Steel for seamless steel pipe, high hydrogen-induced cracking resistance wall thickness greater than 50 mm seamless steel pipe for LNG receiving station and its heat treatment process

Abstract

This invention provides seamless steel pipes, seamless steel pipes with high resistance to hydrogen-induced cracking and a wall thickness greater than 50 mm for LNG receiving stations, and their heat treatment processes. The composition is: C 0.05%–0.10%, Si 0.20%–0.40%, Mn 1.70%–2.00%, Cr 0.30%–0.60%, Mo 0.10%–0.30%, Ni 0.40%–0.60%, Cu 0.030%–0.050%, V 0.10–0.20%, Nb 0.015%–0.035%, Al 0.015%–0.025%, P≤0.015%, S≤0.010%, N 0.0050%~0.0090%, TO≤0.0040%, the remainder being Fe and other unavoidable impurities. The design incorporates a matching heat treatment process, and the steel pipes meet the high-pressure transportation requirements of the LNG receiving station.

Description

Technical Field

[0001] This invention belongs to the field of alloy steel, and particularly relates to seamless steel pipes, seamless steel pipes with high resistance to hydrogen-induced cracking and wall thickness greater than 50mm for LNG receiving stations, and their heat treatment processes. The produced seamless steel pipes have high strength and good cross-sectional uniformity. Background Technology

[0002] Liquefied natural gas (LNG) is widely recognized as the cleanest fossil fuel on Earth. It is colorless, odorless, non-toxic, and non-corrosive, and is a major energy source in my country. During LNG pipeline transportation, LNG terminals are needed at intervals to facilitate operations such as transport, maintenance, and pressurization. Currently, LNG receiving terminals are relatively large, and their scale will further expand as LNG demand increases.

[0003] To increase LNG transport capacity, 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 pressure—welded pipelines cannot meet production requirements. Therefore, the demand for high-strength, thick-walled seamless steel pipes for LNG receiving terminals is growing rapidly.

[0004] Patent CN 110331333A, published on October 15, 2019, describes a production method for large-diameter steel pipe blanks used in X80 pipelines. This patent mainly details the production process of the pipe blanks but does not mention the performance of the product. It only lists the performance of steel pipes with a wall thickness of 38.5mm in the examples, and does not test the steel pipe's resistance to hydrogen-induced cracking.

[0005] Patent CN 102581553A, published on July 18, 2012, describes a method for manufacturing X80 grade large-diameter seamless steel pipes. It specifies the use of centrifugal casting to produce seamless steel pipe blanks, which are then manufactured into steel pipes. However, the patented pipes have a wall thickness of 35mm and a diameter of 650mm. The key aspect of the product remains the production process of the seamless pipe blank; the inspection, evaluation, and heat treatment of the steel pipes are not addressed. Furthermore, the resistance to hydrogen-induced cracking is not mentioned, therefore the application environment of the steel pipes needs further consideration.

[0006] Patent CN 110306120A, published on October 8, 2019, discloses a bending method for X80 grade seamless steel pipe with a diameter of 1422mm and its manufacturing process. It uses electroslag ingots for billet production and describes the bending process. However, the patent's use of electroslag ingots faces the problem of high cost. Furthermore, the maximum wall thickness of the steel pipe is 36mm, and its performance fluctuates greatly, with a maximum strength fluctuation of 70MPa.

[0007] Patent CN 112570487A, published on March 30, 2021, discloses a forming process for producing seamless X80 pipeline steel pipes using a φ800mm large round billet. This patent does not cover the composition or steel production method, only describing the pipe-making process. The patent's feasibility is insufficient.

[0008] Patent CN 112981248A, published on June 18, 2021, refers to a continuously cast round billet used for manufacturing X80 seamless steel pipes and its production method. This patent specifies the production process of the raw material billet used for X80 seamless steel pipes. However, it does not mention key indicators such as the diameter, wall thickness, and performance of the final product.

[0009] Patent CN 110404972A, published on November 5, 2019, discloses a method for producing a seamless steel pipe with a diameter of 1422mm. This patent details the production process of the seamless steel pipe. However, the steel pipe has low impact resistance, which is not conducive to its use in LNG environments.

[0010] Therefore, it is urgent to track the characteristics of seamless steel pipes used in LNG receiving terminals, develop a high-strength, high-toughness, large-diameter, and thick-walled seamless steel pipe resistant to hydrogen-induced cracking, and design a targeted heat treatment process for the steel pipe to solve the national energy equipment security problem. Summary of the Invention

[0011] The purpose of this invention is to provide a seamless steel pipe steel and its production method, which, through composition design, obtains a seamless steel pipe steel with large diameter, large wall thickness, high strength and toughness, and resistance to hydrogen-induced cracking.

[0012] Another objective of this invention is to provide a seamless steel pipe for LNG receiving terminals with a wall thickness greater than 50mm and high resistance to hydrogen-induced cracking, along with its heat treatment process. The pipe is produced using the aforementioned seamless steel pipe steel. Based on the characteristics of the steel used in seamless steel pipes for LNG receiving terminals with a wall thickness greater than 50mm and high resistance to hydrogen-induced cracking, a matching heat treatment process is designed. The resulting seamless steel pipe exhibits a tensile strength ≥700MPa, a yield strength ≥630MPa, and a KV2 ≥240J at -50℃ at 1 / 2 wall thickness; it also possesses good cross-sectional uniformity and a cross-sectional hardness difference ≤20HBW; and simultaneously demonstrates excellent resistance to hydrogen-induced cracking.

[0013] The specific technical solution of this invention is as follows:

[0014] A seamless steel pipe steel comprises the following components by mass percentage: C 0.05%–0.10%, Si 0.20%–0.40%, Mn 1.70%–2.00%, Cr 0.30%–0.60%, Mo 0.10%–0.30%, Ni 0.40%–0.60%, Cu 0.030%–0.050%, V 0.10–0.20%, Nb 0.015%–0.035%, Al 0.015%–0.025%, P≤0.015%, S≤0.010%, N 0.0050%–0.0090%, TO≤0.0040%, with the remainder being Fe and other unavoidable impurities.

[0015] The composition of the steel used for the seamless steel pipe also meets the following requirements:

[0016] 35.0≤A≤55.0, A=(4.5×%C)×(1+3.4×%Mn)×(1+0.7×%Si)×

[0017] (1.2+2.6×%Cu)×(1+2.7×%Ni)×(1+3.1×%Cr)×(1+2.3×%Mo)×(1+1.6×%V+4.6×%N+1.7×%Nb);

[0018] The composition of the steel used for the seamless steel pipe also satisfies: Y≥3.0%; Y=2.5×%Cr+3.8×%Mo+16.5×%Ni+2.5×%Cu+1.2×%V+1.4×%Nb-1×%C-4×%Mn.

[0019] The present invention provides a seamless steel pipe for LNG receiving stations with a wall thickness greater than 50 mm and high resistance to hydrogen-induced cracking, which is produced using the aforementioned seamless steel pipe steel.

[0020] The seamless steel pipes for LNG receiving stations with high resistance to hydrogen-induced cracking and a wall thickness greater than 50mm have a wall thickness > 50mm. The production of thick-walled seamless steel pipes presents several challenges compared to thin-walled pipes: First, thick-walled pipes experience less rolling deformation, resulting in poorer as-cast microstructure fragmentation and a higher likelihood of microstructure inheritance, leading to uneven performance. Second, thick-walled pipes exhibit better heat treatment heating than thin-walled pipes, making it difficult to achieve consistent heating temperatures at the center and edges, resulting in coarser microstructure and grain size at the outer edges and lower performance. Third, thick-walled steel pipes exhibit better cooling than thick-walled pipes, leading to higher cooling intensity at the outer edges and lower cooling intensity at 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 addresses these problems through the above-mentioned component design and heat treatment process.

[0021] 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 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.

[0022] The seamless steel pipes for LNG receiving stations with high resistance to hydrogen-induced cracking and a wall thickness greater than 50mm have a tensile strength ≥700MPa, yield strength ≥630MPa, KV2 ≥240J at -50℃, A ≥20%, Z ≥50% at half the wall thickness, and good cross-sectional uniformity with a cross-sectional hardness difference ≤20HBW, preferably ≤15HBW. Hydrogen-induced cracking testing is conducted according to GB / T 8650, meeting the requirements of CSR ≤1.3% and CLR ≤10% in solution A, preferably CSR ≤1.0% and CLR ≤7%, thus meeting the high-pressure transportation requirements of LNG receiving stations.

[0023] This invention provides a heat treatment method for seamless steel pipes with a wall thickness greater than 50 mm that are highly resistant to hydrogen-induced cracking in LNG receiving stations, including quenching and tempering;

[0024] For the quenching process, the steel pipe's furnace entry temperature is ≤400℃, and the heating temperature is T. 淬火加热 The temperature ranges from 840 to 960℃, and the holding time is t. 淬火保温 The steel pipe wall thickness S and heating temperature T 淬火加热 The decision is to control 160 + (S / 2) - (T) 淬火加热 / 9)≤t 淬火保温 ≤170+(S / 2)-(T 淬火加热 / 9), 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.

[0025] The tempering: tempering temperature T 回火加热 The temperature ranges from 580 to 700℃, and the holding time is 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.

[0026] The steel provided by this invention possesses excellent strength, toughness, and resistance to hydrogen-induced cracking, making it suitable for manufacturing seamless steel pipes (wall thickness > 50 mm) for large-diameter, thick-walled LNG receiving stations. The design concept is as follows:

[0027] Carbon (C): Carbon is the cheapest strengthening element in steel. Each 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 significantly improves hardenability, enabling thick-walled steel pipes to acquire a martensitic structure in the center. However, as its content increases, plasticity and toughness decrease; therefore, the C content is controlled between 0.05% and 0.10%.

[0028] Si: Si is an effective solid solution strengthening element in steel, increasing its strength and hardness. Si also acts as a deoxidizer during steelmaking. However, Si tends to segregate at austenite grain boundaries, reducing grain boundary bonding and causing brittleness. Furthermore, Si easily causes elemental segregation in steel. Therefore, the Si content is controlled between 0.20% and 0.40%.

[0029] 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 1.70% and 2.00%.

[0030] 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 0.30% and 0.60%.

[0031] Mo: Mo primarily improves the hardenability of steel. Mo dissolved in the matrix helps maintain high stability of the steel's microstructure during tempering and effectively reduces the segregation of impurity elements such as P, S, and As at grain boundaries, thereby improving the steel's toughness 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 it can also alter the steel's toughness by changing the precipitation of carbides. Therefore, the Mo content should be controlled between 0.10% and 0.30%.

[0032] 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 0.40% and 0.60%.

[0033] Cu: Cu expands the austenite phase region. Elemental Cu can act as a second phase, significantly improving strength and enhancing the tempering stability and strength of the microstructure. However, excessive Cu content will lead to Cu brittleness. Therefore, the Cu content should be controlled between 0.030% and 0.050%.

[0034] 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 at 0.10% to 0.20%.

[0035] 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, thus strengthening and refining the microstructure. Strengthening the matrix increases resistance to fatigue crack initiation and propagation, thereby improving fatigue strength. The Nb content is controlled between 0.015% and 0.035%.

[0036] Al: Al is the main deoxidizer in steelmaking. Al combines with N to form finely dispersed AlN, which maintains a coherent relationship with the matrix. This strengthens and refines the microstructure, increases resistance to fatigue crack initiation and propagation, and thus improves the endurance strength of steel. The Al content is controlled between 0.015% and 0.025%.

[0037] TO and N: TO forms oxide inclusions in steel, so TO should be controlled at ≤0.0040%; N can form fine precipitates with nitride-forming elements in steel to refine the microstructure, and can also precipitate Fe4N, which has a slow diffusion rate, leading to aging of the steel and reduced processing performance. Therefore, N should be controlled at 0.0050% to 0.0090%.

[0038] In this invention, P≤0.015% and S≤0.010% are controlled as impurity elements.

[0039] The steel pipe of this invention has an outer diameter greater than 965mm, belonging to the category of large-diameter seamless steel pipes. Producing such pipes presents several challenges: firstly, the deformation amount is small when manufacturing steel pipes using continuously cast round billets, making it difficult to achieve a uniform steel microstructure; secondly, the large outer diameter necessitates pipe diameter expansion, placing high demands on the material's ductility and toughness. This invention addresses this by using a rational ratio of elements to reduce segregation during the steel smelting process, resulting in more uniform steel. Furthermore, the synergistic effect of the elements improves the steel's ductility and toughness, thus meeting the requirements of the expanded pipe diameter production process.

[0040] Thick-walled seamless steel pipes, with wall thicknesses exceeding 50mm, require 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, under this composition, Mn is the most effective alloying element in improving hardenability and strength, hence the coefficient is 3.4. Mo also contributes significantly to hardenability and strength by improving tempering stability and interacting with Mn, with a coefficient of 2.3. Cr is a major substitutional solid solution element and carbide-forming element, contributing 3.1 to strength. Ni and Cu do not form carbides in steel, but improve hardenability and strength by altering the crystal morphology through solid solution strengthening, with coefficients of 2.7 and 2.6, respectively. C is a non-metallic element and the most important interstitial solid solution strengthening element in steel, affecting both strength and toughness, hence the coefficient is 4.5. Si is a non-metallic element and also a major solid solution strengthening element in steel, contributing 0.7 to the steel's performance. V, N, and Nb are microalloying elements that improve steel strength through interaction and the formation of a second phase. In addition, N can improve steel strength by altering the crystal lattice of C, hence the coefficients are 1.6, 4.6, and 1.7, respectively. Because the strength, plasticity, and toughness of steel are inversely proportional—that is, 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 35.0 ≤ A ≤ 55.0.

[0041] 35.0≤A≤55.0, A=(4.5×%C)×(1+3.4×%Mn)×(1+0.7×%Si)×(1.2+2.6×%Cu)× (1+2.7×%Ni)×(1+3.1×%Cr)×(1+2.3×%Mo)×(1+1.6×%V+4.6×%N+1.7×%Nb);

[0042] Seamless steel pipes used in LNG receiving terminals require good resistance to hydrogen-induced cracking during service. Therefore, the proportions of C, Mn, Cr, Mo, Ni, Cu, V, and Nb need to be carefully controlled. While C and Mn significantly improve 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. Cr, 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. Cu exhibits good nanoscale bonding with steel, forming a semi-coherent relationship, thus fixing hydrogen and hindering hydrogen-induced cracking. Let Y represent the hydrogen-induced cracking resistance factor in steel. Then Y≥3.0%, Y=2.5×%Cr+3.8×%Mo+16.5×%Ni+2.5×%Cu+1.2×%V+1.4×%Nb-1×%C-4×%Mn.

[0043] This invention designs t 淬火保温 The steel pipe wall thickness S and heating temperature T 淬火加热 The decision is 160 + (S / 2) - (T) 淬火加热 / 9)≤t 淬火保温 ≤170+(S / 2)-(T 淬火加热 / 9); 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). The above-mentioned quenching time limit is designed to effectively make the internal and external grains of the thick-walled tube more uniform, thereby improving the uniformity of the steel pipe in the subsequent process. The tempering temperature is to ensure that the size and amount of carbide precipitation in different parts of the steel pipe are more uniform, thereby ensuring the consistency of the performance of different parts of the steel pipe.

[0044] Compared with existing technologies, this invention, through the above-designed composition, obtains steel with excellent strength, toughness, and resistance to hydrogen-induced cracking. It is suitable for manufacturing seamless steel pipes (wall thickness > 50 mm) for large-diameter, thick-walled LNG receiving stations. After production using a matched heat treatment process, the seamless steel pipe has a tensile strength ≥ 700 MPa, a yield strength ≥ 630 MPa, and a KV2 ≥ 240 J at -50℃ at 1 / 2 wall thickness. It also has good cross-sectional uniformity, with a cross-sectional hardness difference ≤ 20 HBW. Hydrogen-induced cracking testing is conducted according to GB / T 8650, meeting the requirements of CSR ≤ 1.3% and CLR ≤ 10% in solution A, thus meeting the high-pressure transportation requirements of LNG receiving stations. Attached Figure Description

[0045] Figure 1 Reconstruction of large-angle grain boundaries on the outer wall, half-radius, and inner wall of the seamless steel pipe produced in Example 2;

[0046] Figure 2 Microscopic large-angle grain boundary reconstruction of the outer wall, half radius, and inner wall of the seamless steel pipe produced in Comparative Example 2. Detailed Implementation

[0047] The present invention provides a seamless steel pipe steel comprising the following components by weight percentage:

[0048] C 0.05%–0.10%, Si 0.20%–0.40%, Mn 1.70%–2.00%, Cr 0.30%–0.60%, Mo 0.10%–0.30%, Ni 0.40%–0.60%, Cu 0.030%–0.050%, V 0.10–0.20%, Nb 0.015%–0.035%, Al 0.015%–0.025%, P≤0.015%, S≤0.010%, N0.0050%–0.0090%, TO≤0.0040%, with the remainder being Fe and other unavoidable impurities.

[0049] The composition of the steel used for the seamless steel pipe also meets the following requirements:

[0050] 35.0≤A≤55.0, A=(4.5×%C)×(1+3.4×%Mn)×(1+0.7×%Si)×(1.2+2.6×%Cu)× (1+2.7×%Ni)×(1+3.1×%Cr)×(1+2.3×%Mo)×(1+1.6×%V+4.6×%N+1.7×%Nb);

[0051] Y≥3.0%; Y=2.5×%Cr+3.8×%Mo+16.5×%Ni+2.5×%Cu+1.2×%V+1.4×%Nb-1×%C-4×%Mn.

[0052] The production method for producing seamless steel pipes with high resistance to hydrogen-induced cracking and a wall thickness greater than 50mm for LNG receiving stations using the above-mentioned seamless steel pipe steel includes the following process flow: electric arc furnace or converter smelting → LF furnace refining → RH or VD vacuum degassing → round billet continuous casting → round billet slow cooling, round billet blanking → round billet heating → piercing → sizing → tension reduction → heat treatment → flaw detection → grinding → packaging and warehousing.

[0053] The circular billet is continuously cast with a diameter φ ≥ 700 mm. For circular billet diameters ≥ 700 mm, the casting speed is ≤ 0.16 m / min, and for every 100 mm increase in the billet diameter, the casting speed decreases by 0.02 m / min from 0.16 m / min.

[0054] Large-diameter, thick-walled seamless steel pipes for LNG receiving terminals undergo quenching and tempering heat treatment. The key heat treatment process is as follows:

[0055] The quenching process involves: the steel pipe entering the furnace at a temperature ≤400℃, and the heating temperature T. 淬火加热 The temperature ranges from 840 to 960℃, and the holding time is t. 淬火保温 The steel pipe wall thickness S and heating temperature T 淬火加热 The decision is to control 160 + (S / 2) - (T) 淬火加热 / 9)≤t 淬火保温 ≤170+(S / 2)-(T 淬火加热 / 9), 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.

[0056] The tempering: tempering temperature T 回火加热 The temperature ranges from 580 to 700℃, and the holding time is 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.

[0057] Seamless steel pipes produced according to the above method have a tensile strength ≥700MPa, yield strength ≥630MPa, KV2 ≥240J at -50℃, A ≥20%, Z ≥50% at 1 / 2 wall thickness, and good cross-sectional uniformity, with a cross-sectional hardness difference ≤20HBW, preferably ≤15HBW; hydrogen-induced cracking test is carried out according to GB / T 8650, meeting the requirements of CSR ≤1.3% and CLR ≤10% in solution A, preferably CSR ≤1.0% and CLR ≤7%, meeting the high-pressure transportation requirements of LNG receiving stations.

[0058] The present application will be further illustrated below with reference to several specific embodiments and comparative examples.

[0059] Examples 1-3

[0060] 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.

[0061] Comparative Examples 1-3

[0062] 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.

[0063] Table 1 Chemical composition (wt%) of embodiments of the present invention

[0064]

[0065]

[0066] The production methods for seamless steel pipes in the above embodiments and comparative examples include the following process flow: electric arc furnace smelting → LF furnace refining → RH or VD vacuum degassing → continuous casting of round billets → slow cooling of round billets, blanking of round billets → heating of round billets → piercing → sizing → tension reduction → heat treatment → flaw detection → grinding → packaging and warehousing.

[0067] Among them, electric arc furnace smelting: oxygen is determined before tapping, and steel retention operation is adopted during the tapping process to avoid slag feeding;

[0068] LF furnace refining: C, Si, Mn, Cr, Ni, Mo, V, Nb, Cu and other elements are adjusted to target values;

[0069] Vacuum degassing: Pure degassing time ≥ 15 minutes, ensuring that the [H] content after vacuum treatment is ≤ 1.5 ppm, avoiding white spots in the steel and causing hydrogen embrittlement;

[0070] 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; the casting speed for round billets with a diameter of ≥700mm is ≤0.16m / min, and for every 100mm increase in the diameter of the round billet, the casting speed is reduced by 0.02m / min from 0.16m / min.

[0071] Slow cooling of round billets: When the round billet is placed in the slow cooling pit, the surface temperature of the billet is ≥600℃. The slow cooling pit is a hot pit with a pit temperature ≥100℃. After the billet is placed in the pit, it is slowly cooled for 96 hours. The cover is then removed for further cooling. The billet is removed from the pit when its temperature is below 200℃.

[0072] Seamless steel pipe manufacturing route: round billet (700mm diameter) blanking → round billet heating → piercing → sizing → tension reduction → heat treatment → flaw detection → grinding → packaging and warehousing.

[0073] Heat treatment: Bogie furnace heating → heat preservation → quenching → tempering → heat preservation → air cooling.

[0074] The quenching and diameter reduction of the steel pipe semi-finished product is carried out at a furnace temperature ≤400℃, and the heating temperature is T. 淬火加热 The temperature ranges from 840 to 960℃, and the holding time is t. 淬火保温The steel pipe wall thickness S and heating temperature T 淬火加热 The decision is to control 160 + (S / 2) - (T) 淬火加热 / 9)≤t 淬火保温 ≤170+(S / 2)-(T 淬火加热 / 9), 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;

[0075] The tempering: tempering temperature T 回火加热 The temperature ranges from 580 to 700℃, and the holding time is 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.

[0076] The steel pipe is manufactured using the above method, with an outer diameter of 1067mm; the performance testing method is as follows:

[0077] Organization: Samples were taken from the seamless steel pipe body at the outer wall, half thickness (the wall thickness of the seamless steel pipe is 56mm) and inner wall for metallographic, grain size and hardness difference analysis.

[0078] Performance: Samples were taken from the seamless steel tube body, and tensile, impact, and hydrogen-induced cracking specimens were collected at 1 / 2 thickness (thickness of 56 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.

[0079] Table 2. Process details of the embodiments and comparative examples of the present invention.

[0080]

[0081] Table 3 Performance testing results of embodiments and comparative examples of the present invention

[0082]

[0083]

[0084] 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.

[0085] The underlined data above are data that do not meet the requirements of this invention.

[0086] Figure 1 Reconstruction of large-angle grain boundaries on the outer wall, half-radius, and inner wall of the seamless steel pipe produced in Example 2; Figure 2 The microscopic large-angle grain boundary reconstructions of the outer wall, half-radius, and inner wall of the seamless steel pipe produced in Comparative Example 2 are shown. It can be seen that the large-angle grain boundary reconstruction diagrams in the examples show denser black lines and smaller white blocks, indicating that the increased number of large-angle grain boundaries through reasonable composition and processing improves the toughness and crack propagation resistance of the steel. In the comparative example, fewer black lines and larger white blocks indicate poorer toughness and crack propagation resistance.

[0087] The chemical composition and production methods of the steels in Examples 1-3 were appropriately controlled, ensuring that the steel with a chemical composition of 35.0 ≤ A ≤ 55.0 and 3.0% ≤ Y exhibited good strength, plasticity, toughness, and resistance to hydrogen-induced cracking. Comparative Example 1 had a lower Y value and a lower tempering temperature, resulting in reduced product strength, plasticity, and resistance to hydrogen-induced cracking. Comparative Example 2 had an unreasonable composition design, a lower Y value, and insufficient heat treatment time, leading to insufficient strength, toughness, and resistance to hydrogen-induced cracking. Comparative Example 3 had improper chemical composition control and an inappropriate heat treatment process, resulting in excessively low material strength, insufficient plasticity and toughness, and unsatisfactory overall performance due to the inappropriate heat treatment process.

Claims

1. A type of steel for seamless steel pipes, characterized in that, The steel used for the seamless steel pipe comprises the following components by weight percentage: C 0.05%~0.10%, Si 0.20%~0.40%, Mn 1.70%~2.00%, Cr 0.30%~0.60%, Mo 0.10%~0.30%, Ni 0.40%~0.60%, Cu 0.030%~0.050%, V 0.10~0.20%, Nb 0.015%~0.035%, Al 0.015%~0.025%, P≤0.015%, S≤0.010%, N 0.0050%~0.0090%, TO≤0.0040%, with the remainder being Fe and other unavoidable impurities; The composition of the steel used for the seamless steel pipe also satisfies: 35.0≤A≤55.0, A=(4.5×%C) ×(1+3.4×%Mn) ×(1+0.7×%Si) ×(1.2+2.6×%Cu) ×(1+2.7×%Ni) ×(1+3.1×%Cr) ×(1+2.3×%Mo) ×(1+1.6×%V+4.6×%N+1.7×%Nb); The composition of the steel used for the seamless steel pipe also satisfies: Y≥3.0%; Y=2.5×%Cr+3.8×%Mo+16.5×%Ni+2.5×%Cu+1.2×%V+1.4×%Nb-1×%C-4×%Mn.

2. A seamless steel pipe for LNG receiving stations with a wall thickness greater than 50 mm and high resistance to hydrogen-induced cracking, characterized in that... It is produced using the seamless steel pipe steel described in claim 1.

3. The seamless steel pipe for LNG receiving stations with a wall thickness greater than 50mm and high resistance to hydrogen-induced cracking as described in claim 2, characterized in that, The seamless steel pipe for LNG receiving stations with high resistance to hydrogen-induced cracking and a wall thickness greater than 50 mm has an inner wall, a 1 / 2 wall thickness section, and an outer wall that are all 100% tempered sorbite; the grain size is 20-27 μm, and the difference in grain size between the inner wall, the 1 / 2 wall thickness section, and the outer wall is ≤1.5 μm.

4. The seamless steel pipe for LNG receiving stations with a wall thickness greater than 50 mm and high resistance to hydrogen-induced cracking as described in claim 2 or 3, characterized in that, The high-hydrogen-crack-resistant seamless steel pipes for LNG receiving stations with a wall thickness greater than 50mm have a tensile strength ≥700MPa, yield strength ≥630MPa, KV2 ≥240J at -50℃, A ≥20%, Z ≥50%, and cross-sectional hardness difference ≤20HBW at 1 / 2 wall thickness. Hydrogen-induced cracking testing is carried out according to GB / T 8650, and the CSR ≤1.3% and CLR ≤10% are met in solution A.

5. A heat treatment process for seamless steel pipes with a wall thickness greater than 50 mm and high resistance to hydrogen-induced cracking in LNG receiving terminals, as described in any one of claims 2-4, characterized in that... The heat treatment includes quenching and tempering.

6. The heat treatment process according to claim 5, characterized in that, For the quenching process, the steel pipe's furnace entry temperature is ≤400℃, and the heating temperature is T. 淬火加热 The temperature ranges from 840 to 960℃, and the holding time is t. 淬火保温 The steel pipe wall thickness S and heating temperature T 淬火加热 The decision is to control 160 + (S / 2) - (T) 淬火加热 / 9) ≤t 淬火保温 ≤170+(S / 2)-(T 淬火加热 / 9), 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.

7. The heat treatment process according to claim 5 or 6, characterized in that, The tempering: tempering temperature T 回火加热 The temperature ranges from 580 to 700℃, and the holding time is 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.

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