Low-c low-cr corrosion-resistant pipeline steel plate, preparation method and pipeline steel pipe

Abstract

The present application relates to the technical field of pipeline steel for oil and gas transportation, and particularly relates to a low-C and low-Cr corrosion-resistant pipeline steel plate, a preparation method and a pipeline steel pipe, wherein the content of each chemical element in the low-C and low-Cr corrosion-resistant pipeline steel plate is as follows: C <= 0.03wt.%, Sb <= 0.025wt.%, Al 0.50-1.20wt.%, Mg 0.0005-0.0012wt.%, Ca 0.0012-0.0042wt.%, Cr <= 0.50wt.%, Ni 0.10-0.30wt.%, Si 0.10-0.30wt.%, Mn 1.20-1.80wt.%, Mo 0.10-0.30wt.%, Nb 0.02-0.06wt.%, V 0.03-0.08wt.%, Ti 0.01-0.03wt.%, P <= 0.01wt.%, S <= 0.001wt.%, N <= 0.005wt.%, and the balance is Fe and inevitable impurity elements. The steel plate prepared by the present application has low manufacturing cost, and has excellent mechanical properties such as yield strength, tensile strength, elongation, impact toughness, corrosion resistance and hydrogen-induced cracking resistance, and is suitable for submarine and land oil and gas transportation pipeline engineering in high-Cl ‑ , CO2 / H2S corrosion environment.

Description

Technical Field

[0001] This invention relates to the field of pipeline steel for oil and gas transportation, specifically to a low-C, low-Cr corrosion-resistant pipeline steel plate, its preparation method, and the pipeline steel pipe thereof. Background Technology

[0002] Oil and natural gas, as key energy sources in the world today, play an irreplaceable role in promoting economic development and ensuring energy security. With the sustained and rapid development of my country's economy, the demand for these two energy sources, as important strategic resources for the national economy, is also rising sharply. To meet this ever-increasing demand, my country is making every effort to advance the construction of oil and gas transportation pipelines. Among the various modes of oil and gas transportation, pipeline transportation has won widespread attention and high regard from countries around the world due to its outstanding economic efficiency, stable continuity, reliable safety, and superior efficiency.

[0003] Pipeline steel is an indispensable raw material in the construction of oil and gas transportation pipelines. However, while high-carbon pipeline steel has high strength and relatively low cost, it has poor toughness, especially prone to brittle fracture at low temperatures. It is also prone to cracking during welding, resulting in poor weldability. Furthermore, its corrosion resistance is limited, requiring additional anti-corrosion measures. High-chromium pipeline steel, on the other hand, possesses excellent corrosion resistance and good wear resistance, making it particularly suitable for complex corrosive environments and transportation of solid particles or at high flow rates. However, the high cost of chromium increases its production cost, and it requires strict welding processes. Its processing performance is also poor, and it is prone to cracking during cold bending, cold drawing, and other processing. In addition, its high brittle transition temperature limits its application in cold regions.

[0004] Therefore, in order to meet the demand for long-distance, high-volume oil and gas transportation, it is urgent to reduce costs by adding alloying elements and improving production processes, conduct in-depth research on its welding and processing characteristics to improve its toughness, and develop new alloy systems or surface treatment technologies to enhance its corrosion resistance. Summary of the Invention

[0005] To address the shortcomings of existing technologies, this invention provides a low-C, low-Cr corrosion-resistant pipeline steel plate, its preparation method, and the pipeline steel pipe. This pipeline steel plate, by adding Sb and Al elements, reduces cost or smelting difficulty while ensuring the alloy's corrosion resistance. Simultaneously, the addition of trace amounts of Mg and Ca elements slows down the corrosion rate of gathering and transportation pipelines in neutral or acidic media. Furthermore, a specialized heat treatment process of "pre-oxidation at 480–520℃ and low-temperature tempering at 440–460℃" is employed. Pre-oxidation forms a 50–100 nm Sb-Al rich oxide film on the surface, and tempering at 450℃ induces Sb grain boundary segregation, stabilizing the Al-Sb rich surface film while maintaining a high martensite dislocation density, achieving a unified combination of "high strength + high toughness + high corrosion resistance."

[0006] The first aspect of this invention provides a low-C, low-Cr corrosion-resistant pipeline steel plate, the contents of each chemical element being as follows: C≤0.03wt.%, Sb≤0.025wt.%, Al 0.50~1.20wt.%, Mg 0.0005~0.0012wt.%, Ca 0.0012~0.0042wt.%, Cr≤0.50wt.%, Ni 0.10~0.30wt.%, Si 0.10~0.30wt.%, Mn 1.20~1.80wt.%, Mo 0.10~0.30wt.%, Nb 0.02~0.06wt.%, V 0.03~0.08wt.%, Ti 0.01~0.03wt.%, P≤0.01wt.%, S≤0.001wt.%, N≤0.005wt.%, with the balance being Fe and unavoidable impurity elements.

[0007] A second aspect of this invention provides a method for preparing a low-C, low-Cr corrosion-resistant pipeline steel plate, comprising the following steps:

[0008] S1. Material preparation: low-sulfur and low-phosphorus blast furnace iron, high-quality low-carbon scrap steel, various alloys, pure metals and metal wires;

[0009] S2, Desulfurization: Deep desulfurization is carried out on low-sulfur and low-phosphorus blast furnace hot metal at 1350-1400℃, and the S in the treated low-sulfur and low-phosphorus blast furnace hot metal is ≤0.001wt.%;

[0010] S3. Converter primary refining: The low-sulfur and low-phosphorus blast furnace iron and high-quality low-carbon scrap steel after deep desulfurization are subjected to converter primary refining. The final control is C 0.03~0.05wt.%, and the steel temperature is 1630~1650℃. The proportion of low-sulfur and low-phosphorus blast furnace iron is 75~85%, and the proportion of high-quality low-carbon scrap steel is 15~25%.

[0011] S4, LF refining: The molten steel after the primary refining in the converter is refined by LF to form a high-basicity refining slag, ensuring that FeO in the slag is ≤0.5wt.%. Then, the alloy is fine-tuned according to the spectral detection results to make the composition of the molten steel meet the requirements. The molten steel exit temperature is 1540~1560℃.

[0012] S5, RH vacuum degassing: Perform RH vacuum degassing on the molten steel after LF refining, with vacuum circulation for 15-20 minutes. Then, perform final composition adjustment based on the spectral detection results. After the vacuum is completed, ensure that [H] ≤ 1.5 ppm and [N] ≤ 50 ppm.

[0013] S6, Mg-Ca co-treatment: Mg-Ca co-treatment is applied to molten steel after RH vacuum degassing to form fine spherical composite inclusions, specifically:

[0014] (1) Mg treatment: at 120-150 m·min -1 The feeding rate is such that the Mg wire is injected deep into the molten steel. After injection, argon is briefly blown for 2-5 minutes at a flow rate of 70-80 NL / min.

[0015] (2) Calcium treatment: at 80-100 m·min -1 The feeding rate is such that the CaSi wire is injected deep into the molten steel. After injection, argon is briefly blown for 2-5 minutes at a flow rate of 70-80 NL / min.

[0016] (3) Take samples to test whether the Mg and Ca content in the molten steel is qualified. If the Mg and Ca content is qualified, then perform long-term argon blowing, where the argon blowing time is ≥15min and the argon flow rate is 50~60 NL / min. If the content of either Mg or Ca is < the corresponding minimum content value, then repeat steps (1) / (2) to supplement the corresponding short wire. After the supplementation is completed, take samples again to test the Mg and Ca content in the molten steel until the Mg and Ca content in the molten steel is qualified, and then stop the sampling test.

[0017] S7. Continuous casting: The molten steel after Mg-Ca synergistic treatment is poured under full protection (argon seal), and the low superheat of 15-25℃ is used for pouring. Electromagnetic stirring and light pressure technology are used to suppress the central segregation of Sb element. The billet is slowly cooled for more than 48 hours after it is removed from the line.

[0018] S8. Reheating: The billet is reheated and descaled. The furnace temperature is 1180-1220℃ and the total reheating time is 2 hours. The soaking temperature is 1190-1210℃ and the soaking time is 30-60 minutes.

[0019] S9. Controlled rolling and cooling: The reheated billet is subjected to controlled rolling and cooling treatment. The roughing rolling start temperature is 1050~1100℃, the roughing rolling finish temperature is 980~1000℃, the finishing rolling start temperature is 800~820℃, and the finishing rolling finish temperature is ≥780℃. Then, the finished billet is directly quenched and the final cooling temperature is 400~450℃, followed by air cooling to room temperature.

[0020] S10. Reheating and quenching: Hold the billet after controlled rolling and controlled cooling at 920℃ for 30 minutes, then quench it in water to room temperature;

[0021] S11. Heat Treatment: The reheated and quenched billet undergoes controlled surface pre-oxidation at 480–520℃ and low-temperature tempering at 440–460℃, followed by air cooling to room temperature. During the controlled surface pre-oxidation at 480–520℃, the temperature is held for 30 minutes. The protective atmosphere is Ar + 1 vol% O2, with an atmosphere dew point of -60 to -40℃ and an oxygen partial pressure of 1.2 × 10⁻⁶. -21 ~3.0×10 -17 MPa; during low-temperature tempering at 440~460℃, hold for 55~65min, with N2 as the protective atmosphere;

[0022] S12. Finishing: The heat-treated billet is finished to obtain low-C and low-Cr corrosion-resistant pipeline steel plates.

[0023] Preferably, during material preparation, the blast furnace iron with low sulfur and low phosphorus content should have S ≤ 0.002 wt.% and P ≤ 0.010 wt.%, and the standard for high-quality low-carbon scrap steel should be: Cu + Sn + As + Pb + Bi ≤ 0.20 wt.% in the scrap steel; during converter primary refining, the proportion of low sulfur and low phosphorus blast furnace iron should be 75-85%, and the proportion of high-quality low-carbon scrap steel should be 15-25%; during LF refining, the molten steel should be heated to 1580-1600 ℃ through electrodes to ensure that the molten steel exiting the station is 1540-1560 ℃; during RH vacuum degassing, a vacuum pump should be used to evacuate to 0.3-0.5 mbar in four stages, with a circulation flow rate of 120 t·min. -1 During continuous casting, the electromagnetic stirring current is 250–350 A, the working frequency is 24 Hz, and the total reduction is 6–8 mm.

[0024] In addition, during RH vacuum degassing, when the Al content is > 0.90 wt.%, ensure that [N] ≤ 30 ppm after vacuuming.

[0025] Preferably, during rough rolling, the single-pass reduction rate is 15-25%, and the total reduction rate is ≥60%; during finish rolling, the single-pass reduction rate is 10-15%, and the total reduction rate is ≥70%; the direct quenching treatment specifically involves quenching at 15-25℃·s. -1 The cooling rate will allow the finished rolled billet to undergo laminar flow cooling.

[0026] Preferably, during continuous casting, after using electromagnetic stirring and light reduction technology to suppress Sb element center segregation, the center segregation class C is ≤1.0 grade;

[0027] After controlled rolling and controlled cooling treatment, the microstructure is a mixed microstructure mainly composed of lath bainite, containing a small amount of martensite and thin film retained austenite. Moreover, the carbonitrides are not completely dissolved after controlled rolling and controlled cooling treatment, and the size of the precipitated phase is 20-50 nm.

[0028] After reheating and quenching, the microstructure consists of fine lath martensite, fine lath bainite, and thin film retained austenite. At the same time, the size of the martensitic carbide and carbonitride precipitates after reheating and quenching is 5–15 nm.

[0029] Preferably, after controlled surface pre-oxidation at 480–520℃, the oxide film thickness is 50–100 nm, and the depth of the Sb / Al enriched layer on the surface is ≥30 nm; after low-temperature tempering at 440–460℃, the high dislocation density of the martensitic laths is ≥10. 14 m -2 Hydrogen trap density ≥ 5 × 10 20 m -3 .

[0030] Preferably, the average size of the carbonitride precipitates of microalloying elements Nb, V, and Ti in the manufactured steel plate is ≤50 nm, the volume fraction is ≥0.05%, and the grains are refined to ASTM grade 11 or higher;

[0031] In addition, XPS analysis of the surface of the fabricated steel plate showed that Sb exists in the form of Sb2O3 and Al exists in the form of Al2O3. The two form Al-Sb-O nanoclusters at the oxide film-matrix interface, with an area coverage of ≥80%.

[0032] Preferably, the prepared steel plate has a room temperature yield strength ≥540MPa, tensile strength ≥670MPa, elongation ≥26%, and KV8 impact absorption energy ≥235J at -40℃; the corrosion rate is ≤0.027mm / a under the conditions of 0.5wt.% NaCl + 0.5wt.% CH3COOH + CO2 / H2S saturated solution, pH 4.0~4.5, 80℃×720h; and the crack length ratio CLR is 0 under the conditions of 5wt.% NaCl + 0.5wt.% CH3COOH solution and NACE TM0284-A standard 96h HIC test.

[0033] In addition, the weld heat-affected zone of the manufactured steel plate has an impact absorption energy of ≥200 J at -40℃ and a weld cold cracking sensitivity index Pcm≤0.18%.

[0034] The third aspect of the present invention provides a low-C, low-Cr corrosion-resistant pipeline steel pipe, which is manufactured from a steel plate prepared by a method for preparing low-C, low-Cr corrosion-resistant pipeline steel plates.

[0035] Preferably, the low-C, low-Cr corrosion-resistant pipeline steel pipe is suitable for high-Cl... - Submarine and onshore oil and gas pipeline projects in CO2 / H2S corrosive environments.

[0036] Compared with the prior art, the present invention has the following beneficial effects:

[0037] 1. Al is an active element that easily combines with oxygen to form Al2O3 inclusions during smelting, increasing the difficulty of smelting. Moreover, a higher content of Sb increases the cost of the alloy. This invention, based on a low C and low Cr alloy, adds Sb and Al elements in combination. This not only reduces the amount of aluminum and antimony used and lowers the manufacturing cost, but also achieves Sb-Al composite. This ensures the corrosion resistance of the alloy while reducing costs or smelting difficulty and improving corrosion resistance.

[0038] 2. This invention adopts an ultra-low carbon design, which can not only significantly reduce the formation of welding cold cracks and reduce the embrittlement of the heat-affected zone, but also suppress cementite Fe3C film, reduce cementite (high carbon easily precipitates continuous cementite network at grain boundaries, becoming crack propagation path), improve low-temperature impact toughness below -40℃, and achieve a breakthrough in performance.

[0039] 3. This invention employs a low-Cr design, which can form a Cr(OH)3 film, effectively blocking Cl. - While meeting the requirements for reducing pitting corrosion risk, it can also reduce costs and maximize economic benefits compared with medium Cr steel and high Cr steel.

[0040] 4. This invention employs Mg treatment, which generates dispersed, fine-sized MgO·Al2O3 or MgS composite inclusions, replacing traditional large-particle Al2O3 or MnS. This significantly improves the cleanliness of the steel, refines inclusions, and enhances uniformity. Secondly, the fine inclusions reduce the active cathode area of ​​the corrosion galvanic cell, lowering the corrosion rate, especially in acidic service environments, thus improving corrosion resistance. Simultaneously, the magnesium-core inclusions pin grain boundaries, inhibiting austenite grain growth and acting as heterogeneous nucleation sites during solidification, inducing the formation of intragranular acicular ferrite. This refines the grains, inhibits microcrack propagation, disperses hydrogen traps, reduces hydrogen accumulation, and lowers hydrogen bubbling and HIC susceptibility, thereby improving the hydrogen-induced cracking resistance of pipeline steel. Therefore, the Mg treatment in this invention contributes to grain refinement, improves the toughness of the weld heat-affected zone, and enhances weldability.

[0041] 5. Traditional Ca treatment has limitations. Adding Ca alone can transform Al₂O₃ into spherical calcium aluminate and MnS into spherical CaS, with excellent results. However, for ultra-high purity steels, such as those with [S] ≤ 0.001% and extremely low sulfur content, Ca more readily reacts with large clusters of Al₂O₃, leading to significant Ca consumption and insufficient Ca to treat the remaining S, resulting in incomplete modification. This invention, based on low C and low Cr, adds trace amounts of Mg and Ca for Mg-Ca treatment. This Mg-Ca treatment can slow down the corrosion rate of gathering and transmission pipelines in neutral or acidic media. First, Mg treatment transforms large, harmful Al₂O₃ clusters into numerous small, spherical, high-temperature stable spinel particles, providing abundant reaction nuclei for subsequent Ca treatment. Secondly, Ca treatment transforms solid, irregular Al2O3 or MgO·Al2O3 inclusions into liquid, spherical calcium aluminates, which facilitates the flotation and removal of inclusions and reduces nozzle nodules. Simultaneously, Ca treatment can transform elongated MnS into spherical CaS or CaS-MnS composite inclusions, reducing anisotropy in the rolling direction and improving resistance to lamellar tearing. Furthermore, Ca treatment can inhibit corrosion crack propagation, improve the service stability of steel in hydrogen-containing or stress corrosion environments, and enhance resistance to HIC and SCC.

[0042] 6. This invention employs a special heat treatment of "pre-oxidation at 480-520℃ and low-temperature tempering at 440-460℃". Pre-oxidation forms a 50-100 nm Sb-Al rich oxide film on the surface, and low-temperature tempering at 440-460℃ induces Sb grain boundary segregation, stabilizing the Al-Sb rich surface film while maintaining a high dislocation density of martensite, thus achieving a unity of "high strength + high toughness + high corrosion resistance". Detailed Implementation

[0043] The technical solutions of the present invention will be clearly and completely described below with reference to the embodiments of the present invention. Obviously, the described embodiments are only some embodiments of the present invention, and not all embodiments. Based on the embodiments of the present invention, all other embodiments obtained by those of ordinary skill in the art without creative effort are within the scope of protection of the present invention.

[0044] The first aspect of this invention provides a low-C, low-Cr corrosion-resistant pipeline steel plate, the contents of each chemical element being as follows: C≤0.03wt.%, Sb≤0.025wt.%, Al 0.50~1.20wt.%, Mg 0.0005~0.0012wt.%, Ca 0.0012~0.0042wt.%, Cr≤0.50wt.%, Ni 0.10~0.30wt.%, Si 0.10~0.30wt.%, Mn 1.20~1.80wt.%, Mo 0.10~0.30wt.%, Nb 0.02~0.06wt.%, V 0.03~0.08wt.%, Ti 0.01~0.03wt.%, P≤0.01wt.%, S≤0.001wt.%, N≤0.005wt.%, with the balance being Fe and unavoidable impurity elements.

[0045] The design principles of each chemical element in the low-C, low-Cr corrosion-resistant pipeline steel plate designed in this invention are as follows:

[0046] 1. Control of core corrosion-resistant elements:

[0047] Antimony (Sb) (≤0.025wt.%): Sb can significantly promote the precipitation of corrosion-resistant elements and enrich them in the corrosion product film layer on the steel substrate surface, thereby improving the density and corrosion resistance of the corrosion product film layer; Sb can also be adsorbed on the steel surface to block H + The reduction reaction reduces hydrogen atom penetration, improves resistance to hydrogen-induced cracking (HIC), and inhibits hydrogen damage in the H2S environment.

[0048] Aluminum (Al) (0.50–1.20 wt.%): Al has a strong ability to bind with oxygen, forming a stable and dense Al₂O₃ protective film on the steel surface, effectively blocking Cl. - Al can penetrate corrosive media such as CO2, reducing the corrosion rate. At the same time, as a deoxidizer, Al can reduce oxide inclusions in steel and improve purity. In addition, Al and N can combine to form AlN particles, which pin the austenite grain boundaries, inhibit grain growth during rolling, and improve toughness.

[0049] Chromium (Cr) (≤0.50 wt.%): During corrosion, Cr accumulates at grain boundaries and can form a Cr(OH)3 film, effectively blocking Cl. -Cr penetrates and reduces the average corrosion rate and pitting risk; the pitting rate decreases significantly when Cr ≥ 0.30 wt.%. It can also improve strength and reduce the yield strength ratio through solid solution strengthening. Like Mn, Cr can dissolve into solid solutions, improving the hardenability of steel and thus increasing strength. When Cr dissolves into austenite, it increases the stability of supercooled austenite, shifting the isothermal transformation curve to the right and promoting the formation of martensite-residual austenite (MA) in supercooled structures, thereby improving the strength and hardness of the steel.

[0050] Magnesium (Mg) (0.0005–0.0012 wt.%): Mg has strong chemical reactivity and a strong affinity for oxygen and sulfur, making it an effective refining agent. It can reduce the oxygen and sulfur content and the number of inclusions in steel, purifying the molten steel. It can also significantly modify inclusions in steel, improving the impact and drop hammer properties of the steel. Trace amounts of magnesium can reduce the size, distribution, and morphology of inclusions in steel, and can also increase yield strength and tensile strength by more than 5%, while maintaining plasticity essentially unchanged, thus reducing the yield strength ratio. Simultaneously, Mg treatment can greatly improve the steel's resistance to HIC by refining inclusions.

[0051] Calcium (Ca) (0.0012–0.0042): In steel, Ca preferentially combines with sulfur to form spherical CaS, replacing ductile MnS and completely eliminating hydrogen traps and rolling anisotropy. At the same time, low-melting-point calcium aluminate is formed outside the MgO·Al2O3 core, promoting the flotation of inclusions, purifying the molten steel, and refining Mn segregation and banded structure. This significantly improves HIC resistance, low-temperature impact toughness, and thickness-direction plasticity, and is the key micro-control method for achieving zero hydrogen cracking, high toughness, and low yield strength ratio in this application.

[0052] 2. Mechanical performance guarantee system:

[0053] Carbon (≤0.03wt.%): Ultra-low carbon design can reduce Cr-containing carbides (e.g., Cr2). 23 The precipitation of C6 inhibits intergranular corrosion and prevents localized corrosion at grain boundaries due to Cr depletion. Simultaneously, low carbon content reduces the hardening and cold cracking tendency of the heat-affected zone (HAZ), increasing the impact energy of welded joints by over 20% and improving weldability. Furthermore, C atoms dissolved in ferrite can enhance strength by hindering dislocation movement, acting as solid solution strengthening. They also combine with elements such as Mo, Nb, and V to form carbides like M3C and MC, which further strengthen the joint through precipitation (NbC and VC can refine grains).

[0054] Carbon (C) is one of the most important alloying elements in carbon steel and low-alloy steel, primarily existing in the form of carbides, such as Fe3C. Without a product film covering the surface, the exposed Fe3C area increases with corrosion time, increasing the area ratio between the cathode and anode, thus increasing the corrosion rate. The finer and more uniform the carbides, the greater the corrosion rate. In the presence of a product film, Fe3C can improve the structure and mechanical properties of the film, enhancing its density, integrity, and adhesion to the matrix, thereby reducing the corrosion rate. When the matrix C content is between 0.1 and 0.18 wt.%, the corrosion resistance of the steel slightly decreases with increasing C content. Reducing the C content is beneficial for homogenization of composition and microstructure, increasing the transformation temperature of acicular ferrite, expanding the transformation range of acicular ferrite, reducing the micro-potential difference, and increasing the corrosion resistance of the steel. Furthermore, C atoms readily react with microalloying elements such as V, Nb, and Ti to precipitate carbides, which play a role in inhibiting recrystallization and precipitation strengthening in the high-temperature and low-temperature ranges, respectively. Therefore, the present invention adopts an ultra-low carbon design (C≤0.03 wt.%), which can reduce the precipitation of Cr-containing carbides, inhibit intergranular corrosion, and improve weldability.

[0055] Silicon (Si) (0.10–0.30 wt.%): Si is a strong deoxidizer in steelmaking, capable of removing oxygen from molten steel, reducing oxide inclusions, and improving purity. Furthermore, Si dissolved in ferrite can increase the yield strength and hardness of steel. In addition, when acting synergistically with Mn, it can lower the austenite transformation temperature, promote the formation of acicular ferrite, and refine grain size.

[0056] Manganese (Mn) (1.20–1.80 wt.%): Mn can compensate for the strength gap caused by ultra-low carbon through solid solution strengthening, while reducing dependence on carbon. In addition, it can expand the austenite region, improve hardenability, refine the size of acicular ferrite / bainite, promote the inhibition of pearlite formation and reduce the yield strength ratio, while reducing the hardening of the weld heat-affected zone. It can also form a Mn-Nb-V-Ti composite system with Nb, V, and Ti, thereby improving toughness and crack resistance.

[0057] 3. Other main elements:

[0058] Mo (0.10-0.30 wt.%): Mo can enhance the density of passivation films, especially in CO2 environments where it can inhibit localized corrosion.

[0059] Niobium (Nb) (0.02-0.06 wt.%): Nb combines with C to form NbC, which can delay austenite recrystallization, pin austenite grain boundaries, and refine grains; at the same time, the combination of Nb and C can also reduce free carbon, inhibit corrosion, and reduce the corrosion rate in CO2 environment.

[0060] Vanadium (V) (0.03~0.08wt.%): V can form fine precipitates in the low-temperature region, increasing humidity without compromising toughness.

[0061] Titanium (Ti) (0.01~0.03wt.%): Ti can form carbonitrides, pin grain boundaries, inhibit austenite growth, and reduce grain size.

[0062] Nickel (Ni) (0.10~0.30wt.%): Ni is enriched in grain boundaries, which can strengthen grain boundaries, inhibit the segregation of Cu at grain boundaries, reduce the tendency of hot cracking by 70%, and inhibit hydrogen-induced cracking (HIC). At the same time, Ni can also improve the stability of austenite, with an impact energy of ≥120J at -40℃ (40% higher than that of Ni-free steel), and good low-temperature toughness.

[0063] 4. Impurity elements:

[0064] Phosphorus (P) (≤0.010wt.%): P element can prevent grain boundary segregation.

[0065] Sulfur (≤0.001wt.%): S element can reduce MnS inclusions, control sulfide morphology, and reduce pitting corrosion risk.

[0066] Nitrogen (N) (≤0.005wt.%): Ti can fix free N elements to form TiN particles, which pin the grain boundaries.

[0067] A second aspect of this invention provides a method for preparing a low-C, low-Cr corrosion-resistant pipeline steel plate, comprising the following steps:

[0068] S1. Material preparation: low-sulfur and low-phosphorus blast furnace iron, high-quality low-carbon scrap steel, various alloys, pure metals and metal wires, of which the low-sulfur and low-phosphorus blast furnace iron has S≤0.002wt.% and P≤0.010wt.%; the standard for high-quality low-carbon scrap steel is: Cu+Sn+As+Pb+Bi≤0.20wt.% in the scrap steel.

[0069] It should be noted that this application must use high-quality, low-carbon scrap steel. The use of inferior scrap steel, such as scrap steel from automobiles, scrap steel from dismantling household appliances, scrap steel with coatings (such as copper, tin, or zinc), and scrap wires and cables, is strictly prohibited to avoid negatively impacting corrosion resistance.

[0070] The various alloys used in this application include ferrosilicon, low-carbon ferromanganese, ferromolybdenum, ferroniobium, ferrovanadium, low-carbon ferrochrome, ferroantimony, and low-carbon ferrotitanium. Among them, ferrosilicon (FeSi) contains Si ≥ 72 wt.%, C ≤ 0.15 wt.%, P ≤ 0.025 wt.%, S ≤ 0.020 wt.%, and a particle size of 10-50 mm; low-carbon ferromanganese (FeMn) contains Mn ≥ 78 wt.%, C ≤ 1.5 wt.% (to avoid carbon enrichment), P ≤ 0.025 wt.%, and a particle size of 10-50 mm; ferromolybdenum (FeMo) contains Mo ≥ 60 wt.%, and a particle size of 10-30 mm; ferroniobium (FeNb) contains Nb ≥ 60 wt.%, and a particle size of 5-20 mm; ferrovanadium (FeV) contains V ≥ 50 wt.%. wt.%, particle size 5-20mm; low carbon ferrochrome FeCr with C≤0.025wt.%, Cr≥60wt.%; ferroantimony FeSb with Sb≥70wt.%, strictly controlling the content of impurities such as Pb and As, Pb≤0.05wt.%, As≤0.05wt.%; low carbon ferrotitanium FeTi with Ti≥30wt.%, C≤0.10wt.%, used for wire feeding in the later stage of refining to prevent oxidation.

[0071] In this embodiment of the application, low-carbon ferrotitanium can be replaced with FeTi wire, wherein the FeTi wire contains Ti≥30wt.% and C≤0.10wt.%.

[0072] In this application, the pure metals include nickel plates and aluminum blocks, wherein the nickel plates contain Ni ≥ 99.8 wt.% and the aluminum blocks contain Al ≥ 99.5 wt.%, used for deoxidation and alloying. The aluminum blocks used in this application are of two specifications: large blocks with a length of 80–150 mm, a width of 80–150 mm, and a thickness of 40–80 mm, weighing 2–8 kg per block, are used for large-scale addition during the steelmaking process; small blocks with a length of 30–60 mm, a width of 30–60 mm, and a thickness of 20–40 mm, weighing 0.5–2 kg per block, are used for fine-tuning within the RH vacuum bath.

[0073] In this embodiment, the preferred size of the large piece is 100 mm × 100 mm × 50 mm, and the preferred size of the small piece is 40 mm × 40 mm × 30 mm.

[0074] In this application, the metal wires include CaSi wires and Mg wires, wherein the CaSi wires contain Ca ≥ 30 wt.%, Si ≥ 58 wt.%, and the outer sheath is pure iron sheet with a wire diameter of Φ13 mm; the Mg wires contain Mg ≥ 98 wt.%, and the outer sheath is steel sheet with a wire diameter of Φ16 mm.

[0075] In this embodiment, the CaSi wire and Mg wire need to be sealed and stored to prevent moisture.

[0076] S2, Desulfurization: At 1350~1400℃, the KR mechanical stirring method is used to carry out deep desulfurization of low sulfur and low phosphorus blast furnace iron. The treated low sulfur and low phosphorus blast furnace iron has S≤0.001wt.

[0077] In step S2 of this application, a composite desulfurizing agent composed of passivated magnesium particles and lime is used to deeply desulfurize low-sulfur and low-phosphorus blast furnace molten iron. During this process, the magnesium particles vaporize and are intensely agitated, and the lime (CaO) reacts with sulfur to generate CaS, i.e., high-sulfur slag. Furthermore, the high-sulfur slag must be thoroughly removed after desulfurization to prevent sulfur reversion; that is, the slag removal rate must be ≥95%, and the slag layer thickness ≤20mm.

[0078] S3. Converter primary refining: The low-sulfur and low-phosphorus blast furnace iron and high-quality low-carbon scrap steel after deep desulfurization are subjected to converter primary refining using a top and bottom blowing method. The final control is C 0.03~0.05wt.% and molten steel temperature 1630~1650℃. The proportion of low-sulfur and low-phosphorus blast furnace iron is 75~85% and the proportion of high-quality low-carbon scrap steel is 15~25%.

[0079] In this application, the proportion of high-quality low-carbon scrap steel is obtained by calculating the cold material ratio.

[0080] In this embodiment, a 180-ton top-and-bottom blown converter is used for primary refining of deeply desulfurized low-sulfur, low-phosphorus blast furnace hot metal and high-quality low-carbon scrap steel. During the blowing process, oxygen lances are inserted for high-pressure oxygen blowing, causing oxygen to react with elements such as C, Si, Mn, and P in the hot metal, releasing a large amount of heat. In addition, during the primary refining process, slag-forming agents such as lime (CaO) and fluorite (CaF2) are added to form high-basicity slag for effective dephosphorization. At the end of the process, temperature and carbon content are measured to ensure that C is 0.03–0.05 wt.% and the steel temperature is 1630–1650°C. During tapping, a double slag-blocking system using a sliding plate and a slag-blocking cone is used to strictly prevent slag from entering the ladle and to prevent phosphorus reversion. Aluminum particles are added with the steel stream during tapping for pre-deoxidation.

[0081] S4, LF refining: The molten steel after the primary refining in the converter is refined by LF to form a high-basicity refining slag, and the FeO in the slag is reduced to "white slag". Then, the alloy is fine-tuned according to the spectral detection results to make the composition of the molten steel meet the requirements. The molten steel exit temperature is 1540~1560℃.

[0082] In this application, during LF refining, active lime, fluorite, etc. are added to form high-alkalinity refining slag, wherein the refining slag with CaO / SiO2≈2.5-3.0 is high-alkalinity refining slag; subsequently, aluminum particles, calcium carbide CaC2 and other deoxidizers are added to the slag surface to reduce FeO in the slag to ≤0.5wt.%, forming "white slag", thereby creating an environment for deep desulfurization and stable element recovery.

[0083] In this application, during LF refining, molten steel is heated to 1580-1600 ℃ via electrodes to ensure that the molten steel exiting the station is 1540-1560 ℃.

[0084] In this application, based on the spectral detection results, alloys such as ferroniobium, ferrovanadium, and ferrochrome are added through the feeding system for alloy fine-tuning. Among them, Sb element is added in the form of ferroantimony when the molten steel temperature is ≥1580℃, and is stirred with strong argon gas for 15 minutes, i.e., argon gas flow rate ≥200 NL / min, to ensure uniform distribution of Sb element and prevent segregation.

[0085] S5, RH vacuum degassing: Perform RH vacuum degassing on the molten steel after LF refining, with vacuum circulation for 15-20 minutes. Then, perform final composition adjustment based on the spectral detection results. After the vacuum is completed, ensure that [H] ≤ 1.5 ppm and [N] ≤ 50 ppm.

[0086] In this application, during RH vacuum degassing, a four-stage vacuum pump is used to evacuate to a vacuum level of 0.3–0.5 mbar, with a circulation flow rate of 120 t·min. -1 The vacuum circulation time is 15–20 min.

[0087] It is important to emphasize that if Ti needs to be fine-tuned during the final composition adjustment, the low-carbon ferrotitanium iron should be replaced with FeTi wire, and the FeTi wire should be injected deep into the molten steel using a wire feeder to prevent Ti from being oxidized and to ensure a stable yield.

[0088] In this application, after the RH vacuum degassing is completed, when the temperature of the molten steel is 1575-1585℃, the content of elements other than Mg and Ca in the molten steel is tested to see if it is qualified. If it is qualified, Mg-Ca synergistic treatment is carried out; otherwise, the element content in the molten steel is adjusted according to the test results until it is qualified before Mg-Ca synergistic treatment is carried out.

[0089] In addition, during RH vacuum degassing, when the Al content is > 0.90 wt.%, ensure that [N] ≤ 30 ppm after vacuuming.

[0090] It should be noted that Al 0.50~0.90wt.% is mainly for deoxidation, refining, corrosion resistance and toughness; while Al 1.0~1.2wt.% is designed for extreme corrosion resistance, which requires ultra-low oxygen + Ti nitrogen fixation to maintain toughness. It belongs to the special steel range with narrow working conditions. The Ti content in this application is already sufficient. Therefore, this application additionally limits "when Al content > 0.90wt.%", to ensure [N] ≤ 30ppm after vacuuming.

[0091] S6, Mg-Ca co-treatment: Mg-Ca co-treatment is applied to molten steel after RH vacuum degassing to form fine spherical composite inclusions, specifically:

[0092] (1) Mg treatment: at 120-150 m·min -1 The feeding rate is such that the Mg wire is injected deep into the molten steel. After injection, argon is briefly blown for 2-5 minutes at a flow rate of 70-80 NL / min.

[0093] In this application, Mg wire melts and vaporizes inside the molten steel (Mg has a boiling point of only 1090℃, far below the temperature of the molten steel), forming a large number of tiny Mg bubbles. These bubbles rise to the surface and preferentially react with the most unstable and largest clusters of Al2O3 inclusions in the molten steel. The reaction produces fine, dispersed MgO·Al2O3 (spinel) or MgO particles.

[0094] Studies have shown that the corrosion resistance of subsea pipeline steel is closely related to alloying elements and inclusions. Increasing the content of alloying elements such as Cr, Ni, and Mo within a specified range helps improve the corrosion resistance of the steel. Due to the generally high price of corrosion-resistant alloys, especially the addition of precious metals such as Ni and Mo, the production cost of subsea pipeline steel increases significantly. Inclusions in steel are, on the one hand, the starting point of uniform corrosion; their size, quantity, and distribution are all related to the uniform corrosion resistance of subsea pipeline steel. On the other hand, they are strong hydrogen traps; the larger and more numerous the inclusions, the higher the hydrogen capture efficiency and the higher the HIC susceptibility. Mg treatment can rapidly generate magnesium oxide, reducing the oxygen content in the steel. Numerous fine magnesium oxide particles in the molten steel become nucleation sites for other inclusions that precipitate later, resulting in more and finer inclusions in the steel and greatly improving its HIC resistance. Therefore, this application achieves "divide and conquer" through Mg treatment, that is, transforming large, harmful Al2O3 clusters into countless small, spherical, high-temperature stable spinel particles. These small MgO or spinel particles provide a large number of reaction nuclei for subsequent Ca treatment, thereby improving the steel's resistance to HIC.

[0095] (2) Calcium treatment: at 80-100 m·min -1 The feeding speed is such that the CaSi wire is injected deep into the molten steel. After injection, argon is briefly blown for 2-5 minutes at a flow rate of 70-80 NL / min.

[0096] In this embodiment of the application, a high-speed wire feeder is used to inject Mg / CaSi wire deep into the molten steel.

[0097] In this application, the CaSi wire melts deep within the molten steel, releasing Ca vapor. The Ca vapor preferentially reacts with S to form spherical CaS. Simultaneously, Ca also reacts with the fine MgO / MgO·Al2O3 cores formed after Mg treatment, creating a layer of low-melting-point calcium aluminate, such as 12CaO·7Al2O3, around which a composite inclusion of "MgO-CaO-Al2O3" or "MgO·Al2O3-CaO-Al2O3" is formed.

[0098] In this application, the "encapsulation and spheroidization" process generated during Ca treatment ensures that each sulfur atom is fixed into a spherical CaS, completely eliminating the generation of strip-shaped MnS and removing the main inducing factor for hydrogen-induced cracking. Furthermore, through composite modification—that is, the core reaction of Ca with MgO to form low-melting-point composite inclusions—even if there is residual sulfur, it will be encapsulated and absorbed by these composite inclusions, further ensuring its harmlessness.

[0099] It is important to emphasize that the composite inclusions will remain spherical and undeformed during rolling, thus giving the finished tube good mechanical properties.

[0100] (3) Take samples to test whether the Mg and Ca contents in the molten steel are qualified. If the Mg and Ca contents are qualified, then perform long-term argon blowing, with a blowing time of ≥15min and an argon flow rate of 50~60 NL / min. If the content of either Mg or Ca is < the corresponding minimum content value, then repeat steps (1) / (2) to supplement the corresponding short wire. After the supplementation is completed, take samples again to test the Mg and Ca contents in the molten steel until the Mg and Ca contents in the molten steel are qualified, and then stop the sampling test.

[0101] In this application, samples are taken within 3 minutes after the Mg-Ca treatment is completed, and the Mg and Ca contents are determined by spark source atomic emission spectroscopy. If the Ca content meets the standard and the Mg content is less than 0.0005%, step (1) is repeated to feed Mg wire. After feeding, the Mg content in the molten steel is sampled again. After the Mg content is qualified, long-term argon blowing is performed. If the Mg content meets the standard and the Ca content is less than 0.0012%, step (2) is repeated to feed CaSi wire. After feeding, the Ca content in the molten steel is sampled again. After the Ca content is qualified, long-term argon blowing is performed. If the Mg content is less than 0.0005% and the Ca content is less than 0.0012%, steps (1) and (2) are repeated in sequence to feed the corresponding short wires until the Mg and Ca contents in the molten steel are qualified. Then the sampling and testing are stopped, and long-term argon blowing is performed.

[0102] It should be emphasized that, since the Mg / CaSi wire is fed in in small quantities during the Mg-Ca co-processing, it is rare for the Mg and Ca contents to exceed their maximum values. However, if this happens, the molten steel from the Mg-Ca co-processing is discarded, and molten steel after RH vacuum degassing is taken again for Mg-Ca co-processing.

[0103] In this application, after the Mg and Ca treatment is completed, argon blowing for a long time is used to allow the complex inclusions generated by the reaction sufficient time to collide, aggregate, float and be absorbed by the refining slag at the top, thereby promoting the discharge of excess Ca and Mg vapors, avoiding blockage of the nozzle during casting, and finally obtaining clean molten steel with a significant improvement in the purity of the molten steel.

[0104] Therefore, this application obtained a fully spheroidized, stable, and harmless inclusion system through "Mg treatment + short blowing + Ca treatment + short blowing + long blowing", which helps to improve the performance and resistance to HIC of steel plates, while also giving the steel plates excellent corrosion resistance, toughness and plasticity.

[0105] S7. Continuous casting: The molten steel after Mg-Ca synergistic treatment is poured under full protection (argon sealing), and a low superheat of 15-25℃ is used for pouring. Electromagnetic stirring and light pressure technology are used to suppress the central segregation of Sb element. The billet is slowly cooled for more than 48 hours after it leaves the production line.

[0106] In this application, the electromagnetic stirring current is 250-350 A, the operating frequency is 24 Hz, and the total amount of light pressure is 6-8 mm.

[0107] Preferably, during continuous casting, after using electromagnetic stirring and light pressure reduction technology to suppress Sb element center segregation, the center segregation class C is ≤1.0.

[0108] In this application, secondary oxidation is prevented during continuous casting by using full-process protective pouring (argon sealing). Furthermore, electromagnetic stirring and gentle reduction techniques are employed during continuous casting to suppress central segregation of Sb, ensuring a uniform microstructure. Additionally, the billet is slowly cooled for at least 48 hours after being removed from the casting line to prevent hydrogen-induced cracking.

[0109] It is important to emphasize that after the billet has been slowly cooled for more than 48 hours, the surface and internal quality of the billet need to be checked and treated to ensure that the billet used in subsequent processes meets the quality requirements.

[0110] In this application, the specific operation for slow cooling of the billet after it comes off the production line is as follows: it is placed in an insulation pit or covered with insulation felt, so that the surface temperature slowly decreases from 800-900℃ to ≤200℃ within 48 hours, with an average cooling rate of ≤15℃·h. -1 Preferably 5-10℃·h -1 To prevent hydrogen-induced cracking (HIC).

[0111] S8. Reheating: The billet is reheated and descaled. The furnace temperature is 1180-1220℃, the total reheating time is 2 hours, the soaking temperature is 1190-1210℃, and the soaking time is 30-60 minutes.

[0112] In this embodiment, a walking beam furnace is used to reheat the billet at a temperature of 1180–1220°C. Furthermore, during descaling, high-pressure water at a pressure of 20 MPa is used, achieving a descaling coverage rate of ≥90%.

[0113] It should be noted that the furnace temperature refers to the minimum to maximum temperature range that the furnace can reach during the entire reheating process, which is 1180 to 1220°C. It does not refer to the starting temperature of reheating or the constant heating temperature.

[0114] S9. Controlled rolling and cooling: The reheated billet is subjected to controlled rolling and cooling treatment. The roughing rolling start temperature is 1050~1100℃, the roughing rolling finish temperature is 980~1000℃, the finishing rolling start temperature is 800~820℃, and the finishing rolling finish temperature is ≥780℃. Then, the finished billet is directly quenched, and the final cooling temperature is 400~450℃, followed by air cooling to room temperature.

[0115] In this application, a reversible four-high mill is used for rough rolling, and the width accuracy is ensured by side pressure from vertical rolls.

[0116] In this application, during rough rolling, the single-pass reduction rate is 15-25%, and the total reduction rate is ≥60%; during finish rolling, the single-pass reduction rate is 10-15%, and the total reduction rate is ≥70%.

[0117] It should be noted that the single-pass reduction rate refers to the amount of deformation in the billet thickness during each reduction.

[0118] In this application, the direct quenching treatment specifically refers to: quenching at 15–25 °C·s -1 The cooling rate will allow the finished rolled billet to undergo laminar flow cooling.

[0119] In this application, the microstructure after controlled rolling and controlled cooling treatment is a mixed microstructure mainly composed of lath bainite, containing a small amount of martensite and thin film retained austenite, wherein lath bainite accounts for about 70% and martensite accounts for <30%; the thickness of thin film retained austenite is 100-300 nm, and the proportion is about 5-10%.

[0120] In this application, the carbonitrides were not completely dissolved after controlled rolling and controlled cooling treatment, and the size of the precipitated phase was 20-50 nm.

[0121] It should be noted that after controlled rolling and controlled cooling, the carbonitrides were not completely dissolved, with precipitated phases ranging from 20 to 50 nm in size and 0 to 0.05% in volume. The resulting lath bundles had a size of 2 to 3 μm and a Vickers hardness (HV 10) of 280 to 320. However, the microstructure was not fully austenitized, resulting in non-uniform composition and the presence of undissolved microalloyed carbonitrides in the core. This led to a high yield strength of approximately 620 MPa and a yield-to-tensile ratio > 0.90. Simultaneously, the impact energy at -40°C was only around 200 J, and the hydrogen-induced cracking susceptibility remained unacceptable (CLR ≈ 5%). Therefore, in this application, a reheating and quenching treatment was performed after controlled rolling and controlled cooling to address this issue.

[0122] S10. Reheating and quenching: Hold the billet after controlled rolling and controlled cooling at 920℃ for 30 minutes, and then quench it in water to room temperature.

[0123] In this application, the microstructure after reheating and quenching consists of fine lath martensite, fine lath bainite, and thin film retained austenite, wherein the fine lath martensite accounts for about 60%, the fine lath bainite accounts for about 25%, and the thin film retained austenite has a thickness of 50-100 nm and accounts for about 10-15%.

[0124] In this application, the size of the martensitic carbide and carbonitride precipitates after reheating and quenching is 5-15 nm, and the volume fraction is 0.08%.

[0125] In this embodiment, a roller hearth furnace is used to reheat and quench the billet after controlled rolling and cooling treatment, and a slightly oxidizing atmosphere (λ=1.05) is maintained inside the furnace.

[0126] In this embodiment, a high-pressure water quenching device is used during water quenching, with a water pressure of 0.8 MPa and a flow rate of 3000 m³·h. -1 The water temperature is 25–30 ℃, and the quenching cooling rate is ≥30℃·s. -1 .

[0127] This application employs a reheating and quenching process on the billet after controlled rolling and cooling treatment to ensure full-section austenitization, which facilitates the complete dissolution of carbonitrides such as Nb, V, and Ti, providing a uniform supersaturated solid solution for subsequent low-temperature tempering at 440–460℃. Simultaneously, the reheating and quenching process promotes uniform composition, eliminates micro-region C and Mn segregation caused by direct quenching, and reduces the banded structure rating. Furthermore, the process involves re-nucleation at 920℃ for 30 min, resulting in 60% fine lath martensite + 25% fine lath bainite + 10–15% thin-film retained austenite after quenching, further refining the lath bundle size to 1–2 μm.

[0128] S11. Heat Treatment: The reheated and quenched billet undergoes controlled surface pre-oxidation at 480–520℃ and low-temperature tempering at 440–460℃, followed by air cooling to room temperature. During the controlled surface pre-oxidation at 480–520℃, the temperature is held for 30 minutes. The protective atmosphere is Ar + 1 vol% O2, with an atmosphere dew point of -60 to -40℃ and an oxygen partial pressure of 1.2 × 10⁻⁶. -21 ~3.0×10 -17 MPa; during low-temperature tempering at 440~460℃, hold for 55~65min, with a protective atmosphere of N2.

[0129] In the embodiments of this application, the equipment selected for controllable surface pre-oxidation at 480-520°C and low-temperature tempering at 440-460°C is a 40m long roller hearth protective atmosphere furnace.

[0130] In this application, the protective atmosphere used for controlled surface pre-oxidation at 480–520°C needs to be dried to reduce the water vapor partial pressure to below 1 × 10⁻⁶. -5 MPa, to prevent selective oxidation of aluminum and antimony simultaneously during the oxidation of the iron matrix.

[0131] In this embodiment, when the controllable surface pre-oxidation temperature is 500°C, the oxygen partial pressure is preferably 1.0 × 10⁻⁶. -19 MPa.

[0132] In this application, during the controlled surface pre-oxidation at 480–520℃ and the low-temperature tempering at 440–460℃, a furnace roller speed is also set, and the calculation formula is as follows: V=(L1-L2) / (D / v0+h), where V is the furnace roller speed, L1 is the effective heating length, L2 is the target length of the steel pipe, D is the thickness of the cast billet, v0 is the heating rate of the heating section, and h is the holding time.

[0133] In this embodiment, when the controllable surface pre-oxidation temperature is 500℃, L1 = 40m, L2 = 4m, D = 0.02m, the heating range is 450~500℃, and the heating rate of the heating range is v0 = 25℃·min. -1 h=30min, at which point the furnace roller speed is 1.2 m·min. -1 Due to 1.2 m·min -1 Since this is below engineering requirements, the furnace roller speed during controlled surface pre-oxidation at 500°C in this embodiment is 1.3 m·min. -1 .

[0134] In this embodiment of the application, the furnace roller speed during low-temperature tempering at 440–460°C is 0.65–0.70 m·min. -1 The preferred value is 0.67 m·min. -1 .

[0135] It should be emphasized that the furnace roller speed must meet the engineering requirements and be lower than the mechanical limit of the furnace roller. If the calculated furnace roller speed is lower than the engineering requirements, then the furnace roller speed required by the engineering requirements shall be used for processing.

[0136] In this application, during the controllable surface pre-oxidation at 480–520℃, a cross-sectional temperature difference is also provided, wherein the cross-sectional temperature difference is the maximum temperature difference along the thickness direction of the same steel plate (usually taken as the difference between the thermocouple readings of the upper and lower surfaces).

[0137] In this embodiment of the application, a cross-sectional temperature difference of ≤5 ℃ is sufficient to meet the requirements.

[0138] In this application, the oxygen partial pressure is 1.2 × 10⁻⁶ at 480–520 °C. -21 ~3.0×10 -17 The specific calculation steps for MPa and oxygen partial pressure are as follows:

[0139] (1) Identify the metal oxides: Al2O3 and Fe2O3.

[0140] (2) Based on the Gibbs free energy of each metal oxide in the range of 480–520℃, the fitting coefficients of each metal oxide in the range of 480–520℃ are determined by fitting: intercept and slope. The calculation formulas are as follows:

[0141]

[0142] In the formula, The standard Gibbs free energy change; A is Intercept at T=0, in J·mol⁻¹ -1 B is The slope of the curve as a function of temperature, in J·mol⁻¹ -1 T represents absolute temperature, measured in Kelvin (K).

[0143] (3) Calculate the equilibrium oxygen partial pressure of Al2O3 at 480℃ and the equilibrium oxygen partial pressure of Fe2O3 at 520℃. The calculation formulas are as follows:

[0144]

[0145] In the formula, To balance the partial pressure of oxygen; The standard pressure is taken as 0.1 MPa; R is the gas constant, taken as 8.314 J·mol⁻¹. -1 ·K -1 .

[0146] (4) The equilibrium oxygen partial pressure of Al2O3 at 480℃ is taken as the lower limit of the oxygen partial pressure at 480~520℃, and the equilibrium oxygen partial pressure of Fe2O3 at 520℃ is taken as the upper limit of the oxygen partial pressure at 480~520℃, so as to obtain the oxygen partial pressure range at 480~520℃.

[0147] In this embodiment, based on the Gibbs free energies of each metal oxide at 480℃, 500℃, and 520℃, the fitting coefficients of each metal oxide in the range of 480~520℃ are determined by fitting, wherein the fitting coefficient A of Al2O3 is -1.676×10 6 B = 320.3; the fitting coefficient A for Fe2O3 is -8.20 × 10⁻⁶. 5 B=280.5.

[0148] In this application, the equilibrium oxygen partial pressure of Al2O3 at 480℃ is 1.2 × 10⁻⁶. -21 The equilibrium oxygen partial pressure of Fe2O3 at 520℃ is 3.0 × 10 MPa. -17 Therefore, the oxygen partial pressure at 480–520°C in this application is 1.2 × 10 MPa. -21 ~3.0×10 - 17 MPa.

[0149] It should be noted that the metal oxides in this application include Al2O3, Sb2O3, and Fe2O3. Al2O3 has the lowest equilibrium oxygen partial pressure, indicating its highest stability; the equilibrium oxygen partial pressure of Sb2O3 is between that of Al2O3 and Fe2O3, indicating its moderate stability; and the equilibrium oxygen partial pressure of Fe2O3 is the highest, indicating its lowest stability. This means that once the atmosphere reaches the lower limit of Al2O3, Sb has automatically co-oxidized, and there is no need to define a separate boundary for Sb. Therefore, within the temperature range of 480–520°C, the equilibrium oxygen partial pressure of Sb2O3 always lies between that of Al2O3 and Fe2O3. Furthermore, the actual oxygen partial pressure only needs to be within the range of the lower limit of Al2O3 and the upper limit of Fe2O3 to simultaneously ensure film formation of Al and Sb and prevent Fe from peeling, and the oxidation risk of Sb is completely covered by the lower limit of Al. Therefore, the oxygen partial pressure range is determined based on Al2O3 and Fe2O3 in this application.

[0150] Preferably, after controlled surface pre-oxidation at 480–520℃, the oxide film thickness is 50–100 nm, and the depth of the Sb / Al enriched layer on the surface is ≥30 nm; after low-temperature tempering at 440–460℃, the high dislocation density of the martensitic laths is ≥10. 14 m -2 Hydrogen trap density ≥ 5 × 10 20 m -3 .

[0151] In this application, during controlled surface pre-oxidation at 480–520°C, under oxygen partial pressure and water vapor partial pressure conditions, Sb and Al are induced to co-diffuse to the oxide film-substrate interface, forming a 50–100 nm thick Al-Sb-rich oxide nanocluster layer at the steel plate surface-substrate interface, which serves as a precursor for the subsequent corrosion-resistant film.

[0152] In this application, controlled surface pre-oxidation at 480–520℃ induces the co-diffusion of Sb and Al to the oxide film-matrix interface, generating an Al-Sb-O nanocluster layer. The Al2O3 region blocks Cl. - CO2 penetrates and pins the grain boundaries, while H2O3 regions adsorb H2O. + It inhibits hydrogen atom penetration, while providing reversible hydrogen traps to prevent H2 bubble nucleation, thereby reducing the corrosion rate, maintaining the HIC susceptibility rate at CLR = 0, and uniformly deforming the surface hard layer, achieving a combination of corrosion resistance, hydrogen resistance, and low yield strength ratio.

[0153] In this application, N2 atmosphere is used as a protective atmosphere during low-temperature tempering at 440–460℃ to prevent secondary oxidation. In addition, the low-temperature tempering treatment at 440–460℃ can also induce Sb grain boundary segregation, stabilize the Al-Sb rich surface film, maintain high dislocations in martensite laths, realize Ca-Mg inclusion synergistic hydrogen trapping, reduce the hydrogen diffusion coefficient, and improve low-temperature toughness.

[0154] S12. Finishing: The heat-treated billet is finished to obtain low-C and low-Cr corrosion-resistant pipeline steel plates.

[0155] In this application, the finishing process includes hot straightening, cutting the head and tail, cutting to length, flaw detection, and pipe end beveling.

[0156] In this embodiment, during hot straightening, a nine-roller hot straightening machine is used, with a straightening temperature of 400–450°C and a straightening force of 1500 t; during head and tail cutting, double-sided shearing is used, with 200 mm cut at each end; during flaw detection, online UT+ET combined flaw detection is used, with a defect equivalent of Φ2 mm as the rejection criterion; during pipe end beveling, a 30°+1.6 mm blunt edge is milled, with a surface roughness Ra≤12.5 µm.

[0157] It should be emphasized that the purities of the various gases used in the preparation process of this application are: argon ≥ 99.999%, nitrogen ≥ 99.9%, and oxygen ≥ 99.5%. Furthermore, all gases are dried and filtered. Moreover, this application can be completed using a conventional converter-LF-RH-continuous casting-rolling production line, requiring no additional complex equipment, and exhibits strong process adaptability.

[0158] Preferably, the average size of the carbonitride precipitates of microalloying elements Nb, V, and Ti in the prepared steel plate is ≤50 nm, the volume fraction is ≥0.05%, and the grains are refined to ASTM grade 11 or higher.

[0159] In this application, XPS detection of the surface of the manufactured steel plate revealed that Sb exists in the form of Sb2O3 and Al exists in the form of Al2O3. The two form Al-Sb-O nanoclusters at the oxide film-matrix interface, with an area coverage of ≥80%.

[0160] Preferably, the steel plate produced has a room temperature yield strength ≥540MPa, tensile strength ≥670MPa, elongation ≥26%, and KV8 impact absorption energy ≥235J at -40℃.

[0161] Preferably, the corrosion rate of the prepared steel plate is ≤0.027 mm / a under the conditions of 0.5 wt.% NaCl + 0.5 wt.% CH3COOH + CO2 / H2S saturated solution, pH 4.0~4.5, 80℃×720h; and the crack length ratio CLR is 0 under the conditions of 5 wt.% NaCl + 0.5 wt.% CH3COOH solution, NACE TM0284-A standard 96h HIC test.

[0162] Preferably, the weld heat-affected zone of the manufactured steel plate has an impact absorption energy of ≥200 J at -40℃ and a welding cold crack sensitivity index Pcm≤0.18%, which can meet the requirements of high heat input welding.

[0163] The third aspect of the present invention provides a low-C, low-Cr corrosion-resistant pipeline steel pipe, which is manufactured from a steel plate prepared by a method for preparing low-C, low-Cr corrosion-resistant pipeline steel plates.

[0164] Preferably, the low-C, low-Cr corrosion-resistant pipeline steel pipe is suitable for high-Cl... - Submarine and onshore oil and gas pipeline projects in CO2 / H2S corrosive environments.

[0165] Based on the aforementioned low-C, low-Cr corrosion-resistant pipeline steel plate and its preparation method, the present invention conducted the following sets of experiments, wherein the component examples 1-10 are shown in Table 1, and the method examples 1-3 are shown in Table 2. It should be noted that the raw materials used in the following experiments were all commercially available.

[0166]

[0167]

[0168]

[0169] Based on the foregoing embodiments, the present invention also conducted the following comparative experiments on multiple components and preparation methods, with component comparison examples 1-7 shown in Table 3.

[0170]

[0171] In this experiment, the main elements in Component Examples 1-10 and Component Comparative Examples 1-7 are compared as shown in Table 4.

[0172]

[0173] Performance testing: room temperature tensile properties test, -40℃ KV8 / J impact properties test, corrosion weight loss test under 0.5wt.% NaCl + 0.5wt.% CH3COOH + CO2 / H2S saturated solution, pH 4.0~4.5, 80℃×720h conditions, and crack length ratio test under 5wt.% NaCl + 0.5wt.% CH3COOH solution, NACE TM0284-A standard 96h HIC test conditions.

[0174] All performance test samples used in the present invention were cut sequentially along the rolling direction from the same heat-treated mother plate with a thickness of 20 mm, a width of 400 mm, and a length of ≥800 mm. Among them, the room temperature tensile test specimens were 5 pieces per group (including 2 spare pieces), with a size of 200 mm × 25 mm × 20 mm; the -40℃ KV8 impact test specimens were 15 pieces per group (including 3 spare pieces), with a size of 10 mm × 10 mm × 55 mm; the corrosion weight loss test specimens were 5 pieces per group (including 2 spare pieces), with a size of 40 mm × 25 mm × 3 mm; and the HIC test specimens were 5 pieces per group (including 2 spare pieces), with a size of 100 mm × 20 mm × 20 mm.

[0175] It should be noted that the room temperature tensile property test and the -40℃ KV8 / J impact property test are routine tests in this field, so the test process will not be described in detail in this experiment.

[0176] In this experiment, during the room temperature tensile property test, in the first stage (to yield), the machine operated at a speed of 0.00025 s. -1 The initial slow stretching; in the second stage (from yield to fracture), the speed increases to 0.0067 s. -1 It was pulled until it snapped with a "snap". Furthermore, during the -40℃ KV8 / J impact performance test, a 300 J pendulum impact tester was used, and it broke in one strike.

[0177] The corrosion resistance test procedure in this experiment is as follows:

[0178] (1) Polish the surface of the test piece with 120#, 600# and 1000# sandpaper in sequence until it is mirror smooth, and ensure that there is no oxide scale and scratches on the surface.

[0179] (2) Clean the sample with deionized water, anhydrous ethanol and acetone for 5 min each, then dry it with cold air and record the initial mass m0 (0.1 mg accuracy).

[0180] (3) Weigh 5g of NaCl and 5g of glacial acetic acid. First, dissolve the NaCl in about 900g of deionized water to form a simulated seawater chloride ion environment. Then, add acetic acid and mix thoroughly. Add deionized water to bring the total mass to 1 kg. Finally, measure the pH and adjust the pH to 4.0-4.5 with a small amount of NaOH or glacial acetic acid to form a 0.5wt.% NaCl + 0.5wt.% CH3COOH corrosive solution to simulate a deoxygenated acidic oil and gas environment.

[0181] (4) Hang the test piece on the polytetrafluoroethylene support to avoid contact with the reactor wall.

[0182] (5) Inject 1 L of the corrosive solution formed in step (3), namely 0.5 wt.% NaCl + 0.5 wt.% CH3COOH, pH 4.0~4.5, and purge with N2 for 30 min to remove oxygen.

[0183] (6) After heating to 60℃, immediately introduce CO2 / H2S, CO2 to 0.5 MPa (continuously introduce to maintain saturation), then introduce H2S to 0.1 MPa (total pressure 0.6 MPa); then directly raise the temperature to 80℃ in this atmosphere and start timing for 720 h of continuous soaking, replacing the freshly prepared corrosion solution every 72 h, with a total volume of 1 L, to ensure that the medium concentration and pH remain unchanged.

[0184] (7) After 720 h, take it out and clean it with deionized water, ethanol and acetone. After that, dry it with cold air and weigh the final mass m1.

[0185] (8) Calculate the corrosion rate of low-C, low-Cr corrosion-resistant pipeline steel plates in corrosive liquid, as shown in the following formula:

[0186]

[0187] In the formula, V represents the uniform corrosion rate; m0 is the initial mass in g; and m1 is the final mass in g. Density of material, unit: g·cm³ -3 (7.85 g·cm³ for steel) -3 S represents the exposed area, in cm². 2 t represents the test time, in hours (h).

[0188] In this experiment, the NACE TM0284-A standard 96 h HIC test procedure is as follows:

[0189] (1) Polish the surface of the test piece and put it into NACE standard A solution (5wt.% NaCl + 0.5wt.% CH3COOH), 25℃, saturated H2S, for 96 hours.

[0190] (2) After soaking, cut 5 pieces along the thickness direction, polish them and look for cracks under a 100x metallographic microscope.

[0191] (3) Calculate the crack length ratio (CLR) using the following formula:

[0192]

[0193] In the formula, The crack length needs to be measured for each cross-section, in mm. W is the sum of the lengths of all cracks on the cross section, in mm; W is the width of the specimen cross section, in mm.

[0194] In this experiment, based on the chemical element content of a low-C, low-Cr corrosion-resistant pipeline steel plate listed in Component Examples 1-8, a low-C, low-Cr corrosion-resistant pipeline steel plate was prepared using the process described in Method Example 1; based on the chemical element content of a low-C, low-Cr corrosion-resistant pipeline steel plate listed in Component Example 9, a low-C, low-Cr corrosion-resistant pipeline steel plate was prepared using the process described in Method Example 2; based on the chemical element content of a low-C, low-Cr corrosion-resistant pipeline steel plate listed in Component Example 10, a low-C, low-Cr corrosion-resistant pipeline steel plate was prepared using the process described in Method Example 3. Meanwhile, based on the chemical element content of the pipeline steel plates listed in Comparative Examples 2 and 5-6, pipeline steel plates were prepared using the process described in Method Example 1. In Method Example 1, Mg-Ca treatment was not performed, and its heat treatment was replaced by conventional 650 ℃ tempering treatment. Based on the chemical element content of the pipeline steel plates listed in Comparative Examples 1, 3-4, and 7, pipeline steel plates were prepared using the process described in Method Example 1. In Method Example 1, the heat treatment was replaced by conventional 650 ℃ tempering treatment.

[0195] In this experiment, the traditional 650 ℃ tempering treatment is as follows: the cast billet after reheating and quenching is heated to 650 ℃ and held for 60 min, and then air-cooled to room temperature.

[0196] Subsequently, performance tests were conducted on the pipeline steel plates prepared in Component Examples 1-8 and Component Comparative Examples 1-7, as shown in Table 5. Table 5 includes the yield strength, tensile strength, and elongation as comparative items in the room temperature tensile performance test.

[0197]

[0198] As shown in Table 5, the overall yield strength of components 1-8 is 540–565 MPa, tensile strength is 670–695 MPa, and elongation is 26–28%, which is significantly better than that of components 1-7 (yield strength 480–550 MPa, tensile strength 600–680 MPa, and elongation 23–26%). Among them, the yield strength and tensile strength of component 6 (except for the absence of Sb, Al, Mg, and Ca elements, and the absence of Mg-Ca treatment and the heat treatment in this application, the other elemental components are the same as those of component example 6) are the best among components 1-7, highlighting the superiority of the low C, low Cr + upper limit Nb / V / Ti design in this application. In addition, the ultra-low carbon (≤0.03%) + upper limit Nb / V / Ti can precipitate 5–20 nm (Nb,V,Ti)C nanocarbides during the tempering stage at 450℃, pinning dislocations and refining grains. The yield strength and tensile strength of Component Example 6 were optimal among Component Examples 1-8 and Component Comparative Examples 1-7, increasing by 85 MPa compared to conventional 650℃ tempered high-C, high-Cr steel (Component Comparative Example 5), while maintaining the same elongation (also achieving the highest performance of Component Comparative Examples 1-7). This confirms that the low-temperature tempering and nano-precipitation in the dedicated heat treatment of this application are the main reasons for the strength improvement, rather than relying on high carbon, further highlighting the superiority of the low-C, low-Cr design, upper limit Nb / V / Ti, Mg-Ca treatment, and dedicated heat treatment in this application. Furthermore, the yield strength, tensile strength, and elongation of Component Example 1 were all higher than those of Component Comparative Example 1. Given the same composition but different preparation methods, this also demonstrates the superiority of the dedicated heat treatment, especially low-temperature tempering, in this application.

[0199] Tempering at 450℃ retains slender martensitic laths and a 50-100 nm thin film of retained austenite, resulting in high crack passivation performance. The upper limits of Sb (0.025%) and Al (1.20%) segregate at grain boundaries, further hindering intergranular cracking. Table 5 clearly shows that the overall KV8 of components 1-8 is 235-252 J, significantly better than the overall KV8 (190-240 J) of comparative examples 1-7, highlighting the superiority of the Mg-Ca treatment + special heat treatment (low-temperature tempering) in this application. In particular, component example 1 shows an increase of 15-42 J compared to comparative example 1 with the same composition, and component example 8 (with lower limits for multiple elements, Sb and Al contents also at the lower limits) still achieves a KV8 of 235 J, further demonstrating that "low-temperature tempering + Sb-Al grain boundary purification" is the core of low-temperature toughness improvement, independent of the total alloy content.

[0200] Furthermore, the impact absorption energy does not increase monotonically with increasing Sb and Al content: when Sb ≥ 0.025% or Al ≥ 1.0%, Sb enrichment at grain boundaries and excessive AlN pinning cause grain boundary embrittlement and a decrease in crack initiation resistance. At the same time, the stability of residual austenite decreases, resulting in a decline in impact toughness. The composition with slightly lower Sb and moderate Al content takes into account grain boundary purification, hydrogen trapping, and grain refinement. Therefore, although composition example 1 is not the upper limit, it has a higher impact absorption energy. The impact absorption energy of composition comparison example 3 drops sharply to 190 J due to excessive Sb triggering grain boundary embrittlement. The impact value of composition comparison example 4 is higher than some high Al cases because insufficient Al reduces AlN pinning and extends the crack propagation path.

[0201] Pre-oxidation at 500℃ in Ar + 1% O2 followed by heating at 450-500℃ produces a 50-100 nm Al-Sb-rich oxide film: the Al2O3 region blocks Cl. - Osmosis, adsorption of H in the Sb2O3 region + As a reversible hydrogen trap, the film coverage is ≥80% (XPS). Table 5 clearly shows that the overall corrosion rate in Component Examples 1-8 is 0.021–0.027 mm / a, which is an average decrease of ≥60% compared to the overall corrosion rate of 0.060–0.110 mm / a in Comparative Examples 1-7. Furthermore, the corrosion rate of Component Example 3 (upper limit Sb-Al) is 0.021 mm / a, the corrosion rate of Component Example 4 (lower limit Sb-Al) is 0.025 mm / a, and the corrosion rates of Comparative Examples 3 (Sb excess), 5 (no Sb, trace Al), and 6 (no Sb, no Al) are ≥0.85 mm / a, further confirming that "Sb-Al enrichment + pre-oxidation" is the key to the leap in corrosion resistance, rather than solely relying on Cr content.

[0202] Maintain high dislocation density ≥10 at 450℃ tempering 14 m -2 Provide ≥5×10 20 m -3 Reversible hydrogen traps; Mg-Ca treatment (component examples 1-8) replaces MnS with spherical CaS+MgO·Al2O3, completely eliminating hydrogen traps. Table 5 clearly shows that the overall CLR of component examples 1-8 is 0, significantly better than the overall CLR of component comparative examples 1-7 (5.1-16.5%). Compared with component comparative example 1, the CLR of component example 1 decreased from 5.1% to 0; compared with component comparative example 6, the CLR of component example 6 decreased from 10.2% to 0, further demonstrating that the three elements of "low-temperature tempering + high dislocation + no MnS" jointly achieve zero hydrogen cracking, proving that this application has excellent resistance to hydrogen-induced cracking in H2S-containing environments, which cannot be replicated by traditional 650℃ tempering.

[0203] This invention utilizes a "low C (≤0.03 wt%) + low Cr (≤0.50 wt%)" basic system to suppress Cr while ensuring strength. 23 The precipitation of Cr-containing carbides such as C6 avoids intergranular corrosion caused by chromium depletion at grain boundaries. At the same time, the low-Cr, low-Sb and other low-precious-metal design results in a significantly lower cost than traditional medium-Cr or high-Cr pipeline steels.

[0204] This invention is the first to combine Sb (≤0.025 wt%) and Al (0.50~1.20 wt%) in a composite addition. Sb adsorbs on the steel surface, blocking H. + The reduction reaction reduces hydrogen permeation and inhibits hydrogen damage under H2S environment; Al forms a dense Al2O3 protective film on the surface, blocking Cl. - CO2 infiltration, the two working together, can maintain high corrosion resistance while reducing the amount of each, achieving "cost reduction without performance reduction".

[0205] This invention synergistically treats trace amounts of Mg (0.0005–0.0012 wt%) and Ca (0.0012–0.0042 wt%) to generate fine, spherical, and diffusely distributed Ca-Mg-Al-OS composite inclusions, significantly reducing the active cathode area of ​​the corrosion galvanic cell. Moreover, the grain refinement and grain boundary pinning can inhibit crack propagation and hydrogen accumulation, improving anti-HIC performance. Simultaneously, the Mg-Ca synergistic treatment converts solid Al2O3 or MgO·Al2O3 into liquid calcium aluminate, promoting inclusion flotation and reducing nozzle nodule formation.

[0206] This invention employs an ultra-low carbon design combined with Nb, V, and Ti microalloying, which significantly improves the toughness of the weld heat-affected zone.

[0207] This invention employs a specialized heat treatment process of "pre-oxidation at 480–520℃ and low-temperature tempering at 440–460℃". Pre-oxidation forms a 50–100 nm Sb-Al rich oxide film on the surface, and tempering at 450℃ induces Sb grain boundary segregation, stabilizing the Al-Sb rich surface film while maintaining a high dislocation density of martensite, thus achieving a unity of "high strength + high toughness + high corrosion resistance".

[0208] In summary, this invention achieves the desired mechanical properties while maintaining or even improving them, using low cost and a simple process. - It exhibits excellent resistance to complex corrosive media such as CO2 and H2S, and completely solves the problems of poor welding performance, high HIC sensitivity, and high alloy cost of traditional pipeline steel, resulting in significant technical, economic, and safety benefits.

[0209] Finally, it should be noted that the above description is only a preferred embodiment of the present invention and is not intended to limit the present invention. Although the present invention has been described in detail with reference to the foregoing embodiments, those skilled in the art can still modify the technical solutions described in the foregoing embodiments or make equivalent substitutions for some of the technical features. Any modifications, equivalent substitutions, improvements, etc., made within the spirit and principles of the present invention should be included within the protection scope of the present invention.

Claims

1. A method for preparing a low-C, low-Cr corrosion-resistant pipeline steel plate, characterized in that, The chemical element contents of this low-C, low-Cr corrosion-resistant pipeline steel plate are as follows: C≤0.03wt.%, Sb≤0.025wt.%, Al 0.50~1.20wt.%, Mg 0.0005~0.0012wt.%, Ca 0.0012~0.0042wt.%, Cr≤0.50wt.%, Ni 0.10~0.30wt.%, Si 0.10~0.30wt.%, Mn 1.20~1.80wt.%, Mo 0.10~0.30wt.%, Nb 0.02~0.06wt.%, V 0.03~0.08wt.%, Ti 0.01~0.03wt.%, P≤0.01wt.%, S≤0.001wt.%, N≤0.005wt.%, balance being Fe and unavoidable impurity elements; wherein the preparation method of this low-C, low-Cr corrosion-resistant pipeline steel plate includes the following steps: S1. Material preparation: low-sulfur and low-phosphorus blast furnace iron, high-quality low-carbon scrap steel, various alloys, pure metals and metal wires. Among them, various alloys include ferrosilicon, low-carbon ferromanganese, ferromolybdenum, ferroniobium, ferrovanadium, low-carbon ferrochrome, ferroantimony and low-carbon fertitanium. Pure metals include nickel plates and aluminum blocks. Metal wires include CaSi wire and Mg wire. S2, Desulfurization: Deep desulfurization is carried out on low-sulfur and low-phosphorus blast furnace hot metal at 1350-1400℃, and the S in the treated low-sulfur and low-phosphorus blast furnace hot metal is ≤0.001wt.%; S3. Converter primary refining: The low-sulfur and low-phosphorus blast furnace iron and high-quality low-carbon scrap steel after deep desulfurization are subjected to converter primary refining, with the final control of C 0.03~0.05wt.% and molten steel temperature 1630~1650℃. S4, LF refining: The molten steel after the primary refining in the converter is refined by LF to form a high-basicity refining slag, ensuring that FeO in the slag is ≤0.5wt.%. Then, the alloy is fine-tuned according to the spectral detection results to make the composition of the molten steel meet the requirements. The molten steel exit temperature is 1540~1560℃. S5, RH vacuum degassing: Perform RH vacuum degassing on the molten steel after LF refining, with vacuum circulation for 15-20 minutes. Then, perform final composition adjustment based on the spectral detection results. After the vacuum is completed, ensure that [H] ≤ 1.5 ppm and [N] ≤ 50 ppm. S6, Mg-Ca co-treatment: Mg-Ca co-treatment is applied to molten steel after RH vacuum degassing to form fine spherical composite inclusions, specifically: (1) Mg treatment: at 120-150 m·min -1 The feeding rate is such that the Mg wire is injected deep into the molten steel. After injection, argon is briefly blown for 2-5 minutes at a flow rate of 70-80 NL / min. (2) Calcium treatment: at 80-100 m·min -1 The feeding rate is such that the CaSi wire is injected deep into the molten steel. After injection, argon is briefly blown for 2-5 minutes at a flow rate of 70-80 NL / min. (3) Take samples to test whether the Mg and Ca content in the molten steel is qualified. If the Mg and Ca content is qualified, then perform long-term argon blowing, where the argon blowing time is ≥15min and the argon flow rate is 50~60 NL / min. If the content of either Mg or Ca is < the corresponding minimum content value, then repeat steps (1) / (2) to supplement the corresponding short wire. After the supplementation is completed, take samples again to test the Mg and Ca content in the molten steel until the Mg and Ca content in the molten steel is qualified, and then stop the sampling test. S7. Continuous casting: The molten steel after Mg-Ca synergistic treatment is poured under full protection. Low superheat of 15-25℃ is used for pouring. Electromagnetic stirring and light reduction technology are used to suppress the central segregation of Sb element. The billet is slowly cooled for more than 48 hours after it is removed from the line. S8. Reheating: The billet is reheated and descaled. The furnace temperature is 1180-1220℃ and the total reheating time is 2 hours. The soaking temperature is 1190-1210℃ and the soaking time is 30-60 minutes. S9. Controlled rolling and cooling: The reheated billet is subjected to controlled rolling and cooling treatment. The roughing rolling start temperature is 1050~1100℃, the roughing rolling finish temperature is 980~1000℃, the finishing rolling start temperature is 800~820℃, and the finishing rolling finish temperature is ≥780℃. Then, the finished billet is directly quenched and the final cooling temperature is 400~450℃, followed by air cooling to room temperature. S10. Reheating and quenching: Hold the billet after controlled rolling and controlled cooling at 920℃ for 30 minutes, then quench it in water to room temperature; S11. Heat Treatment: The reheated and quenched billet undergoes controlled surface pre-oxidation at 480–520℃ and low-temperature tempering at 440–460℃, followed by air cooling to room temperature. During the controlled surface pre-oxidation at 480–520℃, the temperature is held for 30 minutes. The protective atmosphere is Ar + 1 vol% O2, with an atmosphere dew point of -60 to -40℃ and an oxygen partial pressure of 1.2 × 10⁻⁶. -21 ~3.0×10 -17 MPa; during low-temperature tempering at 440~460℃, hold for 55~65min, with N2 as the protective atmosphere; S12. Finishing: The heat-treated billet is finished to obtain low-C and low-Cr corrosion-resistant pipeline steel plates.

2. The method for preparing a low-C, low-Cr corrosion-resistant pipeline steel plate according to claim 1, characterized in that, During material preparation, the blast furnace hot metal should contain S ≤ 0.002 wt.% and P ≤ 0.010 wt.%, and the standard for high-quality low-carbon scrap steel is: Cu + Sn + As + Pb + Bi ≤ 0.20 wt.%. During converter primary refining, the proportion of low-sulfur, low-phosphorus blast furnace hot metal should be 75–85%, and the proportion of high-quality low-carbon scrap steel should be 15–25%. During LF refining, the molten steel is heated to 1580–1600 ℃ using electrodes to ensure the steel exits the station at a temperature of 1540–1560 ℃. During RH vacuum degassing, a four-stage vacuum pump is used to evacuate to 0.3–0.5 mbar, with a circulation flow rate of 120 t·min. -1 During continuous casting, the electromagnetic stirring current is 250–350 A, the working frequency is 24 Hz, and the total reduction is 6–8 mm. In addition, during RH vacuum degassing, when the Al content is > 0.90 wt.%, ensure that [N] ≤ 30 ppm after vacuuming.

3. The method for preparing a low-C, low-Cr corrosion-resistant pipeline steel plate according to claim 1, characterized in that, During rough rolling, the single-pass reduction rate is 15-25%, and the total reduction rate is ≥60%; during finish rolling, the single-pass reduction rate is 10-15%, and the total reduction rate is ≥70%; direct quenching treatment specifically involves quenching at 15-25℃ for s. -1 The cooling rate will allow the finished rolled billet to undergo laminar flow cooling.

4. The method for preparing a low-C, low-Cr corrosion-resistant pipeline steel plate according to claim 1, characterized in that, During continuous casting, after suppressing the center segregation of Sb element by combining electromagnetic stirring and light reduction technology, the center segregation C class is ≤1.0 grade; After controlled rolling and controlled cooling treatment, the microstructure is a mixed microstructure mainly composed of lath bainite, with a small amount of martensite and thin film retained austenite. Moreover, the carbonitrides are not completely dissolved after controlled rolling and controlled cooling treatment, and the precipitated phase size is 20-50 nm. Among them, lath bainite accounts for 70%, martensite accounts for <30%, and thin film retained austenite accounts for 5-10%. After reheating and quenching, the microstructure consists of fine lath martensite, fine lath bainite, and thin film retained austenite. At the same time, the size of the martensitic carbide and carbonitride precipitates after reheating and quenching is 5–15 nm.

5. The method for preparing a low-C, low-Cr corrosion-resistant pipeline steel plate according to claim 1, characterized in that, After controlled surface pre-oxidation at 480–520℃, the oxide film thickness is 50–100 nm, and the depth of the Sb / Al enriched layer on the surface is ≥30 nm; after low-temperature tempering at 440–460℃, the high dislocation density of the martensitic laths is ≥10. 14 m -2 Hydrogen trap density ≥ 5 × 10 20 m -3 .

6. The method for preparing a low-C, low-Cr corrosion-resistant pipeline steel plate according to claim 1, characterized in that, The average size of the carbonitride precipitates of microalloying elements Nb, V, and Ti in the manufactured steel plate is ≤50 nm, the volume fraction is ≥0.05%, and the grains are refined to ASTM grade 11 or higher. In addition, XPS analysis of the surface of the fabricated steel plate showed that Sb exists in the form of Sb2O3 and Al exists in the form of Al2O3. The two form Al-Sb-O nanoclusters at the oxide film-matrix interface, with an area coverage of ≥80%.

7. The method for preparing a low-C, low-Cr corrosion-resistant pipeline steel plate according to claim 1, characterized in that, The steel plate produced has a room temperature yield strength ≥540MPa, tensile strength ≥670MPa, elongation ≥26%, and KV8 impact absorption energy at -40℃ ≥235J. Under the conditions of 0.5 wt.% NaCl + 0.5 wt.% CH3COOH + CO2 / H2S saturated solution, pH 4.0–4.5, 80℃ × 720h, the corrosion rate is ≤0.027 mm / a; under the conditions of 5 wt.% NaCl + 0.5 wt.% CH3COOH solution, NACE TM0284-A standard 96h HIC test, the crack length ratio CLR = 0. In addition, the weld heat-affected zone of the manufactured steel plate has an impact absorption energy of ≥200 J at -40℃ and a weld cold cracking sensitivity index Pcm≤0.18%.

8. A low-C, low-Cr corrosion-resistant pipeline steel pipe, characterized in that, The steel plate is manufactured using the preparation method of a low-C, low-Cr corrosion-resistant pipeline steel plate according to any one of claims 1-7.

9. A low-C, low-Cr corrosion-resistant pipeline steel pipe according to claim 8, characterized in that, The low-C, low-Cr corrosion-resistant pipeline steel pipe is suitable for high-Cl... - Submarine and onshore oil and gas pipeline projects in CO2 / H2S corrosive environments.

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