Preparation method of corrosion-resistant L555-grade subsea pipeline steel

Abstract

The invention discloses a preparation method of corrosion-resistant L555-grade subsea pipeline steel, belongs to the technical field of subsea pipeline steel manufacturing, and aims to solve the problem that the strength, toughness and corrosion resistance of pipeline steel are difficult to synergistically improve due to insufficient tissue regulation and control capability in an existing L555-grade pipeline steel preparation method. The invention provides a preparation method of corrosion-resistant L555-grade subsea pipeline steel. The preparation method comprises the following steps: step 1, smelting and continuous casting process; step 2, controlling a rolling process; the method comprises the following steps: firstly, heating a continuous casting billet to 1180 + / -10 DEG C, and preserving heat for 30-60 minutes; step 3, a third-stage temperature control cooling process; the rolled plate blank is sequentially subjected to first-stage rapid cooling, second-stage medium-speed cooling-isothermal keeping stage and third-stage air cooling treatment; 4, the rolled subsea pipeline steel is subjected to online self-tempering treatment; the on-line self-tempering treatment process comprises the following steps: after cooling, carrying out on-line tempering treatment on the steel plate at 450-480 DEG C for 3-8 minutes. The corrosion resistance of the L555-grade subsea pipeline steel can be remarkably improved.

Description

Technical Field

[0001] This invention belongs to the field of submarine pipeline steel manufacturing technology, and particularly relates to a method for preparing corrosion-resistant L555 grade submarine pipeline steel. Background Technology

[0002] Submarine pipelines, as key infrastructure for the development and transportation of offshore oil and gas resources, have long been in operation in environments characterized by high salinity, high humidity, high pressure, and high chloride content. - In harsh marine environments with corrosive media such as H2S and CO2, its safety and reliability are directly related to the stable operation of the entire oil and gas extraction system and the protection of the marine ecological environment.

[0003] Currently, traditional methods for manufacturing L555 grade pipeline steel still have certain limitations in terms of microstructure control. The rolling process parameters of traditional methods are not precisely controlled, resulting in insufficient austenite grain refinement during rolling and poor matching between cooling rate and phase transformation process. This leads to the formation of coarse pearlite or network carbides in the steel microstructure, affecting the material's low-temperature toughness and potentially posing a risk of brittle fracture under deep-sea low-temperature conditions. Furthermore, existing methods lack targeted corrosion-resistant strengthening treatments, and current processes do not adequately promote the formation and stability of the passivation film on the steel surface, failing to effectively resist chloride ion corrosion in the seabed environment, leading to frequent localized corrosion phenomena such as pitting and crevice corrosion. Summary of the Invention

[0004] Based on the above analysis, the present invention aims to provide a method for preparing corrosion-resistant L555 grade subsea pipeline steel, which solves the problem that the existing L555 grade pipeline steel preparation methods have insufficient microstructure control capabilities, resulting in the difficulty in synergistically improving the strength, toughness and corrosion resistance of pipeline steel.

[0005] The objective of this invention is mainly achieved through the following technical solutions:

[0006] This invention provides a method for preparing corrosion-resistant L555 grade subsea pipeline steel, comprising the following steps: Step 1: Smelting and continuous casting process; The steel is smelted according to the design composition of L555 grade submarine pipeline steel, using converter smelting + LF refining + RH vacuum degassing process + continuous casting. Step 2: Control the rolling process; First, heat the continuously cast billet to 1180℃±10℃ and hold it for 30-60 minutes; Step 3: Three-stage temperature-controlled cooling process; The rolled slab is subjected to three stages of rapid cooling, medium-speed cooling-isothermal holding, and air cooling in sequence. The first stage of rapid cooling involves cooling the rolled slab to 480-520℃ at a cooling rate of 20-35℃ / s. Step 4: Perform online self-tempering treatment on the rolled submarine pipeline steel; The online self-tempering process is as follows: after cooling, the steel plate is subjected to online tempering treatment at 450-480℃ for 3-8 minutes.

[0007] Furthermore, in step 3, the second stage of medium-speed cooling-isothermal holding includes a medium-speed cooling stage and an isothermal holding stage; Medium-speed cooling stage: The steel plate is cooled from the temperature of 480-520℃ after the first stage of rapid cooling to 400-450℃ at a cooling rate of 8-12℃ / s.

[0008] Furthermore, in step 3, the isothermal holding stage: after the steel plate reaches 400-450℃, it is isothermally held for 5-10 minutes, and then slowly cooled to 320-380℃ at a cooling rate of 1-3℃ / s.

[0009] Furthermore, in step 3, the third stage is air cooling: the steel plate is naturally cooled from 320-380℃ to room temperature.

[0010] Furthermore, in step 1, during smelting and continuous casting, [O] is controlled to be ≤0.003% and the inclusion grade is controlled to be ≤1.0.

[0011] Furthermore, in step 1, the thickness of the continuously cast billet is 200-220 mm, and the center segregation is controlled within Class C, grade 1.0.

[0012] Furthermore, in step 1, during the converter smelting stage, the initial temperature is 1450-1600℃, followed by blowing for 20-45 minutes, and then tapping the steel at a temperature of 1650-1710℃.

[0013] Furthermore, in step 1, during the LF refining stage, the temperature of the molten steel is 1600-1650℃, and the refining time is 15-40 minutes.

[0014] Furthermore, in step 4, the steel plate is subjected to online tempering treatment at 450-480℃ for 3-8 minutes after cooling.

[0015] The present invention also provides a corrosion-resistant L555 grade subsea pipeline steel, which is prepared by the above-mentioned method for preparing corrosion-resistant L555 grade subsea pipeline steel; The composition of this corrosion-resistant L555 grade submarine pipeline steel, by mass percentage, includes: C: 0.03-0.07%, Mn: 1.60-2.20%, Ni: 0.10-0.80%, Cu: 0.10-0.45%, Cr: 0-0.30%, Nb: 0-0.06%, Ti 0-0.02%, P≤0.008%, S≤0.002%, N≤0.004%, with the remainder being Fe and unavoidable impurities.

[0016] Compared with the prior art, the present invention can achieve at least one of the following beneficial effects: (1) The preparation method of the present invention can significantly improve the uniformity of its structure, low temperature toughness and seawater corrosion resistance by adopting controlled rolling and three-stage temperature control cooling process.

[0017] (2) By precisely controlling the smelting and continuous casting process, the present invention strictly controls the oxygen content [O] in the steel to ≤0.003% and the inclusion grade to ≤1.0, and controls the center segregation of the continuous casting billet to within Class C 1.0, effectively reducing the adverse effects of inclusions and segregation on the steel properties, and laying a pure steel foundation for obtaining excellent structure in subsequent rolling and heat treatment.

[0018] (3) In the controlled rolling process of this invention, the continuously cast billet is heated to 1180℃±10℃ and held for 30-60 minutes. This heating regime can fully dissolve the precipitated phases such as carbides in the steel and avoid excessive growth of austenite grains. Combined with subsequent rolling deformation, the austenite grains can be significantly refined, creating conditions for finally obtaining fine ferrite and bainite structures, thereby improving the strength and toughness of pipeline steel.

[0019] (4) The present invention employs segmented controlled cooling, the beneficial effects of which include: firstly, the rapid cooling section inhibits the formation of coarse martensite and coarse bainite in the slab, reducing the tendency for local embrittlement; secondly, the medium-speed cooling section controls the diffusion process of elements in the slab to promote the formation of uniform acicular ferrite, thereby improving the impact toughness and fatigue threshold in the transverse direction; and thirdly, the slow cooling section completes the tempering and precipitate stabilization of the slab, reducing the residual stress of the slab and stabilizing the mechanical properties of the slab. For deep-sea service (low temperature, high salt, cyclic stress), this alloy takes into account the requirements of crack resistance, corrosion resistance and welded joint performance, significantly improving long-term service reliability.

[0020] (5) By setting up online self-tempering treatment, the steel plate is tempered at 450-480℃ for 3-8 minutes after cooling. This not only eliminates the internal stress generated during rolling and rapid cooling, but also promotes the dispersion precipitation of carbides and the stabilization of the matrix structure, further improving the toughness and corrosion resistance of the steel. At the same time, it avoids the problems of increased energy consumption and reduced production efficiency caused by offline tempering.

[0021] In this invention, the above-described technical solutions can be combined with each other to achieve more preferred combinations. Other features and advantages of this invention will be set forth in the following description, and some advantages may become apparent from the description or be learned by practicing the invention. The objects and other advantages of this invention can be realized and obtained from what is particularly pointed out in the description and drawings. Attached Figure Description

[0022] The accompanying drawings are for illustrative purposes only and are not intended to limit the invention. Throughout the drawings, the same reference numerals denote the same parts.

[0023] Figure 1 This is a schematic diagram of the preparation process of the corrosion-resistant L555 grade submarine pipeline steel of the present invention; Figure 2 This is a metallographic diagram of the corrosion-resistant L555 grade submarine pipeline steel of the present invention. Detailed Implementation

[0024] The preferred embodiments of the present invention will now be described in detail with reference to the accompanying drawings, which constitute a part of the present invention and are used together with the embodiments of the present invention to illustrate the principles of the present invention, but are not intended to limit the scope of the present invention.

[0025] In existing technologies, L555 grade pipeline steel cannot effectively resist long-term electrochemical corrosion from seawater. Its low-temperature toughness also falls short of performance requirements in extreme conditions at -40°C and below. These issues collectively limit the application effectiveness and reliability of L555 grade pipeline steel in complex environments such as deep-sea oil and gas exploration and transportation.

[0026] To address the above problems, this invention provides a method for preparing corrosion-resistant L555 grade subsea pipeline steel, which includes the following steps: Step 1: Smelting and continuous casting process; The steel is smelted according to the design composition of L555 grade submarine pipeline steel; the process is converter smelting + LF refining + RH vacuum degassing + continuous casting.

[0027] During the converter (BOF) smelting stage, the initial temperature is 1450-1600℃, followed by blowing for 20-45 minutes. After blowing, the steel is tapped at a temperature of 1650-1710℃.

[0028] The aforementioned converter (BOF) smelting is used for rapid decarburization and to complete basic alloying.

[0029] During the LF refining stage, the temperature of the molten steel is 1600-1650℃, and the refining time is 15-40 min.

[0030] LF refining is used to achieve desulfurization, dephosphorization, inclusion crushing, and homogenization of the dispersion of trace alloying elements (Nb, Ti).

[0031] The RH vacuum degassing stage is carried out at a steel temperature of 1580-1620℃, a vacuum degree of 10-50mbar, and a vacuum degassing time of 8-25 min. The lifting-lowering action is repeated 1-3 times to finally obtain the continuously cast billet.

[0032] RH vacuum degassing is used to remove dissolved hydrogen from molten steel and reduce the oxygen and nitrogen content of molten steel, thereby reducing the risk of HIC / SSC and optimizing inclusion morphology.

[0033] It should be noted that during smelting and continuous casting, [O] should be controlled to be ≤0.003%, and the inclusion grade should be ≤1.0. The thickness of the continuously cast billet should be 200-220mm, and the center segregation should be controlled within Class C 1.0.

[0034] It should be noted that the thickness of the continuously cast billet is limited to 180-220 mm (e.g., 200 mm) in this invention. This helps to control the subsequent hot rolling processing allowance, control center segregation, and ensure the uniformity of heat conduction. If the continuously cast billet is too thick, it will increase thermal inertia and make it difficult to ensure that the surface and internal temperature are consistent; if the continuously cast billet is too thin, it may not meet the requirements of subsequent rolling into finished products.

[0035] Step 2: Control the rolling process; The continuously cast billet is heated to 1180℃±10℃ and held for 30-60 minutes to ensure uniform temperature at the center of the billet and to allow the microalloying elements to fully dissolve the austenite within the billet.

[0036] Rough rolling stage: After holding at the temperature, the continuously cast billet is rolled in the recrystallization zone in 4-6 passes, with a final rolling temperature of 1050℃±10℃. The recrystallization deformation refines the previous austenite grains and breaks up the segregation zone, controls the cumulative strain level and adjusts the micromorphology of austenite.

[0037] Finishing stage: After roughing, the billet is rolled in the non-recrystallization zone for 4-5 passes, with a final rolling temperature of 800℃±10℃. Non-recrystallization deformation is used to induce lattice distortion in the billet, thereby achieving a grain refinement effect and providing a favorable size and distribution basis for subsequent rapid cooling.

[0038] Step 3: Three-stage temperature-controlled cooling process; The first stage is rapid cooling: the rolled slab is cooled to 480-520℃ at a cooling rate of 20-35℃ / s.

[0039] The rapid cooling in the first stage aims to allow the slab to pass through the martensite / bainite transformation zone quickly, promoting the formation of fine and uniformly distributed lower bainite nuclei and suppressing the formation of coarse martensite.

[0040] The second stage is the medium-speed cooling-isothermal holding stage: the medium-speed cooling stage: the steel plate is cooled from the temperature of 480-520℃ after the rapid cooling in the first stage to 400-450℃ at a cooling rate of 8-12℃ / s. This stage mainly promotes the formation of fine and uniform acicular ferrite.

[0041] Isothermal holding stage: After the steel plate reaches 400-450℃, it is held isothermally for 5-10 minutes, and then slowly cooled to 320-380℃ at a cooling rate of 1-3℃ / s. This stage aims to precisely control the precipitation kinetics of microalloying elements (Nb, Ti) carbonitrides (Nb(C,N), TiN), causing them to form smaller, more uniformly distributed, and more thermally stable nanoscale precipitates. Simultaneously, this temperature range also favors the formation of a small amount of stable and uniformly distributed retained austenite.

[0042] The third stage is air cooling: the slab is naturally cooled from 320-380℃ to room temperature; the purpose is to allow the precipitates (such as NbC) and the metallographic structure to complete the tempering evolution, reduce the residual stress of the slab and stabilize the mechanical properties of the slab.

[0043] This invention employs segmented controlled cooling, which offers several advantages: First, the rapid cooling section suppresses the formation of coarse martensite and coarse bainite within the slab, reducing the tendency for localized embrittlement. Second, the medium-speed cooling section controls the diffusion process of elements within the slab to promote the formation of uniform acicular ferrite, thereby improving transverse impact toughness and fatigue threshold. Third, the slow cooling section completes the tempering and precipitate stabilization of the slab, reducing residual stress and stabilizing the slab's mechanical properties. For deep-sea service (low temperature, high salinity, cyclic stress), this alloy meets the requirements for crack resistance, corrosion resistance, and weld joint performance, significantly improving long-term service reliability.

[0044] Step 4: Perform online self-tempering treatment on the rolled submarine pipeline steel; After cooling, the steel plate is subjected to online tempering treatment at 450-480℃ for 3-8 minutes to relieve its residual stress.

[0045] The final microstructure of the subsea pipeline steel of this invention is dominated by fine acicular ferrite, accounting for 85-95% of the total volume, accompanied by 5-15% lower bainite or fragmented bainite. The acicular ferrite is elongated and uniformly distributed, without bulk sorbite or coarse martensite. This microstructure combination ensures high yield strength while significantly improving low-temperature toughness and delaying crack propagation.

[0046] This invention, through a composite alloying design concept, incorporates elements such as Ni, Cu, and Cr, combined with a three-stage controlled cooling thermomechanical process, to achieve an integrated design of strong, tough, and corrosion-resistant L555 grade subsea pipeline steel. Its yield strength is 555-615 MPa, tensile strength is 625-700 MPa, and yield-to-tensile ratio is ≤0.90; impact energy (lateral) at -40℃ is ≥380 J, CTOD value at -40℃ is ≥0.40 mm, and DWTT shear area at -30℃ is ≥85%; impact energy of weld and heat-affected zone (-10℃) is ≥100 J; corrosion rate (3.5% NaCl, 30d) is ≤0.05 mm / a. This steel is suitable for deep-sea riser main pipes, risers, and wellhead connection structures, and can be used in... It can operate for a long time in a low temperature environment of 40℃ and a corrosion environment of 3.5% NaCl, and meets the requirements of international standards API 5L L555 and DNV-OS-F101 deep water pipelines.

[0047] The present invention also provides a corrosion-resistant L555 grade subsea pipeline steel, which is prepared by the above-described preparation method. The composition of the corrosion-resistant L555 grade subsea pipeline steel, by mass percentage, includes: C: 0.03-0.07%, Mn: 1.60-2.20%, Si: 0.10-0.30%, Ni: 0.10-0.80%, Cu: 0.10-0.45%, Cr: 0-0.30%, Nb: 0-0.06%, Ti: 0-0.02%, P≤0.008%, S≤0.002%, N≤0.004%, with the remainder being Fe and unavoidable impurities.

[0048] Preferably, the composition of the above-mentioned corrosion-resistant L555 grade submarine pipeline steel, by mass percentage, includes: C: 0.03-0.059%, Mn: 1.80-2.20%, Si: 0.15-0.30%, Ni: 0.50-0.80%, Cu: 0.20-0.45%, Cr: 0.2-0.30%, Nb: 0.015-0.06%, Ti: 0-0.02%, P≤0.008%, S≤0.002%, N≤0.004%, with the remainder being Fe and unavoidable impurities.

[0049] Preferably, the composition of the above-mentioned corrosion-resistant L555 grade submarine pipeline steel, by mass percentage, includes: C: 0.031-0.045%, Mn: 2.00-2.20%, Si: 0.15-0.25%, Ni: 0.60-0.80%, Cu: 0.31-0.45%, Cr: 0.15-0.25%, Nb: 0.020-0.030%, Ti: 0.010-0.015%, P≤0.008%, S≤0.002%, N≤0.004%, with the remainder being Fe and unavoidable impurities.

[0050] Preferably, the composition of the above-mentioned low-C-high-Mn-Ni-Cu corrosion-resistant L555 grade submarine pipeline steel, by mass percentage, includes: C: 0.031-0.045%, Mn: 2.00-2.10%, Si: 0.15-0.22%, Ni: 0.60-0.80%, Cu: 0.31-0.40%, Cr: 0.15-0.20%, Nb: 0.020-0.030%, Ti: 0.010-0.015%, P≤0.008%, S≤0.002%, N≤0.004%, with the remainder being Fe and unavoidable impurities.

[0051] It should be noted that the preparation method of the present invention, by employing controlled rolling and three-stage temperature-controlled cooling processes, can significantly improve the uniformity of its microstructure, low-temperature toughness, and resistance to seawater corrosion.

[0052] Preferably, the composition of the corrosion-resistant L555 grade submarine pipeline steel of the present invention, by mass percentage, includes: C: 0.031-0.045%, Mn: 2.00-2.20%, Si: 0.15-0.25%, Ni: 0.60-0.80%, Cu: 0.31-0.45%, Cr: 0.15-0.25%, Nb: 0.020-0.030%, Ti: 0.010-0.015%, P≤0.008%, S≤0.002%, N≤0.004%, with the remainder being Fe and unavoidable impurities.

[0053] Preferably, the composition of the corrosion-resistant L555 grade submarine pipeline steel of the present invention, by mass percentage, includes: C: 0.031-0.045%, Mn: 2.00-2.20%, Si: 0.15-0.25%, Ni: 0.60-0.80%, Cu: 0.31-0.45%, Cr: 0.15-0.25%, Nb: 0.020-0.030%, Ti: 0.010-0.015%, P≤0.008%, S≤0.002%, N≤0.004%, with the remainder being Fe and unavoidable impurities.

[0054] The carbon equivalent CE_Pcm is controlled within the range of 0.15-0.17% to ensure the weldability and crack resistance of L555 grade subsea pipeline steel. The formula for calculating carbon equivalent CE_Pcm is as follows:

[0055] The mechanism of action of each alloy component in the L555 grade subsea pipeline steel of this invention is as follows: Carbon (C): As a fundamental element of L555 grade subsea pipeline steel, carbon has a significant impact on its overall performance. A low carbon content (0.031-0.045%) is crucial for ensuring the low-temperature toughness and weldability of L555 grade subsea pipeline steel: ① It can lower the brittle transition temperature of the steel, improving... 40℃ impact energy and CTOD value (low-temperature toughness index); ② It can reduce the hardened structure (such as martensite) in the weld heat-affected zone, reducing the risk of weld cracks. However, C is a strengthening element, and reducing the C content will result in some strength loss, which needs to be compensated for by the solid solution / precipitation strengthening effect of elements such as Mn, Nb, and Ti.

[0056] Mn: Mn is an austenite-forming and solid solution strengthening element that can improve strength. High Mn (2.00-2.20%) is key to balancing strength and toughness: ① Solid solution strengthens austenite, significantly improving yield strength and tensile strength; ② As an austenite-forming element, it delays pearlite transformation, promotes the formation of fine-grained ferrite, and improves impact energy at -40℃ and DWTT shear area at -30℃ (low-temperature toughness).

[0057] Si: Si is a strong deoxidizer, removing oxygen from steel, reducing oxide inclusions, and improving purity; it also strengthens through solid solution, helping to improve strength. However, excessive amounts (>0.5%) will worsen low-temperature toughness (increase the brittle transition temperature and reduce the impact energy at -40℃) and reduce weldability.

[0058] Ni: Ni is the "core element" for improving the low-temperature toughness of steel: ① It significantly reduces the brittle transition temperature of steel and greatly improves the impact energy at -40℃ and the CTOD value (low-temperature toughness); ② Ni works synergistically with Cu to enhance the density of the oxide film on the steel surface and improve its resistance to seawater corrosion.

[0059] Cu: Cu is key to improving seawater corrosion resistance: ① It selectively dissolves in seawater to form a Cu-rich oxide film, effectively reducing the uniform corrosion rate; ② During aging treatment, Cu atoms agglomerate to form a nanoscale ε-Cu precipitate phase, resulting in precipitation strengthening and compensating for the strength loss caused by low C; ③ Cu, Ni, and Cr synergistically enhance the resistance to pitting corrosion.

[0060] Cr: Cr enhances resistance to localized corrosion: ① Improves the stability of the passivation film on the steel surface, resisting the effects of Cl in seawater. - The corrosion caused by Cr is pitting and crevice corrosion; ② A small amount of Cr can be strengthened by solid solution and help improve strength. However, too much Cr will increase brittleness and the tendency for welding cracks. Therefore, this invention strictly controls its content within the range of 0.15-0.25%.

[0061] Nb: Nb has a dual function of microalloying: "grain refinement + strengthening": ① It enables the alloy to precipitate Nb(C,N) particles, inhibits austenite recrystallization, and refines hot-rolled grains (improving low-temperature toughness); ② During welding, it can prevent the growth of austenite grains in the heat-affected zone, improve the uniformity of the heat-affected zone structure, and ensure the impact energy of the weld and the heat-affected zone (≥100J at -10℃); ③ Precipitation strengthening, improving yield strength.

[0062] Ti: Ti can combine with N to form TiN particles, which refines austenite grains (inhibits their growth at high temperatures) and improves low-temperature toughness; ② Ti can fix free-state N and prevent N-induced aging brittleness; ③ During welding, TiN pins the grain boundaries of the heat-affected zone, which can improve the alloy's welding performance.

[0063] P: P tends to segregate at grain boundaries, increasing cold brittleness (reducing impact energy at -40℃ and CTOD value); at the same time, it deteriorates weldability and increases the tendency for cold cracking during welding. Its content must be strictly limited to P≤0.008%.

[0064] S: S forms MnS inclusions with Mn, which can extend along the rolling direction and reduce transverse low-temperature toughness (transverse impact energy at -40℃); in addition, MnS inclusions are prone to become corrosion sources, increasing the pitting corrosion rate, so S ≤ 0.002% must be strictly controlled.

[0065] N: N can combine with Ti to form TiN, which is used to refine grains (improving low-temperature toughness); excessive N content will generate free N, increasing the possibility of aging brittleness and welding cracks. Therefore, the N content must be strictly controlled to ≤0.004%.

[0066] The aforementioned unavoidable impurity elements include O, H, and harmful elements such as Pb, Sn, As, Sb, and Bi.

[0067] Furthermore, the composition of the aforementioned corrosion-resistant L555 grade submarine pipeline steel, by mass percentage, includes: C: 0.031-0.045%, Mn: 2.00-2.20%, Si: 0.15-0.25%, Ni: 0.60-0.80%, Cu: 0.31-0.45%, Cr: 0.15-0.25%, Nb: 0.020-0.030%, Ti: 0.010-0.015%, P≤0.008%, S≤0.002%, N≤0.004%, with the remainder being Fe and unavoidable impurities.

[0068] The mechanical properties of the L555 grade submarine pipeline steel include: yield strength 555-615 MPa, tensile strength 625-700 MPa, and yield-to-tensile ratio ≤0.90.

[0069] The toughness performance indicators of the aforementioned L555 grade subsea pipeline steel are: impact energy (transverse) at -40℃ ≥380 J, CTOD value at -40℃ ≥0.40 mm. 30℃ DWTT shear area ≥85%.

[0070] The welding performance indicators for the aforementioned L555 grade submarine pipeline steel are as follows: impact energy of the weld and heat-affected zone is ( ≥100 J (10℃)

[0071] The corrosion resistance performance index of the L555 grade submarine pipeline steel is as follows: corrosion rate (3.5% NaCl, 30d) ≤ 0.05mm / a.

[0072] The low-C, high-Mn–Ni–Cu corrosion-resistant L555 grade subsea pipeline steel of this invention is suitable for deep-sea oil and gas drilling and transportation systems, especially for deep-water drilling riser main pipes and other pressure-bearing components.

[0073] Compared with existing technologies, this invention, firstly, ensures excellent low-temperature toughness and weldability of steel through a low-carbon (C) design, effectively reducing the brittle transition temperature and minimizing the hardened microstructure in the weld heat-affected zone. To compensate for the strength loss caused by low carbon content, it utilizes solid solution strengthening and precipitation strengthening effects from elements such as high manganese (Mn), niobium (Nb), and titanium (Ti).

[0074] Secondly, the synergistic effect of high nickel (Ni), copper (Cu), and chromium (Cr) in this invention significantly improves the corrosion resistance of steel. The synergistic effect of nickel and copper enhances the density of the oxide film on the steel surface, effectively improving its resistance to uniform seawater corrosion. Copper, through selective dissolution in seawater to form a copper-rich oxide film, and subsequent aging treatment to form a nanoscale ε-Cu precipitate phase, further reduces the uniform corrosion rate, while also synergistically enhancing resistance to pitting corrosion with nickel and chromium. Chromium, by improving the stability of the passivation film on the steel surface, enhances its resistance to localized corrosion.

[0075] Furthermore, the microalloying of niobium (Nb) and titanium (Ti) inhibits austenite recrystallization by precipitating carbonitrides (Nb(C,N) and TiN), thereby refining the hot-rolled grains and significantly improving low-temperature toughness. During welding, these precipitates effectively pinnate grain boundaries in the heat-affected zone, preventing grain growth and improving the uniformity of the weld microstructure (e.g., ...). Figure 2 As shown), it provides precipitation enhancement effects.

[0076] Finally, the present invention strictly controls the carbon equivalent CE Pcm within a narrow range of 0.15~0.17%, ensuring that the steel maintains L555 grade mechanical properties while having excellent weldability and effectively avoiding hardening and embrittlement of the weld heat-affected zone (HAZ).

[0077] It should also be noted that the L555 grade submarine pipeline steel of the present invention does not contain the expensive element Mo; compared with the existing submarine pipeline steel which uses Ni-Mo, its steel cost is reduced by 10-15%.

[0078] Compared with the prior art, the present invention can achieve the following technical effects: (1) High corrosion resistance: Ni, Cu, and Cr synergistically form a dense passivation film, which is effective in the presence of Cl. - The corrosion rate in seawater is significantly lower than that of traditional L555 steel.

[0079] (2) High toughness and uniformity: Three-stage controlled cooling is used to obtain fine needle-like ferrite structure, and the thickness (equivalent size) of needle-like ferrite lamellars or units is controlled at 0.8-1.8 μm; the size of the main carbide / nitride (NbC, TiN) dispersed particles observed under TEM is 20-80 nm; the difference in transverse and longitudinal properties is ≤30 MPa.

[0080] (3) Excellent weldability: To ensure excellent weldability and avoid hardening of the heat-affected zone (HAZ), the carbon equivalent CE_Pcm is controlled at 0.15-0.17%. Within this carbon equivalent range, the matrix contributes strength through high Mn and precipitation strengthening (Nb, Ti) and Ni / Cu solid solution and precipitation, which maintains the mechanical requirements of L555 grade and ensures that the hardness and brittleness of the heat-affected zone (HAZ) are controlled under the welding process.

[0081] (4) Reduced cost: The L555 grade submarine pipeline steel of the present invention belongs to the Mo-free alloy system, which reduces the cost by 10-15% compared with the existing L555 grade submarine pipeline steel (Ni-Mo steel); Extended service life: The fatigue crack propagation rate is reduced by 30%, which can meet the long-term service requirements of 1500m water depth.

[0082] The following analysis will be based on specific implementation cases.

[0083] Example 1 The preparation process of the corrosion-resistant L555 grade subsea pipeline steel in this embodiment specifically includes the following steps: Step 1: Smelting and continuous casting process; The steel is smelted according to the design composition of L555 grade submarine pipeline steel in Table 1 below. Its composition by weight percentage includes: C: 0.039%, Mn: 2.10%, Si: 0.18%, Ni: 0.70%, Cu: 0.40%, Cr: 0.20%, Nb: 0.025%, Ti: 0.012%, P: 0.006%, S: 0.002%, N: 0.003%, with the remainder being Fe and unavoidable impurities.

[0084] In the converter (BOF) smelting stage, the initial temperature is 1460℃, followed by blowing for 35 minutes. After blowing, the steel is tapped at a temperature of 1680℃.

[0085] The aforementioned converter (BOF) smelting is used for rapid decarburization and to complete basic alloying.

[0086] During the LF refining stage, the temperature of the molten steel is 1630℃ and the refining time is 25 min.

[0087] In the RH vacuum degassing stage, the steel liquid temperature was 1600℃, the vacuum degree was 40 mbar, the vacuum degassing time was 17 min, the lifting-lowering action was repeated 3 times, and finally the continuous casting billet was obtained.

[0088] During smelting and continuous casting, [O] should be controlled to be ≤0.003%, and the inclusion grade should be ≤1.0. The thickness of the continuously cast billet should be 200mm, and the center segregation should be controlled within Class C, Grade 1.0.

[0089] Step 2: Control the rolling process; The continuous casting billet is heated to 1180℃ and held for 40 minutes; rough rolling stage: after holding, the continuous casting billet is rolled in 5 passes in the recrystallization zone, with a final rolling temperature of 1050℃; finish rolling stage: after rough rolling, the rolled billet is rolled in 4 passes in the non-recrystallization zone, with a final rolling temperature of 800℃.

[0090] Step 3: Three-stage temperature-controlled cooling process; The first stage of rapid cooling involves cooling the rolled slab to 490°C at a rate of 25°C / s.

[0091] The second stage is the medium-speed cooling-isothermal holding stage: The medium-speed cooling stage: The steel plate is cooled from the temperature of 490°C after the rapid cooling in the first stage to 420°C at a cooling rate of 10°C / s. This stage mainly promotes the formation of fine and uniform acicular ferrite.

[0092] Isothermal holding stage: After the steel plate reaches 420℃, it isothermally held for 7 minutes, and then slowly cooled to 360℃ at a cooling rate of 2℃ / s.

[0093] The third stage is air cooling: the slab is naturally cooled to room temperature.

[0094] Step 4: Perform online self-tempering on the rolled submarine pipeline steel; After cooling, the steel plate is subjected to online tempering at 460℃ for 5 minutes to relieve residual stress.

[0095] The final microstructure of the submarine pipeline steel of the present invention is mainly composed of fine acicular ferrite, accounting for 92% of the total volume, accompanied by 8% lower bainite.

[0096] The subsea pipeline steel prepared in this embodiment has a yield strength of 585 MPa, a tensile strength of 655 MPa, and a yield-to-tensile ratio ≤0.90; an impact energy (transverse) of -40℃ of 410 J, a CTOD value of -40℃ of 0.43 mm, and a DWTT shear area of ​​88% at -30℃; an impact energy of the weld and heat-affected zone (-10℃) of 117 J; and a corrosion rate (3.5% NaCl, 30d) ≤0.041 mm / a.

[0097] Example 2 The preparation process of the corrosion-resistant L555 grade subsea pipeline steel in this embodiment specifically includes the following steps: Step 1: Smelting and continuous casting process; The steel for subsea pipelines is smelted according to the design composition in Table 1; C: 0.037%, Mn: 2.05%, Si: 0.22%, Ni: 0.65%, Cu: 0.35%, Cr: 0.18%, Nb: 0.022%, Ti: 0.013%, P: 0.006%, S: 0.002%, N: 0.003%, with the remainder being Fe and unavoidable impurities.

[0098] In the converter (BOF) smelting stage, the initial temperature is 1580℃, followed by blowing for 40 minutes. After blowing, the steel is tapped at a temperature of 1700℃. Converter (BOF) smelting is used for rapid decarburization and to complete basic alloying.

[0099] In the LF refining stage, the temperature of the molten steel is 1640℃ and the refining time is 35 min. LF refining is used to achieve desulfurization, dephosphorization, inclusion crushing and homogenization of trace alloying elements (Nb, Ti) dispersion.

[0100] In the RH vacuum degassing stage, the steel liquid temperature was 1600℃, the vacuum degree was 45 mbar, the vacuum degassing time was 23 min, the lifting-lowering action was repeated 3 times, and finally the continuous casting billet was obtained.

[0101] During smelting and continuous casting, [O] should be controlled to be ≤0.003%, and the inclusion grade should be ≤1.0. The thickness of the continuously cast billet should be 210mm, and the center segregation should be controlled within Class C, Grade 1.0.

[0102] Step 2: Control the rolling process; The continuous casting billet is heated to 1190℃ and held for 55 minutes; rough rolling stage: after holding, the continuous casting billet is rolled in 6 passes in the recrystallization zone, with a final rolling temperature of 1060℃; finish rolling stage: after rough rolling, the rolled billet is rolled in 5 passes in the non-recrystallization zone, with a final rolling temperature of 810℃.

[0103] Step 3: Three-stage temperature-controlled cooling process; The first stage of rapid cooling involves cooling the rolled slab to 510°C at a rate of 32°C / s.

[0104] The second stage is the medium-speed cooling-isothermal holding stage: The medium-speed cooling stage: The steel plate is cooled from the temperature of 510°C after the rapid cooling in the first stage to 440°C at a cooling rate of 10°C / s. This stage mainly promotes the formation of fine and uniform acicular ferrite.

[0105] Isothermal holding stage: After the steel plate reaches 440℃, it is held isothermally for 9 minutes, and then slowly cooled to 370℃ at a cooling rate of 3℃ / s. This stage aims to precisely control the precipitation kinetics of microalloying elements (Nb, Ti) carbonitrides (Nb(C,N), TiN), causing them to form smaller, more uniformly distributed, and more thermally stable nanoscale precipitates. Simultaneously, this temperature range also favors the formation of a small amount of stable and uniformly distributed retained austenite.

[0106] The third stage is air cooling: the slab is naturally cooled to room temperature.

[0107] Step 4: After rolling, the submarine pipeline steel needs to undergo online self-tempering treatment.

[0108] After cooling, the steel plate is subjected to online tempering at 470℃ for 7 minutes to relieve residual stress.

[0109] The final microstructure of the subsea pipeline steel of the present invention is mainly composed of fine acicular ferrite, accounting for 94% of the total volume, accompanied by 6% of lower bainite or fragmented bainite.

[0110] The subsea pipeline steel prepared in this embodiment has a yield strength of 600 MPa, a tensile strength of 674 MPa, and a yield-to-tensile ratio ≤0.90; an impact energy (transverse) of -40℃ of 395 J, a CTOD value of -40℃ of 0.41 mm, and a DWTT shear area of ​​86% at -30℃; an impact energy of the weld and heat-affected zone (-10℃) of 103 J; and a corrosion rate (3.5% NaCl, 30d) ≤0.045 mm / a.

[0111] Comparative Example 1 Comparative Example 1 uses the existing composition formula of L555 grade subsea pipeline steel (as shown in Table 1), whose components by weight percentage include: C: 0.06%, Si: 0.20%, Mn: 1.7%, Al: 0.03%, Nb: 0.06%, Ti: 0.015%, Ni: 0.12%, Cr: 0.20%, Cu: 0.12%, Mo: 0.1%, P: 0.008%, S: 0.001%.

[0112] Using the same preparation process conditions as in Example 1, the L555 grade subsea pipeline steel obtained the following performance parameters: impact energy at -40℃ is only 320J; corrosion rate is 0.08mm / a; and HAZ hardness is significantly increased (275HV).

[0113] Comparing Example 1 and Comparative Example 1, it can be seen that in terms of low-temperature impact toughness, the impact energy (lateral) at -40℃ of Example 1 of the present invention is 410 J, while the impact energy (lateral) at -40℃ of Comparative Example 1 is 320 J, which is less than the impact energy (lateral) at -40℃ of the present invention. This is because the present invention forms a structure mainly composed of fine needle-like ferrite, which has good toughness and resistance to crack propagation.

[0114] Regarding the corrosion rate, the corrosion rate of Example 1 of the present invention (3.5% NaCl, 30 days) is ≤0.041 mm / a, significantly lower than that of Comparative Example 1 (0.08 mm / a). This is because in Example 1 of the present invention, Ni, Cu, and Cr synergistically form a dense passivation film, which effectively prevents Cl from corroding. - Corrosion of the steel substrate. In Comparative Example 1, the low Ni and Cu content makes it difficult to form an effective passivation film, resulting in corrosion of the steel in Cl-containing substrates. - Corrosion is more likely to occur in seawater.

[0115] Regarding the performance of the weld heat-affected zone (HAZ), Example 1 of the present invention ensured that the hardness and brittleness of the HAZ were controlled during welding by controlling the carbon equivalent CE_Pcm at 0.16%. In contrast, the HAZ hardness of Comparative Example 1 was 275HV, indicating that its carbon equivalent may not be within an appropriate range, leading to the formation of hardened structures in the HAZ during welding, increased hardness, and increased brittleness, thus affecting the performance of the welded joint.

[0116] In summary, the corrosion-resistant L555 grade subsea pipeline steel of this invention is superior to the existing L555 grade subsea pipeline steel in terms of strength, toughness, corrosion resistance and weldability, and can meet the usage requirements of deep-sea riser and main riser systems.

[0117] Table 1. Elemental composition (wt%) of Examples 1 and 2

[0118] Example 3 The composition of the L555 grade submarine pipeline steel in this embodiment, by weight percentage, includes: C: 0.059%, Mn: 1.80%, Si: 0.14%, Ni: 0.50%, Cu: 0.40%, Cr: 0.20%, Nb: 0.035%, Ti: 0.012%, P: 0.006%, S: 0.002%, N: 0.003%, with the remainder being Fe and unavoidable impurities.

[0119] The preparation process in this embodiment is the same as in Example 1. The yield strength of the submarine pipeline steel prepared in this embodiment is 565 MPa, the tensile strength is 635 MPa, and the yield strength ratio is ≤0.89. The impact energy (transverse) at -40℃ is 355 J, the CTOD value at -40℃ is 0.36 mm, and the DWTT shear area at -30℃ is 82%. The impact energy of the weld and heat-affected zone (at -10℃) is 102 J. The corrosion rate (3.5% NaCl, 30d) is ≤0.06 mm / a.

[0120] Example 4 The composition of the L555 grade submarine pipeline steel in this embodiment, by weight percentage, includes: C: 0.060%, Mn: 1.90%, Si: 0.13%, Ni: 0.52%, Cu: 0.22%, Cr: 0.26%, Nb: 0.045%, Ti: 0.012%, P: 0.006%, S: 0.002%, N: 0.003%, with the remainder being Fe and unavoidable impurities.

[0121] The preparation process and conditions in this embodiment are the same as in Example 2. The yield strength of the subsea pipeline steel prepared in this embodiment is 558 MPa, the tensile strength is 628 MPa, and the yield-to-tensile ratio is ≤0.89; the impact energy (transverse) at -40℃ is 340 J, the CTOD value at -40℃ is 0.34 mm, and the DWTT shear area at -30℃ is 80%; the impact energy of the weld and heat-affected zone (at -10℃) is 98 J; and the corrosion rate (3.5% NaCl, 30d) is ≤0.065 mm / a.

[0122] Comparative Example 2 The composition and content of the L555 grade subsea pipeline steel in this comparative example are the same as those in Example 3. The difference is that this comparative example is prepared using existing methods for preparing subsea pipeline steel.

[0123] The L555 grade subsea pipeline steel prepared in this comparative example was tested, and its yield strength was 545 MPa and tensile strength was 610 MPa. The impact energy (lateral) at 40℃ is 300J. At 30℃, the DWTT shear area was 76%, and the corrosion rate (3.5% NaCl, 30d) was 0.082mm / a.

[0124] A comparative analysis of this comparative example and Example 3 shows that, since the same composition system is used, the performance difference mainly stems from the different preparation processes. Example 3, employing the controlled rolling and three-stage temperature-controlled cooling process of this invention, significantly refined the austenite grains and obtained a uniform acicular ferrite structure, making... The impact energy at 40℃ increased from 300 J to 355 J, and the DWTT shear area increased from 76% to 82%. Simultaneously, segmented controlled cooling and precipitation regulation promoted the formation of a dense passivation film, reducing the corrosion rate from 0.082 mm / a to 0.060 mm / a. This fully demonstrates that the preparation method of this invention has significant technical effects in improving corrosion resistance and low-temperature toughness.

[0125] Comparative Example 3 The composition and content of the L555 grade subsea pipeline steel in this comparative example are the same as those in Example 4. The difference is that this comparative example is prepared using existing methods for preparing subsea pipeline steel.

[0126] The L555 grade subsea pipeline steel prepared in this comparative example was tested, and its yield strength was 548 MPa and tensile strength was 615 MPa. The impact energy (lateral) at 40℃ is 305J. At 30℃, the DWTT shear area was 78%, and the corrosion rate (3.5% NaCl, 30d) was 0.080 mm / a.

[0127] A comparative analysis was conducted between this comparative example and Example 4. Example 4, under the same component conditions, utilized the preparation method of the present invention to achieve the desired results. The impact energy at 40℃ increased by 35 J, the DWTT shear area increased by 2%, and the corrosion rate decreased by 19%. These results indicate that the present invention, through the synergistic regulation of rolling and segmented controlled cooling on microstructure, precipitation behavior, and surface corrosion film formation, is key to simultaneously improving corrosion resistance and low-temperature toughness.

[0128] The above description is only a preferred embodiment of the present invention, but the scope of protection of the present invention is not limited thereto. Any changes or substitutions that can be easily conceived by those skilled in the art within the scope of the technology disclosed in the present invention should be included within the scope of protection of the present invention.

Claims

1. A method for preparing corrosion-resistant L555 grade subsea pipeline steel, characterized in that, Includes the following steps: Step 1: Smelting and continuous casting process; The steel is smelted according to the design composition of L555 grade submarine pipeline steel, using converter smelting + LF refining + RH vacuum degassing process + continuous casting. Step 2: Control the rolling process; First, heat the continuously cast billet to 1180℃±10℃ and hold it for 30-60 minutes; Step 3: Three-stage temperature-controlled cooling process; The rolled slab is subjected to three stages of rapid cooling, medium-speed cooling-isothermal holding, and air cooling in sequence. The first stage of rapid cooling: the rolled slab is cooled to 480-520℃ at a cooling rate of 20-35℃ / s; Step 4: Perform online self-tempering treatment on the rolled submarine pipeline steel; The online self-tempering process is as follows: after cooling, the steel plate is subjected to online tempering treatment at 450-480℃ for 3-8 minutes.

2. The method for preparing corrosion-resistant L555 grade subsea pipeline steel according to claim 1, characterized in that, In step 3, the second stage of medium-speed cooling-isothermal holding includes a medium-speed cooling stage and an isothermal holding stage; The medium-speed cooling stage: the steel plate is cooled from the temperature of 480-520℃ after the rapid cooling in the first stage to 400-450℃ at a cooling rate of 8-12℃ / s.

3. The method for preparing corrosion-resistant L555 grade subsea pipeline steel according to claim 2, characterized in that, In step 3, the isothermal holding stage involves holding the steel plate isothermally for 5-10 minutes after it reaches 400-450℃, and then slowly cooling it to 320-380℃ at a cooling rate of 1-3℃ / s.

4. The method for preparing corrosion-resistant L555 grade subsea pipeline steel according to claim 3, characterized in that, In step 3, the third stage is air cooling: the steel plate is naturally cooled from 320-380℃ to room temperature.

5. The method for preparing corrosion-resistant L555 grade subsea pipeline steel according to claim 1, characterized in that, In step 1, during smelting and continuous casting, [O] is controlled to be ≤0.003% and the inclusion grade is controlled to be ≤1.

0.

6. The method for preparing corrosion-resistant L555 grade subsea pipeline steel according to claim 1, characterized in that, In step 1, the thickness of the continuously cast billet is 200-220 mm, and the center segregation is controlled within Class C 1.

0.

7. The method for preparing corrosion-resistant L555 grade subsea pipeline steel according to claim 6, characterized in that, In step 1, during the converter smelting stage, the initial temperature is 1450-1600℃, followed by blowing for 20-45 minutes. After blowing, the steel is tapped at a temperature of 1650-1710℃.

8. The method for preparing corrosion-resistant L555 grade subsea pipeline steel according to claim 7, characterized in that, In step 1, during the LF refining stage, the temperature of the molten steel is 1600-1650℃, and the refining time is 15-40 min.

9. The method for preparing corrosion-resistant L555 grade subsea pipeline steel according to claim 8, characterized in that, In step 4, after cooling, the steel plate is subjected to online tempering treatment at 450-480℃ for 3-8 minutes.

10. A corrosion-resistant L555 grade subsea pipeline steel, characterized in that, It is prepared by the method for preparing corrosion-resistant L555 grade submarine pipeline steel according to any one of claims 1 to 9; The corrosion-resistant L555 grade submarine pipeline steel comprises, by mass percentage: C: 0.03-0.07%, Mn: 1.60-2.20%, Ni: 0.10-0.80%, Cu: 0.10-0.45%, Cr: 0-0.30%, Nb: 0-0.06%, Ti 0-0.02%, P≤0.008%, S≤0.002%, N≤0.004%, with the remainder being Fe and unavoidable impurities.

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