High-grade acid-resistant coiled tubing steel and preparation method thereof

Abstract

The invention relates to high-grade acid-resistant coiled tubing steel and a preparation method, and belongs to the technical field of steel preparation. The steel for the coiled tubing comprises the following chemical components: C, Si, Mn, P, S, Cr, Ni, Cu, Mo, Nb, Ti and a matrix element Fe, the mass part of C is less than or equal to 0.070; the mass part of Mn is 0.70 to 1.20 parts; the mass part of the Cu is 0.20 to 0.60 part. The high-grade steel which is high in acid resistance and meets the strength requirement is successfully developed by accurately controlling chemical components, particularly utilizing a low-carbon low-manganese design to reduce the influence of a banded structure and combining a microalloying technology and copper nano precipitation strengthening. The steel is very suitable for manufacturing coiled tubing needing to bear extreme corrosion environments and high-pressure conditions, the service life of equipment can be effectively prolonged, and operation safety and efficiency are improved.

Description

Technical Field

[0001] The present application relates to the technical field of steel preparation, and in particular to a high-grade acid-resistant steel for continuous oil pipes and a preparation method thereof. Background Art

[0002] Continuous tubing, as a highly flexible tubing, plays a vital role in the field of oil and gas extraction with its versatility and significant advantages of reusability. In recent years, with the continuous deepening of oil and gas resource exploration and development at home and abroad, the number of deep wells, ultra-deep wells and acidic oil and gas fields has increased significantly, which has put forward more stringent requirements on the technical performance and adaptability of extraction equipment. In this context, it is particularly important to improve the steel grade of continuous tubing. On the one hand, the improvement of steel grade helps to reduce the dead weight of steel pipes, making operations under complex geological conditions more convenient and efficient. On the other hand, high-grade continuous tubing can significantly improve its fatigue life, ensure stable performance during long-term, high-intensity extraction operations, and effectively reduce equipment failure rate and maintenance costs.

[0003] However, facing the special environment of acidic oil and gas fields, conventional CT90 coiled tubing is not able to meet the requirements in terms of corrosion resistance. The corrosive media in acidic oil and gas fields pose a severe challenge to tubing materials. Traditional tubing materials are often unable to withstand long-term corrosion, resulting in equipment performance degradation and even safety accidents. Therefore, developing new acid-resistant coiled tubing products to fill the market gap in this field has become an urgent problem to be solved in the current oil and gas production industry. Summary of the invention

[0004] The present application provides a high-level acid-resistant steel for continuous oil pipe and a preparation method thereof to solve the following technical problem: how to improve the acid resistance of steel for continuous oil pipe.

[0005] In a first aspect, an embodiment of the present application provides a high-level acid-resistant steel for a coiled tubing, the chemical composition of the steel for a coiled tubing comprising: C, Si, Mn, P, S, Cr, Ni, Cu, Mo, Nb, Ti, and a matrix element Fe;

[0006] The mass fraction of C is ≤0.070 parts;

[0007] The mass fraction of Mn is 0.70 to 1.20 parts;

[0008] The mass fraction of Cu is 0.20 parts to 0.60 parts.

[0009] Optionally, the chemical composition of the steel for the continuous oil pipe includes, by mass fraction: C≤0.070%, Si: 0.10%~0.50%, Mn: 0.70%~1.20%, P≤0.015%, S≤0.005%, Cr: 0.20%~0.70%, Ni: 0.10%~0.50%, Cu: 0.20%~0.60%, Mo: 0.10%~0.40%, Nb: 0.01%~0.05%, Ti: 0.010%~0.080%.

[0010] Optionally, the chemical composition of the steel for the continuous oil pipe includes, by mass fraction: C: 0.030%~0.070%, Si: 0.10%~0.50%, Mn: 0.80%~1.10%, P≤0.015%, S≤0.005%, Cr: 0.20%~0.70%, Ni: 0.10%~0.50%, Cu: 0.25%~0.55%, Mo: 0.10%~0.40%, Nb: 0.01%~0.05%, Ti: 0.010%~0.080%.

[0011] Optionally, the microstructure of the steel for the coiled tubing includes quasi-polygonal ferrite.

[0012] Optionally, the strength of the steel pipe made of the coiled tubing steel is CT90 steel grade.

[0013] In a second aspect, an embodiment of the present application provides a method for preparing the high-grade acid-resistant coiled tubing steel described in the first aspect, the method comprising:

[0014] Obtaining a slab having the chemical composition;

[0015] Heating the slab to make it reach the furnace discharge temperature;

[0016] The heated slab is rolled, cooled and coiled in sequence to obtain steel for continuous oil pipe.

[0017] Optionally, the furnace temperature is 1200°C to 1280°C.

[0018] Optionally, the final rolling temperature is 830°C to 920°C.

[0019] Optionally, the coiling temperature is 500°C to 650°C.

[0020] The above technical solution provided by the embodiment of the present application has the following advantages compared with the prior art:

[0021] The embodiment of the present application provides a high-level acid-resistant continuous oil pipe steel, the chemical composition of the continuous oil pipe steel includes: C, Si, Mn, P, S, Cr, Ni, Cu, Mo, Nb, Ti, and matrix element Fe; the mass fraction of C is ≤0.070 parts; the mass fraction of Mn is 0.70 parts to 1.20 parts; the mass fraction of Cu is 0.20 parts to 0.60 parts. By precisely controlling the chemical composition, especially using low-carbon and low-manganese design to reduce the influence of banded structure on acid resistance, combined with microalloying technology and copper nano-precipitation strengthening, a high-grade steel with high acid resistance and strength requirements has been successfully developed. This steel is very suitable for the manufacture of continuous oil pipes that need to withstand extreme corrosive environments and high-pressure conditions, which can effectively extend the service life of equipment and improve operational safety and efficiency. BRIEF DESCRIPTION OF THE DRAWINGS

[0022] The accompanying drawings, which are incorporated in and constitute a part of this specification, illustrate embodiments consistent with the present application and, together with the description, serve to explain the principles of the present application.

[0023] In order to more clearly illustrate the technical solutions in the embodiments of the present application or the prior art, the drawings required for use in the embodiments or the description of the prior art will be briefly introduced below. Obviously, for ordinary technicians in this field, other drawings can be obtained based on these drawings without paying any creative labor.

[0024] Figure 1 A schematic flow chart of a method for preparing a high-grade acid-resistant coiled tubing steel provided in an embodiment of the present application. DETAILED DESCRIPTION

[0025] In order to make the purpose, technical solution and advantages of the embodiments of the present application clearer, the technical solution in the embodiments of the present application will be clearly and completely described below in conjunction with the drawings in the embodiments of the present application. Obviously, the described embodiments are part of the embodiments of the present application, not all of the embodiments. Based on the embodiments in the present application, all other embodiments obtained by ordinary technicians in this field without making creative work are within the scope of protection of this application.

[0026] Various embodiments of the present application may be presented in the form of a range; it should be understood that the description in the form of a range is only for convenience and brevity and should not be understood as a rigid limitation on the scope of the present application; therefore, it should be considered that the range description has specifically disclosed all possible sub-ranges and single numerical values ​​within the range; for example, it should be considered that the range description from 1 to 6 has specifically disclosed sub-ranges, such as from 1 to 3, from 1 to 4, from 1 to 5, from 2 to 4, from 2 to 6, from 3 to 6, etc., as well as single numbers within the range, such as 1, 2, 3, 4, 5 and 6, which applies regardless of the range; in addition, whenever a numerical range is indicated in this document, it is meant to include any cited numbers (fractions or integers) within the indicated range.

[0027] In this document, the terms including "including" and "including" mean "including but not limited to". Relational terms such as "first" and "second" are only used to distinguish one entity or operation from another entity or operation, and do not necessarily require or imply any such actual relationship or order between these entities or operations. "And / or" describes the association relationship of associated objects, indicating that there can be three relationships. For example, A and / or B can mean: A exists alone, A and B exist at the same time, and B exists alone; where A and B can be singular or plural. "At least one" means one or more, and "plurality" means two or more; "at least one", "at least one of the following" or similar expressions refer to any combination of these items, including any combination of single or plural items; for example, "at least one of a, b, or c", or "at least one of a, b, and c" can all mean: a, b, c, ab (i.e., a and b), ac, bc, or abc, where a, b, and c can be single or multiple. "Parts" such as parts by weight and parts by mass indicate the proportional relationship between the components. In the proportional relationship involved in this article, the parameters that need to be described by proportion should be understood as the first term of the proportional formula in the order of description, and the proportional numbers should be understood as the second term of the proportional formula. For example, if the mass ratio of substance A, substance B and substance C is 1:2:3, then substance A, substance B and substance C should correspond to the proportional numbers in the proportional formula in the order of description, that is, the mass of substance A: the mass of substance B: the mass of substance C = 1:2:3.

[0028] Unless otherwise specified, various raw materials, reagents, instruments and equipment used in this article can be purchased from the market or prepared by existing methods.

[0029] In a first aspect, an embodiment of the present application provides a high-level acid-resistant steel for a coiled tubing, the chemical composition of the steel for a coiled tubing comprising: C, Si, Mn, P, S, Cr, Ni, Cu, Mo, Nb, Ti, and a matrix element Fe;

[0030] The mass fraction of C is ≤0.070 parts;

[0031] The mass fraction of Mn is 0.70 to 1.20 parts;

[0032] The mass fraction of Cu is 0.20 parts to 0.60 parts.

[0033] The positive effect of limiting the mass fraction of C to ≤ 0.070 parts: C is a basic strengthening element of steel materials and plays a key role in material strength. In the embodiment of the present application, the mass fraction of C is greater than 0.070 parts. On the one hand, it will increase the precipitation of carbides and promote the occurrence of local corrosion, accelerate the corrosion of steel, and on the other hand, it will aggravate the central segregation, thereby significantly reducing the acid resistance.

[0034] The positive effect of limiting the mass fraction of Mn to 0.70 to 1.20 parts: Mn is one of the important alloying elements in steel materials, which can significantly improve the strength and hardness of steel. Mn can form a solid solution with iron, and improve the tensile strength and yield strength of steel through solid solution strengthening. However, the element Mn is also an element that is prone to segregation, and Mn greater than 1.20 parts will be detrimental to the control of center segregation.

[0035] The positive effect of limiting the mass fraction of Cu to 0.20 to 0.60 parts: on the one hand, element Cu is a good corrosion-resistant element, especially in corrosive media containing sulfides; on the other hand, Cu can enhance the strength and hardness of steel, especially in low-carbon steel, and make full use of the precipitation strengthening effect of Cu to achieve a double improvement in strength and acid resistance.

[0036] Fe is a matrix element. The specific content / content range of Fe can be obtained by the upper and lower limit formula of the component, that is:

[0037] The sum of the percentages of the contents of each component in a composition should be equal to 100%, and the content ranges of several components should meet the following conditions: the upper limit value of a component + the lower limit value of other components ≤ 100; the lower limit value of a component + the upper limit value of other components ≥ 100.

[0038] In some embodiments, the chemical composition of the steel for continuous oil pipe includes, by mass fraction: C≤0.070%, Si: 0.10%~0.50%, Mn: 0.70%~1.20%, P≤0.015%, S≤0.005%, Cr: 0.20%~0.70%, Ni: 0.10%~0.50%, Cu: 0.20%~0.60%, Mo: 0.10%~0.40%, Nb: 0.01%~0.05%, Ti: 0.010%~0.080%.

[0039] In some embodiments, the chemical composition of the steel for continuous oil pipe includes, by mass fraction: C: 0.030% to 0.070%, Si: 0.10% to 0.50%, Mn: 0.80% to 1.10%, P≤0.015%, S≤0.005%, Cr: 0.20% to 0.70%, Ni: 0.10% to 0.50%, Cu: 0.25% to 0.55%, Mo: 0.10% to 0.40%, Nb: 0.01% to 0.05%, and Ti: 0.010% to 0.080%.

[0040] The positive effect of limiting the mass fraction of Si to 0.10% to 0.50% is as follows: Si is a solid solution strengthening element in steel and can significantly improve the strength of steel. By adding 0.10% to 0.50% Si, the tensile strength and yield strength of steel can be improved without sacrificing too much toughness. In addition, Si can improve the oxidation resistance and creep resistance of steel at high temperatures, which is especially important for oil pipes that need to work in high temperature environments. For example, the mass fraction of Si can be 0.10%, 0.20%, 0.30%, 0.40%, 0.50%, etc.

[0041] The positive effect of limiting the mass fraction of Cr to 0.20% to 0.70% is that Cr is one of the main alloying elements of stainless steel and acid-resistant steel. Adding 0.20% to 0.70% Cr to steel can significantly improve the corrosion resistance of steel, especially in acidic environments. This is crucial for oil pipe steel that needs to work in corrosive media. For example, the mass fraction of Cr can be 0.20%, 0.30%, 0.40%, 0.50%, 0.60%, 0.70%, etc.

[0042] The positive effect of limiting the mass fraction of Ni to 0.10% to 0.50% is that Ni is an effective corrosion-resistant element. When used in combination with other elements (such as chromium, copper, etc.), it can significantly improve the atmospheric corrosion resistance of steel. Within this mass fraction range, Ni can effectively form a dense oxide film to protect the steel from corrosion by corrosive media. In addition, Ni helps to improve the fatigue resistance of steel, which is particularly important for oil pipe steel that needs to withstand alternating loads. By increasing the mass fraction of Ni, the fatigue life of the oil pipe can be extended and safety accidents caused by fatigue fracture can be reduced. Finally, Ni is a relatively expensive alloying element. By limiting its mass fraction within a reasonable range, the production cost can be effectively controlled while ensuring the performance of the steel, which helps to maximize economic benefits and improve the market competitiveness of the product. Exemplarily, the mass fraction of Ni can be 0.10%, 0.20%, 0.30%, 0.40%, 0.50%, etc.

[0043] The positive effect of limiting the mass fraction of Mo to 0.10% to 0.40% is that Mo can effectively improve the thermal strength of steel, that is, the ability to maintain strength and stability at high temperatures, which is particularly important for oil pipes that need to work in high temperature environments. In addition, Mo helps to reduce the brittle tendency of steel during tempering, so that the steel still has good toughness and plasticity while maintaining high strength, which helps to extend the service life of the oil pipe and reduce safety accidents caused by brittle fracture. Finally, Mo is one of the important alloying elements in stainless steel and acid-resistant steel, which can significantly improve the corrosion resistance of steel. Within this mass fraction range, Mo can effectively resist the erosion of corrosive media such as acids, alkalis, and salts, and protect steel from corrosion damage. In particular, for acid-resistant oil pipes, the addition of Mo can significantly improve its corrosion resistance to formic acid, acetic acid, oxalic acid, hydrogen peroxide, sulfuric acid and other media. Exemplarily, the mass fraction of Mo can be 0.10%, 0.20%, 0.30%, 0.40%, etc.

[0044] The positive effect of limiting the mass fraction of Ti to 0.010% to 0.080% is that Ti easily forms carbides TiC with carbon in steel. These carbide particles have the effect of preventing the growth and coarsening of steel grains, thereby refining the grain structure of steel. The refined grain structure helps to improve the strength and toughness of steel, so that the oil pipe can maintain better stability and reliability when subjected to complex stresses and harsh environments. In addition, Ti can significantly improve the corrosion resistance of steel, especially in stainless steel, where Ti can eliminate or reduce the intergranular corrosion of steel. For oil pipes that need to work in corrosive media, the addition of Ti can significantly improve their corrosion resistance and extend their service life. Finally, the addition of Ti can improve the welding performance of steel and reduce the tendency of welding cracks, which makes it easier to weld and connect oil pipes during manufacturing and installation, improving production efficiency and engineering quality. For example, the mass fraction of Ti may be 0.010%, 0.020%, 0.030%, 0.040%, 0.050%, 0.060%, 0.070%, 0.080%, etc.

[0045] In some embodiments, the microstructure of the coiled tubing steel includes quasi-polygonal ferrite.

[0046] Quasi-polygonal ferrite is a form of steel microstructure, which is between polygonal ferrite and acicular ferrite, and has unique crystal structure and morphological characteristics. In coiled tubing steel, the presence of quasi-polygonal ferrite has an important influence on the performance of steel:

[0047] Improved strength and toughness: The introduction of quasi-polygonal ferrite can improve the strength and toughness of steel, allowing it to meet high strength requirements while maintaining good toughness, making it suitable for more complex underground operating environments.

[0048] Optimized corrosion resistance: The unique crystal structure of quasi-polygonal ferrite helps improve the corrosion resistance of the steel and extend the service life of the coiled tubing.

[0049] Improve fatigue resistance: Quasi-polygonal ferrite can improve the fatigue resistance of steel and reduce the risk of fatigue fracture during continuous operation.

[0050] In some embodiments, the strength of the steel pipe made of the coiled tubing steel is CT90 steel grade.

[0051] CT90 steel grade is a strength grade of steel for coiled tubing, which means that the tensile strength, yield strength and other mechanical properties of the steel have reached specific standards. CT90 steel grade steel pipes are mainly used for the exploitation and transportation of energy such as oil and natural gas. They can withstand high pressure and temperature to ensure the safe operation of the pipeline.

[0052] Figure 1A schematic flow chart of a method for preparing a high-grade acid-resistant coiled tubing steel provided in an embodiment of the present application.

[0053] See also Figure 1 In a second aspect, an embodiment of the present application provides a method for preparing the high-grade acid-resistant coiled tubing steel described in the first aspect, the method comprising:

[0054] S1, obtaining a slab having the chemical composition;

[0055] This step is the basis of the preparation process, and it is necessary to ensure that the chemical composition of the slab accurately meets the subsequent preparation requirements. The chemical composition of the slab should include C, Si, Mn, P, S, Cr, Ni, Cu, Mo, Nb, Ti and the matrix element Fe, and the content of each element must be strictly controlled within a certain range to meet the special performance requirements of high-level acid-resistant continuous oil pipe steel.

[0056] S2, heating the slab to make the slab reach the furnace discharge temperature;

[0057] In the embodiment of the present application, the obtained slab is placed in a heating furnace for heating so that the slab reaches the furnace discharge temperature. The furnace discharge temperature is a key parameter, which directly affects the plasticity and deformation ability of the slab in the subsequent rolling process.

[0058] In some embodiments, the furnace temperature is 1200°C to 1280°C.

[0059] This temperature range can ensure that the slab is fully austenitized and provide good plastic deformation conditions for the subsequent rolling process. At the same time, too high a furnace temperature may cause thicker iron oxide scale on the surface of the slab, which is not conducive to the descaling of the strip surface and affects the surface quality of the finished strip; while too low a furnace temperature may cause insufficient heating of the slab, making it difficult to ensure the dynamic recrystallization process during the rolling process. For example, the furnace temperature can be 1200℃, 1220℃, 1240℃, 1260℃, 1280℃, etc.

[0060] S3. The heated slab is rolled, cooled and coiled in sequence to obtain steel for continuous oil pipe.

[0061] After reaching the furnace temperature, the slab is rolled. Rolling is the process of pressing the slab into the required shape and size. After rolling, the slab is cooled. The cooling process can further adjust the structure and performance of the steel to meet specific usage requirements. After cooling to a certain temperature, the slab is coiled. Coiling is the process of rolling the cooled slab into a cylindrical shape for subsequent transportation and storage. The coiling temperature is a key parameter that directly affects the microstructure and mechanical properties of the steel after coiling. After coiling, the steel for continuous oil pipes can also be processed and tested to ensure that its quality and performance meet the requirements.

[0062] In some embodiments, the final rolling temperature of the rolling is 830°C to 920°C.

[0063] Finish rolling within this temperature range can ensure that the steel obtains an ideal microstructure, such as polygonal ferrite, etc. These structures help to improve the strength and toughness of the steel. In addition, too high or too low a finish rolling temperature may lead to the generation of rolling cracks. Finish rolling within this temperature range can effectively control the occurrence of rolling cracks, which helps to ensure the quality of the steel and the stability of the production process. Exemplarily, the finish rolling temperature of rolling can be 830°C, 840°C, 850°C, 860°C, 870°C, 880°C, 890°C, 900°C, 910°C, 920°C, etc.

[0064] In some embodiments, the coiling temperature is 500°C to 650°C.

[0065] At an appropriate coiling temperature, the grains of the steel can be refined, thereby improving the strength and toughness of the steel. Too high a coiling temperature may cause the grains of the steel to grow, thereby affecting its mechanical properties and corrosion resistance; too low a coiling temperature may increase production difficulty and cost, and may also cause the toughness of the steel to decrease. For example, the coiling temperature may be 500°C, 530°C, 560°C, 590°C, 620°C, 650°C, etc.

[0066] The product prepared by the preparation method of the high-level acid-resistant continuous oil pipe steel is the above-mentioned high-level acid-resistant continuous oil pipe steel. Since the preparation method of the high-level acid-resistant continuous oil pipe steel adopts part or all of the technical solutions of the high-level acid-resistant continuous oil pipe steel embodiment, it at least has all the beneficial effects brought by the technical solutions of the high-level acid-resistant continuous oil pipe steel embodiment, which will not be repeated here one by one.

[0067] The present application is further described below in conjunction with specific examples. The experimental methods in the following examples where specific conditions are not specified are usually measured in accordance with national standards / industry standards; if there are no corresponding national standards / industry standards, they are measured in accordance with common international standards, conventional conditions or conditions recommended by the manufacturer.

[0068] A slab with set chemical composition is obtained, and the chemical composition of the slab is shown in Table 1.

[0069] Table 1 Chemical composition of slab (wt%), the balance is Fe and unavoidable impurities

[0070] Group C Si Mn P S Cr Ni Cu Mo Nb Ti Example 1 0.07 0.10 0.70 0.010 0.001 0.55 0.20 0.40 0.20 0.04 0.01 Example 2 0.05 0.15 0.85 0.010 0.001 0.60 0.15 0.30 0.20 0.035 0.01 Example 3 0.03 0.15 1.20 0.012 0.005 0.60 0.10 0.20 0.10 0.020 0.02

[0071] Based on the chemical composition of the slab of the embodiment, this embodiment also provides a method for preparing high-grade acid-resistant coiled tubing steel, comprising the following steps:

[0072] Obtaining a slab having the chemical composition;

[0073] Heating the slab to make it reach the furnace discharge temperature;

[0074] The heated slab is rolled, cooled and coiled in sequence to obtain steel for continuous oil pipe. Please refer to Table 2 for the preparation process parameters.

[0075] Table 2 Preparation process parameters of coiled tubing steel

[0076] Group Oven temperature (℃) Finishing rolling temperature(℃) Coiling temperature(℃) Thickness of steel for coiled tubing (mm) Example 1 1240 870 530 4.0 Example 2 1260 890 580 3.0 Example 3 1280 870 610 4.0

[0077] Table 3 Performance results of coiled tubing steel

[0078]

[0079] The obtained coiled tubing steel was subjected to performance tests, and the results are shown in Table 3.

[0080] The hydrogen induced cracking (HIC) test is an important method for evaluating the crack propagation resistance of materials in hydrogen sulfide environments. The test simulates a specific corrosion environment and observes the crack formation on the surface of the material to evaluate its hydrogen induced cracking resistance. CLR (crack length percentage): refers to the ratio of the total crack length to the total length of the specimen, which is used to measure the extent of crack propagation on the surface of the material. CTR (crack thickness percentage): refers to the extent of crack propagation in the thickness direction of the material, that is, the ratio of the maximum crack depth to the material thickness. CSR (crack sensitivity percentage): a parameter that comprehensively reflects the crack length and thickness, used to evaluate the crack sensitivity of the material. The HIC test results are divided into different levels according to the values ​​of CLR, CTR and CSR. The higher the level, the worse the material's resistance to hydrogen induced cracking. These parameters are of great significance for evaluating the applicability of materials in hydrogen-containing environments such as oil and natural gas.

[0081] Sulfide stress corrosion cracking (SSCC) is a brittle fracture phenomenon that occurs in metal materials in sulfide media due to the coupling effect of the medium and stress.

[0082] It can be seen from Tables 1 to 3 that the steel for coiled tubing prepared by the embodiments of the present application, on the one hand, has mechanical properties such as strength that meet the technical requirements of users, and on the other hand, no cracks were found in hydrogen induced cracking (HIC) test and sulfide stress corrosion cracking (SSCC) test after pipe making, showing good acid resistance.

[0083] One or more technical solutions in the embodiments of the present invention have at least the following technical effects or advantages:

[0084] The steel for the coiled oil pipe provided by the embodiment of the present invention has high strength, good toughness and corrosion resistance, and is suitable for oil and gas exploitation and transportation under various complex geological conditions.

[0085] The above description is only a specific implementation of the present application, so that those skilled in the art can understand or implement the present application. It will be apparent to those skilled in the art that various modifications to these embodiments are possible, and the general principles defined in the present application can be implemented in other embodiments without departing from the spirit or scope of the present application. Therefore, the present application will not be limited to the embodiments shown in the present application, but will conform to the widest range consistent with the principles and novel features applied for by the present application.

Claims

1. A high-grade acid-resistant steel for coiled tubing, the chemical composition of the steel for coiled tubing comprising: C, Si, Mn, P, S, Cr, Ni, Cu, Mo, Nb, Ti, and matrix element Fe; The mass fraction of C is ≤0.070 parts; The mass fraction of Mn is 0.70 to 1.20 parts; The mass fraction of Cu is 0.20 parts to 0.60 parts.

2. The steel for coiled tubing according to claim 1, characterized in that: Measured by mass fraction, the chemical composition of the steel for continuous oil pipe includes: C≤0.070%, Si: 0.10%~0.50%, Mn: 0.70%~1.20%, P≤0.015%, S≤0.005%, Cr: 0.20%~0.70%, Ni: 0.10%~0.50%, Cu: 0.20%~0.60%, Mo: 0.10%~0.40%, Nb: 0.01%~0.05%, Ti: 0.010%~0.080%.

3. The steel for coiled tubing according to claim 1, characterized in that: Measured in mass fraction, the chemical composition of the steel for continuous oil pipe includes: C: 0.030% ~ 0.070%, Si: 0.10% ~ 0.50%, Mn: 0.80% ~ 1.10%, P≤0.015%, S≤0.005%, Cr: 0.20% ~ 0.70%, Ni: 0.10% ~ 0.50%, Cu: 0.25% ~ 0.55%, Mo: 0.10% ~ 0.40%, Nb: 0.01% ~ 0.05%, Ti: 0.010% ~ 0.080%.

4. The steel for coiled tubing according to claim 1, characterized in that: The microstructure of the steel for the coiled tubing includes quasi-polygonal ferrite.

5. The steel for coiled tubing according to claim 1, characterized in that: The strength of the steel pipe made of the coiled tubing steel is CT90 steel grade.

6. A method for preparing the steel for coiled tubing according to any one of claims 1 to 5, the method comprising: Obtaining a slab having the chemical composition; Heating the slab to make it reach the furnace discharge temperature; The heated slab is rolled, cooled and coiled in sequence to obtain steel for continuous oil pipe.

7. The method according to claim 6, characterized in that The furnace temperature is 1200°C to 1280°C.

8. The method according to claim 6, characterized in that The final rolling temperature of the rolling is 830°C to 920°C.

9. The method according to claim 6, characterized in that The coiling temperature is 500°C to 650°C.

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