Steel for coiled tubing serving in acid environment and production method thereof

Abstract

The invention discloses steel for a coiled tubing serving in an acid environment. The steel comprises the following chemical components in percentage by mass: 0.07%-0.11% of C, 0.15%-0.30% of Si, 1.06%-1.15% of Mn, less than or equal to 0.012% of P, less than or equal to 0.0008% of S, 0.16%-0.19% of Cu, 0.08%-0.10% of Ni, 0.41%-0.51% of Cr, 0.020%-0.030% of Nb, 0.005%-0.010% of Ti, 0.0025%-0.0030% of Ca, 0.020%-0.030% of Al, less than or equal to 0.0030% of N, less than or equal to 0.0004% of B and the balance of Fe and inevitable impurities. The high-strength coiled tubing product is designed on the basis of the API 5CT standard, economic component design is adopted, a corresponding production method is matched, and the obtained coiled tubing serving in the acid-resistant environment has excellent strength and plasticity (the yield strength is larger than or equal to 680 MPa, the tensile strength is larger than or equal to 720 MPa, and the ductility is larger than or equal to 24%), excellent fatigue performance and acid resistance are achieved, and the service life of the coiled tubing is prolonged. And moreover, the hardness of the material can be ensured to be within a standard range while the excellent performance is ensured, and the material can cope with various acidic service well conditions.

Description

Technical Field

[0001] The invention belongs to the field of steelmaking, and in particular relates to a steel for a continuous oil pipe serving in an acidic environment and a production method thereof. Background Art

[0002] Coiled tubing is widely used in oil and gas field workover, drilling, completion, and logging operations. With the development of oil and gas operations, the various service environments have become more complex. The industry is focusing on how to economically design coiled tubing products that can withstand sour service environments. Existing composite steel coiled tubing utilizes a double-layer design. Furthermore, the design incorporates a large amount of alloying elements such as nickel, molybdenum, and chromium, resulting in high production costs and complex processes.

[0003] There are also some acid-resistant continuous oil pipe steels in the existing technology. They are designed with low C and Mn components and add corrosion-resistant alloys such as Cu, Cr, and Mo. The raw materials are produced through controlled rolling and controlled cooling methods. The strength of the prepared continuous oil pipes is limited, which affects the overall fatigue life of the continuous oil pipes. Summary of the Invention

[0004] The technical problem to be solved by the present invention is to provide a continuous oil pipe steel for serving in sour environments in response to the deficiencies in the above-mentioned existing technologies. The steel has a uniform tempered bainite structure, is not only high in strength and good in toughness, but also has suitable hardness and excellent acid resistance, and can cope with a variety of sour service well conditions.

[0005] The technical solution adopted by the present invention to solve the above-mentioned problems is: A coiled tubing steel for service in a sour environment, the chemical composition of which is as follows, calculated by mass percentage: C 0.07%-0.11%, Si 0.15%-0.30%, Mn 1.06%-1.15%, P≤0.012%, S≤0.0008%, Cu 0.16%-0.19%, Ni 0.08%-0.10%, Cr 0.41%-0.51%, Nb 0.020%-0.030%, Ti 0.005%-0.010%, Ca 0.0025%-0.0030%, Al 0.020%-0.030%, N≤0.0030%, B≤0.0004%, and the remainder being Fe and unavoidable impurities.

[0006] Furthermore, the mass percentages of C, Mn, and Cr satisfy (C+Mn+Cr / 2)×10 4 Between 133 and 152, preferably between 140 and 150.

[0007] Furthermore, the mass percentages of Nb and Ti satisfy (Nb+Ti)×10 4Between 2.5 and 4, preferably between 3.2 and 3.8.

[0008] Furthermore, the mass percentage of Si is Si 0.18% to 0.21%.

[0009] Furthermore, the mass percentage of Mo does not exceed 0.003%, preferably does not exceed 0.002%.

[0010] The design concept of the present invention on chemical composition is as follows: Carbon (C): The most economical interstitial solid solution strengthening element, but also the element most susceptible to segregation. Considering the quenching strength, segregation and hardness, the suitable carbon content of the present invention is 0.07% to 0.11%.

[0011] Silicon (Si): It aids in desulfurization and improves the material's corrosion resistance. However, excessive silicon content can cause severe tiger stripes on the surface and affect welding performance. The optimal Si content for this invention is 0.15% to 0.30%.

[0012] Manganese (Mn): As a solid solution strengthening element, it can improve material strength. However, Mn has a low diffusion coefficient in the solid phase (such as austenite), and its diffusion rate is much lower than the propagation rate of the solidification interface. This makes it difficult to distribute evenly, exacerbating segregation and easily causing cracking in segregated areas in acidic service environments. The optimal Mn addition in this invention is 1.06% to 1.15%.

[0013] Phosphorus and sulfur (P, S): P can promote the production of hydrochloric acid corrosion passivation film, but too high a content can easily lead to central segregation of the material and increased brittleness. The suitable P content of the present invention is 0.010% to 0.012%. S can easily lead to the production of Mn / S inclusions, which is a fatal defect for materials serving in acidic conditions. Its content should be strictly limited. The suitable content of the present invention should be S≤0.0008%.

[0014] Copper and Nickel (Cu, Ni): Copper forms a dense oxide layer on the surface of the material, improving its corrosion resistance. Ni improves the material's hardenability and forms high-melting-point compounds with copper, preventing the formation of low-melting-point copper compounds along the grains during heating and cracking during hot rolling. In this invention, Cu content is preferably 0.16% to 0.19%, and Ni content is 0.08% to 0.10%.

[0015] Chromium (Cr): The passive film formed by its corrosion products can inhibit corrosion or reduce the corrosion rate, while also increasing the hardenability of the steel. For economic reasons, the appropriate addition amount of Cr in the present invention is 0.41% to 0.51%.

[0016] Niobium and titanium (Nb, Ti): As grain-refining elements, Nb and Ti can precipitate alone or in combination with Ti within austenite grains. Nb inhibits austenite grain growth while serving as a nucleation site to refine the transformation structure. Excessive additions can result in large-scale TiN and NbTi composite precipitation, affecting the material's acid resistance and impact toughness. Suitable additions for this invention are 0.020% to 0.030% Nb and 0.005% to 0.010% Ti.

[0017] Calcium (Ca): Ca is added primarily to modify inclusions, initially forming small, discrete Ca oxides that serve as nucleation sites for inclusions, thereby refining the inclusions in the steel. Excessive Ca additions can lead to excessive oxide inclusions, affecting the material's acid resistance. The optimal addition level for this method is 0.0025% to 0.0030%.

[0018] Aluminum (Al): Al is added as a deoxidizer. Excessive addition can lead to excessive oxide inclusions in the steel, affecting the steel's purity and adversely affecting the material's acid resistance. The content of Al in this invention is preferably controlled between 0.020% and 0.030%.

[0019] Nitrogen (N): Nitrogen reacts with the steel's Ti to form TiN particles, refining the austenite grains. However, excessive N content can cause the TiN particles to grow rapidly, affecting the material's toughness and acid resistance. The N content in this invention is preferably controlled at ≤0.003%.

[0020] Boron (B): In the present invention, there is no need to add additional B to improve hardenability, and the residual B content in the steel due to the raw material iron alloy is controlled to be ≤0.0004%.

[0021] The present invention adds a certain amount of C, Mn and Cr to the coiled tubing steel used in the sour environment, and (C+Mn+Cr)×10 4 The value of satisfies 133~152, which ensures the basic hardenability of the material. At the same time, a certain amount of Nb and Ti is added, and (Nb+Ti)×10 4 The value of 2.5 to 4 ensures a fine and uniform structure without excessive precipitates. Furthermore, the molten steel undergoes calcium treatment to deform inclusions and enhance the material's acid resistance. Furthermore, the present invention adds certain levels of Cu and Ni to improve the material's overall corrosion resistance, and employs a low-P and ultra-low-S design to reduce internal defects and improve the material's overall service performance.

[0022] The present invention also provides a method for producing the above-mentioned steel for coiled tubing serving in sour environments, comprising the following steps: 1) Smelting and Casting: According to the composition and weight percentage requirements of the steel for coiled tubing serving in sour environment according to the present invention, converter smelting, off-furnace refining and mold slag protection casting are carried out to obtain slabs meeting the composition requirements; 2) Slab heating: The slab is heated in a heating furnace to ensure full solid solution of the alloy and homogenization of the slab composition; 3) Rolling: The heated slab is rough rolled and finished into 3.0-6 mm steel strips. The rough rolling adopts the 1+5 method (i.e., the rough rolling adopts 2 stands, the first stand is rolled in one pass, and the second stand is rolled in 5 passes). The rough rolling temperature is controlled at 1080±30℃; the finishing rolling inlet temperature is controlled between 980-1020℃, and the finishing rolling is carried out using 7 stands. The cumulative deformation of the 3 stands after finishing rolling is ≥30%, and the finishing rolling outlet temperature is controlled at 830±30℃. 4) Controlled cooling: After the steel strip is finished and relaxed for 1 to 3 seconds, it is cooled to 570 to 630°C at a cooling rate of 50 to 70°C / s and then coiled to obtain a steel coil; 5) Pickling: The steel coil is cooled to below 60℃ and sent for pickling to remove the surface oxide scale; 6) Tube making: The steel coil is made into continuous tubing of a certain size and length through a series of tube making processes according to the requirements, and the tube is wound on a disc; 7) Whole pipe heat treatment: The whole coil of steel pipe is uncoiled and heat treated in the heat treatment production line. The specific process includes four steps: preheating of the steel pipe, austenitizing, quenching and induction tempering. The temperature of the preheating section is controlled at 600-700℃, the austenitizing temperature is controlled at 850-920℃, and the austenitizing holding time is 50-60 seconds; after austenitizing, the steel pipe is quenched to room temperature at a quenching rate of 120-150℃ / s; during the tempering process, the heating rate is controlled at 80-120℃ / s, the tempering temperature is controlled at 680-720℃, and the tempering time is 60-75 seconds. After tempering, the steel pipe is air-cooled to less than 150℃ and then water-cooled to room temperature. The steel pipe is coiled to obtain the final product, continuous oil pipe steel (oil pipe).

[0023] According to the above scheme, in step 1), smelting is carried out in a converter, and the P content is precisely controlled within 0.012%; LF+RH double refining is adopted for refining outside the furnace, and the sulfur content of the steel is controlled within 0.0008% through LF deep desulfurization. The harmful gas oxygen content and inclusions in the molten steel are controlled through RH degassing and calcium treatment (calcium treatment is carried out by feeding calcium wire into the molten steel, and the calcium wire input amount is 2-3 kg / t), wherein the O content is ≤0.0020%, the inclusion A is level 0, and the B and C inclusions are ≤level 0.5; a straight arc casting machine is used for protective slag casting, and the superheat is controlled within 20°C to avoid segregation of the slab during the casting process and improve the acid resistance of the material.

[0024] According to the above scheme, in step 2), the slab is heated in a heating furnace in four stages: a preheating stage, a first heating stage, a second heating stage, and a homogenization stage. This ensures full solutionization of the alloy and uniform composition of the slab. The preheating stage temperature is controlled between 600°C and 700°C, with a preheating duration of 35 to 50 minutes. The first heating stage heats the slab to 900°C to 1000°C, with a holding time of 30 to 40 minutes. The second heating stage heats the slab to 1150°C to 1200°C, with a holding time of 40 to 60 minutes. The homogenization stage heats the slab to 1230°C to 1260°C, with a heating time of 40 to 50 minutes. A reducing, negative oxygen atmosphere, primarily hydrogen, should be used throughout the heating furnace. The oxygen volume percentage should be kept below 0.2% to prevent Cu precipitation at grain boundaries, which could affect the material's subsequent service life.

[0025] According to the above scheme, in step 3), during the steel rolling process, rough rolling is performed using a 1+5 rolling method, and the rough rolling temperature is controlled at 1080±30°C to ensure that the slab is fully deformed in the austenite recrystallization zone and the austenite grains are refined. After rough rolling, the slab transfer speed is controlled to ensure that the finishing rolling inlet temperature is between 980 and 1020°C. The cumulative deformation of the three stands after finishing rolling is ≥30%, and the finishing rolling outlet temperature is controlled at 830±30°C to ensure that the austenite undergoes sufficient deformation in the non-recrystallization zone to accumulate distortion energy, thereby preparing for grain refinement during the cooling process.

[0026] According to the above solution, in step 7), the entire coil of steel pipe is uncoiled and heat treated on the heat treatment line. The specific process includes four steps: preheating, austenitizing, quenching, and induction tempering. The temperature in the preheating section is controlled between 600°C and 700°C. This ensures that the pipe temperature can be quickly raised to the austenitizing range while maintaining production speed and ensuring austenitization. The austenitizing temperature is controlled between 850°C and 920°C, and the austenitizing time is 50 to 60 seconds to fully austenitize the pipe. After austenitization, the material is quenched at a cooling rate of 120-150℃ / s to room temperature to refine the quenched structure while ensuring the quenched strength. During the tempering process, the heating rate is controlled at 80-120℃ / s, the tempering temperature is controlled at 680-720℃, and the tempering time is 60-75 seconds. The high heating rate allows the material to quickly reach the tempering temperature range to ensure the tempering effect. At the same time, the use of high-temperature tempering can eliminate quenching losses and improve the material's acid resistance.

[0027] Compared with the prior art, the present invention has the following beneficial effects: First, the present invention designs high-strength coiled tubing products based on the API 5CT standard. By adopting an economical component design and combining it with appropriate production methods, the resulting coiled tubing, designed for service in sour environments, not only exhibits excellent strength and ductility (yield strength ≥ 680 MPa, tensile strength ≥ 720 MPa, elongation ≥ 24%), excellent fatigue performance, and acid resistance, but also maintains these excellent properties while ensuring the material's hardness falls within the standard range, greatly enhancing the material's value for widespread application.

[0028] Secondly, in the production method of the present invention, on the one hand, the rough rolling is carried out in a 1+5 manner during the steel rolling process, firstly, the slab is fully deformed in the austenite recrystallization zone to refine the austenite grains, and then the slab transfer speed is controlled to reach the finishing rolling inlet temperature. After a certain deformation amount is reached, the finishing rolling outlet temperature is controlled to ensure that the austenite undergoes sufficient deformation in the non-recrystallization zone to accumulate distortion energy, preparing for grain refinement during the cooling process; on the other hand, the steel strip is air-cooled for about 2 seconds after finishing rolling for relaxation. During this process, as many nucleation points as possible are formed inside the material to prepare for phase transformation in the subsequent cooling process. A large cooling rate is combined with an appropriate coiling temperature to achieve a good cooling effect. A combined method is used to obtain a fine and uniform structure, which is beneficial to subsequent forming and heat treatment; thirdly, the heat treatment process includes four steps: preheating of the steel pipe, austenitizing, quenching and induction tempering. The preheating section ensures the production rhythm while allowing the steel pipe to quickly heat up to the austenitizing temperature range to ensure the austenitizing effect. The austenitizing temperature and time allow the steel pipe to fully austenitize and quench to room temperature, refining the martensite lath while ensuring the quenching strength. The tempering process uses a high heating rate to allow the material to quickly reach the tempering temperature range to ensure the tempering effect. At the same time, the use of high-temperature tempering can eliminate quenching losses and improve the material's acid resistance. BRIEF DESCRIPTION OF THE DRAWINGS

[0029] Figure 1 and Figure 2 These are the microstructure morphologies of the base material and weld of the present invention, and the microstructure type is tempered bainite. DETAILED DESCRIPTION

[0030] In order to better understand the present invention, the content of the present invention is further illustrated below in conjunction with examples, but the present invention is not limited to the following examples.

[0031] Each embodiment of the present invention is produced according to the following steps: 1) Smelting and Casting: According to the present invention, converter smelting, off-furnace refining and mold slag protection casting are carried out according to the composition and mass percentage requirements to obtain slabs that meet the composition requirements, as shown in Table 1.

[0032] During converter smelting, the P content is precisely controlled within 0.012%. During refining outside the furnace, LF+RH double refining is adopted. The sulfur content of the steel is controlled within 0.0008% through LF deep desulfurization. The harmful gas oxygen content and inclusions in the molten steel are controlled through RH degassing and calcium treatment (calcium treatment is carried out by feeding calcium wire into the molten steel, with a calcium wire input of 2-3 kg / t). The O content is ≤0.0020%. A straight arc casting machine is used for protective slag casting, and the superheat is controlled within 20°C.

[0033] 2) Slab Heating: The slab is heated in a heating furnace in four stages: preheating, first heating, second heating, and homogenization. This ensures full solutionization of the alloy and homogenization of the slab composition. The preheating temperature is controlled between 600°C and 700°C, with a preheating duration of 35 to 50 minutes. The temperature at the end of the first heating stage is controlled between 900°C and 1000°C, with a heating duration of 30 to 40 minutes. The temperature at the end of the second heating stage is controlled between 1150°C and 1200°C, with a heating duration of 40 to 60 minutes. The homogenization stage is controlled between 1230°C and 1260°C, with a heating duration of 40 to 50 minutes. A reducing, negative oxygen atmosphere, primarily hydrogen, should be maintained throughout the heating process. The oxygen volume percentage should be kept below 0.2%.

[0034] 3) Rolling: The heated slab is rough rolled and finished rolled into 3.0-6 mm steel strips. The rough rolling is carried out in a 1+5 manner during the rolling process, and the rough rolling temperature is controlled at 1080±30°C. After rough rolling, the slab transfer speed is controlled to ensure that the finishing inlet temperature is between 980 and 1020°C. Finish rolling is carried out using 7 stands, and the cumulative deformation of the 3 stands after finishing rolling is ≥30%. At the same time, the finishing outlet temperature is controlled at 830±30°C.

[0035] 4) Controlled cooling: After the steel strip is finished and relaxed for 2 seconds, it is cooled to 570-630°C at a cooling rate of 50-70°C / s and coiled to obtain a steel coil; 5) Pickling: The steel coil is cooled to below 60℃ and sent for pickling to remove the surface oxide scale; 6) Tube Making: The steel coil is processed into coiled tubing with a diameter of 50.8mm to 60.3mm and a length of 5000 to 6000m through a series of tubing making processes according to demand, and the tubing is then wound on a disc; 7) Whole pipe heat treatment: The whole coil of steel pipe is uncoiled and heat treated in the heat treatment production line. The specific process includes four steps: preheating, austenitizing, quenching and induction tempering. The temperature of the preheating section is controlled at 600-700℃, the austenitizing temperature is controlled at 850-920℃, and the austenitizing holding time is 50-60 seconds. After austenitizing, the pipe is quenched to room temperature at a quenching rate of 120-150℃ / s. During the tempering process, the heating rate is controlled at 80-120℃ / s, the tempering temperature is controlled at 680-720℃, and the tempering time is 60-75 seconds. After tempering, the pipe is air-cooled to less than 150℃ and then water-cooled to room temperature. The pipe is coiled to obtain the final product (oil pipe).

[0036] The production steps of Comparative Example 1 were carried out with reference to Chinese patent application CN113637925 A.

[0037] Tables 2-1, 2-2, 2-3, and 3 list the main process parameters and corresponding properties for each embodiment and comparative example of the present invention, respectively. The mechanical properties (yield strength, tensile strength, and elongation, as well as impact energy and steel grade) in Table 3 were tested according to ASTM A370, fatigue testing according to group standard T / CPI 13005-2023, hardness testing according to ASTM E18, and acid resistance HIC testing according to NACE TM0284.

[0038] Table 1 Chemical composition of various embodiments and comparative examples of the present invention (unit: wt%)

[0039] Note: In Table 1, a represents (C+Mn+Cr / 2)×10 4 ; b represents (Nb+Ti)×10 4 .

[0040] Table 2-1 List of main process parameters of various embodiments and comparative examples of the present invention

[0041] Table 2-2 List of main process parameters of various embodiments of the present invention and comparative examples

[0042] Table 2-3 Main process parameters of various embodiments and comparative examples of the present invention

[0043] Table 3 Performance data of various embodiments of the present invention and comparative examples

[0044] Note: CLR (crack length percentage), CTR (crack thickness percentage) and CSR (crack sensitivity percentage) are indicators to measure the material's resistance to hydrogen induced cracking (HIC).

[0045] The steels for sour environment resistant coiled tubing prepared in Examples 1 to 5 were tested according to GB T 10561 2005 standard, and the inclusions A were level 0, and the inclusions B and C were ≤ level 0.5.

[0046] like Figure 1 As shown in FIG, the microstructure of the steel (i.e., the parent material) for continuous oil pipes used in acidic environments prepared by the present invention is tempered troostite. Figure 2 As shown, the steel for continuous oil pipe used in acidic environment prepared by the present invention has a microstructure of the weld part as shown in FIG. Figure 2 As shown, the tissue type is basically the same as that of the parent material.

[0047] As shown in Table 3, the coiled tubing for sour environment service produced by the present invention meets the following performance requirements: yield strength ≥ 680 MPa, tensile strength ≥ 720 MPa, elongation ≥ 24%, hardness HRC < 24, -40°C impact energy > 100 J, fatigue life > 340 cycles, steel grade ≥ 99 Ksi, and no cracks were observed in the HIC test specimens using the NACE TM0284 standard. The present invention utilizes a relatively economical alloy design to produce steel for sour environment-resistant coiled tubing at multiple strength levels. The product exhibits both excellent strength, ductility, and acid resistance. Comparative Example 1, which utilizes a higher Mn content, is susceptible to segregation, which compromises its acid resistance, and does not include low-temperature toughness and acid resistance.

[0048] The above is only a preferred embodiment of the present invention. It should be pointed out that for ordinary technicians in this field, several improvements and changes can be made without departing from the creative concept of the present invention, which all fall within the scope of protection of the present invention.

Claims

1. A steel for coiled tubing resistant to acidic environments, characterized in that: The chemical composition by mass percentage is: C 0.07%~0.11%, Si 0.15%~0.30%, Mn 1.06%~1.15%, P≤0.012%, S≤0.0008%, Cu 0.16%~0.19%, Ni 0.08%~0.10%, Cr 0.41%~0.51%, Nb 0.020%~0.030%, Ti 0.005%~0.010%, Ca0.0025%~0.0030%, Al 0.020%~0.030%, N≤0.0030%, B≤0.0004%, and the rest are Fe and unavoidable impurities.

2. The steel for coiled tubing resistant to acidic environment according to claim 1, characterized in that: The mass percentage of C, Mn and Cr satisfies (C+Mn+Cr / 2)×10 4 Between 133 and 152.

3. The steel for coiled tubing resistant to acidic environment according to claim 1, characterized in that: The mass percentage of Nb and Ti satisfies (Nb+Ti)×10 4 Between 2.5 and 4.

4. The method for producing steel for coiled tubing resistant to acidic environment according to claim 1 or 2, characterized in that: The steps include: 1) Smelting and Casting: After converter smelting, refining outside the furnace and pouring, slabs that meet the composition requirements are obtained; 2) Slab heating: The slab is heated in a heating furnace to ensure full solid solution of the alloy and homogenization of the slab composition; 3) Rolling: The heated slab is rough rolled and finished into 3-6 mm steel strips. The rough rolling process adopts the 1+5 method, and the rough rolling temperature is controlled at 1080±30℃; the finishing rolling inlet temperature is controlled between 980-1020℃. The cumulative deformation of the three stands after finishing rolling is ≥30%, and the finishing rolling outlet temperature is controlled at 830±30℃. 4) Controlled cooling: After the steel strip is finished and relaxed for 1 to 4 seconds, it is cooled to 570 to 630°C at a cooling rate of 50 to 70°C / s and then coiled to obtain a steel coil; 5) Pickling and pipe making: The steel coil is cooled to below 60℃ and sent down for pickling to make continuous tubing, which is then wound up on a disc to obtain a whole coil of steel pipe; 6) Whole pipe heat treatment: The whole coil of steel pipe is uncoiled and heat treated in the heat treatment production line. The specific process includes four steps: steel pipe preheating, austenitizing, quenching and induction tempering. The temperature of the preheating section is controlled at 600-700℃, the austenitizing temperature is controlled at 850-920℃, and the austenitizing holding time is 50-60 seconds. After austenitizing, the pipe is quenched to room temperature at a quenching rate of 120-150℃ / s. During the tempering process, the heating rate is controlled at 80-120℃ / s, the tempering temperature is controlled at 680-720℃, and the tempering time is 60-75 seconds. After tempering, the pipe is air-cooled to less than 150℃ and then water-cooled to room temperature. The pipe is coiled to obtain the final product, coiled oil pipe steel.

5. The method for producing steel for coiled tubing resistant to acidic environment according to claim 4, characterized in that: In step 1), smelting is carried out in a converter, and the P content is precisely controlled within 0.012%. LF+RH double refining is adopted for refining outside the furnace, and the sulfur content of the steel is controlled within 0.0008% through LF deep desulfurization. The harmful gas oxygen content in the molten steel and the inclusions in the steel are controlled through RH degassing and calcium treatment, among which the O content is ≤0.0020%, the inclusion A is level 0, and the inclusions of types B and C are ≤0.

5.

6. The method for producing steel for coiled tubing resistant to acidic environment according to claim 4, characterized in that: In step 2), the slab is heated in a heating furnace in four sections: a preheating section, a first heating section, a second heating section, and a homogenization stage. The temperature at the exit of the preheating section is controlled between 600°C and 700°C, and the preheating section lasts for 35 to 50 minutes. The slab is heated to 900°C to 1000°C in the first heating section and held at that temperature for 30 to 40 minutes. The slab is heated to 1150°C to 1200°C in the second heating section and held at that temperature for 40 to 60 minutes. The temperature in the homogenization section is controlled between 1230°C and 1260°C, and the heating time is 40 to 50 minutes. A reducing negative oxygen atmosphere is used throughout the heating furnace, and the oxygen volume percentage is controlled within 0.2%.

7. The method for producing steel for coiled tubing resistant to acidic environments according to claim 4, characterized in that: In step 3), during the steel rolling process, rough rolling is performed in a 1+5 manner, and the rough rolling temperature is controlled at 1080±30°C; the finishing rolling inlet temperature is between 980 and 1020°C, and the cumulative deformation of the three stands after finishing rolling is ≥30%, and the finishing rolling outlet temperature is controlled at 830±30°C.

8. The method for producing steel for coiled tubing resistant to acidic environments according to claim 4, characterized in that: In step 3), the relaxation time is 2 to 3 s.

9. The steel for coiled tubing produced by the method of claim 4, characterized in that: Yield strength ≥680MPa, tensile strength ≥720MPa, elongation ≥24%, hardness HRC>20; acid resistance tested according to NACE TM0284 standard, no cracks on the steel surface.

10. The steel for coiled oil pipe produced by the method of claim 4, characterized in that: The microstructure type is tempered troostite, inclusion A is level 0, and inclusions of type B and type C are ≤ level 0.5.

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