A low-cost, high-quality 500 MPa grade hot-rolled coil for welded pipe and its manufacturing method

Abstract

This invention discloses a low-cost, high-quality 500 MPa grade hot-rolled coil for welded pipes and its manufacturing method. The chemical composition is: C 0.05~0.07%, Si 0.15~0.35%, Mn 1.48~1.78%, P≤0.02%, S≤0.003%, Ti 0.01~0.02%, Nb 0.03~0.06%, V 0.02~0.04%, RE 0.004~0.006%, Cr 0.18~0.30%, Mg 0.002~0.004%, Als 0.015~0.045%, Ca≤0.003%, N≤0.007%, with the remainder being Fe and unavoidable impurities. The manufacturing method includes smelting, slab continuous casting, slab heating, rolling, cooling, and coiling. The R of the steel... t0.5 For 520~610 MPa, R m 620~720 MPa, A 50mm ≥32%, R t0.5 / R m The steel exhibits a strength ≤0.88, an average impact energy (Akv) ≥200 J at -60 °C, an average drop weight dip-weight test (DWTT) ≥90% at -40 °C, a hardness value (HV10) ≤220, and satisfactory cold bending performance. This invention significantly improves the overall performance of steel, providing high strength and toughness, good weldability, and greater suitability for various extreme environments, resulting in enhanced safety in pipeline engineering.

Description

Technical Field

[0001] This invention relates to the field of low-alloy high-strength steel production technology, specifically to a low-cost, high-quality 500 MPa grade hot-rolled coil for welded pipes produced using medium-thickness cast billets, exhibiting good strength and toughness and easy weldability, and its manufacturing method. Background Technology

[0002] With the continuous advancement of continuous rolling technology, the production capacity of high-quality strip steel has significantly improved. Furthermore, continuous innovation in welding and inspection technologies has led to welded steel pipes becoming one of the most widely used steel materials due to their high production efficiency, low cost, reliable quality, wide application, and compliance with environmental requirements. These pipes are used in almost all industries and fields requiring steel pipes, such as pipeline transportation, construction, machinery manufacturing, and bridge construction. In the future, with continuous technological progress and market expansion, the application areas of welded pipes will further expand. Currently, welded pipes with higher usage and technical requirements are mainly used in the oil and natural gas energy sectors. These sectors require welded pipes with higher strength, good toughness, and better corrosion resistance to meet the demands of energy transportation in harsh service environments.

[0003] Hot-rolled coils are the main raw material for welded pipe production, and their quality directly determines the final performance and service life of the welded pipes. To ensure the safety and economy of welded pipes, the quality of the hot-rolled coils used in their manufacture must be strictly controlled. Simultaneously, in the face of fierce market competition, companies need to continuously reduce production costs to maintain competitiveness. Furthermore, with increasing environmental awareness and stricter environmental policies, energy conservation and emission reduction in the hot-rolled coil production process have become important tasks for the industry. Therefore, developing low-cost, high-quality hot-rolled coils for welded pipes with excellent properties such as high strength, high toughness, and ease of welding through alloy composition design and optimized production processes has become a significant trend in the industry.

[0004] Currently, there are many patents both domestically and internationally concerning 500 MPa grade welded pipe steel, especially pipeline steel, but they are all significantly different from this invention. The following is a brief introduction to patents that are relatively close to this invention:

[0005] (1) Chinese Patent CN202011250936.1 "A low-cost hot-rolled coil with a yield strength of 500 MPa and its preparation method". The composition contains C: 0.14~0.20%, Si: 0~0.25%, Mn: 0.35~0.60%, Ti: 0.040~0.055%, Al: 0.025~0.040%, B: 0.0008~0.0020%, P: <0.015%, S: <0.005%, O: <0.0025%, N: ≤0.005%, with the remainder being Fe and unavoidable impurities. The C and Mn content of this patent is high, and only Ti alloying element is added alone, which cannot exert the micro-alloying composite effect of Nb, V, and Ti, reducing the strengthening effect and easily causing product performance fluctuations; secondly, the heating temperature and coiling temperature are high, much higher than the heating and coiling temperatures of this invention; in addition, this patent does not specify the low-temperature toughness index.

[0006] (2) Chinese Patent CN202210120169.5 “A low-cost, high-toughness, 500 MPa yield strength steel plate and its production process”. The composition contains C: 0.15~0.17%, Si: 0.10~0.30%, Mn: 1.45~1.70%, S<0.005%, P<0.018%, Ti: 0.008~0.018%, V: 0.020~0.035%, Nb: 0.020~0.035%, N<0.005%, Als: 0.015~0.050%, with the remainder being Fe and unavoidable impurity elements. The patented component has a high C content and does not contain Cr, Mg, or rare earth elements, which is different from the alloy system of this invention. Secondly, the process uses a high final rolling temperature and a low cooling rate, which cannot fully utilize the water-substitute alloy to reduce costs and improve the strength and toughness of the steel plate. In addition, the low-temperature toughness index is relatively lenient, and this patent only has impact toughness and no low-temperature drop hammer test performance.

[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. The composition of this patent has a low Mn content and only adds Ti alloying element alone, which cannot exert the micro-alloying composite effect of Nb, V, and Ti, reducing the strengthening effect and easily causing product performance fluctuations; at the same time, the requirements for low-temperature toughness index are also relatively lenient, with an impact test temperature of -20 ℃ and a DWTT test temperature of -10 ℃; secondly, the coiling temperature of this patent is high, which is quite different from that of this invention.

[0008] (4) Chinese Patent CN201210022429.1 "A low-cost, high-strength and tough X70 pipeline steel coil and its production method". The composition contains C: 0.03~0.07%, Si: 0.10~0.25%, Mn: 1.10~1.80%, S≤0.005%, P≤0.018%, Nb: 0.05~0.10%, Ti: 0.008~0.020%, Mo: 0.05~0.12%, Cr: 0.15~0.35%, Als: 0.020~0.040%, Ca: 0.0015~0.0030%, N≤0.0060%. The patented composition does not contain V, so it cannot exert the microalloying composite effect of Nb, V and Ti, thus reducing the strengthening effect. At the same time, the composition contains Mo, which increases the alloy cost. In addition, the low temperature toughness requirements of the patent are also relatively lenient, with an impact test temperature of -20 ℃ and a DWTT test temperature of -15 ℃, which is significantly different from the present invention.

[0009] (5) Chinese Patent CN201210408198.8 "High Crack Arresting Toughness Steel Plate with Yield Strength of 500 MPa and its Production Method". The composition contains C: 0.04~0.08%, Si: 0.10~0.50%, Mn: 0.80~1.24%, P≤0.02%, S≤0.01%, Al: 0.053~0.065%, N≤0.005%, Nb: 0.005~0.050%, Ti: 0.005~0.030%, B: 0.0005~0.0030%, Ca: 0.0016~0.0040%, Cu: 0.26~0.35%, Ni: 0.35~0.45%, with the remainder being Fe and unavoidable impurities. The patented component contains the precious metal element Ni, resulting in high alloy costs; it lacks the element V, thus failing to leverage the microalloying composite effect of Nb, V, and Ti, reducing the strengthening effect; furthermore, the composition range is relatively wide, which can easily cause fluctuations in product performance; in addition, the patented component uses a billet thickness of 200~300 mm, which is significantly different from the present invention.

[0010] Currently, there are many publicly available patents for 500 MPa grade welded pipe steel, especially for pipeline steel. However, on the one hand, most of them have added precious 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, making it difficult to meet the requirements for low-temperature fracture toughness below -40 ℃. Summary of the Invention

[0011] The present invention relates to a low-cost, high-quality hot-rolled pipeline steel coil and its manufacturing method, and particularly to a low-cost, high-quality 500 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.

[0012] One of the technical solutions of this invention is to propose a low-cost, high-quality 500 MPa grade hot-rolled coil for welded pipes, which has good strength and toughness and is easy to weld, produced using medium-thickness cast billets. The chemical composition (weight, %) is as follows: C: 0.05~0.07%, Si: 0.15~0.35%, Mn: 1.48~1.78%, P: ≤0.02%, S: ≤0.003%, Ti: 0.01~0.02%, Nb: 0.03~0.06%, V: 0.02~0.04%, RE: 0.004~0.006%, Cr: 0.18~0.30%, Mg: 0.002~0.004%, Als: 0.015~0.045%, Ca: ≤0.003%, N: ≤0.007%, with the remainder being Fe and unavoidable impurities.

[0013] Preferably, the ratio of La to Ce is La / Ce = 2.65~3.10.

[0014] Preferably, more than 80% of the nucleation size of the intracrystalline needle-like ferrite after rare earth treatment is concentrated in the range of 0.5~3.5 μm.

[0015] The reason for selecting the above alloying elements and their contents in this invention is as follows:

[0016] Carbon (C): Carbon enhances the strength and hardness of steel through solid solution strengthening and precipitation strengthening, making it the most economical, fundamental, and effective strengthening element in steel. C atoms dissolve in the iron matrix to form a solid solution, significantly increasing the steel's strength. During subsequent cooling, C combines with alloying elements such as Nb, V, and Ti, precipitating fine carbide particles. These carbide particles are dispersed throughout the steel matrix, hindering dislocation movement and thus improving the steel's strength and hardness. However, excessively high C content reduces the steel's plasticity, toughness, and weldability; therefore, the C content needs to be precisely controlled within a suitable range. Thus, this invention controls the C content to be between 0.05% and 0.07%.

[0017] Si: A commonly used deoxidizing element, it can dissolve in ferrite and austenite, playing a certain role in solid solution strengthening. It can significantly improve the hardness and strength of steel, while also preventing the nucleation and growth of ferrite, shifting the C-curve of steel to the right, thereby improving the hardenability of steel. However, excessive Si content will significantly reduce the plasticity, toughness, and weldability of steel. Therefore, this invention controls the Si content at 0.15~0.35%.

[0018] Mn (Mn) possesses both solid solution strengthening and transformation strengthening properties. It can lower the γ-α transformation temperature, promote bainite formation by inhibiting the austenite transformation temperature, and thus refine the ferrite microstructure. Simultaneously, Mn can compensate for the strength loss caused by the reduction in carbon content, making it the most important and economical strengthening element. Mn also increases the hardenability and toughness of steel, and lowers the ductile-brittle transition temperature. However, excessively high Mn content can exacerbate 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 at 1.48~1.78%.

[0019] P, S, and N are unavoidable impurity elements in steel, and the lower the content, the better. However, excessive reduction in steel purity will significantly increase production costs. Therefore, while ensuring the toughness of this product, the content of P in this invention is ≤0.02%, S≤0.003%, and N≤0.007%.

[0020] Ti: Ti is a strong carbonitride forming element. Adding approximately 0.015% Ti can form high-temperature stable, fine TiN precipitates during slab continuous casting. These dispersed, fine TiN particles are anchored at the austenite grain boundaries, effectively hindering austenite grain growth during the heating process of the continuously cast slab. This leads to the γ→α transformation, forming fine initial austenite grains, which in turn produce fine ferrite grains. Simultaneously, it significantly improves the toughness of the heat-affected zone during steel welding. On the other hand, the addition of Ti also inhibits the formation of NbN, helping to increase the solid solubility of Nb in austenite and exert a strengthening effect. Furthermore, Ti has a significant desulfurization effect in pipeline steel. Ti has a greater affinity for S than Mn, and TiS is more stable than MnS. Therefore, Ti can reduce the brittleness of steel and improve its toughness. However, excessively high content not only increases costs but also easily forms large inclusions. Therefore, this invention controls the Ti content to 0.01~0.02%.

[0021] Nitrogen (Nb) is a strong carbonitride forming element, exhibiting significant grain refinement and precipitation strengthening effects. 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, failing to refine the grains and strengthen the matrix. Furthermore, Nb is a precious metal, and excessively high content increases alloy costs, and the strengthening effect becomes less pronounced after a certain amount is added. Therefore, this invention controls the Nb content to 0.03~0.06%.

[0022] V (Volume) is a strong carbide-forming element. When dissolved in austenite, it improves the hardenability of steel and exhibits strong precipitation strengthening and general grain refinement strengthening effects. The fine carbides formed by V and C are dispersed throughout the steel matrix, hindering dislocation movement and thus improving the steel's strength and toughness. Simultaneously, it can compensate for the deficiencies in Nb precipitation strengthening and improve the post-weld toughness of steel. However, in the alloy design of pipeline steel, it is generally not used alone; its effect is better when combined with Nb and Ti. Excessive V content can lead to an increase in the ductile-brittle transition temperature of steel; therefore, this invention controls the V content to 0.02~0.04%.

[0023] Rare earth elements (REs) added to steel can deoxidize, desulfurize, purify the steel, and modify inclusions. The addition of REs can significantly reduce the O and S content in steel, thereby improving its purity. Rare earth elements have a modifying effect on inclusions, altering their properties, morphology, and distribution, generating fine, dispersed inclusions. These inclusions become heterogeneous nucleation centers, thus refining the grain size. Rare earth inclusions play a crucial role in the solidification and phase transformation of steel, inducing the nucleation of acicular ferrite within the grains and hindering the growth of austenite grains, refining the grain structure of the steel, and significantly improving its strength and toughness. Rare earth elements La and Ce have a synergistic effect in pipeline steel. When they are added in an appropriate ratio (La / Ce=2.65~3.10), they can produce a more significant effect than when added alone. After rare earth treatment, more than 80% of the nucleation size of intracrystalline acicular ferrite is concentrated in the range of 0.5~3.5 μm, with the optimal range being 0.004~0.006%.

[0024] Cr: It can effectively improve the hardenability of steel, inhibit the formation of polygonal ferrite and pearlite, and promote the formation of ferrite or bainite with a large number of dislocations in the grains at medium and low temperatures. Cr can significantly improve the strength and hardness of products, and is cheaper than Mo. When the Cr content exceeds 0.3%, 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. Therefore, the Cr content in this invention is controlled at 0.18~0.30%.

[0025] Mg can reduce the O and S content and the number of inclusions in steel, purifying the molten steel and greatly reducing the harmful effects of inclusions on steel properties. Mg has a significant modifying effect on inclusions in steel, generating fine and dispersed inclusions, which can provide nucleation sites for sulfide and carbonitride precipitation, contributing to the optimized distribution of inclusions in steel. At the same time, it can induce the nucleation of acicular ferrite (AF), thereby refining the microstructure of steel, increasing yield strength and tensile strength by more than 5%, while maintaining plasticity essentially unchanged, reducing the yield strength ratio, and improving the low-temperature toughness of steel. In addition, by utilizing the microalloying effect of Mg and setting a reasonable Mg addition amount, 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%.

[0026] Al (Al₂S) is a strong nitride-forming element. Adding an appropriate amount of Al can form fine and dispersed AlN particles, which is beneficial for refining grains and improving the strength and toughness of steel. At the same time, Al is also an indispensable and excellent deoxidizer in steelmaking. Adding a small amount of Al to steel can refine grains and improve steel strength and impact toughness. However, excessive Al content can lead to increased brittleness in steel and promote graphitization, reducing the strength and toughness of the steel. Therefore, the Al₂S content in this invention is controlled at 0.015~0.045%.

[0027] Ca (Ca) has a strong affinity for sulfur (S) in steel, thus playing a role in desulfurization. Calcium treatment can also alter the morphology of sulfides, improve the anisotropy of steel, and modify inclusions, achieving spheroidization and ensuring the toughness of the steel. However, excessively high Ca content can lead to large inclusion particles; the optimal range is below 0.003%.

[0028] The second technical solution of this invention is to propose a low-cost, high-quality hot-rolled coil for welded pipes of 500 MPa grade, comprising smelting, slab continuous casting, slab heating, rolling, cooling, and coiling, characterized in that:

[0029] Billet heating: The continuously cast slab is directly heated in a hot charging furnace at a temperature of 500~850 ℃, and then heated to 1120~1180 ℃ in a walking beam furnace before being unloaded. Within this heating temperature range, Nb element 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.

[0030] Rolling: Roughing rolling temperature is 1010~1050 ℃, 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. Finishing rolling temperature is 940~960 ℃, and the finishing rolling temperature is 740~780 ℃, with a cumulative reduction rate of 62%~66%. The temperature range of the final rolling is conducive to Nb precipitation, refining the grain size and improving strength and toughness. Using a large reduction rate can generate a large number of dislocations and twins, thereby playing a dislocation strengthening role and significantly improving the yield strength and tensile strength of the steel.

[0031] Preferably, the thickness of the intermediate slab is 40~60 mm, and the thickness of the finished product is 13~18 mm.

[0032] Cooling: After rolling, ultra-fast cooling is adopted, with a final cooling temperature of 430~480 ℃ and a cooling rate of 30~55 ℃ / s. It eventually transforms into a mixed structure of acicular ferrite and MA, which gives the final product high strength, low yield strength ratio and good low temperature toughness.

[0033] Winding: After ultra-fast cooling, a high-power winder is used for winding. The drum tension coefficient is 1.8~2.5, the auxiliary winding roller pressure is 420~550 MPa, the guide roller pressure is 50~80 kN, the drum expansion speed is 0.5 times, the number of skip turns is 4~6, the MD lead rate is 12~14, the WR lead rate is 15~16, the PR lead rate is 10~11, and the coil shape is good with no hard bends.

[0034] 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 employs 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 ≥5 min, and the calming time is ≥10 min.

[0035] 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.

[0036] Hot-rolled coils for welded pipes produced using the above-mentioned chemical composition and process possess excellent comprehensive properties, with a yield strength of 520~610 MPa, tensile strength of 620~720 MPa, elongation after fracture ≥32%, yield strength ratio ≤0.88, average impact energy Akv ≥200 J at -60 ℃, average drop weight test (DWTT) value ≥90% at -40 ℃, hardness value (HV10) ≤220, and qualified cold bending performance. They can be widely used in welded pipes for special purposes such as oil and gas transportation, marine engineering, nuclear power, and aerospace.

[0037] Compared with existing technologies, the composition design of this invention adopts a low C-Nb-V-Ti alloy design, while simultaneously incorporating rare earth elements Ce+La and inexpensive Mg alloys. This reduces or eliminates the addition of precious metals, lowering production costs. Furthermore, the addition of Mg and Ce+La elements purifies the steel, refines the grain structure, and improves inclusions, significantly enhancing the overall performance of pipeline steel. In addition, the process employs low-temperature precision rolling and ultra-rapid cooling after rolling to obtain a fine and uniform acicular ferrite structure, ensuring high strength and toughness as well as good weldability. This makes the product more suitable for various complex and extreme service environments, resulting in higher safety in pipeline engineering.

[0038] This patent breaks away from the existing design concept of 500MPa grade welded pipe steel alloys. It combines rare earth elements and inexpensive alloy Mg to reduce production costs while purifying the steel through Mg, Ce, and La elements, refining the grain structure, and improving inclusions, which helps to enhance the overall performance of pipeline steel and ensure that the product has good strength and toughness. In addition, it adopts a medium billet thickness of 190~210mm, a low final rolling temperature, and an ultra-fast cooling method to ensure that the product has a sufficient compression ratio while giving full play to the role of water-displaced alloys, thereby achieving the goal of reducing costs and improving product strength and toughness.

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

[0040] (1) Break away from the existing design concept of 500 MPa grade welded pipe steel alloy, add rare earth elements and cheap Mg alloy, and reduce the amount of precious metals such as Nb and Ti through micro-alloying to reduce 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 rare earth elements and Nb and Ti 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 1Example 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 embodiments 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, cooling, and coiling in each embodiment, Table 3 shows the AF nucleation dimensions in each embodiment, and Table 4 shows the mechanical property test results for each embodiment.

[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 low-cost, high-quality, easy-to-weld 500MPa grade hot-rolled coils for welded pipes produced by adopting the composition design and process of the present invention have good comprehensive mechanical properties, especially excellent low-temperature toughness.

Claims

1. A low-cost, high-quality 500 MPa grade hot-rolled coil for welded pipes, characterized in that, The chemical composition of the steel plate is as follows: C: 0.05%~0.07%, Si: 0.15%~0.35%, Mn: 1.48%~1.78%, P: ≤0.02%, S: ≤0.003%, Ti: 0.01%~0.02%, Nb: 0.03%~0.06%, V: 0.02%~0.04%, RE: 0.004~0.006%, Cr: 0.18%~0.30%, Mg: 0.002%~0.004%, Als: 0.015%~0.045%, Ca: ≤0.003%, N: ≤0.007%. The ratio of La and Ce in RE is La / Ce = 2.65~3.10, and the remainder is Fe and unavoidable impurities.

2. The low-cost, high-quality 500 MPa grade hot-rolled coil for welded pipes according to claim 1, characterized in that, After rare earth treatment, more than 80% of the intracrystalline needle-like ferrite nucleation size is concentrated in the range of 0.5~3.5 μm.

3. The low-cost, high-quality 500 MPa grade hot-rolled coil for welded pipes according to claim 1, characterized in that, The steel has a yield strength of 520~610 MPa, a tensile strength of 620~720 MPa, an elongation after fracture of ≥32%, a yield strength ratio of ≤0.88, an average impact energy Akv of -60℃ ≥200 J, an average drop weight DWTT of -40℃ ≥90%, a hardness value HV10 ≤220, and all cold bending performances are qualified.

4. A method for manufacturing a low-cost, high-quality 500 MPa grade hot-rolled coil for welded pipe as described in any one of claims 1 to 3, comprising smelting, continuous casting of slabs, heating of slabs, rolling, cooling, and coiling, characterized in that, Billet heating: The continuously cast slab is heated to 1120~1180 ℃ in a walking beam furnace before being taken out of the furnace; Rolling: Roughing rolling temperature 1010~1050 ℃, finishing rolling temperature 940~960 ℃, finishing rolling temperature 740~780 ℃, cumulative reduction rate 62%~66%; Cooling: After rolling, ultra-fast cooling is adopted, with a final cooling temperature of 430~480 ℃ and a cooling rate of 30~55 ℃ / s; Winding: After cooling, a high-strength winder is used for winding. The drum tension coefficient is 1.8~2.5, the auxiliary winding roller pressure is 420~550MPa, and the guide roller pressure is 50~80 kN.

5. The method for manufacturing low-cost, high-quality 500 MPa grade hot-rolled coils for welded pipes according to claim 4, characterized in that, The thickness of the intermediate slab in the finishing mill is 40~60 mm, and the thickness of the finished product is 13~18 mm.

6. The method for manufacturing low-cost, high-quality 500 MPa grade hot-rolled coils for welded pipes according to claim 4, characterized in that, During slab continuous casting, the entire process is protected during pouring, and electromagnetic stirring or dynamic light pressure is used. The slab thickness is 190~210 mm.

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