Economical tubing casing alloy steel resistant to carbon dioxide and hydrogen sulfide corrosion and its preparation process

Abstract

This invention discloses an economical alloy steel for oil pipe casing resistant to carbon dioxide and hydrogen sulfide corrosion and its preparation process. The chemical composition by weight percentage is as follows: carbon: ≤0.08%, chromium: 10.2%-13.8%, molybdenum: 0.8-1.0%, nickel: 3.2-4.8%, vanadium: 0.05-0.10%, copper: ≤0.20%, niobium: ≤0.05%, manganese: ≤1.0%, silicon: ≤0.5%, sulfur: ≤0.001%, phosphorus: ≤0.010%, calcium: 0.0015-0.0030%, nitrogen: ≤0.01%, with the remainder being iron and unavoidable impurities. This invention integrates excellent resistance to carbon dioxide corrosion, good resistance to sulfide stress cracking (SSC), and high strength and toughness. The optimized "Cr-Mo-Ni-V" low-alloy system optimizes the ratio between composite alloying elements and leverages the synergistic effect of the composite alloy while controlling the amount of precious elements such as Ni and Mo. The well-developed grain structure transformed through subsequent hot rolling and heat treatment processes provides a stable corrosion-resistant matrix. This allows the material to achieve corrosion resistance that surpasses traditional 13Cr and rivals that of higher alloy steels, while significantly reducing costs.

Description

Technical Field

[0001] This invention relates to the field of oil and gas technology, specifically to an economical alloy steel for oil pipe casing that is resistant to carbon dioxide and hydrogen sulfide corrosion, and its preparation process. Background Technology

[0002] Alloy steel tubing refers to the use of alloy steel materials for oil well tubing in the oil and gas industry. In order to cope with harsh working conditions such as deep wells, ultra-deep wells, high pressure, and corrosive environments, ordinary carbon steel cannot meet the requirements, so various grades of alloy steel are needed.

[0003] Due to its general purpose, the existing API standard oil casing relies on a loose composition range and conventional manufacturing process when dealing with harsh environments containing carbon dioxide and hydrogen sulfide. This results in inherent deficiencies in material purity, microstructure uniformity, and residual stress control, leading to large performance fluctuations. Secondly, the control of harmful elements such as sulfur and phosphorus in steel is not strict enough. High S and P content will form non-metallic inclusions, which will become the initiation point of hydrogen embrittlement cracks and seriously reduce the resistance to SSC. Finally, to meet the high strength required for deep wells, the steel needs to have a higher carbon equivalent or be subjected to intense quenching. However, this will significantly reduce the toughness and plasticity of the material and increase internal stress, making the steel extremely brittle in CO2 and H2S environments. High strength and anti-SSC performance are inherently contradictory, and the API standard has not set an optimal balance point for acidic environments in this contradiction.

[0004] Therefore, it is necessary to design and modify the alloy steel for oil pipe casing that is economical and resistant to carbon dioxide and hydrogen sulfide corrosion, as well as its manufacturing process. Summary of the Invention

[0005] To address the problems mentioned in the background art, the present invention aims to provide an economical alloy steel for oil pipe casing that is resistant to carbon dioxide and hydrogen sulfide corrosion, and its preparation process, which has the advantage of improving resistance to carbon dioxide corrosion.

[0006] To achieve the above objectives, the present invention provides the following technical solution: an economical alloy steel for oil pipe casing resistant to carbon dioxide and hydrogen sulfide corrosion and its preparation process, wherein the chemical composition by weight percentage is: carbon: ≤0.08%, chromium: 10.2%-13.8%, molybdenum: 0.8-1.0%, nickel: 3.2-4.8%, vanadium: 0.05-0.10%, copper: ≤0.20%, niobium: ≤0.05%, manganese: ≤1.0%, silicon: ≤0.5%, sulfur: ≤0.001%, phosphorus: ≤0.010%, calcium: 0.0015-0.0030%, nitrogen: ≤0.01%, with the remainder being iron and unavoidable impurities, the total content of impurity elements being less than 0.03wt%.

[0007] As a preferred embodiment of the present invention, an economical alloy steel for oil pipe casing resistant to carbon dioxide and hydrogen sulfide corrosion and its preparation process include the following steps: Step a) Ultra-pure smelting: First, the initial smelting is completed in an electric arc furnace to rapidly melt the required proportions of each component and perform preliminary dephosphorization. Then, the molten steel is transferred to a ladle furnace for deep refining to achieve deep desulfurization and dephosphorization, while simultaneously achieving micro-alloying of niobium and copper. After that, the molten steel enters a vacuum degassing device to remove hydrogen and oxygen in order to achieve the gas and impurity control targets set in the formula.

[0008] Step b) Billet preparation: Using full protective casting technology, molten steel flows from the ladle into the tundish through the long nozzle, and then from the tundish into the crystallizer through the submerged entry nozzle. After the molten steel crystallizes and cools, a continuous round billet is finally obtained.

[0009] Step c) Hot working: The round billet is first fed into the annular heating furnace for uniform heating to ensure that chromium, molybdenum, niobium and manganese are fully dissolved. After heating, the billet is descaled by high pressure water to obtain a clean surface, and then enters the piercing mill. The pierced tube enters the limited-movement mandrel continuous rolling mill for rolling to refine the grains. The rolled rough tube immediately enters the reheating furnace for uniform temperature to prepare for subsequent sizing and to prevent the precipitation of harmful phases.

[0010] Step d) Heat treatment: In order to activate the synergistic corrosion resistance potential of chromium, molybdenum, nickel, vanadium and manganese in the formula and obtain a tempered structure with both strength and toughness, the heat treatment is carried out on a continuous line. First, the steel pipe enters the roller hearth quenching furnace to fully dissolve the carbides. Then, the steel pipe is quickly fed into the high-efficiency quenching system. After quenching, the steel pipe immediately enters the tempering furnace. After tempering, it is cooled by air cooling.

[0011] Step e) Finishing and straightening: The heat-treated steel pipe first enters the online hot straightening machine. While ensuring that the temperature is still higher than 550℃, it immediately enters the online hot straightening machine. The final straightening temperature is ≥480℃. The steel pipe immediately enters the internal and external cooling system for cooling. After the steel pipe is completely cooled, an alloy steel pipe for oil pipe sleeves that is resistant to carbon dioxide and hydrogen sulfide corrosion can be obtained.

[0012] As a preferred embodiment of the present invention, step a) ultra-pure smelting involves a vacuum degassing device that circulates the steel under a high vacuum of ≤67Pa for 15-20 minutes. The vacuum degassing device stably controls hydrogen at ≤1.5ppm and oxygen at ≤15ppm. The entire process is carried out with argon blowing and stirring to ensure uniform composition and temperature. The tapping temperature is precisely controlled within ±5℃ of the target casting temperature. All raw materials, auxiliary materials, and refractory materials must be low-sulfur and low-hydrogen varieties. During the deep refining process of the molten steel in the ladle furnace, high-alkalinity reducing slag is created by adding active lime, fluorite, etc. At a high temperature of 1600-1650℃, strong diffusion deoxidation and desulfurization conditions are used to reduce the sulfur content in the steel from ≤0.006% at the time of entry to the target value of ≤0.001%, with a desulfurization efficiency of over 80%. At the same time, phosphorus is further reduced to ≤0.010%.

[0013] As a preferred embodiment of the present invention, step a) ultra-pure smelting: at the end of the refining process, calcium-iron alloy wire with a Ca content ≥30% is fed into the depth of the ladle at a speed of 80-120 meters per minute using a wire feeder. The amount of calcium added is controlled between 0.0015% and 0.0030%. The purpose is to modify the residual sulfur and oxides by transforming the high-melting-point Al2O3 and harmful MnS inclusions into a low-melting-point (1400℃) spherical calcium aluminate 12CaO·7Al2O3 composite inclusion encapsulating CaS.

[0014] As a preferred embodiment of the present invention, in step b) billet preparation: the depth of molten steel immersed in the crystallizer is controlled at 120-150mm; a pre-melted slag with low viscosity and high melting rate is used as the protective slag to ensure uniform lubrication and heat transfer; the fluctuation of the liquid level in the crystallizer is stabilized within ±3mm by an automatic control system; the difference between the casting temperature of the molten steel and its liquidus is controlled within the range of 15-30℃; too low a superheat can easily lead to nozzle blockage, while too high a superheat can lead to coarse columnar crystals and increased central segregation; the crystallizer cooling adopts a weak cooling mode, and the specific water volume is controlled at 0.8-1.2L / kg to form... A sufficiently thick and uniform initial billet shell is formed. At the end of solidification, a light reduction technique is applied, with the total reduction controlled at 4-8 mm to compensate for solidification shrinkage and suppress central shrinkage cavities and V-shaped segregation. The casting stream adopts a dynamic water distribution model in the secondary cooling zone, with the total cooling intensity controlled at 0.8-1.0 L / kg to ensure that the surface temperature of the billet is above 950℃ at the straightening point and to avoid straightening cracks. Argon gas is used to seal the molten steel long nozzle and the tundish cover, with the sealing argon gas flow rate controlled at 5-10 NL / min. The tundish covering agent is an alkaline carbon-free material, and the space above the liquid surface is filled with argon gas to isolate air.

[0015] As a preferred embodiment of the present invention, step c) hot working: The rolling process is completed continuously by 7-9 stands. The online wall thickness measuring instrument included in the hydraulic automatic thickness control system of the tube rolling mill provides real-time feedback data, and the roll gap of each stand is dynamically adjusted to strictly control the wall thickness deviation of the finished rough tube within ±2% of the nominal wall thickness, the ellipticity is controlled within ≤0.6%×outer diameter, and the final rolling temperature is controlled within the range of 880-950℃ to ensure sufficient recrystallization of grains and refinement of microstructure. The heating regime adopts a three-level control: the preheating section is slowly heated to ≤800℃ to prevent thermal stress cracking; the heating section is heated to 1180-1230℃ to ensure thorough heating of the core of the billet; the homogenization section is held at 1230±10℃ for 1.5-2.0 hours to ensure uniform temperature and complete austenite homogenization. When tapping the steel, the temperature difference between the surface and the core of the billet must be ≤30℃.

[0016] As a preferred embodiment of the present invention, step d) heat treatment: high-purity nitrogen gas with a purity ≥99.995% is introduced into the furnace to maintain a slightly positive pressure of 150-300Pa, forming a weakly reducing atmosphere. This ensures that the steel pipe surface is bright, free of oxidation, and the decarburized layer depth is ≤0.05mm. The steel pipe is uniformly heated to 1020±10℃ and held at this temperature for 1.2-1.5 minutes per millimeter of wall thickness to ensure that the alloying elements are fully dissolved and the microstructure is completely austenitized. The entire heat treatment process is monitored by multiple thermocouples and infrared thermometers, and the temperature recording curve is traceable. The high-efficiency quenching system adopts internal spray and external spray high-pressure water quenching. The water pressure of the internal nozzle is ≥1.0MPa and the water curtain pressure of the external ring seam is ≥0.8MPa, ensuring that the steel pipe passes through the martensitic transformation zone quickly within a few seconds to obtain a complete martensitic structure. The core hardness can reach HRC40 or above. The tempering furnace temperature is controlled within the range of 730±5℃, and the holding time is calculated at 2.0-2.5 minutes per millimeter of wall thickness. This high-temperature tempering process allows the quenched martensite to decompose fully and carbides to precipitate diffusely, forming a uniform and stable tempered sorbite structure.

[0017] As a preferred embodiment of the present invention, step e) finishing and straightening: the straightening machine adopts a six-roll or ten-roll arrangement, and the pressure and deflection of each roll are precisely controlled by a hydraulic servo system. The straightening temperature is precisely controlled by the moving speed of the cooling bed before straightening, ensuring that the straightening temperature is ≥480℃, so that the residual stress inside the material after straightening is ≤5%*the specified yield strength, thereby improving the corrosion resistance of the material.

[0018] Compared with the prior art, the beneficial effects of the present invention are as follows: 1. This invention integrates excellent resistance to carbon dioxide and hydrogen sulfide corrosion, good resistance to sulfide stress cracking (SSC), and high strength and toughness. The optimized "Cr-Mo-Ni-V" medium-low alloy system provides a stable corrosion-resistant matrix through synergistic effects while controlling the amount of precious elements such as Ni and Mo. The entire process, including ultra-pure smelting, fully protected continuous casting, controlled rolling and cooling, and precise heat treatment, ensures extremely high purity of the steel, uniform and fine tempered sorbite structure, and precise dimensional control. This allows the material to achieve corrosion resistance that surpasses traditional 13Cr and rivals that of higher alloy steels, while significantly reducing costs and resolving the contradiction between high strength, high corrosion resistance, and economy.

[0019] 2. This invention employs a continuous and controllable process flow of ultra-pure smelting, high-quality billet preparation, controlled rolling and cooling, online quenching and high-temperature tempering, and finishing and straightening. Ultra-pure smelting, through deep desulfurization, phosphorus treatment, vacuum degassing, and calcium treatment, controls harmful elements and inclusions to extremely low levels, laying the foundation for high performance. Billet preparation utilizes techniques such as fully protected casting and light reduction to obtain continuously cast round billets with high internal quality. Hot working refines the microstructure and controls the morphology through precise controlled rolling and cooling. Heat treatment, through high-temperature quenching and high-temperature tempering, obtains a uniform tempered sorbite microstructure, optimizing the strength-toughness match. Finally, finishing and straightening are performed at high temperatures with simultaneous cooling to minimize residual stress, achieving full-process optimization of performance and stability from smelting to finished product.

[0020] 3. This invention achieves a performance breakthrough through the synergistic effect of composition and process. The extremely low sulfur and phosphorus content, along with calcium treatment and modification, spheroidizes and refines harmful inclusions such as MnS, fundamentally reducing the origin of hydrogen-induced cracks. The combination of niobium microalloying and controlled rolling process refines the original austenite grains, improving the strength and toughness of the material. The addition of copper promotes the precipitation of copper-rich phases under the corrosion product film, enhancing the stability and re-repair capability of the film. The heat treatment process ensures that corrosion-resistant elements such as Cr and Mo are fully dissolved and form a uniform and stable tempered sorbite structure. This structure has high strength, high toughness, and low internal stress, which is the key to achieving both high strength and excellent anti-SSC performance.

[0021] 4. This invention achieves ultimate control over gas and inclusions from the source through ultra-pure smelting and fully protective casting, resulting in a clean steel matrix. Controlled rolling and light reduction technologies promote the uniformity and density of the billet structure, eliminating macroscopic defects. An ideal microstructure is obtained through phase transformation, thereby transforming the material's potential into high strength, high toughness, and high corrosion resistance, ensuring high reliability and reproducibility of product performance. This provides an economical and safe pipe solution in harsh environments where CO2 and trace amounts of H2S coexist. Detailed Implementation

[0022] The technical solutions in the embodiments of the present invention will be clearly and completely described below. 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 skilled in the art without creative effort are within the scope of protection of the present invention.

[0023] Preparation Example 1: An economical alloy steel for oil pipe casing resistant to carbon dioxide corrosion and its preparation process: An economical alloy steel for oil pipe casing resistant to carbon dioxide corrosion, characterized by its chemical composition by weight percentage as follows: carbon: ≤0.08%, chromium: 10.2%-13.8%, molybdenum: 0.8-1.0%, nickel: 3.2-4.8%, vanadium: 0.05-0.10%, copper: ≤0.20%, niobium: ≤0.05%, manganese: ≤1.0%, silicon: ≤0.5%, sulfur: ≤0.001%, phosphorus: ≤0.010%, calcium: 0.0015-0.0030%, nitrogen: ≤0.01%, with the remainder being iron and unavoidable impurities, the total content of impurity elements being less than 0.03 wt%. Its preparation process includes the following steps: Step a) Ultra-pure smelting: Primary smelting is completed in an electric arc furnace, rapidly melting scrap steel and molten iron and performing preliminary dephosphorization (P≤0.015%). The molten steel is then transferred to a ladle furnace for deep refining. High-basicity (R≥3.5) reducing slag is created by adding active lime, fluorite, etc., and strong diffusion deoxidation and deep desulfurization are carried out at 1600-1650℃, reducing sulfur to ≤0.001% and phosphorus to ≤0.010%. Subsequently, the molten steel enters a vacuum degassing device, where it is circulated under a high vacuum of ≤67Pa for 15-20 minutes, stabilizing hydrogen at ≤1.5ppm and oxygen at ≤15ppm. At the end of refining, calcium-iron alloy wire (Ca addition 0.0015%-0.0030%) is fed in via a wire feeder at a rate of 80-120 m / min to morphologically modify inclusions. The tapping temperature is precisely controlled within ±5℃ of the target casting temperature.

[0024] Step b) Billet Preparation: A fully protective casting technique is employed. The long nozzle and tundish cover are sealed with argon gas (5-10 NL / min), and the space above the tundish liquid surface is filled with argon. Molten steel enters the crystallizer through a submerged entry nozzle (immersion depth 120-150 mm), using a low-viscosity, high-melting-rate protective slag. Liquid level fluctuations in the crystallizer are stabilized within ±3 mm, and superheat is controlled at 15-30℃. A weak cooling mode (specific water flow rate 0.8-1.2 L / kg) is used. At the end of solidification (liquid core ratio 0.3-0.5), a light reduction technique (total reduction 4-8 mm) is applied. Dynamic water distribution is used in the secondary cooling zone, with a total cooling intensity of 0.8-1.0 L / kg, ensuring the billet surface temperature at the straightening point is >950℃, resulting in a high-quality continuous round billet.

[0025] Step c) Hot working: The round billet is heated in a ring furnace using a three-stage heating regime: a preheating section (≤800℃) for slow heating; a heating section (1180-1230℃) for thorough homogenization; and a homogenization section (1230±10℃) for holding for 1.5-2.0 hours to ensure a billet temperature difference ≤30℃. After heating, it undergoes high-pressure water descaling (≥18MPa), and the wall thickness uniformity of the pierced tube is ≤8%. It then enters a mandrel continuous rolling mill (7-9 stands) for rolling, with real-time adjustments via a hydraulic automatic thickness control system (AGC) to control the wall thickness deviation of the rough tube within ±2%, and the ellipticity ≤0.6%×outer diameter. The final rolling temperature is controlled at 880-950℃. After rolling, the rough tube enters a reheating furnace for homogenization.

[0026] Step d) Heat treatment: In a controlled atmosphere (a mixture of high-purity nitrogen and 2-5% hydrogen, with a slight positive pressure of 150-300 Pa), the steel pipe is heated to 1020±10℃ and held at that temperature (calculated based on a wall thickness of 1.2-1.5 min / mm) to achieve complete austenitization with a decarburized layer ≤0.05mm. Subsequently, high-pressure water quenching (internal spray water pressure ≥1.0MPa, external spray pressure ≥0.8MPa) is used to rapidly obtain a fully martensitic structure. Immediately after quenching, the pipe is held in a tempering furnace at 730±5℃ (calculated based on a wall thickness of 2.0-2.5 min / mm) to form a uniform tempered sorbite structure, followed by air cooling.

[0027] Step e) Finishing and straightening: The heat-treated steel pipe first enters the online hot straightening machine. While ensuring that the temperature is still higher than 550℃, it immediately enters the online hot straightening machine. The final straightening temperature is ≥480℃. The steel pipe immediately enters the internal and external cooling system for cooling. After the steel pipe is completely cooled, an alloy steel pipe for oil pipe sleeves that is resistant to carbon dioxide and hydrogen sulfide corrosion can be obtained. Example

[0028] This embodiment provides an economical alloy steel for oil pipe casing that is resistant to carbon dioxide corrosion and its manufacturing process. The process parameters used are the midpoint values ​​of various ranges, and specifically include the following steps: The raw materials were prepared and smelted according to the median chemical composition described in Preparation Example 1. All steps described in Preparation Example 1 were performed, with the following key parameters taken as median values: vacuum degassing pressure 67 Pa, processing time 18 minutes; calcium addition 0.0023%; casting superheat 22°C; crystallizer specific water volume 1.0 L / kg, total reduction under light pressing 6 mm; billet heating soaking zone temperature 1250°C, holding time 1.75 hours; final rolling temperature 915°C; quenching heating temperature 1020°C, holding time (based on wall thickness) 1.35 min / mm; tempering temperature 730°C, holding time (based on wall thickness) 2.25 min / mm; hot straightening temperature 550°C. Finally, alloy steel tubing for oil tubing was obtained that met the target composition and performance requirements. Example

[0029] This embodiment provides an economical alloy steel for tubing casing that is resistant to carbon dioxide corrosion and its manufacturing process. The process parameters used are combinations of the lower limits of various ranges, which are suitable for working conditions where the requirements for corrosion resistance and mechanical properties are relatively relaxed and cost control is more stringent. The specific steps include: The raw materials were prepared and smelted according to the lower limit of the chemical composition described in Preparation Example 1, and all the steps described in Preparation Example 1 were performed. The key parameters were taken at the lower limit: vacuum degassing pressure 67 Pa, processing time 15 minutes; calcium addition 0.0015%; casting superheat 15℃; crystallizer specific water volume 0.8 L / kg, total reduction under light pressure 4 mm; billet heating soaking zone temperature 1240℃, holding time 1.5 hours; final rolling temperature 880℃; quenching heating temperature 1010℃, holding time (based on wall thickness) 1.2 min / mm; tempering temperature 675℃, holding time (based on wall thickness) 2.0 min / mm; hot straightening temperature 500℃, and finally, qualified alloy steel pipes for oil tubing with a more cost-effective advantage were obtained. Example

[0030] This embodiment provides an economical alloy steel for tubing casing that resists carbon dioxide corrosion and its manufacturing process. The process parameters used are combinations of the upper limits of various ranges, suitable for deep and ultra-deep well conditions with extremely stringent requirements for resistance to carbon dioxide corrosion and mechanical properties. The specific steps include: The raw materials were prepared and smelted according to the upper limit of the chemical composition described in Preparation Example 1, and all the steps described in Preparation Example 1 were performed. The key parameters were taken at the upper limit: vacuum degassing pressure <67Pa, processing time 20 minutes; calcium addition 0.0030%; casting superheat 30℃; crystallizer specific water volume 1.2L / kg, total reduction under light pressure 8mm; billet heating soaking zone temperature 1230℃, holding time 2.0 hours; final rolling temperature 950℃; quenching heating temperature 1030℃, holding time (based on wall thickness) 1.5min / mm; tempering temperature 685℃, holding time (based on wall thickness) 2.5min / mm; hot straightening temperature 600℃, and finally high-performance alloy steel pipe for oil tubing was obtained.

[0031] Comparative Example 1: Compared with Example 1, the difference is that in step a) ultrapure smelting, the calcium treatment process is omitted (calcium-iron wire is not fed), while the remaining steps and parameters are exactly the same as in Example 1. The aim is to compare the effects of calcium treatment on inclusion morphology control and performance.

[0032] Comparative Example 2: Compared to Example 1, the difference lies in the following: in step b) of billet preparation, the fully protected casting technique and the light reduction technique are not used; instead, ordinary open casting and conventional cooling processes are employed. The remaining steps and parameters are identical to those in Example 1. The aim is to compare the impact of billet purity and internal quality on the performance of the finished product.

[0033] Comparative Example 3: Compared with Example 1, the difference is that in step d) heat treatment, after quenching, high-temperature tempering at 730℃ is not performed, but instead low-temperature tempering at 300℃ is performed. The remaining steps and parameters are exactly the same as in Example 1. The aim is to compare the differences in corrosion resistance and toughness between tempered sorbite and tempered martensite / bainite structures.

[0034] Comparative Example 4: This study compares the oil tubing products produced using conventional production processes for traditional 13Cr martensitic stainless steel (such as API 13Cr, with similar composition but without Nb and Cu microalloying, and more relaxed control of S, P, and impurities) (electric arc furnace and AOD smelting, ingot casting or ordinary continuous casting, conventional rolling, and heat treatment). The aim is to provide a comprehensive comparison of the advanced alloy system and processes of this patent.

[0035] Comparative Example 5: The commonly used 22Cr duplex stainless steel (UNSS31803) oil casing was used as a comparison. Its typical chemical composition (wt%) is: C≤0.03, Cr: 21.0-23.0, Ni: 4.5-6.5, Mo: 2.5-3.5, N: 0.08-0.20, Mn≤2.0, Si≤1.0, P≤0.03, S≤0.02. Conventional production processes were employed: electric arc furnace / AOD furnace smelting, continuous casting, hot rolling / extrusion, and solution treatment (1050℃ water quenching).

[0036] Comparative Example 6: Another widely used martensitic stainless steel, "Super 13Cr" (such as the improved API 13Cr), is used as a comparison for oil casing. Its typical chemical composition (wt%) is: C: 0.015-0.030, Cr: 12.0-14.0, Ni: 4.5-6.5, Mo: 1.5-2.5, Cu: 0.5-1.5, N≤0.01. Conventional production processes are employed: electric furnace and VOD smelting, continuous casting, hot rolling / piercing, quenching (950-1000℃ oil quenching / water quenching), and low-temperature tempering (600-650℃).

[0037] Test Example 1: Key Performance Test Mechanical properties, microstructure, purity, and carbon dioxide corrosion resistance of the steel pipe samples obtained in Examples 1-3 and Comparative Examples 1-6 were tested respectively.

[0038] 1. Mechanical property testing: Test methods: In accordance with the API 5CT standard, samples were taken for room temperature tensile testing, Charpy V-notch impact testing (0℃) and hardness testing.

[0039] Example 1 760 860 18 75 28 0.12 Example 2 720 830 20 80 26 0.15 Example 3 800 900 16 70 30 0.10 Comparative Example 1 750 850 16 60 28 0.25 Comparative Example 2 740 840 15 55 28 0.35 Comparative Example 3 950 1050 12 25 38 1.20 Comparative Example 4 655 795 22 50 24 0.45 Comparative Example 5 550 750 25 100 30 0.20 Comparative Example 6 760 850 18 40 28 0.15 Test Results and Analysis: As shown in Table 1, Examples 1-3, under optimized composition and process, achieved excellent overall strength and toughness, especially in impact toughness (70-80J), which was significantly better than the comparative examples. Comparative Example 5 (22Cr duplex steel), although having high elongation, had significantly lower yield strength, and its impact toughness was not superior to the examples due to the potential presence of harmful phases. Under the test conditions, its corrosion rate was higher than Examples 1-3. Comparative Example 6 (super 13Cr) had strength comparable to Example 1, but its impact toughness (35J) was significantly lower than all examples. Its corrosion resistance was comparable to Example 2 but lower than Examples 1 and 3, and it was also costly.

[0040] Test Example 2: Sulfide Stress Cracking (SSC) Sensitivity Test Test Method: The standard tensile test method (Method A) in standard NACETM0177-2016, "Metals in H2S Environment – ​​Laboratory Tests for Resistance to Sulfide Stress Cracking," was followed. The test solution was NACETM0177 standard A solution (5% NaCl, 0.5% CH3COOH, saturated H2S), the temperature was 24±3℃, and the test stress was 85% of the actual yield strength of the specimen. A U-bend specimen (Method B) was used for auxiliary verification. The test period was 720 hours (30 days), and the occurrence and timing of cracking of the specimens were observed and recorded.

[0041] Evaluation criteria: No fracture occurred within 720 hours, resulting in a "pass" rating; fracture time was recorded for sensitivity comparison. Table 2. Results of sulfide stress cracking (SSC) test: Example 1 No cracks No cracks Excellent SSC resistance Example 2 No cracks No cracks Excellent SSC resistance Example 3 No cracks No cracks Excellent SSC resistance Comparative Example 1 Cracking after 480 hours Surface microcracks Highly sensitive Comparative Example 2 360-hour cracking Obvious cracks High sensitivity Comparative Example 3 Cracking within 96 hours Severe cracking Extremely sensitive Comparative Example 4 Cracking after 240 hours Obvious cracks High sensitivity Comparative Example 5 No cracks No cracks Excellent SSC resistance Comparative Example 6 No cracks No cracks Excellent SSC resistance Test Results and Analysis: In Examples 1-3 of this invention, no SSC cracking occurred within 720 hours under harsh stress conditions of 85% yield strength, demonstrating excellent SSC resistance. This is attributed to its extremely low S and P content, uniform and fine tempered sorbite structure, and spheroidized harmless inclusion morphology.

[0042] Comparative Example 1 (no calcium treatment) and Comparative Example 2 (poor billet quality) showed a significant decrease in SSC resistance due to the harmfulness of inclusions or poor microstructure uniformity. Comparative Example 3 (low-temperature tempering) exhibited extremely high SSC susceptibility because its hardness (HRC38) was much higher than the conventional requirements for martensitic stainless steel (≤HRC26) in the NACEMR0175 / ISO15156 standard, and its microstructure was tempered martensite with high internal stress.

[0043] Comparative Example 4 (traditional 13Cr) had relatively low SSC resistance due to its lower purity and microstructure control.

[0044] Comparative Example 5 (22Cr duplex steel) and Comparative Example 6 (super 13Cr) also exhibited excellent SSC resistance, which is an inherent advantage of their high alloying (high Cr, Mo, Ni). However, considering the cost data in Table 1, the examples have a significant cost advantage while achieving similar SSC resistance.

[0045] 3. Purity analysis of molten steel and finished steel: Test method: The gas content, number and size distribution of inclusions in the steel were statistically analyzed using an oxygen, nitrogen and hydrogen analyzer and an in-situ analyzer (such as ASPEX).

[0046] Test results: In Examples 1-3, [TO] ≤ 15 ppm, [N] ≤ 80 ppm, and [H] ≤ 1.5 ppm. The purity indicators of Comparative Examples 1, 2, and 4 were all inferior to those of the Examples to varying degrees. Comparative Example 5 had a high nitrogen content (0.08-0.20%), making purity control difficult. Comparative Example 6 had better purity control, but its overall cost was high.

[0047] 4. Carbon dioxide corrosion resistance test: Test method: Following the standard GB / T23258-2009 "Carbon Dioxide Corrosion Resistant Steel for Oil and Gas Fields", a simulated corrosion experiment was conducted in a high-temperature, high-pressure autoclave. Conditions included: CO2 partial pressure 2 MPa, temperature 90℃, Cl- concentration 100,000 mg / L, solution pH 4, flow rate 1 m / s, and experimental time 168 hours. The uniform corrosion rate was measured, and the morphology of the corrosion product film was observed under SEM.

[0048] Test Results and Analysis: As shown in Table 1, Examples 1-3 exhibited the lowest uniform corrosion rate (0.10-0.15 mm / a) due to their optimized Cr, Mo, Cu, Nb alloy design, high purity, and uniform tempered sorbite microstructure. The corrosion product film was dense and firmly adhered. Comparative Example 5 (22Cr duplex steel) showed a higher corrosion rate (0.20 mm / a) than the Examples due to the tendency for galvanic corrosion caused by the potential difference between the two phases under actual conditions and the possible local sensitization, and its corrosion uniformity was slightly worse. Comparative Example 6 (Super 13Cr) had corrosion resistance (0.15 mm / a) comparable to Example 2, but worse than Examples 1 and 3. The corrosion resistance of Comparative Examples 1, 2, 3, and 4 was inferior to all the Examples of the present invention to varying degrees.

[0049] Test Example 2: Process Stability and Production Statistics During the industrial trial production phase, key parameters of the entire process and finished product performance statistics were monitored for five consecutive batches (1000 tons) of steel pipes produced using the process parameters of Example 1.

[0050] Statistical results: The steel purity compliance rate ([H]≤1.5ppm, [O]≤15ppm, [S]≤0.001%) was 98.5%; the low-magnification microstructure qualification rate of continuously cast billets (central porosity ≤1 grade, segregation ≤1 grade) was 99%; the dimensional accuracy qualification rate of finished steel pipes (wall thickness deviation ±2%, ellipticity ≤0.6%D) was 99.8%; the first-time mechanical property inspection qualification rate was 99.5%; and the first-time carbon dioxide corrosion resistance inspection qualification rate was 100%. The data show that the preparation process is stable, reliable, and reproducible. Compared with Comparative Example 5 (22Cr duplex steel), which has extremely high smelting and hot processing difficulty and large fluctuations in microstructure and properties, and Comparative Example 6 (super 13Cr), which has relatively low yield due to stringent requirements for purity and heat treatment uniformity, the process route adopted in this invention, although requiring precise control, is more mature and stable overall, with higher production efficiency and comprehensive yield, further consolidating its economic advantages.

[0051] 5. Microstructure and grain size observation: Test method: Samples were taken along the cross-section of the steel pipe, ground and polished, and then etched. The microstructure, grain size and inclusion morphology were observed under a metallographic microscope and a scanning electron microscope (SEM).

[0052] Test Results and Analysis: Examples 1-3 all exhibited uniform, fine-grained tempered sorbite microstructures, with the original austenite grain size at level 8-9. Inclusions were predominantly fine-grained spherical calcium aluminate composite inclusions. Comparative Example 1 contained elongated MnS inclusions. Comparative Example 2 showed significant central segregation and porosity. Comparative Example 3 consisted of tempered martensite. Comparative Example 4 consisted of tempered sorbite, but with coarser grains (level 7-8) and visible irregular oxides and sulfides. Comparative Example 5 consisted of α+γ phases, but exhibited localized uneven proportions and a tendency for trace σ phase precipitation. Comparative Example 6 consisted of tempered martensite, but the original austenite grains were coarser than in the examples, and a small amount of undissolved carbides were present.

[0053] 6. Comprehensive mechanical strength and stability test: 1. High-temperature yield strength test: Test method: In accordance with ASTM E21 standard, a tensile test was conducted at 150°C to measure its high-temperature yield strength and evaluate its strength retention rate under downhole temperature rise environment.

[0054] 2. Cross-sectional hardness gradient test: Test method: Along the thickness direction of the steel pipe, at least 5 points are taken at equal intervals from the inner wall to the outer wall to perform Vickers hardness (HV10) test, calculate the hardness range (maximum value - minimum value) and evaluate the uniformity of heat treatment.

[0055] 3. Flattening test: Test method: A flattening test was conducted according to the API 5CT standard to check the surface and internal defects of the steel pipe under severe deformation and to evaluate its plasticity and forming uniformity.

[0056] Table 3. Results of comprehensive mechanical strength and stability tests: Example 1 700 92.1% ≤15 No cracks, in perfect condition Example 2 665 92.4% ≤18 No cracks, in perfect condition Example 3 735 91.9% ≤20 No cracks, in perfect condition Comparative Example 1 690 92.0% ≤25 Microcracks appeared on the inner wall Comparative Example 2 730 91.9% ≤35 Slight cracks appeared Comparative Example 3 Not tested (performance not applicable) - ≤50 Brittle cracking Comparative Example 4 600 91.6% ≤30 The performance was acceptable. Comparative Example 5 460 92.0% ≤40 No cracks, in perfect condition Comparative Example 6 700 92.1% ≤25 No cracks, in perfect condition Test Results and Analysis: The embodiments of the present invention maintain an extremely high strength retention rate (>91.9%) and absolute value at 150°C, which is significantly better than Comparative Examples 4 and 5 and comparable to Comparative Example 6, thus meeting the requirements of deep wells for high-temperature strength.

[0057] The example with the smallest wall thickness hardness range (≤20HV10) demonstrates the best heat treatment uniformity and highly consistent internal structure. In contrast, the hardness ranges of comparative examples 2, 3, and 5 are relatively large, reflecting their relatively poor structural uniformity.

[0058] The flattening test results of the examples were intact, indicating that they have good plasticity and uniform deformation ability. Comparative Example 1 and the inner wall have microcracks, which may be related to the morphology of inclusions; Comparative Example 3, on the other hand, has brittle cracking due to excessive hardness and poor plasticity.

[0059] Based on the above embodiments, comparative examples, and test results, the present invention provides an economical alloy steel for tubing sleeves resistant to carbon dioxide corrosion and its preparation process. Through the design of a "Cr-Mo-Ni-Cu-Nb" alloy system combined with meticulous control throughout the entire process, including "ultra-pure smelting, fully protective casting and light reduction, controlled rolling and cooling, online quenching and high-temperature tempering, and online hot straightening and controlled cooling," the following beneficial effects are successfully achieved: The CO2 corrosion resistance of the material of this invention is superior to that of traditional 13Cr steel (Comparative Example 4), and reaches or surpasses the level of higher alloyed 22Cr duplex steel (Comparative Example 5) and super 13Cr (Comparative Example 6). At the same time, its strength and toughness are matched, especially its impact toughness, which is significantly better than that of Comparative Examples 3, 4 and 6, and comparable to or better than that of Comparative Example 5 (its strength is much higher than that of Comparative Example 5).

[0060] Through precise composition control (significantly reducing the content of precious metals Ni and Mo) and efficient and stable short-process technology, the cost of the material of this invention is much lower than that of 22Cr duplex steel (Comparative Example 5) and super 13Cr (Comparative Example 6), and comparable to that of API13Cr (Comparative Example 4), but its performance is comprehensively surpassed, resulting in extremely high cost performance.

[0061] This invention effectively solves the problem of insufficient corrosion resistance of low-cost oil casing (such as traditional 13Cr) in the prior art, and achieves the best balance between corrosion resistance, toughness, process stability and manufacturing cost, providing a reliable and economical pipe material solution for most oil and gas wells in CO2 corrosive environments.

[0062] It should be noted that, in this document, relational terms such as "first" and "second" are used only to distinguish one entity or operation from another, and do not necessarily require or imply any such actual relationship or order between these entities or operations. Furthermore, the terms "comprising," "including," or any other variations thereof are intended to cover non-exclusive inclusion, such that a process, method, article, or apparatus that comprises a list of elements includes not only those elements but also other elements not expressly listed, or elements inherent to such process, method, article, or apparatus.

[0063] Although embodiments of the invention have been shown and described, it will be understood by those skilled in the art that various changes, modifications, substitutions and alterations can be made to these embodiments without departing from the principles and spirit of the invention, the scope of which is defined by the appended claims and their equivalents.

Claims

1. An economical alloy steel for oil pipe casing resistant to carbon dioxide and hydrogen sulfide corrosion, characterized in that: The chemical composition by weight percentage is as follows: carbon: ≤0.08%, chromium: 10.2%-13.8%, molybdenum: 0.8-1.0%, nickel: 3.2-4.8%, vanadium: 0.05-0.10%, copper: ≤0.20%, niobium: ≤0.05%, manganese: ≤1.0%, silicon: ≤0.5%, sulfur: ≤0.001%, phosphorus: ≤0.010%, calcium: 0.0015-0.0030%, nitrogen: ≤0.01%, with the remainder being iron and unavoidable impurities, the total content of which is less than 0.03 wt%.

2. The preparation process of an economical alloy steel for oil pipe casing resistant to carbon dioxide and hydrogen sulfide corrosion according to claim 1, characterized in that: Includes the following steps: Step a) Ultra-pure smelting: First, the primary smelting is completed in the converter to quickly melt the required proportions of each component and perform preliminary dephosphorization. Then, the molten steel is transferred to the ladle furnace for deep refining to achieve deep desulfurization and dephosphorization, while achieving micro-alloying of niobium and copper. After that, the molten steel enters the vacuum degassing device to remove hydrogen and oxygen in order to achieve the gas and impurity control targets set in the formula. Step b) Billet preparation: Using full protective casting technology, molten steel flows from the ladle into the tundish through the long nozzle, and then from the tundish into the crystallizer through the submerged entry nozzle. After the molten steel crystallizes and cools, a continuous round billet is finally obtained. Step c) Hot working: The round billet is first fed into the annular heating furnace for uniform heating to ensure that carbon, chromium, molybdenum, nickel, vanadium and manganese are fully dissolved. After heating, the billet is descaled by high pressure water to obtain a clean surface, and then enters the piercing mill. The pierced tube enters the limited-movement mandrel continuous rolling mill for rolling to refine the grains. The rolled tube is immediately put into the reheating furnace for uniform temperature to prepare for subsequent sizing and to prevent the precipitation of harmful phases. Step d) Heat treatment: In order to activate the synergistic corrosion resistance potential of chromium, molybdenum, nickel, vanadium and manganese in the formula and obtain a tempered structure with both strength and toughness, the heat treatment is carried out on a continuous line. First, the steel pipe enters the controllable roller hearth quenching furnace to fully dissolve the carbides. Then, the steel pipe is quickly fed into the high-efficiency quenching system. After quenching, the steel pipe immediately enters the tempering furnace. After tempering, it is cooled by air cooling. Step e) Finishing and straightening: After heat treatment, the steel pipe is first put into an online hot straightening machine while ensuring that the temperature is still above 550℃. The final straightening temperature is ≥480℃. The steel pipe is then immediately put into an internal and external cooling system for cooling. After the steel pipe is completely cooled, an alloy steel pipe for oil pipe sleeves that is resistant to carbon dioxide and hydrogen sulfide corrosion can be obtained.

3. The preparation process of an economical alloy steel for oil pipe casing resistant to carbon dioxide and hydrogen sulfide corrosion according to claim 2, characterized in that: Step a) Ultra-pure smelting: Vacuum degassing device, circulating treatment for 15-20 minutes under a high vacuum of ≤67Pa. The vacuum degassing device stabilizes hydrogen at ≤1.5ppm and oxygen at ≤15ppm. The entire process uses argon blowing and stirring to ensure uniform composition and temperature. The tapping temperature is precisely controlled within ±5℃ of the target casting temperature. All raw materials, auxiliary materials, and refractory materials must be low-sulfur and low-hydrogen varieties. During the deep refining process of the molten steel in the ladle furnace, high-alkalinity reducing slag is created by adding active lime, fluorite, etc. At a high temperature of 1600-1650℃, strong diffusion deoxidation and desulfurization conditions are used to reduce the sulfur content in the steel from ≤0.006% at the time of entry to the target value of ≤0.001%, with a desulfurization efficiency of over 80%. At the same time, phosphorus is further reduced to ≤0.010%.

4. The preparation process of an economical alloy steel for oil pipe casing resistant to carbon dioxide and hydrogen sulfide corrosion according to claim 2, characterized in that: Step a) Ultra-pure smelting: At the end of the refining process, calcium-iron alloy wire with a Ca content ≥30% is fed into the ladle at a speed of 80-120 meters per minute using a wire feeder. The amount of calcium added is controlled between 0.0015% and 0.0030%. The purpose is to modify the residual sulfur and oxides, and to transform the high-melting-point Al2O3 and harmful MnS inclusions into a low-melting-point (1400℃) spherical calcium aluminate 12CaO·7Al2O3 composite inclusion that encapsulates CaS.

5. The preparation process of an economical alloy steel for oil pipe casing resistant to carbon dioxide and hydrogen sulfide corrosion according to claim 2, characterized in that: Step b) Billet Preparation: The depth of molten steel immersed in the crystallizer is controlled at 120-150mm. A low-viscosity, high-melting-rate pre-melted slag is used as the protective slag to ensure uniform lubrication and heat transfer. Fluctuations in the crystallizer liquid level are stabilized within ±3mm by an automatic control system. The difference between the molten steel casting temperature and its liquidus temperature is controlled within the range of 15-30℃. Too low a superheat can easily lead to nozzle blockage, while too high a superheat can result in coarse columnar crystals and increased central segregation. A weak cooling mode is used for crystallizer cooling, with a specific water volume controlled at 0.8-1.2L / kg to form a sufficiently thick and uniform billet. The uniform initial billet shell is treated with a light reduction technique at the end of solidification, with the total reduction controlled at 4-8 mm to compensate for solidification shrinkage and suppress central shrinkage cavities and V-shaped segregation. The casting stream adopts a dynamic water distribution model in the secondary cooling zone, with the total cooling intensity controlled at 0.8-1.0 L / kg to ensure that the surface temperature of the billet is above 950℃ at the straightening point and to avoid straightening cracks. Argon gas is used to seal the molten steel long nozzle and the tundish cover, with the sealing argon gas flow rate controlled at 5-10 NL / min. The tundish covering agent is made of alkaline carbon-free material, and the space above the liquid surface is filled with argon gas to isolate air.

6. The preparation process of an economical alloy steel for oil pipe casing resistant to carbon dioxide and hydrogen sulfide corrosion according to claim 2, characterized in that: Step c) Hot working: The rolling process is completed continuously by 7-9 stands. The online wall thickness measuring instrument included in the hydraulic automatic thickness control system of the tube rolling mill provides real-time feedback data, and the roll gap of each stand is dynamically adjusted to strictly control the wall thickness deviation of the finished rough tube within ±2% of the nominal wall thickness, the ellipticity is controlled within ≤0.6%×outer diameter, and the final rolling temperature is controlled within the range of 880-950℃ to ensure sufficient recrystallization of grains and refinement of microstructure. The heating regime adopts three-level control: the preheating section is ≤800℃ with slow heating to prevent thermal stress cracking; the heating section is 1180-1230℃ for sufficient homogenization to ensure thorough heating of the core of the billet. The soaking zone is heated to 1230±10℃ for 1.5-2.0 hours to ensure uniform temperature and complete austenite homogenization. When tapping the steel, the temperature difference between the surface and core of the billet must be ≤30℃.

7. The preparation process of an economical alloy steel for oil pipe casing resistant to carbon dioxide and hydrogen sulfide corrosion according to claim 2, characterized in that: Step d) Heat treatment: A mixture of high-purity nitrogen (≥99.995%) and 2-5% hydrogen is introduced into the furnace to maintain a slightly positive pressure of 150-300 Pa, forming a weakly reducing atmosphere. This ensures that the steel pipe surface is bright, free of oxidation, and the decarburized layer depth is ≤0.05 mm. The steel pipe is uniformly heated to 1020±10℃ and held at this temperature for 1.2-1.5 minutes per millimeter of wall thickness to ensure that the alloying elements are fully dissolved and the microstructure is completely austenitized. The entire heat treatment process is monitored by multiple thermocouples and infrared thermometers, and the temperature recording curves are traceable. The high-efficiency quenching system adopts internal spray and external spray high-pressure water quenching. The water pressure of the internal nozzle is ≥1.0MPa and the water curtain pressure of the external ring seam is ≥0.8MPa, ensuring that the steel pipe passes through the martensitic transformation zone quickly within a few seconds to obtain a complete martensitic structure. The core hardness can reach HRC40 or above. The tempering furnace temperature is controlled within the range of 730±5℃, and the holding time is calculated at 2.0-2.5 minutes per millimeter of wall thickness. This high-temperature tempering process allows the quenched martensite to decompose fully and carbides to precipitate diffusely, forming a uniform and stable tempered sorbite structure.

8. The preparation process of an economical alloy steel for oil pipe casing resistant to carbon dioxide and hydrogen sulfide corrosion according to claim 2, characterized in that: Step e) Finishing and straightening: The straightening machine adopts a six-roll or ten-roll arrangement. The pressure and deflection of each roll are precisely controlled by a hydraulic servo system. The straightening temperature is precisely controlled by the moving speed of the cooling bed before straightening, ensuring that the straightening temperature is ≥480℃. This ensures that the residual stress inside the material after straightening is ≤5%*the specified yield strength, thereby improving the material's corrosion resistance.