Hot-rolled coil plate for high-steel-grade low-cost 550 MPa-grade welded pipe and manufacturing method of hot-rolled coil plate

Abstract

The high-steel-grade low-cost hot-rolled coil plate comprises the following chemical components: 0.040 to 0.065 percent of C, 0.20 to 0.35 percent of Si, 1.48 to 1.75 percent of Mn, less than or equal to 0.02 percent of P, less than or equal to 0.003 percent of S, 0.012 to 0.025 percent of Ti, 0.035 to 0.055 percent of Nb, 0.005 to 0.008 percent of RE, 0.15 to 0.20 percent of Cr, 0.0005 to 0.0020 percent of B, 0.002 to 0.004 percent of Mg, 0.025 to 0.050 percent of Als, less than or equal to 0.003 percent of Ca, less than or equal to 0.008 percent of N and the balance of Fe and inevitable impurities. The technological process comprises the steps of smelting, slab continuous casting, slab heating, rolling, cooling and coiling. Rt0.5 of the steel is 580-660 MPa, Rm is 650-750 MPa, A50 mm is larger than or equal to 28%, Rt0.5 / Rm is smaller than or equal to 0.90, the impact energy mean value Akv at the temperature of-40 DEG C is larger than or equal to 200 J, the drop weight DWTT mean value at the temperature of-20 DEG C is larger than or equal to 90%, the hardness value HV10 is smaller than or equal to 220, and the cold bending performance is qualified. The product provided by the invention has high strength and toughness and good welding performance, so that the product is more suitable for various complex and extreme service environments, and has higher safety in pipeline engineering.

Description

Technical Field

[0001] The present invention relates to the technical field of low alloy high strength steel production, and in particular to a hot rolled coil for a 550 MPa grade welded pipe having good strength and toughness and easy welding, produced by using a medium thickness ingot, and a manufacturing method thereof. Background Art

[0002] In recent years, my country has made significant progress in hot-rolled coil production technology, significantly improving production efficiency and product quality. This has provided technical support for the development of high-grade, low-cost hot-rolled coil for welded pipes. Furthermore, facing increasingly stringent environmental protection and energy conservation and emission reduction requirements, the steel industry is undergoing transformation, upgrading, and high-quality development. The development of high-value-added, high-tech hot-rolled coil for welded pipes has become a key direction for enterprise transformation and upgrading. The development of low-cost hot-rolled coil for welded pipes is a concrete manifestation of this trend. Furthermore, with the accelerated global and domestic infrastructure construction and the continuous growth of energy demand, the demand for high-quality, high-performance steel in the energy, transportation, and construction sectors continues to increase. In particular, the need for long-distance transportation of oil and gas resources has led to a significant increase in the demand for high-quality, high-grade pipeline steel. High-grade, low-cost hot-rolled coil for welded pipes has a very broad market prospect.

[0003] The development and application of high-grade, low-cost hot-rolled coils for welded pipes not only significantly reduces material costs for pipeline projects, improves project economics, and ensures pipeline safety and reliability, but also further reduces production costs and enhances enterprise competitiveness in the face of fierce market competition by optimizing alloy composition design and production processes. However, this places stringent demands on the coil's performance, requiring not only a higher degree of purity but also excellent strength, toughness, and weldability to meet the demands of harsh environments.

[0004] At present, although there are many patents related to 550 MPa grade welded pipe steel, especially pipeline steel, both domestically and internationally, they are significantly different from the present invention. The following briefly introduces several patents that are relatively close to the present invention: (1) Chinese patent CN201610871405.1 “550 MPa grade pipeline steel with high temperature resistance and good low temperature crack arrest toughness and its manufacturing method”. 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%, and the rest is Fe and unavoidable impurities. The patented composition adds a high content of precious metal elements such as Mo and Ni, and the alloy cost is relatively high, which is different from the alloy system of the present invention; in addition, the process uses a high final rolling temperature and a medium cooling rate, which cannot fully utilize the water-substituted alloy, thereby reducing costs and improving strength and toughness.

[0005] (2) Chinese patent CN200810227820.9 “X80 hot-rolled coil for high-strength and toughness spiral submerged arc welded pipe and its production method”. 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%, and the balance is iron and inevitable trace impurities. This patent adds a high content of precious metal elements such as Mo and Ni to the composition, resulting in a high alloy cost, which is different from the alloy system of the present invention; secondly, this patent uses a 230 mm ingot thickness, which is different from the present invention; 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 ℃.

[0006] (3) Chinese patent CN202210198496.2 “A low-cost, easily welded hot-rolled coil of pipeline steel 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%, and the rest is Fe and unavoidable impurities. The Mn content of this patent is relatively low, and only Ti alloying elements are added alone. The microalloying composite effect of Nb, Ti, and B cannot be fully exerted, which reduces the strengthening effect and easily causes product performance fluctuations. At the same time, the low-temperature toughness index requirements are relatively loose, the impact test temperature is -20 °C, and the DWTT test temperature is -10 °C. Secondly, the coiling temperature is relatively high, which is significantly different from the present invention.

[0007] (4) Chinese patent CN201210022461.X “A low-cost, high-strength and tough 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%, and the balance is iron and unavoidable impurities. The patented composition design contains a relatively high Mo precious metal element, resulting in a high alloy cost. In addition, the patent has relatively loose requirements for low-temperature toughness index, with an impact test temperature of -20°C and a DWTT test temperature of -15°C, which are quite different from the present invention.

[0008] (5) Chinese patent CN201010530202.9 “A low-cost, high-strength and tough X80 large deformation-resistant pipeline steel 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%, and the rest is Fe and unavoidable impurities. The composition of this patent contains precious metal elements such as Mo and Ni, resulting in high alloy cost and a wide composition range, which is prone to product performance fluctuations; the cooling process of this patent adopts two-stage cooling, which cannot fully utilize water to replace the alloy, thereby reducing costs and improving strength and toughness; in addition, the thickness of the ingot used in this patent is 210~300 mm, which is quite different from the present invention.

[0009] (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%, and the balance is Fe and trace amounts of unavoidable impurities. The patented composition contains the precious metal element Ni, resulting in high alloy cost; the patented process uses a relatively high final rolling temperature and a medium cooling rate, which cannot fully utilize water to replace the alloy, thereby reducing costs and improving strength and toughness; the patented low-temperature toughness index requirements are relatively loose, with an impact test temperature of -20°C and a DWTT test temperature of -15°C; in addition, the patent uses a billet thickness of 230 mm, which is significantly different from the present invention.

[0010] Although there are many patents for 550 MPa grade welded pipe steel, especially pipeline steel, most of them contain precious elements such as Ni and Mo in their composition design, resulting in high alloy costs. On the other hand, the requirements for low-temperature toughness indicators are relatively loose, and it is basically difficult to meet the low-temperature fracture toughness requirements below -40°C. Summary of the Invention

[0011] The present invention relates to a high-grade, low-cost hot-rolled coil of pipeline steel and a method for manufacturing the same, and more particularly to a high-grade, low-cost, 550 MPa grade welded pipe hot-rolled coil produced from medium-thickness ingots, having good strength and toughness and easy welding, and a method for manufacturing the same.

[0012] One of the technical solutions of the present invention is to provide a high-grade, low-cost 550 MPa welded pipe hot-rolled coil with good strength and toughness and easy welding produced by using a medium-thickness ingot, wherein the chemical composition (by 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%, and the remainder is Fe and unavoidable impurities.

[0013] Preferably, the optimal composite mass ratio of La and Ce is La / Ce=2.8~3.2.

[0014] Preferably, more than 80% of the nucleation size of intracrystalline acicular ferrite after rare earth treatment is concentrated in the range of 0.5-3 μm.

[0015] The present invention selects the above alloying element types and contents because: Carbon is the most economical and effective strengthening element in steel, providing both solid solution strengthening and precipitation strengthening. On the one hand, carbon atoms dissolve in the iron matrix to form a solid solution, significantly increasing the strength of the steel. On the other hand, during cooling, carbon combines with alloying elements such as niobium and titanium to precipitate fine carbide particles, which improve the strength and hardness of the steel through precipitation strengthening. However, excessive carbon content can reduce the steel's plasticity, toughness, and weldability. Therefore, the present invention controls the carbon content to between 0.040% and 0.065%.

[0016] Si: During the steelmaking process, Si is often used as a deoxidizer, helping to reduce non-metallic inclusions in steel and improve its purity and quality. Furthermore, it dissolves into ferrite and austenite, exerting a strong solid solution strengthening effect, significantly increasing the strength and hardness of steel. Furthermore, it inhibits the nucleation and growth of ferrite, shifting the steel's C curve to the right and improving its hardenability. However, excessive Si content can significantly reduce the steel's plasticity, toughness, and weldability. Therefore, the present invention limits the Si content to 0.20-0.35%.

[0017] Mn: It contributes to solid solution strengthening and phase transformation strengthening, compensating for the strength loss caused by a reduction in the carbon content. It also lowers the γ-α transition temperature, expands the austenite region, promotes bainite formation, and refines the ferrite structure. Furthermore, Mn increases the hardenability and toughness of steel, lowering the brittle transition temperature of ductile steel. However, excessive Mn content exacerbates central segregation in the continuously cast slab, leading to anisotropy in the mechanical properties of the steel plate and pipe. Therefore, the present invention limits the Mn content to 1.48-1.75%.

[0018] P, S, and N are inevitable impurity elements in steel. The lower their content, the better. However, too low a purity level will increase production costs. Therefore, while ensuring the toughness of this product, the present invention sets P ≤ 0.02%, S ≤ 0.003%, and N ≤ 0.008%.

[0019] Ti: A strong nitrogen-strengthening element. Adding approximately 0.015% Ti forms high-temperature stable, fine TiN precipitates during slab continuous casting. These dispersed, fine TiN particles pin the austenite grain boundaries, hindering austenite grain growth and effectively refining the structure. The addition of Ti also effectively prevents the formation of NbN and BN, allowing Nb to exert its strengthening effect and B to enhance hardenability. Furthermore, Ti has a strong affinity with sulfur, forming a stable TiS compound. TiS is more stable than MnS. Therefore, Ti significantly reduces the hot brittleness of sulfur and improves the toughness of steel. However, excessive Ti content not only increases costs but also easily forms large inclusions. Therefore, the present invention controls the Ti content to 0.012-0.025%.

[0020] Nb: It has significant grain refinement and precipitation strengthening effects and is a strong carbonitride-forming element. During the rolling process, fine Nb (N, C) particles are formed by strain-induced precipitation, which pins the grain boundaries to suppress the recovery and recrystallization of the deformed austenite. After controlled rolling and controlled cooling, the deformed austenite in the non-recrystallization zone of the finishing stage is transformed into a fine product with a high dislocation density, thereby improving the strength and toughness of the steel. However, if the Nb content is too low, the dispersion precipitation effect is not obvious, and it does not play a role in refining the grains and strengthening the matrix. At the same time, Nb is a precious metal, and too high a content will increase the alloy cost, and after adding a certain amount, the strengthening effect is no longer obvious. Therefore, the present invention controls the Nb content to 0.035~0.055%.

[0021] RE: It plays the role of deoxidation, desulfurization, purification of steel and modification of inclusions in steel. Rare earth elements have deoxidizing and desulfurizing effects, which can reduce and refine inclusions in steel and purify the molten steel. Rare earth elements have a metamorphic effect on inclusions, forming dispersed, fine inclusions. These inclusions serve as heterogeneous nucleation centers, thereby achieving the effect of grain refinement. Rare earth inclusions can also induce the nucleation of intracrystalline acicular ferrite and hinder the growth of austenite grains, refining the grain structure of the steel and significantly improving the strength and toughness of the steel. In addition, the microalloying effect of rare earth elements La and Ce can inhibit 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 the steel, especially high-temperature plasticity. When they are added in a combined ratio (La / Ce=2.8-3.2), they can produce more significant effects than single additions. After rare earth treatment, the size of the intracrystalline acicular ferrite nuclei is concentrated in the range of 0.5-3 μm, with the optimal range being 0.005-0.008%.

[0022] Cr: Cr forms a continuous solid solution with Fe, reducing the austenite phase, 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 crystals in the medium and low temperature ranges. Cr is one of the elements that effectively improves the strength and hardness of steel. Compared to Mo, Cr is less expensive. When the Cr content is too high, it can significantly increase the tensile strength and hardness of the steel, but the improvement in yield strength is limited, and the low-temperature toughness will be significantly reduced. Therefore, excessive addition is not advisable. Therefore, the Cr content of the present invention is controlled to 0.15-0.20%.

[0023] Boron (B) is a high-hardenability element that strongly inhibits the γ-α transformation, shifting the CCT curve to the right and increasing hardenability. Trace amounts of B significantly inhibit ferrite nucleation at austenite grain boundaries. It exhibits a strong synergistic effect with Mo and Nb, making it a viable alternative to 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 bainite to be achieved even within a wide cooling rate range. However, excessive B content can adversely affect weldability, requiring more stringent welding procedures and increasing process complexity. Therefore, the present invention controls the B content to between 0.0005% and 0.0020%.

[0024] Mg: It can reduce the O and S content and the number of inclusions in steel, purifying the molten steel. Mg also has a significant modifier effect on inclusions in steel, generating fine, dispersed inclusions that induce the nucleation of acicular ferrite (AF), thereby refining the steel's microstructure. Mg can increase the yield strength and tensile strength of steel by more than 5%, while keeping the plasticity essentially unchanged, thereby reducing the yield strength ratio. Furthermore, by utilizing the microalloying effect of Mg and setting a reasonable Mg addition level, the use of expensive microalloying metals such as Nb, V, and Ti can be reduced, thereby reducing alloying production costs. Therefore, the Mg content in the present invention is in the range of 0.002 to 0.004%.

[0025] Al (AlS) is an essential and excellent deoxidizer in steelmaking. Adding a small amount of Al to steel refines grain size, improving its strength and impact toughness. It also forms fine, dispersed AlN particles, further refining the grain size and enhancing the steel's toughness. However, excessive AlS content can increase steel brittleness and reduce its strength and toughness. Therefore, the AlS content in this invention is controlled within a range of 0.025% to 0.050%.

[0026] Ca: Desulfurizes and deoxidizes. Calcium treatment also changes the morphology of sulfides, improving the anisotropy of the steel. It also denatures inclusions, spheroidizing them and ensuring the toughness of the steel. However, excessive Ca content can form large inclusions. The optimal range is below 0.003%.

[0027] The second technical solution of the present invention is to propose a method for manufacturing a high-grade, low-cost, 550 MPa grade hot-rolled coil for welded pipes. The production process involves smelting, slab continuous casting, slab heating, rolling, ultra-fast cooling, and coiling, and is characterized by: Slab Heating: Continuously cast slabs are directly heated in a hot-charging furnace at 500-850°C, then heated in a walking beam furnace to 1150-1180°C before being discharged. Within this temperature range, Nb is fully dissolved back, and austenite grain growth is not significant due to the pinning of Ti (C, N) particles at the grain boundaries, which is conducive to obtaining a fine and uniform structure.

[0028] Rolling: Rough rolling is performed at a final rolling temperature of 1000-1060°C over five passes, with a reduction of ≥20% for the first pass and ≥22% for the remaining four passes, with the reduction gradually increasing. Finish rolling begins at a temperature of 920-940°C and ends at a temperature of 730-780°C, with a cumulative reduction of 65%-70%. This final rolling temperature range is conducive to Nb precipitation, refines grain size, and improves strength and toughness. High reductions generate a large number of dislocations and twins, which contribute to dislocation strengthening and significantly improve the yield strength and tensile strength of the steel.

[0029] Preferably, the thickness of the finishing intermediate billet is 50-62 mm, and the thickness of the finished product is 16-20 mm.

[0030] Cooling: After rolling, ultra-fast cooling is adopted, with the final cooling temperature of 400~460℃ and the cooling rate of 30~60℃ / s, and finally the steel is transformed into a mixed structure of acicular ferrite and martensite / MA. This structure makes the final coil have high strength, low yield ratio and good low-temperature toughness.

[0031] Coiling: After ultra-fast cooling, a strong coiler is used for coiling. The coil tension coefficient is 2.0~2.5, the auxiliary roll pressure is 550~620 MPa, the guide pressure is 60~90 kN, the coil speed is 0.5 times, the number of skipping circles is 4~6, the MD lead rate is 15~16, the WR lead rate is 18~19, and the PR lead rate is 10~11. The coil shape is good without hard bends.

[0032] Furthermore, after the molten iron is pretreated, top blowing or top and bottom combined blowing process is adopted for converter smelting, followed by off-furnace refining. Off-furnace refining adopts RH vacuum treatment and LF furnace light desulfurization treatment, and then calcium wire feeding treatment is carried out. 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.

[0033] Furthermore, during continuous casting of slabs, the pouring is protected throughout, and electromagnetic stirring or dynamic soft reduction is used to reduce the center segregation of the continuous casting slab. The slab adopts a medium thickness of 190~210 mm, which can not only ensure the compression ratio, but also make the slab quickly and evenly heated through, reducing energy consumption and saving costs.

[0034] The 550 MPa hot-rolled coil for welded pipe produced using the above chemical composition and process has excellent comprehensive properties, including yield strength of 580~660 MPa, tensile strength of 650~750 MPa, elongation after fracture ≥28%, yield strength ratio ≤0.90, average impact energy value Akv ≥200 J at -40°C, average drop weight test (DWTT) value at -20°C ≥90%, hardness value (HV10) ≤220, and qualified cold bending performance, which can meet the use requirements of welded pipe steel in harsh and complex environments.

[0035] Compared with existing technologies, the composition design concept of this invention adopts a low-C-Nb-Ti-B alloy design, adding small amounts of B and Mg elements while also adding rare earth elements Ce + La. On the one hand, by adding small amounts of B and Mg elements, the addition of precious metals is reduced or eliminated, reducing production costs. On the other hand, the addition of Mg, Ce + La elements can achieve steel purification, refine the grain structure, and modify inclusions, thereby improving the overall performance of pipeline steel. In addition, the production process uses low-temperature finish 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, making it more suitable for various complex and extreme service environments and providing higher safety in pipeline projects.

[0036] Compared with the prior art, the present invention has the following beneficial effects: (1) Break the existing design concept of 550 MPa grade welded pipe steel alloy, adopt low carbon and add a small amount of Nb, Ti, B elements, and at the same time add rare earth elements and cheap Mg alloys. Mg plays its micro-alloying role, reduces the use of precious metals such as Nb and Ti, and reduces production costs; (2) The composite addition of Mg and rare earth elements Ce+La purifies the steel, refines inclusions, improves the morphology and distribution of inclusions, refines grains, enhances overall mechanical properties, and provides higher safety in pipeline engineering. (3) Using medium-thick continuous casting billets of 190-210 mm, while ensuring the compression ratio, the continuous casting billets can be quickly and evenly burned through, saving energy and reducing consumption, with high efficiency and high production flexibility; (4) By adopting a lower final rolling temperature and ultra-fast cooling process after rolling, combined with the effects of Nb, Ti, B and RE elements, a relatively fine and uniform acicular ferrite structure is obtained, ensuring that the product has good strength and toughness properties. BRIEF DESCRIPTION OF THE DRAWINGS

[0037] Figure 1 This is the metallographic optical microstructure of Example 1 - AF (acicular ferrite). AF, through its unique interlaced fine needle structure and high dislocation density, achieves an excellent balance of high strength and high toughness, making it the core structure of existing welded pipe steels. Furthermore, the addition of rare earth elements (Ce + La) further optimizes the nucleation and distribution of AF. DETAILED DESCRIPTION

[0038] In order to make the purpose, technical solutions and advantages of the embodiments of the present invention clearer, the technical solutions in the embodiments of the present invention will be clearly and completely described below in conjunction with the drawings in the embodiments of the present invention. Obviously, the described embodiments are only part of the embodiments of the present invention, rather than all the embodiments. The following description of the exemplary embodiments of the experimental method is actually only illustrative and is in no way intended to limit the present invention and its application or use. Based on the embodiments of the present invention, all other embodiments obtained by those skilled in the art without making creative work are within the scope of protection of the present invention.

[0039] The following examples are intended only to provide some preferred embodiments of the present invention and are not intended to limit the scope of the invention or its technical approach. Table 1 lists the chemical compositions of Examples 1-8, Table 2 lists the heating, rolling, and cooling process parameters for each example, Table 3 lists the AF nucleation size for each example, and Table 4 lists the mechanical property test results for each example.

[0040] Table 1 Chemical composition of example steel (wt, %)

[0041] Table 2 Process parameters for heating, rolling, cooling and coiling of steel in various examples

[0042] Table 3 AF nucleation size of each example steel

[0043] Table 4 Main mechanical properties of steel in various examples

[0044] As can be seen from Tables 1 to 4, the high-grade, low-cost, easy-to-weld 550 MPa grade hot-rolled coil for welded pipe produced by the composition design and rolling and coiling processes of the present invention has good comprehensive mechanical properties, especially excellent low-temperature toughness.

Claims

1. A high-grade, low-cost 550 MPa hot-rolled coil for welded pipe, characterized in that: The chemical composition of the steel plate 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%, and the rest are Fe and unavoidable impurities.

2. The low-cost 550 MPa grade hot-rolled coil for welded pipe according to claim 1, characterized in that: The optimal composite mass ratio of La and Ce is La / Ce=2.8~3.

2.

3. The high-grade, low-cost, 550 MPa hot-rolled coil for welded pipe according to claim 1, characterized in that: After rare earth treatment, more than 80% of the intracrystalline acicular ferrite nucleation size is concentrated in the range of 0.5~3 μm.

4. The high-grade, low-cost, 550 MPa hot-rolled coil for welded pipe according to claim 1, characterized in that: The yield strength of the steel is 580~660 MPa, the tensile strength is 650~750 MPa, the elongation after fracture is ≥28%, the yield strength ratio is ≤0.90, the average impact energy Akv at -40℃ is ≥200 J, the average DWTT of the drop hammer at -20℃ is ≥90%, the hardness value HV10 is ≤220, and the cold bending performance is qualified.

5. A method for manufacturing a high-grade, low-cost, 550 MPa grade hot-rolled coil for welded pipe according to any one of claims 1 to 4, wherein the production process involves smelting, slab continuous casting, slab heating, rolling, ultra-fast cooling, and coiling, characterized in that: Slab heating: The continuous casting slab is heated to 1150~1180℃ in a walking beam heating furnace before being discharged from the furnace; Rolling: rough rolling and finishing temperature 1000~1060℃, finishing rolling start temperature 920~940℃, finishing temperature 730~780℃, cumulative reduction rate 65%~70%; Cooling: After rolling, ultra-fast cooling is adopted, with the final cooling temperature of 400~460℃ and the cooling rate of 30~60℃ / s; Coiling: After cooling, it is coiled on a strong coiler with a roll tension coefficient of 2.0~2.5, an auxiliary roll pressure of 550~620MPa, and a guide ruler pressure of 60~90 kN.

6. The method for manufacturing high-grade, low-cost 550 MPa grade hot-rolled coil for welded pipe according to claim 5, characterized in that: The thickness of the finishing intermediate bar is 50~62 mm, and the thickness of the finished product is 16~20 mm.

7. The method for manufacturing high-grade, low-cost 550 MPa grade hot-rolled coil for welded pipe according to claim 5, characterized in that: During continuous casting of slabs, the pouring is protected throughout the entire process, and electromagnetic stirring or dynamic soft reduction is used. The thickness of the slab is 190~210 mm.

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