Double-phase region strengthening and toughening heat treatment method of seamless steel pipe for high-strength and high-toughness drill rod

Abstract

The invention belongs to the technical field of metal material heat treatment, and particularly relates to a double-phase region strengthening and toughening heat treatment method of a seamless steel pipe for a high-toughness drill pipe, which comprises the following steps: heating the seamless steel pipe to a double-phase region temperature T1, preserving heat, and then quickly cooling; the temperature T1 of the two-phase region is within the range of 30-50 DEG C below the Ac3 temperature; the cooled seamless steel pipe is heated to the tempering temperature T2, heat preservation is conducted, and then air cooling is conducted to the room temperature; the tempering temperature T2 is in the range of 500 DEG C to 20 DEG C below the Ac1 temperature. A combined process of double-phase region quenching and high-temperature tempering is adopted, the heating temperature, the heat preservation time and the cooling mode of a double-phase region are accurately controlled, and a fine ferrite and tempered sorbite double-phase structure with a coordinated proportion is formed in the steel pipe, so that the impact toughness, the fatigue strength and the fracture toughness of the steel pipe are remarkably improved while the high strength is ensured.

Description

Technical Field

[0001] This invention belongs to the field of heat treatment technology for metallic materials, and specifically relates to a two-phase region strengthening and toughening heat treatment method for seamless steel pipes used in high-strength and high-toughness drill pipes. Background Technology

[0002] Drill pipe is a key component in oil and gas drilling. Downhole, it is subjected to complex alternating loads such as tension, compression, bending, torsion, and impact, while its inner wall is eroded by drilling fluid. Therefore, drill pipe materials are required to have high strength, high toughness, good fatigue resistance, and corrosion resistance.

[0003] Currently, seamless steel pipes for drill pipe (such as high-grade S135 steel) typically undergo quenching and tempering heat treatment (quenching + high-temperature tempering) to obtain a tempered sorbite structure, achieving a balance between strength and toughness. However, while pursuing higher strength, traditional quenching and tempering treatment often leads to a decrease in toughness, fatigue life, and resistance to crack propagation. This limits the application of drill pipes in deeper and more complex well conditions.

[0004] To improve the strength-toughness balance of materials, dual-phase heat treatment technology has been developed. Dual-phase heat treatment is a process involving heating and cooling between Ac1 and Ac3 temperatures to obtain a dual-phase microstructure consisting of ferrite and austenite (which transforms into martensite or other structures upon cooling). For example, Chinese patent document CN101020950A discloses a heat treatment process for thin-walled low-carbon steel parts. This process involves heating and quenching thin-walled low-carbon steel parts (wall thickness less than 3 mm) with a carbon content of less than 0.25% in the Ac1-Ac3 dual-phase region to obtain a mixed microstructure of martensite and ferrite, followed by low-temperature tempering below 300°C to improve the room-temperature strength and toughness of the parts.

[0005] However, existing dual-phase heat treatment processes are primarily designed for general-purpose mechanical parts made of low-carbon steel and with thin-walled sections. These processes typically employ low to medium tempering temperatures (≤300℃) to maintain the high hardness of quenched martensite while moderately improving toughness by introducing soft ferrite. Their performance targets and load-bearing conditions are relatively mild. This process is ill-suited to meeting the extremely demanding manufacturing requirements of seamless steel pipes for low-alloy, medium-thick-walled drill pipes.

[0006] Therefore, how to develop a two-phase heat treatment method that can achieve the synergy of ultra-high strength and ultra-high toughness at low temperature for seamless steel pipes used in low-alloy, medium-thick-walled drill pipes is a technical problem that needs to be solved in this field. Summary of the Invention

[0007] To address the shortcomings of existing technologies, this invention provides a dual-phase heat treatment method for seamless steel pipes used in high-strength and high-toughness drill pipes. By employing a combination of dual-phase quenching and high-temperature tempering, and precisely controlling the heating temperature, holding time, and cooling method in the dual-phase region, a fine and proportionally coordinated dual-phase structure of ferrite and tempered sorbite is ultimately formed in the steel pipe. This significantly improves the low-temperature impact toughness, fatigue strength, and fracture toughness of the steel pipe while ensuring high strength.

[0008] The technical solution adopted by this invention to solve the technical problem is: This invention provides a two-phase region strengthening and toughening heat treatment method for seamless steel pipes used in high-strength and toughness drill pipes. The chemical composition of the seamless steel pipe, by weight percentage, includes: C: 0.20%-0.30%. Si: 0.15%-0.35%, Mn: 0.40%-0.70%, Cr: 0.70%-1.20%, P: ≤0.010%, S: ≤0.001%, Mo: 0.70%-1.20%, Ni: ≤0.60%, V: 0.15%-0.30%, Al: 0.005%-0.040%, O: ≤0.0020%; N: ≤0.0050%; H: ≤0.00015%; Pb: ≤0.0150%; Sb: ≤0.0150%; As: ≤0.0150%; Bi: ≤0.0150%; Sn: ≤0.0150%, with the remainder being Fe and unavoidable impurities, including the following steps: (1) Dual-phase quenching: The seamless steel pipe is heated to the dual-phase temperature T1 and held at that temperature, and then rapidly cooled; the dual-phase temperature T1 is 30℃-50℃ below the Ac3 temperature of the steel. (2) High-temperature tempering: The seamless steel pipe cooled in step (1) is heated to the tempering temperature T2 and kept at that temperature, and then air-cooled to room temperature; the tempering temperature T2 is in the range of 500℃ to 20℃ below the Ac1 temperature.

[0009] Preferably, the tempering temperature T2 is 500℃-650℃.

[0010] Preferably, the rapid cooling is achieved by water quenching.

[0011] Preferably, the heat preservation time in step (1) is determined according to the wall thickness of the steel pipe, and the heat preservation time t (minutes) satisfies: t = (1.2 - 1.8) × D, where D is the wall thickness of the steel pipe in millimeters.

[0012] Preferably, the Ac1 and Ac3 temperatures of the seamless steel pipe are obtained by experimental determination using a thermal expansion meter.

[0013] Another aspect of the present invention provides a high-strength and high-toughness seamless steel pipe for drill pipe prepared by the heat treatment method described above, wherein the metallographic structure is mainly a dual-phase structure of fine ferrite and tempered sorbite; the seamless steel pipe has a yield strength of 1140-1275 MPa and a tensile strength of 1200-1413 MPa at room temperature, and a Charpy V-notch impact energy of ≥100 J at -20℃.

[0014] Preferably, the wall thickness of the seamless steel pipe is greater than 5 mm.

[0015] The advantages and positive effects of this invention are: (1) Excellent strength and toughness matching: By synergistically controlling the quenching temperature and tempering temperature in the dual-phase region, a dual-phase structure of fine and uniform tempered sorbite and polygonal ferrite was obtained. The tempered sorbite provides high strength, while the soft and tough ferrite phase can effectively blunt the crack tip and absorb impact energy, thereby significantly improving the impact toughness (Charpy V-type impact energy at -20℃ ≥100J) while maintaining high strength (yield strength ≥1140MPa).

[0016] (2) High fracture toughness and fatigue performance: The dual-phase structure, especially the soft phase ferrite, can hinder the crack propagation path, causing the crack to deflect and bifurcate during the propagation process, consuming more energy, thereby greatly improving the fracture toughness and fatigue life of the material.

[0017] (3) The process window is clear and easy to industrialize: the heating temperature and tempering temperature of the two-phase region are clearly defined, the process is stable and reproducible, and it is easy to implement on existing heat treatment production lines, making it suitable for large-scale industrial production. Attached Figure Description

[0018] Figure 1 Here is a metallographic diagram of the seamless steel pipe in Example 1; Figure 2 The image shows the metallographic structure of the seamless steel pipe in Comparative Example 1. Detailed Implementation

[0019] The present invention will be further described in detail below through specific embodiments. The following embodiments are merely descriptive and not limiting, and should not be used to limit the scope of protection of the present invention.

[0020] Example 1 A high-strength and high-toughness seamless steel pipe for drill pipe has the following chemical composition (wt%): C: 0.25%, Si: 0.35%, Mn: 0.50%, Cr: 1.10%, P: ≤0.010%, S: ≤0.001%, Mo: 0.80%, Ni: 0.30%, V: 0.30%, Al: 0.005%, O: ≤0.0020%, N: ≤0.0050%, H: ≤0.00015%, Pb: ≤0.0150%, Sb: ≤0.0150%, As: ≤0.0150%, Bi: ≤0.0150%, Sn: ≤0.0150%, with Fe as the balance. Taking a 165ksi grade seamless steel pipe for drill pipe (outer diameter 139.7mm, wall thickness 10.54mm) as an example, its Ac1 is calculated to be approximately 758℃, and Ac3 approximately 895℃.

[0021] The steps of the two-phase heat treatment method are as follows: (1) Two-phase quenching: Heat the seamless steel pipe to 860℃ and hold for 25 minutes (wall thickness 10.54mm×1.8 ≈18.972 minutes, rounded to the industry common value of 25 minutes), and then water quench it; (2) High-temperature tempering: Heat the steel pipe cooled in step (1) to 650°C for tempering, keep it at that temperature for 60 minutes, and then air-cool it to room temperature.

[0022] Example 2 A high-strength and high-toughness seamless steel pipe for drill pipes, wherein the chemical composition (wt%) of the seamless steel pipe is the same as that in Example 1.

[0023] The steps of the two-phase heat treatment method are as follows: (1) Two-phase quenching: Heat the seamless steel pipe to 855℃ (40℃ below Ac3), hold for 25 minutes (wall thickness 10.54mm×1.8 ≈18.972 minutes, rounded to the industry common value of 25 minutes), and then water quench; (2) High-temperature tempering: Heat the steel pipe cooled in step (1) to 550°C for tempering, keep it at that temperature for 60 minutes, and then air-cool it to room temperature.

[0024] Example 3 A high-strength and high-toughness seamless steel pipe for drill pipes, wherein the chemical composition (wt%) of the seamless steel pipe is the same as that in Example 1.

[0025] The steps of the two-phase heat treatment method are as follows: (1) Two-phase quenching: Heat the seamless steel pipe to 850℃ (45℃ below Ac3), hold for 25 minutes (wall thickness 10.54mm×1.8 ≈18.972 minutes, rounded to the industry common value of 25 minutes), and then water quench; (2) High-temperature tempering: Heat the steel pipe cooled in step (1) to 500°C for tempering, keep it at that temperature for 60 minutes, and then air-cool it to room temperature.

[0026] Comparative Example 1 The same seamless steel pipes as in Example 1 were used, and the same batch of steel pipes were treated with a traditional quenching and tempering process. (1) Quenching: Heat the seamless steel pipe to 920℃ (the temperature of complete austenitization), hold for 25 minutes, and then quench in water.

[0027] (2) Tempering: Heat the seamless steel pipe cooled in step (1) to 650°C, hold for 60 minutes, and then air cool.

[0028] Comparative Example 2 The only difference from Example 1 is that in step (1), the seamless steel pipe is heated to 770°C.

[0029] Comparative Example 3 The only difference from Example 1 is that in step (1), the seamless steel pipe is heated to 880°C.

[0030] Comparative Example 4 The only difference from Example 1 is that in step (2), the steel pipe cooled in step (1) is heated to 400°C, kept at that temperature for 60 minutes, and then taken out of the furnace and air-cooled to room temperature.

[0031] Comparative Example 5 The only difference from Example 1 is that in step (2), the steel pipe cooled in step (1) is heated to 680°C, kept at that temperature for 60 minutes, and then taken out of the furnace and air-cooled to room temperature.

[0032] The performance standards of 165ksi seamless steel pipes are shown in Table 1, and the performance of seamless steel pipes after the examples and comparative treatments are shown in Table 2.

[0033] Table 1 Performance Standards for 165ksi Seamless Steel Pipes

[0034] Table 2 Performance test results of seamless steel pipes in the examples and comparative examples

[0035] Combining the data in Tables 1 and 2, it can be seen that the steel pipe treated with the method of the present invention, while fully meeting the strength requirements of 165ksi steel grade, exhibits significantly better impact toughness than steel pipes treated with traditional quenching and tempering processes, achieving the goal of high strength and toughness. The metallographic structure of the seamless steel pipe prepared in Example 1 is shown below. Figure 1 As shown, the microstructure is tempered sorbite + ferrite.

[0036] Comparative Example 1: Seamless steel pipes from the same batch were treated using traditional quenching and tempering processes. The metallographic structure of the seamless steel pipes is shown in the figure below. Figure 2 As shown, the microstructure is tempered sorbite.

[0037] Furthermore, Comparative Examples 2 and 3 changed the heating temperature of the two-phase region to 770℃ and 880℃, respectively. These temperatures were outside the preferred range, and the quenching temperatures were too low. Although the temperature of 770℃ was higher than Ac1, the microstructure after quenching was martensite + a large amount of undissolved ferrite. The soft ferrite phase severely reduced the strength. Quenching at 880℃ was close to the Ac3 temperature, but still below Ac3. The quenched material was close to complete austenitization, and the quenched microstructure should be a coarse-grained microstructure after complete austenitization. The coarse austenite grains led to a decrease in tempering strength.

[0038] Furthermore, Comparative Example 4 was tempered at 400℃, which is considered a low-temperature tempering temperature, resulting in a tempered martensite structure. This structure retains the high hardness and strength of quenched martensite, but exhibits extremely poor plasticity and toughness.

[0039] Comparative Example 5 changed the tempering temperature to 680℃. The impact toughness was improved by using a higher tempering temperature, but the coarse grains could not make the strength reach a suitable level.

[0040] The above description is only a preferred embodiment of the present invention. It should be noted that for those skilled in the art, several modifications and improvements can be made without departing from the inventive concept, and these all fall within the protection scope of the present invention.

Claims

1. A method of dual-phase zone heat treatment of a seamless steel pipe for high toughness drill pipe, the seamless steel pipe having a chemical composition, in terms of percentages by weight, comprising: C: 0.20%-0.30%, Si: 0.15%-0.35%, Mn: 0.40%-0.70%, Cr: 0.70%-1.20%, P: ≤0.010%, S: ≤0.001%, Mo: 0.70%-1.20%, Ni: ≤0.60%, V: 0.15%-0.30%, Al: 0.005%-0.040%, O: ≤0.0020%; N：≤0.0050%； H ：≤0.00015%； Pb: ≤0.0150%; Sb: ≤0.0150%; As : ≤0.0150%; Bi: ≤0.0150%; Sn: ≤0.0150%, the rest being Fe and unavoidable impurities, characterized in that it comprises the following steps: (1) dual-phase quenching: heating the seamless steel pipe to a dual-phase temperature T1 and keeping it, then rapidly cooling it; (2) high-temperature tempering: heating the seamless steel pipe cooled in step (1) to a tempering temperature T2 and keeping it, then air cooling it to room temperature; the tempering temperature T2 is in the range of 500°C to 20°C below the Ac1 temperature.

2. The heat treatment method according to claim 1, characterized by, The dual-phase temperature T1 is in the range of 30-50°C below the Ac3 temperature.

3. The heat treatment method according to claim 1, characterized by, The tempering temperature T2 is in the range of 500-650°C.

4. The heat treatment method according to claim 1, characterized by, The rapid cooling uses water quenching.

5. The heat treatment method according to claim 1, characterized by, The keeping time in step (1) is determined according to the wall thickness of the steel pipe, and the keeping time t (minutes) satisfies: t = (1.2-1.8) x D, wherein D is the wall thickness of the steel pipe in millimeters.

6. The heat treatment method according to claim 1, wherein The Ac1 and Ac3 temperatures of the seamless steel pipe are obtained by experimental determination method using a thermal dilatometer.

7. A seamless steel pipe for high toughness drill pipe, characterized by, The metallographic structure is fine dual-phase structure of ferrite and tempered sorbite, the yield strength of the seamless steel pipe at room temperature is 1140-1275 MPa, the tensile strength is 1200-1413 MPa, and the Charpy V-notch impact energy at -20°C is ≥100 J.

8. The high toughness seamless pipe for drill pipe according to claim 7, characterized by The wall thickness of the seamless steel pipe is greater than 5 mm.

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