A steel for an x80 grade high toughness high hydrogen-induced cracking resistant thick wall seamless steel pipe for an lng receiving station and a heat treatment process thereof

Abstract

The application provides a kind of X80 grade high toughness high hydrogen-induced cracking resistance thick wall LNG receiving station seamless steel pipe steel and its heat treatment process, composition: C 0.03%~0.06%, Mn 1.30%~1.50%, Cr 0.20%~0.50%, Mo 0.40%~0.60%, Ni 0.20%~0.40%, V 0.080%~0.150%, Nb 0.010%~0.030%, Al 0.015%~0.025%, P≤0.012%, S≤0.008%, N 0.0060%~0.0100%, T.O≤0.0020%, the rest is Fe and other inevitable impurities, cooperate heat treatment process production, with good strength and toughness and hydrogen-induced cracking resistance performance.

Description

Technical Field

[0001] This invention belongs to the field of alloy steel, specifically relating to an X80 grade high-toughness, high-resistance to hydrogen-induced cracking thick-walled seamless steel pipe for LNG receiving stations and its heat treatment process. Background Technology

[0002] To improve LNG transportation efficiency, pipelines are developing towards higher pressure, larger diameter, and longer service life, leading to increased strength and wall thickness in LNG terminal pipelines. Since welded pipelines have limitations on wall thickness—excessive thickness makes welding uneconomical and increases the risk of weld failure under increased pressure—pipelines produced by welding cannot meet production requirements.

[0003] For the reasons mentioned above, the demand for high-strength, thick-walled seamless steel pipes for LNG receiving terminals is increasingly strong. However, increased pipe strength also increases the risk of hydrogen-induced cracking, especially in cryogenic LNG transport environments. At low temperatures, the crack threshold for hydrogen-induced cracking decreases significantly, making cracking failure more likely. Furthermore, thick-walled pipes have poorer wall thickness uniformity than thin-walled pipes, and this inhomogeneity in performance and microstructure can create weak points, further increasing the likelihood of cracking.

[0004] Patent CN 110331333A, published on October 15, 2019, discloses a billet for large-diameter seamless steel pipes used in X80 pipelines and its production method. The method involves mixing converter molten iron and scrap steel for electric arc furnace smelting to obtain crude steel; then adding silicon-titanium alloy cored wire to the molten steel to induce a silicon-titanium oxide reaction; followed by slagging and desulfurization of the molten steel, and adjustment of the C, S, Si, and Ti contents to obtain refined steel; finally, the refined steel is cast and stretched to form a continuously cast round billet. This continuously cast round billet can be used to produce large-diameter seamless steel pipes for natural gas transmission pipelines, exhibiting good weldability, low-temperature toughness, and excellent corrosion resistance. However, the patent does not mention the product's performance. It only lists the performance of a 38.5mm thick steel pipe in the examples, which requires welding, and does not test the steel pipe's resistance to hydrogen-induced cracking; therefore, it cannot meet the requirements for high-toughness, high-resistance to hydrogen-induced cracking thick-walled LNG receiving stations.

[0005] Patent CN 112570487A, published on March 30, 2021, discloses a forming process for producing X80 pipeline steel seamless pipes using a Ф800mm large round billet. The process includes: first, heating the forging billet; second, piercing; third, vacuum annealing; fourth, straightening; fifth, pickling; sixth, cold rolling; and seventh, secondary treatment. By controlling the heating temperature and filling the centering hole of the billet during piercing, the process solves the problems of difficulty in low-temperature piercing and easy adhesion to the guide plate in existing processes. Vacuum annealing and pickling give the X80 pipeline steel seamless pipe superior mechanical properties. However, this patent does not cover the composition or steel production method, only describing the pipe-making process. Therefore, the patent's feasibility is insufficient.

[0006] Therefore, it is essential to develop a high-strength, high-toughness, large-diameter, and thick-walled seamless steel pipe resistant to hydrogen-induced cracking to meet the requirements of X80 grade high-toughness and high resistance to hydrogen-induced cracking thick-walled LNG receiving stations. Summary of the Invention

[0007] The purpose of this invention is to provide X80 grade high-toughness, high-resistance to hydrogen-induced cracking thick-walled seamless steel pipes for LNG receiving stations. By designing the composition, high-strength and high-toughness seamless steel pipes can be obtained.

[0008] This invention provides a heat treatment process for seamless steel pipes used in X80 grade high-toughness, high-resistance to hydrogen-induced cracking thick-walled LNG receiving terminals. Based on the composition, a matching heat treatment process is designed to obtain high-performance steel that meets the requirements of X80 grade high-toughness, high-resistance to hydrogen-induced cracking thick-walled LNG receiving terminals.

[0009] The specific technical solution of this invention is as follows:

[0010] A type of seamless steel pipe for thick-walled LNG receiving terminals with high toughness and high resistance to hydrogen-induced cracking (X80 grade) comprises the following components by mass percentage:

[0011] C 0.03%–0.06%, Mn 1.30%–1.50%, Cr 0.20%–0.50%, Mo 0.40%–0.60%, Ni 0.20%–0.40%, V 0.080%–0.150%, Nb 0.010%–0.030%, Al 0.015%–0.025%, P≤0.012%, S≤0.008%, N 0.0060%–0.0100%, TO≤0.0020%, with the remainder being Fe and other unavoidable impurities.

[0012] The composition of the seamless steel pipes used in the X80 grade high-toughness, high-resistance to hydrogen-induced cracking thick-walled LNG receiving terminals also meets the following requirements:

[0013] 90.0≤A≤105.0,

[0014] A=467×(%C+%N)+35×%Mn+30×(%Cr+%Ni)+10×%Mo+3×%V+25×%Nb+60×%Al;

[0015] The composition of the seamless steel pipes used in the X80 grade high-toughness, high-resistance to hydrogen-induced cracking thick-walled LNG receiving terminals also meets the following requirements:

[0016] Y≥0.6%, Y=10×%Ni+6×(%V+%Nb)+5×%Mo+15×%N-20×%C-3×%Mn.

[0017] The present invention provides a heat treatment process for X80 grade high-toughness, high-resistance to hydrogen-induced cracking thick-walled seamless steel pipes for LNG receiving terminals, including quenching and tempering.

[0018] The quenching process involves an initial furnace temperature of ≤400℃ and a heating temperature of T. 淬火加热 800~900℃; heat preservation time t 淬火保温 The steel pipe wall thickness S and heating temperature T 淬火加热 Decision: 150 + (S / 2) - (T) 淬火加热 / 8)≤t 淬火保温 ≤180+(S / 2)-(T 淬火加热 / 8), water cooling; where the steel pipe wall thickness S is in mm, and the heating temperature T 淬火加热 The unit is ℃, and the holding time is t. 淬火保温 The unit is min. When calculating the above formula, simply substitute the data before the unit into the formula.

[0019] The tempering: tempering temperature T 回火加热 580~700℃, holding time t 回火保温 The steel pipe wall thickness S and tempering temperature T 回火加热 The decision is 460 + (S / 2) - (T) 回火加热 / 2)≤t 回火保温 ≤480+(S / 2)-(T 回火加热 / 2), water cooling, where the steel pipe wall thickness S is in mm, and the heating temperature T 回火加热 The unit is ℃, and the holding time is t. 回火保温 The unit is min. When calculating the above formula, simply substitute the data before the unit into the formula.

[0020] The X80 grade high-toughness, high-resistance to hydrogen-induced cracking thick-walled seamless steel pipes for LNG receiving stations are produced by the above heat treatment process. The resulting X80 grade high-toughness, high-resistance to hydrogen-induced cracking thick-walled seamless steel pipes for LNG receiving stations have a wall thickness of 60-70mm and a diameter of ≥1016mm.

[0021] The X80 grade high-toughness and high-resistance to hydrogen-induced cracking thick-walled seamless steel pipe for LNG receiving stations has an inner wall, half wall thickness and outer wall of 100% tempered sorbite; the grain size is 20-27μm, and the difference in grain size between the inner wall, half wall thickness and outer wall is ≤1.5μm.

[0022] The X80 grade high-toughness, high-resistance to hydrogen-induced cracking thick-walled seamless steel pipe for LNG receiving stations has a tensile strength ≥620MPa, yield strength ≥560MPa, KV2 ≥280J at -50℃, A ≥21%, and Z ≥52% at 1 / 2 wall thickness; and good cross-sectional uniformity with a cross-sectional hardness difference ≤20HBW, preferably ≤15HBW; hydrogen-induced cracking testing is carried out according to GB / T 8650, meeting the requirements of CSR ≤1.4% and CLR ≤11% in solution A, preferably CSR ≤1.3% and CLR ≤9%, meeting the high-pressure transportation requirements of LNG receiving stations.

[0023] The present invention also provides a method for producing the steel, which has excellent strength, toughness and resistance to hydrogen-induced cracking, and is suitable for manufacturing seamless steel pipes (wall thickness 60-70mm) for large-diameter thick-walled LNG receiving stations.

[0024] Carbon (C): Carbon is an effective strength element; every 0.01% increase in dissolved C can increase strength by approximately 45 MPa. C forms precipitates with alloying elements in steel, resulting in precipitation strengthening. C can promote a rightward shift in the C-curve, thereby improving hardenability and enabling martensitic structures to form in the center of thick-walled steel pipes. However, as its content increases, plasticity and toughness decrease; therefore, the C content is controlled between 0.03% and 0.06%.

[0025] Mn: Mn can play a solid solution strengthening role, but its solid solution strengthening ability is weaker than that of Si. Mn is an austenite stabilizing element that can significantly improve the hardenability of steel and reduce decarburization. Mn combined with S can prevent hot brittleness caused by S. However, excessive Mn will reduce the plasticity of steel. Therefore, the Mn content should be controlled between 1.30% and 1.50%.

[0026] Cr: Cr is a carbide-forming element. Cr can improve the hardenability and strength of steel, but it easily causes temper brittleness. Cr can improve the oxidation resistance and corrosion resistance of steel, but excessive Cr content will increase crack susceptibility. The Cr content should be controlled between 0.20% and 0.50%.

[0027] Mo: Mo dissolved in the matrix enables the steel microstructure to maintain high stability during tempering and effectively reduces the segregation of impurity elements such as P, S, and As at grain boundaries, thereby improving the toughness of the steel and reducing temper brittleness. Mo reduces the stability of M7C3; when the Mo content is high, acicular Mo2C will form, leading to a reduction in the Mo content in the matrix. Mo can improve the strength of steel through the combined effects of solid solution strengthening and precipitation strengthening, and can also change the toughness of steel by altering the precipitation of carbides. Therefore, the Mo content should be controlled between 0.40% and 0.60%.

[0028] Ni: Ni can form an infinitely miscible solid solution with Fe. It is an austenite stabilizing element, expanding the phase region, increasing the stability of supercooled austenite, shifting the C-curve to the right, and improving the hardenability of steel. Ni can refine the width of martensite laths, increasing strength. Ni significantly lowers the ductile-brittle transition temperature of steel and improves low-temperature toughness. Ni is a precious metal element; excessive addition leads to excessively high costs. The Ni content should be controlled between 0.20% and 0.40%.

[0029] V: V is a strong C and N compound-forming element. V (C, N) is finely dispersed and maintains a coherent relationship with the matrix, thus playing a role in strengthening and refining the microstructure. The V content is controlled between 0.080% and 0.15%.

[0030] Nb: Nb is a strong C and N compound forming element. Nb (C, N) is finely dispersed and maintains a coherent relationship with the matrix, which can strengthen and refine the microstructure. Strengthening the matrix can increase the resistance to fatigue crack initiation and propagation. The Nb content is controlled at 0.010% to 0.030%.

[0031] Al: Al combines with N to form finely dispersed AlN, which remains coherent with the matrix, thus strengthening and refining the microstructure and increasing resistance to fatigue crack initiation and propagation. The Al content is controlled between 0.015% and 0.025%.

[0032] TO and N: TO forms oxide inclusions in steel, and TO should be controlled to be ≤0.0020%.

[0033] In steel, nitrogen can form fine precipitates with nitride-forming elements to refine the microstructure, and it can also precipitate Fe4N. However, the diffusion rate is slow, which leads to aging of the steel and reduces its processing performance. Therefore, nitrogen is controlled at 0.0060% to 0.0100%.

[0034] The thick-walled seamless steel pipe of this invention, with a wall thickness exceeding 60mm, requires high toughness and resistance to hydrogen-induced cracking when serving in LNG environments. The strength of the steel can be improved by adding beneficial alloying elements, the toughness can be improved by using effective element ratios, and the resistance to hydrogen-induced cracking can be improved by forming an effective solid hydrogen precipitation phase. This study of the alloy system shows that C and N have strong strengthening effects under this composition, hence the coefficient is 467. Among the alloying elements, Mn is the most effective in improving hardenability and strength, hence the coefficient is 35. Mo also contributes significantly to hardenability and strength by improving tempering stability and interacting with Mn, hence the coefficient is 10. Cr is a major substitution solid solution element and carbide forming element, contributing 30 to strength. Ni does not form carbides in steel, but improves hardenability and strength by changing the crystal morphology through solid solution strengthening, hence the coefficient is 30. V and Nb are microalloying elements that improve the strength of steel through interaction and the formation of a second phase, but the effect of Nb is significantly greater than that of V, hence the coefficients are 3 and 25, respectively. The study found that Al also has a good contribution to steel. On the one hand, Al can effectively control the grain size when combined with N in steel, and on the other hand, Al is a lightweight metal that can effectively change the density of steel and the interatomic solid solution effect, thus significantly improving the strength and toughness of steel, hence the coefficient is 60. Because the strength, plasticity, and toughness of steel are inversely proportional—high strength leads to decreased plasticity and toughness—strength cannot be increased indiscriminately to ensure the overall performance of steel. Let A represent the strengthening factor in steel; then 90.0 ≤ A ≤ 105.0.

[0035] A=467×(%C+%N)+35×%Mn+30×(%Cr+%Ni)+10×%Mo+3×%V+25×%Nb+60×%Al.

[0036] Seamless steel pipes used in LNG receiving terminals require good resistance to hydrogen-induced cracking during service. Therefore, the proportions of C, Mn, Mo, N, Ni, V, and Nb need to be carefully controlled. While C and Mn significantly increase the strength of steel, these elements are prone to misalignment, leading to microstructure inhomogeneity, increased entropy, and localized weakness in the matrix, thus exacerbating hydrogen-induced cracking. Mo, V, and Nb can form a second phase with C and N in the steel. This second phase acts as a fixed source of hydrogen, providing resistance to hydrogen-induced cracking. Ni increases the stacking fault energy, dislocation density, and dislocation slip rate, thereby improving resistance to hydrogen-induced cracking. Let Y represent the hydrogen-induced cracking resistance factor in the steel; then Y ≥ 0.6%.

[0037] Y=10×%Ni+6×(%V+%Nb)+5×%Mo+15×%N-20×%C-3×%Mn.

[0038] This invention provides a method for producing X80 grade high-toughness, high-resistance to hydrogen-induced cracking thick-walled seamless steel pipes for LNG receiving terminals, comprising the following process flow:

[0039] Smelting in an electric arc furnace or converter → refining in an LF furnace → RH or VD vacuum degassing → continuous casting of round billets → slow cooling of round billets, billet blanking → heating of round billets → piercing → sizing → diameter reduction → heat treatment → flaw detection → grinding → packaging and warehousing.

[0040] In this invention, the vacuum degassing time of RH and VD is integrally correlated with the diameter of the continuously cast billet. When the diameter of the continuously cast billet is 700mm, the vacuum degassing time is 15 minutes. For every 100mm increase in the billet diameter, the vacuum degassing time increases by 1.5 minutes, that is, the vacuum degassing time of a round billet with a diameter of 1200mm is 22.5 minutes.

[0041] The challenges in producing the thick-walled seamless steel pipe provided by this invention compared to thin-walled steel pipes include: 1. Thick-walled pipes have smaller rolling deformation, resulting in poorer as-cast microstructure fragmentation compared to thin-walled pipes, making them more prone to microstructure inheritance and leading to uneven performance. 2. Thick-walled pipes experience better heating during heat treatment than thin-walled pipes, but the heating temperature in the middle and edges is not easily consistent, leading to coarser microstructure and grain size on the outer edges, resulting in lower performance. 3. Thick-walled steel pipes exhibit better cooling than thick-walled pipes, resulting in higher cooling intensity at the outer edges and lower cooling intensity in the center. This makes it difficult to obtain martensite in the center, thus hindering the formation of tempered sorbite during tempering and reducing strength and toughness. This invention, through designed composition and matching relationships, combined with the heat treatment described in this application, can obtain a thick-walled seamless steel pipe for LNG receiving stations with uniform microstructure, excellent performance, and X80 grade high toughness and high resistance to hydrogen-induced cracking.

[0042] The steel pipe of this invention has a diameter ≥1016mm. Producing such pipes presents several challenges: 1. The deformation of the steel pipe manufactured using continuously cast round billets is small, making it difficult to achieve a uniform microstructure. 2. The large outer diameter necessitates pipe diameter expansion, placing high demands on the material's ductility and toughness. This invention addresses these challenges by using a rational element ratio to reduce segregation during the smelting process, resulting in a more uniform steel structure. Furthermore, the synergistic effect of the elements improves the steel's ductility and toughness, thus meeting the requirements for expanding pipe diameter production processes.

[0043] Compared with the prior art, the steel provided by this invention has good strength, toughness and resistance to hydrogen-induced cracking. The X80 grade high-toughness and high-resistance to hydrogen-induced cracking thick-walled seamless steel pipe for LNG receiving stations produced by this invention has a tensile strength ≥620MPa, yield strength ≥560MPa, KV2 ≥280J at -50℃, A ≥21%, and Z ≥52% at 1 / 2 wall thickness; and has good cross-sectional uniformity with a cross-sectional hardness difference ≤20HBW, preferably ≤15HBW; according to GB / T 8650, hydrogen-induced cracking test meets the requirements of CSR ≤1.4% and CLR ≤11% in solution A, preferably CSR ≤1.3% and CLR ≤9%, which meets the performance requirements of seamless steel pipe for large-diameter thick-walled LNG receiving stations. Attached Figure Description

[0044] Figure 1 Microstructure reconstruction of the outer wall, half radius, and inner wall of the seamless steel pipe in Example 1;

[0045] Figure 2 Microstructure reconstruction of the outer wall, half radius, and inner wall of the seamless steel pipe in Comparative Example 3. Detailed Implementation

[0046] The present application will be further illustrated below with reference to several specific embodiments and comparative examples.

[0047] Examples 1-3

[0048] A type of seamless steel pipe for thick-walled LNG receiving terminals with high toughness and high resistance to hydrogen-induced cracking, grade X80, comprises the following composition by mass percentage as shown in Table 1. The balance not shown in Table 1 is Fe and other unavoidable impurities.

[0049] Comparative Examples 1-3

[0050] A type of steel for seamless steel pipes comprises the following composition by weight percentage as shown in Table 1, where the balance not shown in Table 1 is Fe and other unavoidable impurities.

[0051] Table 1 Chemical composition (wt%) of embodiments of the present invention

[0052]

[0053]

[0054] Seamless steel pipes are produced using the aforementioned X80 grade high-toughness, high-resistance to hydrogen-induced cracking thick-walled LNG receiving station seamless steel pipes. The production process, including heat treatment, includes the following steps: electric furnace smelting → LF furnace refining → RH or VD vacuum degassing → continuous casting of round billets (round billet diameter ≥ 700 mm) → slow cooling of round billets, round billet blanking → round billet heating → piercing → sizing → tension reduction → heat treatment → flaw detection → grinding → packaging and warehousing.

[0055] The specific production process is as follows:

[0056] Electric furnace smelting: oxygen is determined before tapping, and steel is left in place during tapping to avoid slag discharge;

[0057] LF furnace refining: C, Mn, Cr, Ni, Mo, V and other elements are adjusted to target values;

[0058] Vacuum degassing: For a continuously cast billet with a diameter of 700mm, the vacuum degassing time is 15 minutes. For every 100mm increase in billet diameter, the vacuum degassing time increases by 1.5 minutes, meaning a 1200mm diameter round billet requires 22.5 minutes of vacuum degassing. Ensure the [H] content after vacuum treatment is ≤1.5ppm to avoid white spots in the steel and hydrogen embrittlement.

[0059] Continuous casting of round billets: The target temperature of molten steel in the ladle is controlled at 10-40℃ above the liquidus temperature, and round billets with a diameter of ≥700mm are continuously cast.

[0060] After the above is made into a round billet, the seamless steel pipe manufacturing route is as follows: round billet, 700mm diameter blanking → round billet heating → piercing → sizing → tension reduction → heat treatment → flaw detection → grinding → packaging and warehousing.

[0061] Seamless steel pipe heat treatment: Bogie furnace heating → heat preservation → quenching → tempering → heat preservation → air cooling.

[0062] The steel pipes produced by the above method have a diameter of ≥1016mm.

[0063] The performance testing methods are as follows:

[0064] Organization: Samples were taken from the seamless steel pipe body at the outer wall, half thickness (64 mm), and inner wall for metallographic, grain size, and hardness difference analysis.

[0065] Performance: Samples were taken from the seamless steel tube body, and tensile, impact, and hydrogen-induced cracking specimens were collected at 1 / 2 thickness (64 mm). Performance tests were conducted according to GB / T228, GB / T229, and GB / T 8650. The heat treatment process is shown in Table 2, and the mechanical properties are shown in Table 3.

[0066] Table 2. Process details of the embodiments and comparative examples of the present invention.

[0067]

[0068] Table 3 Performance testing results of embodiments and comparative examples of the present invention

[0069]

[0070]

[0071]

[0072] Cross-sectional hardness fluctuation is determined by measuring Brinell hardness at three locations along the wall thickness of the steel pipe: the outer side, half the wall thickness, and the inner side. The difference in hardness is calculated, and the maximum value is taken.

[0073] The underlined data above are data that do not meet the requirements of this invention.

[0074] The chemical composition and production methods of the steels in Examples 1-3 were appropriately controlled, ensuring that the chemical composition was 90.0 ≤ A ≤ 105.0 and 0.6% ≤ Y. The steel exhibited good strength, plasticity, toughness, and resistance to hydrogen-induced cracking. In Comparative Examples 1 and 3, the chemical composition control was inadequate; although the heat treatment process was reasonable, the overall performance did not meet the requirements. Comparative Example 2 had a reasonable composition ratio, but the heat treatment process was inappropriate, resulting in unsatisfactory performance.

Claims

1. A type of seamless steel pipe for thick-walled LNG receiving terminals with high toughness and resistance to hydrogen-induced cracking (X80 grade), characterized in that... The X80 grade high-toughness, high-resistance to hydrogen-induced cracking thick-walled seamless steel pipes for LNG receiving terminals comprise the following composition by weight percentage: C 0.03%~0.06%, Mn 1.30%~1.50%, Cr 0.20%~0.50%, Mo 0.40%~0.60%, Ni 0.20%~0.40%, V 0.080%~0.150%, Nb 0.010%~0.030%, Al 0.015%~0.025%, P≤0.012%, S≤0.008%, N 0.0060%~0.0100%, TO≤0.0020%, with the remainder being Fe and other unavoidable impurities; The composition of the seamless steel pipes used in the X80 grade high-toughness, high-resistance to hydrogen-induced cracking thick-walled LNG receiving terminals also meets the following requirements: 90.0≤A≤105.0， A=467×(%C+%N)+35×%Mn+30×(%Cr+%Ni)+10×%Mo+3×%V+25×%Nb+60×%Al; The composition of the seamless steel pipes used in the X80 grade high-toughness, high-resistance to hydrogen-induced cracking thick-walled LNG receiving terminals also meets the following requirements: Y≥0.6%, Y=10×%Ni+6×(%V+%Nb)+5×%Mo+15×%N-20×%C-3×%Mn.

2. A heat treatment process for seamless steel pipes used in X80 grade high-toughness, high-resistance to hydrogen-induced cracking thick-walled LNG receiving terminals as described in claim 1, characterized in that... The heat treatment process includes quenching and tempering.

3. The heat treatment process according to claim 2, characterized in that, The quenching process involves an initial furnace temperature of ≤400℃ and a heating temperature of T. 淬火加热 800~900℃; heat preservation time t 淬火保温 The steel pipe wall thickness S and heating temperature T 淬火加热 Decision: 150 + (S / 2) - (T) 淬火加热 / 8) ≤t 淬火保温 ≤180+(S / 2)-(T 淬火加热 / 8), water cooling; where the steel pipe wall thickness S is in mm, and the heating temperature T 淬火加热 The unit is ℃, and the holding time is t. 淬火保温 The unit is min.

4. The heat treatment process according to claim 2 or 3, characterized in that, The tempering: tempering temperature T 回火加热 580~700℃, holding time t 回火保温 The steel pipe wall thickness S and tempering temperature T 回火加热 The decision is 460 + (S / 2) - (T) 回火加热 / 2)≤t 回火保温 ≤480+(S / 2)-(T 回火加热 / 2), water cooling, where the steel pipe wall thickness S is in mm, and the heating temperature T 回火加热 The unit is ℃, and the holding time is t. 回火保温 The unit is min.

5. The heat treatment process according to any one of claims 2-4, characterized in that, The heat-treated X80 grade high-toughness, high-resistance to hydrogen-induced cracking thick-walled seamless steel pipes for LNG receiving stations have a wall thickness of 60-70mm.

6. The heat treatment process according to any one of claims 2-5, characterized in that, The inner wall, half-wall thickness, and outer wall of the seamless steel pipe for LNG receiving stations with high resistance to hydrogen-induced cracking and a wall thickness greater than 50 mm, produced by heat treatment process, are all 100% tempered sorbite; the grain size is 20-27 μm, and the difference in grain size between the inner wall, half-wall thickness, and outer wall is ≤1.5 μm.

7. The heat treatment process according to any one of claims 2-6, characterized in that, X80 grade high-toughness, high-resistance to hydrogen-induced cracking thick-walled seamless steel pipes for LNG receiving stations have a tensile strength at 1 / 2 wall thickness ≥620MPa, yield strength ≥560MPa, KV2 ≥280J at -50℃, A ≥21%, and Z ≥52%; The cross-sectional hardness difference is ≤20HBW. Hydrogen-induced cracking test shall be carried out in accordance with GB / T 8650, and the CSR and CLR shall be ≤1.4% and ≤11% in solution A.

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