600mpa high performance line pipe wide and thick steel plate and method of producing the same

Abstract

This invention belongs to the field of metallic materials and provides a 600MPa high-performance thick steel plate for pipelines and its production method. The composition of the steel plate, by weight percentage, is as follows: C: 0.035%~0.065%, Si: 0.15%~0.45%, Mn: 1.60%~1.74%, Nb: 0.030%~0.050%, V≤0.08%, N: 0.0040%~0.010%, Ni: 0.05%~0.065%. The composition is as follows: Cu: 0.01%–0.15%, Cr: 0.10%–0.30%, Al: 0.005%–0.015%, Ca: 0.0020%–0.0050%, Zr≤0.003%, La≤0.005%, Ti≤0.010%, P≤0.008%, S≤0.0015%, H≤0.00015%, O≤0.0015%, with the balance being iron and unavoidable impurities. The production method includes smelting, continuous casting, heating, rough rolling, finish rolling, and cooling. The steel plates produced using this invention meet the requirements of high-performance pipelines operating under complex conditions, resulting in 600MPa high-performance pipeline wide and thick steel plates and their production method.

Description

Technical Field

[0001] This invention belongs to the field of metallic materials, and in particular relates to a wide and thick steel plate for high-performance pipelines operating under complex conditions of 600MPa and its production method, which is especially suitable for manufacturing energy transmission pipelines under complex operating conditions such as marine, high-altitude and high-low temperature cycles. Background Technology

[0002] Pipelines are the most economical and rational mode of transportation for long-distance energy transmission and a fundamental guarantee for energy strategy. With the continuous development of the economy and society, my country has become the world's largest energy consumer. To ensure energy supply, the pace of oil and gas capacity construction in key areas such as deep energy, non-deep energy, and renewable energy is accelerating, as are the development of new technologies, models, and business forms for renewable energy and energy storage. This has placed more stringent requirements on the service environment and transport media of energy pipelines, and has also brought new opportunities and challenges to the development of pipeline materials for complex conditions such as marine environments, high-altitude cold regions, and high-low temperature cycles.

[0003] To meet service requirements, high-performance pipeline steel plates operating under complex conditions must possess a combination of technical characteristics, including high strength, high uniform elongation, low-temperature toughness, high strain resistance, corrosion resistance, aging resistance, and fatigue resistance. Simultaneously, they must also have thick-walled dimensions to meet high-pressure transportation and safety requirements.

[0004] Currently, there has been some research on high-performance pipeline steel plates for service under complex conditions. Some patents and literature have been found through searching, but the contents recorded therein are significantly different from those of the present invention in terms of composition, production method, performance, and microstructure design. In particular, the present invention has significant advantages in the coupling of multi-dimensional technical features.

[0005] The patent document "Steel with excellent low-temperature toughness and elongation and low yield ratio for high-strength thick pipelines and its manufacturing method" (CN113166905A) provides a high-performance pipeline steel plate with a high Nb (0.08%~0.12%) and high Mo (0.20%~0.40%) design scheme, which results in excessively high alloy costs; the microstructure is mainly composed of acicular ferrite and bainitic ferrite, which leads to weak strain capacity.

[0006] The patent document "An X80M Deep-Sea Strain-Resistant Pipeline Steel Plate and Rolling Process" (CN109234623B) provides a deep-sea pipeline steel plate with a high Ni (0.65%~0.85%) and high Mo (0.31%~0.36%) composition, which results in excessively high alloy content and cost.

[0007] The patent document "An X80 pipeline steel with excellent toughness in the heat-affected zone of the circumferential weld joint and its preparation method" (CN117821851A) provides a high-strength and high-toughness pipeline steel with a high Ni design (0.30%~0.45%) and high Nb (0.09%~0.12%). It has high cost and requires a reduction rate of ≥25% in the last pass of rough rolling and a rolling temperature of ≤960℃ in the last pass, which is not suitable for the production of thick steel plates.

[0008] The patent document "Thick-walled Low-Temperature Resistant Pipeline Steel and Its Manufacturing Method" (CN116288017A) provides a low-temperature resistant pipeline steel, which also adopts a high Ni (0.35%~0.50%) design.

[0009] In summary, existing technologies still fall short in their research on 600MPa thick-walled high-performance pipeline steel plates and their production technologies that possess comprehensive technical characteristics such as high strength, high uniform elongation, low-temperature toughness, high strain, corrosion resistance, aging resistance, and fatigue resistance, which are suitable for complex conditions. Summary of the Invention

[0010] The purpose of this invention is to overcome the above-mentioned problems and deficiencies and provide a 600MPa high-performance pipeline thick steel plate and its production method, which has comprehensive technical characteristics such as high strength, high uniform elongation, low temperature toughness, high strain, corrosion resistance, aging resistance, and fatigue resistance, as well as an ideal microstructure, to meet the requirements of high-performance pipelines serving under complex conditions.

[0011] The objective of this invention is achieved as follows:

[0012] A 600MPa high-performance pipeline thick steel plate, the composition of which, by weight percentage, is as follows: C: 0.035%~0.065%, Si: 0.15%~0.45%, Mn: 1.60%~1.74%, Nb: 0.030%~0.050%, V≤0.08%, N: 0.0040%~0.010%, Ni: 0.05%~0.15%, Cu: 0. 0.1%~0.15%, Cr: 0.10%~0.30%, Al: 0.005%~0.015%, Ca: 0.0020%~0.0050%, Zr≤0.003%, La≤0.005%, Ti≤0.010%, P≤0.008%, S≤0.0015%, H≤0.00015%, O≤0.0015%, with the balance being iron and unavoidable impurities.

[0013] Furthermore, the steel plate contains (Nb+V) at a concentration of 0.040% to 0.120%.

[0014] Furthermore, in the steel plate, (La / 139+Ca / 40) / (S / 32): 2~7.

[0015] Furthermore, the microstructure of the steel plate includes bainitic ferrite and polygonal ferrite, with the polygonal ferrite comprising 20%–75% by volume and an average grain cross-sectional area ≤90 μm. 2 The matrix contains fine precipitates with a size ≤30nm that are diffusely distributed. The precipitates are composed of nitrides and carbides containing one or both of Nb and V elements.

[0016] Furthermore, the microstructure of the steel plate may also include granular bainite with a volume percentage not exceeding 30%.

[0017] Further details: steel plate thickness 21–40 mm, transverse yield strength 450–530 MPa, transverse tensile strength 620–700 MPa, transverse yield-to-tensile ratio <0.78, average transverse impact energy at -60℃ ≥250 J, average transverse impact energy in the weld heat-affected zone at -20℃ ≥160 J, transverse DWTT shear area at -45℃ ≥85%; longitudinal yield strength 430–510 MPa, longitudinal tensile strength 600–670 MPa, longitudinal yield-to-tensile ratio <0.77, longitudinal uniform elongation U EL ≥10%, longitudinal strain hardening index n≥0.11, CTOD≥0.6mm at -45℃, after aging at 280℃ for 4h, longitudinal yield strength 440~530MPa, longitudinal tensile strength 610~690MPa, longitudinal uniform elongation UEL≥9%, longitudinal yield ratio≤0.78, longitudinal strain hardening index n≥0.10, fatigue life meets the requirement of 20000 cycles under simulated service pressure of 4~10MPa and temperature of -40~50℃ without fatigue failure, and SSCC corrosion resistance meets the requirement of no fracture after 720 hours of saturated H2S solution immersion under 90% stress loading and no visible cracks under 10x magnification.

[0018] A method for producing 600MPa high-performance pipeline thick steel plates includes smelting, continuous casting, heating, rough rolling, finish rolling, and cooling.

[0019] heating:

[0020] The continuously cast billet adopts a multi-stage heating process including preheating, heating stage 1, heating stage 2, heating stage 3, and soaking. The average heating rate in heating stages 1 and 2 is 5-12℃ / min, the average heating rate in heating stage 3 is 2-6℃ / min, the average heating rate in the soaking stage is 0.2-0.5℃ / min, the total heating time is 1.0min / mm to 1.8min / mm, the furnace exit temperature is 1100-1150℃, and the temperature difference between the thickness section is ≤30℃.

[0021] Rough rolling:

[0022] Before roughing, 2 to 4 passes of high-pressure water descaling and cooling are performed, with an average cooling rate of 0.5 to 2℃ / s. Roughing consists of two stages, with each rolling pass undergoing spray cooling. The end temperature of the first stage of roughing is 1050 to 1090℃, and the end temperature of the second stage is 960 to 1040℃. The total deformation rate of the second stage of roughing is ≥40%, with a deformation rate of 17% to 21% per pass and a high-low-high cycle of variable passes. The roughing rolling speed is 1.0 to 2.0 m / s.

[0023] Finishing rolling:

[0024] The finishing rolling process consists of two stages. The initial rolling temperature of the first stage is 800–920℃, and the final rolling temperature of the first stage is 770–820℃. Then, after a 20–90 s interval at which the intermediate temperature is maintained, the second stage of finishing rolling begins, with a final rolling temperature of 720–760℃. The deformation rate during the finishing rolling stage is 60%–80%.

[0025] cool down:

[0026] After rolling, the steel plate undergoes two-stage cooling after pre-straightening. The initial water cooling temperature is 690–730℃, and the end temperature of the steel plate head is 150–240℃. The roller conveyor of the water cooling system first decelerates and then accelerates, with a deceleration rate of 0.01–0.05 m / s. The deceleration transition point is when the steel plate exits the cooling system at the head. The final cooling temperature gradually decreases along the length of the steel plate, and the average final cooling temperature at the head of the steel plate is 20–50℃ higher than that at the tail.

[0027] Furthermore, rapid water cooling is applied between the two stages of rough rolling, with an average cooling rate of 2–5°C / s and a cooling time of 10–40s.

[0028] Furthermore, after rough rolling, the intermediate billet is rapidly cooled to 20-50°C above the starting temperature of the first stage of finishing rolling, with an average cooling rate of 2-10°C / s, and then cooled to the starting temperature of the first stage of finishing rolling.

[0029] Furthermore; during water cooling, the water flow rate of the first 5-7 groups of upper manifolds is 360-550 L / m. 2 •min, the remaining upper manifold water flow rate is 200-300L / m 2 ·min.

[0030] The beneficial effects of this invention are as follows:

[0031] (1) The composition of this invention uses C, Mn, and Cr as the basic strengthening elements to ensure strength; by adding elements such as La, Zr, and Ca to control the inclusions, the effects of removing S and O are increased, the number and size of inclusions are reduced, and they are made more uniformly distributed. At the same time, they play the role of nucleation particles and grain refinement. Combined with ultra-low P, S, H, and O content and continuous casting billet quality control, the toughness and corrosion resistance are effectively improved. By adding Nb and V in combination, controlling the amount of N and reducing the addition of Ti and Al, Nb and V are promoted to play the role of microalloy solid solution, precipitation and grain refinement, which improves strength, aging resistance and weldability and plays a certain role of hydrogen trapping, thus improving corrosion resistance. The addition of a small amount of Ni and Cu balances the improvement of strength and corrosion resistance. The Mo-free design promotes the formation of polygonal ferrite and reduces costs.

[0032] (2) This invention creatively proposes a production process such as smelting, heating, rolling and cooling to obtain high strength, high uniform elongation, low temperature toughness, high strain, as well as corrosion resistance, aging resistance and fatigue resistance and ideal microstructure, so as to meet the requirements of high performance pipelines in complex conditions.

[0033] (3) The present invention adopts production methods such as high-purity smelting, high-homogeneity heating, two-stage low-temperature rolling of rough rolling + high-penetration deformation of thickness section, rapid cooling of intermediate waiting billet, two-stage finishing rolling + intermediate waiting + second-stage low-temperature two-phase region rolling and differentiated water cooling to obtain fine bainitic ferrite + polygonal ferrite, and may also include microstructure of granular bainite with a volume percentage not exceeding 30% and small-sized precipitates dispersedly, so that the steel plate has good comprehensive performance.

[0034] (4) The thick steel plate with a width of 1200-4370mm used for high-performance pipelines operating under complex conditions described in this invention has a thickness of 21-40mm, a transverse yield strength of 450-530MPa, a transverse tensile strength of 620-700MPa, a transverse yield-to-tensile ratio <0.78, an average transverse impact energy of ≥250J at -60℃, an average transverse impact energy of ≥160J at -20℃ in the weld heat-affected zone, and a transverse DWTT shear area of ​​≥85% at -45℃; a longitudinal yield strength of 430-510MPa, a longitudinal tensile strength of 600-670MPa, a longitudinal yield-to-tensile ratio <0.77, and a longitudinal uniform elongation U EL ≥10%, longitudinal strain hardening index n≥0.11, CTOD≥0.6mm at -45℃, after aging at 280℃ for 4h, the longitudinal yield strength can reach 440-530MPa, the longitudinal tensile strength can reach 610-690MPa, and the longitudinal uniform elongation U EL≥9%, longitudinal yield strength ratio ≤0.78, longitudinal strain hardening index n≥0.10, fatigue life meets the requirement of 20,000 cycles without fatigue failure within the simulated service pressure range of 4~10MPa and temperature range of -40~50℃, and SSCC corrosion resistance meets the requirement of no fracture after 720 hours of saturated H2S solution immersion under 90% stress loading and no visible cracks under 10x magnification. Attached Figure Description

[0035] Figure 1 This is a metallographic image of the microstructure of Example 3 of the present invention. Detailed Implementation

[0036] The present invention will be further illustrated below through examples.

[0037] A 600MPa high-performance pipeline thick steel plate, the composition of which, by weight percentage, is as follows: C: 0.035%~0.065%, Si: 0.15%~0.45%, Mn: 1.60%~1.74%, Nb: 0.030%~0.050%, V≤0.08%, N: 0.0040%~0.010%, Ni: 0.05%~0.15%, Cu: 0. 0.1%~0.15%, Cr: 0.10%~0.30%, Al: 0.005%~0.015%, Ca: 0.0020%~0.0050%, Zr≤0.003%, La≤0.005%, Ti≤0.010%, P≤0.008%, S≤0.0015%, H≤0.00015%, O≤0.0015%, with the balance being iron and unavoidable impurities.

[0038] Furthermore, the steel plate contains (Nb+V) of 0.040% to 0.120%.

[0039] Furthermore, in the steel plate, (La / 139+Ca / 40) / (S / 32): 2~7.

[0040] Furthermore, the steel plate CE IIW The concentration ranges from 0.360% to 0.410%, CE Pcm The concentration ranges from 0.145% to 0.185%, of which CE IIW =C+Mn / 6+(Cr+Mo+V) / 5+(Ni+Cu) / 15; CE Pcm =C+Si / 30+(Mn+Cu+Cr) / 20+Ni / 60+Mo / 15+V / 10+5B.

[0041] Furthermore, the microstructure of the steel plate includes bainitic ferrite and polygonal ferrite; the volume percentage of polygonal ferrite is 20% to 75%, and the average grain cross-sectional area of ​​polygonal ferrite is ≤90μm.2 The matrix contains fine precipitates with a size ≤30 nm that are diffusely distributed. The precipitates consist of nitrides and carbides containing one or both of Nb and V elements. Preferably, the microstructure may also include granular bainite with a volume percentage not exceeding 30%.

[0042] Furthermore, the steel plate thickness is 21–40 mm, the transverse yield strength can reach 450–530 MPa, the transverse tensile strength can reach 620–700 MPa, the transverse yield-to-tensile ratio is <0.78, the average transverse impact energy at -60℃ is ≥250 J, the average transverse impact energy in the weld heat-affected zone at -20℃ is ≥160 J, and the transverse DWTT shear area at -45℃ is ≥85%; the longitudinal yield strength can reach 430–510 MPa, the longitudinal tensile strength can reach 600–670 MPa, the longitudinal yield-to-tensile ratio is <0.77, and the longitudinal uniform elongation U EL ≥10%, longitudinal strain hardening index n≥0.11, CTOD≥0.6mm at -45℃, after aging at 280℃ for 4h, the longitudinal yield strength can reach 440~530MPa, the longitudinal tensile strength can reach 610~690MPa, and the longitudinal uniform elongation U EL ≥9%, longitudinal yield strength ratio ≤0.78, longitudinal strain hardening index n≥0.10, fatigue life meets the requirement of 20,000 cycles without fatigue failure within the simulated service pressure range of 4~10MPa and temperature range of -40~50℃, and SSCC corrosion resistance meets the requirement of no fracture after 720 hours of saturated H2S solution immersion under 90% stress loading and no visible cracks under 10x magnification.

[0043] The rationale for the design of the components in this invention is as follows:

[0044] C is a fundamental strengthening and phase transformation control element. It can play a role through interstitial solid solution, precipitation by combining with Nb and V, and promoting microstructure transformation at medium and low temperatures. Therefore, it is necessary to ensure the lower limit of C. However, an increase in C will deteriorate plasticity and toughness, leading to an increase in banded structure and segregation. Therefore, in this invention, C is controlled at 0.035% to 0.065%.

[0045] Si acts as a deoxidizer. Since it is necessary to better facilitate the precipitation of Nb, V and N, this invention adopts a low-Al design. Therefore, Si is used to replace Al to perform the deoxidation function. At the same time, it can also play a solid solution strengthening role. However, if its content is too high, it will cause a decrease in toughness, plasticity and weldability. Therefore, the Si range in this invention is 0.15% to 0.45%.

[0046] Mn is an effective and inexpensive strengthening element and is the basic strengthening element in this invention. It can improve hardenability, increase the uniformity of cooling in the thickness direction of thick-walled steel plates, and has a certain grain refinement effect. However, excessive manganese content can easily form hard phase bands, induce central segregation, increase the non-uniformity of the structure, and reduce toughness, weldability and corrosion resistance. Therefore, in this invention, the Mn content is controlled at 1.60% to 1.74%.

[0047] Nitrogen (Nb) is an excellent element for enhancing strength and toughness. During solution treatment, it acts as a solute drag, and during precipitation, it strengthens the grains and inhibits grain growth by refining them. In particular, under appropriate processes, it can form fine Nb (CN) precipitates, which have beneficial effects such as strengthening, refining grains, and reducing aging tendency. However, excessive Nb content will delay the formation of polygonal ferrite, requiring an increase in the heating temperature of the continuously cast billet to ensure the solution treatment effect. This leads to coarse austenite grains, increased energy consumption, and may also worsen weldability and inhibit the formation of V precipitates. Therefore, the Nb content should be controlled between 0.030% and 0.050%.

[0048] V has the effects of solid solution, precipitation, and grain refinement. During the rolling and cooling of pipeline steel, it can promote ferrite nucleation, refine grains, reduce the formation of central hard phase banded structure, improve the uniformity of microstructure of steel plate thickness section, increase hydrogen trapping, and improve acid corrosion resistance. However, excessive V content is detrimental to toughness and weldability. Therefore, in this invention, V is controlled at ≤0.08%.

[0049] Controlling Nb+V between 0.040% and 0.120% ensures the effects of solid solution, fine grains, and phase transformation control. It also makes the precipitated phase fine and dispersed, while improving the economics of product design.

[0050] N: In this invention, N forms fine precipitates with V and Nb, which play a role in strengthening and refining grains. Therefore, N is a key element in this invention, with a content of not less than 0.0040%. However, if the N content is too high, the precipitate will coarsen, toughness will deteriorate, and hard band structure will increase. Therefore, in this invention, the N content is controlled at 0.0040% to 0.010% by combining the addition of Nb, V and other elements.

[0051] Ni can improve low-temperature toughness and strength, prolong the austenite phase transformation, reduce the critical cooling rate, and inhibit the pearlite transformation; it can also reduce Cu embrittlement and improve corrosion resistance. However, on the one hand, Ni is expensive and adding more will affect economic efficiency; on the other hand, Ni will increase the tendency of HIC in low hydrogen pressure environments. Therefore, the Ni content in this invention is controlled at 0.05% to 0.15%.

[0052] Cu can improve strength and corrosion resistance, and increase hardenability, but too high a Cu content is detrimental to toughness and welding. In this invention, the Cu content is controlled at 0.01% to 0.15%.

[0053] Cr can effectively improve tensile strength and is economical; it can increase hardenability, which is beneficial to improving the uniformity of microstructure in the thickness direction. Moreover, it has a weaker inhibition of the transformation of austenite to polygonal ferrite, which is conducive to increasing the "hardness difference" between soft and hard phases in the microstructure, making it easier to control strain and yield strength ratio; at the same time, it is beneficial to improve corrosion resistance. However, excessive Cr content will deteriorate toughness and weldability. In this invention, the Cr content is controlled at 0.10% to 0.30%.

[0054] Al is a deoxygenating element. Excessive content will promote the increase of inclusions. Moreover, Al will reduce free N and affect the formation of NbN and VN. In this invention, the Al content is controlled at 0.005% to 0.015%.

[0055] Ca can change the composition and morphology of inclusions, improve corrosion resistance, improve toughness and performance uniformity, and is also beneficial to improve weldability. However, excessive Ca content will lead to a decrease in cleanliness. Therefore, the Ca content in this invention is controlled at 0.0020% to 0.0050%.

[0056] Zr can play a role in deoxidation and inclusion control. Zr oxides can achieve a fine and dispersed distribution in molten steel, reducing the harmfulness of inclusions and improving toughness. At the same time, Zr compounds can also promote phase deformation nucleation and refine grains, which is beneficial to improving strength and toughness. In addition, Zr can also provide toughness after welding. However, if the Zr content is too high, the toughness will decrease. Therefore, in this invention, Zr ≤ 0.030%.

[0057] La has a deep purifying effect on molten steel. It has a strong tendency to combine with O and S to form fine, dispersed, and highly stable inclusions, reducing stress concentration and deformation cracking tendency, and effectively improving corrosion resistance. La can also inhibit grain growth and promote microstructure refinement during high-temperature heating and phase transformation, effectively improving low-temperature toughness and weldability. Moreover, La can reduce the diffusion rate of H, increase the density of the corrosion-resistant oxide film on the surface, and thus improve corrosion resistance. However, excessively high La content will reduce the inclusion removal and control effect, and is also detrimental to economic efficiency. Therefore, this invention controls the La content to ≤0.005%.

[0058] The ratio of (La / 139+Ca / 40) / (S / 32) should be controlled between 2 and 7 to achieve a good additive effect, effectively control sulfur compound inclusions, and improve corrosion resistance.

[0059] Ti can form high-melting-point precipitates, thereby inhibiting grain growth and refining grains, which is beneficial to improving toughness; however, excessive Ti content will inhibit the formation of Nb and V precipitates. In this invention, the Ti content is controlled to be ≤0.010%.

[0060] P and S are harmful impurity elements; P reduces low-temperature toughness, and this invention controls P to ≤0.008%; an increase in S content will promote the formation of S-containing inclusions, leading to a decrease in corrosion resistance, therefore, S≤0.0015%.

[0061] Increased H and O content can lead to decreased toughness, increased inclusions, and reduced corrosion resistance. Therefore, this invention controls H ≤ 0.00015% and O ≤ 0.0015%.

[0062] This invention CE IIW Controlled within 0.360% to 0.410%, CE Pcm By controlling the content at 0.145% to 0.185%, the strength and toughness requirements of the steel plate can be met, while also reducing the tendency for welding cracking, thus giving the steel plate good weldability.

[0063] The second technical solution of the present invention is to provide a production method for 600MPa high-performance pipeline thick steel plate, including smelting, continuous casting, heating, rough rolling, finish rolling and cooling;

[0064] Smelting: including converter smelting and ladle refining;

[0065] The converter adopts top and bottom blowing and double slag smelting, with slag blocking during steel tapping. The slag layer thickness is ≤35mm. Lime and fluorite are added in a ratio of 4 / 1 to 5 / 1 to form top slag. The basicity of the refining slag is controlled at 4 to 6, and the weight percentage of FeO+MnO in the slag is controlled to be ≤1%. After refining and deoxidation, Ti, La, and Zr are added in that order. The RH vacuum degree is ≤0.5 torr, and the vacuum treatment time is ≥15 min. After vacuum treatment, nitrogen blowing is carried out according to the cost steel plate N content control requirements.

[0066] Converter double-slag smelting and slag-blocking tapping can ensure low-phosphorus steelmaking and reduce phosphorus and sulfur reversion; controlling slag basicity and FeO+MnO in slag can ensure slag reducing capacity and fluidity, fully desulfurize and deoxidize, and reduce inclusions; the order of adding Ti, La and Zr after refining and deoxidation can more effectively improve the element recovery rate; controlling RH vacuum degree and vacuum treatment time can effectively ensure the treatment effect.

[0067] Continuous casting:

[0068] The quenching time before molten steel is loaded onto the casting machine is ≥10 min; the superheating temperature of the continuously cast billet is 5~35℃; the residence time of molten steel in the tundish during casting is ≥380s; the dynamic light reduction of the billet is ≥5mm; the continuous casting speed control range is 0.6~1.2m / min; the center segregation of the continuously cast billet is ≤C0.5 grade; the center porosity is ≤0.5 grade; and the inclusions of categories A, B, C, and D are ≤1.0 grade.

[0069] Pre-cast steel calming, pouring superheat, and tundish dwell time control can effectively remove inclusions, control segregation, and homogenize steel temperature. Dynamic light reduction and constant casting speed control can reduce segregation, decrease crystallizer surface fluctuations, reduce defect generation tendency, and effectively improve billet quality. Control of center segregation, center porosity, and inclusions can effectively improve service performance such as corrosion resistance.

[0070] heating:

[0071] The continuously cast billet undergoes multi-stage heating, including preheating, heating stage 1, heating stage 2, heating stage 3, and soaking. The average heating rate in heating stages 1 and 2 is 5–12℃ / min, the average heating rate in heating stage 3 is 2–6℃ / min, and the average heating rate in the soaking stage is 0.2–0.5℃ / min. The total heating time is 1.0 min / mm–1.8 min / mm, the tapping temperature is 1100–1150℃, and the temperature difference across the thickness section is ≤30℃. The multi-stage heating and control of the total heating time improve heating efficiency. The relatively low tapping temperature inhibits austenite grain growth and allows the main alloying elements to dissolve, while Nb precipitates remain in a partially dissolved state, further preventing grain growth. Controlling the temperature difference across the thickness section ensures heating uniformity.

[0072] Rough rolling:

[0073] Before roughing, 2-4 passes of high-pressure water descaling and cooling are performed, with an average cooling rate of 0.5-2℃ / s. Roughing consists of two stages, with spray cooling performed in each rolling pass. The end temperature of the first stage of roughing is 1050-1090℃, and the end temperature of the second stage is 960-1040℃. Rapid water cooling is performed between the two stages of roughing, with an average cooling rate of 2-5℃ / s and a cooling time of 10-40s. The total deformation rate of the second stage of roughing is ≥40%, and the deformation rate per pass is 17%-21%, using a high-low-high cycle of variable passes. The roughing rolling speed is 1.0m / s-2.0m / s. High-pressure water descaling and cooling before rough rolling can remove the scale generated during heating. At the same time, it lowers the billet temperature and increases the temperature gradient of the thickness section, which is beneficial for low-temperature rough rolling and deformation penetration. The second stage of rough rolling, which combines rapid water cooling before rolling, low-temperature rolling, and process spray cooling, can achieve low-temperature rolling and high-penetration deformation of the thickness section, improving production efficiency. At the same time, it suppresses the tendency of recrystallized austenite growth. Combined with the deformation rate and low-speed rolling in the second stage of rough rolling, it can achieve beneficial effects such as refining the microstructure near the thickness center and improving the uniformity of microstructure properties. The use of high-low-high cycle pass variables in the second stage of rough rolling helps to reduce the risk of increased rolling passes and reduced pass deformation rate caused by the difference between the actual rolling torque and the controlled simulation torque in the low-temperature rolling stage, ensuring the realization of the low-temperature + high pass deformation rate process.

[0074] Finishing rolling:

[0075] After rough rolling, the intermediate preheated billet is rapidly cooled to 20–50°C above the finishing rolling start temperature, with an average cooling rate of 2–10°C / s. Then, it is allowed to reach the finishing rolling start temperature. Finish rolling consists of two stages: the first stage starts at 800–920°C, and the final rolling temperature of the first stage is 770–820°C. Then, after an intermediate warming period of 20–90 seconds, the second stage of finishing rolling begins, with a final rolling temperature of 720–760°C. The deformation rate during the finishing rolling stage is 60%–80%.

[0076] The deformation rate in the finishing rolling stage ensures sufficient deformation of austenite in the finishing rolling zone, increasing deformation energy and nucleation sites, and refining grains; rapid cooling of the intermediate warming billet can effectively suppress austenite growth in the high-temperature zone; the finishing rolling stage adopts a two-stage + intermediate warming + second-stage low-temperature two-phase zone rolling, which on the one hand can promote the precipitation of carbonitrides such as Nb and V, playing a role in strengthening, refining grains, and pinning; on the other hand, it can obtain a sufficient proportion of polygonal ferrite, improving plasticity, toughness and strain performance.

[0077] cool down:

[0078] After rolling, the steel plates undergo two-stage cooling after pre-straightening. The initial water cooling temperature is 690–730℃, and the final water cooling temperature at the top of the steel plate is 150–240℃. During water cooling, the water flow rate in the first 5–7 groups of upper manifolds is 360 L / m. 2·min~550L / m 2 The remaining water flow rate in the upper manifold is 200 L / m. 2 ·min~300L / m 2 In the water-cooling system, the roller conveyor first decelerates and then accelerates, with a deceleration rate of 0.01–0.05 m / s. The deceleration transition point is at the head of the steel plate exiting the cooling system. The final cooling temperature gradually decreases along the length of the steel plate, with the average final cooling temperature at the head being 20–50°C higher than that at the tail. Post-rolling pre-straightening helps improve the cooling uniformity of the steel plate. Controlling the start and end temperatures of water cooling ensures the acquisition of ideal microstructure and substructure. Two-stage cooling can suppress high-temperature phase transformation and refine the hard phase structure, while also reducing internal stress, which is beneficial for shape control. The temperature difference between the head and tail of the steel plate corresponds to the difference in water cooling at different positions of the steel plate, which helps improve performance uniformity.

[0079] According to the component ratio of the technical solution, the embodiments of the present invention carry out smelting, continuous casting, heating, rough rolling, finish rolling and cooling.

[0080] The composition of the steel in this embodiment of the invention is shown in Table 1. The main process parameters for smelting the steel in this embodiment of the invention are shown in Table 2. The main process parameters for continuous casting of the steel in this embodiment of the invention are shown in Table 3. The main process parameters for heating the steel in this embodiment of the invention are shown in Table 4. The main process parameters for rough rolling of the steel in this embodiment of the invention are shown in Table 5. The main process parameters for finish rolling of the steel in this embodiment of the invention are shown in Table 6. The main process parameters for cooling the steel in this embodiment of the invention are shown in Table 7. The microstructure of the steel in this embodiment of the invention is shown in Table 8. The mechanical properties of the steel in this embodiment of the invention are shown in Table 9. The longitudinal tensile properties after aging are shown in Table 10. The fatigue resistance and corrosion resistance of the steel in this embodiment of the invention are shown in Table 11.

[0081] Table 1. Composition (wt%) of steel in embodiments of the present invention

[0082] Example C Si Mn Nb V Nb+V N Ni Cu Cr Al 1 0.041 0.28 1.68 0.038 0.045 0.083 0.0072 0.12 0.08 0.22 0.008 2 0.055 0.18 1.71 0.045 0.025 0.070 0.0048 0.07 0.06 0.15 0.012 3 0.062 0.42 1.62 0.030 0.075 0.105 0.0095 0.14 0.11 0.23 0.006 4 0.037 0.15 1.74 0.050 0.032 0.082 0.0049 0.05 0.09 0.16 0.014 5 0.049 0.33 1.65 0.042 0.063 0.105 0.0081 0.10 0.13 0.19 0.010 6 0.035 0.45 1.74 0.048 0 0.048 0.0055 0.06 0.11 0.12 0.015 7 0.042 0.22 1.60 0.038 0.080 0.118 0.0092 0.15 0.15 0.30 0.007 8 0.056 0.39 1.67 0.035 0.052 0.087 0.0063 0.13 0.09 0.25 0.011 9 0.046 0.27 1.69 0.032 0 0.032 0.0055 0.12 0.10 0.21 0.009 Example Ca Zr La Ti P S H O <![CDATA[CE IIW ]]> <![CDATA[CE Pcm ]]> A 1 0.0038 0.0012 0.0018 0.005 0.005 0.0012 0.00009 0.0012 0.387 0.156 2.9 2 0.0025 0 0 0.008 0.006 0.0010 0.00011 0.0015 0.384 0.161 3.2 3 0.0045 0.0028 0.0037 0 0.005 0.0010 0.00012 0.0010 0.410 0.184 6.6 4 0.0023 0.0010 0 0.005 0 0.0013 0.00011 0.0012 0.375 0.146 2.3 5 0.0042 0.0021 0.0032 0.007 0.005 0.0010 0.00010 0.0013 0.390 0.166 6.1 6 0.0029 0.0015 0 0.010 0.007 0.0011 0.00009 0.0012 0.360 0.150 3.4 7 0.0048 0 0.0045 0 0.008 0.0012 0.00015 0.0010 0.405 0.162 6.0 8 0.0035 0.0022 0.0027 0.006 0.007 0.0012 0.00008 0.0012 0.409 0.177 4.2 9 0.0037 0.0015 0.0034 0.009 0.006 0.0013 0.00010 0.0010 0.384 0.157 4.2

[0083] Note: A = (La / 139 + Ca / 40) / (S / 32)

[0084] Table 2 Main process parameters for steelmaking in the embodiments of the present invention

[0085] Example Top slag limestone / fluorite slag alkalinity FeO + MnO weight percentage in slag / % RH vacuum degree / torr Vacuum processing time / min 1 4.5 4.8 0.90 0.5 18 2 4.8 5.3 0.82 0.5 15 3 4.1 4.6 0.94 0.4 23 4 4.3 4.5 0.92 0.5 19 5 4.0 4.9 0.85 0.3 20 6 4.3 5.2 0.80 0.5 23 7 4.5 4.7 0.82 0.4 25 8 4.6 5.0 0.91 0.4 19 9 4.5 4.9 0.86 0.5 21

[0086] Table 3 Main process parameters for continuous steel casting and heating in the embodiments of the present invention.

[0087] Example Pre-cure time for molten steel / min Casting superheat / °C Intermediate package dwell time / s Dynamic reduction in continuous casting / mm Continuous casting billet casting speed / m / min Center segregation / grade of continuously cast billet Porosity at the center of continuously cast billet / grade The highest level of inclusions is classified as A, B, C, and D. 1 20 18 390 5.5 1.0 C0.5 0 0.5 2 21 23 392 5.8 1.0 C0.5 0 0.5 3 18 12 385 5.2 1.0 C0.5 0 1.0 4 15 25 397 5.9 1.0 C0.5 0 1.0 5 17 21 388 5.3 0.85 C0.5 0.5 1.0 6 15 15 390 5.2 0.85 C0.5 0.5 0.5 7 12 11 383 5.3 0.85 C0.5 0 0.5 8 20 24 405 5.5 0.85 C0.5 0 1.0 9 17 20 394 5.3 1.0 C0.5 0.5 0.5

[0088] Table 4 Main process parameters for steel heating in embodiments of the present invention

[0089] Example Average heating rate in heating stages 1 and 2 / °C / min Average heating rate in three stages / °C / min Average heating rate of the soaking section / °C / min Total heating time / min / mm Furnace temperature / ℃ Thickness section temperature difference / ℃ 1 9.1 5.3 0.22 1.3 1137 17 2 5.8 4.8 0.47 1.7 1121 13 3 8.6 5.1 0.36 1.0 1128 21 4 6.2 3.0 0.45 1.6 1125 19 5 7.0 4.7 0.30 1.5 1119 15 6 9.5 5.0 0.24 1.2 1141 24 7 8.3 4.3 0.21 1.3 1149 17 8 7.9 5.6 0.47 1.0 1135 21 9 6.6 4.0 0.38 1.5 1140 16

[0090] Table 5 Main process parameters for steel roughing in embodiments of the present invention

[0091] Example Number of high-pressure water descaling and cooling passes Average cooling rate of high-pressure water dephosphorization / ℃ / s First-stage roughing finishing temperature / ℃ Second-stage roughing finishing temperature / ℃ Average cooling rate of rapid water cooling between the two stages / ℃ / s Rapid water cooling time between two stages / s Total deformation rate in the second stage of rough rolling ≥ 40% / % Deformation rate per pass in the second stage of rough rolling / % Roughing rolling speed / m / s 1 4 1.4 1058 998 2.3 18 58 17～20 1.8 2 2 0.6 1052 1001 2.4 15 60 18～19 1.8 3 3 1.0 1060 972 2.1 33 56 18～20 1.8 4 2 0.6 1063 988 2.2 25 49 17～20 1.8 5 2 0.7 1065 997 2 21 52 17～21 1.2 6 3 0.9 1072 981 2 36 48 17～19 1.2 7 4 1.2 1070 986 2.2 27 42 17～21 1.2 8 4 1.1 1061 1022 2.1 13 45 18～20 1.2 9 4 1.2 1065 994 2.4 18 48 18～20 1.8

[0092] Table 6 Main process parameters for steel finishing in embodiments of the present invention

[0093] Example Deformation rate during finishing rolling stage / % Termination temperature of rapid cooling of intermediate preform at ℃ B / ℃ Average cooling rate / ℃ / s First-stage finishing rolling temperature / ℃ First-stage finishing rolling temperature / ℃ Intermediate waiting time / s Two-stage finishing rolling temperature / ℃ 1 76 935 34 3.1 901 796 65 750 2 73 942 35 3.6 907 805 78 746 3 70 948 34 2.2 914 798 60 755 4 67 910 45 4.7 865 785 46 741 5 70 897 44 6.3 853 791 38 758 6 68 892 32 6.1 860 782 41 745 7 65 869 42 8 827 774 32 734 8 67 874 40 9.2 834 781 39 741 9 70 905 44 3.1 861 788 35 751

[0094] Note: B = intermediate billet temperature - first-stage finishing rolling temperature

[0095] Table 7 Main process parameters for steel cooling in embodiments of the present invention

[0096] Example Initial water cooling temperature / °C Water-cooled steel plate head temperature / ℃ <![CDATA[Water volume of the first 5 - 7 upper headers / L / m 2 *min]]> <![CDATA[Remaining upper header water volume / L / m 2 *min]]> Average final cooling temperature difference from head to tail along length (°C) 1 712 189 6 groups 520 210 22 2 705 226 7 groups 390 230 39 3 718 220 5 groups 450 270 35 4 711 175 6 groups 520 240 30 5 721 192 6 groups of 500 250 25 6 712 183 7 groups 480 230 28 7 710 195 6 groups 530 270 21 8 716 177 7 groups 510 280 25 9 718 180 5 groups 520 270 33

[0097] Table 8 Microstructure of steel in embodiments of the present invention

[0098] Example Polygonal ferrite volume percentage / % Granular bainite volume percentage / % Average grain cross-sectional area of ​​polygonal ferrite / μm2 Plate thickness / mm 1 50 17 48 21 2 62 21 53 21 3 35 28 42 21 4 48 0 55 26.2 5 31 15 51 26.2 6 45 8 62 32 7 58 12 73 32 8 46 6 65 32 9 36 9 102 26.2

[0099] Table 9 Mechanical properties of steel in the embodiments of the present invention

[0100] Example - Direction Rt0.5 / MPa Rm / MPa Rt0.5 / Rm UEL / % Steel plate KV-60℃ave / J Welding hot zone KV-20℃ave / J DWTTSA-45℃ave / % CTOD-45℃ave / mm n 1-Horizontal 505 675 0.75 -- 276 187 91 -- -- 1-Vertical 475 655 0.73 11.5 -- -- -- 0.83 0.11 2-Horizontal 485 650 0.75 -- 295 176 92 -- -- 2-Vertical 450 635 0.71 13.0 -- -- -- 0.87 0.13 3-Horizontal 520 685 0.76 -- 258 180 89 -- -- 3-Vertical 500 665 0.75 11.0 -- -- -- 0.90 0.11 4-Horizontal 500 660 0.76 -- 302 192 87 -- -- 4-Vertical 475 650 0.73 12.0 -- -- -- 0.82 0.13 5-Horizontal 515 675 0.76 -- 274 201 87 -- -- 5-Vertical 500 665 0.75 12.0 -- -- -- 0.75 0.12 6-Horizontal 480 640 0.75 -- 283 205 89 -- -- 6-Vertical 465 620 0.75 13.0 -- -- -- 0.90 0.12 7-Horizontal 475 650 0.73 -- 276 218 87 -- -- 7-Vertical 450 630 0.71 13.5 -- -- -- 0.96 0.13 8-Horizontal 490 655 0.75 -- 289 195 86 -- -- 8-Vertical 460 630 0.73 12.5 -- -- -- 0.79 0.12 9-Horizontal 505 665 0.76 -- 228 163 85 -- -- 9-Vertical 485 650 0.75 11.0 -- -- -- 0.63 0.10

[0101] Note: The tensile specimen is a full-thickness rectangular specimen with a parallel test section width of 38.1 mm; the impact specimen size is 10*55*55 mm.

[0102] Table 10. Longitudinal tensile properties of embodiments of the present invention after aging.

[0103] Example Rt0.5 / MPa Rm / MPa Rt0.5 / Rm UEL / % n 1 485 660 0.73 11.5 0.11 2 455 635 0.72 12.5 0.12 3 515 670 0.77 10.5 0.10 4 490 660 0.74 11.0 0.11 5 505 670 0.75 11.5 0.11 6 480 625 0.77 12.0 0.12 7 460 630 0.73 13.0 0.12 8 465 630 0.74 13.0 0.12 9 495 650 0.76 10.5 0.10

[0104] Note: The aging process involves holding the temperature at 280℃ for 4 hours.

[0105] Table 11 Fatigue resistance and corrosion resistance of the steel of this invention

[0106] Example fatigue test Anti-SSCC test 1 qualified qualified 2 qualified qualified 3 qualified qualified 4 qualified qualified 5 qualified qualified 6 qualified qualified 7 qualified qualified 8 qualified qualified 9 qualified qualified

[0107] Notes: Fatigue test: 20,000 fatigue cycles were performed under simulated service pressure of 4-10 MPa and temperature of -40-50℃; SSCC resistance test: No fracture occurred after immersion in saturated H2S solution (solution A) for 720 hours under 90% stress loading conditions, and no visible cracks were observed when magnified 10 times.

[0108] As can be seen from the above, the thickness of the wide and thick steel plates produced using this invention is 21-40mm, the transverse yield strength can reach 450-530MPa, the transverse tensile strength can reach 620-700MPa, the transverse yield-to-tensile ratio is <0.78, the average transverse impact energy at -60℃ is ≥250J, the average transverse impact energy at -20℃ in the weld heat-affected zone is ≥160J, and the transverse DWTT shear area at -35℃ is ≥85%; the longitudinal yield strength can reach 430-510MPa, the longitudinal tensile strength can reach 600-670MPa, the longitudinal yield-to-tensile ratio is <0.77, and the longitudinal uniform elongation U EL With a longitudinal strain hardening index n≥0.11, after aging at 250℃ for 1 hour, the longitudinal yield strength can reach 440~530MPa, the longitudinal tensile strength can reach 610~690MPa, and the longitudinal uniform elongation U EL The steel plate exhibits a strength ≥9%, a longitudinal yield strength ratio ≤0.78, a longitudinal strain hardening index n≥0.10, and SSCC corrosion resistance meeting the requirement of not fracturing after 720 hours of immersion in saturated H2S solution under 90% stress loading and showing no visible cracks under 10x magnification. The microstructure of the steel plate includes bainitic ferrite, polygonal ferrite, and may also include a small amount of granular bainite. The volume percentage of polygonal ferrite is 20%–75%, and the average grain cross-sectional area of ​​polygonal ferrite is ≤90μm. 2 The matrix contains fine precipitates with a size ≤30nm that are diffusely distributed.

[0109] To illustrate the present invention, the present invention has been appropriately and sufficiently described above through embodiments. The above embodiments are only for illustrating the present invention and are not intended to limit the present invention. Those skilled in the art can make various changes and modifications without departing from the spirit and scope of the present invention. Any modifications, equivalent substitutions, improvements, etc., should be included within the protection scope of the present invention. The patent protection scope of the present invention should be defined by the claims.

Claims

1. A 600MPa high-performance pipeline wide and thick steel plate, characterized in that, The composition of the steel plate, by weight percentage, is as follows: C: 0.035%–0.065%, Si: 0.15%–0.45%, Mn: 1.60%–1.74%, Nb: 0.030%–0.050%, V≤0.08%, N: 0.0040%–0.010%, Ni: 0.05%–0.15%, Cu: 0.01%–0.15%, Cr: 0.10%–0.30%, Al: 0.005%–0.015%. %, Ca: 0.0020%~0.0050%, Zr≤0.003%, La≤0.005%, Ti≤0.010%, P≤0.008%, S≤0.0015%, H≤0.00015%, O≤0.0015%, with the balance being iron and unavoidable impurities; the microstructure of the steel plate includes bainitic ferrite and polygonal ferrite, with polygonal ferrite comprising 20%~75% by volume and an average grain cross-sectional area ≤90μm. 2 The matrix contains fine precipitates with a size ≤30nm that are diffusely distributed. The precipitates are composed of nitrides and carbides containing one or two of Nb and V elements.

2. The 600MPa high-performance pipeline thick steel plate according to claim 1, characterized in that, The steel plate contains (Nb+V): 0.040%~0.120%.

3. The 600MPa high-performance pipeline thick steel plate according to claim 1, characterized in that, In the steel plate, (La / 139+Ca / 40) / (S / 32): 2~7.

4. The 600MPa high-performance pipeline thick steel plate according to claim 1, characterized in that, The microstructure of steel plates may also include granular bainite with a volume percentage not exceeding 30%.

5. The 600MPa high-performance pipeline thick steel plate according to claim 1, characterized in that, Steel plate thickness 21–40 mm, transverse yield strength 450–530 MPa, transverse tensile strength 620–700 MPa, transverse yield-to-tensile ratio <0.78, average transverse impact energy at -60℃ ≥250 J, average transverse impact energy in the weld heat-affected zone at -20℃ ≥160 J, transverse DWTT shear area at -45℃ ≥85%; longitudinal yield strength 430–510 MPa, longitudinal tensile strength 600–670 MPa, longitudinal yield-to-tensile ratio <0.77, longitudinal uniform elongation U EL ≥10%, longitudinal strain hardening index n≥0.11, CTOD≥0.6mm at -45℃, after aging at 280℃ for 4h, longitudinal yield strength 440~530MPa, longitudinal tensile strength 610~690MPa, longitudinal uniform elongation U EL ≥9%, longitudinal yield strength ratio ≤0.78, longitudinal strain hardening index n≥0.10, fatigue life meets the requirement of 20,000 cycles without fatigue failure within the simulated service pressure range of 4~10MPa and temperature range of -40~50℃, and SSCC corrosion resistance meets the requirement of no fracture after 720 hours of saturated H2S solution immersion under 90% stress loading and no visible cracks under 10x magnification.

6. A method for producing a 600MPa high-performance pipeline thick steel plate as described in any one of claims 1-5, characterized in that: Including smelting, continuous casting, heating, rough rolling, finish rolling, and cooling; heating: The continuously cast billet adopts a multi-stage heating process including preheating, heating stage 1, heating stage 2, heating stage 3, and soaking. The average heating rate in heating stages 1 and 2 is 5-12℃ / min, the average heating rate in heating stage 3 is 2-6℃ / min, the average heating rate in the soaking stage is 0.2-0.5℃ / min, the total heating time is 1.0min / mm to 1.8min / mm, the furnace exit temperature is 1100-1150℃, and the temperature difference between the thickness section is ≤30℃. Rough rolling: Before roughing, 2 to 4 passes of high-pressure water descaling and cooling are performed, with an average cooling rate of 0.5 to 2℃ / s. Roughing consists of two stages, with each rolling pass undergoing spray cooling. The end temperature of the first stage of roughing is 1050 to 1090℃, and the end temperature of the second stage is 960 to 1040℃. The total deformation rate of the second stage of roughing is ≥40%, with a deformation rate of 17% to 21% per pass and a high-low-high cycle of variable passes. The roughing rolling speed is 1.0 to 2.0 m / s. Finishing rolling: The finishing rolling process consists of two stages. The initial rolling temperature of the first stage is 800–920℃, and the final rolling temperature of the first stage is 770–820℃. Then, after a 20–90 s interval, the second stage of finishing rolling begins, with a final rolling temperature of 720–760℃. The deformation rate during the finishing rolling stage is 60%–80%. cool down: After rolling, the steel plate undergoes two-stage cooling after pre-straightening. The initial water cooling temperature is 690–730℃, and the end temperature of the steel plate head is 150–240℃. The roller conveyor of the water cooling system first decelerates and then accelerates, with a deceleration rate of 0.01–0.05 m / s. The deceleration transition point is when the steel plate head exits the cooling system. The final cooling temperature gradually decreases along the length of the steel plate, and the average final cooling temperature at the head of the steel plate is 20–50℃ higher than that at the tail.

7. The method for producing 600MPa high-performance pipeline thick steel plates according to claim 6, characterized in that: Rapid water cooling is applied between the two stages of rough rolling, with an average cooling rate of 2–5℃ / s and a cooling time of 10–40s.

8. The method for producing 600MPa high-performance pipeline thick steel plates according to claim 6, characterized in that: After rough rolling, the intermediate billet is rapidly cooled to 20-50°C above the first stage rolling temperature of finishing rolling, with an average cooling rate of 2-10°C / s. Then, it is allowed to warm up to the first stage rolling temperature of finishing rolling.

9. The method for producing 600MPa high-performance pipeline thick steel plates according to claim 6, characterized in that: When water-cooled, the water flow rate of the first 5 to 7 groups of upper manifolds is 360 to 550 L / m. 2 •min, the remaining upper manifold water flow rate is 200-300L / m 2 ·min.

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