Steel for thick-walled seamless steel pipe for lng receiving station and method for producing seamless steel pipe

Abstract

The present application provides a high-toughness high-crack tip opening displacement LNG receiving station thick-walled seamless steel pipe steel and a method for producing a seamless steel pipe, the composition satisfies: 125.0<=A<=150.0, A=467*(%C+%N)+35*%Mn+30*(%Cr+%Ni)+10*%Mo+3*%V+15*%Si+28*%Cu; Y>=1.0%, Y=10*%Ni+6*%V+5*%Mo+15*%N-20*%C-3*%Mn+8*%Cu-2*%Si. The present application produces a 40-60mm thick-walled seamless steel pipe for LNG receiving station through component design and production method, heat treatment design, the tensile strength of the product at 1 / 2 wall thickness is >=720MPa, the yield strength is >=640MPa, -50 DEG C KV2 is >=200J, and a high-toughness high-crack tip opening displacement thick-walled seamless steel pipe for LNG receiving station is obtained.

Description

TECHNICAL FIELD

[0001] The present application belongs to the field of alloy, relates to the field of steel for seamless pipe, and particularly relates to a steel for thick-walled seamless pipe for LNG receiving station with high strength and toughness and high crack tip opening displacement and a method for producing the seamless pipe. BACKGROUND

[0002] Liquefied natural gas (LNG) is recognized as the cleanest fossil energy on earth, colorless, odorless, non-toxic and non-corrosive, and is a major energy material. In LNG pipeline transportation, LNG stations are needed at intervals for delivery, maintenance, pressure increase and other operations. At present, the LNG receiving station is large in scale, and with the increase of LNG demand, the scale of the receiving station will be further expanded.

[0003] With the increase of LNG delivery pressure, the strength, toughness and crack resistance of the LNG delivery pipeline are required to be improved. Due to the changeable service environment of the LNG delivery pipeline, the pipeline is easy to produce steel pipe micro-defects when being bumped by external objects, and these micro-defects are easy to form crack sources, thereby causing the steel pipe to be broken. Therefore, how to improve the crack tip opening displacement resistance of the steel pipe and improve the crack resistance of the steel pipe is the key to the performance improvement of the steel pipe.

[0004] It is traditionally believed that the crack tip opening displacement resistance of the steel pipe needs to be strictly required for the purity of the steel material, but this method increases the smelting difficulty. In addition, more toughening elements Ni are used to improve the toughness of the material to improve the crack tip opening displacement resistance, but this method directly increases the cost, and the economy needs to be considered. Therefore, how to obtain a LNG seamless steel pipe with high crack tip opening displacement resistance which is easy to realize is increasingly urgent.

[0005] The patent CN 102581553A published on July 18, 2012 points out a manufacturing method of X80 grade large-diameter seamless steel pipe, alloy refined molten steel is cast and centrifugally cast to obtain a large-diameter pipe blank, carbon steel circular parts are fixedly connected at the head and tail ends of the pipe blank, and the inner and outer surfaces of the entire pipe are machined, after heating and dephosphorization, a core rod is inserted into the hollow pipe blank and positioned, and then the pipe blank is sent into a periodic rolling mill to reciprocally roll to obtain a pipe with a required size. After the core rod is removed, the head and tail are cut, and after heating, sizing and finishing, a finished pipe with a qualified size is obtained. However, the steel pipe prepared by the patent has a wall thickness of 35 mm and a diameter of 650 mm. The key point of the product is still the production process of the seamless pipe blank, and the inspection, evaluation and heat treatment of the steel pipe are involved. The crack tip opening displacement performance is not mentioned, and therefore the application environment of the steel pipe needs to be considered.

[0006] Patent CN 110306120A published on October 8, 2019 points out a kind of X80 grade diameter 1422 seamless steel pipe elbow and its manufacturing method, obtains electroslag ingot blank, carries out heating, heat preservation, forging to electroslag ingot blank in turn, obtains bar-shaped pipe blank;First normalizing heat treatment, annealing heat treatment and machining are carried out to bar-shaped pipe blank, and steel pipe blank is obtained;Cold centering, heating, piercing, pipe rolling, expanding, finishing, second normalizing heat treatment are carried out to steel pipe blank in turn, and X80 steel grade D1422mm seamless steel pipe is obtained;Full induction heating bending forming and tempering heat treatment are carried out to seamless steel pipe using medium frequency induction heating elbow bender, and X80 steel grade D1422mm seamless elbow pipe is obtained.But the patent faces the problem of high cost using electroslag ingot.And the maximum wall thickness of the steel pipe is 36mm, and the performance fluctuates greatly, and the maximum fluctuation of strength is 70MPa, which cannot meet the use requirements of large-diameter thick-wall LNG receiving station seamless steel pipe.

[0007] Therefore, it is necessary to develop a high-toughness, large-diameter and thick-wall high-crack tip opening displacement (CTOD) seamless steel pipe steel for manufacturing large-diameter thick-wall LNG receiving station seamless steel pipe. SUMMARY

[0008] The high-toughness high-crack tip opening displacement LNG receiving station thick-wall seamless steel pipe steel provided by the application can be used to manufacture large-diameter thick-wall LNG receiving station seamless steel pipe by component design and matching between components, excellent toughness and high high-crack tip opening displacement (CTOD) performance are obtained.

[0009] Another object of the application is to provide a method for producing seamless steel pipe using high-toughness high-crack tip opening displacement LNG receiving station thick-wall seamless steel pipe steel, and a production method and heat treatment process matched with the components are designed to produce LNG receiving station seamless steel pipe with a wall thickness of 40-60mm, the tensile strength at 1 / 2 wall thickness of the product is ≥720MPa, the yield strength is ≥640MPa, and the KV2 at-50℃ is ≥200J;And has good cross-section uniformity, the cross-section hardness difference is ≤20HBW;At the same time, it has good crack tip opening displacement (CTOD) performance.

[0010] The specific technical solutions of the application are as follows:

[0011] The high-toughness high-crack tip opening displacement LNG receiving station thick-wall seamless steel pipe steel comprises the following components by mass percentage:

[0012] C 0.05% to 0.10%, Si 0.20% to 0.40%, Mn 1.40% to 1.70%, Cr 0.60% to 0.90%, Mo 0.50% to 0.70%, Ni 0.20% to 0.50%, Cu 0.010% to 0.030%, V 0.20% to 0.35%, P ≤ 0.012%, S ≤ 0.008%, N 0.0030% to 0.0070%, T.O ≤ 0.0020%, the rest being Fe and other inevitable impurities.

[0013] The composition of the steel for the high-strength and high-toughness thick-walled seamless steel pipe for LNG receiving stations with high crack tip opening displacement also satisfies:

[0014] 125.0 ≤ A ≤ 150.0, A = 467 × (%C + %N) + 35 × %Mn + 30 × (%Cr + %Ni) + 10 × %Mo + 3 × %V + 15 × %Si + 28 × %Cu;

[0015] The composition of the steel for the high-strength and high-toughness thick-walled seamless steel pipe for LNG receiving stations with high crack tip opening displacement also satisfies:

[0016] Y ≥ 1.0%, Y = 10 × %Ni + 6 × %V + 5 × %Mo + 15 × %N - 20 × %C - 3 × %Mn + 8 × %Cu - 2

[0017] × %Si.

[0018] The method for producing a seamless steel pipe by using the steel for the high-strength and high-toughness thick-walled seamless steel pipe for LNG receiving stations with high crack tip opening displacement provided by the application comprises hot forming and heat treatment of the steel for seamless steel pipes.

[0019] In the hot forming of the steel for seamless steel pipes, the pipe blank heating temperature is controlled to be 1050°C to 1150°C, the pipe piercing deformation is 15% to 30%, the pipe piercing speed is 0.30 to 0.50 s -1 , the single-expanding deformation is 10% to 20%, and the expanding speed is 0.15 to 0.25 s -1 .

[0020] The wall thickness of the produced seamless steel pipe is ≥ 40 mm; preferably, the wall thickness is 40 mm to 60 mm.

[0021] The heat treatment comprises quenching and tempering.

[0022] The quenching is performed at an entering furnace temperature ≤ 400°C and a heating temperature T 淬火加热 820 to 980°C; the holding time t 淬火保温 is determined by the steel pipe wall thickness S and the heating temperature T 淬火加热 , 180 + (S / 2) - (T 淬火加热 / 9) ≤ t 淬火保温≤200+(S / 2)-(T 淬火加热 / 9), water cooling; wherein the wall thickness S of the steel pipe is in mm, the heating temperature T 淬火加热 is in ℃, the holding time t 淬火保温 is in min, and the data before the units are directly brought into the formula calculation when the above formula is calculated.

[0023] The tempering temperature T 回火加热 is 600-720 ℃, and the holding time t 回火保温 is determined by the wall thickness S of the steel pipe and the tempering temperature T 回火加热 , 460+(S / 2)-(T 回火加热 / 3)≤t 回火保温 ≤500+(S / 2)-(T 回火加热 / 3), water cooling, wherein the wall thickness S of the steel pipe is in mm, the heating temperature T 回火加热 is in ℃, the holding time t 回火保温 is in min, and the data before the units are directly brought into the formula calculation when the above formula is calculated.

[0024] Through the above production process, the high-strength and high-toughness high-crack tip opening displacement LNG receiving station thick-walled seamless steel pipe is produced by using the steel with the above composition, the above production method and the heat treatment method.

[0025] The inner wall, 1 / 2 wall thickness and outer wall of the produced seamless steel pipe are all 100% tempered sorbite; the grain size is 20-27 μm, and the difference between the grain sizes of the inner wall, 1 / 2 wall thickness and outer wall is ≤0.5 μm.

[0026] The produced seamless steel pipe has a tensile strength of ≥720 MPa, a yield strength of ≥640 MPa, a KV2 of -50 ℃ of ≥200 J, an A of ≥22%, and a Z of ≥50% at the 1 / 2 wall thickness of the steel pipe, and has good cross-section uniformity, a cross-section hardness difference of ≤20 HBW, preferably ≤15 HBW; according to the CTOD test carried out according to GB / T 21143, the CTOD(δ) is ≥1.10 mm at -20 ℃, and the CTOD(δ) is ≥0.65 mm at -40 ℃, which meets the needs of high-pressure transportation of LNG receiving stations.

[0027] The steel according to the above composition has excellent strength and toughness and high high-crack tip opening displacement (CTOD) performance, and is suitable for manufacturing large-diameter (diameter of 965 mm-1500 mm) thick-walled seamless steel pipes (wall thickness of 40 mm-60 mm) for LNG receiving stations.

[0028] C: C is a strengthening element in steel, and each increase of 0.01% of solid solution C can make the strength increase by about 45 MPa. C and alloying elements in steel form precipitated phase, which plays a role of precipitation strengthening. C can significantly improve the hardenability, so that the center of large wall thickness steel pipe obtains martensite structure. However, with the increase of its content, the plasticity and toughness decrease, so the C content is controlled at 0.05% to 0.10%.

[0029] Si: Si is an effective solid solution strengthening element in steel, which can improve the hardness of steel. Si can play a role of deoxidation in steelmaking and is a commonly used deoxidizer. However, Si is easy to segregate at the austenite grain boundary, which reduces the grain boundary bonding force and causes brittleness. In addition, Si can cause element segregation in steel. Therefore, the Si content is controlled at 0.20% to 0.40%.

[0030] Mn: Mn can play a role of solid solution strengthening, and the solid solution strengthening ability is weaker than Si. Mn is an austenite stabilizing element, which can significantly improve the hardenability of steel and also can reduce the decarburization of steel. Mn combined with S can prevent the hot brittleness caused by S. However, excessive Mn will reduce the plasticity of steel. Therefore, the Mn content is controlled at 1.40% to 1.70%.

[0031] Cr: Cr is a carbide forming element, which can improve the hardenability and strength of steel, but is easy to cause temper brittleness. Cr can improve the oxidation resistance of steel and increase the corrosion resistance, but when the Cr content is too high, the crack sensitivity will increase. The Cr content should be controlled at 0.60% to 0.90%.

[0032] Mo: Mo can effectively reduce the segregation of impurity elements such as P, S and As at the grain boundary, thereby improving the toughness of steel and reducing the temper brittleness. Mo reduces the stability of M7C3, and when the Mo content is high, needle-like Mo2C will be formed, which will reduce the Mo content of the matrix. Mo can improve the strength of steel through the combined action of solid solution strengthening and precipitation strengthening, and can also change the toughness of steel by changing the precipitation of carbide. Therefore, Mo is controlled at 0.50% to 0.70%.

[0033] Ni: Ni can form an infinite solid solution with Fe, which is an austenite stabilizing element, has the effect of expanding the phase region, increases the stability of undercooled austenite, makes the C curve move to the right, and improves the hardenability of steel. Ni can refine the martensite lath width and improve the strength. Ni can significantly reduce the ductile-brittle transition temperature of steel, improve the low temperature toughness, and effectively control the deformation of the steel base after the toughness of the steel is improved, so that sharp cracks do not occur under external force, thereby reducing the crack formation rate and reducing the risk of crack tip expansion. Ni element is a noble metal element, and excessive addition will lead to high cost. The Ni content is controlled at 0.20% to 0.50%.

[0034] V: V is a strong C, N compound forming element, V(C, N) is fine and dispersed, and is in a coherent relationship with the matrix, which can play a role in strengthening and refining the structure. V content is controlled at 0.25% to 0.35%.

[0035] Cu: Cu is to expand the austenite phase region, Cu element can be as a second phase to significantly improve the strength, can improve the structure of the tempering stability and strength. But Cu too high will lead to Cu brittle. Therefore, the Cu content is controlled at 0.010% to 0.030%.

[0036] T.O and N: T.O forms oxide inclusions in steel, and T.O is controlled to be less than or equal to 0.0020%.

[0037] N can form fine precipitated phase with nitride forming elements in steel to refine the structure, and can also precipitate Fe4N, which has a slow diffusion rate, resulting in aging of the steel, reducing the processing performance, therefore N is controlled at 0.0030% to 0.0070%.

[0038] The thick-walled seamless steel pipe is better than the wall thickness of more than 40mm, and needs high toughness and high crack tip opening displacement (CTOD) performance in LNG environment. The strength of the steel can be improved by adding beneficial alloying elements, the toughness of the steel can be improved by effective proportioning of elements, the crack tip opening displacement (CTOD) performance can be improved by forming effective crack propagation resistance in corrosive environment. Through the study of this alloy system under this composition system, the effective coefficient of Mn in alloying elements which improves hardenability and strength is 35; Mo has a great contribution to hardenability and strength through improving tempering stability and interacting with Mn, the coefficient is 10; Cr is the main replacement solid solution element and carbide forming element, the contribution coefficient to strength is 30; Ni and Cu do not form carbides in steel, and change the lattice morphology through solid solution strengthening, thereby improving the hardenability and strength of the steel, the coefficients are 30 and 28 respectively; C and N are non-metallic elements, which are the most important interstitial solid solution strengthening elements in steel, and have an influence on strength and toughness, so the coefficient is 467; Si is a non-metallic element, which is also the main solid solution strengthening element in steel, and the contribution to the performance of the steel is 15; V is a micro-alloying element which improves the strength of the steel through interaction and formation of second phase, so the coefficient is 3. Since the strength of the steel has an inverse proportional relationship with plasticity and toughness, and high strength will lead to low plasticity and toughness, in order to ensure the comprehensive performance of the steel, the strength cannot be increased blindly. Let the strengthening factor in the steel be represented by A, then 125.0≤A≤150.0,

[0039] A=467×(%C+%N)+35×%Mn+30×(%Cr+%Ni)+10×%Mo+3×%V+15×%Si+28

[0040] ×%Cu.

[0041] The seamless steel pipe for LNG receiving station needs good crack tip opening displacement (CTOD) performance during service, so the ratio of C, N, Si, Mn, Mo, Ni, Cu and V needs to be limited. C, Si and Mn can significantly improve the strength of the steel, but these elements are easy to deviate and cause uneven structure, thus increasing the entropy of the material and causing local weakness of the material matrix, thus exacerbating the crack. Mo and V can form a second phase with C and N in the steel, which can control the dislocation slip and migration in the microstructure of the steel, thus improving the crack propagation resistance. Ni can improve the stacking fault energy of the steel, increase the dislocation density and reduce the dislocation slip rate, thus improving the hydrogen-induced cracking resistance. Cu can form a semi-coherent relationship with the steel at the nanoscale, thus fixing harmful elements, controlling crack tip displacement, and hindering the crack. The crack propagation resistance factor of the steel is represented by Y, and Y≥1.0%,

[0042] Y=10×%Ni+6×%V+5×%Mo+15×%N-20×%C-3×%Mn+8×%Cu-2×%Si. The large-diameter steel pipe of the present application has an outer diameter greater than 965mm, and such steel pipe production is difficult: 1. The deformation of the continuous casting round billet for manufacturing steel pipe is small, and the structure of the steel is not easy to be uniform. 2. The outer diameter is large, and the material plasticity and toughness are required to be high. The present application reasonably matches the elements to weaken the segregation effect during smelting, so that the steel is uniform. In addition, through the mutual coordination of the elements, the plasticity and toughness of the steel are improved, so as to meet the production process of expanding the pipe diameter.

[0043] Compared with the prior art, the present application adopts reasonable component design and heat treatment process design, on the one hand, the steel material obtains higher toughness, and the crack propagation resistance of the material is improved; on the other hand, the crack propagation resistance is improved by increasing the fracture crack threshold value of the material. The thick-walled seamless steel pipe for LNG receiving station produced by the present application has a tensile strength of≥720MPa, a yield strength of≥640MPa, and a KV2 of-50℃≥200J at 1 / 2 wall thickness (wall thickness≥40mm); and has good cross-section uniformity, the cross-section hardness difference is≤20HBW; according to GB / T 21143, the CTOD test is carried out, and the CTOD(δ) is≥0.65mm at-40℃, which meets the needs of high-pressure transportation of LNG receiving station. BRIEF DESCRIPTION OF DRAWINGS

[0044] Figure 1 The microstructure and IPF reconstruction of the outer wall, 1 / 2 radius and inner wall of the seamless steel pipe of Example 1 are shown in the following table:

[0045] Figure 2 The low-temperature impact fracture morphology of the outer wall, 1 / 2 radius and inner wall of the seamless steel pipe of Example 1 is shown in the following table:

[0046] Figure 3Microstructure and IPF reconstruction of outer wall, 1 / 2 radius and inner wall of seamless steel pipe of Comparative Example 2;

[0047] Figure 4 Low-temperature impact fracture morphology of outer wall, 1 / 2 radius and inner wall of seamless steel pipe of Comparative Example 2. DETAILED DESCRIPTION

[0048] The present application is further described below in conjunction with several specific examples and comparative examples.

[0049] Examples 1-3

[0050] A high-toughness high-crack-tip-opening-displacement LNG receiving station thick-walled seamless steel pipe steel includes the following mass percentage components: as shown in Table 1, the balance not shown in Table 1 is Fe and other unavoidable impurities.

[0051] Comparative Examples 1-3

[0052] A seamless steel pipe steel includes the following mass percentage components: as shown in Table 1, the balance not shown in Table 1 is Fe and other unavoidable impurities.

[0053] Table 1 Chemical composition of each example and comparative example (wt%)

[0054]

[0055]

[0056] A production method for producing a high-toughness high-crack-tip-opening-displacement LNG receiving station thick-walled seamless steel pipe using a high-toughness high-crack-tip-opening-displacement LNG receiving station thick-walled seamless steel pipe steel, including seamless steel pipe steel hot forming and heat treatment, specifically including the following process flow:

[0057] Electric furnace 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 → reducing sizing → heat treatment → flaw detection → grinding → packaging into warehouse.

[0058] Electric furnace smelting: oxygen is determined before tapping, and the remaining steel operation is adopted during tapping to avoid slagging;

[0059] LF furnace refining: C, Si, Mn, Cr, Ni, Mo, V, Cu and other elements are adjusted to the target value; LF uses strong stirring for desulfurization, decarburization, and stirring uses argon, with a gas flow of 400 L / min-800 L / min; 100-150 meters of calcium line is added for inclusion modification; white slag is made, and the white slag time is controlled at 20-30 minutes; the LF exit temperature is controlled at 1580-1620℃.

[0060] Vacuum degassing: pure degassing time ≥ 15 minutes, ensure that the content of [H] is ≤ 1.5 ppm after vacuum treatment, avoid white spot in steel, cause hydrogen embrittlement phenomenon;

[0061] Round billet continuous casting: the target temperature of the tundish molten steel is controlled at 10-40℃ above the liquidus temperature, and the diameter of the continuous casting round billet is φ=700mm.

[0062] Seamless steel pipe manufacturing route: round billet (diameter 700mm) blanking→round billet heating→piercing→sizing→stretching and reducing diameter→heat treatment→flaw detection→grinding→packaging and warehousing.

[0063] Seamless steel pipe forming process: heating temperature 1050-1150℃, pipe deformation 15-30%, pipe piercing speed 0.30-0.50s -1 , pipe expanding single deformation 10-20%, pipe expanding speed 0.15-0.25s -1 .

[0064] Seamless steel pipe heat treatment: trolley furnace heating→soaking→quenching→tempering→soaking→air cooling.

[0065] The heat treatment method adopted includes quenching and tempering.

[0066] The quenching: furnace entry temperature ≤ 400℃, heating temperature T 淬火加热 820-980℃; soaking time t 淬火保温 is determined by the steel pipe wall thickness S and the heating temperature T 淬火加热 , 180+(S / 2)-(T 淬火加热 / 9)≤t 淬火保温 ≤200+(S / 2)-(T 淬火加热 / 9), water cooling; wherein the unit of the steel pipe wall thickness S is mm, the unit of the heating temperature T 淬火加热 is ℃, and the unit of the soaking time t 淬火保温 is min, and when the above formula is calculated, the data before the unit is directly brought into the formula calculation;

[0067] The tempering: tempering temperature T 回火加热 600-720℃, soaking time t 回火保温 is determined by the steel pipe wall thickness S and the tempering temperature T 回火加热 , 460+(S / 2)-(T 回火加热 / 3)≤t 回火保温 ≤500+(S / 2)-(T 回火加热 / 3), water cooling, wherein the unit of the steel pipe wall thickness S is mm, the unit of the heating temperature T 回火加热 is ℃, and the unit of the soaking time t 回火保温 is min, and when the above formula is calculated, the data before the unit is directly brought into the formula calculation.

[0068] The performance detection method is as follows:

[0069] Tissue: Sampling on the seamless steel pipe body, sampling on the outer wall, 1 / 2 thickness (thickness is 56mm) and inner wall position for metallographic, grain size, hardness difference analysis.

[0070] Performance: Sampling on the seamless steel pipe body, sampling on 1 / 2 thickness (thickness is 56mm) to take tensile, impact, hydrogen induced cracking sample, and performance test is carried out according to GB / T228, GB / T229, GB / T 21143. The heat treatment process is shown in Table 2, and the mechanical properties are shown in Table 3.

[0071] Table 2 Process list of the examples and the comparative examples of the application

[0072]

[0073]

[0074] Table 3 Performance detection list of the examples and the comparative examples of the application

[0075]

[0076]

[0077] The cross-section hardness fluctuation is the hardness difference value calculated by measuring the Brinell hardness at the outer wall, 1 / 2 wall thickness and inner wall respectively in the thickness direction of the steel pipe wall, and taking the maximum value.

[0078] The above underlined data does not meet the requirements of the application.

[0079] The chemical composition, production method of examples 1-3 are properly controlled, the chemical composition ensures 125.0≤A≤150.0, 1.0%≤Y, the strength, plasticity, toughness and CTOD performance of the steel are all good. The comparative examples 1, 2 and 3 are not suitable in chemical composition, the pipe production process and heat treatment process of comparative example 3 are both improper, and the performance is poor.

Claims

1. A type of steel for thick-walled seamless steel pipes used in LNG receiving stations with high strength, toughness, and high crack tip opening displacement, characterized in that... The steel used for high-strength, high-toughness, high-crack-tip-opening-displacement thick-walled seamless steel pipes for LNG receiving stations comprises the following composition by mass percentage: C 0.05%~0.10%, Si 0.20%~0.40%, Mn 1.40%~1.70%, Cr 0.60%~0.90%, Mo 0.50%~0.70%, Ni 0.20%~0.50%, Cu 0.010%~0.030%, V 0.20%~0.35%, P≤0.012%, S≤0.008%, N0.0030%~0.0070%, TO≤0.0020%, with the remainder being Fe and other unavoidable impurities; The composition of the steel used in the high-strength, high-toughness, high-crack-tip-opening-displacement LNG receiving station thick-walled seamless steel pipes also meets the following requirements: 125.0≤A≤150.0, A=467×(%C+%N)+35×%Mn+30×(%Cr+%Ni)+10×%Mo+3×%V+15×%Si+28×%Cu; The composition of the steel used in the high-strength, high-toughness, high-crack-tip-opening-displacement LNG receiving station thick-walled seamless steel pipes also meets the following requirements: Y≥1.0%, Y=10×%Ni+6×%V+5×%Mo+15×%N-20×%C-3×%Mn+8×%Cu-2×%Si.

2. A method for producing seamless steel pipes from the high-strength, high-toughness, high-crack-tip-opening-displacement steel for thick-walled seamless steel pipes used in LNG receiving stations, as described in claim 1, characterized in that... The production method includes hot forming and heat treatment of steel for seamless steel pipes.

3. The method according to claim 2, characterized in that, The hot forming of the seamless steel pipe involves controlling the billet heating temperature to 1050℃~1150℃, the pipe-piercing deformation to 15%~30%, and the pipe-piercing speed to 0.30~0.50s. -1 The deformation amount per expansion cycle is 10%–20%, and the expansion rate is 0.15–0.25 s. -1 .

4. The method according to claim 2 or 3, characterized in that, The heat treatment includes quenching and tempering; the quenching process involves an initial furnace temperature of ≤400℃ and a heating temperature of T. 淬火加热 820~980℃; heat preservation time t 淬火保温 The steel pipe wall thickness S and heating temperature T 淬火加热 Decision: 180 + (S / 2) - (T) 淬火加热 / 9)≤t 淬火保温 ≤200+(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.

5. The method according to claim 4, characterized in that, The tempering: tempering temperature T 回火加热 600~720℃, heat preservation 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 / 3)≤t 回火保温 ≤500+(S / 2)-(T 回火加热 / 3), 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.

6. The method according to any one of claims 2-5, characterized in that, The seamless steel pipes produced are 100% tempered sorbite on the inner wall, half wall thickness, and outer wall; the grain size is 20-27μm, and the difference in grain size between the inner wall, half wall thickness, and outer wall is ≤0.5μm.

7. The method according to any one of claims 2-5, characterized in that, The seamless steel pipes produced have a tensile strength ≥720MPa, yield strength ≥640MPa at 1 / 2 wall thickness, KV2 ≥200J at -50℃, A ≥22%, and Z ≥50%. The cross-sectional hardness difference is ≤20HBW; CTOD test shall be carried out in accordance with GB / T 21143, and the CTOD(δ) shall be ≥1.10mm at -20℃ and ≥0.65mm at -40℃.

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