Ultra-low temperature welded pipe and method of making, and welding wire and welding method for the same

Abstract

The application discloses a super-low-temperature welded pipe and a preparation method thereof, and a welding wire and a welding method for the welded pipe. The steel plate of the welded pipe is composed of C: 0.02-0.03%, Si: 0.10-0.40%, Mn: 2.5-4.0%, Al: 0.05-0.10%, P: ≤0.010%, S: ≤0.002%, other alloy elements ≤1.0%, and the balance of Fe, and the total content of the above components is 100%. A kind of super-low-temperature L450M / L485M steel grade straight seam submerged arc welded pipe with low cost, excellent low-temperature toughness and plasticity is manufactured by using a Mn-Ni-Mo welding wire and a straight seam submerged arc welding method of first outer welding and then inner welding. The material and the weld of the welded pipe have good low-temperature toughness, and the safety of the working process is ensured.

Description

Technical Field

[0001] This invention belongs to the field of pipe manufacturing technology, and relates to an ultra-low temperature welded pipe, as well as a preparation method of the above-mentioned welded pipe. This invention also relates to the welding wire used in the above-mentioned welded pipe and the welding method. Background Technology

[0002] If the release rate of a steel pipe transporting supercritical carbon dioxide fails, leakage or venting of carbon dioxide can cause the pipe wall to reach a dry ice temperature of around -78.5°C. At this low temperature, the steel pipe is extremely prone to brittle fracture and long-range propagation. Compared to natural gas pipelines, carbon dioxide pipelines are more susceptible to rapid-propagation ductile fracture, placing extremely high demands on the low-temperature fracture performance of the materials.

[0003] Currently, the most commonly used supercritical carbon dioxide pipelines in foreign pipeline projects are L450M / L485M grade steel pipes. The ductile-brittle transition temperature of these steel pipes is generally between -30 and -70℃. Due to the limitations of the material itself, these low-temperature steel pipes often experience embrittlement and fracture when the temperature is below the ductile-brittle transition temperature. If carbon dioxide leaks, the pipeline will pose a serious safety hazard.

[0004] Therefore, steel pipes for transporting supercritical carbon dioxide require both the steel and the weld to possess high toughness at low temperatures; that is, the lower the ductile-brittle transition temperature, the higher the safety. Submerged arc welded pipes for supercritical carbon dioxide transport mostly use L450M / L485M steel grades as typical grades. To ensure the low-temperature toughness of L450M / L485M grade welded pipes, steel companies in Japan and Sweden need to add nearly 4% Ni, resulting in high production costs. Summary of the Invention

[0005] The first objective of this invention is to provide an ultra-low temperature L450M / L485M grade steel plate; The second objective of this invention is to provide a method for preparing the above-mentioned ultra-low temperature type L450M / L485M steel grade plate; A third object of the present invention is to provide a Mn-Ni-Mo welding wire for the aforementioned steel plate; The fourth objective of this invention is to provide a method for preparing the above-mentioned Mn-Ni-Mo welding wire; The fifth objective of this invention is to provide a welding method for straight seam submerged arc welding of the above-mentioned steel plate using Mn-Ni-Mo welding wire; The sixth objective of this invention is to provide a method for preparing ultra-low temperature L450M / L485M grade straight seam submerged arc welded pipes using the above-mentioned steel plate, welding wire, and welding method.

[0006] The above objectives solve the problem that low-temperature steel pipes are prone to embrittlement and fracture in the range of -70℃ to -120℃ in the existing technology.

[0007] The first technical solution adopted in this invention is an ultra-low temperature type L450M / L485M grade steel plate, with the following composition by mass percentage: C: 0.02-0.03%, Si: 0.10-0.40%, Mn: 2.5-4.0%, Al: 0.05-0.10%, P: ≤0.010%, S: ≤0.002%, other alloying elements ≤1.0%, balance Fe, and the total content of the above components is 100%.

[0008] The invention is further characterized in that the other alloying elements are the following chemical compositions, in mass percentage: Cr: 0-0.5%, Cu: 0-0.25%, Mo: 0-0.3%, Nb+V+Ti: 0-0.12%.

[0009] The second technical solution adopted in this invention is the preparation method of the above-mentioned ultra-low temperature type L450M / L485M steel grade plate, which is specifically implemented according to the following steps: Step 1: The raw materials are sequentially heated, melted, refined, alloyed and cast to obtain ingots that meet the mass percentage of ultra-low temperature type L450M / L485M steel grade plates. Step 2: The ingot obtained in Step 1 is used to obtain a large-section billet through an extra-thick plate continuous casting machine. Step 3: Roll the billet; Step 4: Heat treat the steel plate obtained in step 3.

[0010] The invention is further characterized by: The specific steps of step 3 are as follows: Step 3-1, Heating: Heat the billet obtained in step 2 to 1150-1200℃; Step 3-2, Rough rolling: Roll the narrow billet 3 to 9 times, with a deformation of ≥20% per pass; then rotate the billet 90° and roll it laterally 2 to 6 times, with a deformation of ≥10% per pass. Step 3-3, Finish rolling: Rotate the slab 90° and finish roll it 6 to 15 times, with a total deformation of ≥60%; Steps 3-4: Final rolling: Roll to the required thickness at a temperature of 800-950℃.

[0011] The specific steps of step 4 are as follows: Step 4-1: Quench the steel plate obtained after rolling in Step 3; Step 4-2, Critical Zone Annealing: The heating temperature is 780~810℃, the holding time is 0.5~2 hours, and then water quenching is performed; Step 4-3, Tempering: Perform a second tempering at a temperature of 520-580℃ for 0.5-2 hours. Step 4-4: After tempering, air cool or water cool to room temperature.

[0012] The third technical solution adopted in this invention is that the Mn-Ni-Mo welding wire used for the above-mentioned ultra-low temperature L450M / L485M grade steel plates has the following composition by mass percentage: C: 0.05%~0.12%, Mn: 1.5%~2%, Ni: 1.5%~2.5%, with the total amount of Mn and Ni ≤4%, V: 0.05%~0.2%, Si: 0.15%~0.2%, Mo: 0.3%~0.5%, of which S≤0.004%, P≤0.012%, and the balance is Fe. The total content of the above components is 100%.

[0013] The fourth technical solution adopted in this invention is the preparation method of the above-mentioned Mn-Ni-Mo welding wire, which is specifically implemented according to the following steps: Step 1: The raw materials are smelted and cast to obtain ingots that meet the above-mentioned mass percentage of Mn-Ni-Mo welding wire; Step 2: Forge the ingot and then roll it into wire rod; Step 3: The wire rod is drawn through a drawing machine in multiple passes to obtain welding wire of the required diameter; Step 4: Heat treat the drawn welding wire; Step 5: Perform surface treatment on the drawn solid welding wire.

[0014] The fifth technical solution adopted in this invention is a welding method for straight seam submerged arc welding of ultra-low temperature L450M / L485M steel plates using the aforementioned Mn-Ni-Mo welding wire, specifically implemented according to the following steps: Step 1, Pre-welding: Mixed gas shielded welding is used, and the pre-welding wire is Mn-Ni-Mo welding wire; Step 2: First, perform external three-wire submerged arc automatic welding, then perform internal three-wire submerged arc automatic welding. Use Mn-Ni-Mo welding wire and a sintered flux with a basicity of 1.8, consisting of CaF2-SiO2-MgO-Al2O3-MnO-TiO2-B2O3 slag system.

[0015] The invention is further characterized in that: the front wire of both the external three-wire submerged arc automatic welding and the internal three-wire submerged arc automatic welding adopts DC reverse polarity, while the middle and rear wires adopt AC polarity, the welding speed V = 1.3~1.5m / min, and the small heat input of 15~25kJ / cm is used.

[0016] The sixth technical solution adopted in this invention is: a method for preparing ultra-low temperature L450M / L485M grade straight seam submerged arc welded pipes using ultra-low temperature L450M / L485M grade steel plates, specifically implemented according to the following steps: Step 1: Before forming, the prepared ultra-low temperature type L450M / L485M steel plate is milled to produce a thick blunt edge, and then the steel plate is pre-bent. Step 2: Perform JCO forming, using a narrow die and multiple bending passes, with a tool holder pressing speed of not less than 0.5 mm / s; Step 3: Use the above-mentioned Mn-Ni-Mo welding wire to perform straight seam submerged arc welding on the JCO-formed steel plate; Step 4: Perform rapid diameter expansion on the welded pipe obtained in Step 3 with an expansion rate of 0.8% to 1.0% and an expansion speed of not less than 0.5 mm / s. Step 5: Routine testing.

[0017] The beneficial effects of this invention are: This invention, based on the composition of conventional low-carbon pipeline steel, mainly adds 2.5-4.0% Mn element to roll it into a low-temperature steel plate. Then, through the TRIP effect (transformation-induced plasticity) of JCO (J-type-C-type-O-type sequential forming), phase transformation toughening is achieved. A new type of low-temperature Mn-Ni-Mo welding wire is used for external submerged arc welding first, followed by internal submerged arc welding. The entire pipe is then rapidly expanded in diameter and subjected to hydrostatic pressure. Finally, a low-cost, ultra-low-temperature L450M / L485M grade straight seam submerged arc welded pipe with excellent low-temperature toughness and plasticity is manufactured. The material and weld of this welded pipe have good low-temperature toughness, ensuring the safety of the working process. Attached Figure Description

[0018] Figure 1 This is a schematic diagram of the internal structure of the L450M grade steel plate of this invention. Detailed Implementation

[0019] The present invention will now be described in detail with reference to the accompanying drawings and specific embodiments.

[0020] This invention relates to ultra-low temperature type L450M / L485M grade steel plates, whose chemical composition by mass percentage is: C: 0.02-0.03%, Si: 0.10-0.40%, Mn: 2.5-4.0%, Al: 0.05-0.10%, P: ≤0.010%, S: ≤0.002%, other alloying elements ≤1.0%, and the balance being Fe, with the total content of all the above components being 100%. Other alloying elements are as follows: Cr: 0–0.5%, Cu: 0–0.25%, Mo: 0–0.3%, Nb+V+Ti: 0–0.12% by mass percentage.

[0021] The steel plate raw material mainly contains 2.5 to 4.0% by mass of Mn element. Mn is the key alloying element for obtaining metastable austenite in the steel of this invention. As an effective austenite stabilizing element, austenite rich in Mn element is more likely to form stable residual austenite, which improves the low temperature toughness of the steel plate, and at the same time can obtain high strength and high plasticity.

[0022] Replacing Ni with Mn can effectively improve the stacking fault energy of steel plates. For every 1% of Mn added, the martensitic transformation point decreases by about 30.4℃. Less than 2.5% Mn results in too little retained austenite, while more than 4% Mn makes the retained austenite structure unstable, increases welding difficulty and weld structure instability, and the steel plate strength will also exceed the standard. This invention limits the mass percentage content of Mn to 2.5% to 4.0% to form some retained austenite and increase the low-temperature toughness of the pipe.

[0023] Carbon can enhance strength through solid solution strengthening or precipitation strengthening, and can also stabilize the austenite phase. However, in order to lower the ductile-brittle transition temperature of the material and reduce the precipitation of carbides on the grain boundaries, ultra-low carbon with a mass percentage of 0.02-0.03% is used to impart plasticity and toughness to the base material.

[0024] Al is a deoxidizing element and can also reduce the number of dissolved nitrogen atoms, thereby improving toughness and resistance to weathering. The formed AlN can also refine the grains, thereby further reducing the ductile-brittle transition temperature. However, excessive addition will form large-sized Al3O2 and AlN and impair low-temperature toughness. In this invention, the Al content (Als) is controlled at 0.05-0.10%.

[0025] Low sulfur (S) and phosphorus (P) content indicates high purity in steel smelting, with clean grain boundaries, which is beneficial for high toughness at low temperatures in sheet metal. Adding small amounts of one or more other microalloying elements compensates for the insufficient strength caused by ultra-low carbon steel, meeting the strength requirements of L450M / L485M grade welded pipes.

[0026] This invention also relates to a method for preparing ultra-low temperature L450M / L485M grade steel plates, specifically implemented according to the following steps: Step 1: The raw materials are sequentially heated in the furnace, smelted, refined, alloyed, and cast to obtain ingots that meet the mass percentage requirements of ultra-low temperature L450M / L485M steel grade plates. The specific steps are as follows: Step 1-1: Place the raw materials in a furnace and heat them to 1500-1650℃. After melting, the composition is uniform. Steps 1-2: Refining the molten steel to remove impurities and gases, thereby improving the purity of the steel. Steps 1-3: Adjust the composition of the refined molten steel by adding appropriate amounts of alloying elements to achieve the mass percentage of the composition of the ultra-low temperature type L450M / L485M steel grade steel plate as described above. Steps 1-4: Pour the molten steel with adjusted composition into the mold and solidify to form an ingot.

[0027] Step 2: Cast the ingot obtained in Step 1 into a large-section billet using an extra-thick plate continuous casting machine. The ingot is placed in a heating furnace and heated to a temperature of 1150-1200℃ for 2-6 hours. The casting process is protected throughout, and the superheat is controlled at 10-18℃. Dynamic light pressure is used, and heavy pressure is used at the end of solidification. The pressure force effectively penetrates into the core, resulting in a large cross-section ingot of 460mm. Controlling the composition and microstructure uniformity of thick-walled, large-section cast billets first is beneficial for controlling center segregation of plates later and improving the uniformity of microstructure in the thickness direction of the base material. Step 3: Roll the billet according to the following specific steps: Step 3-1, Heating: Heat the billet obtained in step 2 to 1150-1200℃; Step 3-2, rough rolling: The narrow billet is rolled 3 to 9 times, with a deformation amount of ≥20% per pass, so that the austenite grains are sufficiently refined, which promotes the nucleation and growth of martensite or bainite during cooling. Then, in order to achieve the width of the finished product, the billet is rotated 90° and rolled transversely 2 to 6 times, with a deformation amount of ≥10% per pass, to control the ratio of the transverse and longitudinal lengths of the grains. Step 3-3, Finish rolling: The slab is rotated 90° and finished rolled for 6 to 15 passes, with a total deformation of ≥60%. This introduces a large number of crystal defects, promotes the decomposition and transformation of austenite and the precipitation of the second phase, and improves strength and toughness. Steps 3-4, final rolling: the temperature is 800-950℃, and the steel pipe is rolled to the required finished thickness.

[0028] Step 4: Perform heat treatment on the steel plate according to the following specific steps: Step 4-1: Quench the steel plate obtained after rolling in Step 3. Quenching directly after rolling will yield a martensitic structure. It mainly suppresses the medium- and high-temperature phase transformation of supercooled austenite, while also suppressing the formation of coarse carbides, which is also beneficial to improving the low-temperature toughness of steel plates; Step 4-2, Critical Zone Annealing: The heating temperature is 780~810℃, the holding time is 0.5~2 hours, and then water quenching is performed; Step 4-3, Tempering: The secondary tempering temperature is 520~580℃, and the holding time is 0.5~2 hours; Step 4-4: After tempering, air cool or water cool to room temperature.

[0029] Multiple heat treatments were performed to control the strength and low-temperature toughness of the steel plate, resulting in an appropriate amount of retained austenite in the plate, which significantly improved the plasticity and low-temperature toughness of this high-Mn-content steel.

[0030] The raw materials are processed through ingot casting, continuous casting, rolling and heat treatment to obtain steel plates for straight seam submerged arc welded pipes with low yield strength ratio L450M / L485M steel grade. At this time, the microstructure of the plate is mainly tempered martensite and fine and stable reverse transformation (residual) austenite. A certain proportion of residual austenite can ensure that the plate has a certain low temperature toughness.

[0031] The present invention also relates to Mn-Ni-Mo welding wire for the above-mentioned ultra-low temperature L450M / L485M grade steel plates, the main elemental composition by mass percentage is C: 0.05%~0.12%, Mn: 1.5%~2%, Ni: 1.5%~2.5%, and Mn+Ni≤4%, V: 0.05%~0.2%, Si: 0.15%~0.2%, Mo: 0.3%~0.5%, wherein S≤0.004%, P≤0.012%, and the balance is Fe, and the total content of the above components is 100%.

[0032] Ni replaces most of the Mn element. Submerged arc welding wire with high Ni and Mn content is beneficial to improving the low-temperature toughness of the weld. S and P should be kept as low as possible to ensure the purity of the weld, thereby improving the low-temperature toughness of the weld.

[0033] The preparation method of the above-mentioned Mn-Ni-Mo welding wire is specifically implemented according to the following steps: Step 1: Obtain ingots conforming to the mass percentage of Mn-Ni-Mo welding wire by smelting and casting the raw materials. The specific steps are as follows: Step 1-1: Place the raw materials in a furnace and heat them to 1500-1600℃. After melting, the composition is uniform. Steps 1-2: Refining the molten metal after smelting to remove impurities and gases and improve the purity of the molten metal. Steps 1-3: Adjust the composition of the refined molten metal by adding appropriate amounts of alloying elements to achieve the mass percentage of the above-mentioned Mn-Ni-Mo welding wire composition. Steps 1-4: Pour the adjusted molten metal into the mold and let it solidify.

[0034] Step 2: Perform simple forging on the ingot, and roll it into wire rod using a hot continuous rolling mill when the ingot temperature range is 1000℃~1100℃. Step 3: The wire rod is drawn through a drawing machine in multiple passes, with the deformation per pass controlled between 10% and 20%. The required welding wire diameter is gradually achieved through multiple drawing processes. Step 4: Heat treat the drawn welding wire. Depending on the specific performance requirements of the welding wire, after annealing, select quenching and tempering treatment. Step 5: Perform surface treatment on the drawn solid welding wire to improve its surface quality and rust resistance. Surface treatment can be copper plating, passivation, or other methods.

[0035] The welding method for straight seam submerged arc welding of ultra-low temperature L450M / L485M grade steel plates using the above-mentioned Mn-Ni-Mo welding wire is implemented according to the following steps: Step 1: Pre-weld before submerged arc welding; Pre-welding uses mixed gas shielded welding, with the shielding gas being 60% CO2 + 40% Ar, and a gas flow rate of 70 L / min; the pre-welding wire is a 2.0 mm diameter Mn-Ni-Mo welding wire; the welding speed V = 4 m / min; Step 2: First perform external three-wire submerged arc welding, then perform internal three-wire submerged arc welding. Welding was performed using Mn-Ni-Mo welding wire with a diameter of 4.0 mm and sintered flux of CaF2-SiO2-MgO-Al2O3-MnO-TiO2-B2O3 slag system with a basicity of 1.8. Both external three-wire submerged arc welding and internal three-wire submerged arc welding use DC reverse polarity for the front wire and AC polarity for the middle and rear wires. The welding speed is V = 1.3~1.5m / min, and a small heat input of 15~25kJ / cm is used.

[0036] In actual submerged arc welding, the mass percentage of Mn in the L450M / L485M grade steel plate of this invention is 2.5% to 4%, which is much higher than the mass percentage of Mn in conventional L450M / L485M grade steel plate (1% to 1.7%). Mn can improve low-temperature toughness, but if the Mn content in the welding wire is too high, the weldability will be poor, and since the weld is in a cast state, it is easy to form coarse structure.

[0037] The Mn-Ni-Mo welding wire of this invention replaces most of the Mn element with Ni element, which is beneficial to low-temperature toughness. Compared with conventional low-temperature welding wire, the Ni content in this welding wire is significantly increased and the Mn content is slightly increased. High Mn and Ni welds have a fast heating rate, high peak temperature, short high-temperature dwell time and uneven cooling rate during submerged arc welding. The welded joint of the outer weld is prone to local hardening, and the degree of embrittlement of the heat-affected zone increases with the increase of heat input.

[0038] The traditional JCOE welded pipe welding sequence of internal welding followed by external welding has been changed. Instead, external welding is performed first, followed by internal welding. The external weld is then heat-treated to reduce its hardness. Using a low heat input of 15–25 kJ / cm, the fine inclusions and acicular ferrite in the high-Mn, Ni weld zone exhibit high-density, large-angle grain boundaries, which slows crack propagation. The low heat input ensures relatively fine grains in the coarse-grained region of the heat-affected zone (HAZ). The retained austenite in the HAZ also undergoes transformation-induced plasticity (TRIP) under impact loads, ensuring the low-temperature impact toughness of both the weld and the HAZ.

[0039] This invention relates to a method for preparing ultra-low temperature L450M / L485M grade straight seam submerged arc welded pipes using ultra-low temperature L450M / L485M grade steel plates, specifically implemented according to the following steps: Step 1: Before forming, mill the edges of the ultra-low temperature type L450M / L485M steel plate to produce a 6-9mm thick blunt edge, and then pre-bend the edges of the steel plate. Step 2: Perform JCO molding (J-type-C-type-O-type sequential molding); JCO forming involves 13 to 35 pressing passes to form a C-shape. Then, a rapid pressing is performed in the middle of the steel plate to create an O-shaped opening. The pressing speed of the tool holder is not less than 0.5 mm / s. Based on the steel plate thickness range, the JCO forming process data expert system software accurately predicts trends such as bending springback, neutral layer offset, and plate thickness changes, and automatically sets the reduction amount and step size for each pass to achieve low-stress, high-dimensional precision forming of pipes. The forming process uses a universal mold, which can automatically adjust the upper mold radius, lower mold opening distance and upper mold pressing amount according to the tube arc design requirements, realize narrow mold multi-pass bending, and progressively and quickly press into a J shape, which improves the forming accuracy and welding bevel accuracy, and helps to ensure welding quality.

[0040] A faster work hardening rate leads to an increase in the strain rate of the pipe sample section, which can delay the occurrence of necking in the pipe, thereby improving the plasticity of the pipe on the one hand and relaxing local stress concentration on the other.

[0041] Rapid forming, or increased strain rate, affects the stability of metastable austenite and the amount of martensite transformation during deformation. During rapid deformation, both twinning-induced plasticity (TWIP) and transformation-induced plasticity (TRIP) occur, increasing the plasticity of the steel pipe base material and simultaneously improving its low-temperature toughness.

[0042] Rapid transformation plastic deformation (TRIP) of the pipe induces the formation of more martensite, that is, metastable austenite undergoes martensitic transformation under the action of stress field. This transformation will alleviate the stress concentration at the crack front, increase the crack propagation resistance, and ensure the low temperature high toughness and low yield strength ratio of L450M / L485M steel grade pipe.

[0043] Step 3: Weld the JCO formed steel plate using the above-mentioned straight seam submerged arc welding method; Step 4: Perform rapid diameter expansion on the welded pipe obtained in Step 3 with an expansion rate of 0.8% to 1.0% and an expansion speed of not less than 0.5 mm / s. The welded pipe was subjected to water pressure strengthening at 100% nominal yield strength. Water pressure strengthening at 100% nominal yield strength not only released the stress of the JCOE pipe, but also continued to induce deformation (TRIP) to form more martensite and twinned (TWIP) structures, thereby continuously increasing the strength, plasticity and low-temperature toughness of the welded pipe steel and weld.

[0044] Step 5: Routine testing; For L450M / L485M grade straight seam submerged arc welded pipes, the following tests must be performed in sequence: weld X-ray inspection, weld ultrasonic inspection, pipe end X-ray inspection, chamfering, pipe end magnetic particle inspection, and appearance quality inspection. All tests must meet the requirements of API SPEC 5L PSL2 standard.

[0045] The prepared ultra-low temperature L450M / L485M grade straight seam submerged arc welded pipes were subjected to chemical composition analysis, bending, tensile, and low-temperature Charpy impact tests according to the inspection batches to ensure that the physical and chemical properties of the L450M / L485M grade straight seam submerged arc welded pipes meet the requirements of API SPEC 5L PSL2 standard and ultra-low temperature toughness requirements. In particular, the high toughness of the base metal, weld and heat-affected zone at low temperatures ensures that the welded pipes have a certain ability to arrest cracks at low temperatures and prevent long-range propagation when transporting supercritical carbon dioxide.

[0046] The ultra-low temperature L450M / L485M grade straight seam submerged arc welded pipe of the present invention was tested, and the results are as follows: Yield strength 455–540 MPa, tensile strength 535–670 MPa, yield strength ratio 0.75–0.85, elongation 30%–48%; impact toughness of the pipe body at -100℃ 350–480 J, weld 250–380 J, hot zone 200–370 J; impact toughness of the pipe body at -120℃ 250–352 J, weld 170–250 J, hot zone 222–302 J; drop hammer shear area at -45℃ 90%–100%.

[0047] The low temperature, high toughness, and low yield strength ratio greatly improve the safety of steel pipes used for transporting supercritical carbon dioxide.

[0048] In summary, this invention, through chemical composition design and changes in manufacturing process, adds Mn element to ultra-low carbon steel and reduces inclusion segregation, smelting it into a large-section, narrow-width cast billet. This ensures that during the rolling of medium-thick plates, the rolling force penetrates along the thickness direction, the core deformation is sufficient, and the microstructure is more uniform. After rolling, the plate is quenched to form a martensitic microstructure, and after annealing, a Mn-rich austenitic second phase with certain stability is obtained. This ensures high impact toughness at low temperatures and high DWTT (drop hammer tear shear area) at low temperatures, producing L450M / L485M grade steel plates. The steel plates are then rapidly formed and expanded in diameter by narrow-die multi-pass bending on the universal mold of a JCO forming machine. This generates deformation-induced phase transformation and phase transformation-induced plasticity effects in the pipe, further increasing the strength, toughness, and plasticity of the pipe. Using a specially invented Mn-Ni-Mo low-temperature submerged arc welding wire, external welding is performed first, followed by internal welding, ultimately producing ultra-low temperature L450M / L485M steel grade straight seam submerged arc welded pipes with excellent low-temperature performance of both the base material and the weld.

[0049] Example 1 In this embodiment, the Mn-Ni-Mo welding wire is manufactured according to the following steps: Step 1: The raw materials are smelted and cast to obtain ingots with the following mass percentages: 0.08% C, 1.9% Mn, 0.19% Si, 2.0% Ni, 0.35% Mo, 0.05% V, 0.011% P, 0.008% S, with the balance being Fe. The total content of the above components is 100%. Step 2: Perform simple forging on the ingot, and roll it into wire rod using a hot continuous rolling mill when the ingot temperature range is 1000℃~1100℃. Step 3: The wire rod is drawn through a drawing machine in multiple passes, with the deformation per pass controlled between 10% and 20%. Through multiple drawing processes, the two types of welding wires with diameters of 2 mm and 4 mm, respectively, required in Examples 2 and 3 are gradually achieved. Step 4: Heat treat the drawn welding wire. Depending on the specific performance requirements of the welding wire, after annealing, select quenching and tempering treatment. Step 5: Perform surface treatment on the drawn solid welding wire to improve its surface quality and rust resistance. The surface treatment can be copper plating.

[0050] Example 2 Table 1 Chemical composition analysis of steel plates (wt%)

[0051] The specific steps for preparing the steel plate according to the composition of the No. 1 ultra-low temperature type L450M steel grade steel plate in Table 1 are as follows: Step 1: The raw materials are sequentially heated, melted, refined, alloyed and cast under vacuum negative pressure to obtain ingots that meet the mass percentage of No. 1 ultra-low temperature type L450M steel grade steel plates in Table 1. Step 2: The ingot obtained in Step 1 is cast into a large cross-section billet with a width of 460mm×2000mm by a continuous casting machine for extra-thick plates; the billet is heated to 1160℃ and held for 2 to 3 hours, with full protection during casting and the superheat is controlled at 10 to 18℃. Step 3: Using dynamic light pressure, rough rolling is performed 7 times, followed by 2 times of 90° horizontal rolling, and then 13 times of 90° finish rolling. The final rolling temperature is 820~950℃, producing a steel plate with a wall thickness of 14.2mm and a wall thickness of 1210mm. Step 4: First, perform critical zone annealing on the L450M steel plate. The annealing heating temperature is 800℃ and the holding time is 1 to 1.5 hours. After heating, the L450M steel plate is water quenched. The tempering temperature is 540 to 580℃ and the holding time is 1 to 1.5 hours. After tempering, it is air-cooled and water-cooled to room temperature respectively.

[0052] The final microstructure of L450M steel plate is tempered martensite and fine reverse-transformed austenite. The microstructure of L450M ultra-low temperature straight seam submerged arc welded pipe steel plate is as follows: Figure 1 As shown.

[0053] The steel plate obtained in the above steps is used to make ultra-low temperature type L450M grade straight seam submerged arc welded pipe with diameter ∅406×14.2mm, specifically according to the following steps: Step 1: Perform 100% ultrasonic testing on the steel plate, then mill the edges of the steel plate. The blunt edge of the L450M steel plate is 6.5mm, the upper bevel angle is 60°, and the lower bevel angle is 90°. Finally, use a pre-bending machine to pre-bend the plate edges so that the plate edge curvature meets the curvature requirements of each pipe diameter. Step 2: Using JCO universal mold, narrow die multi-pass bending for rapid forming, the tool holder pressing speed is 0.6mm / s to quickly complete JCO forming. First, one side of the steel plate is pressed quickly into a J shape, then the other half of the steel plate is pressed quickly into a C shape, and finally the middle of the steel plate is pressed quickly once to form an O-shaped opening. A total of 23 pressing passes are required for L450M steel plate. Step 3: Using the above-mentioned straight seam submerged arc welding method, weld the JCO formed steel plate with Mn-Ni-Mo welding wire. The specific welding steps are as follows: Step 3-1: Feed the JCO-formed steel pipe into the pre-welding machine, adjust the position of the pressure roller of the pre-welding machine, and use mixed gas shielded welding method for welding. The pre-welding wire is the Mn-Ni-Mo welding wire with a diameter of 2.0 mm prepared in Example 1, wherein the gas type is 60% carbon dioxide + 40% argon, and the welding speed V=4m / min to form a continuous and reliable pre-welded seam. Step 3-2: First, perform external three-wire submerged arc automatic welding, then perform internal three-wire submerged arc automatic welding. The welding wire is the Mn-Ni-Mo welding wire with a diameter of 4mm prepared in Example 1, combined with the CaF2-SiO2-MgO-Al2O3-MnO-TiO2-B2O3 slag system sintered flux with a basicity of 1.80 to complete the submerged arc welding. During submerged arc welding, both the inner and outer front wires use DC reverse polarity, while the middle and rear wires use AC polarity. The welding speed is 1.35 m / min, and the line energy of the L450M welded pipe is controlled at 20 kJ / cm to complete the submerged arc welding of the straight seam of the welded pipe. Step 4: Expand the diameter rapidly along the entire length at an expansion rate of 0.8%, with an expansion speed of 0.5 mm / s, and maintain the water pressure at 100% nominal yield strength for 15 seconds; Step 5: Routine testing; Mechanical properties were tested on the L450M steel grade ∅406×14.2mm pipe prepared with ultra-low temperature high toughness and low yield strength ratio. The results showed that the yield strength of the L450M welded pipe was 455~510MPa, the tensile strength was 535~650MPa, the yield strength ratio was 0.75~0.83, the elongation was 30%~42%, the impact toughness of the pipe body at -100℃ was 360~470J, the impact toughness of the weld at -100℃ was 255~340J, the impact toughness of the hot zone at -100℃ was 220~350J, the impact toughness of the pipe body at -120℃ was 263~352J, the weld was 190~250J, and the hot zone was 244~302J. The drop shear area at -45℃ was 98%~100%. X-ray diffraction showed that the volume fraction of austenite in the pipe body base material was 4.57%.

[0054] Example 3 The specific steps for preparing the steel plate according to the composition of the No. 2 ultra-low temperature type L485M steel grade steel plate in Table 1 are as follows: Step 1: The raw materials are sequentially heated, melted, refined, alloyed and cast under vacuum negative pressure to obtain ingots that meet the mass percentage of No. 2 ultra-low temperature type L485M steel grade steel plates in Table 1. Step 2: The ingot obtained in Step 1 is cast into a large cross-section billet with a width of 460mm×2000mm by a continuous casting machine for extra-thick plates; the billet heating temperature is 1170℃, the holding time is 3 to 4 hours, the casting is carried out under full protection, and the superheat is controlled at 10 to 18℃. Step 3: Using dynamic light pressure, rough rolling is performed 7 times, followed by 2 times of 90° horizontal rolling, and then 12 times of 90° finish rolling. The final rolling temperature is 820~950℃, producing a steel plate with a wall thickness of 17.5mm and a wall thickness of 1380mm. Step 4: First, perform critical zone annealing on the L485M steel plate. The annealing heating temperature is 790℃ and the holding time is 1 to 1.5 hours. After heating, the L485M steel plate is water quenched. The tempering temperature is 540 to 580℃ and the holding time is 1 to 1.5 hours. After tempering, it is air-cooled and water-cooled to room temperature respectively.

[0055] The final microstructure of L485M steel plate consists of tempered martensite and fine reverse-transformed austenite.

[0056] The steel plate obtained in the above steps is used to make ultra-low temperature type L485M steel grade straight seam submerged arc welded pipe with diameters of Φ457×17.5mm. The specific steps are as follows: Step 1: Perform 100% ultrasonic testing on the steel plate, then mill the edges of the steel plate. The blunt edge of the L485M steel plate is 7.5mm, the upper bevel angle is 60°, and the lower bevel angle is 90°. Finally, use a pre-bending machine to pre-bend the plate edges so that the plate edge curvature meets the curvature requirements of each pipe diameter.

[0057] Step 2: Using JCO universal mold, narrow die multi-pass bending for rapid forming, the tool holder pressing speed is 0.6mm / s to quickly complete JCO forming. First, press one side of the steel plate to quickly form a J shape, then press the other half of the steel plate to quickly form a C shape, and finally press the middle of the steel plate quickly once to form an O-shaped opening. The L485M steel plate is pressed 25 times in total. Step 3: Using the above-mentioned straight seam submerged arc welding method, weld the JCO formed steel plate with Mn-Ni-Mo welding wire. The specific welding steps are as follows: Step 3-1: Feed the JCO-formed steel plate into the pre-welding machine, adjust the position of the pressure roller of the pre-welding machine, and use mixed gas shielded welding method for welding. The pre-welding wire is the Mn-Ni-Mo welding wire with a diameter of 2.0 mm prepared in Example 1, wherein the gas type is 60% carbon dioxide + 40% argon, and the welding speed V=4m / min to form a continuous and reliable pre-welded seam. Step 3-2: First, perform external three-wire submerged arc automatic welding, then perform internal three-wire submerged arc automatic welding. The welding wire is the Mn-Ni-Mo welding wire with a diameter of 4mm prepared in Example 1, combined with the CaF2-SiO2-MgO-Al2O3-MnO-TiO2-B2O3 slag system sintered flux with a basicity of 1.80 to complete the submerged arc welding. During submerged arc welding, both the inner and outer front wires use DC reverse polarity, while the middle and rear wires use AC polarity. The welding speed is 1.35 m / min, and the line energy of the L485M welded pipe is controlled at 21.5 kJ / cm to complete the submerged arc welding of the straight seam of the welded pipe. Step 4: Expand the diameter rapidly along the entire length at an expansion rate of 0.8%, with an expansion speed of 0.6 mm / s, and maintain the water pressure at 100% nominal yield strength for 15 seconds; Step 5: Routine testing; Mechanical properties were tested on the L485M steel grade ∅457×17.5mm pipe prepared with ultra-low temperature high toughness and low yield strength ratio. The results showed that the yield strength of the L485M welded pipe was 485~540MPa, the tensile strength was 570~670MPa, the yield strength ratio was 0.78~0.85, the elongation was 32%~48%, the impact toughness of the pipe body at -100℃ was 378~480J, the impact toughness of the weld at -100℃ was 262~380J, the impact toughness of the hot zone at -100℃ was 217~370J, the impact toughness of the pipe body at -120℃ was 270~312J, the impact toughness of the weld was 190~250J, and the impact toughness of the hot zone was 249~300J. The drop shear area at -45℃ was 90%~100%. X-ray diffraction showed that the volume fraction of austenite in the pipe body base material was 6.58%.

[0058] Example 4 The manufacturing of Mn-Ni-Mo welding wire is carried out according to the following steps: Step 1: The raw materials are smelted and cast to obtain ingots with the following mass percentages: 0.10% C, 2% Mn, 0.19% Si, 1.7% Ni, 0.40% Mo, 0.04% V, 0.010% P, 0.005% S, with the balance being Fe. The total content of the above components is 100%. Step 2: Perform simple forging on the ingot, and roll it into wire rod using a hot continuous rolling mill when the ingot temperature range is 1000℃~1100℃. Step 3: The wire rod is drawn through a drawing machine in multiple passes, with the deformation per pass controlled between 10% and 20%. Through multiple drawing processes, the two types of welding wires with diameters of 2 mm and 4 mm, respectively, required in Examples 5 and 6 are gradually achieved. Step 4: Heat treat the drawn welding wire. Depending on the specific performance requirements of the welding wire, after annealing, select quenching and tempering treatment. Step 5: Perform surface treatment on the drawn solid welding wire to improve its surface quality and rust resistance. The surface treatment can be copper plating.

[0059] Example 5 Table 2 Chemical composition analysis of steel plates (wt%)

[0060] The specific steps for preparing the steel plate according to the composition of the No. 3 ultra-low temperature type L450M steel grade steel plate in Table 2 are as follows: Step 1: The raw materials are sequentially heated, melted, refined, alloyed and cast under vacuum negative pressure to obtain ingots that meet the mass percentage of grade 3 L450M steel plate in Table 2. Step 2: The ingot obtained in Step 1 is cast into a large cross-section billet with a width of 460mm×2000mm using an extra-thick plate continuous casting machine; the billet heating temperature is 1155℃, the holding time is 2 to 2.5 hours, the casting is carried out under full protection, and the superheat is controlled at 10 to 18℃. Step 3: Using dynamic light pressure, rough rolling is performed 7 times, followed by 2 times of 90° horizontal rolling, and then 14 times of 90° finish rolling. The final rolling temperature is 790~950℃, producing a steel plate with a wall thickness of 12.5mm and a wall thickness of 1200mm. Step 4: First, perform critical zone annealing on the L450M steel plate. The annealing heating temperature is 790℃ and the holding time is 0.5 to 1 hour. After heating, the L450M steel plate is water quenched. The tempering temperature is 520 to 560℃ and the holding time is 0.5 to 1 hour. After tempering, it is air-cooled and water-cooled to room temperature respectively.

[0061] The steel plate obtained in the above steps is used to make ultra-low temperature type L450M grade straight seam submerged arc welded pipe with diameters of Φ406×12.5mm. The specific steps are as follows: Step 1: Perform 100% ultrasonic testing on the steel plate, then mill the edges of the steel plate. The blunt edge of the L450M steel plate is 5.5mm, the upper bevel angle is 60°, and the lower bevel angle is 90°. Finally, use a pre-bending machine to pre-bend the plate edges so that the plate edge curvature meets the curvature requirements of each pipe diameter.

[0062] Step 2: Using JCO universal mold, narrow die multi-pass bending for rapid forming, the tool holder pressing speed is 0.5mm / s to quickly complete JCO forming. First, press one side of the steel plate to quickly form a J shape, then press the other half of the steel plate to quickly form a C shape, and finally press the middle of the steel plate quickly once to form an O-shaped opening. The L450M steel plate is pressed in a total of 23 passes. Step 3: Using the above-mentioned straight seam submerged arc welding method, weld the JCO formed steel plate with Mn-Ni-Mo welding wire. The specific welding steps are as follows: Step 3-1: Feed the JCO-formed steel pipe into the pre-welding machine, adjust the position of the pressure roller of the pre-welding machine, and use mixed gas shielded welding method for welding. The pre-welding wire is the Mn-Ni-Mo welding wire with a diameter of 2.0 mm prepared in Example 4, wherein the gas type is 60% carbon dioxide + 40% argon, and the welding speed V=4m / min to form a continuous and reliable pre-welded seam. Step 3-2: First, perform external three-wire submerged arc welding, then perform internal three-wire submerged arc welding. The welding wire is the Mn-Ni-Mo welding wire with a diameter of 4mm prepared in Example 4, combined with the CaF2-SiO2-MgO-Al2O3-MnO-TiO2-B2O3 slag system sintered flux with a basicity of 1.80 to complete the submerged arc welding. During submerged arc welding, both the inner and outer front wires use DC reverse polarity, while the middle and rear wires use AC polarity. The welding speed is 1.35 m / min, and the line energy of the L450M welded pipe is controlled at 20 kJ / cm to complete the submerged arc welding of the straight seam of the welded pipe. Step 4: Expand the diameter rapidly along the entire length at an expansion rate of 1.0%, with an expansion speed of 0.70 mm / s. Hold the water pressure at 100% of the nominal yield strength for 15 seconds. Step 5: Routine testing; Mechanical properties were tested on the L450M steel grade ∅406×14.2mm pipe prepared with ultra-low temperature high toughness and low yield strength ratio. The results showed that the yield strength of the L450M welded pipe was 455~500MPa, the tensile strength was 535~640MPa, the yield strength ratio was 0.76~0.83, the elongation was 32%~42%, the impact toughness of the pipe body at -100℃ was 350~430J, the impact toughness of the weld at -100℃ was 250~300J, the impact toughness of the hot zone at -100℃ was 200~310J, the impact toughness of the pipe body at -120℃ was 250~321J, the weld was 170~233J, and the hot zone was 222~269J. The drop shear area at -45℃ was 98%~100%. X-ray diffraction showed that the volume fraction of austenite in the pipe body base material was 5.38%.

[0063] Example 6 The specific steps for preparing the steel plate according to the composition of grade L485M ultra-low temperature steel plate in Table 2 are as follows: Step 1: The raw materials are sequentially heated, melted, refined, alloyed and cast under vacuum negative pressure to obtain ingots that meet the mass percentage of grade 4# ultra-low temperature L485M steel plates in Table 2. Step 2: The ingot obtained in Step 1 is cast into a large cross-section billet with a width of 460mm×2000mm by a continuous casting machine for extra-thick plates; the billet is heated to 1165℃ and held for 2 to 3 hours, with full protection during casting and the superheat is controlled at 10 to 18℃. Step 3: Using dynamic light pressure, rough rolling is performed 7 times, followed by 2 times of 90° horizontal rolling, and then 13 times of 90° finish rolling. The final rolling temperature is 790~950℃, producing a steel plate with a wall thickness of 14.2mm and a wall thickness of 1365mm. Step 4: First, perform critical zone annealing on the L485M steel plate. The annealing heating temperature is 800℃, and the holding time is 0.5 to 1 hour. After heating, the L485M steel plate is water quenched. The tempering temperature is 520 to 560℃, and the holding time is 0.5 to 1 hour. After tempering, it is air-cooled and water-cooled to room temperature respectively. Finally, the microstructure of the L485M steel plate is tempered martensite and fine reverse-transformed austenite.

[0064] The steel plate obtained in the above steps is used to make ultra-low temperature type L485M steel grade straight seam submerged arc welded pipe with diameters of Φ457×14.2mm. The specific steps are as follows: Step 1: Perform 100% ultrasonic testing on the steel plate, then mill the edges of the steel plate. The blunt edge of the L485M steel plate is 6.5mm, the upper bevel angle is 60°, and the lower bevel angle is 90°. Finally, use a pre-bending machine to pre-bend the plate edges so that the plate edge curvature meets the curvature requirements of each pipe diameter.

[0065] Step 2: Using JCO universal mold, narrow die multi-pass bending for rapid forming, the tool holder pressing speed is 0.5mm / s to quickly complete JCO forming. First, press one side of the steel plate to quickly form a J shape, then press the other half of the steel plate to quickly form a C shape, and finally press the middle of the steel plate quickly once to form an O-shaped opening. The L485M steel plate is pressed 25 times in total. Step 3: Using the above-mentioned straight seam submerged arc welding method, weld the JCO formed steel plate with Mn-Ni-Mo welding wire. The specific welding steps are as follows: Step 3-1: Feed the JCO-formed steel pipe into the pre-welding machine, adjust the position of the pressure roller of the pre-welding machine, and use mixed gas shielded welding method for welding. The pre-welding wire is the Mn-Ni-Mo welding wire with a diameter of 2.0 mm prepared in Example 4, wherein the gas type is 60% carbon dioxide + 40% argon, and the welding speed V=4m / min to form a continuous and reliable pre-welded seam. Step 3-2: First, perform external three-wire submerged arc welding, then perform internal three-wire submerged arc welding. The welding wire is the Mn-Ni-Mo welding wire with a diameter of 4mm prepared in Example 4, combined with the CaF2-SiO2-MgO-Al2O3-MnO-TiO2-B2O3 slag system sintered flux with a basicity of 1.80 to complete the submerged arc welding. During submerged arc welding, both the inner and outer front wires use DC reverse polarity, while the middle and rear wires use AC polarity. The welding speed is 1.35 m / min, and the heat input of the L485M welded pipe is controlled at 20 kJ / cm to complete the submerged arc welding of the straight seam of the welded pipe. Step 4: Expand the diameter rapidly along the entire length at an expansion rate of 1.0%, with an expansion speed of 0.70 mm / s. Hold the water pressure at 100% of the nominal yield strength for 15 seconds. Step 5: Routine testing; Mechanical properties were tested on the L485M steel grade (∅457×14.2mm) prepared using the above steps, which exhibits high toughness and low yield strength ratio at ultra-low temperatures. The results showed that the L485M welded pipe had a yield strength of 485–535 MPa, tensile strength of 570–660 MPa, yield strength ratio of 0.78–0.85, elongation of 32%–45%, impact toughness of the pipe body at -100℃ of 370–440 J, weld impact toughness of 260–340 J, and hot zone impact toughness of 212–340 J at -100℃. At -120℃, the pipe body impact toughness was 250–271 J, weld impact toughness was 170–203 J, and hot zone impact toughness was 222–269 J. The drop shear area at -45℃ was 90%–100%. X-ray diffraction analysis revealed that the austenite volume fraction of the pipe body base material was 5.35%.

[0066] This invention demonstrates that the steel plates and welds of the L450M grade Φ406×14.2mm and L485M grade Φ457×17.5mm ultra-low temperature straight seam submerged arc welded pipes manufactured by this invention exhibit excellent high toughness and low yield strength ratio at low temperatures, with no weld defects, greatly improving the safety of steel pipes used for transporting supercritical carbon dioxide.

Claims

1. Ultra-low temperature type L450M / L485M grade steel plate, characterized in that, The ingredients are as follows, by weight percentage: C: 0.02-0.03%, Si: 0.10-0.40%, Mn: 2.5-4.0%, Al: 0.05-0.10%, P: ≤0.010%, S: ≤0.002%, other alloying elements ≤1.0%, balance Fe, and the total content of the above components is 100%.

2. The ultra-low temperature type L450M / L485M grade steel plate according to claim 1, characterized in that, The other alloying elements are as follows: Cr: 0-0.5%, Cu: 0-0.25%, Mo: 0-0.3%, Nb+V+Ti: 0-0.12% by mass percentage.

3. The method for preparing the ultra-low temperature L450M / L485M grade steel plate according to claim 1 or 2, characterized in that, The specific steps are as follows: Step 1: The raw materials are sequentially heated, smelted, refined, alloyed and cast to obtain ingots that conform to the mass percentage of the composition of ultra-low temperature type L450M / L485M steel plates. Step 2: The ingot obtained in Step 1 is used to obtain a large-section billet through an extra-thick plate continuous casting machine. Step 3: Roll the billet; Step 4: Heat treat the steel plate obtained in step 3.

4. The method for preparing ultra-low temperature L450M / L485M grade steel plate according to claim 3, characterized in that, The specific steps of step 3 are as follows: Step 3-1, Heating: Heat the billet obtained in step 2 to 1150-1200℃; Step 3-2, Rough rolling: Roll the narrow billet 3 to 9 times, with a deformation of ≥20% per pass; then rotate the billet 90° and roll it laterally 2 to 6 times, with a deformation of ≥10% per pass. Step 3-3, Finish rolling: Rotate the slab 90° and finish roll it 6 to 15 times, with a total deformation of ≥60%; Steps 3-4: Final rolling: Roll to the required thickness at a temperature of 800-950℃.

5. The method for preparing ultra-low temperature L450M / L485M grade steel plate according to claim 3, characterized in that, The specific steps of step 4 are as follows: Step 4-1: Quench the steel plate obtained after rolling in Step 3; Step 4-2, Critical Zone Annealing: The heating temperature is 780~810℃, the holding time is 0.5~2 hours, and then water quenching is performed; Step 4-3, Tempering: Perform a second tempering at a temperature of 520-580℃ for 0.5-2 hours. Step 4-4: After tempering, air cool or water cool to room temperature.

6. The Mn-Ni-Mo welding wire used for the ultra-low temperature L450M / L485M grade steel plate as described in claim 3, characterized in that, The ingredients are as follows, by weight percentage: C: 0.05%~0.12%, Mn: 1.5%~2%, Ni: 1.5%~2.5%, with the total amount of Mn and Ni ≤4%, V: 0.05%~0.2%, Si: 0.15%~0.2%, Mo: 0.3%~0.5%, of which S≤0.004%, P≤0.012%, and the balance is Fe. The total content of the above components is 100%.

7. The method for preparing the Mn-Ni-Mo welding wire according to claim 6, characterized in that, The specific steps are as follows: Step 1: The raw materials are smelted and cast to obtain an ingot that conforms to the mass percentage of the Mn-Ni-Mo welding wire composition as described in claim 6; Step 2: Forge the ingot and then roll it into wire rod; Step 3: The wire rod is drawn through a drawing machine in multiple passes to obtain welding wire of the required diameter; Step 4: Heat treat the drawn welding wire; Step 5: Perform surface treatment on the drawn solid welding wire.

8. A welding method for straight seam submerged arc welding of the ultra-low temperature type L450M / L485M steel grade plate of claim 3 using the Mn-Ni-Mo welding wire of claim 6, characterized in that, The specific steps are as follows: Step 1, Pre-welding: Mixed gas shielded welding is used, and the pre-welding wire is Mn-Ni-Mo welding wire; Step 2: First, perform external three-wire submerged arc automatic welding, then perform internal three-wire submerged arc automatic welding. Use Mn-Ni-Mo welding wire and a sintered flux with a basicity of 1.8, consisting of CaF2-SiO2-MgO-Al2O3-MnO-TiO2-B2O3 slag system.

9. The welding method for straight seam submerged arc welding according to claim 8, characterized in that, Both the front wire of the external three-wire submerged arc automatic welding and the internal three-wire submerged arc automatic welding use DC reverse polarity, while the middle and rear wires use AC polarity. The welding speed is V = 1.3~1.5m / min, and a small heat input of 15~25kJ / cm is used.

10. A method for preparing ultra-low temperature L450M / L485M grade straight seam submerged arc welded pipes using the steel plate described in claim 3, characterized in that, The specific steps are as follows: Step 1: Before forming, the prepared ultra-low temperature type L450M / L485M steel plate is milled to produce a thick blunt edge, and then the steel plate is pre-bent. Step 2: Perform JCO forming, using a narrow die and multiple bending passes, with a tool holder pressing speed of not less than 0.5 mm / s; Step 3: Weld the JCO-formed steel plate using the straight seam submerged arc welding method described in claim 9; Step 4: Perform rapid diameter expansion on the welded pipe obtained in Step 3 with an expansion rate of 0.8% to 1.0% and an expansion speed of not less than 0.5 mm / s. Step 5: Routine testing.

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