A high-grade low-cost 550mpa grade welded pipe hot-rolled coil and its manufacturing method

Abstract

The application discloses a high-steel-grade low-cost 550 MPa-grade hot-rolled plate for welded pipes and a manufacturing method thereof, and chemical components of the plate are as follows: C 0.040-0.065%, Si 0.20-0.35%, Mn 1.48-1.75%, P <=0.02%, S <=0.003%, Ti 0.012-0.025%, Nb 0.035-0.055%, RE 0.005-0.008%, Cr 0.15-0.20%, B 0.0005-0.0020%, Mg 0.002-0.004%, Al 0.025-0.050%, Ca <=0.003%, N <=0.008%, and the rest is Fe and inevitable impurities. A process flow comprises smelting, slab continuous casting, slab heating, rolling, cooling and coiling. The steel has the following properties: R t0.5 580-660 MPa, R m 650-750 MPa, A 50mm >=28%, R t0.5 / R m <=0.90, the average impact energy at -40 DEG C Akv is >=200 J, the average drop weight tear test (DWTT) at -20 DEG C is >=90%, the hardness value HV10 is <=220, and the cold bending performance is qualified. The product has high strength and toughness and good welding performance, is more suitable for various complex and extreme service environments, and has higher safety in pipeline engineering.

Description

TECHNICAL FIELD

[0001] The present application relates to the technical field of low-alloy high-strength steel production, in particular to a high-grade low-cost 550 MPa grade hot-rolled plate for welded pipe with good strength and toughness and easy welding produced by medium-thickness casting blank and a manufacturing method thereof. BACKGROUND

[0002] In recent years, China has made significant progress in hot-rolled plate production technology, with a substantial increase in production efficiency and product quality, which provides technical support for the development of high-grade low-cost hot-rolled plate for welded pipe. At the same time, in the face of increasingly stringent environmental protection and energy conservation and emission reduction requirements, the steel industry is undergoing transformation and upgrading and high-quality development, and the development of high-value-added and high-technology-content hot-rolled plate for welded pipe has become an important direction for enterprise transformation and upgrading, and the development of low-cost hot-rolled plate for welded pipe is a specific embodiment of this trend. In addition, with the accelerated advancement of global and domestic infrastructure construction and the continuous growth of energy demand, the demand for high-quality and high-performance steel in the energy, transportation, and construction sectors continues to increase, especially the demand for long-distance transportation of oil and gas resources, prompting a significant increase in demand for high-quality and high-grade pipeline steel, and high-grade low-cost hot-rolled plate for welded pipe has a very broad market prospect.

[0003] The development and application of high-grade low-cost hot-rolled plate for welded pipe not only can significantly reduce the material cost of pipeline engineering, improve engineering economic benefits, and ensure the safety and reliability of the pipeline, but also, in the face of fierce market competition, through optimization of alloy composition design and production process, can further reduce production cost and enhance enterprise competitiveness. However, this puts strict requirements on the performance of the plate, not only higher purity, but also excellent strength and toughness and welding performance to meet the use requirements in harsh environments.

[0004] Currently, although there are many patents on 550 MPa grade steel for welded pipe, especially pipeline steel, both at home and abroad, they are significantly different from the present application. The following briefly introduces several patents similar to the present application:

[0005] (1) Chinese patent CN201610871405.1 "550 MPa grade pipeline steel with good high temperature resistance and good low temperature crack arrest toughness and manufacturing method thereof". The composition contains C: 0.061~0.120%, Mn: 1.70~2.20%, Mo: 0.15~0.39%, Cu: 0.15~0.30%, Ni: 0.15~0.50%, Nb: 0.035~0.080%, V: 0.005~0.054%, Ti: 0.005~0.030%, Al: 0.015~0.040%, Ca: 0.005~0.035%, the rest is Fe and inevitable impurities. The patent composition contains high content of Mo, Ni and other noble metal elements, the alloy cost is high, which is different from the alloy system of the present application; in addition, higher finishing temperature and medium cooling rate are used in the process, which cannot fully play the role of water substitution alloy, thereby reducing the cost and improving the strength and toughness.

[0006] (2) Chinese patent CN200810227820.9 "X80 hot-rolled coil for high strength and toughness spiral submerged arc welded pipe and production method thereof". The composition contains C: 0.03~0.08%, Si: 0.10~0.25%, Mn: 1.60~1.95%, Nb: 0.09~0.11%, V: 0.020~0.030%, Ti: 0.010~0.020%, Mo: 0.10~0.30%, Cu: 0.10~0.30%, Ni: 0.10~0.30%, Cr: 0~0.30%, P: <0.018%, S: <0.003%, N: <0.006%, B: 0~0.0005%, the balance is iron and inevitable trace impurities. The patent composition contains high content of Mo, Ni and other noble metal elements, the alloy cost is high, which is different from the alloy system of the present application; secondly, the patent uses 230 mm cast slab thickness, which is different from the present application; in addition, the cooling rate is not specified, the low temperature toughness index is relatively loose, the impact test temperature is-20 ℃, and the DWTT test temperature is-15 ℃.

[0007] (3) Chinese Patent CN202210198496.2, “A Low-Cost Easy-to-Weld Pipeline Steel Hot-Rolled Coil and Its Preparation Method”. The composition contains C: 0.050~0.080%, Si: 0.10~0.25%, Mn: 0.50~0.70%, P≤0.020%, S≤0.0040%, Ti: 0.045~0.065%, Als: 0.020~0.050%, N≤0.0050%, with the remainder being Fe and unavoidable impurities. This patent has a low Mn content and only adds Ti alloying element alone, failing to fully utilize the microalloying composite effect of Nb, Ti, and B, reducing the strengthening effect and easily causing fluctuations in product performance. Furthermore, the low-temperature toughness requirements are relatively lenient, with an impact test temperature of -20 ℃ and a DWTT test temperature of -10 ℃. Secondly, the coiling temperature is relatively high, which differs significantly from the present invention.

[0008] (4) Chinese Patent CN201210022461.X “A low-cost, high-strength and high-toughness X80 pipeline steel coil and its production method”. The composition contains C: 0.03~0.07%, Si: 0.10~0.25%, Mn: 1.25~1.95%, P<0.018%, S<0.005%, Nb: 0.06~0.12%, Mo: 0.15~0.30%, Ti: 0.008~0.020%, Cr: 0.15~0.35%, Als: 0.020~0.045%, Ca: 0.0015~0.0035%, N<0.0060%, with the balance being iron and unavoidable impurities. The patented composition contains a high amount of Mo, a precious metal element, resulting in higher alloy costs. Furthermore, the patented low-temperature toughness requirements are relatively lenient, with an impact test temperature of -20°C and a DWTT test temperature of -15°C, which differs significantly from the present invention.

[0009] (5) Chinese Patent CN201010530202.9, “A low-cost, high-strength and tough X80 pipeline steel resistant to large deformation and its production method.” The composition contains C: 0.02~0.08%, Si: <0.40%, Mn: 1.2~2.0%, P: <0.015%, S: <0.004%, Nb: 0.03~0.08%, Ti: 0.005~0.030%, Mo: 0.10~0.30%, Cu: <0.40%, Ni: <0.30%, with the remainder being Fe and unavoidable impurities. The composition of this patent contains precious metal elements such as Mo and Ni, resulting in high alloy costs and a relatively wide composition range, which can easily cause fluctuations in product performance. The cooling process of this patent uses two-stage cooling, which cannot fully utilize water to replace the alloy, thereby reducing costs and improving strength and toughness. In addition, this patent uses a billet thickness of 210~300 mm, which is significantly different from the present invention.

[0010] (6) Chinese Patent CN201310471192.X “A Low-Cost X80 Pipeline Steel and Its Manufacturing Method”. The composition contains C: 0.065~0.085%, Mn: 1.0~2.0%, Si: 0.25~0.35%, Cu: 0.10~0.25%, Ni: 0.10~0.30%, Cr: 0.10~0.50%, Nb: 0.02~0.04%, Ti: 0.005~0.030%, V: 0.02~0.04%, Als: 0.02~0.06%, Ca: <0.006%, P: <0.015%, S: <0.003%, N: <0.012%, with the balance being Fe and trace amounts of unavoidable impurities. The patented composition contains the precious metal element Ni, resulting in high alloy costs. The patented process uses a high final rolling temperature and a moderate cooling rate, which cannot fully utilize the water-displacement alloy to reduce costs and improve strength and toughness. The patented low-temperature toughness requirements are relatively lenient, with an impact test temperature of -20 ℃ and a DWTT test temperature of -15 ℃. In addition, the patented billet thickness is 230 mm, which is significantly different from the present invention.

[0011] Currently, there are many publicly available patents for 550 MPa grade welded pipe steel, especially for pipeline steel. However, on the one hand, most of them have added expensive elements such as Ni and Mo to their composition design, resulting in high alloy costs. On the other hand, the requirements for low-temperature toughness are relatively lenient, and it is basically difficult to meet the requirements for low-temperature fracture toughness below -40 ℃. Summary of the Invention

[0012] The present invention relates to a high-strength, low-cost hot-rolled pipeline steel coil and its manufacturing method, particularly to a high-strength, low-cost 550 MPa grade welded pipe hot-rolled coil with good strength and toughness and easy weldability produced from a medium-thickness cast billet and its manufacturing method.

[0013] One of the technical solutions of this invention is to propose a high-strength, low-cost 550 MPa grade hot-rolled coil for welded pipes, produced using medium-thickness cast billets, which has good strength and toughness and is easy to weld. The chemical composition (weight, %) is as follows: C: 0.040~0.065%, Si: 0.20~0.35%, Mn: 1.48~1.75%, P: ≤0.02%, S: ≤0.003%, Ti: 0.012~0.025%, Nb: 0.035~0.055%, RE: 0.005~0.008%, Cr: 0.15~0.20%, B: 0.0005~0.0020%, Mg: 0.002~0.004%, Als: 0.025~0.050%, Ca: ≤0.003%, N: ≤0.008%, with the remainder being Fe and unavoidable impurities.

[0014] Preferably, the optimal composite mass ratio of La and Ce is La / Ce = 2.8~3.2.

[0015] Preferably, more than 80% of the acicular ferrite nucleation size in the crystal after rare earth treatment is concentrated in the range of 0.5~3 μm.

[0016] The reason for selecting the above alloying elements and their contents in this invention is as follows:

[0017] Carbon (C) is the most economical and effective strengthening element in steel, playing a role in solid solution strengthening and precipitation strengthening. On the one hand, C atoms dissolve in the iron matrix to form a solid solution, which can significantly improve the strength of steel. On the other hand, during cooling, C combines with alloying elements such as Nb and Ti to precipitate fine carbide particles, thereby improving the strength and hardness of steel through precipitation strengthening. However, excessive C content will reduce the plasticity, toughness, and weldability of steel. Therefore, this invention controls the C content to 0.040~0.065%.

[0018] Si: In the steelmaking process, Si is commonly used as a deoxidizer, helping to reduce non-metallic inclusions in steel and improve its purity and quality. Furthermore, it can dissolve in ferrite and austenite, exhibiting strong solid solution strengthening effects and significantly improving the strength and hardness of steel. Simultaneously, it can inhibit the nucleation and growth of ferrite, shifting the steel's C-curve to the right, thereby improving its hardenability. However, excessive Si content can significantly reduce the steel's plasticity, toughness, and weldability. Therefore, this invention controls the Si content to 0.20~0.35%.

[0019] Mn plays a role in solid solution strengthening and phase transformation strengthening, compensating for the strength loss caused by the reduction in carbon content. Simultaneously, it lowers the γ-α phase transformation temperature, expands the austenite region, promotes bainite formation, and thus refines the ferrite microstructure. Furthermore, Mn can increase the hardenability of steel, improve toughness, and lower the brittle-to-brittle transition temperature of tough steel. However, excessively high Mn content exacerbates center segregation in continuously cast billets, leading to anisotropy in the mechanical properties of steel plates and pipes. Therefore, this invention controls the Mn content to 1.48~1.75%.

[0020] P, S, and N are unavoidable impurity elements in steel, and the lower the content, the better. However, excessively low steel purity will increase production costs. Therefore, while ensuring the toughness index of this product, the content of P in this invention is ≤0.02%, S≤0.003%, and N≤0.008%.

[0021] Ti: A strengthening element for nitrogen. Adding approximately 0.015% Ti allows for the formation of high-temperature stable, fine TiN precipitates during slab continuous casting. These dispersed, fine TiN particles anchor at austenite grain boundaries, hindering austenite grain growth and effectively refining the microstructure. Furthermore, the addition of Ti effectively prevents the formation of NbN and BN, allowing Nb to exert its strengthening effect and B to enhance hardenability. In addition, Ti has a strong affinity for sulfur, forming stable TiS compounds. TiS is more stable than MnS; therefore, Ti can significantly reduce the hot brittleness of sulfur and improve the toughness of steel. However, excessive Ti content not only increases costs but also easily leads to the formation of large inclusions. Therefore, this invention controls the Ti content to 0.012~0.025%.

[0022] Nitrogen (Nb) exhibits significant grain refinement and precipitation strengthening effects and is a strong carbonitride forming element. During rolling, strain-induced precipitation forms fine Nb(N,C) particles, which pin grain boundaries and inhibit the recovery and recrystallization of deformed austenite. Through controlled rolling and controlled cooling, the deformed austenite in the non-recrystallization zone during the finishing rolling stage transforms into fine products with high dislocation density, thereby improving the strength and toughness of the steel. However, if the Nb content is too low, the dispersed precipitation effect is not significant, and it fails to refine the grains and strengthen the matrix. Furthermore, Nb is a precious metal; excessively high content increases alloy costs, and the strengthening effect diminishes after a certain amount is added. Therefore, this invention controls the Nb content to be between 0.035% and 0.055%.

[0023] RE: It plays a role in deoxidation, desulfurization, purification of steel, and removal of altered inclusions in steel. Rare earth elements have deoxidizing and desulfurizing effects, which can reduce and refine inclusions in steel and purify molten steel. Rare earth elements have a modifying effect on inclusions, generating dispersed and fine inclusions. These inclusions become heterogeneous nucleation centers, thereby refining the grain size. Rare earth inclusions can also induce the nucleation of acicular ferrite within the grains and hinder the growth of austenite grains, refining the grain structure of steel and significantly improving its strength and toughness. In addition, the microalloying effect of rare earth elements La and Ce can suppress the segregation of P, S and low-melting-point impurities at grain boundaries, helping to strengthen grain boundaries and improve the plasticity and toughness of steel, especially high-temperature plasticity. When they are added in an appropriate ratio (La / Ce=2.8~3.2), they can produce a more significant effect than adding them alone. After rare earth treatment, more than 80% of the acicular ferrite nucleation size is concentrated in the range of 0.5~3 μm, with the optimal range being 0.005~0.008%.

[0024] Cr (Cr) can form a continuous solid solution with Fe, reducing the austenite phase region, significantly improving the hardenability of steel, inhibiting the formation of polygonal ferrite and pearlite, and promoting the formation of ferrite or bainite with a large number of dislocations within the grains in the medium and low temperature ranges. Cr is one of the effective elements for improving the strength and hardness of steel, and compared with Mo, Cr is cheaper. When the Cr content is too high, it can significantly improve the tensile strength and hardness of steel, but the improvement on yield strength is limited, and the low-temperature toughness will decrease significantly. Therefore, it is not advisable to add too much Cr. Therefore, the Cr content in this invention is controlled at 0.15~0.20%.

[0025] B (B) is a highly hardenable element that strongly inhibits the γ-α transformation, shifting the CCT curve to the right and increasing hardenability. Trace amounts of B can significantly inhibit ferrite nucleation at austenite grain boundaries, exhibiting a strong synergistic effect with Mo and Nb. It can replace some expensive alloying elements (such as Ni, Cr, and Mo), reducing production costs while maintaining or improving pipeline steel performance. Furthermore, it flattens the bainite transformation curve, allowing for the acquisition of bainitic microstructure even over a wide range of cooling rates. However, excessively high B content can negatively impact weldability, necessitating more stringent welding processes and increasing process complexity. Therefore, this invention controls the B content to 0.0005~0.0020%.

[0026] Mg: It can reduce the O and S content and the number of inclusions in steel, thus purifying the molten steel; Mg can also significantly modify inclusions in steel, generating fine and dispersed inclusions and inducing acicular ferrite (AF) nucleation, thereby refining the microstructure of steel; Mg can increase the yield strength and tensile strength of steel by more than 5%, while the plasticity remains basically unchanged, thus reducing the yield strength ratio; In addition, by utilizing the microalloying effect of Mg and setting a reasonable amount of Mg added, the amount of expensive microalloying metals such as Nb, V, and Ti can be reduced, thereby reducing the alloying production cost. Therefore, the Mg content range of this invention is 0.002~0.004%.

[0027] Als (Al₂O₃) is an indispensable and excellent deoxidizer in steelmaking. Adding a small amount of Al to steel can refine the grain size, improve the steel's strength and impact toughness; at the same time, it forms fine and dispersed AlN particles, which is beneficial for grain refinement and improving the steel's strength and toughness. However, excessive content can lead to increased brittleness in the steel, reducing its strength and toughness. Therefore, the Al content in this invention is controlled at 0.025~0.050%.

[0028] Ca (Ca) plays a role in desulfurization and deoxidation. Calcium treatment can also alter the morphology of sulfides, improve the anisotropy of steel, and modify inclusions to achieve spheroidization, thus ensuring the toughness of the steel. However, excessive Ca content can lead to the formation of large inclusion particles; the optimal range is below 0.003%.

[0029] The second technical solution of this invention is to propose a method for manufacturing high-strength, low-cost 550 MPa grade hot-rolled coils for welded pipes. The production process involves smelting, slab continuous casting, slab heating, rolling, ultra-fast cooling, and coiling. Its characteristic is:

[0030] Billet heating: The continuously cast slab is directly heated in a hot charging furnace at a temperature of 500~850 ℃, and then heated to 1150~1180 ℃ in a walking beam furnace before being unloaded. Within this temperature range, Nb has been fully dissolved, and the austenite grains do not grow significantly due to the pinning of Ti(C,N) particles at the grain boundaries, which is conducive to obtaining a fine and uniform microstructure.

[0031] Rolling: The roughing rolling temperature is 1000~1060 ℃, with 5 passes. The reduction rate of the first single pass is ≥20%, and the reduction rate of the remaining 4 passes is ≥22%, with the reduction rate gradually increasing. The finishing rolling temperature is 920~940 ℃, and the finishing rolling temperature is 730~780 ℃, with a cumulative reduction rate of 65%~70%. This final rolling temperature range is conducive to Nb precipitation, refines grain size, and improves strength and toughness. The use of a large reduction rate can generate a large number of dislocations and twins, thereby playing a role in dislocation strengthening and significantly improving the yield strength and tensile strength of the steel.

[0032] Preferably, the thickness of the intermediate slab is 50~62 mm, and the thickness of the finished product is 16~20 mm.

[0033] Cooling: After rolling, ultra-fast cooling is adopted, with a final cooling temperature of 400~460 ℃ and a cooling rate of 30~60 ℃ / s. It eventually transforms into a mixed structure of acicular ferrite and martensite / MA, which gives the final coil high strength, low yield strength ratio and good low temperature toughness.

[0034] Winding: After ultra-fast cooling, a high-power winder is used for winding. The drum tension coefficient is 2.0~2.5, the auxiliary winding roller pressure is 550~620 MPa, the guide roller pressure is 60~90 kN, the drum expansion speed is 0.5 times, the number of skip turns is 4~6, the MD lead rate is 15~16, the WR lead rate is 18~19, the PR lead rate is 10~11, and the coil shape is good with no hard bends.

[0035] Furthermore, after the molten iron is pretreated, it is smelted in a converter using a top-blowing or top-bottom combined blowing process, followed by ladle refining. The ladle refining process uses RH vacuum treatment and LF furnace light desulfurization treatment, followed by calcium wire feeding treatment. To ensure the spheroidization effect of sulfide and oxide inclusions, the wire feeding speed is ≥3.5 m / s, the net argon blowing time is ≥8 min, and the calming time is ≥10 min.

[0036] Furthermore, during slab continuous casting, the entire process is protected during pouring, and electromagnetic stirring or dynamic light reduction is used to reduce center segregation of the continuously cast slab. The slab is of medium thickness, 190~210 mm, which can ensure the compression ratio and allow the slab to be heated quickly and evenly, reducing energy consumption and saving costs.

[0037] The 550 MPa hot-rolled coils for welded pipes produced using the above-mentioned chemical composition and process possess excellent comprehensive properties, with a yield strength of 580~660 MPa, tensile strength of 650~750 MPa, elongation after fracture ≥28%, yield strength ratio ≤0.90, average impact energy Akv ≥200 J at -40 ℃, average drop weight test (DWTT) value ≥90% at -20 ℃, hardness (HV10) ≤220, and qualified cold bending performance, which can meet the requirements for use of welded pipe steel in harsh and complex environments.

[0038] Compared with existing technologies, the composition design concept of this invention adopts a low-C-Nb-Ti-B alloy design. While adding small amounts of B and Mg elements, it also incorporates rare earth elements Ce and La. On the one hand, by adding small amounts of B and Mg, the addition of precious metals is reduced or eliminated, lowering production costs. On the other hand, the addition of Mg, Ce, and La elements completes steel purification, refines grain structure, and modifies inclusions, thereby improving the overall performance of pipeline steel. Furthermore, the production process employs low-temperature precision rolling and ultra-rapid cooling after rolling to obtain a fine and uniform acicular ferrite structure, ensuring the product has high strength and toughness as well as good weldability. This makes it more suitable for various complex and extreme service environments, resulting in higher safety in pipeline engineering.

[0039] Compared with the prior art, the present invention has the following advantages:

[0040] (1) Break away from the existing design concept of 550 MPa grade welded pipe steel alloy, adopt low carbon and add a small amount of Nb, Ti and B elements, and at the same time add rare earth elements and cheap Mg alloy. Mg plays its micro-alloying role, reducing the amount of precious metals such as Nb and Ti used and reducing production costs.

[0041] (2) The composite addition of Mg and rare earth elements Ce+La plays a role in purifying steel, refining inclusions, improving the morphology and distribution of inclusions, refining grains, improving overall mechanical properties, and providing higher safety in pipeline engineering.

[0042] (3) Using medium-thickness continuous casting billets of 190~210 mm, while ensuring the compression ratio, the continuous casting billets can be quickly and evenly burned through, resulting in energy saving, high efficiency, and high production flexibility.

[0043] (4) By adopting a lower final rolling temperature and ultra-fast cooling process after rolling, and by combining the effects of Nb, Ti, B and RE elements, a finer and more uniform acicular ferrite structure is obtained, which ensures that the product has good strength and toughness. Attached Figure Description

[0044] Figure 1 Example 1 shows the metallographic optical microstructure - AF (acicular ferrite). The AF microstructure, through its unique interlaced distribution of fine acicular structures and high dislocation density, achieves a good balance between high strength and high toughness, making it the core microstructure in existing welded pipe steels. Furthermore, the addition of rare earth elements (Ce + La) can further optimize the nucleation and distribution of AF. Detailed Implementation

[0045] To make the objectives, technical solutions, and advantages of the embodiments of the present invention clearer, the technical solutions of the embodiments of the present invention will be clearly and completely described below with reference to the accompanying drawings. Obviously, the described embodiments are only some embodiments of the present invention, and not all embodiments. The following description of exemplary embodiments of the experimental method is merely illustrative and is in no way intended to limit the present invention or its application or use. Based on the embodiments of the present invention, all other embodiments obtained by those skilled in the art without creative effort are within the scope of protection of the present invention.

[0046] The following examples are merely some preferred embodiments of the present invention and do not limit the scope and technical means of the invention in any way. Table 1 shows the chemical composition of Examples 1-8, Table 2 shows the process parameters for heating, rolling, and cooling in each example, Table 3 shows the AF nucleation size in each example, and Table 4 shows the mechanical property test results for each example.

[0047] Table 1 Chemical composition (wt, %) of the steel in the examples

[0048]

[0049] Table 2. Steel heating, rolling, cooling, and coiling process parameters for each embodiment.

[0050]

[0051] Table 3 AF nucleation dimensions of steels in each embodiment

[0052]

[0053] Table 4 Main mechanical properties of steel in each embodiment

[0054]

[0055] As can be seen from Tables 1-4, the hot-rolled coils for high-strength, low-cost, easy-to-weld 550 MPa welded pipes produced by adopting the composition design, rolling and coiling process of the present invention have good comprehensive mechanical properties, especially excellent low-temperature toughness.

Claims

1. A high-strength, low-cost 550 MPa grade hot-rolled coil for welded pipes, characterized in that, The chemical composition of the steel plate is as follows: C: 0.040%~0.065%, Si: 0.22%~0.35%, Mn: 1.48%~1.75%, P: ≤0.02%, S: ≤0.003%, Ti: 0.012%~0.025%, Nb: 0.035%~0.055%, La+Ce: 0.005~0.008%, Cr: 0.15%~0.20%, B: 0.0005%~0.0020%, Mg: 0.002%~0.004%, Als: 0.025%~0.050%, Ca: ≤0.003%, N: ≤0.008%. The optimal composite mass ratio of La and Ce is La / Ce = 2.8~3.2, with the remainder being Fe and unavoidable impurities. The yield strength of the steel is 580~660 mm. MPa, tensile strength 650~750 MPa, elongation after fracture ≥28%, yield strength ratio ≤0.90, average impact energy Akv ≥200 J at -40 ℃, average drop weight DWTT ≥90% at -20 ℃, hardness HV10 ≤220, and cold bending performance are all qualified. The manufacturing method of high-strength, low-cost 550 MPa grade hot-rolled coils for welded pipes involves a production process that includes smelting, slab continuous casting, slab heating, rolling, ultra-rapid cooling, and coiling. Continuous casting: During the continuous casting of slabs, the entire process is protected during pouring, and electromagnetic stirring or dynamic light pressure is used. The thickness of the slab is 190~210mm. Billet heating: The continuously cast slab is heated to 1150~1180 ℃ in a walking beam furnace before being taken out of the furnace; Rolling: Roughing rolling temperature is 1022~1060 ℃, intermediate billet thickness is 50~62 mm, finishing rolling temperature is 920~940℃, finishing rolling temperature is 730~780 ℃, cumulative reduction rate is 65%~70%, and finished product thickness is 16~20 mm. Cooling: After rolling, ultra-fast cooling is adopted, with a final cooling temperature of 400~460 ℃ and a cooling rate of 30~60 ℃ / s; Winding: After cooling, a high-strength winder is used for winding. The drum tension coefficient is 2.0~2.5, the auxiliary winding roller pressure is 550~620MPa, and the guide roller pressure is 60~90 kN.

2. The high-strength, low-cost 550 MPa grade hot-rolled coil for welded pipes according to claim 1, characterized in that, After rare earth treatment, more than 80% of the nucleation size of intracrystalline needle-like ferrite is concentrated in the range of 0.6~1.2 μm.

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