Anti-fatigue high-strength petroleum drill rod and preparation method thereof

Abstract

The invention relates to an anti-fatigue high-strength petroleum drill rod and a preparation method thereof. The anti-fatigue high-strength petroleum drill rod is prepared from, by mass, 0.43%-0.46% of C, 12%-16% of Mn, 2.1%-2.5% of Cr, 0.12%-0.16% of Mo, 0.16%-0.19% of V, 0.001%-0.003% of B, iron and inevitable impurities. Wherein the result is 0.15 lt; (Cr + Mo + V) / (C + Mn) lt; 0.23, 0.23; the anti-fatigue high-strength petroleum drill rod structurally comprises a first drill rod unit and a second drill rod unit, one end of the first drill rod unit is provided with a first thickening section, and a mortise is formed in the first thickening section; at least one end of the second drill rod unit is provided with a second thickening section, and a tenon in interference fit with the mortise is arranged on the second thickening section. The drill rod can be freely pulled, bent and twisted according to the stress condition, stress concentration is avoided, and the anti-fatigue performance is remarkably improved.

Description

Technical Field

[0001] This invention relates to the field of oil drill pipe manufacturing technology, and in particular to a fatigue-resistant high-strength oil drill pipe and its preparation method. Background Technology

[0002] The core differences between oil drill pipe and geological drill pipe, coal mine drill pipe, water well drill pipe, and engineering exploration drill pipe lie in their completely different service conditions, stress conditions, material strength, connection structure, sealing, and safety standards, making them unsuitable for general use. Specifically, oil drill pipe is a core channel component in oil and gas drilling, used in deep wells, ultra-deep wells, horizontal wells, deviated wells, and high-pressure, high-temperature wells. It withstands combined loads such as tension, compression, torsion, and impact, requiring characteristics of load-bearing capacity, torsion resistance, and high pressure resistance, making it highly susceptible to fatigue failure. In contrast, drill pipes used in other fields such as geological exploration, coalfields, and water wells operate in shallow to medium-deep wells, conventional formations, and under low pressure and low torque, focusing on drilling and rock breaking. Their requirements for strength, fatigue resistance, sulfur resistance, and sealing are far lower than those for oil drill pipes.

[0003] As the core of oil and gas drilling operations, drill pipe directly determines drilling safety, efficiency, and cost. During drill pipe manufacturing, higher strength and impact toughness are often ensured by reducing P and S content and adding more Cr. However, this improvement has limited effect on enhancing the drill pipe's fatigue resistance, and excessively high Cr content leads to a severe tendency for stress concentration in drill pipes that are predominantly martensitic in structure. In response to this situation, existing drill pipe and manufacturing solutions include: See CN102773603A, which uses existing friction welding technology to manufacture drill pipes. However, due to the significant differences in strength and toughness between the weld, heat-affected zone, and base material caused by friction welding, stress concentration is easily formed at the welded area, resulting in limited overall fatigue resistance. See CN102773603A, drill pipes are manufactured using solid bar stock or thick-walled tubular materials. While this avoids the poor microstructure and properties caused by friction welding and heat treatment of the weld area, it requires connection to equipment such as hanger slots and thick-walled slips, as well as thread machining, overall heat treatment of the drill pipe, and localized heat treatment of the joint area. This process is cumbersome, and localized heat treatment inevitably leads to softening of the transition zone, easily causing stress concentration and reducing fatigue resistance. See CN116537710A, whose drill pipe joints include threaded male and female joints, which can improve the torque resistance of the drill pipe joint. However, the problem of severe stress concentration at the threads remains unresolved, and fatigue resistance is still low. See CN118346188A, which uses titanium alloy, with thickened ends and external threads, and connects to the joint via a special thread. While this improves the thread anti-sticking and fatigue resistance, issues such as the fusion of the binary interface between titanium alloy and steel materials mean that the performance of the fusion zone differs from both the titanium and steel base materials, resulting in poor fatigue resistance stability. See CN117987685A, which improves the raw material composition ratio of the drill pipe and enhances the impact resistance of the titanium alloy drill pipe through process improvements. However, Ti, Al, V, Mo, Zr, and Ru alloys are expensive and have complex manufacturing processes, failing to address the weak performance of the transition section between the high-strength titanium alloy drill pipe and the joint, and thus the overall fatigue resistance cannot be guaranteed.

[0004] In summary, there is an urgent need to develop a high-strength oil drill pipe with excellent fatigue resistance. Summary of the Invention

[0005] To address the shortcomings of existing technologies, this invention provides a fatigue-resistant high-strength oil drill pipe and its preparation method, thereby solving the technical problem of low fatigue resistance of existing high-strength oil drill pipes.

[0006] The technical solution adopted in this invention is as follows: This invention provides a fatigue-resistant, high-strength oil drill pipe, the material of which comprises the following components by mass percentage: C: 0.43%~0.46%, Mn: 12%~16%, Cr: 2.1%~2.5%, Mo: 0.12%~0.16%, V: 0.16%~0.19%, B: 0.001%~0.003%, And iron and unavoidable impurities; Among them, 0.15 < (Cr + Mo + V) / (C + Mn) < 0.23; The fatigue-resistant high-strength oil drill pipe structure includes a first drill pipe unit and a second drill pipe unit, wherein the first drill pipe unit and the second drill pipe unit are interference-fitted. One end of the first drill pipe unit is provided with a first thickened section, on which a mortise is provided; The second drill rod unit has a second thickened section at at least one end, on which a tenon is provided that is interference fit with the mortise; The thickness of the first and second thickened sections is the same.

[0007] The preferred technical solution is: The tenon includes an outwardly protruding structure extending axially from the end face of the second thickened section, the outwardly protruding structure being cylindrical or conical; the mortise includes a groove extending axially from the end face of the first thickened section that matches the outwardly protruding structure; The outer circumferential surface of the cylindrical or conical shape is provided with at least one positioning protrusion, and the inner circumferential surface of the groove is provided with a positioning groove that mates with the positioning protrusion.

[0008] The ratio of the depth of the mortise to the length of the first thickened section is 1:2; the ratio of the length of the tenon to the length of the second thickened section is 1:2.

[0009] The interference fit is 0.15mm to 0.25mm.

[0010] The outer diameter of the first thickened section is 127mm~152mm, and the inner diameter is 89mm~115mm; the outer diameter of the non-thickened section of the first drill pipe unit is 127mm~152mm, and the inner diameter is 118mm~143mm.

[0011] The outer diameter of the second thickened section is 89mm~115mm, and the inner diameter is 52mm~77mm; the outer diameter of the non-thickened section of the second drill pipe unit is 89mm~115mm, and the inner diameter is 80mm~106mm.

[0012] This invention also provides a method for preparing the aforementioned fatigue-resistant high-strength oil drill pipe, comprising the following steps: Raw materials are prepared according to the set component ratio, and two drill pipe blanks are prepared using the raw materials according to the set structural dimensions; the first drill pipe unit and the second drill pipe unit are respectively made using the two drill pipe blanks. The first drill rod unit is hot-extruded at one end to keep the outer diameter constant, increase the inner wall thickness, and reduce the inner diameter to obtain a first thickened section. A mortise is machined on the first thickened section, and an interference fit is retained. The second drill rod unit is hot-extruded at both ends to keep the outer diameter constant and increase the inner wall thickness to obtain a second thickened section with the same thickness as the first thickened section. The mating surfaces of the tenon and the mortise are lubricated. The two drill pipe units were subjected to a first heat treatment at 210℃~350℃, and then air-cooled to room temperature; The two drill pipe units were subjected to a second heat treatment at 1050℃~1150℃, followed by a cooling treatment. The two drill pipe units were subjected to a third heating treatment at 210℃~350℃, and then air-cooled to room temperature; The first drill pipe unit is heated to expand and deform it, and the mortise is inserted into the tenon of the second drill pipe unit. After cooling, a seamless mortise and tenon connection is achieved.

[0013] The hot extrusion temperature is 500℃~700℃.

[0014] During the first heat treatment, the holding time is 50 min to 120 min; During the second heating process, the heating rate is 100℃ / h~140℃ / h, the holding time is 100min~210min, and the cooling process includes: transferring the drill rod unit from the heating furnace to cooling water at 10℃~35℃, with a transfer time ≤35s, cooling to 210℃ and then air cooling to room temperature; During the third heating process, the holding time is 120 min to 230 min.

[0015] The heating temperature for causing the first drill pipe unit to expand and deform is 100℃~300℃.

[0016] The technical solution of the present invention can achieve at least some of the following beneficial effects: This invention, through a synergistic approach of improved proportions and structural modifications, significantly reduces the performance difference between the drill pipe connection and the base material. This allows the drill pipe to freely adapt to tension, compression, bending, and torsion under stress during operation, avoiding stress concentration. The overall fatigue resistance of the drill pipe is excellent, greatly improving and extending the service life of high-strength drill pipes. Specifically, based on the improved raw material proportions, this invention prepares drill pipe units with a uniform deformation rate of over 35% at room temperature. This provides a reliable foundation for improving the drill pipe connection structure, ensuring that the expansion joint of the drill pipe unit has sufficient uniform deformation reserve, ultimately achieving high strength, impact resistance, and fatigue resistance in the drill pipe after the joint connection.

[0017] The material formulation of this invention achieves a balance between the strength, machinability, impact resistance, and fatigue resistance of the material, and transforms the microstructure of the drill pipe matrix from F-type to A-type, providing a fundamental guarantee for high-strength oil drill pipe products to have large deformation capacity and reduce the tendency of stress concentration during deformation.

[0018] This invention's manufacturing method achieves the fabrication of a tenon-and-mortise connection structure by increasing the pipe end thickness and employing a unique heat treatment. This weld-free tenon-and-mortise connection structure avoids stress concentration and even cracking caused by drill pipe joint threads and welds on the pipe body. It significantly reduces the performance difference between the drill pipe connection area and the base material, allowing the drill pipe to freely adapt to tension, compression, bending, and torsion under stress during operation, thus preventing stress concentration. The drill pipe exhibits excellent overall fatigue resistance, with a 10 million-cycle tensile and compressive fatigue limit strength at the drill pipe joint ≥350MPa, a room temperature yield strength ≥750MPa, a room temperature tensile strength ≥880MPa, and an impact energy (CVN) ≥35J at -40℃. This greatly improves and extends the service life of high-strength drill pipes.

[0019] The material formulation of this invention eliminates the need for the use of precious elements such as W, Ti, Ni, and Ru, while simultaneously reducing the amount of Mo added. Furthermore, by precisely controlling the content and combination of elements, the manufacturing cost and the complexity of production organization are reduced.

[0020] Other features and advantages of the invention will be set forth in the following description or may be learned by practicing the invention. Attached Figure Description

[0021] Figure 1 This is an exploded structural diagram of the drill pipe according to an embodiment of the present invention.

[0022] Figure 2 This is a schematic cross-sectional view of the first thickened section in an embodiment of the present invention.

[0023] Figure 3 This is a schematic cross-sectional view of the second thickened section in an embodiment of the present invention.

[0024] Figure 4This is a longitudinal cross-sectional view of the drill pipe assembly structure according to an embodiment of the present invention.

[0025] Explanation of reference numerals in the attached drawings: 1. First drill rod unit; 2. Second drill rod unit; 11. First thickened section; 21. Second thickened section; 111. Mortising; 112. Positioning groove; 211. Tenon; 212. Positioning protrusion. Detailed Implementation

[0026] The specific embodiments of the present invention are described below with reference to the accompanying drawings.

[0027] This application provides a fatigue-resistant, high-strength oil drill pipe, the material of which comprises the following components by mass percentage: C: 0.43%~0.46%, Mn: 12%~16%, Cr: 2.1%~2.5%, Mo: 0.12%~0.16%, V: 0.16%~0.19%, B: 0.001%~0.003%, And iron and unavoidable impurities; Among them, 0.15 < (Cr + Mo + V) / (C + Mn) < 0.23 is a key parameter for balancing the strength, machinability, impact resistance, and fatigue resistance of materials. If it is less than 0.15, the product's strength and machinability will be poor, and surface quality defects will easily occur. If it is greater than 0.23, the product's impact resistance will be reduced, leading to an increased tendency for stress concentration. Furthermore, it breaks the conventional limitation of Mn not exceeding 2.0% in high-strength oil drill pipe design. Combined with the synergistic effect of C, Cr, Mo, V, and B, it transforms the matrix structure of the drill pipe from F-type to A-type, providing a fundamental guarantee for high-strength oil drill pipe products to have large deformation capacity and reduce the tendency for stress concentration during deformation. See Figures 1 to 4 The fatigue-resistant high-strength oil drill pipe structure includes a first drill pipe unit 1 (i.e., the female drill pipe) and a second drill pipe unit 2 (i.e., the male drill pipe), which are interference-fitted together. Figure 1 (a) and (b) are partial structural diagrams of the first drill rod unit and the second drill rod unit near their ends, respectively. One end of the first drill rod unit 1 is provided with a first thickened section 11, on which a mortise 111 is provided. At least one end of the second drill rod unit 2 is provided with a second thickened section 21, on which a tenon 211 is provided that is interference-fitted with the mortise 111. Preferably, the thicknesses of the first and second thickened sections are the same.

[0028] In one specific embodiment, the fatigue-resistant high-strength oil drill pipe joint has a tensile and compressive fatigue limit strength of ≥350MPa after 10 million cycles, a yield strength at room temperature of ≥750MPa, a tensile strength at room temperature of ≥880MPa, and an impact energy of ≥35J at -40℃. This significantly improves the service life and performance of the high-strength drill pipe under combined alternating loads such as tension, compression, bending, and torsion, as well as under conditions of corrosion, wear, temperature, and pressure.

[0029] The drill pipe in this embodiment is first prepared based on an improved raw material ratio to obtain a drill pipe unit with a uniform deformation rate of more than 35% under room temperature conditions. This provides a reliable basis for improving the drill pipe connection structure, enabling the expansion tenon and mortise connection of the drill pipe unit to have sufficient uniform deformation reserve, and ultimately achieving high strength, impact resistance and fatigue resistance of the drill pipe after the tenon and mortise connection.

[0030] As a preferred embodiment, the tenon 211 includes an outwardly convex structure extending axially from the end face of the second thickened section 21, the outwardly convex structure being conical or cylindrical; the mortise 111 includes a groove extending axially from the end face of the first thickened section 11, matching the outwardly convex structure. Preferably, at least one positioning protrusion 212 is provided on the outer circumferential surface of the conical or cylindrical shape, and a positioning groove 112 that engages with the positioning protrusion 212 is provided on the inner circumferential wall of the groove.

[0031] The angle of the mating surfaces of the tenon and mortise can be set according to actual needs.

[0032] The machining lengths of the tenon 211 and mortise 111 are matched with the lengths of the thickened sections. As a preferred embodiment, the ratio of the depth of the mortise 111 to the length of the first thickened section 11 is 1:2; the ratio of the length of the tenon 211 to the length of the second thickened section 21 is 1:2.

[0033] As a preferred embodiment, the interference fit is 0.15mm to 0.25mm.

[0034] In one specific embodiment, the outer diameter of the first thickened section 11 is 127mm~152mm, and the inner diameter is 89mm~115mm; the outer diameter of the non-thickened section of the first drill pipe unit 1 is 127mm~152mm, and the inner diameter is 118mm~143mm. The outer diameter of the second thickened section 21 is 89mm~115mm, and the inner diameter is 52mm~77mm; the outer diameter of the non-thickened section of the second drill pipe unit 2 is 89mm~115mm, and the inner diameter is 80mm~106mm.

[0035] This application also provides a method for preparing the fatigue-resistant high-strength oil drill pipe described above, comprising the following steps: S1. Drill pipe unit preparation: Raw materials are prepared according to the set component ratio, and two drill pipe blanks are prepared using the raw materials and according to the set structural dimensions; the two drill pipe blanks are used to make the first drill pipe unit and the second drill pipe unit respectively.

[0036] In one specific embodiment, the drill pipe billet preparation adopts a converter or electric furnace + ladle refining + round steel manufacturing + steel pipe forming: ladle refining time ≥ 50 min; casting superheat ≤ 25℃; round steel drawing speed ≤ 1.2 m / min; billet forming temperature ≤ 1150℃, to obtain a high-cleanliness drill pipe billet.

[0037] S2. Connection structure processing: Hot extrusion is performed on one end of the first drill rod unit to keep the outer diameter of the end constant, increase the inner wall thickness, and reduce the inner diameter to obtain a first thickened section. A mortise is machined on the first thickened section, and an interference fit is retained. Hot extrusion is performed on both ends of the second drill rod unit to keep the outer diameter of the end constant and increase the inner wall thickness to obtain a second thickened section with the same thickness as the first thickened section. The mating surfaces of the tenon and the mortise are lubricated.

[0038] In one specific embodiment, a dedicated hydraulic forging press is used to thicken the end, and the hot extrusion temperature is 500℃~700℃.

[0039] As a preferred method, the mating surfaces of the tenon and mortise are carburized to increase the surface compressive stress of the mating surfaces and improve fatigue resistance.

[0040] As a preferred approach, a rounded transition is used between all thickened sections and the drill pipe unit base material (body).

[0041] S3. Heat treatment: The two drill pipe units were subjected to a first heat treatment at 210℃~350℃, and then air-cooled to room temperature; The two drill pipe units were subjected to a second heat treatment at 1050℃~1150℃, followed by a cooling treatment. The two drill pipe units were subjected to a third heating treatment at 210℃~350℃, and then air-cooled to room temperature.

[0042] As a preferred method, during the first heat treatment, the holding time is 50 min to 120 min; during the second heat treatment, the heating rate is 100℃ / h to 140℃ / h, and the holding time is 100 min to 210 min. The cooling treatment includes: transferring the drill rod unit from the heating furnace to cooling water at 10℃ to 35℃, cooling it to 210℃ before removing it from the water, and then air-cooling it to room temperature, wherein the transfer time is ≤35 s; during the third heat treatment, the holding time is 120 min to 230 min. In one specific embodiment, the first heating treatment can be performed using an electromagnetic induction coil or a box furnace; the second heating treatment can be performed using a box furnace; water cooling can be performed using an internal spray + external spray method or a water tank immersion method; and the drill rod can be conveyed from the furnace to the water using a roller conveyor or a transverse conveyor chain.

[0043] S4. Assembly: The first drill pipe unit is heated to expand and deform it, and the mortise is inserted into the tenon of the second drill pipe unit. After cooling, a seamless mortise and tenon connection is achieved.

[0044] As a preferred method, the heating head can be preheated using electromagnetic induction at a temperature of 100℃ to 300℃. The heating head is then used to heat the first drill rod unit, causing it to expand and deform. In one specific embodiment, when heating the first drill rod unit, the tail end is first fixed, and heating begins from the head end to expand and deform the mortise. Then, it is inserted into the tenon of the second drill rod unit, and the inserted end is fixed again. The tail end continues to expand and deform until the entire first drill rod unit has expanded and deformed, ensuring the uniformity of its thermal expansion.

[0045] In one specific embodiment, the tenon-and-mortise connection between the two drill rod units is completed by hydraulic transmission.

[0046] The following specific embodiments further illustrate the fatigue-resistant high-strength oil drill pipe and its preparation method.

[0047] Example 1

[0048] The fatigue-resistant high-strength oil drill pipe of this embodiment comprises the following components by mass percentage: C: 0.43%, Mn: 15%, Cr: 2.3%, Mo: 0.14%, V: 0.17%, B: 0.0021%, and the balance being iron and unavoidable impurities; wherein, (Cr+Mo+V) / (C+Mn) = 0.17; The method for preparing fatigue-resistant high-strength oil drill pipe in this embodiment includes the following steps: S1. Drill rod unit manufacturing: According to the above-mentioned material composition, and according to the set dimensions, drill rod blanks are manufactured using a converter or electric furnace + ladle refining + round steel manufacturing + steel pipe forming: ladle refining time 55min; casting superheat controlled at 25℃; round steel drawing speed 1.15m / min; drill rod blank forming temperature 1100℃, resulting in multiple high-cleanliness drill rod blanks with different sizes; corresponding drill rod units are manufactured using drill rod blanks of different sizes. S2. Machining of drill pipe unit connection structure: A drill rod unit with a larger outer diameter is selected as the first drill rod unit. One end of the first drill rod unit is hot-extruded to increase the inner wall thickness and reduce the inner diameter while keeping the outer diameter unchanged, thus obtaining the first thickened section. The first thickened section is then mortised with an interference fit of 0.16 mm. A drill rod unit with a smaller outer diameter is selected as the second drill rod unit. Both ends of the second drill rod unit are hot-extruded to increase the wall thickness at both ends while keeping the outer diameter unchanged, thus obtaining the second thickened section with the same thickness as the first thickened section. The second thickened section is then tenoned. All tenons and mortises are lubricated. The hot extrusion temperature is 550℃.

[0049] S3. Heat Treatment: The two drill pipe units are first heated at 230℃ for 60 minutes and then air-cooled to room temperature. The second heating is carried out at 1080℃ at a heating rate of 115℃ / h and a holding time of 100 minutes. Then, they are water-cooled at 17℃. The transfer time from the drill pipe to the water is 35 seconds. The drill pipe is water-cooled to 210℃ and then air-cooled to room temperature. Finally, the third heating is carried out at 250℃ for 210 minutes and then air-cooled to room temperature.

[0050] S4. Drill pipe assembly: The first drill pipe unit is heated to expand and deform it, and the mortise is inserted into the tenon of the second drill pipe unit. After cooling, a seamless mortise and tenon connection is achieved.

[0051] The drill pipe product prepared in this embodiment has a yield strength of 880 MPa at room temperature, a tensile strength of 970 MPa at room temperature, and an impact energy of 64 J at -40℃. Its ultimate tensile and compressive fatigue strength after 10 million cycles is 368 MPa.

[0052] Referring to the scheme of Example 1, Examples 2 to 16 are set up.

[0053] Example 2:

[0054] The differences between this embodiment and Example 1 are as follows, provided that all other parameters and process steps are the same as in Example 1: (1) In terms of component ratio: C: 0.44%, Mn: 12%, Cr: 2.5%, Mo: 0.16%, V: 0.19%, B: 0.001%, of which (Cr+Mo+V) / (C+Mn)=0.23.

[0055] (2) When manufacturing the drill rod unit, the ladle refining time is 57 min; the casting superheat is controlled at 23℃; the round steel pulling speed is 1.2 m / min; and the drill rod billet forming temperature is 1050℃.

[0056] (3) When processing the drill rod unit connection structure, the hot extrusion temperature is 630℃ and the interference is reserved 0.21mm.

[0057] (4) During heat treatment, the first heat treatment temperature is 280℃ and the holding time is 95min; the second heat treatment temperature is 1105℃, the heating rate is 120℃ / h, the holding time is 120min, the cooling water temperature is 21℃, and the transfer time from the drill rod leaving the furnace to entering the water is 30s; the third heat treatment temperature is 310℃ and the holding time is 120min.

[0058] The drill pipe product prepared in this embodiment has a yield strength of 835 MPa at room temperature, a tensile strength of 923 MPa at room temperature, and an impact energy of 95 J at -40℃. Its fatigue strength after 10 million tensile and compressive cycles is 355 MPa.

[0059] Example 3

[0060] The differences between this embodiment and Example 1 are as follows, provided that all other parameters and process steps are the same as in Example 1: (1) In terms of component ratio: C: 0.45%, Mn: 13%, Cr: 2.1%, Mo: 0.12%, V: 0.16%, B: 0.0025%, of which (Cr+Mo+V) / (C+Mn)=0.18.

[0061] (2) When manufacturing the drill rod unit, the ladle refining time is 50 min; the casting superheat is controlled at 24℃; the round steel pulling speed is 1.1 m / min; and the drill rod billet forming temperature is 1150℃.

[0062] (3) When processing the drill rod unit connection structure, the hot extrusion temperature is 500℃ and the interference is reserved 0.23mm.

[0063] (4) During heat treatment, the first heat treatment temperature is 260℃ and the holding time is 70min; the second heat treatment temperature is 1050℃, the heating rate is 125℃ / h, the holding time is 140min, the cooling water temperature is 10℃, and the transfer time from the drill rod leaving the furnace to entering the water is 25s; the third heat treatment temperature is 350℃ and the holding time is 160min.

[0064] The drill pipe product prepared in this embodiment has a yield strength of 750 MPa at room temperature, a tensile strength of 880 MPa at room temperature, and an impact energy of 129 J at -40℃. Its fatigue strength after 10 million tensile and compressive cycles is 365 MPa.

[0065] Example 4 The differences between this embodiment and Example 1 are as follows, provided that all other parameters and process steps are the same as in Example 1: (1) In terms of component ratio: C: 0.46%, Mn: 14%, Cr: 2.46%, Mo: 0.15%, V: 0.18%, B: 0.0028%, of which (Cr+Mo+V) / (C+Mn)=0.19.

[0066] (2) When manufacturing the drill rod unit, the ladle refining time is 59 min; the casting superheat is controlled at 22℃; the round steel pulling speed is 1.13 m / min; and the drill rod billet forming temperature is 1000℃.

[0067] (3) When processing the drill rod unit connection structure, the hot extrusion temperature is 570℃ and the interference is reserved 0.19mm.

[0068] (4) During heat treatment, the first heat treatment temperature is 210℃ and the holding time is 106min; the second heat treatment temperature is 1130℃, the heating rate is 104℃ / h, the holding time is 160min, the cooling water temperature is 30℃, and the transfer time from the drill rod leaving the furnace to entering the water is 21s; the third heat treatment temperature is 330℃ and the holding time is 140min.

[0069] The drill pipe product prepared in this embodiment has a yield strength of 811 MPa at room temperature, a tensile strength of 905 MPa at room temperature, and an impact energy of 107 J at -40℃. Its fatigue strength after 10 million tensile and compressive cycles is 370 MPa.

[0070] Example 5 The differences between this embodiment and Example 1 are as follows, provided that all other parameters and process steps are the same as in Example 1: (1) In terms of component ratio: C: 0.43%, Mn: 12.5%, Cr: 2.28%, Mo: 0.13%, V: 0.19%, B: 0.0012%, of which (Cr+Mo+V) / (C+Mn)=0.20.

[0071] (2) When manufacturing the drill rod unit, the ladle refining time is 58 min; the casting superheat is controlled at 23℃; the round steel pulling speed is 1.06 m / min; and the drill rod billet forming temperature is 1130℃.

[0072] (3) When processing the drill rod unit connection structure, the hot extrusion temperature is 650℃ and the interference is reserved 0.15mm.

[0073] (4) During heat treatment, the first heat treatment temperature is 250℃ and the holding time is 50min; the second heat treatment temperature is 1150℃, the heating rate is 100℃ / h, the holding time is 190min, the cooling water temperature is 12℃, and the transfer time from the drill rod leaving the furnace to entering the water is 32s; the third heat treatment temperature is 210℃ and the holding time is 190min.

[0074] The drill pipe product prepared in this embodiment has a yield strength of 937 MPa at room temperature, a tensile strength of 1038 MPa at room temperature, and an impact energy of 35 J at -40℃. Its fatigue strength after 10 million tensile and compressive cycles is 350 MPa.

[0075] Example 6 The differences between this embodiment and Example 1 are as follows, provided that all other parameters and process steps are the same as in Example 1: (1) In terms of component ratio: C: 0.45%, Mn: 16%, Cr: 2.17%, Mo: 0.14%, V: 0.16%, B: 0.0023%, of which (Cr+Mo+V) / (C+Mn)=0.15.

[0076] (2) When manufacturing the drill rod unit, the ladle refining time is 56 min; the casting superheat is controlled at 21℃; the round steel pulling speed is 1.02 m / min; and the drill rod billet forming temperature is 1110℃.

[0077] (3) When processing the drill rod unit connection structure, the hot extrusion temperature is 520℃ and the interference is reserved 0.17mm.

[0078] (4) During heat treatment, the first heat treatment temperature is 320℃ and the holding time is 100min; the second heat treatment temperature is 1070℃, the heating rate is 118℃ / h, the holding time is 170min, the cooling water temperature is 35℃, and the transfer time from the drill rod leaving the furnace to entering the water is 23s; the third heat treatment temperature is 340℃ and the holding time is 215min.

[0079] The drill pipe product prepared in this embodiment has a yield strength of 858 MPa at room temperature, a tensile strength of 943 MPa at room temperature, and an impact energy of 80 J at -40℃. Its fatigue limit strength after 10 million tensile and compressive cycles is 367 MPa.

[0080] Example 7 The differences between this embodiment and Example 1 are as follows, provided that all other parameters and process steps are the same as in Example 1: (1) In terms of component ratio: C: 0.44%, Mn: 12.1%, Cr: 2.48%, Mo: 0.16%, V: 0.18%, B: 0.0026%, of which (Cr+Mo+V) / (C+Mn)=0.22.

[0081] (2) When manufacturing the drill rod unit, the ladle refining time is 60 min; the casting superheat is controlled at 20℃; the round steel pulling speed is 1 m / min; and the drill rod billet forming temperature is 1090℃.

[0082] (3) When processing the drill rod unit connection structure, the hot extrusion temperature is 610℃ and the interference is reserved 0.25mm.

[0083] (4) During heat treatment, the first heat treatment temperature is 300℃ and the holding time is 115min; the second heat treatment temperature is 1090℃, the heating rate is 133℃ / h, the holding time is 210min, the cooling water temperature is 26℃, and the transfer time from the drill rod leaving the furnace to entering the water is 26s; the third heat treatment temperature is 290℃ and the holding time is 230min.

[0084] The drill pipe product prepared in this embodiment has a yield strength of 779 MPa at room temperature, a tensile strength of 890 MPa at room temperature, and an impact energy of 122 J at -40℃. Its fatigue strength after 10 million tensile and compressive cycles is 350 MPa.

[0085] Example 8 The differences between this embodiment and Example 1 are as follows, provided that all other parameters and process steps are the same as in Example 1: (1) In terms of component ratio: C: 0.46%, Mn: 13.1%, Cr: 2.19%, Mo: 0.16%, V: 0.17%, B: 0.0019%, of which (Cr+Mo+V) / (C+Mn)=0.19.

[0086] (2) When manufacturing the drill rod unit, the ladle refining time is 52 min; the casting superheat is controlled at 22℃; the round steel pulling speed is 0.98 m / min; and the drill rod billet forming temperature is 1070℃.

[0087] (3) When processing the drill rod unit connection structure, the hot extrusion temperature is 700℃ and the interference is reserved 0.2mm.

[0088] (4) During heat treatment, the first heat treatment temperature is 340℃ and the holding time is 80min; the second heat treatment temperature is 1060℃, the heating rate is 140℃ / h, the holding time is 205min, the cooling water temperature is 14℃, and the transfer time from the drill rod leaving the furnace to entering the water is 35s; the third heat treatment temperature is 270℃ and the holding time is 145min.

[0089] The drill pipe product prepared in this embodiment has a yield strength of 870 MPa at room temperature, a tensile strength of 962 MPa at room temperature, and an impact energy of 68 J at -40℃. Its fatigue strength after 10 million tensile and compressive cycles is 353 MPa.

[0090] Example 9 The differences between this embodiment and Example 1 are as follows, provided that all other parameters and process steps are the same as in Example 1: (1) In terms of component ratio: C: 0.43%, Mn: 12%, Cr: 2.5%, Mo: 0.16%, V: 0.19%, B: 0.0014%, of which (Cr+Mo+V) / (C+Mn)=0.23.

[0091] (2) When manufacturing the drill rod unit, the ladle refining time is 54 min; the casting superheat is controlled at 19℃; the round steel pulling speed is 0.95 m / min; and the drill rod billet forming temperature is 1150℃.

[0092] (3) When processing the drill rod unit connection structure, the hot extrusion temperature is 540℃ and the interference is reserved 0.16mm.

[0093] (4) During heat treatment, the first heat treatment temperature is 350℃ and the holding time is 50min; the second heat treatment temperature is 1050℃, the heating rate is 130℃ / h, the holding time is 105min, the cooling water temperature is 19℃, and the transfer time from the drill rod leaving the furnace to entering the water is 33s; the third heat treatment temperature is 220℃ and the holding time is 225min.

[0094] The drill pipe product prepared in this embodiment has a yield strength of 925 MPa at room temperature, a tensile strength of 1025 MPa at room temperature, an impact energy of 39 J at -40℃, and a fatigue limit of 380 MPa after 10 million tensile and compressive cycles.

[0095] Example 10 The differences between this embodiment and Example 1 are as follows, provided that all other parameters and process steps are the same as in Example 1: (1) In terms of component ratio: C: 0.45%, Mn: 12.7%, Cr: 2.43%, Mo: 0.13%, V: 0.18%, B: 0.0027%, of which (Cr+Mo+V) / (C+Mn)=0.21.

[0096] (2) When manufacturing the drill rod unit, the ladle refining time is 51 min; the casting superheat is controlled at 25℃; the round steel pulling speed is 1.12 m / min; and the drill rod billet forming temperature is 1140℃.

[0097] (3) When processing the drill rod unit connection structure, the hot extrusion temperature is 690℃ and the interference is reserved 0.22mm.

[0098] (4) During heat treatment, the first heat treatment temperature is 220℃ and the holding time is 85min; the second heat treatment temperature is 1070℃, the heating rate is 127℃ / h, the holding time is 110min, the cooling water temperature is 23℃, and the transfer time from the drill rod leaving the furnace to entering the water is 31s; the third heat treatment temperature is 320℃ and the holding time is 180min.

[0099] The drill pipe product prepared in this embodiment has a yield strength of 826 MPa at room temperature, a tensile strength of 917 MPa at room temperature, and an impact energy of 100 J at -40℃. Its fatigue strength after 10 million tensile and compressive cycles is 381 MPa.

[0100] Example 11 The differences between this embodiment and Example 1 are as follows, provided that all other parameters and process steps are the same as in Example 1: (1) In terms of component ratio: C: 0.44%, Mn: 13.8%, Cr: 2.32%, Mo: 0.15%, V: 0.16%, B: 0.0016%, of which (Cr+Mo+V) / (C+Mn)=0.18.

[0101] (2) When manufacturing the drill rod unit, the ladle refining time is 53 min; the casting superheat is controlled at 23℃; the round steel pulling speed is 1.14 m / min; and the drill rod billet forming temperature is 1120℃.

[0102] (3) When processing the drill rod unit connection structure, the hot extrusion temperature is 530℃ and the interference is reserved 0.18mm.

[0103] (4) During heat treatment, the first heat treatment temperature is 240℃ and the holding time is 90min; the second heat treatment temperature is 1150℃, the heating rate is 109℃ / h, the holding time is 115min, the cooling water temperature is 25℃, and the transfer time from the drill rod leaving the furnace to entering the water is 29s; the third heat treatment temperature is 230℃ and the holding time is 320min.

[0104] The drill pipe product prepared in this embodiment has a yield strength of 896 MPa at room temperature, a tensile strength of 991 MPa at room temperature, and an impact energy of 55 J at -40℃. Its fatigue strength after 10 million tensile and compressive cycles is 356 MPa.

[0105] Example 12 The differences between this embodiment and Example 1 are as follows, provided that all other parameters and process steps are the same as in Example 1: (1) In terms of component ratio: C: 0.46%, Mn: 14.5%, Cr: 2.24%, Mo: 0.14%, V: 0.17%, B: 0.002%, of which (Cr+Mo+V) / (C+Mn)=0.17.

[0106] (2) When manufacturing the drill rod unit, the ladle refining time is 61 min; the casting superheat is controlled at 20℃; the round steel pulling speed is 1.17 m / min; and the drill rod billet forming temperature is 1080℃.

[0107] (3) When processing the drill rod unit connection structure, the hot extrusion temperature is 670℃ and the interference is reserved 0.24mm.

[0108] (4) During heat treatment, the first heat treatment temperature is 270℃ and the holding time is 120min; the second heat treatment temperature is 1110℃, the heating rate is 113℃ / h, the holding time is 130min, the cooling water temperature is 28℃, and the transfer time from the drill rod leaving the furnace to entering the water is 34s; the third heat treatment temperature is 240℃ and the holding time is 170min.

[0109] The drill pipe product prepared in this embodiment has a yield strength of 903 MPa at room temperature, a tensile strength of 1004 MPa at room temperature, and an impact energy of 48 J at -40℃. Its fatigue strength after 10 million tensile and compressive cycles is 361 MPa.

[0110] Example 13 The differences between this embodiment and Example 1 are as follows, provided that all other parameters and process steps are the same as in Example 1: (1) In terms of component ratio: C: 0.43%, Mn: 16%, Cr: 2.1%, Mo: 0.12%, V: 0.18%, B: 0.003%, of which (Cr+Mo+V) / (C+Mn)=0.15.

[0111] (2) When manufacturing the drill rod unit, the ladle refining time is 50 min; the casting superheat is controlled at 24℃; the round steel pulling speed is 1.19 m / min; and the drill rod billet forming temperature is 1060℃.

[0112] (3) When processing the drill rod unit connection structure, the hot extrusion temperature is 510℃ and the interference is reserved 0.19mm.

[0113] (4) During heat treatment, the first heat treatment temperature is 215℃ and the holding time is 55min; the second heat treatment temperature is 1055℃, the heating rate is 105℃ / h, the holding time is 150min, the cooling water temperature is 32℃, and the transfer time from the drill rod leaving the furnace to entering the water is 28s; the third heat treatment temperature is 250℃ and the holding time is 120min.

[0114] The drill pipe product prepared in this embodiment has a yield strength of 889 MPa at room temperature, a tensile strength of 980 MPa at room temperature, and an impact energy of 59 J at -40℃. Its fatigue strength after 10 million tensile and compressive cycles is 374 MPa.

[0115] Example 14 The differences between this embodiment and Example 1 are as follows, provided that all other parameters and process steps are the same as in Example 1: (1) In terms of component ratio: C: 0.45%, Mn: 14%, Cr: 2.3%, Mo: 0.15%, V: 0.19%, B: 0.001%, of which (Cr+Mo+V) / (C+Mn)=0.18.

[0116] (2) When manufacturing the drill rod unit, the ladle refining time is 54 min; the casting superheat is controlled at 23℃; the round steel pulling speed is 1.18 m / min; and the drill rod billet forming temperature is 1040℃.

[0117] (3) When processing the drill rod unit connection structure, the hot extrusion temperature is 680℃ and the interference is reserved 0.15mm.

[0118] (4) During heat treatment, the first heat treatment temperature is 335℃ and the holding time is 65min; the second heat treatment temperature is 1065℃, the heating rate is 122℃ / h, the holding time is 165min, the cooling water temperature is 34℃, and the transfer time from the drill rod leaving the furnace to entering the water is 27s; the third heat treatment temperature is 220℃ and the holding time is 130min.

[0119] The drill pipe product prepared in this embodiment has a yield strength of 914 MPa at room temperature, a tensile strength of 1016 MPa at room temperature, and an impact energy of 41 J at -40℃. Its ultimate tensile and compressive fatigue strength after 10 million cycles is 377 MPa.

[0120] Example 15 The differences between this embodiment and Example 1 are as follows, provided that all other parameters and process steps are the same as in Example 1: (1) In terms of component ratio: C: 0.44%, Mn: 15%, Cr: 2.5%, Mo: 0.13%, V: 0.16%, B: 0.002%, of which (Cr+Mo+V) / (C+Mn)=0.18.

[0121] (2) When manufacturing the drill rod unit, the ladle refining time is 58 min; the casting superheat is controlled at 22℃; the round steel pulling speed is 1.16 m / min; and the drill rod billet forming temperature is 1030℃.

[0122] (3) When processing the drill rod unit connection structure, the hot extrusion temperature is 530℃ and the interference is reserved 0.23mm.

[0123] (4) During heat treatment, the first heat treatment temperature is 225℃ and the holding time is 75min; the second heat treatment temperature is 1075℃, the heating rate is 136℃ / h, the holding time is 175min, the cooling water temperature is 10℃, and the transfer time from the drill rod leaving the furnace to entering the water is 24s; the third heat treatment temperature is 300℃ and the holding time is 150min.

[0124] The drill pipe product prepared in this embodiment has a yield strength of 841 MPa at room temperature, a tensile strength of 930 MPa at room temperature, and an impact energy of 89 J at -40℃. Its fatigue strength after 10 million tensile and compressive cycles is 372 MPa.

[0125] Example 16 The differences between this embodiment and Example 1 are as follows, provided that all other parameters and process steps are the same as in Example 1: (1) In terms of component ratio: C: 0.46%, Mn: 13%, Cr: 2.2%, Mo: 0.16%, V: 0.17%, B: 0.0017%, of which (Cr+Mo+V) / (C+Mn)=0.19.

[0126] (2) When manufacturing the drill rod unit, the ladle refining time is 62 min; the casting superheat is controlled at 21℃; the round steel pulling speed is 1.11 m / min; and the drill rod billet forming temperature is 1020℃.

[0127] (3) When processing the drill rod unit connection structure, the hot extrusion temperature is 660℃ and the interference is 0.2mm.

[0128] (4) During heat treatment, the first heat treatment temperature is 315℃ and the holding time is 110min; the second heat treatment temperature is 1085℃, the heating rate is 138℃ / h, the holding time is 180min, the cooling water temperature is 35℃, and the transfer time from the drill rod leaving the furnace to entering the water is 26s; the third heat treatment temperature is 300℃ and the holding time is 200min.

[0129] The drill pipe product prepared in this embodiment has a yield strength of 862 MPa at room temperature, a tensile strength of 959 MPa at room temperature, and an impact energy of 73 J at -40℃. Its fatigue strength after 10 million tensile and compressive cycles is 379 MPa.

[0130] Comparative Example This comparative example provides a conventional method for manufacturing high-strength oil drill pipe, including: The raw materials were prepared according to the following mass percentage composition: C: 0.26%, Si: 0.22%, Mn: 0.98%, P: 0.01%, S: 0.003%, Cr: 1.09%, Ni: 1.11%, Mo: 0.25%, Al: 0.012%. Processing technology: Using the above raw materials, the comparative drill pipe products are obtained through steelmaking, continuous casting, piercing, hot rolling, primary heat treatment, pipe end thickening, full pipe body tempering heat treatment, drill pipe friction welding and heat treatment.

[0131] The piercing and hot rolling processes include: heating the continuously cast billet obtained from continuous casting in an annular heating furnace at a temperature of 1180℃~1230℃ for 100~120 minutes, hot piercing it at a temperature of 1150℃~1200℃, hot rolling it at a temperature of 960℃~1170℃, and then sequentially performing sizing, straightening, cooling, and sawing.

[0132] The initial heat treatment includes: quenching at a heating temperature of 890℃~910℃, holding time of 40~60 minutes, internal and external water quenching, and cooling rate of 20℃ / s~30℃ / s; high-temperature tempering at a temperature of 630℃~680℃, tempering time of 90~120 minutes; water cooling after high-temperature tempering; followed by sizing and hot straightening at a temperature of 570℃~600℃, and then water cooling.

[0133] The entire pipe body undergoes a quenching and tempering heat treatment process, which includes: quenching at a heating temperature of 880℃~900℃, holding for 40~60 minutes, internal and external water spray quenching, and a cooling rate of 20℃ / s~30℃ / s; the high-temperature tempering temperature is controlled at 620℃~680℃, and the tempering time is 90~120 minutes; followed by water cooling after high-temperature tempering.

[0134] The friction welding and heat treatment of the drill pipe includes: using inertial friction welding; immediately after friction welding upsetting, spraying quenching liquid on the outer surface of the weld area and compressed air on the inner surface for quenching, then tempering by medium-frequency induction heating to 680℃~700℃; and finally processing the flash and burrs inside and outside the weld area.

[0135] The comparative drill pipe product has a tensile and compressive fatigue limit of 30MPa to 170MPa after 10 million cycles, a yield strength of 655MPa to 1074MPa at room temperature, a tensile strength of 720MPa to 1143MPa at room temperature, a uniform deformation rate of 5% to 9%, and an impact energy (CVN) of 20J to 88J at -40℃ for the weld.

[0136] Table 1 below compares the performance of the oil drill pipes prepared in the embodiments of the present invention with those prepared in the comparative examples.

[0137] Table 1. Comparison of the performance of oil drill pipe produced by the process of this invention and conventional processes.

[0138] Comparative analysis shows that the oil drill pipe prepared by this invention employs a novel composition and process design, resulting in excellent overall fatigue resistance. The drill pipe joint exhibits a tensile-compressive fatigue limit strength of ≥350 MPa after 10 million cycles, a yield strength at room temperature ≥750 MPa, a tensile strength at room temperature ≥880 MPa, a uniform deformation rate (Agt) ≥35%, and an impact energy (CVN) at -40℃ ≥35 J. Compared to high-strength oil drill pipe products manufactured using conventional processes, the drill pipe product of this invention has a higher uniform deformation rate, achieves a more uniform stress distribution during expansion and deformation of the tenon and mortise joints, and demonstrates superior fatigue limit strength, significantly improving and extending the service life of high-strength oil drill pipes.

[0139] It will be understood by those skilled in the art 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 foregoing embodiments, those skilled in the art can still modify the technical solutions described in the foregoing 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 fatigue-resistant, high-strength oil drill pipe, characterized in that, The fatigue-resistant high-strength oil drill pipe is made of the following components by mass percentage: C：0.43%～0.46%， Mn: 12%~16%, Cr：2.1%～2.5%， Mo: 0.12%~0.16%, V：0.16%～0.19%， B：0.001%～0.003%， And iron and unavoidable impurities; Among them, 0.15 < (Cr + Mo + V) / (C + Mn) < 0.23; The fatigue-resistant high-strength oil drill pipe structure includes a first drill pipe unit and a second drill pipe unit, wherein the first drill pipe unit and the second drill pipe unit are interference-fitted. One end of the first drill pipe unit is provided with a first thickened section, on which a mortise is provided; The second drill rod unit has a second thickened section at at least one end, on which a tenon is provided that is interference fit with the mortise; The thickness of the first and second thickened sections is the same.

2. The fatigue-resistant high-strength oil drill pipe according to claim 1, characterized in that, The tenon includes an outwardly protruding structure extending axially from the end face of the second thickened section, the outwardly protruding structure being cylindrical or conical; the mortise includes a groove extending axially from the end face of the first thickened section that matches the outwardly protruding structure; The outer circumferential surface of the cylindrical or conical shape is provided with at least one positioning protrusion, and the inner circumferential surface of the groove is provided with a positioning groove that mates with the positioning protrusion.

3. The fatigue-resistant high-strength oil drill pipe according to claim 2, characterized in that, The ratio of the depth of the mortise to the length of the first thickened section is 1:2; the ratio of the length of the tenon to the length of the second thickened section is 1:

2.

4. The fatigue-resistant high-strength oil drill pipe according to claim 1, characterized in that, The interference fit is 0.15mm to 0.25mm.

5. The fatigue-resistant high-strength oil drill pipe according to claim 1, characterized in that, The outer diameter of the first thickened section is 127mm~152mm, and the inner diameter is 89mm~115mm; the outer diameter of the non-thickened section of the first drill pipe unit is 127mm~152mm, and the inner diameter is 118mm~143mm.

6. The fatigue-resistant high-strength oil drill pipe according to claim 1, characterized in that, The outer diameter of the second thickened section is 89mm~115mm, and the inner diameter is 52mm~77mm; the outer diameter of the non-thickened section of the second drill pipe unit is 89mm~115mm, and the inner diameter is 80mm~106mm.

7. A method for preparing fatigue-resistant high-strength oil drill pipe according to any one of claims 1 to 6, characterized in that, Includes the following steps: Raw materials are prepared according to the set component ratio, and two drill pipe blanks are prepared using the raw materials according to the set structural dimensions; the first drill pipe unit and the second drill pipe unit are respectively made using the two drill pipe blanks. The first drill rod unit is hot-extruded at one end to keep the outer diameter constant, increase the inner wall thickness, and reduce the inner diameter to obtain a first thickened section. A mortise is machined on the first thickened section, and an interference fit is retained. The second drill rod unit is hot-extruded at both ends to keep the outer diameter constant and increase the inner wall thickness to obtain a second thickened section with the same thickness as the first thickened section. The mating surfaces of the tenon and the mortise are lubricated. The two drill pipe units were subjected to a first heat treatment at 210℃~350℃, and then air-cooled to room temperature; The two drill pipe units were subjected to a second heat treatment at 1050℃~1150℃, followed by a cooling treatment. The two drill pipe units were subjected to a third heating treatment at 210℃~350℃, and then air-cooled to room temperature; The first drill pipe unit is heated to expand and deform it, and the mortise is inserted into the tenon of the second drill pipe unit. After cooling, a seamless mortise and tenon connection is achieved.

8. The preparation method according to claim 7, characterized in that, The hot extrusion temperature is 500℃~700℃.

9. The preparation method according to claim 7, characterized in that, During the first heat treatment, the holding time is 50 min to 120 min; During the second heating process, the heating rate is 100℃ / h~140℃ / h, the holding time is 100min~210min, and the cooling process includes: transferring the drill rod unit from the heating furnace to cooling water at 10℃~35℃, with a transfer time ≤35s, cooling to 210℃ and then air cooling to room temperature; During the third heating process, the holding time is 120 min to 230 min.

10. The preparation method according to claim 7, characterized in that, The heating temperature for causing the first drill pipe unit to expand and deform is 100℃~300℃.

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