Low-cost high-strength drill rod and preparation method thereof

Abstract

The invention belongs to the field of metal materials and manufacturing thereof, and provides a low-cost and high-strength drill rod and a preparation method thereof.The low-cost and high-strength drill rod comprises, by mass, 0.26%-0.35% of C, 0.25%-0.30% of Si, 1.35%-1.55% of Mn, smaller than or equal to 0.02% of P, smaller than or equal to 0.05% of S, 0.7%-0.9% of Cr, 0.002%-0.004% of B, 0.050%-0.06% of Ti, 0.050%-0.080% of V, 0.030%-0.060% of Nb, 0.0040%-0.0060% of N, 0.04%-0.07% of B / Ti, 0.5%-2.0% of V / Nb and the balance Fe and inevitable impurities. On the premise that expensive elements such as Mo and Ni are completely abandoned, the comprehensive performance that the yield strength of the drill rod material is 931-1138 MPa, the tensile strength is larger than or equal to 1000 MPa, and meanwhile the longitudinal impact energy is larger than or equal to 100 J at the low temperature of-21 DEG C is achieved, the heat treatment process window is widened to 40 DEG C, the critical cooling speed is smaller than or equal to 30 DEG C / s, and the production cost and the process difficulty are remarkably reduced.

Description

Technical Field

[0001] This invention belongs to the field of metallic materials and their manufacturing, specifically relating to a low-cost, high-strength drill pipe and its preparation method. Background Technology

[0002] With the development of the petroleum industry, the depth of oil and gas wells is constantly increasing, leading to increasingly complex drilling conditions. Drill pipes are subjected to tensile, compressive, bending, torsional, and shear loads, as well as their combined alternating loads. They also need to cope with the long-term effects of low temperatures, high pressures, and corrosive media at the bottom of the well, significantly increasing the probability of failure. Drill pipe failure can lead to wellbore abandonment, significant economic losses, and even casualties.

[0003] Advances in drilling technology have driven parameter enhancement, continuously increasing the performance requirements for drill pipes and necessitating the development of new materials more resistant to harsh operating conditions. However, traditional high-strength drill pipes generally employ high levels of expensive alloying elements such as Cr and Mo, significantly increasing raw material costs. Their stringent heat treatment process windows (e.g., narrow quenching temperature range and high critical cooling rate requirements) also increase the difficulty of production control and scrap rates, making it difficult to meet the demand for cost-effective drill pipes in large-scale deep well development. Furthermore, under the high temperature, high pressure, acidic media, and severe vibration and impact conditions of deep and ultra-deep wells, conventional drill pipe materials are prone to stress corrosion cracking, fatigue fracture, and early wear, placing near-stringent requirements on the strength-toughness balance, low-temperature impact toughness, and corrosion resistance of drill pipes. Therefore, developing a new drill pipe manufacturing technology that can significantly reduce alloy costs and process sensitivity while ensuring high strength (Rm≥1000MPa), excellent low-temperature toughness (longitudinal 10×10 impact energy≥100J at -21℃), and good service safety has become a key challenge that the industry urgently needs to overcome. Summary of the Invention

[0004] The purpose of this invention is to overcome the high cost problem caused by the reliance on expensive alloying elements such as Mo and Ni in existing technologies, as well as the technical defects of traditional Cr-Mo steel, such as a narrow heat treatment process window and difficulty in meeting the low-temperature toughness requirements for deep well drilling. This invention provides a low-cost, high-strength drill pipe and its preparation method. This method, through the synergistic optimization of microalloying design and rolling-heat treatment processes, enables the drill pipe material to achieve a yield strength Rt without completely eliminating expensive alloying elements such as Mo and Ni. 0.7 With a tensile strength of 931-1138 MPa and a tensile strength R m The invention achieves a combined performance of ≥1000 MPa and longitudinal impact energy ≥100 J at a low temperature of -21℃. Furthermore, it broadens the heat treatment process window to 40℃ and reduces the critical cooling rate to ≤30℃ / s, thereby significantly reducing production costs and process control difficulty.

[0005] To achieve the above objectives, the technical solution adopted by the present invention is as follows: This invention provides a low-cost, high-strength drill pipe, the chemical composition of which, by mass percentage, is: C: 0.26-0.35%, Si: 0.25-0.30%, Mn: 1.35-1.55%, P≤0.02%, S≤0.05%, Cr: 0.7-0.9%, B: 0.002-0.004%, Ti: 0.050-0.06%, V: 0.050-0.080%, Nb: 0.030-0.060%, N: 0.0040-0.0060%, B / Ti=0.04-0.07, V / Nb=0.5-2.0, with the balance being Fe and unavoidable impurities.

[0006] Preferably, the metallographic structure of the drill pipe comprises tempered sorbite.

[0007] Preferably, the high-strength drill pipe has the following mechanical properties: room temperature yield strength Rt0.7: 931-1138 MPa, tensile strength Rm ≥ 1000 MPa, and longitudinal strength of 10 at -21℃. 10 Impact energy ≥ 100J.

[0008] The functions and limitations of each chemical component are as follows: C: 0.26-0.35%: Carbon is the main element that ensures the strength of steel. If the carbon content is too low, the strength will be insufficient; if it is too high, the toughness and weldability will decrease, while the hardenability will increase. This invention controls the C content at 0.26-0.35% to balance high strength with good toughness and weldability.

[0009] Mn: 1.35-1.55%: Manganese can improve the hardenability of steel and increase the martensite content, thereby improving strength and toughness. This invention selects an Mn content of 1.35-1.55% to obtain a better balance of strength and toughness.

[0010] Cr: 0.7-0.9%: Chromium is a strong carbide-forming element. Therefore, in addition to solid solution strengthening and improving hardenability, adding chromium to steel can also promote carbide precipitation during tempering, thus playing a precipitation strengthening role. The present invention controls the content to 0.7-0.9%.

[0011] B: 0.002-0.004%: Boron can significantly improve the hardenability of steel. A small amount of boron can enable steel to obtain a martensitic structure after quenching, thereby increasing its strength. Therefore, it should be controlled within 0.002-0.004%.

[0012] Ti: 0.050-0.060%: Titanium is the main strengthening element, which forms a composite compound with N and C to precipitate and strengthen. In this invention, the content is controlled at 0.050-0.060%.

[0013] V: 0.050-0.080%: Vanadium precipitates V(C,N) during tempering, resulting in precipitation strengthening and increasing the strength of the steel. Controlling the content within 0.050-0.080% can achieve good strengthening effects, while also significantly improving tempering stability and reducing the difficulty of heat treatment.

[0014] Nb: 0.030-0.060%: Niobium can inhibit austenite grain growth, refine grains, and precipitate Nb (C,N) during rolling, resulting in precipitation strengthening and improving the strength and toughness of steel.

[0015] N: 0.0040-0.0060%: Nitrogen forms nitrides or carbonitrides with Ti, Nb, V, etc., which plays a role in refining grains and precipitation strengthening. If the nitrogen content is too low, the precipitation strengthening effect will not be obvious; if it is too high, it will easily form defects such as porosity. Therefore, it is controlled at 0.0040-0.0060%.

[0016] Another aspect of the present invention provides a method for manufacturing a low-cost, high-strength drill pipe, comprising the following steps: (1) Steelmaking: The raw materials are prepared according to the above chemical composition, and the steel is smelted in a converter or electric arc furnace. Then, the steel is refined by LF and vacuum degassing by VD to obtain qualified molten steel. (2) Continuous casting: The molten steel obtained in step (1) is continuously cast into round tube billets; (3) Billet heating: The round billet is heated in a walking beam furnace at a temperature of 1180-1250℃ and held for 6-8 hours to ensure that the billet is fully heated; (4) Piercing: The ring furnace is heated to 1180-1250℃ to ensure that the billet is fully heated. The heated billet is pierced to obtain the tube. The piercing temperature is 1150-1200℃. In this temperature range, austenite has good plasticity and the wear of tools and dies is minimized. (5) Rolling: After the tube temperature drops to below 950℃, the tube is rolled using continuous rolling or tension reduction process, and the final rolling temperature is controlled at 850-920℃ to obtain the rolled tube. (6) Heat treatment: The rolled tube is quenched and tempered, with a quenching temperature of 880-920℃ and a holding time of 1.0-2.0 min / mm. Then it is water quenched, tempered at 550-590℃ and held for 2.0-3.0 min / mm, and air cooled to room temperature to obtain a low-cost, high-strength drill rod.

[0017] The key technical point of this invention is: A moderate C content (0.26-0.35%) ensures the strength of the solid solution-strengthened matrix, while Mn (1.35-1.55%) improves hardenability, and Cr (0.7-0.9%) promotes carbide precipitation during tempering, together forming a high-strength matrix.

[0018] Synergistic effect of B-Ti-V-Nb microalloying: B (0.002-0.004%): Significantly improves hardenability, reduces the critical cooling rate to ≤20℃ / s, allows for conventional water quenching, and reduces equipment and process complexity. It shows a significant improvement compared to traditional Cr-Mo steel, further reducing the requirements for water quenching in actual production. Ti (0.050-0.06%) and N (0.0040-0.0060%): form high-temperature stable TiN particles, effectively pinning austenite grain boundaries and strongly inhibiting grain growth during heating and rolling, which is the core of achieving grain refinement. At the same time, by controlling the V / Nb ratio, tempering stability is improved, and the temperature drop after piercing is strictly controlled to ≤950℃. Final rolling temperature control (850-920℃): final rolling is carried out in the non-crystallized region to promote the accumulation of deformed austenite, creating conditions for obtaining fine ferrite / martensite structure after phase transformation.

[0019] Strict control over the content of B-Ti-V-Nb elements and the controlled rolling process allows for a wide quenching temperature window of up to 40℃ during heat treatment, significantly reducing process sensitivity. A relatively high tempering temperature (550-590℃) ensures sufficient carbide precipitation to maintain strength while effectively eliminating quenching stress, significantly improving toughness, especially low-temperature impact toughness. It maintains an impact energy of ≥100J even at -21℃, making it more suitable for drill pipe applications under extreme conditions. This solves the problem of Cr-Mo steel's sensitivity to heat treatment temperature, reducing production difficulty.

[0020] The advantages and beneficial effects of this invention are: (1) The seamless steel pipe of this invention has a low alloy composition and no high-valence elements such as Mo and Ni. By synergistically controlling the content of B-Ti-V-Nb elements and combining it with strict temperature control, precipitation and grain refinement effects are enhanced, achieving a synergistic improvement in high strength and high toughness. The manufactured drill pipe meets the following performance requirements: yield strength 931-1138MPa, tensile strength greater than 1000MPa, longitudinal strength of 10 at -21℃ 10 Impact energy greater than 100J, -21℃ transverse 10 5. The impact energy is greater than 40J, which can meet the mechanical performance requirements of high-strength drill pipes.

[0021] (2) Through the synergistic effect of B-Ti-V-Nb microalloying, the heat treatment process has a wider process window, reduces process sensitivity, improves production stability, and has low cost. Attached Figure Description

[0022] Figure 1 The image shows a microscopic tissue photograph of Example 1; Figure 2 Microscopic tissue photographs of Example 2; Figure 3 This is a microscopic tissue photograph of Comparative Example 1. Detailed Implementation

[0023] 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.

[0024] The present invention will be further illustrated by specific embodiments below. These embodiments are for illustrative purposes only, and the scope of protection of the present invention is not limited to these embodiments.

[0025] A low-cost, high-strength drill pipe, wherein the chemical composition of the drill pipe, by mass percentage, is: C: 0.26-0.35%, Si: 0.25-0.30%, Mn: 1.35-1.55%, P≤0.02%, S≤0.05%, Cr: 0.7-0.9%, B: 0.002-0.004%, Ti: 0.050-0.06%, V: 0.050-0.080%, Nb: 0.030-0.060%, N: 0.0040-0.0060%, B / Ti=0.04-0.07, V / Nb=0.5-2.0, with the balance being Fe and unavoidable impurities.

[0026] A method for manufacturing a low-cost, high-strength drill pipe includes the following steps: (1) Steelmaking: The raw materials are prepared according to the above chemical composition, and the steel is smelted in a converter or electric arc furnace. Then, the steel is refined by LF and vacuum degassing by VD to obtain qualified molten steel. (2) Continuous casting: The molten steel obtained in step (1) is continuously cast into round tube billets; (3) Billet heating: The round billet is heated in a walking beam furnace at a temperature of 1180-1250℃ and held for 6-8 hours to ensure that the billet is fully heated; (4) Piercing: The ring furnace is heated to 1180-1250℃ to ensure that the billet is fully heated. The heated billet is pierced to obtain the tube. The piercing temperature is 1150-1200℃. In this temperature range, austenite has good plasticity and the wear of tools and dies is minimized. (5) Rolling: After the tube temperature drops to below 950℃, the tube is rolled using continuous rolling or tension reduction process, and the final rolling temperature is controlled at 850-920℃ to obtain the rolled tube. (6) Heat treatment: The rolled tube is quenched and tempered, with a quenching temperature of 880-920℃ and a holding time of 1.0-2.0 min / mm. Then it is water quenched, tempered at 550-590℃ and held for 2.0-3.0 min / mm, and air cooled to room temperature to obtain a low-cost, high-strength drill rod.

[0027] The chemical composition and weight percentage of the drill pipes in the examples and comparative examples are shown in Table 1, the rolling and heat treatment process parameters are shown in Table 2, and the mechanical properties are shown in Table 3.

[0028] Table 1. Weight percentage (%) of chemical components in the embodiments and comparative examples of the present invention

[0029] Table 2 Rolling and heat treatment process parameters of the embodiments and comparative examples of the present invention

[0030] Table 3 Mechanical properties of embodiments and comparative examples of the present invention

[0031] As can be seen from the above, the seamless steel pipe manufactured using this invention meets the following properties: yield strength greater than 931 MPa, tensile strength greater than 1000 MPa, and longitudinal strength of 10 mm at -21℃. Charpy impact energy greater than 100J, -21℃, transverse 10 5. Charpy impact energy is greater than 40 J. The seamless steel pipe prepared by this invention meets the requirements of high-strength drill pipe.

[0032] The tissue diagram of Example 1 is as follows Figure 1 As shown, the microstructure is tempered sorbite.

[0033] Microscopic tissue photographs of Example 2 are shown below. Figure 2 As shown, the microstructure is tempered sorbite.

[0034] Comparative Example 1: Microscopic tissue photographs as follows Figure 3 As shown, the microstructure consists of tempered sorbite and unmelted ferrite. Insufficient vanadium content reduces carbide precipitation, weakening the inhibition of grain growth and causing austenite grain coarsening. This may raise the Ac3 phase transformation critical point, leading to insufficient austenite homogenization. Because the phase transformation driving force of coarse-grained steel is reduced, and the phase transformation temperature tends to rise, some undissolved ferrite remains.

[0035] Compared to Example 1, Comparative Examples 1 and 2 changed the amount of V outside the range, resulting in increased strength but significantly decreased impact toughness. Excessive vanadium content led to excessive grain refinement, significantly increasing the strength and hardness of the steel, but significantly decreasing its plasticity and toughness. It also increased hardenability, potentially causing quenching cracks and reducing tempering stability. Furthermore, the precipitation of vanadium as carbides at high temperatures may reduce resistance to hydrogen corrosion. In addition, excessive vanadium forms a hard and brittle phase, exacerbating cold brittleness and increasing material costs and processing difficulty. Insufficient vanadium content had the following main negative effects: insufficient grain refinement, leading to reduced strength and toughness, and decreased wear resistance; weakened hardenability and tempering stability, affecting the overall mechanical properties after heat treatment; insignificant secondary hardening effect, limiting the ability to retain hardness at high temperatures; and insufficient vanadium weakened the inhibition of overheating sensitivity, potentially leading to grain coarsening and reduced low-temperature toughness.

[0036] In Comparative Examples 3 and 4, altering the amount of Nb outside the acceptable range resulted in a decrease in the impact toughness of the steel pipe. Excessive niobium may form coarse carbides, which in turn promotes grain growth and reduces the material's plasticity and toughness, especially in the weld heat-affected zone. Simultaneously, excessive niobium may improve hardenability, but it easily leads to an increase in retained austenite and microstructure inhomogeneity, increasing the risk of deformation. Insufficient niobium content weakens the grain refinement effect: it cannot effectively suppress grain growth during high-temperature heating, resulting in coarse grains that reduce toughness and fatigue strength.

[0037] In Comparative Examples 5 and 6, the alteration of carbon content outside the acceptable range significantly impacted strength and toughness. Excessive carbon content increased the steel's strength and hardness but significantly reduced its plasticity and ductility, leading to brittleness and increased susceptibility to cracking. In high-temperature applications, it may also reduce corrosion resistance and oxidation resistance. Conversely, insufficient carbon content improved the steel's plasticity and toughness, but resulted in inadequate strength and hardness, affecting its load-bearing capacity. For seamless steel pipes requiring high strength, insufficient carbon content could prevent meeting design requirements.

[0038] Comparative Example 7 shows that lowering the final rolling temperature below the range (840℃) will cause a decrease in the toughness of the steel pipe. Excessively low final rolling temperatures significantly reduce the plasticity of the metal, drastically increase the deformation resistance, and lead to stress concentration within the material during rolling, easily exceeding its fracture limit and causing cracks or fissures. Simultaneously, excessively low temperatures may cause rolling to occur in a non-single-phase region, affecting the recrystallization process and resulting in coarse grains or banded structures, reducing the material's uniformity. Coarse grains will worsen strength and toughness, deteriorating deep-drawing performance and processing properties.

[0039] Comparative Example 8 shows that changing the final rolling temperature above the range (930℃) will cause a decrease in both strength and toughness. Excessive final rolling temperature of steel pipe mainly leads to coarse austenite grains. Coarse grains reduce the grain boundary area and weaken the grain boundary's resistance to dislocation movement, resulting in a decrease in strength and hardness. At the same time, grain refinement usually improves toughness. Coarse grains may cause uneven microstructure and mixed grain phenomenon. Therefore, coarse grains will reduce toughness and plasticity and increase the risk of brittle fracture.

[0040] In Comparative Example 9, if the temperature of the capillary tube is not lowered to below 950℃, the rolling temperature of the steel pipe will be too high, resulting in a decrease in the strength properties of the steel pipe. Excessive rolling temperature leads to coarse grains, thus reducing toughness and plasticity. Furthermore, excessively high final rolling temperature can disrupt austenite recrystallization, resulting in coarse grains and directly reducing the yield strength, tensile strength, and other mechanical properties of the steel.

[0041] In Examples 9-10, changing the tempering temperature of the same alloy composition had little impact on performance, indicating low process sensitivity and reduced production difficulty.

[0042] 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 low-cost, high-strength drill pipe, characterized in that, The chemical composition of the drill pipe, by mass percentage, is as follows: C: 0.26-0.35%, Si: 0.25-0.30%, Mn: 1.35-1.55%, P≤0.02%, S≤0.05%, Cr: 0.7-0.9%, B: 0.002-0.004%, Ti: 0.050-0.06%, V: 0.050-0.080%, Nb: 0.030-0.060%, N: 0.0040-0.0060%, B / Ti=0.04-0.07, V / Nb=0.5-2.0, with the balance being Fe and unavoidable impurities.

2. The low-cost, high-strength drill pipe according to claim 1, characterized in that, The metallographic structure of the drill pipe is tempered sorbite.

3. The low-cost, high-strength drill pipe according to claim 1, characterized in that, The high-strength drill pipe has the following mechanical properties: room temperature yield strength Rt 0.7 The strength is 931-1138 MPa, the tensile strength Rm≥1000 MPa, and the longitudinal strength at -21℃ is 10. 10 Impact energy ≥ 100J.

4. A method for preparing a low-cost, high-strength drill pipe as described in any one of claims 1-3, characterized in that, Includes the following steps: (1) Steelmaking: The raw materials are batched according to the chemical composition, and the steel is smelted in a converter or electric arc furnace. After LF refining and VD vacuum degassing, qualified molten steel is obtained. (2) Continuous casting: The molten steel from step (1) is continuously cast into round tube billets; (3) Heating of tube blank: The round tube blank is kept at 1180-1250℃ in a walking beam furnace for 6-8 hours; (4) Piercing: The ring furnace is heated to 1180-1250℃ to ensure that the steel billet is fully heated. The heated billet is then pierced to obtain the rough tube. (5) Rolling: The tube is rolled by continuous rolling or tension reduction process, and the final rolling temperature is controlled at 850-920℃ to obtain the rolled tube; (6) Heat treatment: The rolled tube is quenched, then water quenched, tempered at 550-590℃, and held for 2.0-3.0 min / mm. It is then air-cooled to room temperature to obtain a low-cost, high-strength drill pipe.

5. The preparation method according to claim 4, characterized in that, The tube needs to be cooled to below 950°C before rolling.

6. The preparation method according to claim 4, characterized in that, The temperature of the perforation is 1150-1200℃.

7. The preparation method according to claim 4, characterized in that, The quenching temperature is 880-920℃, and the holding time is 1.0-2.0 min / mm.

8. The preparation method according to claim 4, characterized in that, The tempering temperature is 550-590℃, and the holding time is 2.0-3.0 min / mm.

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