X70 grade steel for outer load-bearing tube of tube bundle and production method thereof

Abstract

The invention relates to an X70 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 0.16-0.20%, Si 0.20-0.35%, Mn 1.75-1.85%, P≤0.012%, S≤0.003%, V 0.040-0.060%, Nb 0.035-0.055%, Sb 0.10-0.15%, RE 0.04-0.06%, Al 0.045-0.06%, N≤0.003%, Ca 0.004-0.009%, wherein RE / V is ≥0.5, Ca / S is ≥1.8, and the remainder is Fe and unavoidable impurities. The yield strength of the present invention is 485-540MPa, the tensile strength is 570-620MPa, and the elongation is 24%-28%; the impact absorption energy at -20℃ is 180-280J; the yield strength at a high temperature of 650℃ is 485-550MPa, the tensile strength is 570-625MPa, and the elongation is 25%-29%.

Description

Technical Field

[0001] The present invention relates to the field of metal materials, and in particular to an X70 grade steel for an external load-bearing tube of a tube bundle 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 will construct numerous new deep-sea oil and gas fields in the future, and the demand for large-diameter steel for external load-bearing pipes is expected to reach 300,000 tons. As seawater depths increase, the wall thickness of these pipes will continue to increase, which, in addition to cost, will also create numerous other challenges. To mitigate this increase in wall thickness, increased strength is required. However, my country still relies on imported X70-grade steel for external load-bearing pipes, primarily due to significant high-temperature creep, a significant drop in strength after high temperatures, and an inadequate balance of strength and toughness.

[0004] Foreign countries have developed steel for external load-bearing pipes of tube bundles, X70 grade large-wall thickness and large-diameter external load-bearing pipes of tube bundles. They are produced using medium and thick plate technology and straight seam submerged arc welding technology. They have excellent low-temperature impact toughness and no high-temperature creep phenomenon, which is significantly different from casing steel.

[0005] 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 in terms of composition, production method, high-temperature thermal stability, low-temperature toughness and other aspects described in the technical solution of the present invention.

[0006] Related Patent 1: "A 2250mm production line for producing H40 oil casing steel and its production method"

[0007] (CN201911408022.0) only provides hot-rolled coils and their preparation methods for casing in oil and gas pipeline projects with a diameter of less than 711 mm and thickness specifications covering 3-19 mm. It does not have high-temperature performance indicators and does not comply with the manufacturing process of medium and thick plates.

[0008] Related Patent 2: "A Steel for Heavy-Walled High-Toughness Resistance Welding Casing and Its Manufacturing Method" (CN202010765824.3) provides a manufacturing method for heavy-walled high-toughness resistance welding casing steel with a steel wall thickness greater 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.

[0009] Related Patent 3: "A Cr Microalloyed Steel for Petroleum Casing and Its Manufacturing Method" (CN201710990236.8) provides a high-strength steel for petroleum casing 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 manufacturing process of medium and thick plates.

[0010] Related Patent 4: Acicular Ferrite Low-Temperature-Resistant N80-Grade Petroleum Casing Steel and Its Preparation Method

[0011] (CN201811577443.1) discloses an acicular ferrite-type, low-temperature-resistant N80-grade oil casing steel and its preparation method, which exhibits excellent low-temperature impact resistance and HIC corrosion resistance. However, the steel is still a hot-rolled coil, lacking high-temperature performance indicators and not suitable for medium and heavy plate manufacturing processes.

[0012] In summary, the existing technology for medium and thick plates with a wall thickness of more than 23 mm used for the X70 grade tube bundle outer support tube produced by submerged arc welding process has insufficient low-temperature resistance and high toughness, and the strength drops significantly after high temperature. The strength-toughness match is insufficient and cannot meet the material performance requirements of submarine oil and gas fields. Summary of the Invention

[0013] The purpose of the present invention is to overcome the above-mentioned problems and shortcomings and provide an X70 grade steel for outer load-bearing pipes of tube bundles and a preparation method thereof that meets the material requirements of submarine oil and gas fields, has low alloy cost, is simple and easy to process, does not require heat treatment, and is easy to achieve industrial mass production.

[0014] This invention addresses the challenges faced by X70-grade steel used in bundle-supported pipes during oil and gas production and transportation in submarine oil and gas fields, including significant high-temperature creep and poor post-creep strength-toughness matching. By comprehensively addressing material composition design, ladle metallurgy, and microstructural control, this invention provides X70-grade steel for bundle-supported external pipes and its production method, resolving these challenges and meeting the material requirements of submarine oil and gas fields. This product is suitable for thick-walled submerged arc welded pipes, using medium-thick steel plates with acicular ferrite as the microstructure.

[0015] In order to achieve the above object, the present invention adopts the following technical solutions:

[0016] An X70 grade steel for outer load-bearing tubes of tube bundles, wherein the chemical composition of the steel is as follows by weight: C 0.16%-0.20%, Si 0.20%-0.35%, Mn 1.75%-1.85%, P≤0.012%, S≤0.003%, V 0.040%-0.060%, Nb 0.035%-0.055%, Sb 0.10%-0.15%, Ti 0.01%-0.02%, RE 0.04%-0.06%, Als 0.045%-0.06%, N≤0.003%, Ca 0.004%~0.009%, H≤0.00015%, O≤0.0020%, among which RE / V≥0.5, Ca / S≥1.8, and the rest are Fe and unavoidable impurities.

[0017] The following is a detailed description of the mechanism of action of the various alloy components in the X70 grade steel for the outer load-bearing tube of the tube bundle according to the present invention, wherein the percentage symbol % represents weight percentage:

[0018] C is an essential element for steel strength. A content of 0.16% or higher provides excellent room-temperature and high-temperature strength. However, exceeding a certain level degrades weldability. Therefore, the upper limit is 0.20%. For economic and product performance considerations, the optimal C content is between 0.16% and 0.20%.

[0019] 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 increases strength while also raising the ductile-brittle transition temperature. Therefore, the Si content is limited to 0.20% to 0.35%.

[0020] Mn: An essential element for ensuring the strength and toughness of steel. Mn combines with sulfur to form MnS, preventing the formation of FeS at grain boundaries and the resulting 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 increase slab segregation and deteriorate the low-temperature toughness of the coarse-grained heat-affected zone (CGHAZ). Therefore, the Mn content should be controlled between 1.75% and 1.85%.

[0021] 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%, and the preferred upper limit is 0.012% or less.

[0022] If the S content exceeds 0.01%, a large number of MnS inclusions will form in the steel. These MnS inclusions are the source of pitting corrosion, thereby reducing the corrosion resistance of the steel. Therefore, measures must be taken to minimize the S content in the steel. Therefore, in the present invention, the upper limit of the S content is determined to be 0.003%.

[0023] Al: As a deoxidizing and grain refining element, it is generally added at a content of more than 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 0.015% to 0.045%.

[0024] 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 0.003%, 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 0.035-0.055%.

[0025] V: V has a strong affinity for both O and N and is a strong carbide-forming element. Generally, V has a high dispersion and is extremely stable, facilitating deoxidation and degassing to achieve a dense, fine-grained structure, improving plasticity, toughness, and strength. Its impact resistance and fatigue strength are superior to those of vanadium-free steel. The high dispersion of vanadium carbide prevents coarsening of weld grains, improving the weldability of the steel. However, heating to the V dissolution temperature causes intense grain growth. When dissolved in solid solution at high temperatures, V increases hardenability; it enhances the tempering stability of quenched steel and produces a secondary hardening effect. V increases the solubility of rare earth elements in steel, thereby enhancing the steel's corrosion resistance. In the present invention, V content is controlled to 0.040% to 0.060%.

[0026] Ti: Added to improve the toughness of steel and welds, it is a strong nitrogen-binding element that readily forms TiN, enhancing the weld metal's resistance to nitrogen porosity. Ti content below 0.005% has minimal effect, while content 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, so the Ti addition range is 0.01% to 0.02%.

[0027] 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 pitting sensitivity. In the present invention, Ca is 0.004% to 0.009%, and Ca / S is ≥1.8.

[0028] RE: Rare earth (RE) atoms are active and have strong binding force. 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 0.04% to 0.06%.

[0029] 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+ Hydrolysis produces H + The content of the present invention is controlled at 0.10% to 0.15%.

[0030] 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%.

[0031] 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%.

[0032] A method for producing X70 grade steel for outer load-bearing tubes of tube bundles, comprising the following steps:

[0033] 1) Smelting: Use deep desulfurized molten iron with sulfur content ≤ 0.002%. After the molten iron reaches the converter, adopt the process of combining "double slag" dephosphorization with "slag skimming" of molten steel after the furnace. The final slag basicity is controlled at R = 3.1 ~ 4.3. Through effective slag blocking operation, a large amount of slag is prevented. The steel release time is not less than 5 minutes.

[0034] 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.

[0035] 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 granules, 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 feed rate of 1.2 to 2.0 meters per ton of steel.

[0036] 2) Continuous casting: superheat ≤ 20℃, weak cooling is adopted for secondary cooling, the casting process is operated according to the matching of temperature and casting speed, the continuous casting billet casting speed is 1.0-1.6m / min, and the casting slab thickness is 200-360mm.

[0037] 3) Heating: The steel slab is heated to 1150-1230°C. Temperatures below 1150°C are insufficient for complete dissolution of alloying elements into the austenite, preventing the required finishing temperature for hot rolling. Temperatures above 1230°C significantly coarsen the original austenite grains, reducing the low-temperature toughness of the steel.

[0038] 4) Rolling: In order to meet the mechanical property requirements of the tube steel, the 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 is guaranteed to be no less than 60%.

[0039] The starting rolling temperature in the recrystallization zone is 1080-1150°C, and the final rolling temperature is 980-990°C.

[0040] Different rolling temperatures and cooling rates in the non-recrystallized zone can be selected based on the mechanical properties of the steel plate. For example, for high-strength steel with an X70 yield strength requirement and an impact toughness temperature requirement of -20°C, the hot-rolled non-recrystallized zone rolling temperature should be below 880°C, and the final rolling temperature should be above 790°C.

[0041] 5) Cooling: The cooling method is laminar cooling, the air cooling time after rolling is 12-26s, the laminar cooling temperature is 765-780℃, the cooling time is 10-15s, and the cooling rate is controlled at 8-10℃ / s, and the steel plate is cooled to 480℃~530℃.

[0042] The steel plate for the outer load-bearing pipe of the present invention has a thickness of 23 mm to 35 mm. The steel plate has an acicular ferrite structure. The steel has a yield strength of 485 to 540 MPa, a tensile strength of 570 to 620 MPa, and an elongation of 24% to 28%. The impact energy absorption at -20°C is 180 to 280 J. At 650°C, the yield strength is 485 to 550 MPa, the tensile strength is 570 to 625 MPa, and the elongation is 25% to 29%.

[0043] Compared with the prior art, the present invention has the following beneficial effects:

[0044] 1) The X70-grade outer load-bearing tube of the tube bundle prepared by the present invention has good matching strength and toughness properties: the yield strength is 485-540 MPa, the tensile strength is 570-620 MPa, and the elongation is 24%-28%; the impact absorption energy at -20°C is 180-280 J; the yield strength at a high temperature of 650°C is 485-550 MPa, the tensile strength is 570-625 MPa, and the elongation is 25%-29%.

[0045] 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.

[0046] 3) The present invention adopts two-stage controlled rolling, and controls the microstructure of the outer load-bearing pipe steel of the tube bundle by adjusting the final rolling temperature of the X70 grade steel plate, thereby maintaining ideal performance and eliminating high-temperature creep, while significantly reducing production costs.

[0047] 4) The maximum thickness of the X70 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. DETAILED DESCRIPTION

[0048] 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.

[0049] This invention primarily targets X70-grade steel for supporting pipes used in subsea oil and gas production and transportation. This material combines a large thickness (over 23 mm), excellent low-temperature toughness (meeting -20°C), high-temperature creep resistance, and stable yield strength in the heat-affected zone during welding. Mechanical property evaluation of the parent material produced using this invention demonstrated excellent overall mechanical properties and high-temperature creep resistance.

[0050] 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.

[0051] Table 1 Chemical composition of steel smelting in each example (%)

[0052]

[0053] Table 2 Process parameters of each example steel

[0054]

[0055] Table 3 Performance and structure of steel plates in Example

[0056]

[0057]

[0058] Table 4 Mechanical properties of the inventive examples after high temperature treatment at 650°C

[0059]

Claims

1. An X70 grade steel for outer load-bearing tubes of tube bundles, characterized in that: The chemical composition of the steel by weight is: C 0.16%~0.20%, Si 0.20%~0.35%, Mn 1.75%~1.85%, P≤0.012%, S≤0.003%, V 0.040%~0.060%, Nb 0.035%~0.055%, Sb 0.10%~0.15%, Ti 0.01%~0.02%, RE 0.04%~0.06%, Als 0.045%~0.06%, N 0.0010%~0.0030%, Ca 0.004%~0.009%, H≤0.00015%, O≤0.0020%, among which RE / V≥0.5, Ca / S≥1.8, and the rest is Fe and unavoidable impurities.

2. The X70 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 X70 grade steel for outer load-bearing tubes of tube bundles according to claim 1, characterized in that: The steel plate structure is acicular ferrite.

4. The X70 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 bundle is 485-540 MPa, the tensile strength is 570-620 MPa, the elongation is 24%-28%; and the impact absorption energy at −20°C is 180-280 J.

5. The X70 grade steel for outer load-bearing tubes of tube bundles according to claim 1, characterized in that: The steel used for the outer load-bearing tube of the tube bundle has a yield strength of 485-550 MPa at a high temperature of 650°C, a tensile strength of 570-625 MPa, and an elongation of 25%-29%.

6. A method for producing the X70 grade steel for outer load-bearing tubes of tube bundles according to any one of claims 1 to 5, characterized in that: The steps include: 1) Heating: Heat the continuous casting billet to 1150-1230℃; 2) Rolling: Controlled rolling in two stages at 1150-980℃ in the austenite recrystallization zone and the non-recrystallization zone, and ensure that the cumulative reduction in the two stages is not less than 60%; The hot rolling temperature of the non-recrystallized zone is below 880℃, and the final rolling temperature is above 790℃; 3) Cooling: The cooling method is laminar cooling, and the cooling rate is controlled at 8-10℃ / s, and the steel plate is cooled to 480-530℃.

7. The method for producing X70 grade steel for outer load-bearing tubes of tube bundles according to claim 6, characterized in that: The steelmaking process uses 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, and the steel placement time is not less than 5 minutes. 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 basicity of the final slag is controlled above 2.

3. After the LF is completed, Ca treatment is carried out, and the wire is fed at 1.2 to 2.0 meters per ton of steel.

8. The method for producing X70 grade steel for outer load-bearing tubes of tube bundles according to claim 6, characterized in that: The continuous casting process superheat is ≤20℃, and the continuous casting billet drawing speed is 1.0~1.6m / min.

9. The method for producing X70 grade steel for outer load-bearing tubes of tube bundles according to claim 6 or 8, characterized in that: The thickness of the continuous casting slab is 200 to 360 mm.

10. The method for producing X70 grade steel for outer load-bearing tubes of tube bundles according to claim 6, characterized in that: The air cooling time after rolling is 12 to 26 seconds, the laminar cooling temperature is 765 to 780°C, and the cooling time is 10 to 15 seconds.

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