X80 grade steel for pipe bundle outer carrier pipe and production method thereof

Abstract

The present application relates to a kind of X80 level pipe bundle outer load bearing tube steel and its production method, steel chemical composition is as follows according to percentage by weight: C 0.13%~0.17%, Si 0.20%~0.40%, Mn 1.75%~2.00%, P 0.003%~0.012%, S≤0.002%, Al 0.01%~0.04%, V 0.040%~0.060%, Mo 0.08%~0.25%, Nb 0.045%~0.07%, Ti 0.01%~0.02%, Ni 0.35%~0.45%, Cu 0.15%~0.25%, Ca 0.004%~0.009%, RE 0.06%~0.08%, H≤0.00015%, O≤0.0020%, wherein Ca / S≥1.8, the rest is Fe and inevitable impurities.The yield strength of the present application pipe bundle outer load bearing tube steel is 555~600MPa, tensile strength is 625~690MPa, elongation is 23%~28%;-20 ℃ impact absorption is 160~220J;650 ℃ high temperature yield strength is 555~585MPa, tensile strength is 625~680MPa, elongation is 24%~29%.

Description

Technical Field

[0001] This invention relates to the field of metallic materials, and in particular to an X80 grade steel for external load-bearing tubes of tubular bundles and its production method. Background Technology

[0002] With increasing environmental awareness and a growing demand for clean energy, oil and natural gas, as among the cleanest energy sources, are receiving increasing attention worldwide for their development and utilization. Due to the limited availability of terrestrial resources, the extraction of seabed oil and gas resources is increasing annually. In China's offshore oil fields, crude oil often exhibits high viscosity, high pour point, and high wax content, thus requiring hot oil pipeline transportation. Pipe bundles offer economical and high-quality insulation, meeting the needs of these transportation processes. Currently, many of China's offshore oil fields are located in shallow waters (less than 200 meters deep), relatively close to the shore. Multiple oil, gas, and water subsea pipelines and cables are often required between platforms within the oil field or between platforms and single-point moorings. Pipe bundles offer significant advantages, allowing for the design and offshore installation of multiple pipes in a single operation. A pipe bundle system refers to two or more pipelines converging together, prefabricated and installed as a single pipe. Against this backdrop, a type of steel for large-diameter pipe bundle external bearing pipes has significant research value. Tube bundle systems have been used worldwide for over 20 years, with the most applications in the North Sea and the Gulf of Mexico. The first tube bundle system was designed and installed in the North Sea in 1980, using the controlled depth towing method (CDTM) for offshore installation. The outer diameter of the load-bearing tube was 12.75 inches, and the total length was only 800 meters. Subsequently, many tube bundle systems of different scales have been designed and installed in the North Sea, the Gulf of Mexico, and around the world. Their complexity, total length, number of internal pipelines, and water depth have all increased rapidly, currently exceeding 80. With the development of tube bundle applications, tube bundles are trending towards thicker walls and larger diameters. The largest tube bundle currently designed and installed has an outer diameter of 50 inches and a wall thickness of 35 mm. my country has not yet conducted research and development on steel for the outer load-bearing tube of large-diameter tube bundles. Currently, oil casing steel is used in my country's shallow seabed oilfields as a substitute for the outer load-bearing tube steel in tube bundles. However, oil casing is usually relatively thin, often produced from hot-rolled strip steel, which has problems such as poor strength-toughness matching with medium-thick plates, thinner specifications, and high-temperature creep. In addition, the steel for casing currently developed is mainly hot-rolled coil with a thickness between 12 and 23 mm. There are no reports of steel plates for casing with a thickness of more than 23 mm.

[0003] my country will build several new deep-sea oil and gas fields in the future, and the demand for steel for large-diameter tubing external bearing pipes is expected to reach 300,000 tons. As seawater depth increases, the wall thickness of the external bearing pipes needs to be continuously increased, which will bring many other problems in addition to cost. In order to reduce the increase in wall thickness, it is necessary to improve strength. X80 grade high-strength tubing external bearing pipe steel is currently unavailable, especially in terms of high-temperature thermal stability and low-temperature toughness.

[0004] Foreign countries have developed steel for external load-bearing tubes of tube bundles, which is thick-walled and large-diameter. It is produced using medium-thick plate technology and straight seam submerged arc welding. It requires excellent low-temperature impact toughness and no high-temperature creep, which is significantly different from the steel used for casing.

[0005] Currently, there is some research on steel for external load-bearing tubes of tube bundles both domestically and internationally. Some patents and literature have been found through searches, but the content recorded therein is significantly insufficient compared to the technical solution of this invention in terms of composition, production method, high-temperature thermal stability, and low-temperature toughness.

[0006] Related Patent 1: "A Method for Preparing H40 Oil Casing Steel for a 2250mm Production Line"

[0007] (CN201911408022.0) only provides hot-rolled coils for casing in oil and gas pipeline projects with diameters less than 711mm and thicknesses ranging from 3 to 19mm, as well as their preparation methods. It does not have high-temperature performance indicators and does not conform to the manufacturing process of medium and heavy plates.

[0008] Related patent 2: "A steel for thick-walled high-toughness resistance welding sleeve and its manufacturing method" (CN202010765824.3) provides a manufacturing method for steel with a wall thickness >13mm, which is a steel for thick-walled high-toughness resistance welding sleeve. The resistance welding material is also hot-rolled coil with a diameter of less than 711mm and a thickness of less than 23mm. It does not have high-temperature performance indicators and does not meet the manufacturing process of medium and heavy plates.

[0009] Related patent 3: "A Cr microalloyed oil casing steel and its manufacturing method" (CN201710990236.8) provides a high-strength oil casing steel and its preparation method. By adding a certain Cr content, such as 0.40-0.50%, the strength requirements of the steel pipe are met, and the corrosion resistance of the steel is increased to a certain extent. The material used is hot-rolled coil, which does not have high-temperature performance indicators and does not meet the manufacturing process of medium and heavy plates.

[0010] Related Patent 4: "Needle-shaped ferritic low-temperature resistant N80 grade oil casing steel and its preparation method"

[0011] CN201811577443.1 discloses a needle-like ferritic low-temperature resistant N80 grade oil casing steel and its preparation method, which has good low-temperature impact resistance and HIC corrosion resistance. However, it is still a hot-rolled coil and lacks high-temperature performance indicators, thus not conforming to medium-thick plate manufacturing processes.

[0012] In summary, existing technologies for medium-thick plates with wall thicknesses of 23mm or more used in submerged arc welding for the production of 555MPa-grade outer load-bearing tubes for tubular bundles still have shortcomings in terms of low-temperature resistance, high toughness, and high-temperature thermal stability, and cannot meet the material performance requirements of subsea oil and gas fields. Summary of the Invention

[0013] The purpose of this invention is to overcome the above-mentioned problems and deficiencies and provide a material that meets the material requirements of subsea oil and gas fields, with low alloy cost, simple and easy process, no need for tempering heat treatment, and easy industrial mass production of X80 grade tube bundle external bearing steel and its preparation method.

[0014] This invention addresses the shortcomings of X80 grade steel in oil and gas extraction and transportation processes, particularly in areas such as high-temperature thermal stability and low-temperature toughness. It provides a comprehensive approach, encompassing material composition design, ladle metallurgy, and microstructure control, to offer X80 grade steel for external bearing tubes and its manufacturing method. This solution overcomes the aforementioned challenges and meets the material requirements of submerged arc welded pipes. The product is suitable for thick-walled submerged arc welded pipes, using medium-thick steel plates with a microstructure of acicular ferrite + MA.

[0015] To achieve the above objectives, the present invention employs the following technical solution:

[0016] A type of X80 grade steel for external load-bearing tubes of tubular bundles, the chemical composition of which, by weight percentage, is: C 0.13%–0.17%, Si 0.20%–0.40%, Mn 1.75%–2.00%, P 0.003%–0.012%, S≤0.002%, N 0.0010%–0.0030%, Al 0.01%–0.04%, V 0.040%–0.060%, Mo 0.08%–0.25%, Nb 0.045%–0.07%, Ti 0.01%–0.02%, Ni 0.35%–0.45%, Cu 0.15%–0.25%, Ca 0.004%–0.009%, RE 0.06%~0.08%, H≤0.00015%, O≤0.0020%, of which Ca / S≥1.8, the remainder being Fe and unavoidable impurities.

[0017] The following details the mechanism of action of each alloy component in an X80 grade steel for external load-bearing tubes of this invention, where the percentage symbol % represents a weight percentage:

[0018] Carbon (C) is an essential element for ensuring the strength of steel. A content above 0.13% gives the steel good strength at both room temperature and high temperatures, while also providing excellent wear resistance. However, weldability deteriorates when the content exceeds a certain level. Therefore, the upper limit is 0.17%. From both economic and product performance perspectives, it is preferable to control the C content between 0.13% and 0.17%.

[0019] Si is the main deoxidizing component in steelmaking. To achieve sufficient deoxidation, it must contain more than 0.10%. However, if it exceeds the upper limit, it will reduce the toughness of the base material and the weld. Si in solid solution form can increase strength and also increase the ductile-brittle transition temperature. Therefore, the Si content should be 0.20% to 0.40%.

[0020] Mn (manganese) is an essential element for ensuring the strength and toughness of steel. Mn combines with sulfur (S) to form MnS, preventing hot cracking caused by FeS formation at grain boundaries. Mn is also a good deoxidizer. While manganese is a low-cost strengthening and toughening element, too low a content cannot guarantee the material's strength. However, excessively high Mn content can exacerbate segregation in the cast billet and worsen the low-temperature toughness of the coarse-grained heat-affected zone (CGHAZ). Therefore, the Mn content should be controlled between 1.75% and 2.00%.

[0021] P is an unavoidable impurity element in steel, which deteriorates the steel's toughness and weldability. Studies have shown that when the P content is higher than 0.015%, its corrosivity on the upper deck under acidic gaseous conditions decreases significantly. The upper limit of this invention is below 0.012%.

[0022] If the sulfur (S) content exceeds 0.01%, it will form a large number of MnS inclusions in the steel. These MnS inclusions are the origin of pitting corrosion, thus reducing the steel's corrosion resistance. Therefore, measures must be taken to minimize the S content in the steel. Thus, in this invention, the upper limit for the S content is set at 0.002%.

[0023] Al: As a deoxidizing and grain refining element, the content is generally above 0.01%, but when it exceeds 0.06%, it is easy to cause hot cracks in the billet, forming a large number of inclusions, and at the same time, the toughness of the steel is reduced. Therefore, the upper limit of Al content is 0.055%, and the content range of this invention is 0.01% to 0.04%.

[0024] Vanadium (VC) has a strong affinity for both oxygen (O) and nitrogen (N), making it a strong carbide-forming element. VC generally exhibits high dispersion and is extremely stable, thus facilitating deoxidation and degassing to achieve a dense, fine-grained structure, improving plasticity, toughness, and strength. Its impact resistance and fatigue strength are higher than those of vanadium-free steel. The high dispersion of vanadium carbide prevents coarse grain growth in the weld, thus improving the weldability of the steel. However, heating to the VC melting temperature will cause strong grain growth in the steel. When dissolved in a solid solution at high temperatures, it increases hardenability; vanadium increases the tempering stability of quenched steel and produces a secondary hardening effect. VC can increase the solid solution content of rare earth elements in steel, thereby improving its corrosion resistance. In this invention, its content is controlled at 0.040%–0.060%.

[0025] Mo: As an element that significantly improves hardenability and bainitrification tendency, appropriate addition of Mo to steel can inhibit the formation of pearlite. Its effect on improving the strengthening of steel is similar to that of Cr, improving the strength and wear resistance of steel through the formation of carbides. Mo also has good "red hardness," allowing steel to maintain high strength and hardness at high temperatures. However, excessive addition can also lead to deterioration of weldability and low-temperature toughness. Therefore, the preferred Mo content range is between 0.08% and 0.25%.

[0026] Nitrogen b (Nb) is one of the key elements for grain refinement and strengthening. Its grain refinement effect manifests in two aspects: firstly, it significantly delays austenite recrystallization, increasing the recrystallization temperature and preventing the growth of recrystallized austenite; secondly, as the rolling temperature decreases, Nb C and N compounds disperse and precipitate before the austenite transforms into ferrite, becoming ferrite nucleation sites. This allows ferrite to form under low undercooling conditions, making it less prone to growth and refining the ferrite grain size. As an element that extends the non-recrystallization temperature range, Nb can improve crack arrest toughness by increasing high-angle grain boundaries through grain refinement. Therefore, the preferred Nb content range is between 0.045% and 0.07%.

[0027] Ti (TiO2) is added to improve the toughness of steel and welded parts. As a strong nitrogen-fixing element, it easily forms TiN, which improves the weld metal's resistance to nitrogen porosity. The effect is minimal below 0.005%, and beyond 0.055%, large TiN particles are formed, negating its effectiveness. To achieve low-temperature toughness under high heat input, the Ti content in steel plates needs to be controlled; therefore, the added Ti content ranges from 0.01% to 0.02%.

[0028] Ca: The combination of Ca and S to form CaS can coat inclusions such as alumina, achieving the modification and spheroidization of the inclusions, which is beneficial to improving corrosion resistance, toughness and fatigue resistance. At the same time, the fine and dispersed CaS formed in the early stage can reduce the proportion of MnS formation. CaS reacts with H2O to dissociate alkaline OH- ions, which can reduce the acidification degree of corrosion pits and reduce the sensitivity to pitting corrosion. In this invention, Ca: 0.004%~0.009%, Ca / S≥1.8.

[0029] Ni: As an austenite stabilizing element, increasing Ni can improve the solid solution strengthening effect. Nickel can lower the ferrite phase transformation temperature, refine the ferrite grain size, and play a role in grain refinement strengthening. In addition, it can promote the formation of acicular ferrite, which, while generating phase transformation strengthening, can also increase the grain boundary area, improve fracture toughness and crack arrest ability. Therefore, the preferred Ni content is between 0.35% and 0.45%.

[0030] Cu can significantly improve the hardenability and corrosion resistance of steel plates. It is also an austenite stabilizing element in steel. Appropriate addition can refine the microstructure of TMCP steel plates and improve low-temperature toughness. However, excessive addition will cause "copper embrittlement" tendency, making it easy for cracks to appear on the surface and inside of the billet, reducing the mechanical properties of the rolled steel plate, and reducing toughness, causing the steel plate to become embrittled. Therefore, the Cu content in this invention is controlled at 0.15% to 0.25%.

[0031] RE: Rare earth (RE) atoms are highly reactive and have strong binding forces. Adding rare earth to steel can improve solidification structure, change solid-state phase transformation structure, form harmless low-melting-point inclusions, strengthen interfaces through segregation, and passivate surface rust layers. Rare earth can increase the self-corrosion potential and polarization resistance of weathering steel, thereby inhibiting the anodic reaction, increasing the resistance of the entire electrochemical reaction, and significantly reducing the corrosion rate of steel. Rare earth is enriched at grain boundaries through diffusion mechanisms, inhibiting the segregation of inclusions at grain boundaries, and improving the low-temperature performance and corrosion resistance of steel. However, rare earth is a scarce resource, and its addition amount must be controlled. The RE content in this invention is controlled at 0.06% to 0.08%.

[0032] Nitrogen (N) can form fine precipitates with Nb and Ti, which can strengthen and refine the grains, improving strength and toughness. However, excessive nitrogen content can deteriorate toughness. It is advisable to control the nitrogen content between 0.0010% and 0.0030%.

[0033] In this invention, H and O are unavoidable harmful impurity elements; an increase in their content will lead to an increased tendency for hydrogen-induced cracking, an increase in inclusions, and a decrease in corrosion resistance and fatigue resistance. Therefore, this invention controls H ≤ 0.00015% and O ≤ 0.0020%.

[0034] A method for producing X80 grade steel for external load-bearing tubes of tubular bundles includes the following steps:

[0035] 1) Smelting: Deeply desulfurized molten iron with a sulfur content ≤0.002% is used. After the molten iron arrives at the converter, a process combining "double slag" dephosphorization and "slag removal" of molten steel after the furnace is adopted for smelting. The final slag basicity is controlled at R=3.1~4.3. Through effective slag blocking operation, a large amount of slag is prevented from falling. The steel discharge time is not less than 5 minutes.

[0036] The smelting process adopts a high-pulling carbon one-time blowing method. The main elements of the steel are adjusted to the range of this invention in the converter, and other alloy components are added as required for smelting.

[0037] The molten steel removed from the converter undergoes secondary refining to further reduce the content of harmful impurities such as O, S, and non-metallic inclusions. During the LF refining process, aluminum granules, silicon carbide, and calcium carbide are used to adjust the slag, and the final slag basicity is controlled above 2.3. After LF refining, Ca treatment is performed, and wire is fed at a rate of 1.2–2.0 meters per ton of steel.

[0038] 2) Continuous casting: superheat ≤20℃, weak cooling is used for secondary cooling, and the casting process is operated according to the matching of temperature and casting speed. The casting speed of the continuous casting billet is 1.0~1.6m / min, and the thickness of the cast slab is 200~360mm.

[0039] 3) Heating: The steel slab is heated to 1150℃~1230℃. This is because temperatures below 1150℃ are insufficient to allow the alloying elements to completely dissolve into the austenite, thus failing to guarantee the final rolling temperature required for hot rolling. Temperatures above 1230℃ cause significant coarsening of the original austenite grains, which reduces the low-temperature toughness of the steel plate.

[0040] 4) Rolling: To meet the mechanical property requirements of tubular steel, rolling is controlled in two stages at 1150-980℃ in the austenite recrystallization zone and the non-recrystallization zone, and the cumulative reduction in the two stages of rolling is not less than 60%.

[0041] The initial rolling temperature in the recrystallization zone is 1080–1150℃, and the final rolling temperature is 980–990℃.

[0042] Different rolling temperatures and cooling rates in the non-recrystallization zone can be selected based on the mechanical property requirements of the steel plate. For example, for high-strength steel with a yield strength requirement of X80 grade and an impact toughness temperature requirement of -20℃, the hot-rolled non-recrystallization zone rolling temperature is below 880℃, and the final rolling temperature is above 780℃.

[0043] 5) Cooling: The cooling method is laminar flow cooling. The air cooling time after rolling is 12-18s, the laminar flow cooling start temperature is 760-770℃, the cooling time is 10-15s, and the cooling rate is controlled at 10-13℃ / s to cool the steel plate to 450℃-480℃.

[0044] The steel plate used for the outer load-bearing tube of this invention has a thickness of 23mm to 35mm. The microstructure of the steel plate is acicular ferrite + MA (Mao Island). The yield strength of the steel for the outer load-bearing tube of the tube bundle is 555 to 600 MPa, the tensile strength is 625 to 690 MPa, and the elongation is 23% to 28%; the impact absorption energy at -20℃ is 160 to 220 J; the yield strength at 650℃ is 555 to 585 MPa, the tensile strength is 625 to 680 MPa, and the elongation is 24% to 29%.

[0045] Compared with the prior art, the beneficial effects of the present invention are:

[0046] 1) The X80 grade tube bundle external bearing tube prepared by the present invention has good strength and toughness matching: yield strength is 555-600MPa, tensile strength is 625-690MPa, elongation is 23%-28%; impact absorption energy at -20℃ is 160-220J; at a high temperature of 650℃, yield strength is 555-585MPa, tensile strength is 625-680MPa, elongation is 24%-29%.

[0047] 2) This invention is simple to operate, and no tempering heat treatment is required after online controlled cooling, which improves production efficiency and saves production costs, making it easy to achieve industrialized mass production.

[0048] 3) This invention adopts a two-stage controlled rolling process. The microstructure of the steel used for the outer bearing tube of the tube bundle is controlled by the final rolling temperature of the X80 grade tube bundle outer bearing tube steel plate, so as to maintain ideal performance, eliminate high-temperature creep, and significantly reduce production costs.

[0049] 4) The maximum thickness of the suitable X80 grade tube bundle outer bearing tube steel plate described in this invention reaches 35mm, which has good low-temperature toughness and high-temperature thermal stability, and meets the technical requirements of the materials required for manufacturing large wall thickness tube bundle outer bearing tubes. Detailed Implementation

[0050] To make the objectives, technical solutions, and advantages of this invention clearer, the specific implementation methods of this invention will be further described below in conjunction with the embodiments. The following embodiments are used to specifically illustrate the content of this invention. These embodiments are only general descriptions of the content of this invention and do not limit the content of this invention.

[0051] This invention primarily addresses the use of X80-grade steel for bearing tubes in offshore oil and gas fields during oil and gas extraction and transportation. This material possesses characteristics such as large thickness (over 23 mm), excellent low-temperature toughness (meeting -20℃), high-temperature creep resistance, and stable yield strength in the heat-affected zone during welding. Mechanical property evaluation of the base material prepared using this invention shows good overall mechanical properties and high-temperature creep resistance.

[0052] Examples are shown in Tables 1-3, where Table 1 shows the chemical composition of each example; the rolling process of each example is shown in Table 2; the mechanical properties of each example are shown in Table 3; and the mechanical properties of the steel plates of the corresponding examples after being subjected to a high temperature of 650℃ are shown in Table 4.

[0053] Table 1. Chemical composition (%) of steel smelting in each embodiment

[0054]

[0055] Table 2 Process parameters of steel in each embodiment

[0056]

[0057] Table 3. Steel plate properties in the examples

[0058]

[0059]

[0060] Table 4 Mechanical properties of the embodiments of the invention after being subjected to high temperature of 650℃

[0061]

Claims

1. A steel for X80 grade pipe bundle outer carrier pipe, characterized in that, The chemical composition of the steel is as follows in percentage by weight: C 0.16%-0.17%, Si 0.20%-0.40%, Mn 0.183%-2.00%, P 0.003%-0.012%, S≤0.002%, N 0.0010%-0.0030%, Al 0.01%-0.04%, V 0.051%-0.060%, Mo 0.08%-0.25%, Nb 0.055%-0.07%, Ti 0.01%-0.02%, Ni 0.35%-0.45%, Cu 0.15%-0.25%, Ca 0.004%-0.009%, RE 0.06%-0.08%, H≤0.00015%, O≤0.0020%, wherein Ca / S≥1.8, and the rest is Fe and inevitable impurities; The yield strength of the steel for the outer load-bearing tube of the tube bundle is 555-600 MPa, the tensile strength is 625-690 MPa, and the elongation is 23%-28%; the impact absorption energy at -20 ℃ is 160-220 J; the yield strength at 650 ℃ is 555-585 MPa, the tensile strength is 625-680 MPa, and the elongation is 24%-29%; The production method of the X80 grade steel for the outer load-bearing tube of the tube bundle comprises the following steps: two-stage controlled rolling in the austenite recrystallization zone and the non-recrystallization zone at 1150-980 ℃, and ensuring that the cumulative reduction in the two-stage rolling is not less than 60%; The rolling temperature in the non-recrystallization zone of hot rolling is below 880 ℃, and the finish rolling temperature is 780-788 ℃.

2. The steel for X80 grade pipe bundle outer carrier pipe according to claim 1, characterized in that, The thickness of the steel plate for the outer load-bearing tube of the tube bundle is 23-35 mm.

3. The steel for X80 grade pipe bundle outer carrier pipe according to claim 1, characterized in that, The microstructure of the steel plate is acicular ferrite+MA.

4. A method of producing a steel for an X80 grade pipe bundle outer carrier pipe according to any one of claims 1 to 3, characterized in that, The method comprises the following steps: 1) heating: heating the continuous casting slab to 1150-1230 ℃; 2) rolling: two-stage controlled rolling in the austenite recrystallization zone and the non-recrystallization zone at 1150-980 ℃, and ensuring that the cumulative reduction in the two-stage rolling is not less than 60%; The rolling temperature in the non-recrystallization zone of hot rolling is below 880 ℃, and the finish rolling temperature is 780-788 ℃; 3) cooling: air cooling for 12-18 s after rolling, and then performing laminar flow cooling, the starting temperature of the laminar flow cooling water cooling is 760-770 ℃, the water cooling rate is controlled at 10-13 ℃ / s, and the water cooling time is 10-15 s, so as to cool the steel plate to 450-480 ℃.

5. A method of producing a steel for an X80 grade pipe bundle outer carrier pipe according to claim 4, characterized by, The molten steel is smelted by using deep desulfurization molten iron with a sulfur content of not more than 0.002%, and the final slag basicity in the converter is controlled at R=3.1-4.3; The molten steel taken out from the converter is subjected to secondary refining, and aluminum particles, silicon carbide and calcium carbide are used for slag adjusting during the LF refining process, and the final slag basicity is controlled at not less than 2.3; Ca treatment is performed after the LF ends, and 1.2-2.0 meters of wire is fed per ton of steel.

6. A method of producing a steel for an X80 grade pipe bundle outer carrier pipe according to claim 5, characterized by, The time for discharging the molten steel from the converter is not less than 5 min.

7. The method of producing a steel for an X80 grade pipe bundle outer carrier pipe according to claim 4, characterized by, The superheating degree of continuous casting is not more than 20 ℃, and the continuous casting speed is 1.0-1.6 m / min.

8. A method of producing a steel for an X80 grade pipe bundle outer carrier pipe according to claim 4 or 7, characterized in that, The thickness of the continuous casting slab is 200-360 mm.

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