X60 grade steel for outer load-bearing tube of tube bundle and production method thereof

Abstract

The invention relates to an X60 grade steel for an outer load-bearing tube of a tube bundle and a production method thereof. The chemical composition of the steel is as follows, by weight percentage: C 0.11%-0.14%, Si 0.25%-0.45%, Mn 0.85%-1.15%, P≤0.012%, S≤0.005%, Nb 0.02%-0.03%, Ti 0.01%-0.02%, Sb 0.05%-0.08%, RE 0.06%-0.08%, Als 0.01%-0.05%, N 0.0010%-0.0030%, H≤0.00015%, O≤0.0020%, Ca 0.004%-0.009%, wherein Sb+RE≥0.12%, Ca / S≥1.8, and the remainder is Fe and unavoidable impurities. This product is suitable for thick-walled submerged arc welded pipes, which are medium and thick plates with a structure of ferrite + a small amount of pearlite. The alloy produced by this technology has low cost, simple process, no need for tempering heat treatment, and is easy to achieve industrial mass production.

Description

Technical Field

[0001] The present invention relates to the field of medium and thick plate / thick plate manufacturing, and in particular to an X60 grade steel for external load-bearing tubes of tube bundles and a production method thereof. Background Art

[0002] With growing environmental awareness and the demand for clean energy, the development and utilization of oil and natural gas, one of the cleanest energy sources, is gaining increasing attention worldwide. Due to limited land resources, the extraction of submarine oil and gas resources is increasing annually. In China's offshore oilfields, crude oil is often highly viscous, highly viscous, and waxy, necessitating the use of hot oil pipelines for transportation. Tube bundles offer economical and high-quality thermal insulation, adapting to these transportation requirements. Currently, many offshore oilfields are located in shallow waters (less than 200 meters deep) and relatively close to shore. Multiple submarine oil, gas, and water pipelines and cables are often required between platforms within the field or between platforms and single-point moorings. The tube bundle approach offers significant advantages, enabling the simultaneous design and offshore installation of multiple pipelines. A tube bundle system involves the consolidation of two or more pipelines, prefabricated and installed as a single pipeline. Against this backdrop, a large-diameter steel material for the outer bearing of tube bundles holds significant research value. Tube bundle systems have been used worldwide for over 20 years, with the largest applications occurring in the North Sea and the Gulf of Mexico. The first tube bundle system was designed and installed in the North Sea in 1980. The offshore installation utilized the Controlled Depth Tow Method (CDTM), with a load-bearing pipe outer diameter of 12.75 inches and a total length of just 800 meters. Subsequently, numerous tube bundle systems of varying sizes were 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 depths have rapidly increased, reaching over 80 systems to date. With the development of tube bundle applications, bundles have become increasingly thicker and larger in diameter. The largest bundle currently designed and installed has an outer outer diameter of 50 inches and a wall thickness of 35 mm. my country has yet to initiate research and development of steel for large-diameter bundle outer load-bearing pipes. Currently, shallow offshore oilfields in my country are replacing these with steel for oil casing. However, oil casing is typically thin and often produced from hot-rolled strip steel. This leads to poor compatibility between strength and toughness with medium and heavy plate, thin gauges, and high-temperature creep. In addition, the casing steel developed at this stage is mainly hot-rolled coil with a thickness between 12 and 23 mm. There are no reports on casing steel plates with a thickness of more than 23 mm.

[0003] my country plans to build numerous new deep-sea oil and gas fields in the future, and demand for large-diameter external pipe steel is expected to reach 300,000 tons. While this type of steel has been developed overseas, significantly improving subsea oil and gas production and transportation efficiency while reducing costs, it remains a niche technology in my country, posing a potential threat to China's continued viability in offshore oil and gas production. X60-grade, thick-walled, large-diameter external pipe steel for pipes is produced using the medium-plate process and the longitudinal submerged arc welding process. It requires excellent low-temperature impact toughness, significantly different from casing steel.

[0004] At present, there are some studies on steel for external load-bearing tubes of tube bundles at home and abroad. After searching, some patents and literature were found. However, the contents recorded therein are obviously insufficient with respect to the composition, production method, high-temperature creep, thermal stability, low-temperature toughness and other aspects described in the technical solution of the present invention.

[0005] Related patent 1: CN201911408022.0 "A 2250mm production line for producing H40 oil casing steel and its preparation method" only provides hot-rolled coils and their preparation methods for casing in oil and gas pipeline projects with a diameter less than 711mm and thickness specifications covering 3-19mm. It has no high-temperature performance indicators and does not conform to the manufacturing process of medium and thick plates.

[0006] Related patent 2: CN202010765824.3 "A thick-walled high-toughness steel for resistance welding casing and its manufacturing method" provides a manufacturing method for thick-walled high-toughness steel for resistance welding casing with a steel wall thickness of more than 13mm. The material for resistance welding is also a hot-rolled coil with a diameter less than 711mm and a thickness less than 23mm. It has no high-temperature performance indicators and does not conform to the manufacturing process of medium and thick plates.

[0007] Related patent 3: CN201710990236.8 "A Cr micro-alloyed steel for oil casing and its manufacturing method" provides a high-strength oil casing steel and its preparation method. By adding a certain Cr content such as 0.40-0.50%, the requirements for steel pipe strength are met and the corrosion resistance of the steel is increased to a certain extent. The material used is hot-rolled coil, which has no high-temperature performance indicators and does not conform to the medium and thick plate manufacturing process.

[0008] Related Patent 4: CN201811577443.1, "Acicular Ferrite-Type Low-Temperature-Resistant N80-Grade Oil Casing Steel and Its Preparation Method," discloses acicular ferrite-type low-temperature-resistant N80-grade oil casing steel and its preparation method. The steel exhibits excellent low-temperature impact resistance and HIC corrosion resistance. However, the steel is still hot-rolled coil and lacks high-temperature performance, making it unsuitable for medium and thick plate manufacturing.

[0009] In summary, the existing technology is still insufficient in developing stable performance in the low-temperature and high-toughness heating zone for medium and thick plates with a wall thickness of more than 23 mm used for the X60-grade large-diameter tube bundle outer support tube produced by the submerged arc welding process, and cannot meet the material performance requirements of submarine oil and gas fields. Summary of the Invention

[0010] This invention addresses the challenges faced by submarine oil and gas fields in oil and gas production and transportation, such as thin thickness specifications, poorly matched high-temperature creep, and poorly matched strength and toughness properties for tube bundle support pipes. By comprehensively considering material composition design, ladle metallurgy, and microstructural control, this invention provides an X60-grade steel for external tube bundle support pipes and its production method, addressing these challenges and meeting the material requirements of submarine oil and gas fields. This product is suitable for thick-walled submerged arc welded pipes, which are medium-heavy plates with a microstructure of ferrite with a small amount of pearlite. This technology offers low alloy production costs and a simple, easy-to-implement process, facilitating industrialized mass production.

[0011] In order to achieve the above object, the present invention adopts the following technical solutions:

[0012] Disclosed is an X60 grade steel for outer load-bearing tubes of tube bundles. The chemical composition of the steel is, by weight percentage, as follows: C 0.11%-0.14%, Si 0.25%-0.45%, Mn 0.85%-1.15%, P≤0.012%, S≤0.005%, Nb 0.02%-0.03%, Ti 0.01%-0.02%, Sb 0.05%-0.08%, RE 0.06%-0.08%, Als 0.01%-0.05%, N 0.0010%-0.0030%, H≤0.00015%, O≤0.0020%, and Ca 0.004%-0.009%, wherein Sb+RE≥0.12%, Ca / S≥1.8, and the remainder is Fe and unavoidable impurities.

[0013] The following details the mechanism of action of each alloy component in the steel of the present invention:

[0014] C is an essential element for steel strength. A content of 0.11% or higher provides excellent room-temperature and high-temperature strength. However, exceeding a certain level degrades weldability. Therefore, the upper limit is 0.14%. For economic and product performance reasons, the optimal C content is between 0.11% and 0.14%.

[0015] Si: A major deoxidizing component in the steelmaking process, Si must contain at least 0.10% to achieve a sufficient deoxidizing effect. However, exceeding this limit reduces the toughness of the base metal and welds. Si in solid solution improves strength while also raising the ductile-brittle transition temperature. Therefore, the Si content is limited to 0.25-0.45%.

[0016] Mn: An essential element for ensuring the strength and toughness of steel. Mn combines with S to form MnS, preventing the formation of FeS at grain boundaries and resulting in hot cracking. Mn is also a good deoxidizer. Manganese is a low-cost strengthening and toughening element. Too low a content of manganese cannot guarantee the material's strength. However, excessive Mn content can aggravate slab segregation and worsen the low-temperature toughness of the coarse-grained heat-affected zone (CGHAZ). Therefore, the Mn content should be controlled between 0.85% and 1.15%.

[0017] Phosphorus (P) is an unavoidable impurity element in steel that degrades its toughness and weldability. Studies have shown that when P content exceeds 0.015%, its corrosiveness in the acidic vapor phase of the upper deck decreases significantly. Therefore, the upper limit is 0.015%. The upper limit in this invention is 0.012% or less.

[0018] If the S content exceeds 0.01%, a large number of MnS inclusions will form in the steel. The MnS inclusions are the source of pitting corrosion, thereby reducing the corrosion resistance of the steel. Therefore, measures must be taken to reduce the S content in the steel as much as possible. Therefore, in the present invention, the upper limit of the S content is determined to be 0.01%, and the upper limit of the present invention is 0.005%.

[0019] Al: As a deoxidizing and grain refining element, the added content is generally above 0.01%. However, when it exceeds 0.06%, hot cracks of the ingot are likely to occur, a large number of inclusions are formed, and the toughness of the steel is reduced. Therefore, the upper limit of the Al content is 0.055%. The content range of the present invention is 0.01 to 0.05%.

[0020] Nb: It effectively refines the grain size of steel and is added to improve its strength and toughness. When the Nb content is less than 0.005%, it has little effect on the strength and toughness of steel. However, when it exceeds 0.065%, MA brittle components are easily generated during submerged arc welding with high heat input, reducing the steel's weldability and low-temperature toughness. Therefore, the Nb content range is 0.02-0.03%.

[0021] Ti: This component is added to improve the toughness of steel and welds. As a strong nitrogen-binding element, it easily forms TiN, which improves the weld metal's resistance to nitrogen porosity. Below 0.005% has little effect, while exceeding 0.055% tends to form large TiN particles, negating the effect. To achieve low-temperature toughness under high heat input, Ti content in steel must be controlled. Therefore, the Ti addition range is 0.01-0.02%.

[0022] Ca: Ca combines with S to form CaS, which can coat inclusions such as alumina, achieve inclusion modification and spheroidization, and 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 into alkaline OH -Ions can reduce the acidification degree of corrosion pits and reduce the sensitivity to pitting corrosion. In the present invention, Ca: 0.004% to 0.009%, Ca / S ≥ 1.8.

[0023] RE: Rare earth (RE) atoms are active and have strong binding forces. Adding rare earth to steel can improve the solidification structure, change the solid phase transformation structure, form harmless low-melting point inclusions, strengthen the interface through segregation, and passivate the surface rust layer. 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 the grain boundaries through a diffusion mechanism, inhibiting the segregation of inclusions at the grain boundaries, and improving the low-temperature performance and corrosion resistance of the steel. However, rare earth is a scarce resource, and its addition amount must be controlled. The RE content of the present invention is controlled at 0.06-0.08%.

[0024] Sb: Antimony (Sb) in steel at austenitic temperature precipitates at MnS inclusions and along the original austenite grain boundaries, thereby inhibiting the enrichment and precipitation of MnS inclusions at the grain boundaries. Antimony can also refine the size of secondary recrystallized grains, refine the steel structure and improve toughness, thereby improving the corrosion resistance of the steel. Sb forms a Sb2O5 corrosion-resistant oxide film on the steel surface, which effectively prevents the interaction between the matrix and the corrosive medium and inhibits the corrosion of the steel in the corrosive medium. Sb obtained after hydration 3+ The precipitates formed in the anode micro-area fill the corrosion cracks or cavities, improving the barrier to corrosion. - Sb can also penetrate through Cl - Hydration in environmental media inhibits Fe 3+ The process of generating H+ by hydrolysis improves the pH value of the corrosion micro-area and alleviates the anodic dissolution process. The content of H+ in the present invention is controlled at 0.05% to 0.08%.

[0025] N can form fine precipitates with Nb and Ti, play a strengthening and grain refining role, and improve strength and toughness. However, too high a content will deteriorate the toughness. Its content should be controlled within 0.0010% to 0.0030%.

[0026] H and O are unavoidable harmful impurity elements in the present invention; an increase in their content will lead to an increased tendency of hydrogen-induced cracking, an increase in inclusions, and a decrease in corrosion resistance and fatigue resistance. Therefore, the present invention controls H≤0.00015% and O≤0.0020%.

[0027] The steel plate used for the outer load-bearing pipe of the tube bundle of the present invention has a thickness of 23 mm to 35 mm. It has a yield strength of 415 to 450 MPa, a tensile strength of 515 to 530 MPa, and an elongation of 26% to 31%. Its impact energy absorption at -20°C is 200 to 268 J. At 650°C, its yield strength is 415 to 425 MPa, its tensile strength is 500 to 515 MPa, and its elongation is 28% to 32%.

[0028] A method for producing X60 grade steel for outer load-bearing tubes of tube bundles, comprising the following steps:

[0029] 1) Smelting: Use deep desulfurized molten iron with a sulfur content of ≤0.002%. After the molten iron reaches the converter, a process combining "double slag" dephosphorization and "slag skimming" of the molten steel at the end of the furnace is adopted. The final slag basicity is controlled at R=3.1~4.3. Through effective slag blocking operation, a large amount of slag is prevented, and the steel release time is not less than 5 minutes.

[0030] The smelting process adopts a high-draw carbon one-shot point blowing method, and the main elements of the steel are adjusted to the range of the present invention in the converter, and other alloy components are added according to requirements for smelting.

[0031] Molten steel removed from the converter undergoes secondary refining to further reduce harmful impurities such as O, S, and non-metallic inclusions. During the LF refining process, aluminum particles, silicon carbide, and calcium carbide are used to condition the slag, maintaining a final slag basicity of above 2.3. After LF, calcium treatment is performed, with a wire feed rate of 1.2 to 2.0 meters per ton of steel.

[0032] 2) Continuous casting: superheat ≤ 20℃, weak cooling is adopted for secondary cooling, the casting process is operated according to the matching of temperature and drawing speed, the continuous casting billet drawing speed is 1.0-1.6m / min, and the casting slab thickness is 200-360mm.

[0033] 3) Heating: The heating temperature for steel slabs is 1160-1260°C. This is because temperatures below 1150°C are insufficient for alloying elements to fully dissolve into the austenite, preventing the required finishing temperature for hot rolling. Temperatures above 1260°C significantly coarsen the original austenite grains, reducing the low-temperature toughness of the steel plate.

[0034] 4) Rolling: To meet the mechanical property requirements of the tube steel, it is preferred to carry out controlled rolling in two stages at temperatures between 1150°C and 980°C in the austenite recrystallization zone and the non-recrystallization zone, and ensure that the cumulative reduction in the two stages is not less than 50%.

[0035] The starting rolling temperature in the recrystallization zone is 1080-1150°C, and the final rolling temperature is 980-1000°C.

[0036] Different rolling temperatures and cooling rates in the pre-recrystallization zone can be selected based on the required mechanical properties of the steel plate. For this invention, the yield strength requirement is 415 MPa for high-strength steel, and the impact toughness temperature requirement is -20°C. The hot rolling pre-recrystallization zone rolling temperature is preferably below 900°C, and the finishing temperature is above 800°C. The steel plate microstructure is ferrite with a small amount of pearlite.

[0037] 5) Cooling: The cooling method is laminar cooling, and the cooling rate is controlled at 5-8°C / s, and the steel plate is cooled to 630°C-650°C.

[0038] Compared with the prior art, the present invention has the following beneficial effects:

[0039] 1) The thick-walled outer load-bearing tubes prepared using the present invention exhibit well-matched strength and toughness properties: a yield strength of 415-450 MPa, a tensile strength of 515-530 MPa, and an elongation of 26-31%. The impact energy absorption at -20°C is 200-268 J. At 650°C, the yield strength is 415-425 MPa, the tensile strength is 500-515 MPa, and the elongation is 28-32%.

[0040] 2) The present invention is simple to operate and does not require tempering heat treatment after online controlled cooling, thereby improving production efficiency while saving production costs and easily realizing industrialized mass production.

[0041] 3) The present invention adopts two-stage controlled rolling, and controls the microstructure of the steel for the outer load-bearing pipe of the tube bundle by adjusting the final rolling temperature of the X60 grade steel plate for the outer load-bearing pipe of the tube bundle, thereby maintaining ideal performance and eliminating high-temperature creep, while significantly reducing production costs.

[0042] 4) The maximum thickness of the X60 grade steel plate suitable for the outer supporting tube of the tube bundle described in the present invention reaches 35 mm, and it has good low-temperature toughness and high-temperature thermal stability, meeting the technical requirements of the material required for the manufacture of the outer supporting tube of the tube bundle with large wall thickness. BRIEF DESCRIPTION OF THE DRAWINGS

[0043] Figure 1 This is the microstructure diagram of the steel plate of the present invention. DETAILED DESCRIPTION

[0044] In order to make the purpose, technical solutions and advantages of the present invention more clearly understood, the specific implementation methods of the present invention are further described below in conjunction with examples. The following examples are used to specifically illustrate the contents of the present invention. These examples are only general descriptions of the contents of the present invention and do not limit the contents of the present invention.

[0045] This invention primarily targets steel used in large-diameter pipe bundles for oil and gas production and transportation in submarine oil and gas fields. This material combines a large thickness (over 23 mm), excellent low-temperature toughness (meeting -20°C), and high-temperature creep resistance. Mechanical property evaluation of the parent material produced using this invention demonstrated excellent overall mechanical properties and high-temperature creep resistance.

[0046] The embodiments are shown in Tables 1 to 3, wherein Table 1 shows the chemical composition of each embodiment; the rolling process of the embodiment is shown in Table 2; the mechanical properties of each embodiment are shown in Table 3; the mechanical properties of the steel plates of the corresponding embodiments after being subjected to a high temperature of 650°C are shown in Table 4.

[0047] Table 1 Chemical composition of steel smelting in each example (%)

[0048]

[0049]

[0050] Table 2 Process parameters of each example steel

[0051]

[0052] Table 3 Performance and structure of steel plates in Example

[0053]

[0054] Table 4 Mechanical properties of the inventive examples after high temperature treatment at 650°C

[0055]

Claims

1. An X60 grade steel for outer load-bearing pipes of tube bundles, characterized in that: The chemical composition of the steel is calculated by weight as follows: C 0.11% to 0.14%, Si 0.25% to 0.45%, Mn 1.06% to 1.15%, P ≤ 0.012%, S ≤ 0.005%, Nb 0.02% to 0.03%, Ti 0.01% to 0.02%, Sb 0.05% to 0.08%, RE 0.06% to 0.08%, Als 0.01% to 0.05%, N 0.0010% to 0.0030%, H ≤ 0.00015%, O ≤ 0.0020%, Ca 0.004% to 0.009%, of which Sb + RE ≥ 0.12%, Ca / S ≥ 1.8, and the rest is Fe and unavoidable impurities; The steel plate structure is ferrite + pearlite; The steel used for the outer load-bearing tube of the tube bundle has a yield strength of 415-425 MPa at a high temperature of 650°C, a tensile strength of 500-515 MPa, and an elongation of 28%-32%.

2. The X60 grade steel for outer load-bearing tubes of tube bundles according to claim 1, characterized in that: The thickness of the steel plate used for the outer load-bearing pipe of the tube bundle is 23 to 35 mm.

3. The X60 grade steel for outer load-bearing tubes of tube bundles according to claim 1, characterized in that: The yield strength of the steel used for the outer load-bearing pipe of the tube bundle is 415-450 MPa, the tensile strength is 515-530 MPa, the elongation is 26%-31%; the impact absorption energy at -20°C is 200-268 J.

4. A method for producing the X60 grade steel for outer load-bearing tubes of tube bundles according to any one of claims 1 to 3, characterized in that: The steps include: 1) Smelting: Use deep desulfurized hot metal with a sulfur content of ≤0.002%. After the hot metal reaches the converter, the final slag basicity is controlled at R=3.1-4.3; 2) Continuous casting: superheat ≤ 20°C, continuous casting billet drawing speed 1.0-1.6 m / min; 3) Heating: The heating temperature of the steel slab is 1160-1260°C; 4) Rolling: controlled rolling in two stages at 1150-980°C in the austenite recrystallization zone and the non-recrystallization zone, and ensuring that the cumulative reduction in the two stages is not less than 50%; The rolling temperature of the hot rolling non-recrystallization zone is below 900℃, and the final rolling temperature is above 800℃; 5) Cooling: The cooling method is laminar cooling, and the cooling rate is controlled at 5-8°C / s, and the steel plate is cooled to 630°C-650°C.

5. The method for producing X60 grade steel for outer load-bearing tubes of tube bundles according to claim 4, characterized in that: The smelting process adopts high-carbon one-time point blowing method, and the steel placing time in the converter is not less than 5 minutes.

6. The method for producing X60 grade steel for outer load-bearing tubes of tube bundles according to claim 4, characterized in that: After LF is completed, Ca treatment is carried out and the wire is fed at a rate of 1.2 to 2.0 meters per ton of steel.

7. The method for producing X60 grade steel for outer load-bearing tubes of tube bundles according to claim 4, characterized in that: The molten steel removed from the converter undergoes secondary refining. During the LF refining process, aluminum particles, silicon carbide and calcium carbide are used to adjust the slag, and the final slag basicity is controlled above 2.

3.

8. The method for producing X60 grade steel for outer load-bearing tubes of tube bundles according to claim 4, characterized in that: The thickness of continuous casting slab is 200-360mm.

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