A low-carbon-equivalent high-toughness normalized steel plate for extra-thick offshore wind power and a manufacturing method thereof

Abstract

This invention discloses a low-carbon equivalent extra-thick high-toughness normalized steel plate for offshore wind power and its manufacturing method. The steel plate is 100-150mm thick, and its chemical composition by weight percentage is C: 0.12-0.18%, Si: 0.20-0.40%, Mn: 1.20-1.60%, P≤0.020%, S≤0.005%, combined with trace amounts of Ni, Mo, and Cr tempering and Nb+V+Ti ternary composite microalloying, with strict control of B≤0.0005% and carbon equivalent CEV≤0.43%. The steel plate has a yield strength ≥355MPa, tensile strength 470-560MPa, and a single Charpy impact energy of -50℃ in both transverse and longitudinal directions ≥50J. The manufacturing method includes smelting and casting, heating of continuously cast billets, rolling, and normalizing heat treatment. The rolling compression ratio is ≥3.0, the reduction rate of the last three passes of rough rolling is >9%, the finishing rolling temperature is 780-820℃, the normalizing temperature is 870-900℃, and the holding time coefficient is 1.5-2.5 min / mm. This invention solves the problems of insufficient strength and toughness and uneven thickness properties of extra-thick plates with low carbon equivalent. It has good weldability, low production cost, and is suitable for offshore wind power monopile foundation structural components, and can be mass-produced.

Description

Technical Field

[0001] This invention belongs to the field of iron and steel metallurgy technology, and relates to a low-carbon equivalent extra-thick normalized steel plate for offshore wind power and its manufacturing method. Background Technology

[0002] With the development of the global clean energy industry, offshore wind power projects are moving towards the deep sea. As a core structural component, the service environment of offshore wind power foundation piles is more severe than that of onshore wind farms. They must not only withstand seawater corrosion and wave impact, but also adapt to the low temperature (-50℃) environment at sea, which puts forward stringent requirements on the performance of structural steel.

[0003] Offshore wind turbine monopile foundations have become the mainstream foundation type due to their simple manufacturing process, quick installation, and low cost. To reduce overall manufacturing costs, the raw material steel plates used for monopiles tend to be extra-thick steel plates of 100-150mm with large single weight. These extra-thick steel plates must simultaneously meet the following core performance requirements: 1) Pass flaw detection and have good resistance to lamellar tearing; 2) Excellent impact toughness at -50℃; 3) Low carbon equivalent to ensure good weldability and avoid welding cracks; 4) Uniform properties in the thickness direction with no obvious segregation.

[0004] In existing technologies, research and development of steel for offshore wind power mainly focuses on conventional thicknesses below 80mm. For extra-thick steel plates of 100-150mm, the following technical challenges exist: 1) Although low-carbon equivalent composition design can improve weldability, it is difficult to guarantee the strength and low-temperature toughness of the steel plate under normalizing heat treatment; 2) During the rolling process of extra-thick plates, the reduction per pass is limited, deformation penetration is insufficient, and the grain structure after rolling is coarse with large differences in grain size in the thickness direction. After normalizing heat treatment, the structure inheritance is prone to occur, and the problem of coarse grains cannot be solved; 3) Existing extra-thick plates mostly use quenching and tempering treatment (quenching + tempering) to improve performance. Although it can guarantee strength and toughness, the production cost is high, and the high amount of alloy additives leads to a high carbon equivalent, which reduces weldability; 4) Domestic medium and heavy plate manufacturers have not yet formed a mature production plan for offshore wind power steel plates with "low carbon equivalent + extra-thick specifications + normalizing process". The uniformity of performance in the thickness direction and the low-temperature toughness are difficult to meet the engineering requirements.

[0005] Therefore, developing a low-carbon equivalent, 100-150mm thick, normalized steel plate with high and low temperature toughness for offshore wind power, and solving the problems of difficulty in balancing weldability and toughness and uneven thickness properties of ultra-thick plates in existing technologies, has become an urgent technical issue to be addressed in this field. Summary of the Invention

[0006] The technical problem to be solved by the present invention is to provide a low-carbon equivalent extra-thick normalized steel plate for offshore wind power and its manufacturing method, which is in contrast to the above-mentioned prior art. The steel plate has a thickness of 100-150mm, is suitable for offshore wind power monopile structural components, can be used in low-temperature offshore environments, has good weldability, and has high strength and good low-temperature toughness.

[0007] The technical solution adopted by this invention to solve the above problems is as follows: a low-carbon equivalent extra-thick structural steel plate for offshore wind turbine monopiles, wherein the chemical composition of the steel plate, by weight percentage, is C: 0.12-0.18%, Si: 0.20-0.40%, Mn: 1.20-1.60%, P: ≤0.020%, S: ≤0.005%, Ni: 0.05-0.20%, Mo: 0.04-0.10%, Cr: 0.05-0.20%, Nb: 0.01-0.04%, V: 0.01-0.03%, Ti: 0.01-0.03%, Alt: 0.02-0.05%, B: ≤0.0005%, with the balance being Fe and unavoidable impurity elements. The carbon equivalent (CEV) of the steel plate is ≤0.43%, and the carbon equivalent is calculated using the formula: CEV(%)=C+Mn / 6+(Cr+Mo+V) / 5+(Ni+Cu) / 15; the thickness of the steel plate is 100-150mm, the yield strength is ≥355MPa, the tensile strength is 470MPa-560MPa, and the single value of transverse and longitudinal Charpy impact energy at -50℃ at 1 / 4 of the thickness is ≥50J.

[0008] The composition design of this invention focuses on low carbon and high manganese, trace composite micro-alloying, and strict control of harmful elements. The role and content limitation principle of each element are as follows: C: 0.12~0.18%. Carbon is the core element to ensure the strength of steel plates. If the content is too low, the strength of the steel plate will be insufficient. If the content is too high, it will increase the hardening tendency of steel, increase the carbon equivalent, reduce weldability, and easily form pearlite banded structure, which will reduce low temperature toughness. Therefore, it should be controlled in the low carbon range of 0.12~0.18%.

[0009] Si: 0.20-0.40%, as a deoxidizer and solid solution strengthening element, can improve the strength of steel plate. If the content is too low, the deoxidation effect is not good, and if the content is too high, it will reduce the plasticity and toughness of steel plate, especially the low temperature toughness. Therefore, it is limited to 0.20-0.40%.

[0010] Mn: 1.20~1.60%, as the main solid solution strengthening and grain refining element, can significantly improve the strength and low temperature toughness of steel plates, while inhibiting the formation of pearlite banded structure; the high manganese combined with low carbon design ensures strength while avoiding excessive carbon equivalent, so it is controlled at 1.20~1.60%.

[0011] P and S: P≤0.020%, S≤0.005%. P and S are harmful elements in steel. P can cause cold brittleness and reduce low-temperature toughness. S can form MnS inclusions, which can reduce the steel’s resistance to lamellar tearing and its toughness. Therefore, their content must be strictly controlled, especially S should be controlled below 0.005% to improve the steel plate’s resistance to lamellar tearing.

[0012] Ni: 0.05-0.20%. Trace amounts of Ni can improve the low-temperature toughness of steel and improve the plasticity-toughness ratio of steel. It has little effect on carbon equivalent and will not reduce weldability. However, excessive content will increase production costs. Therefore, it is limited to 0.05-0.20%.

[0013] Mo and Cr: Mo: 0.04-0.10%, Cr: 0.05-0.20%. Trace amounts of Mo and Cr can refine austenite grains, improve the strength and tempering stability of steel, and further enhance the strength and toughness of steel plates when combined with normalizing process. However, excessive content will increase the carbon equivalent and reduce weldability.

[0014] Nb, V, Ti: Nb: 0.01-0.04%, V: 0.01-0.03%, Ti: 0.01-0.03%. A ternary composite microalloying design is employed. Nb inhibits austenite grain growth, refining the grains after rolling and normalizing; V forms carbonitrides, resulting in precipitation strengthening and increased strength; Ti combines with N to form TiN, fixing N in the steel and preventing B from combining with N to form BN, thus reducing the effect of B. Simultaneously, TiN refines austenite grains. The synergistic effect of these three compounds achieves both grain refinement and precipitation strengthening, significantly improving the strength, toughness, and thickness-direction uniformity of the steel plate.

[0015] Alt: 0.02~0.05%, as a final deoxidizer, can effectively remove oxygen in steel and form fine Al2O3 inclusions, avoiding the influence of coarse oxide inclusions on the toughness of steel plates. At the same time, Alt can refine grains and improve low-temperature toughness.

[0016] B: ≤0.0005%. Although B can improve the hardenability of steel, excessive content will increase the hardening tendency of steel, increase the sensitivity to welding cracks, and easily combine with N to form BN, which reduces the toughness of steel. Therefore, the content of B should be strictly controlled to ≤0.0005%.

[0017] Carbon Equivalent (CEV) ≤ 0.43%: By limiting the content of each element, the carbon equivalent is controlled at ≤ 0.43%, ensuring good weldability of the steel plate. No preheating is required or only low-temperature preheating is needed during welding, which meets the on-site welding requirements of offshore wind power monopile foundations.

[0018] The manufacturing method of the above-mentioned low-carbon equivalent extra-thick high-toughness normalized steel plate for offshore wind power includes the following steps: Smelting and Casting: A pure steel smelting process is adopted, using a combination of converter smelting + LF refining + RH vacuum treatment to control the A, B, C and D non-metallic inclusions in the molten steel to ≤1.5 grade, reducing the impact of inclusions on the toughness of the steel plate; after RH vacuum breaking, the soft blowing argon time is ≥20min to achieve homogenization of steel composition and temperature and remove microbubbles in the steel; the superheat of molten steel in the continuous casting process is 20-30℃ to avoid excessive superheat leading to coarse columnar crystals and severe segregation in the continuous casting billet; an argon-protected casting process + dynamic light reduction technology is used to suppress the generation of center segregation in the continuous casting billet, ensuring that the center segregation of C grade is ≤1.0 grade and porosity is ≤0.5 grade; after the continuous casting billet comes off the line, it undergoes slow cooling treatment for 12-24h at a slow cooling temperature of 300-400℃ to eliminate internal stress in the billet and prevent cracking.

[0019] Heating of continuously cast billets: The continuously cast billets are heated in stages using a walking beam furnace, with a total heating time of 300-550 min to ensure uniform internal temperature of the billet and avoid underheating or overheating. The temperature of the second heating section is 1175-1245℃, with a heating rate of 5-8℃ / min, to achieve the initial formation of austenitization in the billet. The temperature of the soaking section is 1165-1235℃, with a temperature fluctuation range of ≤±10℃. The total heating time of the second heating section and the soaking section is ≥200 min to ensure sufficient homogenization of austenite in the billet, eliminate segregation in the billet structure, and provide a uniform austenitic structure for subsequent rolling.

[0020] Rolling: A medium-thick plate rolling mill is used, with a rolling compression ratio ≥3.0 to ensure sufficient deformation of the cast billet and refine the grain size. Rolling is divided into two stages: roughing and finishing. Rough rolling stage: High penetration rolling process is adopted, with an initial rolling temperature of 1120-1130℃ and a final rolling temperature of 950-960℃. The reduction rate of the last three passes is >9%. By forcibly increasing the penetration of rolling deformation in the thickness direction of the steel plate through the large reduction rate, the coarse columnar crystals of the billet are broken, the microstructure difference in the thickness direction is reduced, and the initial grain refinement is achieved. Finishing rolling stage: The thickness of the steel plate to be heated is 2 to 3 times the thickness of the finished steel plate, the finishing rolling temperature is 780 to 820℃, the reduction rate per pass is 5 to 8%, and the total reduction rate is ≥40%. Under the condition that the rolling force is sufficient, the rolling temperature should be reduced as much as possible to increase the deformation energy of the steel, so that austenite can generate a large number of deformation bands, which provides nucleation cores for grain refinement in subsequent normalizing treatment, while avoiding excessive finishing rolling temperature that leads to grain growth.

[0021] Normalizing heat treatment: The rolled steel plate is sent to a normalizing furnace for normalizing heat treatment. The heating rate is 3-5℃ / min to avoid excessive internal stress caused by rapid heating. The normalizing temperature is 870-900℃, which ensures sufficient homogenization of austenite in the steel plate while preventing austenite grain growth. The holding time coefficient is 1.5-2.5min / mm, and the holding time is calculated according to the thickness of the steel plate to ensure sufficient heat penetration in the thickness direction and uniform austenitization. After holding, the plate is removed from the furnace and air-cooled at a cooling rate of 8-15℃ / min to room temperature, so that the austenite transforms into a fine and uniform ferrite + pearlite structure, achieving secondary grain refinement. After air cooling, the steel plate is subjected to surface flaw detection, and the flaw detection standard meets the requirements of BS EN10160 standard S2E3, resulting in the final low carbon equivalent extra-thick high toughness normalized steel plate for offshore wind power.

[0022] The manufacturing process of this invention is based on "composition design + process synergy". Through full-process process control of smelting, heating, rolling and normalizing, it solves the problems of coarse grains, uneven thickness properties and insufficient strength and toughness under low carbon equivalent in extra-thick plates. The process design principle is as follows: Smelting and casting: Pure steel smelting process is adopted to reduce the content of inclusions. Soft blowing argon, low superheat continuous casting, argon-protected casting + dynamic light reduction technology eliminate segregation and porosity of continuous casting billets from the source, ensure the internal quality of billets, and provide high-quality billets for subsequent rolling and heat treatment; slow cooling treatment eliminates internal stress of billets and prevents billet cracking.

[0023] Continuous casting billet heating: segmented heating + long-term uniform heating ensures that the austenite in the billet is fully homogenized, eliminates the casting structure and segregation of the billet, avoids uneven steel plate structure after rolling due to uneven heating, and provides a uniform austenitic structure for rolling.

[0024] Rolling: Rolling compression ratio ≥3.0 ensures sufficient deformation of the billet; rough rolling with high penetration increases the penetration of thickness deformation through a large reduction rate, breaking the coarse columnar crystals; finish rolling with low temperature increases deformation energy, forming a large number of deformation bands, providing nucleation cores for normalizing and refining grains, achieving preliminary grain refinement, and solving the problem of uneven thickness properties in extra-thick plates.

[0025] Normalizing heat treatment: Precise normalizing temperature and holding time ensure that austenite is fully homogenized and does not grow, forming a synergistic effect with the deformation refinement of rolling, realizing secondary refinement of ferrite grains, and obtaining a fine and uniform ferrite + pearlite structure, ultimately ensuring the high strength and high and low temperature toughness of the steel plate under low carbon equivalent.

[0026] Compared with the prior art, the present invention has the following beneficial effects: (1) The steel plate of the present invention has a thickness of 100-150mm, a carbon equivalent CEV≤0.43%, good weldability, and does not require preheating or only low-temperature preheating during welding; yield strength ≥355MPa (actual measurement 365~370MPa, exceeding the standard by more than 30MPa), tensile strength 470~560MPa, meeting the requirements of 355 steel grade for offshore wind power; at -50℃, the single value of Charpy impact energy in the transverse and longitudinal directions of 1 / 4 plate thickness is ≥144J, which is much higher than the standard requirement of ≥50J, and the low-temperature toughness is greatly improved; the steel plate flaw detection meets the requirements of BS EN10160 standard S2E3, the performance in the thickness direction is uniform, the resistance to lamellar tearing is good, and the feedback after rolling, bending and welding is good.

[0027] (2) This invention is the first to adopt the composition system of "low carbon high manganese + Nb + V + Ti ternary composite microalloying". While controlling the carbon equivalent ≤0.43%, it avoids pearlite banding structure from the source of composition, laying the foundation for grain refinement. The design of the synergistic process of "high penetration rough rolling + low temperature fine rolling + precision normalizing" solves the technical problems of hereditary structure, coarse grains and uneven thickness properties of low carbon equivalent ultra-thick plates under the traditional normalizing process. The process synergy logic is original.

[0028] (3) The present invention uses conventional continuous casting billet production, and the rolling compression ratio only needs to be ≥3.0, without the need for special rolling equipment; the normalizing heat treatment process is adopted, which reduces the production cost by more than 30% compared with the quenching and tempering treatment (quenching + tempering); the alloying elements are all added in trace amounts, there are no precious metals, the raw material cost is low, and industrial mass production can be realized.

[0029] (4) The steel plate of the present invention is designed for offshore wind power monopile foundations and is suitable for extra-thick specifications of 100-150mm. It can meet the harsh service environment requirements of deep-sea offshore wind power, solve the technical gap of domestic medium and heavy plate manufacturers in this field, and can replace imported similar steel plates, with significant economic and social benefits. Attached Figure Description

[0030] Figure 1 The image shows the metallographic structure (200X) of the steel plate obtained in Example 1 of this invention. As can be seen from the image, the metallographic structure of the steel plate is fine and uniform ferrite with a small amount of pearlite, without pearlite banding. The ferrite grain size is 8-10 μm, and the grain refinement effect is significant.

[0031] Figure 2 The image shows the metallographic structure (500X) of the steel plate obtained in Example 1 of this invention. It can be clearly seen from the image that the ferrite grains are fine and uniform, the pearlite is distributed in a dotted or flaky manner, and there are no coarse inclusions or segregation, which ensures the high strength and high and low temperature toughness of the steel plate. Detailed Implementation

[0032] The technical solution of the present invention will be described in more detail below with reference to preferred embodiments. However, these embodiments are merely descriptions of preferred implementations of the present invention and should not be construed as limiting the scope of the present invention. Example 1

[0033] The low-carbon equivalent extra-thick high-toughness steel plate for offshore wind power in this embodiment has a thickness of 120mm. Its chemical composition by weight percentage is: C: 0.15%, Si: 0.25%, Mn: 1.45%, P: 0.018%, S: 0.0008%, Ni: 0.12%, Mo: 0.01%, Cr: 0.06%, Nb: 0.035%, V: 0.005%, Ti: 0.015%, Alt: 0.032%, B: 0.0005%, with the balance being Fe and unavoidable impurity elements. The carbon equivalent (CEV) is 0.42% (CEV(%) = C + Mn / 6 + (Cr + Mo + V) / 5 + (Ni + Cu) / 15).

[0034] The manufacturing process of this steel plate is as follows: (1) Smelting and casting process: A pure steel smelting process is adopted to control the non-metallic inclusions of Class A, B, C and D in the molten steel to be ≤1.5 grade; after RH venting, the soft blowing argon time is 25 min, and the superheat of the molten steel in the continuous casting process is 20℃. Through argon-protected casting process and dynamic light reduction technology, the segregation of Class C in the center of the continuous casting billet is guaranteed to be below 1.0 grade and the porosity is below 0.5 grade. The continuous casting billet is slowly cooled after it is removed from the line.

[0035] (2) Heating process: The continuous casting billet adopts segmented heating, with a total heating time of 360 min. The temperature of the second heating section is 1175~1200℃, and the temperature of the soaking section is 1200~1235℃. The total heating time of the second heating section and the soaking section is 250 min.

[0036] (3) Rolling process: Rolling is divided into two stages: roughing and finishing. The roughing stage adopts a high-penetration rolling process, with an initial rolling temperature of 1120℃ and a final rolling temperature of 960℃. The reduction rates of the last three passes are >9%, namely 9.2%, 10.3%, and 10.6%. The finishing stage requires a thickness of 2.0 times that of the finished steel plate, and the final rolling temperature is 790℃.

[0037] (4) Heat treatment process: The normalizing process is adopted, the normalizing temperature is 890℃, the holding time is 240min, and the furnace is air cooled after being removed from the furnace.

[0038] The 120mm thick offshore wind power steel plate produced by the above manufacturing process has well-matched mechanical properties, which are detailed in Table 1. The steel plate meets the requirements of BS EN10160 standard S2E3. Example 2

[0039] The low-carbon equivalent extra-thick high-toughness steel plate for offshore wind power in this embodiment has a thickness of 150mm. Its chemical composition by weight percentage is: C: 0.13%, Si: 0.30%, Mn: 1.55%, P: 0.018%, S: 0.0008%, Ni: 0.15%, Mo: 0.01%, Cr: 0.06%, Nb: 0.045%, V: 0.025%, Ti: 0.015%, Alt: 0.032%, B: 0.0005%, with the balance being Fe and unavoidable impurity elements. The carbon equivalent (CEV) is 0.42% (CEV(%) = C + Mn / 6 + (Cr + Mo + V) / 5 + (Ni + Cu) / 15).

[0040] The manufacturing process of this steel plate is as follows: (1) Smelting and casting process: A pure steel smelting process is adopted to control the non-metallic inclusions of Class A, B, C and D in the molten steel to be ≤1.5 grade; after RH venting, the soft blowing argon time is 23 min, and the superheat of the molten steel in the continuous casting process is 15℃. Through argon-protected casting process and dynamic light pressure reduction technology, the segregation of Class C in the center of the continuous casting billet is guaranteed to be below 1.0 grade and the porosity is below 0.5 grade. The continuous casting billet is slowly cooled after it is removed from the line.

[0041] (2) Heating process: The continuous casting billet adopts segmented heating, with a total heating time of 535 min. The temperature of the second heating section is 1185~1210℃, and the temperature of the soaking section is 1220~1250℃. The total heating time of the second heating section and the soaking section is 300 min.

[0042] (3) Rolling process: Rolling is divided into two stages: roughing and finishing. The roughing stage adopts a high-penetration rolling process, with an initial rolling temperature of 1130℃ and a final rolling temperature of 950℃. The reduction rate of the last three passes is >9%, namely 11.2%, 12.2%, and 13.5%. The finishing stage requires a thickness of 2.0 times the thickness of the finished steel plate, and the final rolling temperature is 780℃.

[0043] (4) Heat treatment process: The normalizing process is adopted, the normalizing temperature is 890℃, the holding time coefficient is 316min, and the furnace is air cooled after being removed from the furnace.

[0044] The 150mm thick offshore wind power steel plate produced by the above manufacturing process has well-matched mechanical properties, which are detailed in Table 1. The steel plate meets the requirements of BS EN10160 standard S2E3.

[0045] Table 1 Mechanical properties of the steel plates produced in each embodiment

[0046] The above are merely specific embodiments of the present invention, but the scope of protection of the present invention is not limited thereto. 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 be included within the scope of protection of the present invention. Therefore, the scope of protection of the present invention should be determined by the scope of the claims.

Claims

1. A low-carbon equivalent extra-thick high-toughness normalized steel plate for offshore wind power, characterized in that: The chemical composition of the steel plate, by weight percentage, is: C: 0.12–0.20%, Si: 0.20–0.40%, Mn: 1.20–1.60%, P: ≤0.020%, S: ≤0.005%, Ni: 0.05–0.20%, Mo: 0.04–0.10%, Cr: 0.05–0.20%, Nb: 0.01–0.04%, V: 0.01–0.03%, Ti: 0.01–0.03%, Alt: 0.02–0.05%, B: ≤0.0005%, with the balance being Fe and unavoidable impurity elements.

2. The low-carbon equivalent extra-thick high-toughness normalized steel plate for offshore wind power according to claim 1, characterized in that, The carbon equivalent (CEV) of the steel plate is ≤0.43%, and the carbon equivalent is calculated using the formula: CEV (%) = C + Mn / 6 + (Cr + Mo + V) / 5 + (Ni + Cu) / 15.

3. The low-carbon equivalent extra-thick high-toughness normalized steel plate for offshore wind power according to claim 1, characterized in that, The steel plate has a thickness of 100-150mm, a yield strength ≥355MPa, a tensile strength of 470MPa-560MPa, and a single Charpy impact energy at -50℃ at 1 / 4 of the thickness ≥50J.

4. A method for manufacturing a low-carbon equivalent extra-thick high-toughness normalized steel plate for offshore wind power as described in any one of claims 1 to 3, characterized in that, Includes the following steps: (1) Smelting and casting: Pure steel smelting process is adopted, and the non-metallic inclusions of Class A, Class B, Class C and Class D in the molten steel are controlled to be ≤1.5 grade; the soft blowing time of argon gas after RH venting is ≥20min, and the superheat of molten steel in the continuous casting process is 20~30℃; Argon gas protection casting + dynamic light pressure reduction technology is adopted to ensure that the segregation of Class C in the center of the continuous casting billet is ≤1.0 grade and the porosity is ≤0.5 grade, and the continuous casting billet is slowly cooled after it is removed from the line; (2) Heating of continuous casting billet: segmented heating is adopted, with a total heating time of 300-550 min, the temperature of the second heating section is 1175-1245℃, the temperature of the soaking section is 1165-1235℃, and the total heating time of the second heating section and the soaking section is ≥200 min; (3) Rolling: The rolling compression ratio is ≥3.0, and it is divided into two stages: roughing and finishing. The roughing adopts a high-penetration rolling process, and the reduction rate of the last three passes is >9%. The thickness of the steel plate to be heated in the finishing stage is 2 to 3 times the thickness of the finished steel plate, and the final rolling temperature is 780 to 820℃. (4) Normalizing heat treatment: normalizing temperature 870~900℃, holding time coefficient 1.5~2.5min / mm, after holding, remove from the furnace and air cool to room temperature to obtain the normalized steel plate.

5. The method for manufacturing a low-carbon equivalent extra-thick high-toughness normalized steel plate for offshore wind power according to claim 4, characterized in that, In step (1), the soft blowing argon time is 23-25 ​​min, the slow cooling time of the continuous casting billet is 12-24 h, and the slow cooling temperature is 300-400℃.

6. The method for manufacturing a low-carbon equivalent extra-thick high-toughness normalized steel plate for offshore wind power according to claim 4, characterized in that, In step (2), the heating rate of the second heating section is 5-8℃ / min, and the temperature fluctuation range of the heat spread section is ≤±10℃.

7. The method for manufacturing a low-carbon equivalent extra-thick high-toughness normalized steel plate for offshore wind power according to claim 4, characterized in that, In step (3), the initial rolling temperature of the roughing mill is 1120-1130℃, and the final rolling temperature of the roughing mill is 950-960℃; the pass reduction rate of the finishing mill is 5-8%, and the total reduction rate of the finishing mill is ≥40%.

8. The method for manufacturing a low-carbon equivalent extra-thick high-toughness normalized steel plate for offshore wind power according to claim 4, characterized in that, In step (4), the heating rate of the normalizing is 3-5℃ / min, the cooling rate of the air cooling after exiting the furnace is 8-15℃ / min, and the steel plate is subjected to surface flaw detection after air cooling to room temperature. The flaw detection standard meets the requirements of BS EN10160 standard S2E3.

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