A seamless steel pipe resistant to co2 corrosion and a method for manufacturing the same

Abstract

The application discloses a kind of anti-CO2 corrosion seamless steel tube and manufacturing method thereof, belong to seamless steel tube and its manufacturing technical field, according to mass percentage, the chemical composition of steel is as follows: C:0.22%~0.32%;Si:0.25%~0.50%;Mn:0.35%~0.65%;P:≤0.015%;S:≤0.008%;Cu:0.05%~0.20%;Ni:0.10%~0.25%;Cr:2.50%~3.50%;Mo:0.02%~0.60%;V:0.02%~0.06%;Nb:0.001%~0.050%;Ti:0.001%~0.050%;B:0.0005%~0.0080%;Al:0.010%~0.030%;RE:0.002%~0.030%;The RE is Ce-La mixed rare earth, Ce-La mass ratio=(1~2.75):1;RE / S mass ratio is 1~4.5;The balance is Fe and inevitable impurities.The application can resist the electrochemical corrosion caused by CO2 and carbonate medium;Under the premise of controlling cost, guarantee the toughness of steel pipe;Strictly control RE / S stoichiometry, while preventing rare earth excessive large size hard brittle inclusion, avoid continuous casting nozzle nodular risk of plugging;Guarantee the strength, plasticity and low temperature impact toughness of steel pipe.

Description

Technical Field

[0001] This invention relates to a CO2 corrosion resistant seamless steel pipe and its manufacturing method, belonging to the field of seamless steel pipe and its manufacturing technology. Background Technology

[0002] During the oil and gas extraction process, CO2 gas is commonly generated as an associated gas in the oil and gas medium. When CO2 comes into contact with groundwater, it forms carbonic acid, which can cause strong electrochemical corrosion to the well pipe, resulting in thinning of the pipe wall, pitting and perforation, and pipe string failure accidents, seriously threatening the safe production of oil and gas fields.

[0003] Currently, high-chromium stainless steels such as 13Cr and 22Cr exhibit excellent corrosion resistance in CO2 corrosion environments, but their high alloy costs make large-scale use uneconomical for low-permeability, low-grade oil and gas fields with limited extraction cycles. 3Cr-based economical CO2-resistant seamless steel pipes have become the industry's preferred material. Existing 3Cr steels achieve a balance between corrosion resistance and strength through Cr and Mo alloying, but several technical challenges remain to be addressed.

[0004] First, long, linear MnS sulfide inclusions in steel are a significant source of localized CO2 pitting corrosion. MnS inclusions preferentially dissolve in corrosive media, becoming a source of pitting corrosion initiation, disrupting the continuity of the passivation film, and significantly accelerating localized corrosion failure of steel pipes. While some existing patents propose adding rare earth elements to improve inclusions, the control of rare earth element ratios is crude, presenting two major problems: First, the total amount of rare earth added is too high, with some schemes reaching an upper limit of 0.2%. Excessive rare earth easily forms high-melting-point composite inclusions during continuous casting, causing blockages at the casting nozzles, deteriorating production continuity, and large-sized rare earth inclusions can also fracture the matrix, reducing the steel's toughness and corrosion resistance. Second, the ratio of Ce to La is not specified. These two rare earth elements have different functional characteristics: Ce has strong deoxidation, desulfurization, and sulfide modification capabilities, while La focuses on improving the corrosion product film. An imbalance in their ratio fails to fully leverage the synergistic modification effect, making it difficult to simultaneously achieve sulfide inclusion modification and improve the stability of the corrosion product film using only a single rare earth element.

[0005] Secondly, most existing technologies do not address the metrological matching relationship of the RE / S ratio. Rare earth elements and sulfur have a very strong chemical affinity. If the RE / S ratio is too low, the rare earth elements are insufficient to fully consume the sulfur in the steel, and the long MnS inclusions cannot be fully spheroidized and modified, thus failing to effectively eliminate the source of pitting corrosion. If the RE / S ratio is too high, the excess rare earth elements, in addition to causing nozzle blockage, will generate hard and brittle large rare earth oxygen-sulfur composite inclusions, which will fracture the steel matrix, worsen impact toughness, and impair CO2 corrosion resistance. Existing published 3Cr steel patents lack RE-S stoichiometric constraints, only simply providing a rare earth mass range. The smelting process is affected by fluctuations in S content, resulting in highly unstable inclusion modification effects. For the same composition, the pitting corrosion resistance and impact performance of steel from different heats show large dispersion, which is not conducive to stable industrial-scale production.

[0006] In addition, traditional 3Cr steel usually contains a high content of Mo to ensure pitting corrosion resistance. Mo is a precious alloying element, which significantly increases the material cost. The boron content of some steel grades is not controlled so loosely. Boron segregation at the grain boundaries brings the risk of quenching cracks. Some steel grades have low strength and can only be used in shallow well conditions, making it difficult to adapt to the service conditions of medium and deep wells.

[0007] In conclusion, the existing technology obviously has inconveniences and defects in practical use, so it is necessary to improve it. Summary of the Invention

[0008] This invention addresses the shortcomings of the prior art by providing a CO2-resistant seamless steel pipe and its manufacturing method, which can resist electrochemical corrosion caused by CO2 and carbonic acid media; ensures the strength and toughness of the steel pipe while controlling costs; strictly controls the RE / S stoichiometric ratio to avoid insufficient rare earth leading to inadequate inclusion modification, while preventing excessive rare earth from generating large-sized hard and brittle inclusions, thus avoiding the risk of nodule blockage in continuous casting nozzles; and ensures the strength, plasticity, and low-temperature impact toughness of the steel pipe.

[0009] To solve the above technical problems, the present invention adopts the following technical solution: A seamless steel pipe resistant to CO2 corrosion, with the following chemical composition of the steel by weight percentage: C: 0.22%~0.32%; Si: 0.25%~0.50%; Mn: 0.35%~0.65%; P: ≤0.015%; S: ≤0.008%; Cu: 0.05%~0.20%; Ni: 0.10%~0.25%; Cr: 2.50%~3.50%; Mo: 0.02%~0.60%; V: 0.02%~0.06%; Nb: 0.001%~0.050%; Ti: 0.001%~0.050%; B: 0.0005%~0.0080%; Al: 0.010%~0.030%; RE: 0.002%~0.030%; The RE is a Ce-La mixed rare earth element with a Ce-La mass ratio of (1-2.75):1; the RE / S mass ratio is 1-4.5; and the balance is Fe and unavoidable impurities.

[0010] Furthermore, the mass percentage of Mo is 0.02% to 0.10%.

[0011] Furthermore, the mass percentage of B is 0.0005% to 0.0020%.

[0012] Furthermore, the CO2 corrosion resistant seamless steel pipe has a yield strength of 552-758 MPa, a tensile strength of 689-889 MPa, an elongation of ≥15%, a transverse full-size Charpy impact energy of ≥60 J or a longitudinal full-size Charpy impact energy of ≥80 J at 0℃, and a hardness of ≥18.0 HRC.

[0013] A method for manufacturing a CO2-resistant seamless steel pipe includes the following steps: Step S1, rough refining in an electric arc furnace: Raw material selection: Use high-quality scrap steel with low phosphorus and low sulfur to adjust carbon content with pig iron; control the residual Cu and Ni content in raw materials; do not add large amounts of Cr, Mo, V, and Nb alloys during the electric arc furnace stage; Melting oxidation: Add lime and fluorite to create oxidizing slag, raise the temperature to 1580-1620℃; blow oxygen to dephosphorize, slag basicity R≥2.5; control the carbon content (C) at the final tapping point to be ≥0.15%; Step S2, refining in a ladle refining furnace: To produce reducing white slag: lime, fluorite, and aluminum slag are added, and the slag basicity R = 2.8–3.5; submerged arc heating is used for SiC and aluminum particle diffusion deoxidation; the white slag is maintained for ≥25 minutes, and desulfurization is achieved to S≤0.008%; Add alloys in batches: In the early and middle stages of slag stabilization and steel purity improvement, add ferrochrome, ferromolybdenum, nickel plate, cathode copper, ferrovanadium, and ferroniobium to the ladle; after the slag stabilizes and the purity of the molten steel is improved, add ferrotitanium and stir for 3-5 minutes after adding ferrotitanium; add aluminum granules for preliminary pre-deoxidation. Step S3, vacuum degassing treatment in the degassing furnace: vacuum degree ≤76Pa, vacuum holding time 10-15min; weak stirring of molten steel after breaking the vacuum; Step S4, Precision micro-alloying and final deoxidation: Complete the following steps in sequence. Final deoxidation of aluminum wire: control acid-soluble Al: 0.010~0.030%; Adjust the Ti content as needed; Add Ce-La mixed rare earth alloy; Gently stir with weak argon gas for 5-10 minutes, and simultaneously add B. Step S5, Protective casting: Casting temperature 1530~1560℃, liquidus temperature 1490~1510℃, casting temperature is 30~50℃ higher than liquidus temperature; long nozzle and argon gas seal for protective casting, casting into a continuously cast round billet; Step S6, Rolling and Heat Treatment: The continuously cast round billet is heated, pierced, rolled, and sized to obtain a rough tube, which is then heat-treated to obtain a finished CO2 corrosion-resistant seamless steel pipe.

[0014] Furthermore, in step S1, during tapping, the tapping temperature is 1620-1650℃; silicon-manganese alloy is added during tapping for pre-deoxidation; slag is blocked during tapping; small amounts of Cr, Mo, Ni, and Cu are added; and the electric arc furnace tapping temperature is controlled to P≤0.013%.

[0015] Furthermore, in step S2, the final control of the ladle refining furnace is as follows: the temperature inside the ladle refining furnace is 1630-1650℃, the composition of the molten steel is close to the target range for steel pipes, the white slag is stable, and the aluminum feed wire is initially deoxidized.

[0016] Compared with the prior art, the present invention, by adopting the above technical solution, has the following advantages: Ce-La mixed rare earth elements in steel are proportioned at a Ce-La mass ratio of (1–2.75):1, with the RE / S mass ratio strictly controlled between 1 and 4.5. Ce preferentially reacts with oxygen and sulfur in the molten steel, transforming and spheroidizing elongated MnS sulfide inclusions into fine, dispersed spherical rare earth oxygen-sulfur composite inclusions, thus eliminating the hazard of MnS as a pitting corrosion initiation source. La further improves the density and stability of the corrosion product film, with both elements working synergistically. Strict control of the RE / S stoichiometric ratio avoids insufficient rare earth leading to inadequate inclusion modification, while preventing excessive rare earth from generating large, hard, and brittle inclusions, thus mitigating the risk of nodule blockage in continuous casting nozzles.

[0017] Through precise alloy composition design, 2.50% to 3.50% Cr is used as the basic corrosion-resistant element, combined with the synergistic effect of Ni and Cu, to form a protective passivation film on the surface of the steel pipe, resisting electrochemical corrosion caused by CO2 and carbonic acid media; by reducing the Mo content, the strength and hardenability are compensated by the C, V, Nb, Ti, and B microalloying system, ensuring the strength and toughness of the steel pipe while controlling costs.

[0018] The boron content is strictly limited to 0.0005% to 0.0020%. Trace amounts of boron improve the hardenability of the steel while avoiding the risk of quenching cracks caused by boron segregation at grain boundaries. The three microalloying elements V, Nb, and Ti precipitate sequentially in different temperature ranges. Ti precipitates at high temperatures to form TiN / TiC, which inhibits austenite grain growth. Nb precipitates at medium temperatures to form Nb(C,N), which inhibits recrystallization. VC precipitates at low temperatures to achieve precipitation strengthening. This multi-scale grain refinement results in a microstructure dominated by tempered sorbite (≥99%) and supplemented by retained austenite (≤1%), ensuring the strength, plasticity, and low-temperature impact toughness of the steel pipe.

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

[0020] Figure 1 This is a flowchart of the manufacturing method of the CO2 corrosion resistant seamless steel pipe in this invention. Detailed Implementation

[0021] To provide a clearer understanding of the technical features, objectives, and effects of the present invention, specific embodiments of the present invention will now be described with reference to the accompanying drawings.

[0022] This invention provides a seamless steel pipe resistant to CO2 corrosion. The chemical composition of the steel, by weight percentage, is as follows: C: 0.22%~0.32%; Si: 0.25%~0.50%; Mn: 0.35%~0.65%; P: ≤0.015%; S: ≤0.008%; Cu: 0.05%~0.20%; Ni: 0.10%~0.25%; Cr: 2.50%~3.50%; Mo: 0.02%~0.60%; V: 0.02%~0.06%; Nb: 0.001%~0.050%; Ti: 0.001%~0.050%; B: 0.0005%~0.0080%; Al: 0.010%~0.030%; RE: 0.002%~0.030%; The RE is a Ce-La mixed rare earth element with a Ce-La mass ratio of (1-2.75):1; the RE / S mass ratio is 1-4.5; and the balance is Fe and unavoidable impurities.

[0023] Ce-La mixed rare earth elements in steel are proportioned at a Ce-La mass ratio of (1–2.75):1, with the RE / S mass ratio strictly controlled between 1 and 4.5. Ce preferentially reacts with oxygen and sulfur in the molten steel, transforming and spheroidizing elongated MnS sulfide inclusions into fine, dispersed spherical rare earth oxygen-sulfur composite inclusions, thus eliminating the hazard of MnS as a pitting corrosion initiation source. La further improves the density and stability of the corrosion product film, with both elements working synergistically. Strict control of the RE / S stoichiometric ratio avoids insufficient rare earth leading to inadequate inclusion modification, while preventing excessive rare earth from generating large, hard, and brittle inclusions, thus mitigating the risk of nodule blockage in continuous casting nozzles.

[0024] In the optimized embodiment, the mass percentage of Mo is 0.02% to 0.10%; reducing the upper limit of Mo content to 0.10% reduces the Mo content and significantly reduces costs.

[0025] In a further optimized embodiment, the mass percentage of B is 0.0005% to 0.0020%; the upper limit of B content is reduced to 0.0020% to avoid quenching cracks and improve toughness.

[0026] The CO2 corrosion resistant seamless steel pipe has a yield strength of 552-758 MPa, a tensile strength of 689-889 MPa, an elongation of ≥15%, a transverse full-size Charpy impact energy of ≥60 J or a longitudinal full-size Charpy impact energy of ≥80 J at 0℃, and a hardness of ≥18.0 HRC.

[0027] The microstructure of the seamless steel pipe includes tempered sorbite and retained austenite, wherein the mass percentage of tempered sorbite is ≥99% and the mass percentage of retained austenite is ≤1%.

[0028] The CO2-resistant seamless steel pipe was tested for 168 hours in 5 wt% NaCl aqueous solution at 80°C, CO2 partial pressure of 0.3 MPa, and stirring rate of 1.5 m / s, according to the ASTM G111 corrosion test guideline and the ASTM G31 metallic materials laboratory immersion corrosion test method. The average corrosion rate in the carbon dioxide environment was ≤0.050 mm / a.

[0029] This invention improves corrosion resistance through precise alloy composition design, particularly by optimizing the Cr content to a medium-high chromium range of 2.50–3.50%, while significantly reducing the Mo content to 0.02–0.10%, optimizing the synergistic ratio of elements such as Ni, V, Nb, and Ti, precisely controlling the B content to 0.0005–0.002% to avoid quenching cracking, and optimizing the RE addition amount to 0.002–0.030% to balance the inclusion modification effect with smelting operability.

[0030] The roles and proportions of each alloying element in this invention are based on the following: Carbon (C): Carbon is the most economical and fundamental strengthening element in steel, improving its strength through solid solution strengthening and precipitation strengthening. This invention controls the C content to 0.22–0.32%, ensuring sufficient strength while avoiding the decrease in toughness and corrosion resistance caused by excessively high carbon content. Simultaneously, it can compensate for strength loss even with a significant reduction in Mo content.

[0031] Si (Silicon): Silicon is an effective deoxidizing element in steelmaking and can also improve the strength of steel. This invention controls the Si content at 0.25-0.50%, which ensures good deoxidation and strengthening effects while avoiding excessive content that could increase the steel's tendency to become brittle.

[0032] Mn (manganese): Manganese has beneficial effects such as expanding the austenite phase region, increasing hardenability, and refining grain size. In this invention, the Mn content is controlled at 0.35-0.65%, which is beneficial for obtaining a good balance of strength and toughness.

[0033] P (phosphorus) and S (sulfur): Phosphorus and sulfur are harmful elements in steel, adversely affecting its corrosion resistance, hot workability, and toughness. This invention strictly controls P ≤ 0.015% and S ≤ 0.008% to ensure the purity and overall performance of the steel.

[0034] Cr (chromium): Chromium is the core corrosion-resistant element in this invention. Cr can form a dense protective oxide film on the surface of the steel pipe, thereby significantly improving the material's resistance to CO2 corrosion. This invention controls the Cr content to 2.50-3.50%, within which good CO2 corrosion resistance can be achieved, while avoiding the cost increase caused by excessively high Cr content.

[0035] Ni (Ni): Nickel can significantly improve the performance of the passivation film and enhance the corrosion resistance of steel. Simultaneously, the synergistic effect of Ni and Cr helps to further improve resistance to CO2 corrosion. This invention controls the Ni content at 0.10–0.25%, and through the synergistic substitution effect of Ni and Cr, it reduces the Mo content while maintaining corrosion resistance, further reducing alloy costs.

[0036] Mo (Mo): Molybdenum effectively increases the pitting corrosion resistance of steel and also has a good solid solution strengthening effect, which is beneficial to improving the overall performance of steel. Considering that Mo is a relatively expensive alloying element, this invention controls the Mo content at a low level (0.02-0.10%). Through the synergistic substitution effect of Ni and Cr and the increase of carbon content, the cost of the alloy is effectively reduced while ensuring corrosion resistance.

[0037] Cu (copper): Copper can improve the atmospheric corrosion resistance of steel. This invention controls the Cu content at 0.05% to 0.20%, which helps to improve corrosion resistance while avoiding cracks during hot working caused by excessive Cu content.

[0038] Vanadium (V), niobium (Nb), and titanium (Ti): all three are microalloying elements that effectively refine grains and improve steel strength through precipitation strengthening. This invention adds all three elements simultaneously, utilizing their precipitation characteristics at different temperature ranges—TiN / TiC precipitates in the high-temperature range (>1200℃), inhibiting austenite grain growth; Nb(C,N) precipitates in the medium-temperature range (~1000℃), delaying recrystallization; and VC precipitates in the low-temperature range (~700℃), producing precipitation strengthening—achieving grain refinement and precipitation strengthening effects across the entire temperature range. This invention controls V at 0.02–0.06%, Nb at 0.001–0.05%, and Ti at 0.001–0.05%. The synergistic effect of these three elements results in a fine grain structure and a good balance of strength and toughness, while effectively controlling alloy costs.

[0039] Boron (B): Trace amounts of boron can significantly improve the hardenability of steel. However, excessive boron content can lead to the formation of low-melting-point eutectics at grain boundaries, significantly increasing the steel's susceptibility to quenching cracking and negatively impacting toughness. This invention strictly controls the boron content within an extremely low range of 0.0005–0.002% (5–20 ppm), ensuring a stable improvement in hardenability while effectively avoiding the risk of cracking and toughness loss caused by excessive boron addition.

[0040] Al (aluminum): Aluminum is an important deoxidizing element in the steelmaking process and also helps to refine grains. In this invention, the Al content is controlled at 0.010–0.030%.

[0041] RE (Rare Earth Elements): The rare earth elements added in this invention are a mixture of cerium (Ce) and lanthanum (La), with the Ce:La weight ratio controlled at (1-2.75):1. Ce has higher chemical activity than La, and is more effective in purifying molten steel and removing modified inclusions; La, on the other hand, has certain advantages in improving the density of corrosion product films. When mixed in an appropriate ratio, the two can exert a synergistic effect: Ce preferentially reacts with oxygen and sulfur, transforming elongated MnS inclusions into fine, dispersed spherical rare earth oxysulfides; La further promotes the stability of the protective corrosion product layer.

[0042] The preferred ratio of rare earth elements (RE) to sulfur (S) is (1–4.29):1. Because rare earth elements have a strong affinity for sulfur, if the RE / S ratio is too low, the sulfide inclusions will not fully deteriorate; if the RE / S ratio is too high, excess rare earth elements can easily form large inclusions, which can impair toughness and corrosion resistance, and increase the risk of nozzle blockage. Controlling the RE / S ratio at (1–4.29):1 achieves stoichiometric matching, ensuring sufficient deterioration of the sulfides.

[0043] This invention controls the total amount of RE (Ce+La) at 0.002-0.030%, which takes into account the fluctuation of rare earth burn-off rate during electric furnace smelting, and avoids the problem of increased cost and continuous casting nozzle blockage caused by excessive addition.

[0044] Table 1 below shows the specific chemical composition and content of five embodiments of the seamless steel pipe of the present invention:

[0045] Table 2 below shows the test results of mechanical properties and CO2 corrosion resistance of the seamless steel pipes prepared in Examples 1-5:

[0046] Table 3 below shows the specific chemical composition and content of two comparative seamless steel pipes:

[0047] Table 4 below shows the mechanical properties and CO2 corrosion resistance tests of the prepared comparative seamless steel pipes. The results are as follows:

[0048] As shown in Table 2, the seamless steel pipes obtained in Examples 1-5 of this invention have a yield strength of 565-740 MPa, a tensile strength of 695-825 MPa, an elongation of 18-26%, a longitudinal Charpy impact energy of 85-123 J at 0℃ (10*3.3*55 mm), and a hardness of 18.5-24.0 HRC, all meeting the design requirements. They comprehensively cover the yield strength range of 552-758 MPa and the tensile strength range of 689-889 MPa, exhibiting a good balance of strength and toughness. Under simulated CO2 corrosion conditions (80℃, 5wt% NaCl solution, CO2 partial pressure 0.3 MPa, 168 h), the uniform corrosion rate of each example is extremely low (0.030-0.040 mm / a), better than the requirement of ≤0.050 mm / a, indicating that the steel pipes described in this invention have excellent CO2 corrosion resistance.

[0049] The comparison between Comparative Example 1 and Example 1 shows that without RE, the corrosion rate increased from 0.036 mm / a to 0.058 mm / a, exceeding the requirement of ≤0.050 mm / a in this invention, proving that the addition of RE has a significant contribution to the modification of inclusions and the improvement of corrosion resistance.

[0050] A comparison between Comparative Example 2 and Example 1 shows that when the Mo content increases from 0.06% to 0.30%, the yield strength increases from 585 MPa to 630 MPa, and the corrosion rate decreases from 0.036 mm / a to 0.024 mm / a, while the Mo addition increases by 400%. This demonstrates that the improvement in corrosion resistance is limited when the Mo content exceeds 0.24%, but the cost increases significantly, thus proving the rationality and economic efficiency of controlling the Mo content at 0.02–0.10% in this invention.

[0051] The manufacturing method of the above-mentioned CO2 corrosion resistant seamless steel pipe includes the following steps: Step S1, rough refining in an electric arc furnace: Raw material selection: Use high-quality scrap steel with low phosphorus and low sulfur to adjust carbon content with pig iron; control the residual Cu and Ni content in raw materials; do not add large amounts of Cr, Mo, V, and Nb alloys during the electric arc furnace stage; Melting oxidation: Add lime and fluorite to create oxidizing slag, raise the temperature to 1580-1620℃; blow oxygen to dephosphorize, slag basicity R≥2.5; control the carbon content (C) at the final tapping point to be ≥0.15%; Step S2, refining in a ladle refining furnace: To produce reducing white slag: lime, fluorite, and aluminum slag are added, and the slag basicity R = 2.8–3.5; submerged arc heating is used for SiC and aluminum particle diffusion deoxidation; the white slag is maintained for ≥25 minutes, and desulfurization is achieved to S≤0.008%; Add alloys in batches: In the early and middle stages of slag stabilization and steel purity improvement, add ferrochrome, ferromolybdenum, nickel plate, cathode copper, ferrovanadium, and ferroniobium to the ladle; after the slag stabilizes and the purity of the molten steel is improved, add ferrotitanium and stir for 3-5 minutes after adding ferrotitanium; add aluminum granules for preliminary pre-deoxidation. Step S3, vacuum degassing treatment in the degassing furnace: vacuum degree ≤76Pa, vacuum holding time 10-15min; weak stirring of molten steel after breaking the vacuum; Step S4, Precision micro-alloying and final deoxidation: Complete the following steps in sequence. Final deoxidation of aluminum wire: control acid-soluble Al: 0.010~0.030%; Adjust the Ti content as needed; Add Ce-La mixed rare earth alloy; Gently stir with weak argon gas for 5-10 minutes, and simultaneously add B. Step S5, Protective casting: Casting temperature 1530~1560℃, liquidus temperature 1490~1510℃, casting temperature is 30~50℃ higher than liquidus temperature; long nozzle and argon gas seal for protective casting, casting into a continuously cast round billet.

[0052] Step S6, Rolling and Heat Treatment: The continuously cast round billet is heated, pierced, rolled, and sized to obtain a rough tube, which is then heat-treated to obtain a finished CO2 corrosion-resistant seamless steel pipe.

[0053] In step S1, the tapping temperature is 1620–1650℃; silicon-manganese alloy is added during tapping for pre-deoxidation; slag is blocked during tapping; small amounts of Cr, Mo, Ni, and Cu are added; and P is controlled to ≤0.013% during electric arc furnace tapping. In step S2, the final control of the ladle refining furnace is as follows: the temperature inside the ladle refining furnace is 1630-1650℃, the composition of the molten steel is close to the target range for steel pipes, the white slag is stable, and the aluminum feed wire is initially deoxidized.

[0054] This preparation method employs an electric arc furnace, a ladle refining furnace, and a degassing furnace smelting route. The electric arc furnace stage controls dephosphorization and prevents excessive oxidation of molten steel. The ladle refining furnace performs deep desulfurization by reducing white slag. The degassing furnace smelting furnace removes hydrogen and nitrogen under vacuum, reducing the gas content in the steel, decreasing the precipitation of TiN and BN, and improving the effective utilization rate of boron. The alloys are added in stages and sequentially, with precious alloys added under a reducing atmosphere, which significantly improves the yield of Cr, Mo, V, and Nb alloys and reduces smelting costs.

[0055] Rare earth, boron, aluminum, titanium and other easily oxidized alloys are all smelted in the degassing furnace within a short time after the furnace is vented. With the help of weak argon gas for soft stirring, secondary oxidation of the molten steel is suppressed, inclusions are promoted to float, the purity of the molten steel is high and the composition control is highly precise.

[0056] The continuous casting round billets prepared by this method, after rolling and heat treatment, produce seamless steel pipes with excellent comprehensive mechanical properties. The rare earth elements modify the inclusions, and with a reasonable alloy ratio, the products have excellent resistance to CO2 corrosion and are suitable for use in CO2 corrosion conditions in oil and gas fields.

[0057] The above description provides examples of the preferred embodiments of the present invention, and any parts not described in detail are common knowledge to those skilled in the art. The scope of protection of the present invention is determined by the claims, and any equivalent modifications made based on the technical teachings of the present invention are also within the scope of protection of the present invention.

Claims

1. A seamless steel pipe resistant to CO2 corrosion, characterized in that: The chemical composition of steel, by mass percentage, is as follows: C: 0.22%~0.32%; Si: 0.25%~0.50%; Mn: 0.35%~0.65%; P: ≤0.015%; S: ≤0.008%; Cu: 0.05%~0.20 %; Ni: 0.10% ~ 0.25%; Cr: 2.50% ~ 3.50%; Mo: 0.02% ~ 0.60%; V: 0.02% ~ 0.06%; Nb: 0.001% ~ 0.050%; Ti: 0.001%~0.050%; B：0.0005%～0.0080%； Al：0.010%～0.030%； RE: 0.002%~0.030%; The RE is a Ce-La mixed rare earth element with a Ce-La mass ratio of (1-2.75):1; the RE / S mass ratio is 1-4.5; and the balance is Fe and unavoidable impurities.

2. The CO2 corrosion-resistant seamless steel pipe as described in claim 1, characterized in that: The mass percentage of Mo is 0.02% to 0.10%.

3. The CO2 corrosion-resistant seamless steel pipe as described in claim 2, characterized in that: The mass percentage of B is 0.0005% to 0.0020%.

4. The CO2 corrosion-resistant seamless steel pipe as described in claim 3, characterized in that: The CO2 corrosion resistant seamless steel pipe has a yield strength of 552-758 MPa, a tensile strength of 689-889 MPa, an elongation of ≥15%, a transverse full-size Charpy impact energy of ≥60 J or a longitudinal full-size Charpy impact energy of ≥80 J at 0℃, and a hardness of ≥18.0 HRC.

5. The method for manufacturing a CO2-resistant seamless steel pipe as described in any one of claims 1 to 4, characterized in that: Includes the following steps, Step S1, rough refining in an electric arc furnace: Raw material selection: Use high-quality scrap steel with low phosphorus and low sulfur to adjust carbon content with pig iron; control the residual Cu and Ni content in raw materials; do not add large amounts of Cr, Mo, V, and Nb alloys during the electric arc furnace stage; Melting oxidation: Add lime and fluorite to create oxidizing slag, raise the temperature to 1580-1620℃; blow oxygen to dephosphorize, slag basicity R≥2.5; control the carbon content (C) at the final tapping point to be ≥0.15%; Step S2, refining in a ladle refining furnace: To produce reducing white slag: lime, fluorite, and aluminum slag are added, and the slag basicity R = 2.8–3.5; submerged arc heating is used for SiC and aluminum particle diffusion deoxidation; the white slag is maintained for ≥25 minutes, and desulfurization is achieved to S≤0.008%; Add alloys in batches: In the early and middle stages of slag stabilization and steel purity improvement, add ferrochrome, ferromolybdenum, nickel plate, cathode copper, ferrovanadium, and ferroniobium to the ladle; after the slag stabilizes and the purity of the molten steel is improved, add ferrotitanium and stir for 3-5 minutes after adding ferrotitanium; add aluminum granules for preliminary pre-deoxidation. Step S3, vacuum degassing treatment in the degassing furnace: vacuum degree ≤76Pa, vacuum holding time 10-15min; weak stirring of molten steel after breaking the vacuum; Step S4, Precision micro-alloying and final deoxidation: Complete the following steps in sequence. Final deoxidation of aluminum wire: control acid-soluble Al: 0.010~0.030%; Adjust the Ti content as needed; Add Ce-La mixed rare earth alloy; Gently stir with weak argon gas for 5-10 minutes, and simultaneously add B. Step S5, Protective casting: Casting temperature 1530~1560℃, liquidus temperature 1490~1510℃, casting temperature is 30~50℃ higher than liquidus temperature; long nozzle and argon gas seal for protective casting, casting into a continuously cast round billet; Step S6, Rolling and Heat Treatment: The continuously cast round billet is heated, pierced, rolled, and sized to obtain a rough tube, which is then heat-treated to obtain a finished CO2 corrosion-resistant seamless steel pipe.

6. The method for manufacturing a CO2-resistant seamless steel pipe as described in claim 5, characterized in that: In step S1, the tapping temperature is 1620-1650℃; silicon-manganese alloy is added during tapping for pre-deoxidation; slag is blocked during tapping; small amounts of Cr, Mo, Ni, and Cu are added; and P is controlled to be ≤0.013% during electric arc furnace tapping.

7. The method for manufacturing a CO2-resistant seamless steel pipe as described in claim 5, characterized in that: In step S2, the final control of the ladle refining furnace is as follows: the temperature inside the ladle refining furnace is 1630-1650℃, the composition of the molten steel is close to the target range for steel pipes, the white slag is stable, and the aluminum feed wire is initially deoxidized.

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