extrusion molding, the temperature can be precisely controlled, so that the grain size and microstructure of the tube body are consistent, improving the quality of the finished product and providing a good foundation for subsequent heat treatment. The heat-treated tube has high strength and good toughness, reduces deformation and increases yield.

[0026] (4) The solution treatment + aging heat treatment process is simple, the heat treatment effect is good, the deformation is small, which makes it have high strength and good toughness, good welding performance, no need for preheating or post-treatment, and low production cost.

[0027] (5) Before casting, the present invention adds Ca, Zr and B to the molten steel. Adding these three easily oxidized active elements under vacuum greatly reduces the degree of alloy burn-off and results in a very high and stable yield. At the same time, Ca is a very strong deoxidizer and desulfurizer. When added under vacuum, it can generate CaO and CaS, which can completely remove dissolved oxygen and sulfur from the melt. Zr has a very strong affinity for oxygen. After being added, it forms fine ZrO2 particles, which further reduces the residual oxygen in the melt and reduces the risk of grain boundary oxidation embrittlement. After adding B, it can strengthen the grain boundaries, block crack propagation, improve the quality of finished products, and reduce the scrap rate.

[0028] In summary, this invention can improve metal purity, refine bar stock grains, reduce component segregation, enhance material density, stabilize mechanical properties, eliminate shrinkage cavities and cracks, improve finished product quality, and enable seamless steel to maintain stable performance in high-temperature and corrosive environments (such as marine engineering), making it suitable for extreme working conditions and applicable to the production of high-performance seamless steel pipes. Attached Figure Description

[0029] Figure 1 This is a flowchart of the production process of the present invention. Detailed Implementation

[0030] The present invention will now be described in detail with reference to the embodiments and accompanying drawings. The scope of protection of the present invention is not limited to the embodiments, and any modifications made by those skilled in the art within the scope defined by the claims also fall within the scope of protection of the present invention.

[0031] Example 1

[0032] This embodiment describes a T250 seamless steel pipe with an outer diameter of φ310mm and a wall thickness of 53mm. Its chemical composition is shown in Table 1.

[0033]

[0034] Note: The remainder consists of Fe and unavoidable impurities. When adding Ca, Zr, and B, the content of these three elements needs to be analyzed and the measured values ​​reported.

[0035] like Figure 1 As shown, the production process of the φ310mm×53mm T250 seamless steel pipe in this embodiment includes the following steps:

[0036] S1. Billet preparation: Using scrap steel as raw material, steel ingots with a diameter of φ600mm are formed by double vacuum smelting process VIM+VAR.

[0037] S2. Forging Bars: The steel ingots from S1 are heated and forged into forged bars. The heating process is as follows: the steel ingot is heated to a furnace temperature of 500℃~550℃ and held for 4 hours; then the furnace temperature is increased to 850℃~900℃ at a heating rate of 80℃~100℃ / h and held for 3 hours; then the furnace temperature is increased to 1140℃~1180℃ at a heating rate of 100℃~120℃ / h, so that the initial forging temperature of the steel ingot is 1180℃ and the final forging temperature is 900℃. During forging, the compression per pass is 30mm~50mm, the feed rate is 50mm~70mm, and the forging compression ratio is 4.1, so that a φ600mm steel ingot is heated and forged into a φ425mm forged bar.

[0038] S3. Solution treatment of forged bars: The forged bars obtained in S2 are solution treated in a chamber heating furnace at a temperature of 810℃~850℃ for 6 hours. After that, the bars are taken out of the furnace and air-cooled to room temperature to reduce the hardness of the forged bars.

[0039] S4. Forging bar processing: After S3 solution treatment, the outer diameter of the forging bar is machined by 6mm on one side, the center hole is machined to a diameter of 65mm, the oxide scale on the outer surface is removed, and a forging bar with an outer diameter of φ415mm / inner diameter of φ65mm and a length of 960mm is produced.

[0040] S5. Extrusion Molding: The cold-formed forged bar obtained in S4 is heated twice in an electromagnetic induction furnace. The first heating power is 350KW, raising the outer surface temperature of the forged bar to 1000℃~1050℃. The second heating power is 700KW, raising the outer surface temperature of the forged bar to 1100℃~1150℃. After heating, it is descaled by 23MPa high-pressure water, followed by lubrication with glass powder. GN26 is used for internal lubrication, 844-7 for external lubrication, and HDK-27 glass gasket powder is used for external lubrication. The steel pipe is extruded using a 63MN horizontal extrusion press. The extrusion process includes: reaming with a reamer at a speed of 150mm~250mm / s and an reaming ratio of 1.1~1.3; and extrusion with an extrusion press at a speed of 150mm~250mm / s and an extrusion ratio of 2~5.

[0041] S6. Solution heat treatment: The steel pipe obtained from S5 is heated to 840℃ in a roller hearth atmosphere protection furnace, held for 60 minutes, removed from the furnace and air cooled, and then pressure straightened.

[0042] S7. Aging heat treatment: The steel pipe obtained in S6 is heated to 510℃, held for 240 minutes, and then air-cooled to obtain a T250 seamless steel pipe with a diameter of φ310mm×53mm.

[0043] The properties of the φ310mm×53mm T250 seamless steel pipe produced using the above steps are as follows: hardness ≤34HRC, grain size ≥5, tensile strength ≥1760MPa, yield strength Rp0.2 ≥1660MPa, elongation ≥6.0%, reduction of area ≥45.0%, and average full-size impact energy of longitudinal Charpy V-notch at room temperature ≥14J.

[0044] Two φ310mm×53mm T250 seamless steel pipes (1#-1 and 1#-2) produced in the above steps were randomly selected for performance testing. The test results are as follows:

[0045] Table 2 Performance test results of T250 seamless steel pipe with φ310mm×53mm

[0046]

[0047] As shown in Table 2, the T250 seamless steel pipe has a tensile strength greater than 1760 MPa, a yield strength of Rp0.2 greater than 1660 MPa, an elongation greater than 6%, a reduction of area greater than 45%, a full-size impact energy of Charpy V-notch at room temperature greater than 14 J, a hardness less than 34 HRC, and a grain size greater than grade 5, all of which meet the standards.

[0048] Example 2

[0049] This embodiment describes a T250 seamless steel pipe with an outer diameter of φ317mm and a wall thickness of 34mm. Its chemical composition is shown in Table 3.

[0050] Table 3 Chemical composition (mass percentage) of T250 seamless steel pipe with φ317mm×34mm

[0051]

[0052] Note: The remainder consists of Fe and unavoidable impurities. When adding Ca, Zr, and B, the content of these three elements needs to be analyzed and the measured values ​​reported.

[0053] The production process of the φ317mm×34mm T250 seamless steel pipe in this embodiment includes the following steps:

[0054] S1. Billet preparation: Using scrap steel as raw material, steel ingots with a diameter of φ680mm are formed by double vacuum smelting process VIM+VAR.

[0055] S2. Forging Bar: The steel ingot from S1 is heated and forged into a forging bar. The heating process is as follows: the steel ingot is heated to a furnace temperature of 500℃~550℃ and held for 4 hours; then the furnace temperature is raised to 850℃~900℃ at a heating rate of 80℃~100℃ / h and held for 3 hours; then the furnace temperature is raised to 1140℃~1180℃ at a heating rate of 100℃~120℃ / h, so that the initial forging temperature of the steel ingot is 1140℃ and the final forging temperature is 950℃. The compression per pass is 30mm~50mm, the feed rate is 50mm~70mm, and the forging compression ratio is 4.5. A φ680mm steel ingot is heated and forged into a φ425mm forging bar.

[0056] S3. Solution treatment of forged bars: The forged bars obtained in S2 are solution treated in a chamber heating furnace at a temperature of 810℃~850℃ for 12 hours. After that, the bars are removed from the furnace and air-cooled to room temperature to reduce the hardness of the forged bars.

[0057] S4. Forging bar processing: After S3 solution treatment, the outer diameter of the forging bar is machined by 10mm on one side, the center hole is machined to a diameter of 65mm, the oxide scale on the outer surface and the expansion guide hole are removed, and a forging bar with an outer diameter of φ415mm / inner diameter of φ65mm and a length of 700mm is produced.

[0058] S5. Extrusion Molding: The cold-formed forged bar obtained in S4 is heated twice in an electromagnetic induction furnace. The first heating power is 350KW, raising the outer surface temperature of the forged bar to 1000℃~1050℃. The second heating power is 700KW, raising the outer surface temperature of the forged bar to 1100℃~1150℃, completing the heating process. Then, it undergoes descaling with 23MPa high-pressure water, followed by lubrication with glass powder. GN26 is used for internal lubrication, 844-7 for external lubrication, and HDK-27 glass gasket powder is used for external lubrication. The steel pipe is extruded using a 63MN horizontal extrusion press. The extrusion process includes: reaming with a reamer at a speed of 150mm~250mm / s and an reaming ratio of 1.1~1.3; and extrusion with an extrusion press at a speed of 150mm~250mm / s and an extrusion ratio of 2~5.

[0059] S6. Solution heat treatment: The steel pipe obtained from S5 is heated to 830℃ in a roller hearth atmosphere protection furnace, held for 120 minutes, removed from the furnace and air cooled, and then pressure straightened.

[0060] S7. Aging heat treatment: The steel pipe obtained in S6 is heated to 490℃, held for 360 minutes, and then air-cooled to obtain a T250 seamless steel pipe with a diameter of φ317mm×34mm.

[0061] The properties of the φ317mm×34mm T250 seamless steel pipe produced using the above steps are as follows: hardness ≤34HRC, grain size ≥5, tensile strength ≥1760MPa, yield strength Rp0.2 ≥1660MPa, elongation ≥6.0%, reduction of area ≥45.0%, and average full-size impact energy of longitudinal Charpy V-notch at room temperature ≥14J.

[0062] Two φ317mm×34mm T250 seamless steel pipes (2#-1 and 2#-2) produced in the above steps were randomly selected for performance testing. The test results are as follows:

[0063] Table 4 Performance test results of T250 seamless steel pipe with φ317mm×34mm

[0064]

[0065] As shown in Table 4, the T250 seamless steel pipe has a tensile strength greater than 1760 MPa, a yield strength of Rp0.2 greater than 1660 MPa, an elongation greater than 6%, a reduction of area greater than 45%, a full-size impact energy of Charpy V-notch at room temperature greater than 14 J, a hardness less than 34 HRC, and a grain size greater than grade 5, all of which meet the standards.

[0066] Example 3

[0067] This embodiment describes a T250 seamless steel pipe with an outer diameter of φ219mm and a wall thickness of 17mm. Its chemical composition is shown in Table 1.

[0068] Table 5 Chemical composition (mass percentage) of T250 seamless steel pipe with φ219mm×17mm

[0069]

[0070] Note: The remainder consists of Fe and unavoidable impurities. When adding Ca, Zr, and B, the content of these three elements needs to be analyzed and the measured values ​​reported.

[0071] The production process of the φ219mm×17mm T250 seamless steel pipe in this embodiment includes the following steps:

[0072] S1. Billet preparation: Using scrap steel as raw material, steel ingots with a diameter of φ550mm are formed by double vacuum smelting process VIM+VAR.

[0073] S2. Forging Bar: The steel ingot from S1 is heated and forged into a forged bar. The heating process is as follows: the steel ingot is heated to a furnace temperature of 500℃~550℃ and held for 4 hours; then the furnace temperature is increased to 850℃~900℃ at a heating rate of 80℃~100℃ / h and held for 3 hours; then the furnace temperature is increased to 1140℃~1180℃ at a heating rate of 100℃~120℃ / h, so that the initial forging temperature of the steel ingot is 1150℃ and the final forging temperature is 930℃. During forging, the compression amount per pass is 30mm~50mm, the feed rate is 50mm~70mm, and the forging compression ratio is 4.3. A φ550mm steel ingot is heated and forged into a φ368mm forged bar.

[0074] S3. Solution treatment of forged bars: The forged bars obtained in S2 are solution treated in a chamber heating furnace at a temperature of 810℃~850℃ for 10 hours. After that, the bars are removed from the furnace and air-cooled to room temperature to reduce the hardness of the forged bars.

[0075] S4. Forging bar machining: Machining 6mm on one side of the outer diameter of the forged bar after S3 solution treatment, machining a diameter of 65mm in the center hole, removing the oxide scale on the outer surface, and machining a forged bar with an outer diameter of φ358mm / inner diameter of φ65mm × length of 730mm.

[0076] S5. Extrusion Molding: The cold-formed forged bar obtained in S4 is heated twice in an electromagnetic induction furnace. The first heating power is 350KW, raising the outer surface temperature of the forged bar to 1000℃~1050℃. The second heating power is 700KW, raising the outer surface temperature of the forged bar to 1100℃~1150℃, completing the heating process. Then, it undergoes descaling with 23MPa high-pressure water, followed by lubrication with glass powder. GN26 is used for internal lubrication, 844-7 for external lubrication, and HDK-27 glass gasket powder is used for external lubrication. The steel pipe is extruded using a 63MN horizontal extrusion press. The extrusion process includes: reaming with a reamer at a speed of 150mm~250mm / s and an reaming ratio of 1.1~1.3; and extrusion with an extrusion press at a speed of 150mm~250mm / s and an extrusion ratio of 2~5.

[0077] S6. Solution heat treatment: The steel pipe obtained from S5 is heated to 820℃ in a roller hearth atmosphere protection furnace, held for 100 minutes, removed from the furnace and air cooled, and then pressure straightened.

[0078] S7. Aging heat treatment: Heat the steel pipe obtained in S6 to 500℃, hold for 300min, remove from the furnace and air cool to obtain a T250 seamless steel pipe with a diameter of φ219mm×17mm.

[0079] The properties of the φ310mm×53mm T250 seamless steel pipe produced using the above steps are as follows: hardness ≤34HRC, grain size ≥5, tensile strength ≥1760MPa, yield strength Rp0.2 ≥1660MPa, elongation ≥6.0%, reduction of area ≥45.0%, and average full-size impact energy of longitudinal Charpy V-notch at room temperature ≥14J.

[0080] Two φ219mm×17mm T250 seamless steel pipes (3#-1 and 3#-2) produced in the above steps were randomly selected for performance testing. The test results are as follows:

[0081] Table 6 Performance test results of T250 seamless steel pipe with φ219mm×17mm

[0082]

[0083] As shown in Table 6, the T250 seamless steel pipe has a tensile strength greater than 1760 MPa, a yield strength of Rp0.2 greater than 1660 MPa, an elongation greater than 6%, a reduction of area greater than 45%, a full-size impact energy of Charpy V-notch at room temperature greater than 14 J, a hardness less than 34 HRC, and a grain size greater than grade 5, all of which meet the standards.

[0084] It should be noted that the above description is merely a preferred embodiment of the present invention and is not intended to limit the present invention. Although the present invention has been described in detail with reference to the above embodiments, those skilled in the art can still modify the technical solutions described in the above embodiments or make equivalent substitutions for some of the technical features. Any modifications, equivalent substitutions, improvements, etc., made within the spirit and principles of the present invention should be included within the protection scope of the present invention.

Claims

1. A production process for T250 seamless steel pipe, characterized in that, The composition of the steel used for seamless steel pipes, by mass percentage, is as follows: C: ≤0.007%, Si: ≤0.12%, Mn: ≤0.13%, P: ≤0.01%, S: ≤0.007%, Ni: 17.8~20.2%, Cr: ≤0.60%, Co: ≤0.60%, Mo: 2.65~3.35%, Ti: 1.1~1.7%, Al: 0.03~0.17%, Cu: ≤0.60%, O: ≤0.0025%, N: ≤0.0025%, with the remainder being Fe; The properties of seamless steel pipes are as follows: hardness ≤34HRC, grain size ≥5, tensile strength ≥1760MPa, yield strength ≥1660MPa (Rp0.2), elongation ≥6.0%, reduction of area ≥45.0%, and average full-size impact energy of longitudinal Charpy V-notch at room temperature ≥14J. The outer diameter of the seamless steel pipe is φ114~φ325mm, and the wall thickness is 15mm~55mm; The production process includes the following steps: S1. Billet preparation: Raw materials are formed into steel ingots through a double vacuum smelting process, VIM+VAR. S2, Forging Bar: The steel ingot in S1 is heated and forged into a forging bar. The forging compression ratio is 2.0 to 5.0, the initial forging temperature is 1140℃ to 1180℃, the final forging temperature is 900℃ to 950℃, the compression amount per pass is 30mm to 50mm, and the feed rate is 50mm to 70mm. S3, Solution treatment of forged bars: The forged bars obtained in S2 are subjected to solution heat treatment to reduce hardness; S4. Forging bar processing: Machining 6mm to 10mm on one side of the outer diameter of the forged bar after S3 solution treatment, machining the center hole with a diameter of 45mm to 80mm, and removing the oxide scale on the outer surface. S5. Extrusion molding: The forged bar obtained in S4 is heated to 1100℃~1150℃, descaled, lubricated with glass powder, and extruded into steel pipe using a horizontal extrusion press. S6. Solution heat treatment: Heat the steel pipe obtained from S5 to 830±10℃, hold for 60min~120min, remove from the furnace and air cool, and then straighten under pressure. S7. Aging heat treatment: The steel pipe obtained in S6 is heated to 500±10℃, held for 240min~360min, and then air-cooled after being taken out of the furnace to obtain the production process of T250 seamless steel pipe.

2. The production process of T250 seamless steel pipe according to claim 1, characterized in that, Before casting, 0.05% Ca, 0.02% Zr, and 0.003% B are added to the molten steel.

3. The production process of T250 seamless steel pipe according to claim 2, characterized in that, In S2, the steel ingot is heated to a furnace temperature of 500℃~550℃ and held for 4 hours; then the furnace temperature is raised to 850℃~900℃ at a heating rate of 80℃~100℃ / h and held for 3 hours; then the furnace temperature is raised to 1140℃~1180℃ at a heating rate of 100℃~120℃ / h and held for 4.5 hours.

4. The production process of T250 seamless steel pipe according to claim 3, characterized in that, In S3, the solution heat treatment temperature of the forged bar is 810℃~850℃, and it is held at that temperature for 6h~12h, and then it is taken out of the furnace and air-cooled to room temperature.

5. The production process of T250 seamless steel pipe according to claim 4, characterized in that, In S5, during the extrusion molding stage, the cold-formed forged bar is heated twice in an electromagnetic induction furnace. The first heating power is 350kW, which heats the outer surface temperature of the forged bar to 1000℃~1050℃. The second heating power is 700kW, which heats the outer surface temperature of the forged bar to 1100℃~1150℃.

6. The production process of a T250 seamless steel pipe according to any one of claims 1-5, characterized in that, In S5, the process of extruding steel pipes includes: expanding the hole with a reamer at a speed of 150mm to 250mm / s and an expansion ratio of 1.1 to 1.3; and extruding with a blasting machine at a speed of 150mm to 250mm / s and an extrusion ratio of 2 to 5.

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