A high-strength seamless steel pipe for trenchless drilling rods and a method for manufacturing the same

Abstract

The application discloses a high-strength seamless steel pipe for trenchless drilling rod and a manufacturing method. The seamless steel pipe for trenchless drilling rod contains the following components in percentage by weight: C 0.23-0.25%, Si 1.15-1.35%, Mn 1.05-1.35%, Cr 0.35-0.50%, Ti 0.026-0.039%, Als 0.016-0.029%, P≤0.016%, S≤0.011%, and the balance of Fe and other inevitable impurities. The manufacturing method comprises preparation of a pipe blank, hot rolling of the steel pipe, preparation of the seamless steel pipe, and the microstructure phase composition of the seamless steel pipe obtained through the manufacturing method is short fine needle-shaped tempered sorbite; the combination of a reasonable steelmaking process, a rolling process and a heat treatment process effectively optimizes the microstructure phase composition, refines the grain size, reduces the inclusion grade and improves the distribution.

Description

TECHNICAL FIELD

[0001] The present application relates to the field of seamless pipe manufacturing technology, in particular to a high-strength seamless steel pipe for trenchless drilling rod and a manufacturing method thereof. BACKGROUND

[0002] In engineering construction, it is often necessary to place a pipe or cable for a certain purpose at a certain depth underground, so it is necessary to excavate a trench on the ground for pipe installation, and then backfill. But in some cases, this method of excavating a trench is very costly or actually not allowed. This requires new technical methods that do not require excavation of a trench on the ground for pipe installation but directly underground construction. Trenchless drilling is a modern technology that can directly install a pipe of a certain range and specification underground. The key tool used in trenchless drilling, the drill rod, is made of seamless steel pipe. Most trenchless drill rods are made by using the production and processing technology of oil drill rods, and the rod body pipe end is thickened, and then the end is threaded or welded with a joint. There are great differences between the non-excavation drill rod column and the ordinary oil drill rod column in the running-in and pulling-out mode during construction and the bearing capacity of the drilling machine, and it is more prone to torsional fatigue fracture. At the same time, because the trenchless drill rod is often used repeatedly during construction, it is easily affected by atmospheric corrosion, which affects the service life. Therefore, in combination with the actual working conditions of the trenchless drill rod, the material has good mechanical properties, fatigue properties, and corrosion resistance, which are the key factors to ensure the safety of trenchless construction and improve the service life of the material.

[0003] CN 103820737 B discloses a steel for trenchless drill rod, the composition of the steel is C 0.26%~0.29%, Si 0.17%~0.37%, Mn 1.05%~1.30%, Cr 0.80%~1.10%, Mo 0.20%~0.25%, V 0.07%~0.15%, Cu≤0.1%, As≤0.010%, Sn≤0.010%, Pb≤0.003%, Sb≤0.010%, the balance being Fe and unavoidable impurities; the invention has a high content of Cr, Mo and V, and has a high cost. Although the mechanical properties reach the level of 135 Ksi, the impact performance is low.

[0004] CN 112226694 A discloses a trenchless drill pipe and its heat treatment process. The trenchless drill pipe contains 0.30~0.33% C, 0.15~0.35% Si, 0.95~1.2% Mn, 0.95~1.2% Cr, 0.3~0.38% Mo, P≤0.015%, S≤0.005%, V≤0.2%, with the balance being Fe. The heat treatment process includes sequentially quenching, water quenching, tempering, and cooling the trenchless drill pipe. However, this invention contains relatively high levels of Cr, Mo, and V, resulting in high cost. Furthermore, the invention does not disclose its strength level and has slightly lower impact toughness.

[0005] CN 105861932 B discloses an ultra-high strength and ultra-high toughness drill pipe joint and its manufacturing method. The chemical element mass content of the ultra-high strength and ultra-high toughness drill pipe joint is as follows: C: 0.33-0.40%; Si: 0.1-0.5%; Mn: 0.7-1.5%; Cr: 0.7-1.5%; Mo: 0.82-0.98%; V: 0.01-0.10%; Nb: 0.01-0.05%; P≤0.015%; S≤0.005%; with the remainder being Fe and unavoidable impurities. The manufacturing method, by adding multiple alloying elements such as Cr, Mo, V, and Nb, achieves excellent strength and toughness, but the large amount of alloying required results in high cost. Its application is limited to oil exploration drill pipe joints and is not suitable for widespread use in trenchless applications.

[0006] CN113789474 A discloses an economical seamless steel pipe for trenchless drill pipes and its manufacturing method. The composition of the economical seamless steel pipe for trenchless drill pipes is: C: 0.27%–0.35%; Si: 0.26%–0.42%; Mn: 1.10%–1.30%; Cr: 0.55%–0.75%; Mo: 0.07%–0.15%; Al: 0.020%–0.045%; P≤0.015%; S≤0.010%; the remainder being iron and unavoidable impurities. This invention adds alloys such as Cr and Mo, but does not mention fatigue performance.

[0007] CN109136483 B discloses a seamless steel pipe for trenchless integral drill pipe and its preparation method. The seamless steel pipe for trenchless integral drill pipe has the following composition: C 0.27-0.32%, Si 0.15-0.35%, Mn 1.60-2.00%, Cr 0.8-1.8%, V 0.20-0.26%, with the balance being Fe and unavoidable impurities. The preparation method significantly increases the product cost by adding a high content of alloys such as Cr, V, and Mn.

[0008] CN116179954 B discloses a steel pipe for drill pipe bodies. The composition of the steel pipe for drill pipe bodies is: C: 0.26%~0.28%; Si: 0.25%~0.28%; Mn: 0.85%~0.90%; Ni: ≤0.04%; Cr: 1.00%~1.05%; Mo: 0.24%~0.27%; Cu: ≤0.02%; Al: 0.020%~0.030%; V: 0.07%~0.09%; P: ≤0.013%; S: ≤0.003%; H: ≤0.0002%, with the remainder being iron and unavoidable impurities. Because the steel pipe contains a high content of Mo, it can damage the stability of the surface passivation film, resulting in poor atmospheric corrosion resistance.

[0009] The high Mo content in existing trenchless drill pipes necessitates the addition of various alloying elements to compensate for the degradation of the surface passivation film caused by the high Mo content, resulting in poor atmospheric corrosion resistance. To address this issue, a high-strength seamless steel pipe for trenchless drill pipes and its manufacturing method are proposed. The steel pipe features a finer grain size, lower inclusion grade, and a good oxide layer composition, significantly improving its comprehensive mechanical properties and atmospheric corrosion resistance. This avoids the problems of oxidation embrittlement and high cost associated with the high Mo content in existing trenchless drill pipes. Summary of the Invention

[0010] To solve the above-mentioned technical problems, this invention proposes a high-strength seamless steel pipe for trenchless drill rods and its manufacturing method. It differs significantly from the aforementioned patents in chemical composition and production process. It has good impact and fatigue performance, and its metallographic structure is tempered sorbite. After heat treatment, the microstructure has uniform and fine grains, which is beneficial to improving its mechanical properties, fatigue performance and corrosion resistance.

[0011] According to one aspect of the present invention, a high-strength seamless steel pipe for trenchless drill pipe is provided, wherein the seamless steel pipe for trenchless drill pipe contains the following components by weight percentage: C 0.23%–0.25%, Si 1.15%–1.35%, Mn 1.05%–1.35%, Cr 0.35%–0.50%, Ti 0.026%–0.039%, Als 0.016%–0.029%, P ≤0.016%, S ≤0.011%, with the balance being Fe and other unavoidable impurities. Its main chemical composition and functions are as follows:

[0012] C is a carbide-forming element that can improve the strength of steel. To ensure the necessary strength, the lower limit is set at 0.23%. However, too high a carbon content will exacerbate banding and affect low-temperature impact performance, so the upper limit is set at 0.25%. Therefore, the C content is 0.23% to 0.25%.

[0013] Si can effectively improve the yield strength and fatigue strength of steel. At the same time, the addition of Si can also effectively improve the protective performance of the oxide layer and the resistance to atmospheric corrosion. Therefore, the Si content is 1.15%~1.35%.

[0014] Mn is an austenite-forming element used to improve the strength of steel. It can compensate for the loss of yield strength due to the reduction of carbon content. While improving strength, Mn can also improve the toughness of steel and lower the ductile-brittle transition temperature. Therefore, the Mn content is 1.05%~1.35%.

[0015] Cr can form stable carbides, slowing down carbon diffusion and forming a solid oxide scale. When Cr is incorporated into the α-FeOOH structure, it significantly improves the protective properties of the rust layer, enhances resistance to atmospheric corrosion, and reduces decarburization. Cr can improve the hardenability of steel, which is beneficial for increasing its strength. Excessive Cr content increases temper brittleness, while insufficient Cr reduces its strengthening effect; therefore, the Cr content should be between 0.35% and 0.50%.

[0016] Ti can prevent austenite grain growth, refine grains, and form carbides, while improving the strength and toughness of steel. However, when the content exceeds a certain amount, its strengthening effect is not obvious. Excessive content can easily form coarse TiN, which is detrimental to the overall performance of steel. Therefore, the Ti content is 0.026%~0.039%.

[0017] P is an impurity element that promotes center segregation, significantly reduces the impact toughness of steel, increases the ductile-brittle transition temperature of steel, and also deteriorates the weldability of steel. Therefore, its content should be minimized, and the P content should be ≤0.016%.

[0018] S is an impurity element that easily forms sulfides, which has a significant impact on impact performance. Furthermore, the sulfides formed severely affect the fatigue performance of steel. Therefore, the S content should be ≤0.011%.

[0019] Furthermore, the phase transition point A of the high-strength seamless steel pipe for trenchless drill pipes is... c3 The temperature is 840~860℃;

[0020] The metallographic structure is short, fine acicular tempered sorbite with a needle length ≤20μm. The short, fine acicular tempered sorbite is composed of ferrite and carbides, wherein the proportion of ferrite is 95.5%~96.5% and the proportion of carbides is 3.5%~4.5%.

[0021] Furthermore, the seamless steel pipe has a two-layer oxide layer: a loose outer rust layer composed of γ-FeOOH and a dense inner rust layer composed of α-FeOOH nanocrystals, with an α / γ mass ratio ≥2.5, where α refers to α-FeOOH and γ refers to γ-FeOOH, and the Cr content of the inner rust layer is not less than 6%.

[0022] Furthermore, the diameter of oxide inclusions in the seamless steel pipe at a distance t / 5 from the inner and outer walls is ≤1μm, wherein the wall thickness t is 8~25mm.

[0023] According to a second aspect of the present invention, a method for manufacturing a seamless steel pipe for trenchless drill pipe is provided, the method comprising the following steps:

[0024] Step (1): The raw material is added to the converter for smelting, and then refined by LF and RH outside the furnace. After continuous casting, it is cooled to room temperature and then added to the heating furnace for continuous rolling to obtain the tube billet.

[0025] Step (2): The tube blank obtained in step (1) is placed in an annular furnace for heating, and then rolled through piercing, PQF continuous rolling, online normalizing, and tension reduction processes. After cooling, the hot-rolled steel pipe is obtained.

[0026] Step (3): The hot-rolled steel pipe obtained in step (2) is quenched and heated, quenched and held at the temperature, cooled, and then tempered and heated, tempered and held at the temperature, and cooled again to obtain a seamless steel pipe.

[0027] Furthermore, the tapping temperature of the steel produced in step (1) is 1675~1690℃;

[0028] In step (1), the LF furnace inlet temperature in the external LF refining process is 1640~1655℃, the basicity of the LF top slag is 3.7~4.0, and argon gas is blown and stirred throughout the process.

[0029] Further, in step (1), the RH steel temperature rise in the external RH refining process is 5~9℃, the refining time is 25min~35min, the O content in the steel leaving the refining station is 6~8ppm, and the ladle is covered throughout the process before entering the tundish. The working vacuum degree of the external RH refining process is 60~65Pa, and the vacuum time after deoxidation is controlled to be 8~10min.

[0030] In step (1), the superheat during continuous casting is 20℃~22℃, the argon blowing time is 8~9 min, the billet pulling speed is 0.9m / min and the pulling speed is kept constant for each batch, and the slow cooling time of continuous casting is ≥9h.

[0031] Furthermore, the heating temperature of the annular furnace in step (2) is 1200~1230℃;

[0032] The piercing temperature in step (2) is 1180~1200℃. The rough tube after continuous rolling of PQF is cooled on a small cooling bed to an outer surface temperature of ≤500℃ and then put into a walking furnace for heating. The temperature of the walking furnace is 900~920℃ and the heating time of the walking furnace is 30~40min.

[0033] Furthermore, the outlet steel pipe rolled by the reduction diameter process in step (2) has a temperature of 740℃~800℃.

[0034] Furthermore, the outer diameter of the hot-rolled steel pipe in step (2) is 108mm~180mm and the wall thickness is 8mm~25mm.

[0035] Furthermore, the seamless steel pipe obtained in step (3) has a microstructure of short, fine needle-like tempered sorbite, a yield strength ≥1100MPa, a tensile strength ≥1200MPa, and room temperature impact performance of 3 / 4 size specimens ≥90J and 1 / 2 size specimens ≥70J; -40℃ impact performance of 3 / 4 size specimens ≥70J and 1 / 2 size specimens ≥60J; torsional fatigue cycle count ≥40000N / cycle; and corrosion weight gain ≤16mg / cm³. 2 .

[0036] Furthermore, the quenching heating temperature in step (3) is 880~910℃;

[0037] The quenching and holding time in step (3) is 60~75min;

[0038] The quenching cooling rate in step (3) is ≥10℃ / s;

[0039] The tempering temperature in step (3) is 490~530℃;

[0040] The tempering and heat preservation time mentioned in step (3) is 80~100 min;

[0041] The tempering cooling rate in step (3) is ≥20℃ / s.

[0042] Compared with the prior art, the beneficial effects of the present invention are as follows:

[0043] (1) The high-strength seamless steel pipe for trenchless drill rods obtained by the technical solution disclosed in this invention, compared with existing products, is obtained by strictly controlling the steelmaking process parameters through the LF+RH route, reducing the inclusion grade and improving the distribution.

[0044] (2) Through optimized rolling process and controlled cooling technology, the metallographic phase composition of this invention is good, consisting of short, fine needle-like tempered sorbite with a needle length ≤20μm. It exhibits excellent mechanical properties, guaranteeing a yield strength ≥1100MPa and a tensile strength ≥1200MPa. Its room temperature impact performance is ≥90J for 3 / 4-size specimens and ≥70J for 1 / 2-size specimens. Its -40℃ impact performance is ≥70J for 3 / 4-size specimens and ≥60J for 1 / 2-size specimens. The torsional fatigue cycle count is over 40000N / cycle. (Loading method: axial stress ratio Rσ=-1, shear stress ratio Rτ=-1, in-phase proportional loading, τa / Sa=0.7, phase angle: 0°, frequency: f=1Hz, axial stress amplitude is 500Sa / MPa, torsional stress amplitude is 350τa / MPa, conducted in room temperature air.) Atmospheric dry-wet cycle accelerated corrosion test (using 0.01 mol L...) -1 Sodium bisulfite (NaHSO3) solution was used as the corrosive agent, and the corrosion cycle was 90 times. The sample size was 20×20×4mm. The corrosion weight gain was ≤16mg / cm³. 2 It possesses excellent tensile strength, low-temperature toughness, fatigue performance, and corrosion resistance, and can be widely used in seamless steel pipes for trenchless drill rods.

[0045] (3) The manufacturing process of the product of the present invention is easy to implement, the product performance has good uniformity and stability, it is good as an oil casing pipe, and has a broad market prospect. Attached Figure Description

[0046] Figure 1 Metallographic photograph of the microstructure at 1 / 2 wall thickness of the seamless steel pipe prepared in Example 1 of the present invention;

[0047] Figure 2 Metallographic photograph of the microstructure at 1 / 2 wall thickness of the seamless steel pipe prepared in Example 2 of the present invention. Detailed Implementation

[0048] The present invention will be further described below with reference to specific embodiments, but this does not limit the present invention in any way.

[0049] The present invention provides a high-strength seamless steel pipe for trenchless drill pipes in the specific embodiments section. The seamless steel pipe contains the following components by weight percentage: C 0.23%~0.25%, Si 1.15%~1.35%, Mn 1.05%~1.35%, Cr 0.35%~0.50%, Ti 0.026%~0.039%, Als 0.016%~0.029%, P ≤0.016%, S ≤0.011%, with the balance being Fe and other unavoidable impurities.

[0050] The present invention also provides a method for manufacturing a high-strength seamless steel pipe for trenchless drill rods in the specific embodiments section, the manufacturing method comprising the following steps:

[0051] Step (1): The raw material is added to the converter for smelting, and then refined by LF and RH outside the furnace. After continuous casting, it is cooled to room temperature and then added to the heating furnace for continuous rolling to obtain the tube billet.

[0052] Step (2): The tube blank obtained in step (1) is placed in an annular furnace and heated to 1200℃~1230℃. After piercing, PQF continuous rolling, online normalizing, and tension reduction rolling processes, it is cooled to obtain hot-rolled steel pipe.

[0053] Step (3): Heat the hot-rolled steel pipe obtained in step (2) to 880~910℃ (quenching heating), hold for 60~75min (quenching holding), cool to room temperature with water at a cooling rate ≥10℃ / s, heat the steel pipe again to 490~530℃ (tempering heating), hold for 80min~100min (tempering holding), cool to room temperature with water at a cooling rate ≥20℃ / s, and obtain a seamless steel pipe.

[0054] Based on the above technical solution, the tapping temperature of the converter smelting in step (1) is 1675~1690℃.

[0055] Based on the above technical solution, in step (1), the LF furnace inlet temperature in the external LF refining process is ≥1640℃, the top slag basicity (CaO / SiO2) is controlled at 3.7~4.0, and argon gas is used for stirring throughout the process.

[0056] Based on the above technical solution, in step (1), the temperature rise of molten steel in the RH refining outside the furnace is <10℃, and the refining time is 25min~35min; the O content in the molten steel leaving the refining station is <8ppm, and it is covered throughout the process before entering the tundish. The working vacuum degree of RH is 60~65Pa, and the vacuum time is controlled to be 8~10min after deoxidation.

[0057] Based on the above technical solution, the superheat during continuous casting in step (1) is controlled at 20℃~22℃, the argon blowing time is 8~9 min, the billet pulling speed is 0.9m / min and the pulling speed is kept constant for each batch, and the slow cooling time of continuous casting is ≥9h.

[0058] Based on the above technical solution, the piercing temperature in step (2) is 1180~1200℃. The rough tube after continuous rolling of PQF is cooled to below 500℃ on the outer surface on a small cooling bed and then put into a walking furnace for heating. The temperature of the walking furnace is 900~920℃ and the heating time of the walking furnace is 30~40min.

[0059] Based on the above technical solution, the outlet steel pipe temperature of the reduced diameter process rolled in step (2) is 740℃~800℃.

[0060] Based on the above technical solution, the outer diameter of the hot-rolled steel pipe in step (2) is 108mm~180mm and the wall thickness is 8mm~25mm.

[0061] Based on the above technical solution, the seamless steel pipe obtained in step (3) has a metallographic structure of short, fine needle-like tempered sorbite, a yield strength ≥1100MPa, a tensile strength ≥1200MPa, and a room temperature impact performance of ≥90J for 3 / 4 size specimens and ≥70J for 1 / 2 size specimens; an impact performance of ≥70J for 3 / 4 size specimens and ≥60J for 1 / 2 size specimens at -40℃; a torsional fatigue cycle count ≥40000N / cycle; and a corrosion weight gain ≤16mg / cm³. 2 .

[0062] Example

[0063] The chemical composition, outer diameter, and wall thickness of the seamless steel pipes described in the embodiments and comparative examples of this invention are shown in Table 1. The manufacturing method of the seamless steel pipes includes the following steps: converter smelting—ladle refining LF+RH—billet continuous casting—cooling to room temperature and then entering the heating furnace—continuous rolling to obtain tube billets—annular furnace heating—forming steel pipes using piercing, PQF continuous rolling, online normalizing, tension reduction, and heat treatment processes. The steelmaking process parameters (converter tapping temperature, LF inlet temperature, LF top slag basicity, RH refining temperature rise during RH refining, refining time, O value of molten steel leaving the refining station, RH working vacuum degree, RH deoxidation vacuum time, superheat, argon blowing time, and billet drawing speed) described in the embodiments and comparative examples of this invention are shown in Table 2. The annular furnace heating temperature, piercing temperature, and other parameters during the continuous rolling process of the embodiments and comparative examples of this invention are also shown in Table 2. The parameters for sizing temperature and rolling elongation coefficient are shown in Table 3; the parameters for quenching heating temperature, quenching holding time, tempering heating temperature, and tempering holding time in the heat treatment processes of the embodiments and comparative examples of this invention are shown in Table 4; the mechanical properties of the high-strength seamless steel pipes for trenchless drill rods prepared in Examples 1-3 of this invention are shown in Table 5; the mechanical properties of the high-strength seamless steel pipes for trenchless drill rods prepared in Examples 4-5 of this invention and comparative examples are shown in Table 6; the fatigue properties and atmospheric corrosion resistance of the high-strength seamless steel pipes for trenchless drill rods prepared in the embodiments and comparative examples of this invention are shown in Table 7; the metallographic structure and inclusions of the high-strength seamless steel pipes for trenchless drill rods prepared in the embodiments and comparative examples of this invention are shown in Table 8. Furthermore, the microstructure at 1 / 2 the wall thickness of the high-strength seamless steel pipe for trenchless drill rods prepared in Example 1 is as follows... Figure 1 As shown, the microstructure at 1 / 2 the wall thickness of the high-strength seamless steel pipe for trenchless drill pipe prepared in Example 2 is as follows. Figure 1 As shown.

[0064] Table 1 shows the chemical composition, outer diameter, and wall thickness of the seamless steel pipes used for high-strength trenchless drill pipes in the examples and comparative cases.

[0065] .

[0066] Table 2 shows the steelmaking process parameters for the high-strength seamless steel pipes used in the examples and comparative examples.

[0067] .

[0068] Table 3. Process parameters during the continuous rolling process of high-strength seamless steel pipes for trenchless drill pipes in the embodiments and comparative examples.

[0069] .

[0070] Table 4. Process parameters for the heat treatment of high-strength seamless steel pipes for trenchless drill pipes in the examples and comparative cases.

[0071] .

[0072] Table 5 Mechanical properties of seamless steel pipes for high-strength trenchless drill pipes described in Examples 1-3

[0073] .

[0074] Table 6 Mechanical properties of seamless steel pipes for high-strength trenchless drill pipes described in Examples 4-5 and Comparative Examples

[0075] .

[0076] Table 7 shows the fatigue performance and atmospheric corrosion resistance of the seamless steel pipes used for high-strength trenchless drill pipes described in the examples and comparative examples.

[0077] .

[0078] Table 8 shows the metallographic structure, oxide layer, and inclusions of the seamless steel pipes used for high-strength trenchless drill pipes described in the examples and comparative examples.

[0079] .

[0080] This invention, through Examples 1-5 and Comparative Example 1, prepared seamless steel pipes for trenchless drill rods with different element contents. By optimizing relevant parameters in the seamless steel pipe manufacturing process, a method with a large rolling elongation coefficient was adopted for steel pipe rolling. The outer diameter of the steel pipe was φ108mm~180mm, and the wall thickness was in the range of 8mm~25mm. The seamless steel pipe of this invention has excellent mechanical properties and fatigue performance, and low cost. It guarantees a yield strength ≥1100MPa, tensile strength ≥1200MPa, elongation ≥17%, and its room temperature impact performance is ≥90J for 3 / 4-size specimens and ≥130J for full-size specimens. Its -40℃ impact performance is ≥70J for 3 / 4-size specimens and ≥100J for full-size specimens. The torsional fatigue cycle count is 40000. N / or more (loading method: axial stress ratio Rσ=-1, shear stress ratio Rτ=-1, in-phase proportional loading, τa / Sa=0.7, phase angle: 0°, frequency: f=1Hz, axial stress amplitude of 500Sa / MPa, torsional stress amplitude of 350τa / MPa, conducted in room temperature air) Atmospheric wet and dry cycle accelerated corrosion test (using 0.01 mol L -1 Sodium bisulfite (NaHSO3) solution was used as the corrosive agent, with a corrosion cycle of 90 times (sample size 20×20×4mm). Atmospheric corrosion test showed a corrosion weight gain ≤16mg / cm³. 2 (Use 0.01 mol L) -1 Sodium bisulfite (NaHSO3) solution was used as the corrosive liquid, with a corrosion cycle of 90 times (sample size 20×20×4mm). The combination of reasonable steelmaking, rolling and heat treatment processes effectively optimized the phase composition of the metallographic structure, refined the grain size, reduced the grade and improved the distribution of inclusions, and generated a microstructure that effectively improved the comprehensive mechanical properties of the steel pipe. It also greatly improved the atmospheric corrosion resistance. The seamless steel pipe for trenchless drill pipe prepared by this invention has excellent tensile strength, low-temperature toughness, fatigue performance and corrosion resistance. It can be widely used in the application of seamless steel pipe for trenchless drill pipe. The manufacturing process is easy to implement, and the product performance has good uniformity and stability. It is well used as oil casing and has broad market prospects.

[0081] The above description is merely a few embodiments of the present invention and is not intended to limit the present invention in any way. Although the present invention has been disclosed above with reference to preferred embodiments, it is not intended to limit the present invention. Any modifications or alterations made by those skilled in the art without departing from the scope of the technical solution of the present invention using the disclosed technical content are equivalent to equivalent implementation cases and fall within the scope of the technical solution.

Claims

1. A high-strength seamless steel pipe for trenchless drill rods, characterized in that, By weight percentage, the seamless steel pipe for trenchless drill pipe contains the following components: C 0.23%–0.25%, Si 1.15%–1.35%, Mn 1.05%–1.35%, Cr 0.35%–0.50%, Ti 0.026%–0.039%, Als 0.016%–0.029%, P ≤0.016%, S ≤0.011%, with the balance being Fe and other unavoidable impurities. The phase transformation point Ac3 of the high-strength seamless steel pipe for trenchless drill pipe is 840–860℃. The metallographic structure is short, fine acicular tempered sorbite with a needle length ≤20μm. The short, fine acicular tempered sorbite is composed of ferrite and carbides, wherein the proportion of ferrite is 95.5%~96.5% and the proportion of carbides is 3.5%~4.5%. The seamless steel pipe has a two-layer oxide layer: a loose outer rust layer composed of γ-FeOOH and a dense inner rust layer composed of α-FeOOH nanocrystals, with an α / γ mass ratio ≥2.5, where α refers to α-FeOOH and γ refers to γ-FeOOH. The Cr content of the inner rust layer is not less than 6%. The diameter of oxide inclusions in the seamless steel pipe at a distance t / 5 from the inner and outer walls is ≤1μm, and the wall thickness t is 8~25mm.

2. The method for manufacturing the seamless steel pipe for trenchless drill rods as described in claim 1, characterized in that, The manufacturing method includes the following steps: Step (1): The raw material is added to the converter for smelting, and then refined by LF and RH outside the furnace. After continuous casting, it is cooled to room temperature and then added to the heating furnace for continuous rolling to obtain the tube billet. Step (2): The tube blank obtained in step (1) is placed in an annular furnace for heating, and then rolled through piercing, PQF continuous rolling, online normalizing, and tension reduction processes. After cooling, the hot-rolled steel pipe is obtained. Step (3): The hot-rolled steel pipe obtained in step (2) is quenched and heated, quenched and held at the temperature, cooled, and then tempered and heated, tempered and held at the temperature, and cooled again to obtain a seamless steel pipe.

3. The manufacturing method according to claim 2, characterized in that, The tapping temperature of the steel produced in step (1) is 1675~1690℃; In step (1), the LF furnace inlet temperature in the external LF refining process is 1640~1655℃, the basicity of the LF top slag is 3.7~4.0, and argon gas is blown and stirred throughout the process.

4. The manufacturing method according to claim 2, characterized in that, In step (1), the RH steel temperature rise in the external RH refining process is 5~9℃, the refining time is 25min~35min, the O content in the steel leaving the refining station is 6~8ppm, and the ladle is covered throughout the process before entering the tundish. The working vacuum degree of the external RH refining process is 60~65Pa, and the vacuum time after deoxidation is controlled to be 8~10min. In step (1), the superheat during continuous casting is 20℃~22℃, the argon blowing time is 8~9 min, the billet pulling speed is 0.9m / min and the pulling speed is kept constant for each batch, and the slow cooling time of continuous casting is ≥9h.

5. The manufacturing method according to claim 2, characterized in that, The heating temperature of the ring furnace in step (2) is 1200~1230℃; The piercing temperature in step (2) is 1180~1200℃. The rough tube after continuous rolling of PQF is cooled on a small cooling bed to an outer surface temperature ≤500℃ and then put into a walking furnace for heating. The temperature of the walking furnace is 900~920℃ and the heating time of the walking furnace is 30~40min. Preferably, the outlet temperature of the steel pipe rolled by the reduction diameter process in step (2) is 740℃~800℃; Preferably, the outer diameter of the hot-rolled steel pipe in step (2) is 108mm~180mm and the wall thickness is 8mm~25mm.

6. The manufacturing method according to claim 2, characterized in that, The seamless steel pipe obtained in step (3) has a microstructure of short, fine needle-like tempered sorbite, a yield strength ≥1100MPa, a tensile strength ≥1200MPa, and a room temperature impact performance of ≥90J for 3 / 4-size specimens and ≥70J for 1 / 2-size specimens; an impact performance at -40℃ of ≥70J for 3 / 4-size specimens and ≥60J for 1 / 2-size specimens; a torsional fatigue cycle count N ≥40000 times, and a corrosion weight gain ≤16mg / cm³. 2 .

7. The manufacturing method according to claim 2, characterized in that, The quenching heating temperature in step (3) is 880~910℃; The quenching and holding time in step (3) is 60~75min; The quenching cooling rate in step (3) is ≥10℃ / s; The tempering temperature in step (3) is 490~530℃; The tempering and heat preservation time mentioned in step (3) is 80~100 min; The tempering cooling rate in step (3) is ≥20℃ / s.

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