A production method of a CT70 grade hot-rolled pickling steel strip for coiled tubing

Abstract

This invention discloses a method for producing hot-rolled pickled steel strip for CT70 grade coiled tubing, comprising: smelting and continuous casting, wherein the chemical composition and weight percentage of the continuously cast billet are: C: 0.06~0.10%, Si: 0.10~0.30%, Mn: 0.60~0.90%, P: ≤0.018%, S: ≤0.005%, Cu: 0.20~0.30%, Ni: 0.10~0.25%, Cr: 0.45~0.65%, Mo: 0.10~0.30%, Nb: 0.008~0.015%, with the remainder being Fe and impurities; heating the continuously cast billet; hot rolling; cooling and coiling. This invention, by adding appropriate amounts of Mn, Cu, Cr, Ni, Mo, and Nb, ensures that the product is matched with a reasonable controlled rolling and cooling process and pickling process, resulting in a product with uniform and fine microstructure, stable performance, and good surface quality and strip shape.

Description

Technical Field

[0001] This invention belongs to the field of coiled tubing technology, and particularly relates to a method for producing hot-rolled pickled steel strip for CT70 grade coiled tubing. Background Technology

[0002] Coiled tubing (CT), also known as flexible tubing, coiled tubing, or serpentine tubing, is a type of tubing that is connected to the wellbore via threads. Its total length can reach hundreds or even thousands of meters. It is a continuous, jointless tubing composed of several flexible pipe sections, each over 100 meters in length, welded together using butt welding or bevel welding techniques. The wall thickness is typically 2–7 mm, and the commonly used diameter is 19–127 mm. Primarily used in oilfield operations, it can perform various tasks such as sand flushing, paraffin removal, acidizing, logging, and drilling. It offers significant advantages, especially in horizontal and micro-wellbore operations, and together with its associated equipment, it is referred to as a "universal workpiece," finding widespread application in the petroleum industry.

[0003] Coiled tubing is attracting increasing attention due to its advantages such as reduced operating costs and increased production in special operations. It is imperative to expand the localization of steel for coiled tubing by mass production to meet the required performance.

[0004] To meet production and market demands, we developed the CT70 pickled coil, which meets market requirements. Utilizing advanced 2250mm rolling equipment and rolling processes tailored to the specific needs of the product, the product boasts excellent performance indicators and has received high praise from customers.

[0005] Zhang Chuanguo et al. (Zhang Chuanguo et al., A CT70 Grade Steel for Coiled Tubing, Authorized Patent, May 11, 2011) researched a CT70... A method for producing steel for continuous tubing, comprising smelting, casting, slab reheating, controlled rolling, cooling, and coiling processes, as well as reasonable process parameters; its chemical composition, by weight percentage, is: C: 0.035–0.09%, Si: 0.10–0.60%, Mn: 0.45–1.80%, Cr: 0.32–1.50%, Nb: 0.015–0.100%, Ti: 0.01–0.03%, V: 0.01–0.10%, Mo: ≤0.30%, P: ≤0.015%, S: ≤0.005%, Cu: ≤0.30%, Ni: ≤0.15%, Al: 0.01–0.05%, Ca: ≤0.005%, with the balance being Fe. And the unavoidable inclusions, the high V content in its composition design is detrimental to the toughness and weldability of the product; the yield strength ratio of its steel plates is all above 0.90, the deformability is poor, and it cannot meet the repeated plasticizing required in the operation of this type of product. In addition, the product design process route is not detailed, and there is not much introduction to the pickling production situation.

[0006] Xu Zhenzhen et al. (Xu Zhenzhen et al., Numerical simulation of residual stress distribution of CT70 continuous steel pipe under high frequency resistance welding under different welding parameters, paper, January 2020) studied the residual stress value and distribution law of CT70 continuous oil pipe after high frequency resistance welding, as well as the influence of welding parameters such as welding speed and extrusion amount on residual stress. The shortcoming is that the smelting, rolling and pickling production of the product are not introduced.

[0007] In summary, while the aforementioned patents and papers have described the production and welding processes of CT70, they have failed to clarify the detailed production process of CT70 coiled tubing throughout the entire production line and the product's microstructure and properties. Summary of the Invention

[0008] The purpose of this invention is to provide a method for producing hot-rolled pickled steel strip for CT70 grade coiled tubing. By adding appropriate amounts of Mn, Cu, Cr, Ni, Mo, and Nb, the product is matched with a reasonable controlled rolling and cooling process and pickling process. The product has a uniform and fine microstructure, stable performance, good surface quality and strip shape, and the customer feedback indicates good application results.

[0009] To solve the above-mentioned technical problems, the present invention adopts the following technical solution:

[0010] This invention discloses a method for producing hot-rolled pickled steel strip for CT70 grade coiled tubing, comprising:

[0011] The smelting and continuous casting process includes: hot metal pretreatment, converter composite blowing, LF refining, RH and slab continuous casting to form a continuous casting billet. The chemical composition and weight percentage of the continuous casting billet are as follows: C: 0.06~0.10%, Si: 0.10~0.30%, Mn: 0.60~0.90%, P: ≤0.018%, S: ≤0.005%, Cu: 0.20~0.30%, Ni: 0.10~0.25%, Cr: 0.45~0.65%, Mo: 0.10~0.30%, Nb: 0.008~0.015%, with the balance being Fe and unavoidable impurities.

[0012] Heating of continuously cast billets: The heating temperature is controlled at 1220~1270℃, the soaking temperature is 1210~1260℃, the furnace exit temperature is controlled at 1250±20℃ when the thickness is ≤3.50mm, and the furnace exit temperature is controlled at 120±20℃ when the thickness is 3.51~5.50mm. The time the billet is in the furnace is controlled within 180~240min.

[0013] Hot rolling employs a two-stage controlled rolling process: roughing and finishing. Roughing uses a 3+5 pattern with an initial rolling temperature of 980–1120℃. Finishing begins with an intermediate slab thickness of 40–55 mm for a thickness of 2.77–3.50 mm and a final rolling temperature of 875–1050℃. The intermediate slab thickness is 45–60 mm for a thickness of 3.51–5.50 mm and a final rolling temperature of 865–895℃. Finishing is performed using a 7-stand continuous variable crown mill with an initial rolling temperature of 980–1130℃.

[0014] Cooling and winding: For thicknesses of 2.77~3.50mm, the cooling adopts a "12" mode laminar flow cooling method, which is front-dispersed, with a cooling method of alternating two cooling cycles and a cooling rate controlled at 20~40℃ / s, and a winding temperature of 600~635℃; For thicknesses of 3.51~5.50mm, the cooling adopts a "12" mode laminar flow cooling method, which is front-dispersed, with a cooling method of alternating two cooling cycles and a cooling rate controlled at 25~45℃ / s, and a winding temperature of 595~625℃.

[0015] Furthermore, the chemical composition and weight percentage of the continuously cast billet are as follows: C: 0.06~0.08%, Si: 0.12~0.22%, Mn: 0.70~0.80%, P: ≤0.01%, S: ≤0.002%, Cu: 0.20~0.30%, Cr: 0.48~0.58%, Ni: 0.12~0.22%, Mo: 0.12~0.22%, Nb: 0.008~0.015%, with the balance being Fe and unavoidable impurities.

[0016] Furthermore, the chemical composition and weight percentage of the continuously cast billet are as follows: C: 0.07%, Si: 0.17%, Mn: 0.76%, P: 0.009%, S: 0.001%, Cu: 0.21%, Cr: 0.50%, Ni: 0.16%, Mo: 0.13%, Nb: 0.015%, Ca: 0.0017%, Al 0.035%, with the balance being Fe and unavoidable impurities.

[0017] Furthermore, the chemical composition and weight percentage of the continuously cast billet are as follows: C: 0.06%, Si: 0.18%, Mn: 0.75%, P: 0.008%, S: 0.002%, Cu: 0.20%, Cr: 0.49%, Ni: 0.15%, Mo: 0.12%, Nb: 0.016%, Ca: 0.0020%, Al 0.033%, with the balance being Fe and unavoidable impurities.

[0018] Furthermore, the molten iron undergoes desulfurization pretreatment. The converter employs top-and-bottom combined blowing smelting technology, adding ferromanganese, copper plates, and ferrochrome alloys for deoxidation and alloying, adjusting the composition to meet steelmaking requirements. The converter tapping temperature is ≥1620℃. The molten steel after converter smelting is then transported to the LF refining process for further treatment. Ferromanganese, ferrosilicon, ferrochrome, ferroaluminum, and ferromolybdenum alloys are added according to the steel composition to adjust to the target composition. The molten steel is then transported for RH vacuum treatment, during which the composition and temperature are fine-tuned. The vacuum level is required to be ≤2.6 mbar, and the vacuum time is maintained for ≥10 min, ensuring a pure degassing time of at least 8 min. After vacuum treatment, calcium treatment is performed, and after wire feeding, a soft blowing time of at least 8 min is ensured. Before steel loading, the argon flow is adjusted. The molten steel is kept in a soft-blowing state to prevent it from being exposed. The superheat of the slab continuous casting is 33℃. After that, the slab is cleaned, slowly cooled, and the quality of the continuously cast slab is inspected. The slab heating temperature is 1240℃, the soaking temperature is 1235℃, the tapping temperature is 1230℃, and the heating time is 209 minutes. The heated slab is then descaled with high-pressure water. The width is fixed by a fixed-width press, using two roughing mills and seven CVC finishing mills. The intermediate slab thickness is 45.0mm, and the finishing rolling temperature is 890℃. Laminar flow cooling adopts a front-dispersed cooling mode "12", with the upper and lower nozzles alternately opened, the cooling rate is 38℃ / s, the coiling temperature is 620℃, the finished product thickness is 4.0mm, and the pickling elongation is controlled at 4.0%.

[0019] Furthermore, the molten iron undergoes desulfurization pretreatment. The converter employs top-and-bottom combined blowing smelting technology, adding ferromanganese, copper plates, and ferrochrome alloys for deoxidation and alloying, adjusting the composition to meet steelmaking requirements. The converter tapping temperature is ≥1620℃. The molten steel after converter smelting is then transported to the LF refining process for further treatment. Ferromanganese, ferrosilicon, ferrochrome, ferroaluminum, and ferromolybdenum alloys are added according to the steel composition to adjust to the target composition. The molten steel is then transported for RH vacuum treatment, during which the composition and temperature are fine-tuned. The vacuum level is required to be ≤2.6 mbar, and the vacuum time is maintained for ≥10 min, ensuring a pure degassing time of at least 8 min. After vacuum treatment, calcium treatment is performed, and after wire feeding, a soft blowing time of at least 8 min is ensured. Before steel loading, the argon flow is adjusted. The molten steel is kept in a soft-blowing state to prevent it from being exposed. The superheat of the slab continuous casting is 33℃. After that, the slab is cleaned, slowly cooled, and the quality of the continuously cast slab is inspected. The slab heating temperature is 1245℃, the soaking temperature is 1240℃, the tapping temperature is 1235℃, and the heating time is 210 minutes. The heated slab is then descaled with high-pressure water. The width is fixed by a fixed-width press, using two roughing mills and seven CVC finishing mills. The intermediate slab thickness is 46.0mm, and the finishing rolling temperature is 885℃. Laminar flow cooling adopts a front-dispersed cooling mode "12", with the upper and lower nozzles alternately opened, the cooling rate is 38℃ / s, the coiling temperature is 615℃, the finished product thickness is 4.5mm, and the pickling elongation is controlled at 4.0%.

[0020] Furthermore, the microstructure of the steel strip is ferrite and a small amount of pearlite, with a grain size of 12.5.

[0021] Furthermore, the elongation of the steel strip satisfies the following condition: ≥35%.

[0022] Functions of each element:

[0023] Carbon (C): The most basic strengthening element. Carbon dissolves in steel to form interstitial solid solutions, providing solid solution strengthening. When it combines with strong carbide-forming elements to form carbide precipitates, it provides precipitation strengthening. However, excessively high C levels are detrimental to the ductility, toughness, and weldability of steel, and also affect the strengthening effect of nitrogen (Nb). Conversely, excessively low C levels reduce the strength of the steel. Therefore, C is typically controlled between 0.06% and 0.10%.

[0024] Silicon (Si): It mainly improves the strength of steel through solid solution strengthening. It is also a deoxidizing element in steel. However, if the content is too high, it will deteriorate the weldability of steel. Therefore, the content of Si is controlled between 0.10% and 0.30%.

[0025] Manganese (Mn): Mn enhances the strength of steel through solid solution strengthening and is the most important and economical strengthening element for compensating for strength loss caused by reduced carbon content. Mn also expands the γ-phase region, lowering the γ→α phase transformation temperature of steel, which helps obtain finer phase transformation products and improves the toughness of the steel. The Mn content in this steel grade ranges from 0.60% to 0.90%.

[0026] Niobium (Nb) is one of the most important elements in modern microalloyed steels, and its effect on grain refinement is significant. During hot rolling, NbC strain-induced precipitation hinders the recovery and recrystallization of deformed austenite. Controlled rolling and cooling transform the deformed austenite structure in the non-recrystallization zone into finer phase transformation products during phase transformation, resulting in high strength and toughness in the steel. Too low an Nb content results in insufficient strengthening, and excessively high Nb content is also limited by carbon content; therefore, the Nb content is controlled between 0.008% and 0.015%.

[0027] Chromium (Cr): An important element for improving the hardenability of steel, effectively increasing its strength, and when the Cr content is above 0.20%, it can effectively improve the corrosion resistance of steel. However, excessive addition of chromium and manganese to steel simultaneously can lead to the formation of low-melting-point Cr-Mn composite oxides, resulting in surface cracks during hot working and severely deteriorating weldability. In this invention, the Cr content should be limited to 0.45%-0.65%.

[0028] Molybdenum (Mo): An element that improves hardenability, second only to manganese (Mn). Mo also helps overcome temper brittleness and improves heat treatment and fatigue properties. In high-strength low-alloy steels, yield strength increases with increasing Mo content; therefore, excessively high Mo content impairs plasticity. In this invention, the Mo content is controlled at 0.10~0.30%.

[0029] Sulfur and phosphorus (S and P) are unavoidable impurity elements in steel, and their levels should be as low as possible. By controlling the inclusion morphology of sulfides through ultra-low sulfur treatment (less than 30 ppm) and Ca treatment, the steel plate can maintain good impact toughness. However, excessively high S content can easily form inclusions such as MnS that impair mechanical properties. Therefore, in this invention, the S content range is ≤ 0.005%, and the P content range is ≤ 0.018%.

[0030] Copper and nickel (Cu and Ni): These can improve the strength of steel through solid solution strengthening. Cu also improves the corrosion resistance of steel, while the addition of Ni mainly mitigates the hot brittleness easily caused by Cu in steel and is beneficial to toughness. In this invention, the Cu content ranges from 0.20% to 0.30%, and the Ni content ranges from 0.10% to 0.25%.

[0031] Aluminum (Al): Al is an element added to steel for deoxidation. Adding an appropriate amount of Al helps refine the grains and improve the strength and toughness of the steel. In this invention, the Al content is 0.02% to 0.05%.

[0032] Calcium (Ca): Ca treatment can control the morphology of sulfides, improve the anisotropy of steel plates, and enhance low-temperature toughness. In this invention, the Ca content is ≤ 0.005%.

[0033] Compared with the prior art, the beneficial technical effects of the present invention are as follows:

[0034] This invention utilizes a micro-alloying design system based on C, Mn, Cu, Cr, Ni, and Mo. By controlling various corrosion-resistant and strengthening elements, the produced products exhibit excellent corrosion resistance, weldability, strength, and toughness. The inventors have discovered that by using the raw material composition and mass ratio provided in this invention, combined with controlled rolling and cooling processes, a 2250mm hot-rolling production line has been used to develop CT70 steel for continuous tubing. This steel possesses excellent corrosion resistance, strength, toughness, and weldability, and has been successfully applied in oilfield operations such as drilling, logging, and well completion, offering the following beneficial effects:

[0035] This invention provides a hot-rolled pickled steel strip for CT70 grade coiled tubing and its manufacturing method. Through a reasonable composition system design, the steel strip contains Cu: 0.20~0.30%, Cr: 0.48~0.58%, Ni: 0.12~0.22%, Mo: 0.12~0.22%, Nb: 0.008~0.015%, Cr: 0.90~1.00%, Mo: 0.40~0.50%, etc., with a microstructure of ferrite + pearlite and a grain size of approximately 12.5. It features high strength, good weldability, and corrosion resistance, making it suitable for coiled tubing steel. The mechanical and technological properties meet the following requirements: tensile strength and yield strength 420~550MPa, yield ratio ≤0.90, elongation ≥35%, 180° cold bending test D=2t, non-metallic inclusions ≤1.5 grade, banded structure ≤1.0 grade, and HIC test meets the requirements. Attached Figure Description

[0036] The present invention will be further described below with reference to the accompanying drawings.

[0037] Figure 1 This is a typical metallographic structure diagram of an embodiment of the present invention.

[0038] Figure 2 For product application images, Detailed Implementation

[0039] A method for producing hot-rolled pickled steel strip for CT70 grade coiled tubing, including the following production process:

[0040] KR desulfurization → converter → LF furnace → RH furnace → slab continuous casting;

[0041] Heating furnace—Rough rolling high-pressure water descaling—Fixed width press—E1R1 rough rolling mill—E2R2 rough rolling mill—(insulation cover)—Flying shear—Finish rolling high-pressure water descaling—F1~F7 finish rolling mill—Dense laminar flow cooling—Coiling—Pallet transport system—Sampling and inspection;

[0042] Pickling and uncoiling—welding—straightening—pickling—rinsing—drying—edge trimming—oiling—winding—sampling and inspection—weighing—labeling—bundling—packaging—warehousing.

[0043] Example 1: A hot-rolled pickled steel strip for CT70 grade coiled tubing and its production method.

[0044] The molten iron undergoes desulfurization pretreatment. The converter employs top-and-bottom blowing smelting technology, adding ferromanganese, copper plates, and ferrochrome alloys for deoxidation and alloying. The composition is adjusted to meet steelmaking requirements, with a converter tapping temperature ≥1620℃. The molten steel after converter smelting is then transferred to the LF refining process for further treatment. Ferromanganese, ferrosilicon, ferrochrome, ferroaluminum, and ferromolybdenum alloys are added to adjust the composition to the target level. The molten steel is then transported for RH vacuum treatment, during which the composition and temperature are fine-tuned. The vacuum level is required to be ≤2.6 mbar, maintained for ≥10 min, and a pure degassing time of at least 8 min is ensured. After vacuum treatment, calcium treatment is performed. After wire feeding, a soft blowing time of at least 8 min is ensured. Before steel loading, the argon flow rate is adjusted to maintain the molten steel in a soft blowing state to prevent exposed steel. Smelting is carried out according to the chemical composition shown in Table 1, with a slab superheat of 33℃ for continuous casting. Following this, the slab is cleaned, slowly cooled, and the continuous casting slab quality is inspected. The slab was heated to 1240℃, the soaking temperature was 1235℃, the exit temperature was 1230℃, and the heating time was 209 minutes. The heated slab was then subjected to high-pressure water descaling. Width was determined using a width-fixing press, and a 2-stand roughing mill and a 7-stand CVC finishing mill were employed. The intermediate slab thickness was 45.0 mm, and the finishing rolling temperature was 890℃. Laminar flow cooling adopted a front-dispersive cooling mode "12" (two upper and one lower nozzle alternately), with a cooling rate of 38℃ / s. The coiling temperature was 620℃, the finished product thickness was 4.0 mm, and the pickling elongation was controlled at 4.0%. The mechanical property test results of the steel strip prepared in the example are shown in Table 2.

[0045] Example 2: A hot-rolled pickled steel strip for CT70 grade coiled tubing and its production method.

[0046] The molten iron undergoes desulfurization pretreatment. The converter employs top-and-bottom blowing smelting technology, adding ferromanganese, copper plates, and ferrochrome alloys for deoxidation and alloying. The composition is adjusted to meet steelmaking requirements, with a converter tapping temperature ≥1620℃. The molten steel after converter smelting is then transferred to the LF refining process for further treatment. Ferromanganese, ferrosilicon, ferrochrome, ferroaluminum, and ferromolybdenum alloys are added to adjust the composition to the target level. The molten steel is then transported for RH vacuum treatment, during which the composition and temperature are fine-tuned. The vacuum level is required to be ≤2.6 mbar, maintained for ≥10 min, and a pure degassing time of at least 8 min is ensured. After vacuum treatment, calcium treatment is performed. After wire feeding, a soft blowing time of at least 8 min is ensured. Before steel loading, the argon flow rate is adjusted to maintain the molten steel in a soft blowing state to prevent exposed steel. Smelting is carried out according to the chemical composition shown in Table 1, with a slab superheat of 33℃ for continuous casting. Following this, the slab is cleaned, slowly cooled, and the continuous casting slab quality is inspected. The slab was heated to 1245℃, the soaking temperature was 1240℃, the exit temperature was 1235℃, and the heating time was 210 min. The heated slab was then subjected to high-pressure water descaling. Width was determined using a width-fixing press, and a 2-stand roughing mill and a 7-stand CVC finishing mill were employed. The intermediate slab thickness was 46.0 mm, and the finishing rolling temperature was 885℃. Laminar flow cooling adopted a front-dispersive cooling mode "12" (two nozzles on top and one on the bottom), with a cooling rate of 38℃ / s. The coiling temperature was 615℃, the finished product thickness was 4.5 mm, and the pickling elongation was controlled at 4.0%. The mechanical property test results of the steel strip prepared in the example are shown in Table 2.

[0047] Figure 1 This is a typical microstructure diagram of the steel strip prepared in this embodiment. As can be seen from the diagram, the microstructure consists of ferrite and a small amount of pearlite, with a grain size of approximately 12.5. The HIC test results should meet the following requirements: CSR≤0%, CLR≤0%, CTR≤0%. The mechanical property test results of the steel strip prepared in this embodiment are shown in Table 2.

[0048] Table 1. Chemical composition (wt%) of various embodiments of the present invention

[0049]

[0050] Table 2. Test results of mechanical properties and corrosion resistance of the steel strips prepared in Examples 1-2 of this invention.

[0051]

[0052] Table 3. Control of inclusions, grain size, and banded structure in the examples

[0053]

[0054] As can be seen from the results of the above embodiments, the pickled steel strip for coiled tubing of the present invention has excellent mechanical properties and corrosion resistance.

[0055] The embodiments described above are merely preferred embodiments of the present invention and are not intended to limit the scope of the present invention. Various modifications and improvements made by those skilled in the art to the technical solutions of the present invention without departing from the spirit of the present invention should fall within the protection scope defined by the claims of the present invention.

Claims

1. A method for producing hot-rolled pickled steel strip for CT70 grade coiled tubing, characterized in that: include: The smelting and continuous casting process includes: hot metal pretreatment, converter composite blowing, LF refining, RH and slab continuous casting to form a continuous casting billet. The chemical composition and weight percentage of the continuous casting billet are as follows: C: 0.06~0.10%, Si: 0.10~0.30%, Mn: 0.60~0.90%, P: ≤0.018%, S: ≤0.005%, Cu: 0.20~0.30%, Ni: 0.10~0.25%, Cr: 0.45~0.65%, Mo: 0.10~0.30%, Nb: 0.008~0.015%, with the balance being Fe and unavoidable impurities. Heating of continuously cast billets: The heating temperature is controlled at 1220~1270℃, the soaking temperature is 1210~1260℃, the furnace exit temperature is controlled at 1250±20℃ when the thickness is ≤3.50mm, and the furnace exit temperature is controlled at 120±20℃ when the thickness is 3.51~5.50mm. The time the billet is in the furnace is controlled within 180~240min. Hot rolling employs a two-stage controlled rolling process: roughing and finishing. Roughing uses a 3+5 pattern with an initial rolling temperature of 980–1120℃. Finishing begins with an intermediate slab thickness of 40–55 mm for a thickness of 2.77–3.50 mm and a final rolling temperature of 875–1050℃. The intermediate slab thickness is 45–60 mm for a thickness of 3.51–5.50 mm and a final rolling temperature of 865–895℃. Finishing is performed using a 7-stand continuous variable crown mill with an initial rolling temperature of 980–1130℃. Cooling and winding: For thicknesses of 2.77~3.50mm, the cooling adopts a "12" mode laminar flow cooling method, which is front-dispersed, with a cooling method of alternating two cooling cycles and a cooling rate controlled at 20~40℃ / s, and a winding temperature of 600~635℃; For thicknesses of 3.51~5.50mm, the cooling adopts a "12" mode laminar flow cooling method, which is front-dispersed, with a cooling method of alternating two cooling cycles and a cooling rate controlled at 25~45℃ / s, and a winding temperature of 595~625℃.

2. The method for producing hot-rolled pickled steel strip for CT70 grade coiled tubing according to claim 1, characterized in that: The chemical composition and weight percentage of the continuously cast billet are as follows: C: 0.06~0.08%, Si: 0.12~0.22%, Mn: 0.70~0.80%, P: ≤0.01%, S: ≤0.002%, Cu: 0.20~0.30%, Cr: 0.48~0.58%, Ni: 0.12~0.22%, Mo: 0.12~0.22%, Nb: 0.008~0.015%, with the balance being Fe and unavoidable impurities.

3. The method for producing hot-rolled pickled steel strip for CT70 grade coiled tubing according to claim 2, characterized in that: The chemical composition and weight percentage of the continuously cast billet are as follows: C: 0.07%, Si: 0.17%, Mn: 0.76%, P: 0.009%, S: 0.001%, Cu: 0.21%, Cr: 0.50%, Ni: 0.16%, Mo: 0.13%, Nb: 0.015%, Ca: 0.0017%, Al 0.035%, with the balance being Fe and unavoidable impurities.

4. The method for producing hot-rolled pickled steel strip for CT70 grade coiled tubing according to claim 2, characterized in that: The chemical composition and weight percentage of the continuously cast billet are as follows: C: 0.06%, Si: 0.18%, Mn: 0.75%, P: 0.008%, S: 0.002%, Cu: 0.20%, Cr: 0.49%, Ni: 0.15%, Mo: 0.12%, Nb: 0.016%, Ca: 0.0020%, Al 0.033%, with the balance being Fe and unavoidable impurities.

5. The method for producing hot-rolled pickled steel strip for CT70 grade coiled tubing according to claim 3, characterized in that: The molten iron undergoes desulfurization pretreatment. The converter employs top-and-bottom combined blowing smelting technology. Ferromanganese, copper plates, and ferrochrome alloys are added to the converter for deoxidation and alloying, adjusting the composition to meet steel tapping requirements. The converter tapping temperature is ≥1620℃. The molten steel after converter smelting is then transferred to the LF refining process for further treatment. Ferromanganese, ferrosilicon, ferrochrome, ferroaluminum, and ferromolybdenum alloys are added according to the steel composition to adjust to the target composition. The molten steel is then transported for RH vacuum treatment, during which the composition and temperature are fine-tuned. The vacuum level is required to be ≤2.6 mbar, and the vacuum time is maintained for ≥10 min, ensuring a pure degassing time of at least 8 min. After vacuum treatment, calcium treatment is performed. After wire feeding, a soft blowing time of at least 8 min is ensured. Before steel loading, the argon gas flow rate is adjusted to ensure the steel... Water is used in a soft-blowing state to prevent molten steel from being exposed; the superheat of the slab continuous casting is 33℃; then the slab is cleaned, slowly cooled, and the quality of the continuously cast slab is inspected; the slab heating temperature is 1240℃, the soaking temperature is 1235℃, the furnace exit temperature is 1230℃, and the heating time is 209min. The heated slab is then subjected to high-pressure water descaling; the width is fixed by a width-fixing press, using 2 roughing mills and 7 CVC finishing mills; the intermediate slab thickness is 45.0mm, and the finishing rolling temperature is 890℃; laminar flow cooling adopts a front-dispersive cooling mode "12", with the upper and lower nozzles alternately opened, the cooling rate is 38℃ / s, the coiling temperature is 620℃, the finished product thickness is 4.0mm, and the pickling elongation is controlled at 4.0%.

6. The method for producing hot-rolled pickled steel strip for CT70 grade coiled tubing according to claim 4, characterized in that: The molten iron undergoes desulfurization pretreatment. The converter employs top-and-bottom combined blowing smelting technology. Ferromanganese, copper plates, and ferrochrome alloys are added to the converter for deoxidation and alloying, adjusting the composition to meet steel tapping requirements. The converter tapping temperature is ≥1620℃. The molten steel after converter smelting is then transferred to the LF refining process for further treatment. Ferromanganese, ferrosilicon, ferrochrome, ferroaluminum, and ferromolybdenum alloys are added according to the steel composition to adjust to the target composition. The molten steel is then transported for RH vacuum treatment, during which the composition and temperature are fine-tuned. The vacuum level is required to be ≤2.6 mbar, and the vacuum time is maintained for ≥10 min, ensuring a pure degassing time of at least 8 min. After vacuum treatment, calcium treatment is performed. After wire feeding, a soft blowing time of at least 8 min is ensured. Before steel loading, the argon gas flow rate is adjusted to ensure the steel... Water is used in a soft-blowing state to prevent molten steel from being exposed; the superheat of the slab continuous casting is 33℃; then the slab is cleaned, slowly cooled, and the quality of the continuously cast slab is inspected; the slab heating temperature is 1245℃, the soaking temperature is 1240℃, the furnace exit temperature is 1235℃, and the heating time is 210min. The heated slab is then subjected to high-pressure water descaling; the width is fixed by a width-fixing press, using 2 roughing mills and 7 CVC finishing mills; the intermediate slab thickness is 46.0mm, and the finishing rolling temperature is 885℃; laminar flow cooling adopts a front-dispersive cooling mode "12", with the upper and lower nozzles alternately opened, the cooling rate is 38℃ / s, the coiling temperature is 615℃, the finished product thickness is 4.5mm, and the pickling elongation is controlled at 4.0%.

7. The method for producing hot-rolled pickled steel strip for CT70 grade coiled tubing according to claim 1, characterized in that: The microstructure of the steel strip consists of ferrite and a small amount of pearlite, with a grain size of 12.

5.

8. The method for producing hot-rolled pickled steel strip for CT70 grade coiled tubing according to claim 1, characterized in that: The elongation of the steel strip must meet the following requirement: ≥35%.

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