Steel for resisting carbon dioxide corrosion with low yield ratio of 500mpa and its manufacturing method and application

Abstract

The application discloses 500MPa-grade low-yield-ratio anti-carbon-dioxide-corrosion steel and a manufacturing method and application thereof, and belongs to the technical field of metal materials and metallurgical engineering. The application adopts low-carbon and low-manganese design, has low carbon content, small hardening tendency of a heat-affected zone during welding, and is not prone to welding cracks; Cr and Al are coordinated, Cr forms dense Cr2O3 on a metal surface, Al forms Al2O3 film, when the two are combined, the high stability of Cr2O3 and the self-healing property of Al2O3 are combined, and the carbon dioxide corrosion resistance is significantly improved; a Zn-Al solid solution enhances the adhesion of a surface layer and improves the carbon dioxide corrosion resistance; Cr and Mo are jointly strengthened to generate Cr2O3, Mo2C and composite carbide, the carbon dioxide corrosion resistance (especially pitting corrosion and crevice corrosion) is improved, and the matrix is synergistically strengthened. The cooling speed and the final cooling temperature are combined, the cooling speed inhibits pearlite transformation; a two-stage cooling mode is adopted, bainite structure with low strength and hardness is obtained in the first stage, and refined bainite with high strength and hardness is obtained by accelerated cooling in the second stage.

Description

Technical Field

[0001] This invention belongs to the field of metallic materials and metallurgical engineering technology, specifically relating to 500MPa grade low yield strength ratio steel resistant to carbon dioxide corrosion and its manufacturing method and application. Background Technology

[0002] CCUS (Carbon Capture, Utilization, and Storage) technology is a key technology for addressing global climate change. Carbon dioxide transportation is a crucial link in the CCUS industry chain, connecting carbon dioxide capture and storage. The efficiency and cost of carbon dioxide transportation directly affect the overall scale and economic benefits of CCUS.

[0003] The critical pressure of pure carbon dioxide is 7.38 MPa, and its critical temperature is 31.1℃. Supercritical transport refers to transport at pressures higher than the critical pressure. It possesses high density and low viscosity. When the entire pipeline transport process is in a supercritical state, transportation is most efficient, and wear is minimal. However, the presence of free water and impurity gases in supercritical carbon dioxide pipelines results in extremely strong corrosiveness. Existing API carbon steel pipelines used for natural gas transport are susceptible to occasional corrosion, potentially leading to leaks and failures. Pipe materials are fundamental to ensuring safe pipeline transport. Pipe material costs are a significant portion of the total pipeline construction investment. Factors such as corrosion and third-party damage can cause pipe walls to thin or fail, potentially leading to various safety accidents. Key technical requirements for supercritical pipe materials include: low-temperature brittleness when carbon dioxide leaks and the temperature drops rapidly to -50℃; and the corrosion rate in a supercritical carbon dioxide environment.

[0004] Chinese Patent CN112941422A discloses a CO2 corrosion-resistant steel plate and its preparation method. The composition includes C: 0.03%-0.07%, Cr: 4.0%-6.0%, Ni: 0.15%-2.50%, Nb: 0.01%-0.06%, P≤0.005%, and S≤0.0050%. This invention, disclosed by Beijing University of Science and Technology, is a method for producing carbon dioxide corrosion-resistant steel plates. The product is manufactured using a medium-thick plate rolling mill, and the rolled steel plate requires a quenching and tempering heat treatment process. However, this invention has a high Cr content, classifying it as stainless steel, making it unsuitable for conventional smelting and continuous casting. Furthermore, the excessively high Cr content prevents straight-seam welding, and the steel plate's impact toughness is low, failing to meet the crack-stopping requirement of -50°C during carbon dioxide leakage.

[0005] Chinese Patent CN106498279A discloses a low-Cr economical X65 pipeline steel resistant to CO2 corrosion and its production method. The composition includes C: 0.04%-0.05%, Si: 0.18%-0.22%, Mn: 0.50%-0.60%, Cr: 0.1%-0.2%, Mo: 0.10%-0.15%, Nb: 0.035%-0.050%, V: 0.020%-0.030%, Ti: 0.010%-0.020%, P≤0.01%, and S≤0.0030%. This invention is a production method for X65 hot-rolled coils resistant to carbon dioxide corrosion disclosed by Wuhan Iron and Steel Co., Ltd. However, the low Mn content in this invention results in poor hardenability during subsequent straight seam welding, failing to guarantee the -50℃ low-temperature toughness of the weld and heat-affected zone. Furthermore, the base material exhibits only moderate low-temperature toughness at -20℃, which also fails to meet the -50℃ crack arrest requirement during carbon dioxide leakage.

[0006] Currently produced 500MPa grade pipeline steel coils do not have resistance to carbon dioxide corrosion, cannot adapt to the service environment of supercritical carbon dioxide transportation, have a high yield strength ratio, and poor safety. Summary of the Invention

[0007] Therefore, the purpose of this invention is to provide a 500MPa grade low yield strength ratio steel resistant to carbon dioxide corrosion, its manufacturing method and application, to solve the problems of current 500MPa grade pipeline steel coils lacking carbon dioxide corrosion resistance, being unable to adapt to the supercritical carbon dioxide transportation service environment, and having a high yield strength ratio.

[0008] To achieve the above objectives, the present invention provides the following technical solution:

[0009] In a first aspect, the present invention provides a 500MPa grade steel with low yield strength ratio and resistance to carbon dioxide corrosion, which is composed of the following components by weight percentage: C: 0.02%-0.05%, Si: 0.10%-0.30%, Mn: 0.60%-0.90%, P: ≤0.020%, S: ≤0.002%, Cr: 0.10%-0.30%, Mo: 0.70%-1.00%, Nb: 0.03%-0.05%, Al: 0.10%-0.30%, Zn: 0.16%-0.20%, Mg: 0.03%-0.08%, N: ≤0.005%, with the remainder being Fe and unavoidable impurities.

[0010] Based on the above technical solution, the Cr / Al value is further 1.20-2.25, the Mo / Cr value is 2.5-9.0, and the Mg / C value is 0.6-4.0.

[0011] Among them, C (Carbon) is a carbide-forming element and the most effective element for ensuring strength. It can improve hardenability and guarantee the strength and hardness of the material. Carbon significantly improves strength through solid solution strengthening and phase transformation strengthening. C preferentially forms carbides with Cr, improving the strength and resistance to carbon dioxide corrosion of the steel. C interacts with Mo to synergistically form carbides, improving hardenability and the uniformity of the microstructure. Excessive content can easily cause center segregation in the steel plate, which is detrimental to the corrosion resistance and crack arrest toughness of the steel plate and will affect the weldability of the product. The optimal range is 0.02%-0.05%.

[0012] Si (Si) can dissolve in ferrite and austenite, playing a role in solid solution strengthening. It significantly improves the hardness and tensile strength of steel, while also promoting ferrite grain coarsening and reducing the anisotropy of steel plates in the transverse and longitudinal directions. SiO2 and dense ferrosilicon inclusions can disperse on the steel surface like "ceramic particles," hindering the contact between CO2 molecules and the steel matrix, reducing carbon dioxide corrosion reactions, and improving resistance to carbon dioxide corrosion. However, increasing the silicon content reduces the low-temperature toughness, plasticity, and weldability of the steel; the optimal range is 0.10%-0.30%.

[0013] Mn (manganese): Manganese has a solid solution strengthening effect. The solid solution formed by manganese and iron increases the hardness and strength of ferrite and austenite in steel. It is also a carbide-forming element, entering cementite to replace some iron atoms. In steel, manganese lowers the critical transformation temperature, increases austenite stability, strongly increases hardenability, promotes bainite formation, and effectively ensures the strength and toughness of the steel. Manganese can compensate for the strength decrease caused by reduced carbon content, making it the most important and economical strengthening element. Experiments have shown that when the manganese content is below 1.0%, it significantly reduces the segregation level of continuously cast billets and improves the product's resistance to carbon dioxide corrosion. Excessive manganese content can increase the tendency for center segregation in continuously cast billets, leading to an increase in banded structures in the steel plate, increasing brittleness and reducing plasticity. The optimal range is 0.60%-0.90%.

[0014] P, S, and N are unavoidable impurity elements in steel, and it is desirable to keep them as low as possible. However, excessively low requirements will increase production costs. In this invention, P ≤ 0.020%, S ≤ 0.002%, and N ≤ 0.006%.

[0015] Cr: Chromium can improve strength through solid solution strengthening. Like Mn, Cr can dissolve into solid solutions, improving the hardenability of steel and thus increasing strength. After dissolving into austenite, Cr increases the stability of supercooled austenite, promotes bainite formation, significantly increases the tensile strength of steel, and reduces the yield strength ratio. Chromium has excellent corrosion resistance; tests show that the corrosion rate decreases with increasing Cr content. In CO2 corrosion systems, chromium forms a multilayer film structure of mixed phases of FeCO3 and Cr(OH)3. The complete and dense corrosion product film Cr(OH)3 not only hinders electrode activation reactions but also inhibits ion diffusion, effectively reducing the corrosion rate. Chromium can also combine with oxygen to form a dense chromium oxide layer (Cr2O3). This passivation layer prevents further erosion by oxygen, water, and other corrosive media, thus providing protection against carbon dioxide corrosion. Chromium can also form various stabilizing compounds such as Cr7C3 and Cr2S3. The presence of these compounds can form a hard film on the steel surface, thereby improving the steel's resistance to carbon dioxide corrosion. Adding Cr and Al in combination, maintaining a Cr / Al ratio of 1.25-2.25, effectively improves resistance to carbon dioxide corrosion. However, excessive chromium content significantly increases the brittle transition temperature of the steel, reduces elongation, and easily forms coarse carbides, leading to a deterioration in toughness. The suitable range is 0.10%-0.30%.

[0016] Mo (Mo): Molybdenum improves the hardenability of steel, thereby increasing the strength of the base material. Mo expands the γ-phase region, lowering the γ→α phase transformation temperature of steel. The phase transformation temperature gradually decreases with increasing Mo content, improving the tensile strength and toughness of the steel. Mo refines the grain size of steel, significantly improving its hardenability. Adding Mo in combination with Cr, maintaining a Mo / Cr ratio between 2.5 and 9.0, can significantly improve tensile strength, thereby noticeably reducing the yield strength ratio and achieving stronger resistance to carbon dioxide corrosion. Experiments have shown that Mo forms MoO3 (a dense surface film) and MoS2 (a lubricating and protective phase) in corrosive environments, with some existing in the matrix as solid solution. In steel, it is mostly uniformly dispersed or forms composite corrosion product films with Fe, Cr, etc. These compounds can combine with FeCO3 to form a dense Fe-Mo-C composite film, blocking the film pores and hindering H2O. + HCO3 - The diffusion of corrosion ions significantly improves resistance to CO2 corrosion. However, excessively high Mo content increases alloy costs and impairs plasticity and toughness; the optimal range is 0.7%-1.0%.

[0017] Nb: During the austenitization stage, stable NbC / Nb(C,N) particles are formed, pinning grain boundaries and inhibiting austenite grain growth, thus achieving fine-grain strengthening. During the cooling stage after rolling, nano-sized NbC precipitates, resulting in precipitation strengthening and improving the creep strength of the steel. Its optimal range is 0.03%-0.05%.

[0018] Al: Aluminum is a commonly used deoxidizer. Adding a small amount of aluminum to steel can refine the grain, improve strength and impact toughness, and also enhance the steel's corrosion resistance. In steel, it mainly forms Al₂O₃ and AlN. The Al-rich oxide film on the surface can firmly adhere to the substrate, isolating H₂ generated from CO₂. + CO 2- Corrosion ions prevent the substrate from being eroded; the internally dispersed Al compounds refine the grains and reduce corrosion channels. Simultaneously, they synergistically work with elements such as Cr to further enhance the density of the corrosion product film and reduce the corrosion rate. The effect is particularly enhanced when used in combination with Mo, Si, and Cr elements. The Al content of this invention is 0.1%-0.3%.

[0019] Zinc (Zn): Zinc solid solution strengthening improves processing performance. Zn dissolves in ferrite / austenite to form a solid solution, hindering dislocation movement and increasing yield strength and tensile strength by 20-60 MPa and 30-80 MPa, respectively. Zn reduces the deformation resistance of steel, improves plastic flowability, and reduces the risk of cracking. ZnO forms a dense passivation film on the steel surface, physically isolating CO2 molecules from the steel matrix while inhibiting anodic dissolution and slowing corrosion rates. Zinc has a lower electrode potential than iron and preferentially dissolves in corrosive environments, providing weak "sacrificial anodic protection" to the steel matrix, slowing localized corrosion, and improving resistance to carbon dioxide corrosion. Its optimal range is 0.16%-0.20%.

[0020] Mg: Magnesium can combine with nitrogen and oxygen in steel to form fine, dispersed inclusions such as MgO and Mg3N2, which enhance the strength of steel through precipitation strengthening and grain refinement. These inclusions can hinder dislocation movement, inhibit the growth of austenite grains during heating, refine the final grain structure, and indirectly improve strength. The grain-refining effect of appropriate amounts of magnesium can significantly improve the low-temperature toughness of steel and lower the ductile-brittle transition temperature. Magnesium oxide inclusions can reduce the amount of free sulfur and phosphorus in steel, mitigating pitting corrosion to some extent. Dispersed MgO particles can form a thin, dense oxide film on the steel surface, physically blocking CO2 molecules and H2 produced by hydrolysis. +Upon contact with the steel substrate, magnesium slows down the corrosion rate. When magnesium participates in the corrosion process, it promotes the formation of a Fe3O4-MgO composite corrosion film on the steel surface. The addition of MgO fills the tiny pores in the film, reducing its permeability, enhancing the stability of the corrosion product film, and reducing film detachment. The Mg / C ratio should be in the range of 1.5-2.0, ideally controlled at 0.6-4.0, while simultaneously improving toughness and resistance to carbon dioxide corrosion. The optimal range is 0.03%-0.08%.

[0021] Based on the above technical solution, further, the transverse / longitudinal yield strength is ≥500MPa, the tensile strength is ≥630MPa, the yield strength ratio is ≤0.78, the elongation is ≥29%, the impact energy at -50℃ is ≥250J, the drop weight DWTT at -20℃ is ≥90%, and the average corrosion rate in a 14.5MPa supercritical carbon dioxide environment is ≤0.10mm / a.

[0022] Secondly, the present invention provides a method for manufacturing the 500MPa grade low yield strength ratio carbon dioxide corrosion resistant steel as described above, comprising the following steps: S1: Molten steel is smelted, refined outside the furnace, hydrogen and oxygen content is controlled in the RH furnace, light desulfurization and calcium treatment are carried out in the LF furnace, and slabs are continuously cast to obtain continuously cast billets. S2: Heat the continuous casting billet to 1000℃-1100℃ and hold for 150min-250min; S3: Continuously cast billets are rolled; roughing rolling temperature is 960℃-1000℃, single-pass reduction is 20%-40%, finishing rolling temperature is 900℃-940℃, and finishing rolling temperature is 760℃-830℃. S4: After rolling, the coil is cooled and coiled to obtain a coil plate; a two-stage cooling method is adopted. The first stage cooling rate is 15℃ / s-20℃ / s, and the first stage cooling is 320℃-350℃. The second stage cooling rate is 30℃ / s-40℃ / s, and the final cooling temperature is 280℃-320℃.

[0023] Among them: light desulfurization and calcium treatment in LF furnace to control the morphology of inclusions and improve the ductility, toughness and cold bending performance of steel; Slab continuous casting employs electromagnetic stirring or dynamic light reduction; Heating is carried out in a heating furnace; rolling is carried out using thermomechanical rolling. Heating the continuously cast billet to 1000℃-1100℃ and holding it for 150min-250min allows alloys such as Nb and Cr to be fully dissolved, which is beneficial for improving yield strength and tensile strength. The roughing and finishing rolling temperature is 960℃-1000℃. This temperature is beneficial to prevent austenite grain growth, make the grains uniform, and improve the uniformity of microstructure and properties. The single-pass reduction rate is 20%-40%, ensuring the grain breaking effect and improving strength and toughness; The initial rolling temperature of the finishing rolling is 900℃-940℃, and the final rolling temperature is 760℃-830℃. This temperature range refines the flattening of austenite grains, thereby refining the bainite grain size and ensuring high strength and low-temperature impact toughness. A two-stage cooling method is adopted. The first stage has a cooling rate of 15℃ / s-20℃ / s, and the second stage has a cooling rate of 30℃ / s-40℃ / s. The final cooling temperature is 280℃-350℃. This cooling rate can suppress the pearlite transformation. The combination of cooling rate and final cooling temperature allows the first stage to obtain a bainite structure with low strength and hardness, while the second stage accelerates the cooling to obtain bainite with high strength and hardness, thereby ensuring that the yield strength ratio is less than 0.78. The final microstructure of the rolled plate is bainitic.

[0024] Based on the above technical solution, the smelting conditions in S1 are further as follows: smelting temperature 1500℃-1700℃, basicity 2.6-3.8, top and bottom combined blowing of N2 or Ar2, flow rate 0.06m³ / h. 3 t -1 min -1 -0.08m 3 t -1 min -1 .

[0025] Based on the above technical solution, the conditions for ladle refining in S1 are: arrival temperature ≥1550℃, heating time ≤30min, total processing time 40min-50min, soft blowing time ≥9min, and settling time 5min-7min.

[0026] Based on the above technical solution, further, the conditions for controlling the hydrogen and oxygen content in the RH furnace in S1 are as follows: the final hydrogen content is less than 1.0 ppm and the oxygen content is less than 15 ppm.

[0027] Based on the above technical solution, further, the conditions for controlling the hydrogen and oxygen content in the RH furnace in S1 are: working vacuum degree <60Pa, holding time 10min-25min, final hydrogen content <1.0ppm, and oxygen content <15ppm.

[0028] Based on the above technical solution, further, the conditions for light desulfurization treatment in the LF furnace in S1 are: slag basicity > 4.0, of which CaO > 50%, temperature 1580℃-1650℃, stirring for 20min-30min; The conditions for calcium treatment in the LF furnace in S1 are as follows: using CaSi or pure Ca wire with a Ca content of 20%-30%, the Ca addition amount is 0.60 kg / t-0.80 kg / t, and soft blowing argon for 30 m. 3 / h-50m 3 / h, time ≥6 minutes.

[0029] Thirdly, the present invention provides the application of the above-mentioned 500MPa grade low yield strength ratio carbon dioxide corrosion resistant steel in the preparation of low yield strength ratio pipelines.

[0030] Compared with the prior art, the present invention has the following beneficial effects: 1. This invention employs a low-carbon, low-manganese design. The low carbon content results in a low hardening tendency in the heat-affected zone during welding, reducing the likelihood of welding cracks. Cr and Al work synergistically, with Cr forming a dense Cr2O3 film on the metal surface and Al forming an Al2O3 film. The high stability of Cr2O3 combined with the self-healing properties of Al2O3 significantly improves resistance to carbon dioxide corrosion. Zn-Al solid solution enhances surface adhesion and improves resistance to carbon dioxide corrosion. Cr and Mo work together to strengthen the matrix, generating Cr2O3, Mo2C, and composite carbides, further enhancing resistance to carbon dioxide corrosion (especially pitting and crevice corrosion) and synergistically strengthening the matrix. The resulting steel exhibits a transverse / longitudinal yield strength ≥500MPa, tensile strength ≥630MPa, yield ratio ≤0.78, elongation ≥29%, impact energy at -50℃ ≥250J, drop weight DWTT at -20℃ ≥90%, and an average corrosion rate ≤0.10mm / a in a 14.5MPa supercritical carbon dioxide environment.

[0031] 2. This invention combines cooling rate and final cooling temperature, with the cooling rate suppressing pearlite transformation; it adopts a two-stage cooling method, the first stage first obtains a bainitic structure with low strength and hardness, and the second stage accelerates cooling to obtain refined bainite with high strength and hardness, realizing the bainitic structure design, and the material has a better strength and toughness match; the bainitic single structure has good uniformity, weak microcouple effect, and better resistance to carbon dioxide corrosion. Attached Figure Description

[0032] To more clearly illustrate the embodiments of the present invention, the accompanying drawings involved in the embodiments will be briefly described below.

[0033] Figure 1 This is a metallographic micrograph of the intermediate product after the first stage of cooling in Example 2 of the present invention; Figure 2 This is a metallographic micrograph of the rolled plate of Embodiment 2 of the present invention. Detailed Implementation

[0034] The present invention will be described in detail below with reference to the embodiments. However, the implementation of the present invention is not limited thereto. Obviously, the embodiments described below are only some embodiments of the present invention. For those skilled in the art, other similar embodiments can be obtained without creative effort and all fall within the protection scope of the present invention.

[0035] Based on the process flow of the present invention, using the weight percentage components in Table 1 as raw materials and the process conditions in Table 2, hot-rolled coils of Examples 1 to 8 were prepared.

[0036] The process flow of this invention is as follows: S1: Molten steel is smelted, refined outside the furnace, hydrogen and oxygen content is controlled in the RH furnace, light desulfurization and calcium treatment are carried out in the LF furnace, and slabs are continuously cast to obtain continuously cast billets. S2: Heat the continuous casting billet to the corresponding temperature and hold for 150-250 minutes; S3: The continuously cast billet is rolled; S4: After rolling, the coil is cooled and wound to obtain a coil; a two-stage cooling method is adopted.

[0037] Mechanical properties were tested on the hot-rolled coils from Examples 1 to 8. The test results are shown in Table 3. The test results from Examples 1 to 8 are all suitable for the supercritical carbon dioxide transportation service environment.

[0038] The corrosion rate was obtained by a carbon dioxide corrosion resistance test. The medium was a standard NACE A solution, the test temperature was 30℃, the CO2 pressure was 2.0MPa, the stirring speed was 3m / s, the test time was 72h, and the corrosion rate was determined by the weight loss method.

[0039] Metallographic images were acquired based on the intermediate product after the first stage of cooling in Example 2 and the resulting rolled plate. For example... Figure 1 and 2 As shown, the first stage first obtains a bainitic structure with low strength and hardness, and the second stage accelerates cooling to obtain bainitic structure with high strength and hardness; the final structure of the coil is bainitic.

[0040] Table 1: Chemical composition of the examples / wt%.

[0041]

[0042] Table 2: Heating, rolling, and cooling process parameters.

[0043]

[0044] Table 3: Mechanical property test results.

[0045]

[0046] Finally, it should be noted that the above embodiments are only used to illustrate the technical solutions of the present invention, and not to limit them; although the present invention has been described in detail with reference to the foregoing embodiments, those skilled in the art should understand that modifications can still be made to the technical solutions described in the foregoing embodiments, or equivalent substitutions can be made to some or all of the technical features; and these modifications or substitutions do not cause the essence of the corresponding technical solutions to deviate from the scope of the technical solutions of the embodiments of the present invention.

Claims

1. A 500MPa grade steel with low yield strength ratio and resistance to carbon dioxide corrosion, characterized in that, Composed of the following components by weight percentage composition: C: 0.02%-0.05%, Si: 0.10%-0.30%, Mn: 0.60%-0.90%, P: ≤0.020%, S: ≤0.002%, Cr: 0.10%-0.30%, Mo: 0.70%-1.00%, Nb: 0.03%-0.05%, Al: 0.10%-0.30%, Zn: 0.16%-0.20%, Mg: 0.03%-0.08%, N: ≤0.005%, with the remainder being Fe and unavoidable impurities.

2. The 500MPa grade low yield strength ratio carbon dioxide corrosion resistant steel according to claim 1, characterized in that, The Cr / Al ratio is 1.20-2.25, the Mo / Cr ratio is 2.5-9.0, and the Mg / C ratio is 0.6-4.

0.

3. The 500MPa grade low yield strength ratio carbon dioxide corrosion resistant steel according to claim 1, characterized in that, Transverse / longitudinal yield strength ≥500MPa, tensile strength ≥630MPa, yield ratio ≤0.78, elongation ≥29%, impact energy at -50℃ ≥250J, drop weight DWTT at -20℃ ≥90%, average corrosion rate in a 14.5MPa supercritical carbon dioxide environment ≤0.10mm / a.

4. The method for manufacturing 500MPa grade low yield strength ratio carbon dioxide corrosion resistant steel as described in any one of claims 1 to 3, characterized in that, Includes the following steps: S1: Molten steel is smelted, refined outside the furnace, hydrogen and oxygen content is controlled in the RH furnace, light desulfurization and calcium treatment are carried out in the LF furnace, and slabs are continuously cast to obtain continuously cast billets. S2: Heat the continuous casting billet to 1000℃-1100℃ and hold for 150min-250min; S3: Continuously cast billets are rolled; roughing rolling temperature is 960℃-1000℃, single-pass reduction is 20%-40%, finishing rolling temperature is 900℃-940℃, and finishing rolling temperature is 760℃-830℃. S4: After rolling, the coil is cooled and coiled to obtain a coil plate; a two-stage cooling method is adopted. The first stage cooling rate is 15℃ / s-20℃ / s, and the first stage cooling is 320℃-350℃. The second stage cooling rate is 30℃ / s-40℃ / s, and the final cooling temperature is 280℃-320℃.

5. The method according to claim 4, characterized in that, The smelting conditions in S1 are: smelting temperature 1500℃-1700℃, basicity 2.6-3.8, top and bottom combined blowing of N2 or Ar2, and flow rate 0.06m³. 3 t -1 min -1 -0.08m 3 t -1 min -1 .

6. The method according to claim 4, characterized in that, The conditions for ladle refining in S1 are: arrival temperature ≥ 1550℃, heating time ≤ 30min, total processing time 40min-50min, soft blowing time ≥ 9min, and settling time 5min-7min.

7. The method according to claim 4, characterized in that, The conditions for controlling the hydrogen and oxygen content in the RH furnace in S1 are as follows: working vacuum degree <60Pa, holding time 10min-25min, final hydrogen content <1.0ppm, and oxygen content <15ppm.

8. The method according to claim 4, characterized in that, The conditions for light desulfurization treatment in the LF furnace in S1 are: slag basicity > 4.0, of which CaO > 50%, temperature 1580℃-1650℃, stirring for 20min-30min; The conditions for calcium treatment in the LF furnace in S1 are as follows: using CaSi or pure Ca wire with a Ca content of 20%-30%, the Ca addition amount is 0.60 kg / t-0.80 kg / t, and soft blowing argon for 30 m. 3 / h-50m 3 / h, time ≥6 minutes.

9. The application of the 500MPa grade low yield strength ratio carbon dioxide corrosion resistant steel as described in any one of claims 1 to 3 in the preparation of low yield strength ratio pipelines.

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