A low yield ratio high toughness steel for coiled tubing and a method for manufacturing the same

Abstract

This invention belongs to the technical field of steel for the petroleum industry, specifically relating to a low yield strength ratio and high strength-toughness coiled tubing steel and its preparation method, particularly suitable for oil and gas extraction in deep wells, ultra-deep wells, and harsh corrosive environments. The chemical composition of the steel is: C, Si, Mn, P, S, Alt, Cr, Mo, Ti, B, Cu, N, O, Pr+Ca: 0.0024%–0.0038%, with B / N = 2.35–5.17, Pr+Ca / S = 1.2–3.8, and the remainder being Fe and unavoidable impurities. The coiled tubing steel provided by this invention features improved burst pressure, resistance to crushing, torsional strength, excellent corrosion resistance, and fatigue life. It also boasts an economical and low-cost composition design.

Description

Technical Field

[0001] This invention belongs to the field of steel technology for the petroleum industry, specifically relating to a low yield strength ratio and high strength and toughness coiled tubing steel and its preparation method, which is particularly suitable for oil and gas extraction in deep wells, ultra-deep wells and harsh corrosive environments. Background Technology

[0002] Coiled tubing is a type of thin-walled, small-diameter continuous tubing that can be wound onto a large-diameter (generally over 1 meter) drum. It consists of several flexible tubes, each over 100 meters long, welded together to form a length of hundreds to thousands of meters without joints. The wall thickness is typically 2–7 mm, and the commonly used diameter is 19–127 mm. It is mainly used in oilfield operations, employing specialized equipment for operation. It offers advantages such as high mobility, high efficiency, low cost, and reusability.

[0003] Coiled tubing is subjected to repeated bending, friction, clamping, and stretching deformations during use, resulting in complex stress states and thus placing high demands on its materials. Early coiled tubing was made of carbon steel, which, due to its low strength, numerous welds, and poor fatigue resistance, led to frequent operational accidents. With advancements in metallurgical and welding technologies, the application of low-alloy high-strength steel and oblique butt welding techniques has significantly improved the reliability of coiled tubing, greatly promoting its development.

[0004] At present, oil and gas exploration is shifting to the deep sea. With the continuous increase in the depth of oil and gas wells, coiled tubing is required to withstand extremely high internal and external pressures, and also needs to maintain sufficient toughness and fatigue resistance in complex geological environments.

[0005] Research results show that high strength can effectively improve the burst pressure, crush resistance, torsional resistance, and fatigue life of coiled tubing, while also increasing the operating depth. This trend towards higher strength coiled tubing has promoted the development of relevant standards for high-strength coiled tubing.

[0006] Existing coiled tubing suffers from a series of problems, including high carbon content, reduced weldability and toughness, fatigue failure, and stress corrosion cracking. There is an urgent need to develop new types of steel for coiled tubing to meet the demands of the high-end market. Summary of the Invention

[0007] The purpose of this invention is to provide a low yield strength ratio, high strength and toughness steel for coiled tubing and its preparation method. The coiled tubing steel provided by this invention features improved burst pressure, resistance to crushing, torsional strength, excellent corrosion resistance, and fatigue life. The chemical composition of the coiled tubing steel includes C, Si, Mn, P, S, and Alt, with added strengthening and corrosion-resistant elements including Ti, B, Cr, Cu, Mo, Pr, and Ca, as well as the base element Fe. Expensive alloying elements Ni, V, and Nb are not added, resulting in an economical and low-cost composition design.

[0008] To achieve the above objectives, the present invention employs the following technical solution: A low yield strength ratio and high strength-toughness coiled tubing steel, the chemical composition of which, by weight percentage, is: C: 0.13%–0.15%, Si: 0.52%–0.64%, Mn: 1.52%–1.74%, P≤0.010%, S≤0.002%, Alt: 0.024%–0.046%, Cr: 1.54%–1.68%, Mo: 0.42%–0.52%, Ti: 0. 0.12%–0.15%, B: 0.0052%–0.0062%, Cu: 0.62%–0.76%, N: 0.0012%–0.0022%, O: 0.0005%–0.0015%, Pr+Ca: 0.0024%–0.0038%, with B / N = 2.35–5.17 and Pr+Ca / S = 1.2–3.8. The remainder is Fe and unavoidable impurities. Inclusions in steel: B: 0–0.5 grade, D: 0–0.5 grade.

[0009] The thickness of the steel plate is 2.0 to 6.0 mm.

[0010] The reason for selecting the above alloying elements and their contents in this invention is as follows: C: In coiled tubing, C can improve the hardenability of austenite, promote bainite transformation, and obtain a sufficient amount of MA (matrix) component. Since the MA component in coiled tubing is key to obtaining a low yield strength ratio, a high work hardening index, and plasticity, a certain C content should be ensured in the steel. However, excessively high C content can easily lead to core segregation, resulting in severe banded structure and deteriorating ERW (efficiency hardening) performance. Under stress, brittle fracture is likely to occur, and excessively high carbon content can also reduce the weldability of the steel, easily causing defects such as cracks and porosity. In this invention, the C content is limited to 0.13–0.15%.

[0011] Si (Si): Inhibits the precipitation of cementite during bainite transformation, causing carbon to accumulate in untransformed austenite, forming carbon-rich MA components, and promoting the formation of polygonal ferrite. Si dissolves in ferrite and austenite, improving the strength and hardness of steel through solid solution strengthening. Furthermore, Si enhances the corrosion resistance of steel by forming a dense oxide protective film on the steel surface, preventing corrosive media from eroding the steel. When added in combination with Cr, it forms a Cr₂Si₃ hard phase with free Cr in the steel, precipitating micron-sized (20–50 μm) hard phases near grain boundaries or within grains, improving the hardness and wear resistance of the steel. However, excessive Si addition reduces the plasticity and toughness of the steel, causing deterioration in weldability. Therefore, the Si content in this invention is limited to 0.52%–0.64%.

[0012] Mn: Mn enhances the strength of steel plates through solid solution strengthening. It enters the cementite and replaces some Fe atoms, thus playing a role in solid solution strengthening, refining the cementite and microstructure. Mn can also improve the hardenability of austenite, which is beneficial for obtaining bainite and MA components. However, for steel used in coiled oil well tubing, due to the high C content, a high Mn content easily leads to banded microstructure. Therefore, to improve the plasticity and strength of the material, the Mn content in this invention is set at 1.52%–1.74%.

[0013] P: Generally speaking, low P and S content is beneficial to improving the low-temperature impact toughness and fatigue resistance of materials. P easily causes cold brittleness in steel at low temperatures, leading to brittle fracture under stress and affecting the safety and service life of the steel. P plays a solid solution strengthening role in steel and is one of the elements that improve corrosion resistance. However, when the content is too high, it is easy to cause segregation at grain boundaries, which can damage the weldability, plasticity, and toughness of the steel plate, causing defects such as cracks and porosity in the weld. This invention limits the P content in steel to ≤0.010%.

[0014] Sulfides (S) have several effects. First, they tend to form low-melting-point eutectics during hot working of steel, leading to hot brittleness, reduced toughness and ductility, and increased susceptibility to cracking. S also increases the likelihood of cracking during welding, particularly in the heat-affected zone of the weld. Second, S forms a dense Cu₂S passivation film with Cu, improving corrosion resistance. However, S reacts with Mn to form MnS inclusions, deteriorating the toughness, ductility, and hot working properties of the steel. This effect is particularly pronounced with increasing strength. Furthermore, S can react with Ti to precipitate coarse Ti₄C₂S₂ two-phase particles at high temperatures, weakening the role of Ti. Therefore, in this invention, S is limited to ≤0.002%.

[0015] Cr: On the one hand, Cr is an austenite stabilizing element, improving hardenability and promoting bainite formation. Furthermore, Cr contributes more to increasing tensile strength than yield strength, thus reducing the yield-to-tensile ratio. On the other hand, Cr forms a continuous solid solution with Fe in steel, exhibiting solid solution strengthening effects, and also forms various types of carbides with C, such as M3C, M7C3, and M... 23 C6 and other compounds produce a secondary strengthening effect. Cr can improve the passivation ability of steel, promote the formation of a dense passivation film or protective rust layer on the steel surface, and its enrichment in the rust layer can improve the selective permeability to corrosive media; at the same time, Cr increases the self-corrosion potential and atmospheric corrosion resistance of steel. However, adding more Cr will increase manufacturing costs. Therefore, in this invention, the Cr content is controlled at 1.54%~1.68%.

[0016] Cu plays a role in precipitation strengthening and solid solution strengthening in steel. Because Cu has a higher electrochemical potential than Fe, it promotes the formation of a dense rust layer on the steel surface, which is beneficial for improving corrosion resistance. Especially in sulfide-containing corrosive media, Cu can enhance the strength and hardness of steel. Utilizing the precipitation strengthening effect of Cu, both strength and acid corrosion resistance are improved. Simultaneously, Cu combines with residual S in the steel to form a Cu₂S protective film, and can also form a dense Cu-P compound protective rust layer on the substrate surface with P, further inhibiting the penetration of corrosive media. Cu also dissolves in Fe to form a substitution solid solution, which has a solid solution strengthening effect, thereby improving the strength of the steel plate. However, when the Cu content is too high, it easily causes "copper embrittlement" defects and increases costs. In this invention, Pr and Ca are intentionally added to disperse them and combined with a heating process to avoid the occurrence of "copper embrittlement" defects, achieving a wear-resistant effect. This invention limits the Cu content to 0.62%~0.76%.

[0017] Al (Al) acts as a strong deoxidizer in steel, which helps refine grains and improve the strength and toughness of the steel. It also increases the corrosion potential of the steel, thus inhibiting corrosion. Simultaneously, the formation and aggregation of complex nanoscale oxides containing Al and Si in the inner rust layer increases charge-mass transfer resistance, inhibiting the corrosion process. However, Al reduces austenite stability and austenite undercooling, causing rapid growth of new phase nuclei, thereby reducing hardenability and increasing the critical quenching rate. Furthermore, as a ferrite-forming element, excessive Al reduces the strength of the steel plate and increases the brittleness of ferrite, leading to a decrease in steel toughness. This invention limits the content of Al to 0.024%~0.046%.

[0018] Pr and Ca: The purpose of adding them is to (1) purify the molten steel, change the morphology and type of inclusions, reduce the potential difference between inclusions and the matrix, reduce the tendency of electrochemical corrosion caused by inclusions, and effectively improve the corrosion resistance of the steel plate; (2) disperse Cu distribution, improve the density of the rust layer, avoid the occurrence of "copper embrittlement" defects on the one hand, and improve Cu utilization on the other hand, thereby increasing the ratio of the protective phase α-FeOOH / γ-FeOOH and improving the density of the rust layer; (3) help to inhibit residual The decomposition of austenite and precipitation of carbides significantly improve the plasticity and forming properties of steel plates, making it easier for the material to bend and other processing in the later stages; (4) It improves the segregation of Cr, promotes the uniform precipitation of hard phases Cr2C3 and Cr2Si3 and inhibits the aggregation and growth of these phases, thereby improving the wear resistance of steel plates; (5) In order to suppress the harmful effects of S, it is specified that (Pr+Ca) / S=1.2~3.8; In summary, the present invention limits the range of Pr+Ca to Pr+Ca: 0.0024%~0.0038%.

[0019] Ti plays three main roles. First, in steel, it combines with C and N to form TiC, TiN, and Ti(C, N). During heating, Ti preferentially forms TiN with N, inhibiting austenite grain growth and refining the grain size. Subsequently, it combines with C and N to form fine carbides and carbonitrides, preventing grain growth and coarsening. Refined grains contribute to improved steel strength and toughness, allowing the tubing to maintain better stability and reliability under complex stress and harsh environments. Second, Ti significantly improves the corrosion resistance of steel, especially for tubing operating in corrosive media. Ti eliminates or reduces intergranular corrosion, improving its corrosion resistance and extending its service life. Finally, the addition of Ti improves the weldability of steel, reducing the tendency to produce weld cracks, making it easier to weld connections during tubing manufacturing and installation, improving production efficiency and engineering quality. This invention limits the Ti content to 0.12%~0.15%.

[0020] B: It can improve hardenability, but improper use of B will reduce the toughness of the steel plate and increase the risk of cracking. B and N form BN particles. BN has good stability, is not easily dissolved at high temperatures, and has a stable effect on improving wear and corrosion resistance. The B / N ratio should be controlled at 2.35~5.17. In addition, B segregates at the austenite grain boundaries to suppress the precipitation of proeutectoid ferrite, allowing carbides to precipitate diffusely in the low-temperature region. In this invention, B is limited to 0.0052%~0.0062%.

[0021] Mo (Mo) enhances the hot strength of steel, increasing its strength and creep resistance at high temperatures, which is particularly important for oil pipes operating at high temperatures. Mo helps reduce the brittleness of steel during tempering, allowing it to maintain high strength while retaining good toughness and plasticity, thus extending the service life of oil pipes and reducing safety accidents caused by brittle fracture. Mo improves the stability of austenite, inhibits the formation of polygonal ferrite, and forms a certain amount of acicular ferrite and bainite during laminar cooling. Furthermore, Mo significantly improves the corrosion resistance of steel, effectively resisting the erosion of acids, alkalis, salts, and other corrosive media. Especially for acid-resistant oil pipes, the addition of Mo can significantly improve their resistance to formic acid, acetic acid, oxalic acid, hydrogen peroxide, sulfuric acid, and other media. Therefore, this invention limits the Mo content to 0.42%–0.52%.

[0022] Nitrogen (N) forms nitrides with Al and Ti in steel. These fine precipitates act as grain boundary pinning agents, thus refining austenite grains. Higher N content readily combines with Al to form AlN, increasing the amount of nitrides in the steel. When AlN exists as a non-metallic inclusion in steel, it disrupts the continuity of the steel matrix. This is especially true when the Al content is high, resulting in a greater quantity of AlN and a more aggregated distribution, leading to the formation of oxides with poor plasticity. Furthermore, higher N content tends to accumulate at defects, worsening low-temperature impact toughness. Therefore, this invention limits the N content to 0.0012%~0.0022%.

[0023] O: forms oxide inclusions in steel, reducing the toughness and wear resistance of the material. Therefore, the present invention limits O to 0.0005%~0.0015%.

[0024] The preparation method of steel for coiled tubing with low yield strength ratio and high strength and toughness includes smelting, continuous casting, heating and rolling; specific methods include: S1. Smelting: After refining, Pr-Fe alloy is added, and Si-Ca wire feeding is performed simultaneously to reduce O and S content. Soft blowing time is 16-20 min to ensure that inclusions float to the surface and undergo modification treatment. Slab continuous casting: Wire feeding is performed during continuous casting. Because rare earth Pr has a strong affinity, it easily forms a large number of rare earth compounds with O and S in steel, affecting the fluidity of molten steel. Therefore, the casting speed is controlled at 1.12-1.31 m / min. To reduce center segregation of the continuously cast slab, the superheat is controlled at 22-26℃, and electromagnetic stirring and dynamic light reduction techniques are used. A slab with the aforementioned chemical composition is obtained; the chemical composition of the slab needs to be precisely controlled to meet the requirements for steel used in coiled tubing. This includes the contents of C, Si, Mn, P, S, Cr, Cu, Mo, Ti, B, O, N, Pr, Ca, and the base element Fe (iron).

[0025] S2. Heat the slab to reach the furnace exit temperature; then place the slab into the heating furnace for uniform heating. Precise control of the exit temperature ensures ideal heating of the steel, providing a good foundation for subsequent processes. The heating time is 150-180 minutes, and the exit temperature is 1175℃-1195℃. This ensures uniform temperature distribution within the slab. This helps reduce temperature gradients during rolling, preventing cracks or deformation due to uneven heating. The exit temperature must not be too high to prevent overheating and burning. Overheating weakens grain boundaries, while burning damages the grain structure, affecting mechanical properties and service life.

[0026] S3. The heated slab is sequentially rolled, cooled, and coiled to obtain steel for continuous tubing. The heated slab is rolled using roughing and finishing mills to change its shape and dimensions. After rolling, the steel needs to be rapidly cooled to inhibit grain growth and improve its strength and toughness. The cooling method can be selected according to actual needs, such as water cooling or air cooling. The cooled steel is then coiled using a coiler to obtain steel for continuous tubing.

[0027] The final rolling temperature is 805℃~835℃. During the rolling process, the final rolling temperature must be strictly controlled to ensure that the finishing rolling process is carried out in the non-recrystallization zone, so as to prepare the microstructure for the subsequent laminar cooling.

[0028] A three-stage laminar flow cooling mode is adopted. In the first stage, the water cooling is stopped when the temperature reaches 650℃. In the second stage, the air cooling is performed for 3-5 seconds, and then the laminar flow cooling water is turned on. In the third stage, the coiling temperature range is 415℃ to 445℃. The positive effects of this coiling temperature range are: ensuring the uniformity of the strip structure, giving the strip good finished hardness and mechanical properties, thereby ensuring high fatigue life and corrosion resistance. The adverse effect of too high a temperature value is that the mechanical properties of the strip may exceed the lower limit requirement, while the adverse effect of too low a temperature value is that the mechanical properties of the strip may exceed the upper limit requirement.

[0029] The preparation method of this coiled tubing steel includes steps such as slab heating, rolling, cooling, and coiling. By precisely controlling the furnace exit temperature, final rolling temperature, and coiling temperature, the quality and performance of the steel are ensured. This steel has broad application prospects in the petroleum industry, especially in the manufacture of tubing requiring high strength and good corrosion resistance.

[0030] The yield strength of steel plates used for coiled tubing is 585–615 MPa, the tensile strength is 775–805 MPa, and the elongation after fracture is A. 50≥24%, yield strength ratio ≤0.78, low-temperature impact energy at -40℃ ≥120J. Inclusions in the steel: B: 0~0.5 grade, D: 0~0.5 grade, CSR%=0, CLR%=0, CTR%=0 as determined by NACE-TM0284 standard, the fatigue life of the steel pipe reaches more than 1200 cycles under an internal pressure of 6.9MPa.

[0031] In steel smelting, processes such as pure steel smelting, Pr+Ca treatment, dynamic light reduction, and electromagnetic stirring are employed to effectively reduce the S and P content in the steel, lessen the number of inclusions and segregation in continuously cast billets, and improve the welding quality, fatigue performance, and corrosion resistance of the products. In hot continuous rolling control, a two-stage rolling and controlled cooling process is used to obtain a refined microstructure and uniform mechanical properties. Simultaneously, CVC (Continuous Dynamic Control) plate shape control technology is used to reduce the crown of the steel coil, improve the wall thickness accuracy of the finished coiled tubing, obtain good surface quality, and contribute to improving the fatigue life of the coiled tubing.

[0032] The microstructure of the steel used for coiled tubing includes equiaxed ferrite and granular bainite; in the microstructure, banded structure is ≤ grade 1.0.

[0033] Banding is a common structural defect in steel, usually caused by factors such as compositional segregation and uneven cooling rates during rolling or heat treatment. Banding affects the strength and toughness of steel, particularly reducing longitudinal impact toughness, and may lead to quenching cracks during heat treatment. Therefore, the banding level needs to be strictly controlled in steel used for coiled tubing. In this embodiment, according to the standard rating chart, a banding level ≤ 1.0 means that the banding in the steel is relatively light and has a relatively small impact on the steel's properties.

[0034] In summary, the microstructure characteristics of steel for coiled tubing involve strict control over equiaxed ferrite, granular bainite, and banded structures. The optimized combination of these microstructures helps improve the overall performance of the steel plates and pipes, including strength, toughness, low-temperature impact resistance, fatigue performance, corrosion resistance, and weldability.

[0035] Compared with the prior art, the beneficial effects of the present invention are: 1) The microstructure of the steel used for coiled tubing includes equiaxed ferrite and granular bainite, which has a unique crystal structure and morphological characteristics. It can improve strength and toughness, and while meeting the high strength requirements, it maintains good toughness, thus making it suitable for more complex downhole operating environments.

[0036] 2) Through precise control of composition and microstructure, steel plates for coiled tubing with a thickness of 2.0–6.0 mm were ultimately obtained with a yield strength of 585–615 MPa, a tensile strength of 775–805 MPa, and an elongation after fracture of A. 50≥24%, yield strength ratio ≤0.78, -40℃ low temperature impact energy ≥120J, inclusions in steel B: 0~0.5 grade, D: 0~0.5 grade, HIC measured according to NACE-TM0284 standard: CSR (%)=0, CLR (%)=0, CTR (%)=0, SSC measured, no cracks, the steel pipe fatigue life reaches more than 1200 cycles under 6.9MPa internal pressure.

[0037] 3) Optimized corrosion resistance: The unique crystal structure of equiaxed ferrite helps to improve the corrosion resistance of steel and extend the service life of coiled tubing.

[0038] 4) Improve fatigue resistance: This structure can improve the fatigue resistance of steel and reduce the risk of fatigue fracture during continuous operation.

[0039] 5) Compared with existing steel grades, it features economic and low-cost design. It does not contain expensive alloys such as Ni, V and Nb. Through fine-tuning of the composition, it achieves improved strength and plasticity of steel for coiled tubing. Attached Figure Description

[0040] Figure 1 This is a CCT phase transformation temperature curve of the steel grade of the present invention.

[0041] Figure 2 This is a microscopic tissue photograph of Embodiment 1 of the present invention.

[0042] Figure 3 This is a schematic flowchart of the preparation method in the embodiments of this application.

[0043] Figure 4 The dimensions of inclusions under a scanning electron microscope according to the present invention.

[0044] Figure 5 This is a secondary electron image of the steel plate sample of the present invention.

[0045] Figure 6 Curve showing the relationship between inclusion length and the probability of hydrogen-induced crack formation. Detailed Implementation

[0046] The present application is further illustrated below with reference to specific embodiments. Experimental methods in the following embodiments that do not specify specific conditions are generally determined according to national / industry standards; if there is no corresponding national / industry standard, they are performed according to general international standards, conventional conditions, or conditions recommended by the manufacturer.

[0047] Molten iron is smelted in a converter to obtain molten steel, which is then continuously cast to obtain slabs with a set chemical composition. The chemical composition of the slabs is shown in Table 1 and Table 1 (continued).

[0048] Table 1. Composition (wt / %) of the embodiments of the present invention: Continued from Table 1: Based on the chemical composition of the slabs in Examples 1-5, this example also provides a method for preparing steel for coiled tubing, including the following steps: A slab with the above-mentioned chemical composition is obtained; The slab is heated to bring it to the furnace exit temperature. The heated slab was sequentially rolled, cooled, and coiled to obtain steel for continuous tubing. The manufacturing process parameters are shown in Table 2. The mechanical test results for Examples 1-5 of this invention are shown in Table 3. The test results for inclusions, banded structure, acid corrosion resistance, and fatigue life for Examples 1-5 of this invention are shown in Table 4.

[0049] Table 2. Manufacturing process parameters for steel used in coiled tubing: Table 3 Mechanical property testing of steel for coiled tubing: Table 4. Inspection of inclusions, banded structure, acid corrosion resistance, and fatigue life of steel for coiled tubing: Appendix Figure 1 Appendix Figure 2 Appendix Figure 3 Detailed explanation: Figure 1 The CCT phase transformation temperature curve of the steel in the embodiments of the present invention is the basis for formulating the rolling process. Figure 1 In this diagram, A represents austenite, B represents bainite, M represents martensite, and F+P represents ferrite + pearlite; the austenitizing temperature is 950℃, held for 5 minutes; Ms is the martensitic transformation initiation temperature, Ms=440℃, A c1 Ac1 is the temperature at which pearlite begins to transform into austenite, where Ac1 = 715℃. c3 A is the temperature at which all ferrite dissolves into austenite and the microstructure becomes 100% austenite. c3 =840℃. Figure 2 The image shows the microstructure of the steel for coiled tubing in Embodiment 1 of the present invention. The microstructure of the steel for coiled tubing includes equiaxed ferrite and granular bainite, with a banded structure grade of 0.5. Figure 3 This is a schematic flowchart of the preparation method according to an embodiment of the present invention, including: S1 yields a slab with the above-mentioned chemical composition; S2 heats the slab to bring it to the furnace exit temperature; S3 sequentially rolls, cools, and coils the heated slab to obtain steel for continuous tubing.

[0050] One or more technical solutions in the embodiments of the present invention have at least the following technical effects or advantages: the steel for continuous tubing provided in the embodiments of the present invention, after undergoing the pipe manufacturing process, meets the strength of CT100 steel grade. The pipe material has reached a high standard in terms of strength performance and can withstand higher pressure and tension, thereby improving mining efficiency.

[0051] The steel tubing for continuous tubing provided in this embodiment of the invention has a long fatigue life due to its high strength and good acid corrosion resistance. As a result of the extended service life, the frequency of tubing replacement is reduced, thereby reducing operating costs.

[0052] The steel for coiled tubing provided in this invention uses a low-alloy design, which meets the requirements of high strength and toughness while ensuring acid corrosion resistance, thus realizing the development of low-cost corrosion-resistant, high-strength and tough coiled tubing products.

[0053] Inclusions in steel: Experimental studies on the influence of inclusions on hydrogen-induced cracking, conducted through sampling, show that inclusions in hot-rolled steel strips for coiled tubing are mainly oxide inclusions. The crack length caused by these inclusions is comparable to the length of the inclusion itself. Specifically, for example... Figure 4 , Figure 5 As shown in Table 5. Figure 4 In the image: 1, 2, 3, 4, and 5 are inclusions of different lengths under a scanning electron microscope. Inclusion 1 has a length of 462 µm, inclusion 2 has a length of 118 µm, inclusion 3 has a length of 65 µm, inclusion 4 has a length of 150 µm, and inclusion 5 has a length of 61 µm. Figure 5 In the figures, 1, 2, and 3 represent the hydrogen-induced crack morphology as shown in the secondary electron image. To determine the limiting inclusion size for hydrogen-induced cracking, the inclusion size and number of hydrogen-induced cracks were statistically analyzed in 60 samples from 20 groups after hydrogen sulfide corrosion. A mathematical model was established between inclusion size and the probability of hydrogen-induced cracking. The statistical model shows that both small and large inclusions can potentially induce hydrogen-induced cracking in hydrogen sulfide corrosion solution, but the probability increases with the increase of inclusion size. For example, the probability of hydrogen-induced cracking for inclusions with a length of 0–20 μm is only 8%, but for inclusions with a length of 500–1000 μm, the probability reaches 100%. The relationship between the probability of hydrogen-induced cracking and inclusion size is as follows: ; Where: Y is the probability of hydrogen-induced cracking; The length of the inclusion is in μm.

[0054] Statistical analysis shows that when the inclusion size is greater than 200 μm, the probability of hydrogen-induced cracking exceeds 50%. Figure 6As shown, cracks of 200 μm are measurable under an optical microscope. Therefore, the upper limit requirement for Class B inclusions in hot-rolled steel strip for coiled tubing is set at 200 μm, corresponding to a rating standard of 1.5.

[0055] Table 5. EDS composition analysis of inclusions: Based on the analysis of the influence of inclusions on hydrogen-induced cracking, the design principle for hydrogen sulfide resistant pipeline steel in hot-rolled steel strip for coiled tubing is proposed: non-metallic inclusions A, B, C, and D ≤ 1.0 grade.

[0056] The above description is merely a specific embodiment of this application, enabling those skilled in the art to understand or implement this application. Various modifications to these embodiments will be readily apparent to those skilled in the art, and the general principles defined in this application can be implemented in other embodiments without departing from the spirit or scope of this application. Therefore, this application is not to be limited to the embodiments shown in this application, but is to be accorded the widest scope consistent with the principles and novel features claimed in this application.

Claims

1. A steel for coiled tubing with low yield strength ratio and high strength and toughness, characterized in that, The chemical composition of the steel, by weight percentage, is as follows: C: 0.13%–0.15%, Si: 0.52%–0.64%, Mn: 1.52%–1.74%, P≤0.010%, S≤0.002%, Alt: 0.024%–0.046%, Cr: 1.54%–1.68%, Mo: 0.42%–0.52%, Ti: 0.12%–0.15%, B: 0.0052%–0.0062%, Cu: 0.62%–0.76%, N: 0.0012%–0.0022%, O: 0.0005%–0.0015%, Pr+Ca: 0.0024%–0.0038%, with B / N = 2.35–5.17 and Pr+Ca / S = 1.2–3.

8. The remainder is Fe and unavoidable impurities.

2. The steel for coiled tubing with low yield strength ratio and high strength and toughness according to claim 1, characterized in that, The thickness of the steel plate is 2.0 to 6.0 mm.

3. The steel for coiled tubing with low yield strength ratio and high strength and toughness according to claim 1, characterized in that, The yield strength of steel plates used for coiled tubing is 585–615 MPa, the tensile strength is 775–805 MPa, and the elongation after fracture is A. 50 ≥24%, yield strength ratio ≤0.78, low-temperature impact energy at -40℃ ≥120J.

4. The steel for coiled tubing with low yield strength ratio and high strength and toughness according to claim 1, characterized in that, The inclusions in the steel are grade B (0-0.5) and grade D (0-0.5). According to the NACE-TM0284 standard, CSR%=0, CLR%=0, and CTR%=0. The fatigue life of the steel pipe under an internal pressure of 6.9MPa reaches more than 1200 cycles.

5. The steel for coiled tubing with low yield strength ratio and high strength and toughness according to claim 1, characterized in that, The microstructure of the steel used for coiled tubing includes equiaxed ferrite and granular bainite; in the microstructure, banded structure is ≤ grade 1.

0.

6. The method for preparing low yield strength ratio, high strength and toughness coiled tubing steel as described in any one of claims 1-5, comprising smelting, continuous casting, heating and rolling; characterized in that, Specific methods include: The final rolling temperature is 805℃~835℃; a three-stage laminar flow cooling mode is adopted. In the first stage, when the temperature reaches 650℃, the water cooling is stopped. In the second stage, the air cooling lasts for 3~5 seconds, and then the laminar flow cooling water is turned on. In the third stage, the coiling temperature range is 415℃~445℃.

7. The method for preparing low yield strength ratio and high strength and toughness coiled tubing steel according to claim 6, characterized in that, After refining, Pr-Fe alloy is added, and SiCa wire feeding is performed simultaneously. Soft blowing time: 16-20 min.

8. The method for preparing low yield strength ratio and high strength and toughness coiled tubing steel according to claim 6, characterized in that, The continuous casting speed is controlled at 1.12–1.31 m / min; the superheat is controlled at 22–26 °C.

9. The method for preparing low yield strength ratio and high strength and toughness coiled tubing steel according to claim 6, characterized in that, Heating time is 150~180min, and the oven temperature is 1175℃~1195℃.

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