A method for producing a large-wall-thickness pipeline steel with improved carbon dioxide corrosion resistance and toughness

Abstract

The present application relates to the technical field of metallurgy, and particularly relates to a production method of a large-wall-thickness pipeline steel with improved carbon dioxide corrosion resistance and strength and toughness, and specific measures include: (1) controlling the bottom blowing mode and end point requirement in the converter process; (2) controlling the tapping time and replacing the tapping hole opportunity in the tapping process; (3) feeding the CAS aluminum wire and controlling the CAS argon blowing time; (4) controlling the top slag addition amount, heating time and outgoing mode in the LF refining process; (5) controlling the wire feeding parameters, soft blowing time and outgoing requirement of RH in the RH refining process; (6) controlling the precision requirement, casting speed fluctuation and crystallizer liquid level fluctuation of the continuous casting equipment in the continuous casting process to obtain the casting blank. Through controlling the parameters of each process in the smelting and rolling process, the present application realizes the whole-process steel cleanliness control and ladle grain refinement, and further improves the strength, low-temperature toughness, HIC resistance and carbon dioxide corrosion resistance of the large-wall-thickness pipeline steel.

Description

Technical Field

[0001] This invention relates to the field of metallurgical technology, specifically to a method for producing thick-walled pipeline steel with improved resistance to carbon dioxide corrosion and enhanced toughness. Background Technology

[0002] Carbon capture, utilization and storage (CCUS) technology refers to the process of separating CO2 from industrial processes, energy use or the atmosphere and using it directly or injecting it into the formation to achieve permanent CO2 emission reduction. CCUS technology can achieve large-scale greenhouse gas emission reduction and promote the efficient use of fossil energy, and is an important carbon dioxide emission control strategy.

[0003] Since CCUS technology requires high-pressure transportation of captured carbon dioxide in a liquid dense phase, the transportation pipeline is required to have a large wall thickness. At the same time, the mechanical properties, HIC resistance (resistance to hydrogen-induced cracking) and corrosion resistance of the transportation pipeline are required to be extremely high. Therefore, the corresponding pipeline steel is required to have good low-temperature toughness, high crack arrest performance and carbon dioxide corrosion resistance.

[0004] In existing technologies, the main method to improve the carbon dioxide corrosion resistance of pipeline steel is to add a high content of chromium (Cr) to the steel raw materials. Conventional smelting, rolling, and cooling processes are mainly used in the smelting and rolling processes. However, pipeline steel with a high Cr content has an increased hardening tendency, resulting in lower toughness and plasticity of the welded joint and poor weldability after welding. This fails to meet the requirements of simultaneously possessing good low-temperature toughness, high crack arrest performance, and carbon dioxide corrosion resistance. Furthermore, existing technologies do not propose production methods for thick pipeline steel that can improve carbon dioxide corrosion resistance and HIC resistance while maintaining excellent mechanical properties. Summary of the Invention

[0005] To address the shortcomings of existing pipeline steel production technologies, which fail to simultaneously meet the requirements of good low-temperature toughness, high crack arrest performance, and resistance to carbon dioxide corrosion (HCC), and which lack a production method for thick-walled pipeline steel that can improve HCC resistance and HIC resistance while maintaining excellent mechanical properties, this invention provides a production method for thick-walled pipeline steel that improves HCC resistance and toughness. By controlling the parameters of each process in the smelting and rolling stages, the cleanliness of the steel throughout the entire process is controlled, and the grain size of the ladle is refined, thereby simultaneously improving the strength, low-temperature toughness, HIC resistance, and HCC resistance of thick-walled pipeline steel.

[0006] The technical solution of this invention is as follows:

[0007] A method for producing thick-walled pipeline steel with improved resistance to carbon dioxide corrosion and high toughness includes smelting and rolling processes. The smelting process sequentially includes converter, tapping, CAS treatment, LF refining, RH refining, and continuous casting. Specific measures include:

[0008] (1) The converter process adopts full argon bottom blowing to control the temperature at the converter end point ≥1640℃, C≤0.030wt.%, P<0.007wt.%, S≤0.008wt.%, and the carbon control mode at the converter end point adopts the low carbon microalloy steel mode. The converter adopts full argon bottom blowing and does not adopt the conventional bottom blowing nitrogen mode, which can ensure that the nitrogen content in the molten steel is low enough, avoid the nitrogen content in the final product steel plate being too high, thereby improving the low temperature toughness of the steel plate. The control of various indicators at the converter end point can ensure that the molten steel is low in phosphorus, low in sulfur and low in carbon, improve the cleanliness of the steel plate, and ensure the low temperature toughness and corrosion resistance of the final product steel plate.

[0009] (2) The tapping time of the tapping process is ≥4.5min. If the tapping process is interrupted, the tapping port shall be replaced immediately. It is strictly forbidden to add slag to the converter. On the one hand, this will prevent the molten steel from being over-oxidized during the tapping process, and on the other hand, it will prevent the converter slag with high phosphorus content from entering the ladle. This will achieve low phosphorus content control throughout the process and improve the toughness and carbon dioxide corrosion resistance of the final product.

[0010] (3) In the CAS processing process, 0.5-1.0 m / t of aluminum wire is fed into the CAS and the CAS argon blowing time is ≥12 min. The aluminum content of the molten steel is adjusted by feeding in aluminum wire to achieve deoxidation of the molten steel. The longer CAS argon blowing time can ensure the uniformity of the temperature and composition of the molten steel.

[0011] (4) In the LF refining process, the amount of LF top slag added is: 7-8 kg / ton of lime and 0.8-1.5 kg / ton of fluorite, which can quickly form slag during the tapping process, achieve large slag volume and high oxidizing slag, and provide good conditions for desulfurization; in the LF refining process, the ladle is blown with argon without exposing the molten steel to prevent the molten steel from absorbing oxygen and oxidizing; the total heating time of LF is ≤40 min to avoid the molten steel from absorbing nitrogen and oxygen due to the excessive heating time of LF electrodes; the requirement for molten steel to leave the LF station is S≤0.0010wt.%, and the outlet temperature is 1605-1625℃; the S in the steel grade will combine with Mn to form easily deformable MnS inclusions, which form long strip-shaped inclusions during the rolling process of steel plates, greatly deteriorating the low temperature toughness of the steel plates, and forming hydrogen traps, which are important defects in the steel plates' resistance to hydrogen-induced cracking and carbon dioxide corrosion. Limiting the LF outlet requirements can meet the ultra-low sulfur content requirements of molten steel, and at the same time, the molten steel can maintain a low superheat during the subsequent continuous casting process;

[0012] (5) In the RH refining process, the amount of calcium-aluminum wire fed is 1-2.5 m / t and the feeding speed is 100-180 m / min. During the RH refining process, the hydrogen content of the molten steel is controlled to be ≤1.5 ppm and the nitrogen content of the molten steel is controlled to be ≤40 ppm. Feeding calcium wire in RH can modify the inclusions such as sulfides in the steel, making them into spherical inclusions, which is beneficial to reduce the elongation and deformation of inclusions after rolling and reduce the harm of inclusions. Therefore, controlling the reasonable amount and feeding speed of calcium-aluminum wire, and strictly controlling the content of harmful elements hydrogen and nitrogen in the molten steel, can effectively improve the low temperature toughness and hydrogen-induced crack sensitivity of the final product steel plate.

[0013] (6) In the continuous casting process, protective nozzles are used for pouring molten steel from the ladle to the tundish, and pouring is not done in an open manner. The superheat of the tundish is controlled at 10-25℃. During the continuous casting process, the liquid level fluctuation in the crystallizer is ≤±5mm, the nitrogen increase is ≤5ppm, the casting speed fluctuation range is ≤0.1m / min, and the arc accuracy is controlled at ≤±0.1mm. The light reduction of the continuous casting machine is 3-5mm, and the reduction position is at the end of solidification. The continuous casting produces a billet. The casting speed fluctuation of the continuous casting machine and the liquid level fluctuation in the crystallizer are all unsteady conditions in the continuous casting process, which have a significant impact on the internal and surface quality of the billet. This technical solution specifies detailed processes regarding the fluctuation range of continuous casting machine speed, the fluctuation of liquid level in the crystallizer, and the setting of light reduction. It also provides comprehensive and unique production methods for aspects such as low-magnification quality of the cast billet and the arc alignment accuracy of the continuous casting machine, thereby ensuring good cleanliness of the steel and good internal quality of the cast billet. Ultimately, this improves the carbon dioxide corrosion resistance and low-temperature toughness of thick-walled pipeline steel. Since the nitrogen content in the molten steel inevitably increases during continuous casting, this invention is designed to increase nitrogen by ≤5ppm, ensuring that the nitrogen content in the molten steel remains within a low range to improve the overall mechanical properties of the final steel plate.

[0014] Furthermore, the steel rolling process sequentially includes billet heating, rough rolling, finish rolling, pre-straightening, rapid cooling, hot straightening, cooling bed cooling, and flaw detection. Specific measures include:

[0015] (a) The furnace temperature for the billet heating process is 1160-1190℃, and the heating time is 9-15 min / cm;

[0016] (b) Before the last two passes of rough rolling, the billet is rapidly cooled by high-pressure water descaling in the mill. The surface temperature of the billet after cooling is ≤1000℃. The reduction rate of the last two passes of rough rolling is ≥25%, and the final rolling temperature of rough rolling is 900-1000℃. The use of high-pressure water descaling in the mill to rapidly cool the billet in the last two passes of rough rolling can achieve the purpose of deformation penetration to the center during the rolling process, thereby improving the uniformity of the grain structure in the thickness direction of the steel plate and making the grains sufficiently refined, thereby improving the strength, toughness and carbon dioxide corrosion resistance of the steel plate.

[0017] (c) The final rolling temperature of the finishing rolling process is 850-880℃;

[0018] (d) The pre-straightening process is carried out using a 9-roll pre-straightening machine at a straightening speed of 4-6 m / s to obtain pre-straightened steel plates;

[0019] (e) The starting temperature of the rapid cooling process is 820-860℃, and rapid water cooling is adopted. During the cooling process, the edges of the straightened steel plate are shielded by 830-930mm. The ratio of water spray volume of the upper manifold to the lower manifold of the water cooling equipment is 1:1.3-1.5, the total water volume is 2800-3200L / s, and the cooling rate is 15-30℃ / s.

[0020] (f) The hot straightening process involves 1-3 passes of hot straightening;

[0021] The combination of pre-straightening, rapid cooling and hot straightening processes can achieve high strength and toughness and shape control for thick-walled pipeline steel. Combined with the smelting process, the steel plate has good resistance to carbon dioxide corrosion.

[0022] (g) The cooling process of the cooling bed adopts a walking cooling bed for 1-1.5 hours to obtain thick-walled pipeline steel plates;

[0023] (h) The flaw detection process uses ultrasonic flaw detection equipment to inspect the internal defects of the steel plate. The flaw detection method conforms to the requirements of NB / T47013.3 standard. The obtained thick-walled pipeline steel plate meets the requirements of Special Class I flaw detection. There are no delamination or inclusion defects within 300mm of the beginning and end and 100mm of the side of the thick-walled pipeline steel plate. This technical solution adopts the most stringent NB / T 47013.3 ultrasonic flaw detection standard in this technical field to perform ultrasonic flaw detection on the finished steel plate. At the same time, it specifies the special requirements for flaw detection quality within a certain range of the beginning, end and two side of the plate to ensure good internal structure of the steel plate, so as to improve the carbon dioxide corrosion resistance and toughness of the thick-walled pipeline steel.

[0024] Furthermore, the chemical composition of the billet by weight percentage is as follows: C: 0.02%-0.05%, Si: 0.20%-0.60%, Mn: 1.2%-1.40%, V: 0.030%-0.040%, Al: 0.020%-0.050%, Cr: 0.40%-0.50%, Ni: 0.10%-0.20%, Nb: 0.040%-0.055%, P≤0.0080%, S≤0.002%, N≤0.0030%, H≤0.0002%, 1.7%≤Mn+Ni+Cr≤2.0%, Nb+V+Al≤0.140%, with the remainder being Fe and unavoidable impurities.

[0025] Furthermore, the converter process uses scrap steel as the main raw material. Nickel plates and copper plates are added to the converter along with the scrap steel for smelting. Before tapping the steel, aluminum blocks, metallic manganese, ferrosilicon, ferrochrome, and ferroniobium are added to the converter in sequence.

[0026] Furthermore, in the converter process of step (1), the oxygen lance position is immediately lowered to the carbon removal lance position after the TSC measurement is completed, ensuring that the low lance position time is 1-2 minutes. This can quickly reduce the carbon in the molten steel to a lower range, while ensuring that the oxygen content in the molten steel is low, so as not to cause over-oxidation of the molten steel, thereby improving the quality of the molten steel.

[0027] Furthermore, in step (6), after continuous casting, at least 6 meters of billet head and at least 4 meters of billet tail are removed from the billet head. The resulting low-magnification sample of the billet has a center segregation of no more than Class C 1.5 and a center porosity of ≤0.5 grade, with no obvious inclusions or other internal crack defects.

[0028] Furthermore, the total rolling compression ratio of the roughing and finishing rolling processes is 7.5-8.

[0029] Furthermore, the thickness of the steel plate for the large-walled pipeline is 30-40mm, Rt0.5≥500MPa, Rt1.5≥510MPa, Rm≥600MPa, impact energy value at -40℃≥300J, shear area of ​​impact test at -40℃≥80%, and shear area of ​​DWTT at -20℃≥90%.

[0030] Furthermore, under conditions of 80℃, CO2 partial pressure of 2MPa, and flow rate of 2.0m / s, the thick-walled pipeline steel plate exhibits a carbon dioxide corrosion rate of ≤0.05mm / a; and HIC resistance properties of CLR≤15%, CTR≤5%, and CSR≤2%.

[0031] The beneficial effects of this invention are as follows:

[0032] The present invention provides a method for producing thick-walled pipeline steel with improved resistance to carbon dioxide corrosion and high toughness. By controlling the parameters of each process in the smelting and rolling stages, the method achieves full-process steel cleanliness control and ladle grain refinement, thereby simultaneously improving the strength, low-temperature toughness, HIC resistance, and carbon dioxide corrosion resistance of the thick-walled pipeline steel. The resulting thick-walled pipeline steel has a thickness of 30-40 mm, Rt0.5≥500MPa, Rt1.5≥510MPa, Rm≥600MPa, impact energy value at -40℃≥300J, impact shear area at -40℃≥80%, and DWTT shear area at -20℃≥90%. Under conditions of 80℃, CO2 partial pressure of 2MPa, and flow rate of 2.0m / s, the carbon dioxide corrosion rate is ≤0.05mm / a; the HIC resistance is CLR≤15%, CTR≤5%, and CSR≤2%. Detailed Implementation

[0033] To enable those skilled in the art to better understand the technical solutions of this invention, the technical solutions in the embodiments of this invention will be clearly and completely described below. Obviously, the described embodiments are only some embodiments of this invention, and not all embodiments. Based on the embodiments of this invention, all other embodiments obtained by those skilled in the art without creative effort should fall within the scope of protection of this invention.

[0034] Examples 1-5

[0035] A method for producing thick-walled pipeline steel with improved resistance to carbon dioxide corrosion and high toughness, wherein the chemical composition by weight percentage of the cast billet is shown in Table 1.

[0036] Table 1. Weight percentage of chemical composition of the cast billets in each embodiment (%)

[0037] C Si Mn P S Al Nb Ni Cr V N H Example 1 0.040 0.20 1.36 0.0051 0.0010 0.028 0.049 0.17 0.45 0.032 0.0025 0.0001 Example 2 0.032 0.21 1.22 0.0068 0.0010 0.035 0.041 0.12 0.41 0.036 0.0028 0.0001 Example 3 0.035 0.22 1.20 0.0070 0.0009 0.040 0.045 0.15 0.49 0.035 0.0025 0.0002 Example 4 0.042 0.25 1.28 0.0075 0.0008 0.029 0.047 0.18 0.44 0.031 0.0026 0.0001 Example 5 0.025 0.28 1.35 0.0059 0.0010 0.035 0.048 0.20 0.40 0.039 0.0029 0.0002

[0038] The production processes in Examples 1-5 include smelting and rolling. The smelting process includes, in sequence, converter, tapping, CAS treatment, LF refining, RH refining, and continuous casting. The rolling process includes, in sequence, billet heating, rough rolling, finish rolling, pre-straightening, rapid cooling, hot straightening, cooling bed cooling, and flaw detection. Specific measures include:

[0039] (1) The converter process uses scrap steel as the main raw material. Nickel plates and copper plates are added to the converter for smelting at the same time as scrap steel. Before tapping, aluminum blocks, metallic manganese, ferrosilicon, ferrochrome and ferroniobium are added to the converter in sequence. The converter process adopts full argon bottom blowing to control the temperature at the converter end point ≥1640℃, C≤0.030wt.%, P<0.007wt.%, S≤0.008wt.%. The carbon control mode at the converter end point adopts the low carbon microalloy steel mode.

[0040] (2) The tapping time of the tapping process is ≥4.5 min. If the tapping process is interrupted, the tapping port shall be changed immediately. Slag feeding into the converter is strictly prohibited.

[0041] (3) In the CAS processing procedure, the CAS feed aluminum wire is 0.5-1.0m / t, and the CAS argon blowing time is ≥12min;

[0042] (4) In the LF refining process, the amount of LF top slag added is: 7-8 kg / ton of lime and 0.8-1.5 kg / ton of fluorite; the steel ladle is not exposed to argon during the LF refining process; the total heating time of LF is ≤40 min; the requirements for molten steel leaving the LF station are S≤0.0010wt.% and the temperature at the station is 1605-1625℃;

[0043] (5) In the RH refining process, the calcium-aluminum wire feed rate is 1-2.5 m / t and the feed rate is 100-180 m / min; during the RH refining process, the hydrogen content of the molten steel is controlled to be ≤1.5 ppm and the nitrogen content of the molten steel is controlled to be ≤40 ppm.

[0044] (6) During the continuous casting process, the steel ladle is used to pour molten steel into the tundish using a protective nozzle and the pouring is not open. The superheat of the tundish is controlled at 10-25℃. During the continuous casting process, the liquid level fluctuation in the crystallizer is ≤±5mm, the nitrogen increase is ≤5ppm, the casting speed fluctuation range is ≤0.1m / min, and the arc accuracy is controlled at ≤±0.1mm. The light reduction of the continuous casting machine is 3-5mm, and the reduction position is at the end of solidification. The billet obtained by continuous casting has at least 6 meters cut off from the billet head and at least 4 meters cut off from the billet tail. The center segregation of the low magnification sample of the obtained billet is not higher than Class C1.5, the center porosity is ≤0.5 grade, and there are no obvious inclusions or other internal crack defects.

[0045] (7) The furnace temperature for the billet heating process is 1160-1190℃, and the heating time is 9-15 min / cm;

[0046] (8) Before the last two passes of rough rolling, the billet is rapidly cooled by high-pressure water descaling. The surface temperature of the billet after cooling is ≤1000℃. The reduction rate of the last two passes of rough rolling is ≥25%. The final rolling temperature of rough rolling is 900-1000℃.

[0047] (9) The final rolling temperature of the finishing rolling process is 850-880℃, and the total rolling compression ratio of the roughing rolling process and the finishing rolling process is 7.5-8;

[0048] (10) The pre-straightening process is carried out using a 9-roll pre-straightening machine with a straightening speed of 4-6 m / s to obtain pre-straightened steel plates;

[0049] (11) The starting temperature of the rapid cooling process is 820-860℃, and rapid water cooling is adopted. During the cooling process, the edges of the straightened steel plate are shielded by 830-930mm. The ratio of the water spray volume of the upper manifold to the lower manifold of the water cooling equipment is 1:1.3-1.5, the total water volume is 2800-3200L / s, and the cooling rate is 15-30℃ / s.

[0050] (12) The hot straightening process involves 1-3 passes of hot straightening;

[0051] (13) The cooling process of the cooling bed adopts a step cooling bed for 1-1.5 hours to obtain thick-walled pipeline steel plates;

[0052] (14) The flaw detection process uses ultrasonic flaw detection equipment to inspect the internal defects of the steel plate. The flaw detection method conforms to the requirements of NB / T47013.3 standard. The obtained thick-walled pipeline steel plate meets the requirements of Class I flaw detection. There are no delamination and inclusion defects within 300mm of the head and tail and 100mm of the side of the thick-walled pipeline steel plate.

[0053] The performance of the thick-walled pipeline steel plates prepared in Examples 1-5 was tested, as shown in Table 2. The conditions for measuring the CO2 corrosion resistance rate were an ambient temperature of 80°C, a CO2 partial pressure of 2 MPa, and a flow rate of 2.0 m / s.

[0054] Table 2 Properties of Thick-Walled Pipeline Steel Prepared in Each Embodiment

[0055] Steel plate thickness (mm) Rt0.5 (MPa) Rt1.5 (MPa) Rm (MPa) Transverse impact energy value (J) at -40℃ -40℃ Transverse impact shear area (%) -20℃ DWTT shear area (%) <![CDATA[CO2 corrosion rate (mm / a)]]> Anti-HIC performance Example 1 30 530 536 597 390 100 96 0.035 CLR: 0%, CTR: 0%, CSR: 0% Example 2 38 527 519 586 416 100 99 0.042 CLR: 1.2%, CTR: 0.2%, CSR: 0.1% Example 3 40 517 534 600 399 100 95 0.038 CLR: 0%, CTR: 0%, CSR: 0% Example 4 38 518 551 610 418 100 99 0.038 CLR: 2.2%, CTR: 0.9%, CSR: 0.2% Example 5 38 527 510 608 421 100 100 0.028 CLR: 0%, CTR: 0%, CSR: 0%

[0056] Although the present invention has been described in detail by way of preferred embodiments, the present invention is not limited thereto. Various equivalent modifications or substitutions can be made to the embodiments of the present invention by those skilled in the art without departing from the spirit and essence of the invention, and such modifications or substitutions should all be within the scope of the present invention. Any variations or substitutions that can be easily conceived by those skilled in the art within the technical scope disclosed in the present invention should also be covered within the protection scope of the present invention.

Claims

1. A method for producing a large wall thickness pipeline steel with improved resistance to carbon dioxide corrosion and strength and toughness, comprising smelting, casting and rolling steps, characterized in that, The smelting and casting process includes, in sequence, converter, tapping, CAS treatment, LF refining, RH refining, and continuous casting. Specific steps include: (1) The converter process adopts full argon bottom blowing to control the temperature at the converter endpoint ≥1640℃, C≤0.030wt.%, P<0.007wt.%, S≤0.008wt.%, and the carbon control mode at the converter endpoint adopts the low carbon microalloy steel mode; (2) The tapping time of the tapping process is ≥4.5 min. If the tapping process is interrupted, the tapping port shall be changed immediately. Slag feeding into the converter is strictly prohibited. (3) In the CAS processing procedure, the CAS feed aluminum wire is 0.5-1.0m / t, and the CAS argon blowing time is ≥12min; (4) In the LF refining process, the amount of LF top slag added is: 7-8 kg / ton of lime and 0.8-1.5 kg / ton of fluorite; the steel ladle is not exposed to argon during the LF refining process; the total heating time of LF is ≤40 min; the requirements for molten steel leaving the LF station are S≤0.0010wt.% and the temperature at the station is 1605-1625℃; (5) In the RH refining process, the calcium-aluminum wire feed rate is 1-2.5 m / t and the feed rate is 100-180 m / min; during the RH refining process, the hydrogen content of the molten steel is controlled to be ≤1.5 ppm and the nitrogen content of the molten steel is controlled to be ≤40 ppm. (6) During the continuous casting process, a protective nozzle is used for pouring molten steel from the ladle to the tundish, and the pouring is not open. The superheat of the tundish is controlled at 10-25℃. During the continuous casting process, the liquid level fluctuation in the crystallizer is within ±5ppm, the nitrogen increase is ≤5ppm, the casting speed fluctuation range is ≤0.1m / min, and the arc accuracy is controlled within ±0.1mm. The light reduction of the continuous casting machine is 3-5mm, and the reduction position is at the end of solidification. The billet is obtained by continuous casting. The steel rolling process includes, in sequence, billet heating, rough rolling, finish rolling, pre-straightening, rapid cooling, hot straightening, cooling bed cooling, and flaw detection. Specific steps include: (a) The furnace temperature for the billet heating process is 1160-1190℃, and the heating time is 9-15 min / cm; (b) Before the last two passes of rough rolling, the billet is rapidly cooled by high-pressure water descaling in the rolling mill. The surface temperature of the billet after cooling is ≤1000℃. The reduction rate of the last two passes of rough rolling is ≥25%, and the final rolling temperature of rough rolling is 900-1000℃. (c) The final rolling temperature of the finishing rolling process is 850-880℃; (d) The pre-straightening process is carried out using a 9-roll pre-straightening machine at a straightening speed of 4-6 m / s to obtain pre-straightened steel plates; (e) The starting temperature of the rapid cooling process is 820-860℃, and rapid water cooling is adopted. During the cooling process, the edges of the straightened steel plate are shielded by 830-930mm. The ratio of water spray volume of the upper manifold to the lower manifold of the water cooling equipment is 1:1.3-1.5, the total water volume is 2800-3200L / s, and the cooling rate is 15-30℃ / s. (f) The hot straightening process involves 1-3 passes of hot straightening; (g) The cooling process of the cooling bed adopts a walking cooling bed for 1-1.5 hours to obtain thick-walled pipeline steel plates; (h) The internal defects of the steel plate are checked by using an ultrasonic flaw detection device, the flaw detection method meets the process requirements of the NB / T47013.3 standard, and the obtained large-wall-thickness pipeline steel plate meets the special I-grade flaw detection requirements, and there is no any delamination and inclusion defect within 300 mm of the head and tail and 100 mm of the side of the large-wall-thickness pipeline steel plate; The chemical composition of the casting blank by weight percentage is C: 0.02%-0.05%, Si: 0.20%-0.60%, Mn: 1.2%-1.40%, V: 0.030%-0.040%, Al: 0.020%-0.050%, Cr: 0.40%-0.50%, Ni: 0.10%-0.20%, Nb: 0.040%-0.055%, P≤0.0080%, S≤0.002%, N≤0.0030%, H≤0.0002%, 1.7%≤Mn+Ni+Cr≤2.0%, Nb+V+Al≤0.140%, and the rest is Fe and inevitable impurities.

2. The method for producing thick-walled pipeline steel with improved carbon dioxide corrosion resistance and toughness as described in claim 1, characterized in that, The converter process uses scrap steel as the main raw material, adds the nickel plate and the scrap steel into the converter at the same time for smelting, and sequentially adds aluminum blocks, metal manganese, silicon iron, chromium iron and niobium iron into the converter before tapping.

3. The method for producing thick-walled pipeline steel with improved carbon dioxide corrosion resistance and toughness as described in claim 1, characterized in that, In the converter process of step (1), the oxygen lance position is immediately lowered to the carbon-lowering lance position after the TSC measurement is completed, and the low-lance position time is ensured to be 1-2 min.

4. The method for producing thick-walled pipeline steel with improved carbon dioxide corrosion resistance and toughness as described in claim 1, characterized in that, After the casting blank is obtained by continuous casting in step (6), the blank head is cut off at least 6 meters, and the blank tail is cut off at least 4 meters, the center segregation of the obtained casting blank is not higher than C class 1.5, the center porosity is ≤0.5 level, there is no obvious inclusion and other internal crack defects.

5. The method for producing thick-walled pipeline steel with improved carbon dioxide corrosion resistance and toughness as described in claim 1, characterized in that, The total rolling compression ratio of the rough rolling process and the finish rolling process is 7.5-8.

6. The method for producing thick-walled pipeline steel with improved carbon dioxide corrosion resistance and toughness as described in claim 1, characterized in that, The large-wall-thickness pipeline steel plate has a thickness of 30-40 mm, Rt0.5≥500 MPa, Rt1.5≥510 MPa, Rm≥600 MPa, -40℃ impact energy value≥300 J, -40℃ impact test shear area≥80%, and -20℃ DWTT shear area≥90%.

7. The method for producing thick-walled pipeline steel with improved carbon dioxide corrosion resistance and toughness as described in claim 1, characterized in that, The large-wall-thickness pipeline steel plate has a carbon dioxide corrosion resistance rate of ≤0.05 mm / a under the conditions of 80℃, 2 MPa of CO2 partial pressure, and a flow rate of 2.0 m / s; and the HIC resistance performance is CLR≤15%, CTR≤5%, and CSR≤2%.