An ultra-high strength stainless steel coiled tubing and method of processing the same

Abstract

The present application relates to the technical field of oil and gas pipe, and particularly relates to a super-high-strength stainless steel coiled tubing and a processing method. The super-high-strength stainless steel coiled tubing has the following chemical element components in percentage by weight: C<=0.03%, Si<=1.00%, Mn: 0.5~1.0%, P<=0.03%, S<=0.03%, Ni: 3.50~5.50%, Cr: 12.00~13.50%, Mo: 0.50~1.00%, Cu<=0.5%, and the balance is Fe and inevitable impurities. The present application realizes the yield strength of 758~965 MPa, the tensile strength of 798~1034 MPa, the elongation of >=17%, the outer diameter range of Phi 50.8~Phi 88.9 mm, the wall thickness range of 3.7~6.4 mm, the pipe length range of 1000~10000 m, and the hardness of <=32 HRC of the manufactured coiled tubing through the processes of continuous forming, laser-TIG composite welding and whole pipe body heat treatment, so that the coiled tubing can meet the development of deep wells and super-deep wells containing corrosion medium.

Description

Technical Field

[0001] This invention relates to the field of oil and gas pipeline technology, and in particular to an ultra-high strength stainless steel continuous tubing and its processing method. Background Technology

[0002] Coiled tubing (CT) is a new type of oil pipe with excellent flexibility, also known as flexible tubing. Unlike conventional threaded tubing, it is a type of tubing with a single length of several kilometers and no threaded connections. Due to its high efficiency, low cost, wide operating range, and small footprint, coiled tubing technology offers numerous technical and operational advantages. When used in conjunction with coiled tubing installation machines, it can perform dozens of operations in oil and gas field workover, logging, drilling, completion, and oil and gas transportation, playing a vital role in oil and gas field exploration, development, operation, and production enhancement.

[0003] With the increasing difficulty of oil and gas exploration and development, and the growing number of unconventional oil and gas resource developments, many complex oil and gas well engineering challenges will emerge. For example, in the Tarim Basin, oil and gas wells combine ultra-deep, ultra-high pressure, ultra-high temperature, and high corrosion, further increasing the requirements for the comprehensive performance of coiled tubing in terms of strength, collapse resistance, and corrosion resistance. The latest round of oil and gas resource assessment results shows that the Tarim Basin is rich in ultra-deep oil and gas resources, with oil and natural gas resources buried at depths exceeding 6,000 meters accounting for 83.2% and 63.9% of the national total, respectively. The total amount of ultra-deep oil and gas resources accounts for approximately 19% of the global total, indicating enormous exploration and development potential.

[0004] In the development of deep and ultra-deep wells containing corrosive media, ultra-high strength coiled tubing made of ordinary carbon steel is prone to uniform corrosion and stress corrosion caused by H2S and CO2 during oil and gas well operations due to its poor corrosion resistance. This leads to thinning of the tubing wall and sudden fracture when the stress is far below its yield strength. Coiled tubing made of 2205 duplex stainless steel, austenitic stainless steel, and nickel-based alloys has better corrosion resistance, but due to the low strength of the materials, the yield strength of the manufactured coiled tubing is generally only around 600 MPa, which only meets the mechanical performance requirements of 80Ksi grade coiled tubing. The maximum well depth is less than 6000m, which cannot meet the well depth requirements of deep and ultra-deep wells. Coiled tubing made of titanium alloys and iron-nickel alloys can meet the well depth requirements and has good corrosion resistance, but their high material cost prevents large-scale application in oil and gas field development. Summary of the Invention

[0005] To address the aforementioned problems, the purpose of this invention is to provide an ultra-high strength stainless steel coiled tubing and its processing method. The ultra-high strength stainless steel coiled tubing of this invention has a microstructure of high-strength tempered martensite with a small amount of dispersed austenite. While improving the strength of the tubing and increasing the tubing's running depth, it also effectively matches the tubing's strength and plasticity, ensuring that the coiled tubing has a good low-cycle, high-strain fatigue life during repeated bending operations in oil and gas fields. It is suitable for the development of deep and ultra-deep wells containing corrosive media.

[0006] The technical solution of the present invention is as follows: an ultra-high strength stainless steel continuous oil tubing, wherein the chemical element composition of the stainless steel continuous oil tubing is as follows by weight percentage: C≤0.03%, Si≤1.00%, Mn:0.5~1.0%, P≤0.03%, S≤0.03%, Ni:3.50~5.50%, Cr:12.00~13.50%, Mo:0.50~1.00%, Cu≤0.5%, with the balance being Fe and unavoidable impurities.

[0007] The selection criteria for the chemical composition of the aforementioned ultra-high strength stainless steel coiled tubing are as follows:

[0008] The design range for chromium (Cr) is 12.0% to 13.5%. This is because Cr is the main alloying element in martensitic stainless steel that plays a decisive role in corrosion resistance. As the Cr content increases, the corrosion resistance of stainless steel in oxidizing corrosive media increases accordingly, and Cr effectively reduces the steel's susceptibility to pitting corrosion. However, Cr is also a strong ferrite-forming element, which can significantly reduce the austenite phase region in the phase diagram. Excessive Cr can promote the formation of ferrite in the microstructure of coiled tubing, resulting in a lack of a fully martensitic structure (containing some δ-ferrite) after solution treatment, thus impairing the toughness and stress corrosion resistance of the coiled tubing. Therefore, the Cr content in ultra-high strength stainless steel coiled tubing must be strictly controlled between 12% and 13.5%.

[0009] The design range for nickel (Ni) is 3.50% to 5.50%. This is because, as previously known, the presence of Cr promotes ferrite formation. Therefore, an appropriate amount of Ni needs to be added to ultra-high strength stainless steel coiled tubing to expand the austenite phase region, ensuring the formation of martensite during cooling after solution treatment, rather than a single ferrite or other phases. Simultaneously, Ni has a positive effect on improving the toughness of the tubing. When the tubing contains an appropriate amount of Ni, the martensitic transformation cannot be completely completed after solution treatment and cooling to room temperature, resulting in a small amount of retained austenite in the microstructure. Retained austenite is softer than martensite, thus improving the toughness of the coiled tubing. Therefore, a suitable Ni content is crucial for a proper balance between strength and plasticity in ultra-high strength coiled tubing. Furthermore, Ni can enhance the passivation tendency of Fe-Cr alloys and improve the corrosion resistance of steel in reducing media. Therefore, the Ni content in this invention is controlled at 3.50-5.50%.

[0010] The design range for molybdenum (Mo) is 0.50% to 1.00%. This is because adding Mo to martensitic stainless steel benefits the steel's strength, toughness, and corrosion resistance. In certain reducing corrosive media, Mo can promote the passivation of Cr, effectively improving the corrosion resistance of ultra-high strength coiled tubing in reducing acids and inhibiting Cl-. - The pitting corrosion effect of ions improves the intergranular corrosion resistance of steel. However, like Ni, Mo is an austenite stabilizing element, and excessive addition can lead to the formation of residual austenite. Therefore, the Mo content in this invention is controlled at 0.50-1.00%.

[0011] The design range for copper (Cu) is ≤0.5%. This is because Cu can form an enriched layer beneath the oxide layer of stainless steel, thus preventing corrosion from penetrating deeper into the metal. The addition of Cu provides excellent resistance to sulfate-reducing bacteria (SRB) corrosion in coiled tubing operating in oil and gas fields, and also improves the steel's resistance to stress corrosion. However, excessive Cu content can cause copper embrittlement during hot working; therefore, the Cu content in this invention is controlled at Cu≤0.5%.

[0012] The design range for manganese (Mn) is 0.5% to 1.0%. This is because Mn is an element that expands the austenite phase region in the phase diagram. In stainless steel, Mn's stabilizing ability for austenite is second only to Ni, significantly improving the hardenability of stainless steel. Therefore, in coiled tubing, adding appropriate amounts of Mn can partially replace the expensive Ni alloying element, reducing tubing development costs. However, the corrosion resistance of stainless steel with low Cr content will slightly decrease with the addition of Mn, but this effect is no longer significant when the Cr content in the stainless steel is sufficiently high (17%). Therefore, in this invention, the Mn content is controlled at 0.5% to 1.0%.

[0013] The design range for silicon (Si) is S ≤ 1.0%, because Si can inhibit temper brittleness in tubing and the decomposition of residual austenite during tempering. At high temperatures or in strongly oxidizing media, adding a certain amount of Si to stainless steel can form a Si-rich SiO2 oxide layer on the surface, thus significantly improving the oxidation resistance or corrosion resistance of coiled tubing. Adding Si can also inhibit the oxidation of stainless steel by Cl... - Pitting corrosion is common in ionic media, but if the Si content in the coiled tubing is too high, it will greatly increase the brittleness of the tubing. Therefore, in this invention, the Si content is controlled to be Si≤1.00%.

[0014] The design range for carbon (C) is S≤0.03%. The reason is that while C contributes to improving the strength of stainless steel, if corrosion resistance is the primary objective, lower C content results in higher corrosion resistance. C and Cr can form various compounds such as Cr23C6, which appear in the intergranular space, causing a decrease in the corrosion resistance of coiled tubing. Therefore, the C content in this invention is controlled at C≤0.04%.

[0015] The design range for sulfur (S) is S≤0.03%. This is because S segregates significantly in stainless steel, deteriorating the steel's quality. It is a harmful element that reduces the plasticity of stainless steel at high temperatures. It exists in the form of FeS, which has a low melting point, so its content is controlled below 0.03%.

[0016] The design range for phosphorus (P) is: P ≤ 0.03%. This is because P segregates significantly in stainless steel, increasing temper brittleness, significantly reducing the steel's plasticity and toughness, making it prone to cracking during cold working. Phosphorus also negatively affects weldability. Phosphorus is a harmful element and should be strictly controlled; its content is generally not greater than 0.030%.

[0017] The above-mentioned processing method for ultra-high strength stainless steel continuous tubing includes the following steps:

[0018] S1: Preparation of stainless steel coils;

[0019] S2: Stainless steel coil, longitudinally cut and butt-welded length;

[0020] S3: Using laser-TIG composite welding, prepare straight seam ultra-high strength stainless steel continuous tubing;

[0021] S4: The longitudinal welds and the entire pipe body are heat-treated.

[0022] The specific steps for preparing the stainless steel coil in S1 are as follows:

[0023] S11: Steelmaking: Steelmaking raw materials are melted into molten iron. The chemical composition of the molten iron by weight percentage is C≤0.03%, Si≤1.00%, Mn:0.5~1.0%, P≤0.03%, S≤0.03%, Ni:3.50~5.50%, Cr:12.00~13.50%, Mo:0.50~1.00%, Cu≤0.5%, with the balance being Fe and unavoidable impurities. The molten iron is refined in a vacuum oxygen blowing decarburization refining furnace at 1600~1660℃ to obtain the first molten steel. Then, the first molten steel is refined in a ladle refining furnace at 1550~1700℃ to obtain the second molten steel.

[0024] S12: Cast slab: The second molten steel after ladle refining is poured into a continuously cast slab, which is then cast into a slab with a thickness of 20-60mm using continuous casting technology and electromagnetic stirring control, and then cooled under controlled conditions.

[0025] S13: Coil: The slab is rolled into a hot-rolled plate with a thickness of 2.4 to 6.35 mm at a temperature of 1000 to 1050℃, then cooled and tempered at a temperature range of 580 to 750℃, pickled and then rolled to finally produce coils with a length of >400 meters.

[0026] The process of longitudinally cutting and butt welding the stainless steel coil in S2 is as follows: the stainless steel coil is longitudinally cut into steel strips of 150~400mm according to the specifications of the continuous tubing; then, according to the specified length requirements of the continuous tubing, multiple steel strips are butt welded together in sequence, and the weld seam is heat treated after welding.

[0027] When welding long steel strips, first process the ends of the steel strips into a 45° bevel, and then open an I-shaped or V-shaped bevel on the bevel. Weld using gas metal arc welding (GMAW). Before welding, preheat the bevel under an argon atmosphere at a temperature of 150–300°C. During welding, the shielding gas is 95% Ar (volume fraction) + 5% CO2 (volume fraction). The welding current is 220–240A, the welding voltage is 28–32V, the welding speed is 200–350 mm / min, and the gas flow rate is 18–25 L / min.

[0028] After the steel strip is welded, the weld is pushed into wood ash for slow cooling. After the weld is cooled to ≤150℃, the weld is subjected to conditioning heat treatment. The conditioning heat treatment first heats the weld to 980~1050℃, holds it for 1~10min, and then quenches it. Then the weld is heated to 580~650℃, holds it for 2~20min, and then air-cooled.

[0029] The specific process for preparing the straight-seam ultra-high strength stainless steel continuous tubing in S3 is as follows:

[0030] S31: Based on the outer diameter and wall thickness requirements of the coiled tubing, mill the edges on both sides of the extended steel strip and open I-type and V-type bevels;

[0031] S32: The UOE roller forming method is used to control the continuous and stable forming of steel strip. During the forming of steel strip, the bevel gap is ≤0.3mm, the misalignment is ≤0.5mm, and the weld center offset is ±0.6mm.

[0032] S33: Longitudinal welding of continuous tubing using laser-TIG. The bevel is preheated to ≤150℃. The shielding gas is: 95% Ar volume fraction + 5% CO2 volume fraction. The TIG welding current is 150~240A, the TIG welding voltage is 22~32V, the laser defocusing amount is -5~+5mm, the laser power is 1Kw~5Kw, the welding speed is 1.5~5m / min, and the gas flow rate is 18~25L / min.

[0033] The heat treatment process for the S4 longitudinal weld and the entire pipe body is as follows:

[0034] S41: Locally heat the weld to 980-1050℃, and apply a certain deformation to the weld by sizing the pipe using extrusion rollers, followed by quenching.

[0035] S42: After quenching, the entire pipe is heated to 590-750℃ using medium-frequency induction heating, held for 5-20 minutes, and then air-cooled to below 150℃. The entire pipe is then heated to 600-680℃ for secondary tempering, held for 5-10 minutes, and then air-cooled to room temperature.

[0036] S43: After the entire tubing is heat-treated, the stainless steel continuous tubing is pickled, cleaned, and coiled.

[0037] The microstructure of the ultra-high strength stainless steel continuous tubing consists of tempered martensite and dispersed inverted austenite, with the volume fraction of the austenite being ≤10%.

[0038] The coiled tubing has a yield strength of 758~965 MPa, a tensile strength of 798~1034 MPa, an elongation of ≥17%, an outer diameter range of Φ50.8~Φ88.9 mm, a wall thickness range of 3.7~6.4 mm, a tubing length range of 1000~10000 m, and a hardness of ≤32 HRC.

[0039] The laser-TIG composite welding technology in this invention is a process that combines a laser beam and a non-consumable electrode tungsten inert gas welding arc as a composite heat source for welding.

[0040] The beneficial effects of this invention are as follows:

[0041] 1. This invention effectively controls the size of the austenite phase region in martensitic stainless steel by controlling the content of Ni and Cr elements, ensuring the formation of martensite structure during cooling after solution treatment, rather than the formation of a single ferrite or other phases. By controlling the content of Mo and Cu elements, the corrosion resistance of ultra-high strength coiled tubing in reducing acids can be effectively improved, and the formation of Cl can be inhibited. - The pitting corrosion effect of ions improves the steel's resistance to intergranular corrosion, while also exhibiting excellent resistance to sulfate-reducing bacteria (SRB) corrosion; 2. This invention utilizes laser-TIG... The composite welding technology for longitudinal welding of coiled tubing after forming, compared to existing single laser welding, can increase the weld penetration, reduce weld depression, and make the weld fuller. Simultaneously, the introduction of an electric arc heat source in laser-TIG composite welding expands the heat application range, increases the amount of molten metal, enhances bridging ability, reduces the requirements for workpiece assembly precision, reduces the application cost of laser welding technology, improves weld quality, and eliminates obvious burrs on the inner wall of the tubing after welding, increasing the inner diameter for oil and gas flow within the coiled tubing and avoiding phenomena such as groove corrosion. 3. This invention regulates the microstructure of the tubing through full-body heat treatment, forming a microstructure of high-strength tempered martensite with a small amount of dispersed austenite. The martensite microstructure has high strength, increasing the tubing's run-in depth, while the austenite microstructure, as a soft phase in the coiled tubing, has good plasticity and toughness, ensuring good low-cycle, high-strain fatigue life during repeated bending operations in oil and gas fields. 4. This invention produces ultra-high strength stainless steel coiled tubing with a yield strength of 758~965. MPa, tensile strength 793~1034MPa, elongation ≥17%. It possesses excellent corrosion resistance, high strength, high compressive strength, and long low-cycle fatigue life, making it suitable for environments containing H2S, CO2, and Cl. - It can be used in deep and ultra-deep wells, and is also suitable for high-pressure operations. It has high operating efficiency and can effectively increase the production of oil and gas wells. Attached Figure Description

[0042] Figure 1 This is a photograph of the longitudinal weld structure of an ultra-high strength stainless steel continuous tubing according to an embodiment of the present invention.

[0043] Figure 2 This is a photograph of the microstructure of a base material for an ultra-high strength stainless steel continuous tubing according to an embodiment of the present invention.

[0044] Figure 3 This is a comparison diagram of the yield strength and compressive strength of an ultra-high strength stainless steel coiled tubing and a duplex stainless steel coiled tubing according to an embodiment of the present invention.

[0045] Figure 4 This is a comparison diagram of the internal and external pressure resistance of an ultra-high strength stainless steel coiled tubing and a duplex stainless steel coiled tubing according to an embodiment of the present invention.

[0046] Figure 5 This is a comparison diagram of the well run-in depth of an ultra-high strength stainless steel coiled tubing and a duplex stainless steel coiled tubing according to an embodiment of the present invention.

[0047] Figure 6 This invention presents a comparison of the corrosion rates of an ultra-high strength stainless steel coiled tubing and a duplex stainless steel coiled tubing under CO2 injection conditions. Detailed Implementation

[0048] The present invention will be further described in detail below with reference to embodiments:

[0049] Example 1

[0050] The chemical element composition of the stainless steel continuous tubing, by weight percentage, is as follows: C≤0.03%, Si≤1.00%, Mn: 0.5~1.0%, P≤0.03%, S≤0.03%, Ni: 3.50~5.50%, Cr: 12.00~13.50%, Mo: 0.50~1.00%, Cu≤0.5%, with the balance being Fe and unavoidable impurities.

[0051] This invention effectively controls the size of the austenite phase region in martensitic stainless steel by controlling the content of Ni and Cr elements, ensuring the formation of martensite structure during cooling after solution treatment, rather than the formation of a single ferrite or other phases. Furthermore, by controlling the Mo element content, this invention can effectively improve the corrosion resistance of ultra-high strength coiled tubing in reducing acids and inhibit Cl... - The pitting corrosion effect of ions improves the intergranular corrosion resistance of steel. This invention, by controlling the Cu element content, can effectively improve the corrosion resistance of ultra-high strength coiled tubing against sulfate-reducing bacteria (SRB) corrosion. The ultra-high strength coiled tubing of this invention is suitable for applications containing H2S, CO2, and Cl... - The development of deep and ultra-deep wells in corrosive media.

[0052] Example 2

[0053] A method for processing ultra-high strength stainless steel continuous tubing includes the following steps:

[0054] S1: Preparation of stainless steel coils;

[0055] S2: Stainless steel coil, longitudinally cut and butt-welded length;

[0056] S3: Using laser-TIG composite welding, prepare straight seam ultra-high strength stainless steel continuous tubing;

[0057] S4: The longitudinal welds and the entire pipe body are heat-treated.

[0058] The specific steps for preparing the stainless steel coil in S1 are as follows:

[0059] S11: Steelmaking: Steelmaking raw materials are melted into molten iron. The chemical composition of the molten iron by weight percentage is C≤0.03%, Si≤1.00%, Mn:0.5~1.0%, P≤0.03%, S≤0.03%, Ni:3.50~5.50%, Cr:12.00~13.50%, Mo:0.50~1.00%, Cu≤0.5%, with the balance being Fe and unavoidable impurities. The molten iron is refined in a vacuum oxygen blowing decarburization refining furnace at 1600~1660℃ to obtain the first molten steel. Then, the first molten steel is refined in a ladle refining furnace at 1550~1700℃ to obtain the second molten steel.

[0060] S12: Cast slab: The second molten steel after ladle refining is poured into a continuously cast slab, which is then cast into a slab with a thickness of 20-60mm using continuous casting technology and electromagnetic stirring control, and then cooled under controlled conditions.

[0061] S13: Coil: The slab is rolled into a hot-rolled plate with a thickness of 2.4 to 6.35 mm at a temperature of 1000 to 1050℃, then cooled and tempered at a temperature range of 580 to 750℃, pickled and then rolled to finally produce coils with a length of >400 meters.

[0062] The process of longitudinally cutting and butt welding the stainless steel coil in S2 is as follows: the stainless steel coil is longitudinally cut into steel strips of 150~400mm according to the specifications of the continuous tubing; then, according to the specified length requirements of the continuous tubing, multiple steel strips are butt welded together in sequence, and the weld seam is heat treated after welding.

[0063] When welding long steel strips, first process the ends of the steel strips into a 45° bevel, and then open an I-shaped or V-shaped bevel on the bevel. Weld using gas metal arc welding (GMAW). Before welding, preheat the bevel under an argon atmosphere at a temperature of 150–300°C. During welding, the shielding gas is 95% Ar (volume fraction) + 5% CO2 (volume fraction). The welding current is 220–240A, the welding voltage is 28–32V, the welding speed is 200–350 mm / min, and the gas flow rate is 18–25 L / min.

[0064] After the steel strip is welded, the weld is pushed into wood ash for slow cooling. After the weld is cooled to ≤150℃, the weld is subjected to conditioning heat treatment. The conditioning heat treatment first heats the weld to 980~1050℃, holds it for 1~10min, and then quenches it. Then the weld is heated to 580~650℃, holds it for 2~20min, and then air-cooled.

[0065] The specific process for preparing the straight-seam ultra-high strength stainless steel continuous tubing in S3 is as follows:

[0066] S31: Based on the outer diameter and wall thickness requirements of the coiled tubing, mill the edges on both sides of the extended steel strip and open I-type and V-type bevels;

[0067] S32: The UOE roller forming method is used to control the continuous and stable forming of steel strip. During the forming of steel strip, the bevel gap is ≤0.3mm, the misalignment is ≤0.5mm, and the weld center offset is ±0.6mm.

[0068] S33: Longitudinal welding of continuous tubing using laser-TIG. The bevel is preheated to ≤150℃. The gas is 95% Ar by volume and 5% CO2 by volume. The TIG welding current is 150~240A, the TIG welding voltage is 22~32V, the laser defocusing amount is -5~+5mm, the laser power is 1Kw~5Kw, the welding speed is 1.5~5m / min, and the gas flow rate is 18~25L / min.

[0069] Traditionally, after forming stainless steel coiled tubing, the longitudinal weld is mainly welded using a separate laser. However, this separate laser welding requires high precision in beveling the tubing, and the continuous welding process, lasting over ten hours, makes it difficult to guarantee the beveling quality. This increases the risk of defects such as misalignment, unevenness, and lack of fusion. This invention employs laser-TIG composite welding technology for longitudinal welding after coiled tubing formation. Compared to existing separate laser welding, this increases weld penetration, reduces weld depression, and results in a fuller weld. Simultaneously, the introduction of an electric arc heat source in the laser-TIG composite welding expands the heat application range, increases the amount of molten metal, enhances bridging capabilities, reduces the requirements for workpiece assembly precision, lowers the application cost of laser welding technology, and improves weld quality. The welded tubing has no obvious burrs on the inner wall, increases the inner diameter for oil and gas flow within the coiled tubing, and avoids trench corrosion.

[0070] The heat treatment process for the S4 longitudinal weld and the entire pipe body is as follows:

[0071] S41: Locally heat the weld to 980-1050℃, and apply a certain deformation to the weld by sizing the pipe using extrusion rollers, followed by quenching.

[0072] S42: After quenching, the entire pipe is heated to 590-750℃ using medium-frequency induction heating, held for 5-20 minutes, and then air-cooled to below 150℃. The entire pipe is then heated to 600-680℃ for secondary tempering, held for 5-10 minutes, and then air-cooled to room temperature.

[0073] S43: After the entire tubing is heat-treated, the stainless steel continuous tubing is pickled, cleaned, and coiled.

[0074] The microstructure of the ultra-high strength stainless steel continuous tubing consists of tempered martensite and dispersed inverted austenite, with the volume fraction of the austenite being ≤10%.

[0075] The coiled tubing has a yield strength of 758~965 MPa, a tensile strength of 798~1034 MPa, an elongation of ≥17%, an outer diameter range of Φ50.8~Φ88.9 mm, a wall thickness range of 3.7~6.4 mm, a tubing length range of 1000~10000 m, and a hardness of ≤32 HRC.

[0076] This invention utilizes full-body heat treatment to regulate the microstructure of the tubing, resulting in a structure composed of high-strength tempered martensite with a small amount of dispersed austenite. The martensite structure provides high strength, increasing the tubing's run-in depth. The austenite structure, acting as a soft phase in the coiled tubing, exhibits good plasticity and toughness, ensuring excellent low-cycle, high-strain fatigue life during repeated bending operations in oil and gas fields. Through continuous forming, laser-TIG composite welding, and full-body heat treatment, this invention produces coiled tubing with a yield strength of 758~965 MPa, tensile strength of 798~1034 MPa, elongation ≥17%, outer diameter range of Φ50.8~Φ88.9 mm, wall thickness range of 3.7~6.4 mm, tubing length range of 1000~10000 m, and hardness ≤32 HRC, meeting the requirements for deep and ultra-deep well development containing corrosive media.

[0077] According to the ultra-high strength stainless steel coiled tubing described in Example 1 above, the ultra-high strength stainless steel coiled tubing is processed using the processing method of the ultra-high strength stainless steel coiled tubing described in Example 2, as detailed in Examples 3 and 4.

[0078] Example 3

[0079] A method for processing ultra-high strength stainless steel continuous tubing includes the following steps:

[0080] S1: Preparation of stainless steel coils;

[0081] S2: Stainless steel coil, longitudinally cut and butt-welded length;

[0082] S3: Using laser-TIG composite welding, prepare straight seam ultra-high strength stainless steel continuous tubing;

[0083] S4: The longitudinal welds and the entire pipe body are heat-treated.

[0084] The specific steps for preparing the stainless steel coil in S1 are as follows:

[0085] S11: Steelmaking: Steelmaking raw materials are melted into molten iron. The chemical composition of the molten iron by weight percentage is C: 0.03%, Si: 0.90%, Mn: 0.9%, P: 0.02%, S: 0.02%, Ni: 5.00%, Cr: 13.50%, Mo: 0.80%, Cu: 0.4%, with the balance being Fe and unavoidable impurities. The molten iron is refined at 1650℃ in a vacuum oxygen blowing decarburization refining furnace to obtain the first molten steel. Then, the first molten steel is refined at 1630℃ in a ladle refining furnace to obtain the second molten steel.

[0086] S12: Cast slab: The second molten steel after ladle refining is poured into a continuous casting slab, which is then cast into a 30mm thick slab using continuous casting technology and electromagnetic stirring control, followed by controlled cooling.

[0087] S13: Coil: The slab is rolled into a 4mm thick hot-rolled plate at 1020℃, then cooled, tempered at a temperature range of 630℃, pickled, and finally rolled to produce coils with a length >500 meters.

[0088] The process of longitudinally cutting and butt welding the stainless steel coil in S2 is as follows: the stainless steel coil is longitudinally cut into 153mm steel strips according to the specifications of the continuous tubing; then, according to the specified length requirements of the continuous tubing, multiple steel strips are butt welded together in sequence, and the weld seam is heat treated after welding.

[0089] When welding long steel strips, first process the ends of the steel strips into a 45° bevel, and then open an I-shaped groove on the bevel. Weld using gas metal arc welding (GMAW). Before welding, preheat the groove under an argon atmosphere at a temperature of 200°C. During welding, the shielding gas is 95% Ar by volume + 5% CO2 by volume. The welding current is 230A, the welding voltage is 30V, the welding speed is 260mm / min, and the gas flow rate is 20L / min.

[0090] After the steel strip is welded, the weld is pushed into wood ash for slow cooling; after the weld is cooled to ≤150℃, the weld is subjected to conditioning heat treatment. The conditioning heat treatment first heats the weld to 1020℃, holds it for 8 minutes, and then quenches it; then heats the weld to 620℃, holds it for 10 minutes, and then air cools it.

[0091] The specific process for preparing the straight-seam ultra-high strength stainless steel continuous tubing in S3 is as follows:

[0092] S31: Based on the outer diameter and wall thickness requirements of the coiled tubing, mill the edges on both sides of the extended steel strip and open an I-bevel.

[0093] S32: The UOE roller forming method is used to control the continuous and stable forming of steel strip. During the forming of steel strip, the bevel gap is ≤0.2mm, the misalignment is ≤0.3mm, and the weld center offset is ±0.6mm.

[0094] S33: Longitudinal welding of continuous tubing is performed using laser-TIG. The bevel is preheated to 150℃, the gas is 95% Ar volume fraction + 5% CO2 volume fraction, the TIG welding current is 180A, the TIG welding voltage is 24V, the laser defocusing amount is +3mm, the laser power is 1.5Kw, the welding speed is 3.5m / min, and the gas flow rate is 22L / min. The final product is a straight seam welded stainless steel pipe with a diameter of Φ50.8 mm and a wall thickness of 4mm.

[0095] The heat treatment process for the S4 longitudinal weld and the entire pipe body is as follows:

[0096] S41: The weld is locally heated to 1020℃, and a certain deformation is applied to the weld by sizing the pipe using extrusion rollers, followed by quenching.

[0097] S42: After quenching, the entire pipe is heated to 700°C using medium-frequency induction heating, held for 12 minutes, and then air-cooled to below 150°C. The entire pipe is then heated to 650°C for a second tempering, held for 8 minutes, and then air-cooled to room temperature.

[0098] S43: After the entire pipe body is heat-treated, citric acid is used to acidify the pipe. After pickling, deionized water is used to clean the pipe. After cleaning, the pipe is wound onto a drum of appropriate core diameter by a coiler to continuously produce 7200m of ultra-high strength stainless steel continuous tubing for transportation and use.

[0099] The microstructure of the ultra-high strength stainless steel continuous tubing consists of tempered martensite and dispersed inverted austenite, with the volume fraction of the austenite being ≤10%.

[0100] Pipe microstructure such as Figure 1 As shown, the coiled tubing has a yield strength of 865 MPa, a tensile strength of 935 MPa, an elongation of 25%, a hardness of 31 HRC, an external pressure resistance of 152.9 MPa, an internal pressure resistance of 171.3 MPa, an outer diameter range of Φ50.8 mm, a wall thickness range of 4 mm, and a maximum running depth of 7254 m under a safety factor of 1.5. Figures 3-5 As shown.

[0101] Example 4

[0102] A method for processing ultra-high strength stainless steel continuous tubing includes the following steps:

[0103] S1: Preparation of stainless steel coils;

[0104] S2: Stainless steel coil, longitudinally cut and butt-welded length;

[0105] S3: Using laser-TIG composite welding, prepare straight seam ultra-high strength stainless steel continuous tubing;

[0106] S4: The longitudinal welds and the entire pipe body are heat-treated.

[0107] The specific steps for preparing the stainless steel coil in S1 are as follows:

[0108] S11: Steelmaking: Steelmaking raw materials are melted into molten iron. The chemical composition of the molten iron by weight percentage is C: 0.01%, Si: 0.60%, Mn: 0.7%, P: 0.02%, S: 0.02%, Ni: 3.80%, Cr: 12.80%, Mo: 0.70%, Cu: 0.3%, with the balance being Fe and unavoidable impurities. The molten iron is refined at 1620℃ in a vacuum oxygen blowing decarburization refining furnace to obtain the first molten steel. Then, the first molten steel is refined at 1600℃ in a ladle refining furnace to obtain the second molten steel.

[0109] S12: Cast slab: The second molten steel after ladle refining is poured into a continuous casting slab, which is then cast into a 40mm thick slab using continuous casting technology and electromagnetic stirring control, followed by controlled cooling.

[0110] S13: Coil: The slab is rolled into a hot-rolled plate with a thickness of 5.2mm at 1040℃, then cooled, tempered at a temperature range of 680℃, pickled, and then rolled to finally produce coils with a length of >400 meters.

[0111] The process of longitudinally cutting and butt welding the stainless steel coil in S2 is as follows: the stainless steel coil is longitudinally cut into 222mm steel strips according to the specifications of the continuous tubing; then, according to the specified length requirements of the continuous tubing, multiple steel strips are butt welded together in sequence, and the weld seam is heat treated.

[0112] When welding long steel strips, first process the ends of the steel strips into a 45° bevel, and then open a V-shaped groove on the bevel. Weld using gas metal arc welding (GMAW). Before welding, preheat the groove under an argon atmosphere at a temperature of 250°C. During welding, the shielding gas is 95% Ar by volume + 5% CO2 by volume. The welding current is 235A, the welding voltage is 31V, the welding speed is 240mm / min, and the gas flow rate is 20L / min.

[0113] After the steel strip is welded, the weld is pushed into wood ash for slow cooling; after the weld is cooled to ≤150℃, the weld is subjected to conditioning heat treatment. The conditioning heat treatment first heats the weld to 1030℃, holds it for 10 minutes, and then quenches it; then heats the weld to 640℃, holds it for 18 minutes, and then air cools it.

[0114] The specific process for preparing the straight-seam ultra-high strength stainless steel continuous tubing in S3 is as follows:

[0115] S31: Based on the outer diameter and wall thickness requirements of the coiled tubing, mill the edges on both sides of the extended steel strip and open an I-bevel.

[0116] S32: The UOE roller forming method is used to control the continuous and stable forming of steel strip. During the forming of steel strip, the bevel gap is ≤0.2mm, the misalignment is ≤0.4mm, and the weld center offset is ±0.6mm.

[0117] S33: Longitudinal welding of continuous tubing was performed using laser-TIG (Thin-Seam Welding). The bevel was preheated to 150℃, with a gas mixture of 95% Ar and 5% CO2. The TIG welding current was 210A, the TIG welding voltage was 27V, the laser defocusing depth was +1mm, the laser power was 2kW, the welding speed was 3 m / min, and the gas flow rate was 24L / min. The final product was a straight-seam welded stainless steel pipe with a diameter of Φ73 mm and a wall thickness of 5.2 mm.

[0118] The heat treatment process for the S4 longitudinal weld and the entire pipe body is as follows:

[0119] S41: The weld is locally heated to 1040℃, and a certain deformation is applied to the weld by sizing the pipe using extrusion rollers, followed by quenching.

[0120] S42: After quenching, the entire pipe is heated to 730°C using medium-frequency induction heating, held for 18 minutes, and then air-cooled to below 150°C. The entire pipe is then heated to 670°C for a second tempering, held for 10 minutes, and then air-cooled to room temperature.

[0121] S43: After the entire pipe body is heat-treated, citric acid is used to acidify the pipe. After pickling, deionized water is used to clean the pipe. After cleaning, the pipe is wound onto a drum of appropriate core diameter by a coiler to continuously produce 6000m of ultra-high strength stainless steel continuous tubing for transportation and use.

[0122] The microstructure of the ultra-high strength stainless steel continuous tubing consists of tempered martensite and dispersed inverted austenite, with the volume fraction of the austenite being ≤10%.

[0123] Pipe microstructure such as Figure 2 As shown, the coiled tubing has a yield strength of 795 MPa, a tensile strength of 889 MPa, an elongation of 23%, a hardness of 30 HRC, an external pressure resistance of 200.9 MPa, an internal pressure resistance of 219.1 MPa, an outer diameter range of Φ73 mm, a wall thickness range of 5.2 mm, and a maximum running depth of 6667 m under a safety factor of 1.5. Figures 3-5 As shown.

[0124] Comparative Example 1

[0125] Currently, the most commonly used stainless steel continuous tubing in China is duplex stainless steel continuous tubing. Its microstructure is mainly composed of ferrite and austenite, with a volume fraction ratio of approximately 50%:50% for ferrite and austenite. The chemical composition by mass percentage is: C≤0.03%, Mn≤2.0%, Si≤1.0%, Cr: 22~23%, Ni: 4.5~6.5%, N: 0.14~0.2%, Mo: 3.0~3.5%, P≤0.030%, S≤0.020%.

[0126] The main manufacturing methods for duplex stainless steel continuous pipes are as follows:

[0127] S1: Manufacturing duplex stainless steel coils

[0128] It is made into hot-rolled sheets with a thickness of 2.4 to 6.35 mm, rolled, and finally made into coils with a length of more than 400 meters;

[0129] S2: Slitting of coiled plates and splicing of steel strips

[0130] The prepared duplex stainless steel coils are cut into steel strips of 50–300 mm using a slitting machine according to the specifications of continuous pipes. The ends of the two steel strips are processed to 45° and then beveled appropriately. The bevels are I-shaped, V-shaped, or U-shaped. The steel strips are butt-welded using gas-protected laser welding, argon arc welding, or plasma welding. After the weld cools, the weld surface is ground and cleaned. Then, the weld and heat-affected zone are reheated to 1020–1150°C in a protective atmosphere, held for 30–300 seconds, and then cooled to room temperature at a cooling rate greater than 7°C / s.

[0131] S3: Forming and Welding

[0132] Based on the final outer diameter and wall thickness requirements of the duplex stainless steel continuous tube, the steel strip is milled into an I-shaped bevel on the side using a milling method. The width of the steel strip and the perpendicularity of the plate edge are precisely controlled. The UOE roller forming method is used to control the forming of the steel strip. Laser welding technology is used to precisely control parameters such as defocusing amount, laser power, and welding speed. The forming bevel gap is controlled to be ≤0.1 mm, the defocusing amount is -5 to 10 mm, the laser power is 2 kW to 16 kW, and the welding speed is 1.5 to 10 m / min. The formed steel strip is longitudinally welded to ensure welding quality. Duplex stainless steel continuous tubes with a diameter of Φ25.4 to Φ88.9 mm and a wall thickness of 2.4 to 6.4 mm are welded.

[0133] S4: Heat treatment of the entire tube body

[0134] After welding, the entire duplex stainless steel continuous pipe is heated to 1020-1150℃ using medium-frequency induction heating and held for 0.5-10 minutes. Then, under H2 atmosphere protection, the pipe is rapidly cooled to room temperature at a cooling rate of ≥7℃ / s.

[0135] S5: Continuous curling:

[0136] After heat treatment, duplex stainless steel continuous tubes are wound onto a drum of appropriate core diameter using a coiler to continuously produce duplex stainless steel continuous tubes with lengths of 1000 to 10000 m.

[0137] According to the manufacturing method described in Comparative Example 1 above, the duplex stainless steel continuous tubing produced has a yield strength of 552-689 MPa and a tensile strength of 689-800 MPa.

[0138] Taking a duplex stainless steel continuous pipe with an outer diameter of Φ50.8 mm and a wall thickness of 4 mm, manufactured according to the manufacturing method described in Comparative Example 1, as an example, its yield strength is 600 MPa, tensile strength is 760 MPa, elongation reaches 33%, hardness is 31 HRC, external pressure resistance is 101.3 MPa, internal pressure resistance is 144.5 MPa, and the maximum well depth under a safety factor of 1.5 is 4969 m. Figure 5 As shown.

[0139] As can be seen from the pipe performance of Examples 3, 4, and Comparative Example 1, the yield strength, tensile strength, and well depth of the ultra-high strength stainless steel coiled tubing of the present invention are higher than those of duplex stainless steel coiled tubing, while the hardness is basically the same.

[0140] Using the ultra-high strength stainless steel coiled tubing and duplex stainless steel coiled tubing described in this invention, a 168-hour simulated corrosion test was conducted on the CO2 injection tubing in the CCUS-EOR technology of an oilfield. The test conditions are shown in Table 1. The corrosion rate of the samples was calculated after the test. Figure 6 As shown, the corrosion rate of the ultra-high strength stainless steel coiled tubing of the present invention is 0.0061 mm / a, and the corrosion rate of the duplex stainless steel coiled tubing is 0.0067 mm / a. The corrosion rates of the two corrosion-resistant coiled tubings are basically the same, both less than the corrosion rate requirements specified by oilfield users (<0.076 mm / a).

[0141] Table 1 shows the service conditions of CO2 injection pipes.

[0142]

[0143] The above description is only a preferred embodiment of the present invention, but the scope of protection of the present invention is not limited thereto. Any changes or substitutions that can be easily conceived by those skilled in the art within the scope of the technology disclosed in the present invention should be included within the scope of protection of the present invention.

Claims

1. An ultra-high strength stainless steel coiled tubing characterized by: The chemical element composition of the stainless steel coiled tubing is as follows in percentage by weight: C≤0.03%, Si≤1.00%, Mn: 0.5~1.0%, P≤0.03%, S≤0.03%, Ni: 3.50~5.50%, Cr: 12.00~13.50%, Mo: 0.50~1.00%, Cu≤0.5%, and the balance of Fe and inevitable impurities. S1: preparing the stainless steel coil plate; S2: longitudinal cutting and butt welding of the stainless steel coil plate; S3: preparing the straight seam super high strength stainless steel coiled tubing by using laser-TIG hybrid welding; S4: heat treatment of the longitudinal weld and the whole pipe body, and the process of the heat treatment of the longitudinal weld and the whole pipe body is as follows: S41: locally heating the weld to 980~1050℃, and applying a certain deformation to the weld by using the sizing method of the extrusion roller, and then quenching; S42: after the quenching, heating the whole pipe body to 590~750℃ by using the intermediate frequency induction heating method, and keeping the temperature for 5~20min, and then air cooling to below 150℃, and then heating the whole pipe body to 600~680℃ for secondary tempering, and keeping the temperature for 5~10min, and then air cooling to room temperature; S43: after the heat treatment of the whole pipe body, carrying out pickling, cleaning and coiling of the stainless steel coiled tubing.

2. The ultra-high strength stainless steel coiled tubing of claim 1, wherein: The specific steps of preparing the stainless steel coil plate in S1 are as follows: S11: steelmaking: melting the steelmaking raw materials into molten iron, the chemical composition of the molten iron is as follows in percentage by weight: C≤0.03%, Si≤1.00%, Mn: 0.5~1.0%, P≤0.03%, S≤0.03%, Ni: 3.50~5.50%, Cr: 12.00~13.50%, Mo: 0.50~1.00%, Cu≤0.5%, and the balance of Fe and inevitable impurities, refining the molten iron at 1600~1660℃ in a vacuum oxygen blowing decarburization refining furnace to obtain first molten steel, and then refining the first molten steel at 1550~1700℃ in a ladle refining furnace to obtain second molten steel; S12: slabbing: casting the second molten steel after ladle refining into a continuous casting slab, and applying electromagnetic stirring to control the casting into a slab with a thickness of 20~60mm by using continuous casting technology and controlled cooling; S13: coiling: finishing the slab to a hot-rolled plate with a thickness of 2.4~6.35mm at a temperature of 1000~1050℃, controlled cooling, tempering, pickling and coiling after the temperature range of 580~750℃, and finally preparing a coil plate with a length of >400m.

3. The ultra-high strength stainless steel coiled tubing of claim 1, wherein: The process of longitudinal cutting and butt welding of the stainless steel coil plate in S2 is as follows: cutting the stainless steel coil plate into steel strips with a length of 150~400mm according to the requirements of the coiled tubing; and then butt welding a plurality of steel strips according to the length requirements of the coiled tubing, and performing quenching and tempering treatment on the weld after the butt welding.

4. The ultra-high strength stainless steel coiled tubing of claim 3, wherein: The steel strip is butt welded for a long time, the end of the steel strip is first processed into a 45° bevel, an I-shaped or V-shaped groove is opened on the bevel, and a gas shielded welding mode is adopted for welding, the groove is preheated under the protection of argon gas before welding, the preheating temperature is 150-300℃, the protective gas is Ar volume fraction 95%+CO2 volume fraction 5% gas during welding, the welding current is 220-240A, the welding voltage is 28-32V, the welding speed is 200-350mm / min, and the gas flow is 18-25L / min.

5. The ultra-high strength stainless steel coiled tubing of claim 3, wherein: After the steel strip is welded, the weld is pushed into wood ash for slow cooling, and the weld is subjected to tempering heat treatment after being cooled to ≤150℃, the weld is heated to 980-1050℃ first, held for 1-10min, quenched, and then heated to 580-650℃, held for 2-20min, and air cooled.

6. The ultra-high strength stainless steel coiled tubing of claim 1, wherein: The specific process for preparing the straight-welded ultra-high-strength stainless steel coiled tubing in S3 is as follows: S31: according to the outer diameter and wall thickness requirements of the coiled tubing, the two sides of the lengthened steel strip are milled, and an I-shaped or V-shaped groove is opened; S32: the UOE roll forming method is adopted to control the continuous and stable forming of the steel strip, the gap of the groove is ≤0.3mm, the misalignment amount is ≤0.5mm, and the weld center offset amount is ±0.6mm during the forming of the steel strip; S33: laser-TIG is used for continuous longitudinal welding of the coiled tubing, the groove is preheated to ≤150℃, the protective gas is Ar volume fraction 95%+CO2 volume fraction 5% gas, the TIG welding current is 150-240A, the TIG welding voltage is 22-32V, the laser defocusing amount is -5-+5mm, the laser power is 1Kw-5Kw, the welding speed is 1.5-5m / min, and the gas flow is 18-25L / min.

7. The ultra-high strength stainless steel coiled tubing of claim 1, wherein: The microstructure of the ultra-high-strength stainless steel coiled tubing is tempered martensite structure plus dispersed reversed austenite structure, and the volume fraction of the austenite structure is ≤10%.

8. The ultra-high strength stainless steel coiled tubing of claim 1, wherein: The yield strength of the coiled tubing is 758-965MPa, the tensile strength is 798-1034MPa, the elongation is ≥17%, the outer diameter ranges from Φ50.8mm to Φ88.9mm, the wall thickness ranges from 3.7mm to 6.4mm, the pipe length ranges from 1000m to 10000m, and the hardness is ≤32HRC.

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