Ultrahigh-strength and high-toughness seamless steel pipe for drill rod and manufacturing method thereof

Abstract

The invention provides an ultrahigh-strength and high-toughness seamless steel pipe for a drill rod and a manufacturing method thereof, and relates to the technical field of ferrous metallurgy, and the manufacturing method comprises the following manufacturing processes: smelting and producing a continuous casting blank, rolling the continuous casting blank into a round pipe blank, rolling a steel pipe, and carrying out heat treatment on a sample; the continuous casting billet is a medium-specification continuous casting square billet, the billet type is 300 mm * 400 mm, and the rolling ratio of the sectional area of the continuous casting billet to the sectional area of the steel pipe obtained after the steel pipe rolling step exceeds 20: 1. The seamless steel pipe manufactured through the manufacturing method solves the problems that an existing petroleum drill pipe is low in strength, insufficient in obdurability, difficult in mass production and the like.

Description

Technical Field

[0001] This invention relates to the field of iron and steel metallurgy technology, and in particular to a seamless steel pipe for ultra-high strength and high toughness drill pipe and its manufacturing method. Background Technology

[0002] The drilling system used in oil and gas extraction mainly consists of drill pipe, weighted drill pipe, drill collars, screw drills, and drill bits. The drill pipe, as an intermediate connecting component, bears the responsibility of transmitting drilling power and providing system support. With the advancement of oil and gas exploration technology, oil and gas extraction has evolved from shallow surface wells with vertical depths of several hundred meters to deep wells, ultra-deep wells, and deep-sea drilling at depths of several thousand meters. As drilling depth increases, the complexity of actual downhole conditions is usually higher than initially estimated, often involving complex geological conditions, high temperatures and pressures, and corrosive environments. Therefore, when conducting deep, ultra-deep, and deep-sea drilling operations, it is necessary to ensure the safe and reliable use of the drill pipe. The design should retain sufficient strength and toughness margins to withstand the overload impact of abrupt changes in complex geological formations on the drill string. The highest strength drill pipe grade in the API 5DP standard is V150 steel, with a PSL3 level impact resistance of only ≥54J at -20℃; GB29166 The specification stipulates a higher steel grade for drill pipe, U165, with a minimum yield strength of 1138MPa, which should meet the requirements of high-strength applications. However, it also only requires an impact energy of greater than or equal to 54J at room temperature, which is insufficient to meet the strength and toughness requirements of drill pipes in deep and ultra-deep wells.

[0003] Patent CN102268609A primarily employs a process of double quenching and single tempering to produce 165 steel grade drill pipes. This patent uses lower C and Mn content in the steel, omits Nb, and significantly increases V content. This composition system enhances product strength through solid solution strengthening with alloying elements and increased precipitation strengthening effect of V. However, due to the absence of Nb, adding a high content of V (0.1%-0.30%) for microalloying may easily lead to grain boundary segregation of V and other harmful elements, resulting in unstable impact performance. Data shows that the highest impact strength at -20℃ is only 104J, with a small impact margin and inability to consistently reach over 100J. Furthermore, the drill pipe's double high-temperature heating followed by quenching heat treatment process results in low production efficiency.

[0004] Patent CN103938095B proposes a high-strength, high-toughness drill pipe of 165ksi steel grade and its manufacturing method. The main effective components by mass percentage are C: 0.20%-0.40%, Mn: 0.50%-1.40%, Cr: 0.50%-1.40%, Mo: 0.60%-2.10%, Ti: 0.005%-0.025%, Nb: 0.03%-0.10%, V: 0.02%-0.15%, Si: 0.20%-0.40%, P≤0.015%, S≤0.008%, [Nb]+[V]+[Ti]≤0.25%, 1%≤[Cr]+[Mo]≤3%. The yield strength of the drill pipe can reach 1138MPa (165ksi), and the impact energy at -20℃ is ≥80J. However, the provided example data cannot all reach 100J or higher. Meanwhile, the patent has too wide a range of control for C, Mn, Cr, and Mo components, and the provided examples involve multiple C content components. In reality, for drill pipes that have undergone upsetting at both ends, the C content fluctuates too much, and the corresponding heat treatment process needs to be significantly adjusted, which is not conducive to the stability of heat treatment process control.

[0005] Patent CN108277440B proposes a steel pipe for drill pipes with a yield strength greater than 1138 MPa and its manufacturing method. The main effective components are C: 0.22%-0.28%, Si: 0.15%-0.20%, Mn: 0.4%-0.7%, Cr: 1.3%-1.6%, Mo: 0.7%-1.0%, Ni: 1.5%-1.9%, Nb: 0.07%-0.15%, S≤0.004%, and P≤0.01%. The yield strength of the heat-treated steel pipe is 1208 MPa-1255 MPa (meeting 165 KSI), and the impact absorption energy at -20℃ is 1101-17 J, all of which can reach over 100 J. However, this patent mainly improves impact toughness by adding a high amount of Ni. Although it can achieve the goal of significantly improving impact toughness, there is a greater risk of cracking in high-Ni steel during quenching, which is not conducive to on-site heat treatment control. In addition, the heat treatment process suggested uses water cooling for tempering. In reality, the drill pipe is water-cooled after being taken out of the furnace at the 580℃-620℃ provided by the patent. If the steel pipe is not rotated quickly during the water cooling process to ensure uniform cooling around the circumference, it is very easy to cause the pipe to bend, which increases the workload of subsequent straightening and stress relief, resulting in low product production efficiency. At the same time, major domestic drill pipe manufacturers do not have a rapid water cooling device after the entire drill pipe is tempered and taken out of the furnace.

[0006] Therefore, this invention provides a seamless steel pipe for ultra-high strength and high toughness drill pipe of U165 grade and its manufacturing method, which solves the problems of low strength, insufficient toughness and difficulty in mass production of existing oil drill pipes, and is of great significance. Summary of the Invention

[0007] The purpose of this invention is to provide a seamless steel pipe for ultra-high strength and high toughness drill pipe and its manufacturing method. The seamless steel pipe manufactured by this method solves the problems of low strength, insufficient toughness and difficulty in mass production of existing oil drill pipes.

[0008] To achieve the above objectives, the present invention adopts the following technical solution: The first aspect of the present invention provides a method for manufacturing a seamless steel pipe for ultra-high strength and high toughness drill pipe, comprising the following manufacturing processes: smelting to produce a continuous casting billet, rolling the continuous casting billet into a round tube billet, rolling the steel pipe, and heat treating the sample; The continuously cast billet is a medium-sized continuously cast square billet with a shape of 300mm×400mm. The cross-sectional area ratio of the continuously cast billet to the steel pipe obtained after the steel pipe rolling step exceeds 20:1.

[0009] Furthermore, the seamless steel pipe comprises the following components by mass percentage: C: 0.20%-0.35%, Mn: 0.30%-0.45%, Si≤0.40%, P≤0.013%, S≤0.005%, Cr: 0.8%-1.30%, Mo: 0.80%-1.35%, Ni: 0.30%-0.80%, V: 0.10%-0.35%, Nb: 0.01%-0.05%, with the remainder being iron and unavoidable impurities.

[0010] Furthermore, the seamless steel pipe has an outer diameter of ∮88.9mm-∮254mm, a wall thickness of 6.35mm-20mm, and a length of 7m-13.5m; And / or, the manufacturing method further includes the steps of surface flaw detection, physical and chemical inspection, sawing, ultrasonic flaw detection, finishing, and packaging and warehousing of the billet; the surface flaw detection of the billet is performed between the rolling of the continuously cast billet into a round billet and the rolling of the steel pipe; the physical and chemical inspection, sawing, ultrasonic flaw detection, finishing, and packaging and warehousing of the sample are performed after the sample is heat treated.

[0011] Furthermore, the specific steps for smelting and producing continuously cast billets are as follows: blast furnace ironmaking, hot metal desulfurization treatment, converter steelmaking, LF furnace refining, RH vacuum degassing, and continuous casting.

[0012] Furthermore, in the hot metal desulfurization treatment step, the sulfur content of the hot metal entering the furnace is ≤0.005 wt%, and the sulfur content after desulfurization is controlled to be ≤0.002 wt%. And / or, the refining time in the LF furnace is ≥45 min; And / or, the RH vacuum degassing time is ≥20 min.

[0013] Further, the specific steps for rolling the continuously cast billet into a round tube billet are as follows: heating with a walking beam furnace, rolling with a large reduction mill, and reducing the diameter with a sizing mill; the heating temperature of the walking beam furnace is controlled at 1210℃-1280℃, and the soaking time is 90min-150min; the rolling ratio of the continuously cast billet to the round tube billet is ≥3:1; preferably, the rolling ratio of the continuously cast billet to the intermediate square billet obtained after rolling with a large reduction mill is 1.85:1-2.0:1; the rolling ratio of the intermediate square billet obtained after rolling with a large reduction mill to the round tube billet obtained after reducing the diameter is 1.8:1-2.1:1.

[0014] Furthermore, the specific steps of the steel pipe rolling process are: billet heating, skew rolling piercing, CPE jacking, and diameter reduction.

[0015] Furthermore, the tube blank is heated using a ring furnace, with the temperature of the soaking section controlled at 1200℃-1280℃; the soaking time is 100min-150min. And / or, the CPE jacking pipe is jacked by a CPE jacking machine on the perforated tube to obtain a rough pipe. The CPE jacking pipe adopts a high-speed jacking mode, with a rolling time of ≤20 seconds, an outer diameter dimensional accuracy of -0.5%D to +1%D, and a wall thickness dimensional accuracy of -5%S to +15%S. And / or, the total production time for the skew rolling piercing and CPE jacking pipe is 2 min-4 min.

[0016] Furthermore, the heat treatment of the sample includes quenching and tempering in sequence; the quenching temperature is 840℃-900℃, the holding time is 50min-90min, and water quenching is performed; the tempering temperature is 600℃-640℃, the holding time is 50min-90min, and air cooling is performed to room temperature.

[0017] In a second aspect, the present invention provides a seamless steel pipe manufactured by the manufacturing method of the first aspect, wherein the seamless steel pipe has a yield strength ≥1138 MPa, a tensile strength ≥1200 MPa, an elongation ≥15%, a minimum single longitudinal impact energy Kv2 at -20℃ ≥100 J, and a grain size of grade 8-11.

[0018] The seamless steel pipe for ultra-high strength and high toughness drill pipe and its manufacturing method provided by this invention have the following advantages: (1) A medium-sized continuous casting square billet is used. The continuous casting square billet is rolled into an intermediate square billet by a rolling mill with a roll diameter of 1350mm. Then, it is rolled into a round tube billet by a sizing mill. This large rolling deformation effectively reduces material segregation, thereby ensuring the uniformity of subsequent heat treatment.

[0019] (2) Medium-sized continuous casting square billets are used, and the compression of the cross-sectional length and width relative to the outer diameter of the steel pipe is controlled at 2-3:1. Compared with conventional continuous casting round billets for pipe forming, there is a larger deformation in the diameter direction. At the same time, the use of medium-sized continuous casting square billets avoids the segregation effect of traditional large square billets. Finally, the cross-sectional area rolling ratio of the continuous casting square billet to the steel pipe exceeds 20:1, while the conventional continuous casting round billet-hot rolled seamless steel pipe forming rolling ratio is usually within 10:1. This process effectively reduces the segregation of seamless steel pipes and improves material uniformity.

[0020] (3) The new composition design takes into account the hardenability requirements of seamless steel pipe after being upsetting at both ends, and at the same time needs to meet the strength and toughness of different wall thickness sections of the whole pipe. By minimizing the content of easily segregated element Mn and appropriately controlling the C composition range, the hardenability of the upsetting and thickened pipe ends is improved. At the same time, the combination design can effectively improve the design impact toughness of elements such as Cr, Mo, and Ni. Meanwhile, the addition of trace element Nb is used to refine the grains, achieving high strength and toughness of U165 under ultra-high strength. The addition of appropriate amount of V element improves the tempering stability and the thermal stability of the drill pipe in the high temperature environment of ultra-deep wells. Compared with the U165 steel grade drill pipe products currently under manufacturing and research at home and abroad, this patent strengthens by trace elements, while meeting the requirements of yield strength greater than or equal to 1138MPa and impact at -20℃ greater than or equal to 100J, while significantly reducing the amount of precious metals Mo and Ni elements added. It has good economic efficiency and does not add extra steps to the heat treatment process, making it suitable for mass production applications.

[0021] (4) Hot rolling forming allows for flexible specification changes and is suitable for the production of seamless steel pipes for drill rods in batches of multiple specifications.

[0022] (5) The continuous casting square billet is rolled into a round tube billet. After the tube billet passes the surface flaw detection, it is hot rolled into a seamless steel pipe again. Compared with the commonly used continuous casting round billet directly forming the pipe, it can effectively eliminate the original surface defects of the billet and achieve zero-defect quality assurance of U165 grade ultra-high strength and toughness seamless steel pipe for oil drill pipe.

[0023] (6) The length of the hot-rolled seamless pipe can reach 10 meters or more, which can meet the requirements of oil drill pipe length. The number of on-site connection joints used by users is small, and the reliability is higher.

[0024] (7) The hot-rolled seamless steel pipe has high dimensional accuracy, with an outer diameter deviation of -0.5%D to +1%D and a wall thickness deviation of -5%S to +15%S, which meets the drill pipe size requirements specified in GB / T 29166 / API 5DP. Detailed Implementation

[0025] To make the objectives, technical solutions, and advantages of this invention clearer, the technical solutions in the embodiments of this invention will be clearly and completely described below in conjunction with the embodiments of this invention. Those skilled in the art should understand that the embodiments described are merely illustrative of the invention and should not be considered as specific limitations thereof. All other embodiments obtained by those skilled in the art based on the embodiments of this invention without creative effort are within the scope of protection of this invention. Process parameters not specifically specified in the following embodiments are generally performed under conventional conditions.

[0026] The endpoints and any values ​​of the ranges disclosed in this invention are not limited to the precise ranges or values, and these ranges or values ​​should be understood to include values ​​close to these ranges or values. For numerical ranges, the endpoint values ​​of the various ranges, the endpoint values ​​of the various ranges and individual point values, and individual point values ​​can be combined with each other to obtain one or more new numerical ranges, which should be considered as specifically disclosed in this invention.

[0027] According to a first aspect of the present invention, the present invention provides a method for manufacturing a seamless steel pipe for ultra-high strength and high toughness drill pipe, comprising the following manufacturing processes: continuous casting billet smelting, continuous casting billet rolling into round tube billet, surface flaw detection, steel pipe rolling, and sample heat treatment; The continuously cast billet adopts a medium-sized continuously cast square billet with a billet shape (length × width) of 300mm × 400mm. The cross-sectional area rolling ratio of the continuously cast square billet to the steel pipe obtained after rolling exceeds 20:1.

[0028] This invention uses medium-sized continuously cast square billets, with the compression ratio of the cross-sectional length and width relative to the outer diameter of the steel pipe controlled at 2-3:1 (here, the compression ratio of the cross-sectional length of the continuously cast square billet relative to the outer diameter of the steel pipe is the length of the continuously cast square billet divided by the outer diameter of the steel pipe; the compression ratio of the width of the continuously cast square billet relative to the outer diameter of the steel pipe is the width of the continuously cast square billet divided by the outer diameter of the steel pipe). Compared with conventional continuously cast round billets for pipe forming, there is a larger deformation in the diameter direction. At the same time, the use of medium-sized continuously cast square billets avoids the segregation effect of traditional large square billets. Finally, the cross-sectional area rolling ratio of the continuously cast square billet to the steel pipe obtained after the rolling process exceeds 20:1, while the conventional continuously cast round billet-hot rolled seamless steel pipe forming rolling ratio is usually within 10:1. The process flow adopted in this invention can effectively reduce the segregation of seamless steel pipes and improve material uniformity.

[0029] As an optional embodiment of the manufacturing method of the seamless steel pipe of the present invention, the seamless steel pipe comprises the following components by mass percentage: C: 0.20%-0.35%, Mn: 0.30%-0.45%, Si≤0.40%, P≤0.013%, S≤0.005%, Cr: 0.8%-1.30%, Mo: 0.80%-1.35%, Ni: 0.30%-0.80%, V: 0.10%-0.35%, Nb: 0.01%-0.05%, with the remainder being iron and unavoidable impurities.

[0030] This invention employs a novel compositional design, considering the hardenability requirements of seamless steel pipes after end-to-end upsetting, while simultaneously ensuring strength and toughness across different wall thicknesses. By minimizing the content of easily segregating element Mn and appropriately controlling the C content range, the hardenability of the upset and thickened pipe ends is improved. The combined design effectively enhances the impact toughness of elements such as Cr, Mo, and Ni. Furthermore, the addition of trace element Nb refines the grain structure, achieving high strength and toughness at the ultra-high strength of U165. The addition of appropriate amounts of V improves tempering stability and enhances the thermal stability of the drill pipe operating in the high-temperature environment of ultra-deep wells. Compared to existing U165 steel-grade drill pipe products currently under development and production both domestically and internationally, this invention, through trace element strengthening, significantly reduces the amount of precious metals Mo and Ni added while meeting the requirements of a yield strength greater than or equal to 1138 MPa and an impact energy greater than or equal to 100 J at -20℃. This results in good economic efficiency without requiring additional heat treatment processes, making it suitable for mass production applications.

[0031] As an optional embodiment of the manufacturing method of the seamless steel pipe of the present invention, the seamless steel pipe has an outer diameter of ∮88.9mm-∮254mm, a wall thickness of 6.35mm-20mm, and a length of 7m-13.5m; and / or, the manufacturing method further includes the steps of surface flaw detection, physical and chemical inspection, sawing, ultrasonic flaw detection, finishing, and packaging and warehousing.

[0032] This invention uses medium-sized continuously cast square billets to roll into round tube billets. After the tube billets pass surface flaw detection, they are hot-rolled again into seamless steel pipes, effectively eliminating the original surface defects of the billets and achieving zero-defect quality assurance for U165 grade ultra-high strength and toughness seamless steel pipes for oil drill pipes.

[0033] As an optional embodiment of the manufacturing method of the seamless steel pipe of the present invention, the specific steps of the continuous casting billet smelting are: blast furnace ironmaking, hot metal desulfurization treatment, converter steelmaking, LF furnace refining, RH vacuum degassing, and continuous casting.

[0034] As an optional embodiment of the manufacturing method of the seamless steel pipe of the present invention, in the hot metal desulfurization treatment step, the sulfur content of the hot metal entering the furnace is ≤0.005 wt%, and the sulfur content after desulfurization can be controlled to ≤0.002 wt%. And / or, the refining time in the LF furnace is ≥45 min; And / or, the RH vacuum degassing time is ≥20 min.

[0035] As an optional embodiment of the manufacturing method of the seamless steel pipe of the present invention, the specific steps of rolling the continuously cast billet into a round billet are as follows: heating with a walking beam furnace, rolling with a large reduction mill, and reducing the diameter with a sizing mill; the heating temperature of the walking beam furnace is controlled at 1210℃-1280℃ (e.g., 1220℃, 1230℃, 1240℃, 1250℃, 1260℃, 1270℃), and the soaking time is about 90min-150min (e.g., 100min, 110min, 120min, 130min, 140min); the rolling ratio of the continuously cast billet to the round billet is ≥3:1; preferably, the rolling ratio of the continuously cast billet to the intermediate square billet obtained after rolling with a large reduction mill is 1.85:1-2.0:1; the rolling ratio of the intermediate square billet obtained after rolling with a large reduction mill to the round billet obtained after reducing the diameter is 1.8:1-2.1:1. The rolling ratio described in this invention is the ratio of the cross-sectional area of ​​the workpiece to be rolled to the cross-sectional area of ​​the rolled tube.

[0036] As an optional embodiment of the manufacturing method of the seamless steel pipe of the present invention, the specific steps of the steel pipe rolling are: billet heating, skew rolling piercing, CPE jacking, and diameter reduction.

[0037] As an optional embodiment of the manufacturing method of the seamless steel pipe of the present invention, the billet is heated by an annular furnace, the temperature of the soaking section is controlled at 1200℃-1280℃ (e.g., 1210℃, 1220℃, 1230℃, 1240℃, 1250℃, 1260℃, 1270℃), and the soaking time is 100min-150min (e.g., 110min, 120min, 130min, 140min). And / or, the CPE jacking pipe is jacked by a CPE jacking machine on the perforated tube to obtain a rough pipe. The CPE jacking pipe adopts a high-speed jacking mode, with a rolling time of ≤20 seconds and dimensional accuracy of: outer diameter -0.5%D to +1%D, wall thickness -5%S to +15%S. And / or, the total production time for the skew rolling piercing and CPE jacking pipe is 2 min-4 min (e.g., 2.5 min, 3 min, 3.5 min).

[0038] As an optional embodiment of the manufacturing method of the seamless steel pipe of the present invention, the heat treatment of the sample is carried out by quenching and tempering; the quenching temperature is 840℃-900℃ (e.g., 850℃, 860℃, 870℃, 880℃, 890℃), the holding time is 50min-90min (e.g., 60min, 70min, 80min), and water quenching is used; the tempering temperature is 600℃-640℃ (e.g., 610℃, 620℃, 630℃), the holding time is 50min-90min (e.g., 60min, 70min, 80min), and air cooling is used to room temperature.

[0039] According to a second aspect of the present invention, the present invention provides a seamless steel pipe manufactured by the manufacturing method of the first aspect, wherein the seamless steel pipe has a yield strength ≥1138 MPa, a tensile strength ≥1200 MPa, an elongation ≥15%, a minimum single longitudinal impact energy Kv2 at -20℃ ≥100 J, and a grain size of grade 8-11.

[0040] The present invention will now be described in further detail with reference to specific embodiments and comparative examples.

[0041] Example 1 A method for manufacturing a U165 grade seamless steel pipe for oil drill pipe with a diameter of 139.7mm × 9.65mm includes smelting to produce a continuously cast billet, rolling the continuously cast billet into a round tube billet, rolling the steel pipe, heat treatment of the sample, physical and chemical testing, sawing, ultrasonic testing, finishing, and packaging and warehousing. The composition of the seamless steel pipe, by mass percentage, includes: C: 0.25%, Mn: 0.45%, Si: 0.40%, P: 0.013%, S: 0.005%, Cr: 0.8%, Mo: 0.80%, Ni: 0.30%, V: 0.25%, Nb: 0.02%, with the remainder being iron and unavoidable impurities.

[0042] The specific manufacturing steps include: (1) Continuous casting billet smelting: including blast furnace ironmaking, hot metal desulfurization treatment, electric furnace steelmaking, LF furnace refining, RH vacuum degassing and continuous casting process.

[0043] Hot metal desulfurization treatment: Hot metal KR desulfurization treatment is adopted to control the sulfur content of hot metal entering the electric furnace to 0.005 wt.%.

[0044] Converter steelmaking: Molten iron with S≤0.002wt.% after desulfurization is transferred to the converter for steelmaking.

[0045] LF furnace refining: The molten steel obtained from electric furnace steelmaking is refined in the LF furnace. The refining process adopts the whole process of argon blowing, and the LF furnace refining time is 45 minutes.

[0046] RH vacuum degassing: Vacuum degassing time is 20 minutes to reduce inclusions and gas content in molten steel, making the composition of molten steel uniform.

[0047] Continuous casting process: The molten steel after RH vacuum degassing is continuously cast to obtain a continuously cast square billet with a length × width of 300mm × 400mm.

[0048] (2) Rolling continuously cast square billets into round tube billets: including walking beam furnace heating, large reduction rolling, and sizing.

[0049] Walking beam furnace heating: The continuously cast billet is heated by a walking beam furnace. The temperature of the soaking zone is controlled at 1260℃ and the soaking time is about 90 minutes.

[0050] Large reduction rolling: The billet is rolled by a rolling mill with a roll diameter of 1350mm. Through large rolling deformation, the side length of the middle square billet is about 250mm.

[0051] Sizing: Multiple sizing mills are used to finally roll a round tube blank with a diameter of 195mm.

[0052] Non-destructive testing: Infrared surface testing is used to inspect the surface quality of the round tube blank. If defects are found on the surface, they are repaired by grinding or peeling.

[0053] (3) Steel pipe rolling: The rolled 195mm round billet is heated, pierced, CPE jacking pipe and sizing to obtain a steel pipe with a specification of ∮139.7mm×9.65mm.

[0054] Tube billet heating: The round tube billet is heated by a ring furnace, with the temperature of the soaking section controlled at 1280℃ and the soaking time being 100min.

[0055] Piercing: The heated round tube blank is pierced by oblique rolling using a conical piercing machine to obtain a tube with a diameter of ∮198mm×19mm.

[0056] CPE jacking pipe: After CPE jacking pipe, a rough pipe with dimensions of ∮162mm×12mm is obtained.

[0057] Sizing and reducing: The seamless steel pipe obtained by skew rolling is sizing and reducing using a 2-roll micro-tension reducing mill to obtain a steel pipe with a specification of ∮139.7mm×9.65mm. The outer diameter accuracy of the steel pipe is ±0.8%D. The wall thickness accuracy of the steel pipe is ±7.5%S.

[0058] (4) Heat treatment of the sample: A 400mm long steel pipe was used. The heat treatment of the sample was carried out by quenching and tempering. The quenching temperature was 850℃ and the holding time was 80 min. The sample was water-cooled. The tempering temperature was 640℃ and the holding time was 60 min. The sample was air-cooled to room temperature.

[0059] (5) Physical and chemical testing: Take 130mm-150mm samples from both ends of the steel pipe to test whether the mechanical properties meet the requirements.

[0060] (6) Straightening: A straightening machine is used for calibration.

[0061] Non-destructive testing: Ultrasonic testing of steel pipes shall be carried out in accordance with GB / T5777 standard, with acceptance level U2-C.

[0062] (7) Overall quality inspection: Inspection of the surface quality and dimensional accuracy of the steel pipe.

[0063] (8) Packaging and warehousing: After weighing and marking, the steel pipes are packaged and stored.

[0064] Example 2 A method for manufacturing a seamless steel pipe of U165 grade with a diameter of ∮152.4×9.65mm for oil drill pipe includes smelting to produce a continuously cast billet, rolling the continuously cast billet into a round tube billet, heat treatment of the sample, physical and chemical testing, sawing, ultrasonic testing, finishing, and packaging and warehousing. The composition of the seamless steel pipe by mass percentage includes: C: 0.30%, Mn: 0.45%, Si: 0.30%, P: 0.013%, S: 0.005%, Cr: 1.0%, Mo: 0.80%, Ni: 0.50%, V: 0.25%, Nb: 0.02%, with the remainder being iron and unavoidable impurities.

[0065] The specific manufacturing steps include: (1) Continuous casting billet smelting: including blast furnace ironmaking, hot metal desulfurization treatment, electric furnace steelmaking, LF furnace refining, RH vacuum degassing and continuous casting process.

[0066] Hot metal desulfurization treatment: Hot metal KR desulfurization treatment is adopted to control the sulfur content of hot metal entering the electric furnace to 0.005 wt.%.

[0067] Converter steelmaking: Molten iron with S≤0.002wt.% after desulfurization is transferred to the converter for steelmaking.

[0068] LF furnace refining: The molten steel obtained from electric furnace steelmaking is refined in the LF furnace. The refining process adopts the whole process of argon blowing, and the LF furnace refining time is 45 minutes.

[0069] RH vacuum degassing: Vacuum degassing time is 22 minutes to reduce inclusions and gas content in molten steel, making the composition of molten steel uniform.

[0070] Continuous casting process: The molten steel after RH vacuum degassing is continuously cast to obtain a continuously cast square billet with a size of 300mm×400mm.

[0071] (2) Rolling continuously cast square billets into round tube billets: including walking beam furnace heating, large reduction rolling, and sizing.

[0072] Walking beam furnace heating: The continuously cast billet is heated by a walking beam furnace. The temperature of the soaking zone is controlled at 1210℃ and the soaking time is about 120min.

[0073] Large reduction rolling: The billet is rolled by a rolling mill with a roll diameter of 1350mm. Through large rolling deformation, the side length of the middle square billet is about 250mm.

[0074] Diameter reduction: Multiple diameter reduction mills are used to finally roll the tube into a round tube blank with a diameter of 210mm.

[0075] Non-destructive testing: Infrared surface testing is used to inspect the surface quality of the round tube blank. If defects are found on the surface, they are repaired by grinding or peeling.

[0076] (3) Steel pipe rolling: The rolled 210mm round billet is heated, pierced, CPE jacking pipe and sizing to obtain a steel pipe with a specification of ∮152.4mm×9.65mm.

[0077] Tube blank heating: The round tube blank is heated by a ring furnace, the temperature of the soaking section is controlled at 1200℃, and the soaking time is 120min.

[0078] Piercing: The heated round tube blank is pierced by oblique rolling using a conical piercing machine to obtain a ∮203mm×20mm tube.

[0079] CPE jacking pipe: After CPE jacking pipe, a rough pipe with dimensions of ∮163mm×12mm is obtained.

[0080] Sizing and reducing: The seamless steel pipe obtained by skew rolling is sizing and reducing using a 2-roll micro-tension reducing mill to obtain a steel pipe with a specification of ∮152.4mm×9.65mm. The outer diameter accuracy of the steel pipe is ±0.8%D. The wall thickness accuracy of the steel pipe is ±7.5%S.

[0081] (4) Heat treatment of the sample: A 400mm long steel pipe was used. The heat treatment of the sample was carried out by quenching and tempering. The quenching temperature was 850℃ and the holding time was 80 min. The sample was water-cooled. The tempering temperature was 640℃ and the holding time was 60 min. The sample was air-cooled to room temperature.

[0082] (5) Physical and chemical testing: Take 130mm-150mm samples from both ends of the steel pipe to test whether the mechanical properties meet the requirements.

[0083] (6) Straightening: A straightening machine is used for calibration.

[0084] Non-destructive testing: Ultrasonic testing of steel pipes shall be carried out in accordance with GB / T5777 standard, with acceptance level U2-C.

[0085] (7) Overall quality inspection: Inspection of the surface quality and dimensional accuracy of the steel pipe.

[0086] (8) Packaging and warehousing: After weighing and marking, the steel pipes are packaged and stored.

[0087] Example 3 The difference between this embodiment and Embodiment 2 is as follows: (2) In the step of rolling continuously cast square billets into round tube billets Walking beam furnace heating: The continuously cast billet is heated by a walking beam furnace. The temperature of the soaking zone is controlled at 1260℃ and the soaking time is about 120 minutes.

[0088] (3) In the steel pipe rolling process: Tube billet heating: The round tube billet is heated by a ring furnace, with the temperature of the soaking section controlled at 1260℃ and the soaking time being 120min.

[0089] Comparative Example 1 The difference between this comparative example and Example 1 is that: Step (2) continuous casting process: the molten steel after RH vacuum degassing is continuously cast to obtain a continuously cast square billet with a specification of 240mm×240mm. In step (2), the continuously cast square billet is rolled into a round tube billet with a diameter of 195mm by a sizing mill. (No intermediate square billet process).

[0090] Step (3) Sizing and reducing: Steel pipe rolling: The rolled round billet with a diameter of 195mm is heated, pierced, CPE jacked and sizing and reduced to obtain a steel pipe with a specification of ∮139.7mm×9.65mm.

[0091] Tube billet heating: The round tube billet is heated by a ring furnace, with the temperature of the soaking section controlled at 1280℃ and the soaking time being 100min.

[0092] Piercing: The heated round tube blank is pierced by oblique rolling using a conical piercing machine to obtain a tube with a diameter of ∮198mm×18mm.

[0093] CPE jacking pipe: After CPE jacking pipe, a rough pipe with dimensions of ∮162mm×12mm is obtained.

[0094] Sizing and reducing: The seamless steel pipe obtained by skew rolling is sizing and reducing using a 2-roll micro-tension reducing mill to obtain a steel pipe with a specification of ∮139.7mm×9.65mm. The outer diameter accuracy of the steel pipe is ±0.8%D. The wall thickness accuracy of the steel pipe is ±7.5%S.

[0095] That is, the rolling ratio of the cross-sectional area of ​​the continuously cast billet in step (2) and the steel pipe obtained in step (3) is 14.6:1.

[0096] Comparative Example 2 The comparative example uses a conventional continuous casting round billet-hot-rolled seamless steel pipe forming process. The composition of the continuous casting round billet by mass percentage includes: C: 0.25%, Mn: 0.45%, Si: 0.40%, P: 0.013%, S: 0.005%, Cr: 0.8%, Mo: 0.80%, Ni: 0.30%, V: 0.25%, Nb: 0.02%, with the remainder being iron and unavoidable impurities.

[0097] Steel pipes with dimensions of ∮139.7mm × 9.65mm are obtained by heating, piercing and continuous rolling using ∮195mm continuously cast round billets.

[0098] Heating of continuously cast round billets: The round billets are heated in a ring furnace at a temperature of 1180℃-1260℃ for 120 minutes.

[0099] Piercing: The heated round billet is pierced by skew rolling using a conical piercing machine to obtain a ∮176mm×16mm tube.

[0100] Continuous rolling: The tube enters a walking beam reheating furnace for heating. The heating and holding temperature is 850℃-910℃, and the holding time is 30 minutes. After heating, the tube exits the furnace and is descaled by high-pressure water. Then it enters a 12-stand tension reducing mill to reduce the diameter, resulting in a steel pipe with a specification of ∮139.7mm×9.65mm. The outer diameter accuracy of the steel pipe is ±0.8%D; the wall thickness accuracy is ±7.5%S; and the rolling ratio of the cross-sectional area of ​​the continuously cast round billet to the obtained steel pipe is 7.57:1.

[0101] Comparative Example 3 The only difference between this comparative example and Example 1 is the heating of the tube blank in step (3): the temperature of the soaking section is controlled at 1100℃.

[0102] Performance testing The properties of the steel pipes manufactured in Examples 1-3 and Comparative Examples 1-3 are shown in Table 1.

[0103] Table 1

[0104] The testing standards for yield strength are: ASTM A370 & ASTM E8. The testing standards for tensile strength are: ASTM A370 & ASTM E8. The testing standards for elongation are: ASTM A370 & ASTM E8 The testing standards for impact energy at -20℃ are: ASTM A370 & ASTM E23. The standard for grain size testing is: ASTM E112 As shown in Table 1, the seamless steel pipes manufactured using the method of this invention have a yield strength ≥1138 MPa, tensile strength ≥1200 MPa, elongation ≥15%, and a minimum single longitudinal impact energy Kv2 at -20℃ ≥100 J. In Comparative Example 1, the cross-sectional area rolling ratio of the continuously cast billet to the final steel pipe is 14.6:1, which does not meet the requirement of a cross-sectional area rolling ratio exceeding 20:1 as described in this application. The yield strength and tensile strength of the seamless steel pipes manufactured using this method are significantly lower than those of Example 1, and the longitudinal impact energy Kv2 at -20℃ is 89 J, 91 J, and 86 J, respectively, all below 100 J. Furthermore, the grain size is also lower than that of the steel pipes manufactured using the method of this application. Comparative Example 2 uses a conventional continuously cast round billet-hot-rolled seamless steel pipe forming process, with a cross-sectional area rolling ratio of 7.57:1. The yield strength, tensile strength, and elongation of the seamless steel pipes manufactured using this method are significantly lower than those of Example 1. The tensile strength and elongation of the steel pipes manufactured in Comparative Example 3 were significantly lower than those of Example 1, and the longitudinal impact energy Kv2 at -20℃ was 81J, 86J, and 83J, respectively, all below 100J. The grain size of the steel pipes manufactured in Comparative Example 3 was also lower than that of the steel pipes manufactured by the method of this application. In the steel pipe rolling process of Comparative Example 3, the temperature of the soaking section of the billet heating step was controlled at 1100℃, which is not within the range of 1200℃-1280℃ described in this invention. The yield strength, tensile strength, and elongation of the seamless steel pipes manufactured in Comparative Example 3 were significantly lower than those of Example 1, and the longitudinal impact energy Kv2 at -20℃ was 73J, 70J, and 79J, respectively, all far below 100J. The grain size of the steel pipes manufactured in Comparative Example 3 was also significantly lower than that of the steel pipes manufactured by the method of this application.

[0105] 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 manufacturing a seamless steel pipe for ultra-high strength and high toughness drill pipes, characterized in that, The manufacturing process includes the following steps: smelting to produce continuously cast billets, rolling continuously cast billets into round tube billets, rolling steel pipes, and heat treatment of samples. The continuously cast billet is a medium-sized continuously cast square billet with a shape of 300mm×400mm. The cross-sectional area ratio of the continuously cast billet to the steel pipe obtained after the steel pipe rolling step exceeds 20:

1.

2. The method for manufacturing seamless steel pipe according to claim 1, characterized in that, The seamless steel pipe comprises the following components by mass percentage: C: 0.20%-0.35%, Mn: 0.30%-0.45%, Si≤0.40%, P≤0.013%, S≤0.005%, Cr: 0.8%-1.30%, Mo: 0.80%-1.35%, Ni: 0.30%-0.80%, V: 0.10%-0.35%, Nb: 0.01%-0.05%, with the remainder being iron and unavoidable impurities.

3. The method for manufacturing seamless steel pipe according to claim 1, characterized in that, The seamless steel pipe has an outer diameter of ∮88.9mm-∮254mm, a wall thickness of 6.35mm-20mm, and a length of 7m-13.5m; And / or, the manufacturing method further includes the steps of surface flaw detection, physical and chemical inspection, sawing, ultrasonic flaw detection, finishing, and packaging and warehousing of the billet; the surface flaw detection of the billet is performed between the rolling of the continuously cast billet into a round billet and the rolling of the steel pipe; the physical and chemical inspection, sawing, ultrasonic flaw detection, finishing, and packaging and warehousing of the sample are performed after the sample is heat treated.

4. The method for manufacturing seamless steel pipe according to claim 1, characterized in that, The specific steps for producing continuously cast billets are as follows: blast furnace ironmaking, hot metal desulfurization treatment, converter steelmaking, LF furnace refining, RH vacuum degassing, and continuous casting.

5. The method for manufacturing seamless steel pipe according to claim 4, characterized in that, In the hot metal desulfurization process, the sulfur content of the hot metal entering the furnace is ≤0.005 wt%, and the sulfur content after desulfurization is controlled to ≤0.002 wt%. And / or, the refining time in the LF furnace is ≥45 min; And / or, the RH vacuum degassing time is ≥20 min.

6. The method for manufacturing seamless steel pipe according to claim 1, characterized in that, The specific steps for rolling the continuously cast billet into a round tube billet are as follows: heating with a walking beam furnace, rolling with a large reduction mill, and reducing the diameter with a sizing mill; the heating temperature of the walking beam furnace is controlled at 1210℃-1280℃, and the soaking time is 90min-150min; the rolling ratio of the continuously cast billet to the round tube billet is ≥3:1; preferably, the rolling ratio of the continuously cast billet to the intermediate square billet obtained after rolling with a large reduction mill is 1.85:1-2.0:1; the rolling ratio of the intermediate square billet obtained after rolling with a large reduction mill to the round tube billet obtained after reducing the diameter is 1.8:1-2.1:

1.

7. The method for manufacturing seamless steel pipe according to claim 1, characterized in that, The specific steps of the steel pipe rolling process are: billet heating, skew rolling piercing, CPE jacking, and diameter reduction.

8. The method for manufacturing a seamless steel pipe according to claim 7, characterized in that, The tube blank is heated using a ring furnace, with the temperature of the soaking section controlled at 1200℃-1280℃ and the soaking time being 100min-150min. And / or, the CPE jacking pipe is jacked by a CPE jacking machine on the perforated tube to obtain a rough pipe. The CPE jacking pipe adopts a high-speed jacking mode, with a rolling time of ≤20 seconds, an outer diameter dimensional accuracy of -0.5%D to +1%D, and a wall thickness dimensional accuracy of -5%S to +15%S. And / or, the total production time for the skew rolling piercing and CPE jacking pipe is 2 min-4 min.

9. The method for manufacturing a seamless steel pipe according to claim 1, characterized in that, The heat treatment of the sample includes quenching and tempering in sequence; the quenching temperature is 840℃-900℃, the holding time is 50min-90min, and water quenching is performed; the tempering temperature is 600℃-640℃, the holding time is 50min-90min, and air cooling is performed to room temperature.

10. A seamless steel pipe manufactured by any one of claims 1-9, characterized in that, The seamless steel pipe has a yield strength ≥1138 MPa, tensile strength ≥1200 MPa, elongation ≥15%, longitudinal impact energy Kv2 at -20℃ with a minimum single value ≥100 J, and grain size of grade 8-11.

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