Large-thickness submarine pipeline steel plate and production method therefor

Abstract

The present invention relates to a large-thickness submarine pipeline steel plate and a production method therefor. Steel plate components are designed by means of optimization of low-carbon low-alloy components, and the production method uses a matching special controlled rolling process of five-stage heating, low-temperature high-reduction rolling in a recrystallization region, instant cooling of intermediate slabs and high-temperature high-reduction rolling in a non-recrystallization region, and a matching multi-stage controlled cooling process of high-temperature region air cooling for redistribution, ferrite region cooling, temperature maintaining, bainite region cooling and air cooling self-tempering cooling. The steel plate product of the present invention has a relatively high proportion of acicular ferrite, and a relatively fine average ferrite grain size of 3-6μm, thus endowing the steel plate with good combination of strength and toughness. The steel plate has high strength, wherein the yield strength is ≥500MPa, the tensile strength is ≥630MPa, and the impact energy KV2 at -60℃ is ≥250J.

Description

A thick submarine pipeline steel plate and its production method Technical Field

[0001] This invention relates to a thick submarine pipeline steel plate and its production method, belonging to the technical field of cast steel. Background Technology

[0002] In recent years, with the rapid development of my country's economy, the demand for energy sources such as oil and natural gas has also grown rapidly. The huge gap in these resources can only be filled by importing from abroad or by finding new resources domestically. Currently, onshore oil and gas fields worldwide are gradually entering the middle and late stages of exploitation, increasing the difficulty and cost of exploration and development. Meanwhile, offshore oil and gas resources have enormous potential and promising exploration prospects, leading to a growing demand for subsea oil and gas pipelines. As the fastest, most economical, and most efficient mode of transporting offshore oil and gas, subsea oil and gas pipelines are an indispensable lifeline for offshore oil and gas field development and production, creating a huge demand for steel used in subsea pipelines.

[0003] Submarine pipelines operate in the complex marine environment for extended periods, subjected to environmental loads such as external currents, seabed erosion and sedimentation, and slippage, as well as internal fluid pressure and corrosion. A rupture in a submarine pipeline can lead to catastrophic events including economic losses, environmental pollution, and ecological damage. Therefore, high requirements are placed on the steel used in submarine pipelines, such as requiring large thicknesses, high strength, low yield strength ratios, excellent low-temperature toughness, and corrosion resistance.

[0004] Patent publication number CN117165870A discloses a hot-rolled steel plate for thick-walled subsea pipelines and its manufacturing method. The patent provides the following chemical composition by weight percentage for the hot-rolled steel plate: C: 0.03%–0.05%, Si: 0.16%–0.24%, Mn: 1.60%–1.70%, P≤0.015%, S≤0.004%, Cr: 0.14%–0.18%, Ni: 0.11%–0.15%, Mo: 0.09%–0.13%, Nb: 0.05%–0.065%, Ti: 0.01%–0.02%, Al: 0.02%–0.05%, P… cm ≤0.19%, with the remainder being Fe and impurity elements. Through a process of KR desulfurization-BOF converter-LF refining-RH degassing-CCM continuous casting-slab reheating-rough rolling-intermediate billet cooling-finish rolling-accelerated cooling-hot straightening-natural cooling-warm and cold straightening-off-line cooling and ultrasonic testing-finishing, it can meet the specifications of X70 grade submarine pipelines with thicknesses of 30mm and above.

[0005] Patent publication number CN104357766A discloses an extra-thick, high-strength, high-toughness subsea pipeline steel for ultra-deep-sea applications. The steel's chemical composition by weight percentage is as follows: C: 0.020%~0.050%, Si≤0.10%, Mn: 1.00%~1.50%, P≤0.010%, S≤0.0010%, Cu: 0.10%~0.30%, Cr: 0.10%~0.25%, Ni: 0.31%~0.50%, Mo≤0.15%, Nb: 0.030%~0.065%, V: 0.015%~0.040%, Ti: 0.010%~0.025%, Al: 0.010%~0.050%, N≤0.008%, with the remainder being Fe. The manufacturing method is as follows: 1) casting; 2) rough rolling; 3) finish rolling; 4) cooling; 5) relaxation. Extra-thickness pipeline steel thickness ≥ 35mm. The initial finishing rolling temperature is controlled at 820~900℃, with a cumulative reduction rate of 60~75%; the final rolling temperature is 760~840℃; after rolling, the steel plate undergoes two-stage accelerated cooling: Stage I cooling rate 5~10℃ / s, final cooling temperature 720~750℃; Stage II cooling rate 20~40℃ / s, final cooling temperature 100~300℃; the yield strength (R) of the steel plate... t0.5） ≥485MPa, tensile strength (R) m） ≥570MPa, yield strength ratio (R) t0.5 / R m） ≤0.85, elongation (A) 50mm） ≥40%, -20℃ KV8≥400J, -20℃ DWTT shear area fraction≥85%.

[0006] Patent publication number CN103451536B discloses a low-cost, thick-gauge steel plate for submarine pipelines and its manufacturing method. The steel plate's chemical composition by weight percentage is as follows: C: 0.04%–0.06%, Si: 0.15%–0.20%, Mn: 1.43%–1.47%, P≤0.008%, S≤0.003%, Ni: 0.10%–0.15%, Nb: 0.030%–0.040%, Ti: 0.012%–0.023%, Al: 0.015%–0.025%, N≤0.006%, Nb+V+Ti≤0.15%, with the remainder being iron and unavoidable impurities. The method employs techniques such as transverse forming of continuously cast billets, low-temperature heating of continuously cast billets, low-temperature controlled rolling, and relaxation cooling control. The produced steel plate exhibits a dual-phase microstructure, achieving high strength and a low yield strength ratio, while also possessing good toughness. During rolling in the recrystallization zone, the final rolling temperature is controlled at 990~1020℃, with a total reduction of 65%~75%, and a reduction of 20%~30% in the last three passes. In the non-recrystallization zone, the total reduction is 60%~70%, and the final rolling temperature is 750~810℃. After rolling, the steel plate is relaxed and slowly cooled, followed by online quenching using MULPIC, with a cooling rate controlled at 20~30℃ / s and a final cooling temperature controlled at 350~450℃. The steel plate thickness is 28.6~31.8mm.

[0007] Patent publication number CN102676925A discloses a steel plate with a large wall thickness for submarine pipelines and a method for producing the steel plate. The steel plate is composed of the following components by weight percentage: C: 0.05%~0.07%, Si: 0.15%~0.25%, Mn: 1.42%~1.48%, P≤0.010%, S≤0.002%, Ni: 0.13%~0.18%, Nb: 0.043%~0.048%, Al: 0.020%~0.040%, Ti: 0.014%~0.024%, Mo: 0.13%~0.18%, with the balance being Fe and unavoidable impurities. The thickness of the steel plate for thick-walled subsea pipelines is 25–30.2 mm. It is rolled using a two-stage rolling process: a recrystallization zone and a non-recrystallization zone. The first stage rolling temperature is 950–1100℃, with a single-pass reduction of 10%–20% and a cumulative reduction of 30%–50%. The second stage rolling temperature is 840–880℃, with a cumulative reduction of 30%–50%. The post-rolling water immersion temperature is 740–780℃, and the red-hot temperature is 450–500℃, resulting in the steel plate. Mechanical properties: yield strength between 470–570 MPa, tensile strength between 535–760 MPa, elongation >30%, impact energy at -20℃ >200 J, DWTT shear area fraction >75%, and yield strength ratio <0.90.

[0008] Currently, there are numerous patents related to steel for submarine pipelines.

[0009] In terms of composition, some submarine pipeline steels use alloy composition systems such as V and Cu, such as patents CN111979497A, CN109234487B, CN111607747A, CN110863145A, CN108504931B, CN105132833B, CN105132807B, CN104357766A, CN103993240A, CN101701315B, CN111748741B, etc., all of which contain 0.015%~0.060% V and / or 0.10%~0.30% Cu. Their high alloy content leads to increased production costs.

[0010] In terms of manufacturing process, some submarine pipeline steel is produced using a coil production line to obtain pipeline steel coils or strips. Rough rolling uses 1 to 2 stands, while finish rolling uses 5 to 7 stands, and coiling is required after rolling, as seen in patents CN111979497A, CN113278880A, CN110578091B, CN111607747A, and CN108504931B. The submarine pipeline steel coils or strips produced by these patents are relatively thin, generally 3 to 20 mm, which is insufficient to meet the requirements of the harsh submarine environment and high-pressure pipeline transportation. Some submarine pipeline steel is rolled using medium-thick plate or wide-thick plate rolling mills, resulting in relatively thick steel plates, such as patents CN103993240A and CN106566991B, with thicknesses ranging from 20 to 48 mm. After being rolled into steel plates, these plates need to be stacked and cooled slowly for 12 to 48 hours, which is a long stacking time and affects production efficiency.

[0011] In terms of performance, some pipeline steel patents have lower strength, such as patents CN109234487B and CN107988562A, which are X65 grade submarine pipeline steels with lower strength, yield strength ≥450MPa. Some patents also have lower toughness, such as patents CN105132807B and CN103834874B, with an impact energy ≥250J at -20℃, but no requirement for impact energy at -40℃. Conversely, some patents have higher yield strength to tensile strength ratios, such as patents CN104357766A, CN103993240A, CN109234487B, and CN111607747A, with a yield strength to tensile strength ratio ≤0.90, mostly between 0.85 and 0.90.

[0012] Existing patents for submarine pipeline steel suffer from problems such as high alloy content, thin thickness, long stacking time, low strength, low toughness, and high yield strength ratio. These issues lead to high production costs, slow production pace, long cycles, and low safety in the current submarine pipeline steel production, thus limiting the development of the submarine pipeline engineering industry. Summary of the Invention

[0013] To address the aforementioned problems, this invention discloses a thick submarine pipeline steel plate and its production method, the specific technical solution of which is as follows:

[0014] A thick-walled subsea pipeline steel plate, comprising, by mass percentage: C: 0.031–0.061%, Si: 0.08–0.16%, Mn: 1.64–1.72%, P: 0.0090–0.0140%, S: 0.0015–0.0038%, Cr: 0.12–0.20%, Ni: 0.12–0.20%, Mo: 0.07–0.13%, Nb: 0.054–0.064%, Ti: 0.01 1~0.019%, Alt: 0.021~0.049%, N: 0.0031~0.0051%, the remainder being iron and unavoidable impurities. The chemical composition of the steel plate also meets the following requirements: carbon equivalent CEV=C+Mn / 6+(Cr+Mo+V) / 5+(Cu+Ni) / 15 is 0.350~0.427, Pcm=C+Si / 30+(Mn+Cu+Cr) / 20+Ni / 60+Mo / 15+V / 10+5B is 0.127~0.174.

[0015] Furthermore, the continuously cast billet is subjected to a special controlled rolling process, which involves five stages of heating, low-temperature rolling with large reduction in the recrystallization zone, intermediate billet cooling, and high-temperature rolling with large reduction in the non-recrystallization zone. It is also subjected to a multi-stage controlled cooling process, which involves air cooling and redistribution in the high-temperature zone after rolling, cooling in the ferrite zone, waiting to reach a certain temperature, cooling in the bainite zone, and air cooling and self-tempering. This process is used to produce submarine pipeline steel plates.

[0016] Furthermore, the five-stage heating process is as follows: the continuously cast billet is placed in a walking beam furnace for heating. The furnace is 55m long and employs a five-stage heating process consisting of a heat recovery section, a preheating section, a first heating section, a second heating section, and a soaking section. Specifically, the heat recovery section temperature is ≤850℃, and the heating time is 0.37–0.41 min / mm; the preheating section temperature is ≤950℃, and the heating time is 0.32–0.35 min / mm; the first heating section temperature is 1050±15℃, and the heating time is 0.22–0.25 min / mm; the second heating section temperature is 1150±15℃, and the heating time is 0.17–0.20 min / mm; and the soaking section temperature is T1, where T1 ≥ T... NbN And T NbN ≤T1≤T NbN At +25℃, the heating time in the soaking zone is 0.07–0.14 min / mm, and the total heating time is 1.15–1.35 min / mm, where lg(Nb×N)=3.7-10800 / (T) NbN +273.15).

[0017] Furthermore, the specific process of low-temperature high-reduction rolling in the recrystallization zone is as follows: the initial rolling temperature is T2 - 10℃ ~ T2 + 30℃, and the final rolling temperature is T2 - 80℃ ~ T2 - 40℃; in the low-temperature section within the recrystallization zone temperature range, a high-reduction rolling process is adopted, with a reduction per pass ≥ 31mm, a total of 4 to 6 roughing passes, a temperature difference of 12℃ to 18℃ between adjacent passes, and the reduction of the previous pass is less than or equal to the reduction of the next pass, i.e., R n ≤R n+1 R n and R n+1 The reduction amounts for the nth and (n+1)th passes, respectively; the intermediate billet thickness t after rolling. z , satisfying 3.4t≤t z ≤4.4t, where t is the thickness of the finished steel plate, and T2 is the recrystallization temperature, T2=887+464C+6445Nb-644 +890Ti+363Al-357Si.

[0018] Furthermore, the intermediate billet cooling process involves the following steps: After rolling in the austenite recrystallization zone, the intermediate billet is immediately cooled by water using an intermediate billet cooling device. The water pressure in the cooling manifold is 0.4~0.6MPa, the water flow rate in the upper manifold is 50~150m³ / h, the water flow rate in the lower manifold is 100~300m³ / h, the water flow rate ratio between the lower and upper manifolds is ≥2, and the cooling water temperature is 10~30℃. The intermediate billet passes back and forth through the cooling device for cooling, and the roller speed of the cooling device is 0.4~6m / s. The billet is cooled to the non-recrystallization rolling temperature range, i.e., T2-240℃ ~ T2-220℃.

[0019] Furthermore, the high-temperature, high-reduction rolling in the non-recrystallization zone involves: an initial rolling temperature of T2-240℃ ~ T2-220℃ and a final rolling temperature of T3 ~ T3 + 25℃; a high-reduction rolling process is employed in the high-temperature section within the non-recrystallization zone temperature range, i.e., T2-280℃ ~ T2-240℃. In this high-temperature zone, 4 to 6 rolling passes are performed at a rolling speed of 3 to 5 m / s, with a reduction per pass ≥ 21 mm, and the reduction in each pass is greater than or equal to the reduction in the next pass, i.e., R. m ≥R m+1 R m and R m+1The reduction amounts for the m-th and m+1-th passes are respectively, and the thickness of the rolled billet is t+3mm ~ t+12mm. In the low-temperature section within the non-recrystallization temperature range, a small reduction rolling process is adopted. The low-temperature section is defined as T3 +5℃ ~ T3 +30℃. In the low-temperature section, the number of rolling passes is 1 to 2, the rolling speed is 4 to 6m / s, the reduction per pass is ≤6mm, and the thickness of the finished steel plate after rolling is t, where t≥35mm. During the rolling process in the non-recrystallization region, water cooling is performed at the interval between passes using an intermediate billet cooling device. Here, T3 is the temperature at which the transformation from austenite to ferrite begins during cooling, T3=910-310C-80Mn-15Cr-80Mo.

[0020] Furthermore, the high-temperature zone air-cooling redistribution after rolling: After rolling, the steel plate is air-cooled on the conveyor rollers. The conveyor rollers are 60m long, the roller speed is 2-5m / s, and the indoor temperature is 5-39℃. The steel plate is air-cooled to T3 - 50℃ ~ T3 - 30℃, and then enters the ultra-fast cooling system.

[0021] Furthermore, the ferrite region cooling process involves the steel plate entering the ultra-fast cooling system through an ultra-fast cooling inlet. The ultra-fast cooling system has 24 sets of cooling manifolds, each with a cooling length of 1m. Manifolds 1 to 24 are opened for cooling at a rate of 10 to 20℃ / s, a water pressure of 0.15 to 0.20 MPa, a cooling water temperature of 10 to 30℃, a water flow rate of 60 to 200 m³ / h for the upper and lower manifolds, a water flow rate ratio of 1.1 to 1.5 for the lower manifold to the upper manifold, and a cooling roller speed of 1.8 to 2.2 m / s. The cooling temperature is reduced to T4 + 5℃ to T4 + 25℃, where T4 = 830-270C-90Mn-37Ni-70Cr-83Mo.

[0022] Furthermore, the waiting temperature is specifically as follows: after cooling to the intermediate temperature T4 + 5℃ ~ T4 + 25℃, the upper and lower cooling water manifolds are closed, the steel plate exits the ultra-fast cooling system from the inlet of the ultra-fast cooling system, and swings on the ultra-fast cooling inlet roller conveyor to wait for the temperature, that is, it swings back and forth on the roller conveyor at a speed of 0.5 to 1.5 m / s, the back and forth movement distance is 5 to 10 m, and the waiting time is 8 to 15 seconds.

[0023] Furthermore, the bainitic region cooling is specifically as follows: the steel plate re-enters the ultra-fast cooling system from the ultra-fast cooling inlet, with a total of 24 sets of cooling manifolds, each manifold having a cooling length of 1m; sets 1 to 24 are opened for cooling, with a water pressure of 0.30 to 0.50 MPa, a cooling rate of 8 to 20℃ / s, a cooling water temperature of 10 to 26℃, a water flow rate of 100 to 400 m³ / h for the upper and lower manifolds, a water flow rate ratio of 1.2 to 1.4 for the lower manifold to the upper manifold, and a cooling roller speed of 0.6 to 1.4 m / s for the ultra-fast cooling system, cooling to T4 - 285℃ ~ T4 - 265℃.

[0024] Furthermore, the air-cooled self-tempering cooling specifically involves: after cooling to T4 - 285℃ ~ T4 - 265℃, the steel plate exits the ultra-fast cooling system from the outlet of the ultra-fast cooling system; then it is straightened and subjected to air-cooled self-tempering on a cooling bed until it is cooled to room temperature; the air-cooled self-tempering conditions are that the cooling bed is open and windless, the cooling bed is covered with hot steel plates, the spacing between the steel plates is 1 to 2 meters, and the temperature of the steel plates is T4 - 305℃ ~ T4 - 105℃.

[0025] Furthermore, by using continuously cast billets with a thickness of ≤320mm, center segregation of no more than 0.5, and inclusions of types A, B, C, and D of no more than 1, steel plates with a thickness of ≥35mm and a compression ratio of ≤9.14 are produced.

[0026] This invention also seeks to protect the thick submarine pipeline steel plate produced by the above-described method for producing thick submarine pipeline steel plates.

[0027] Furthermore, the microstructure of the thick subsea pipeline steel plate is a multiphase structure of quasi-polygonal ferrite + acicular ferrite + bainite + tempered bainite. The proportion of MA self-tempering decomposition in bainite is ≤20%, the average grain size of ferrite is 3~6μm, the proportion of quasi-polygonal ferrite is 5%~15%, the proportion of acicular ferrite is 25%~40%, the proportion of bainite is 25%~65%, the proportion of tempered bainite is 5%~20%, the yield strength of the steel plate is ≥500MPa, the tensile strength is ≥630MPa, the elongation is ≥35%, the yield ratio is ≤0.80, the impact energy KV2 at -40℃ is ≥300J, the impact energy KV2 at -60℃ is ≥250J, the hardness is ≤208HV10, the DWTT drop shear area fraction at -10℃ is 100%, the DWTT drop shear area fraction at -20℃ is ≥90%, and the DWTT ductile-brittle transition temperature is below -20℃.

[0028] The beneficial effects of this invention are:

[0029] (1) The chemical composition of this invention does not contain expensive alloys such as V and Cu, and the content of alloys such as Ni and Mo is low. Harmful elements such as P and S do not need to be controlled at ultra-low content, making it easy to produce steel and with low production cost.

[0030] (2) The production process adopts controlled rolling and controlled cooling, which has the advantages of short process flow, low production cost and fast delivery, avoiding the problems of long production process, slow production rhythm and long delivery cycle caused by stacking. At the same time, the process of low temperature large reduction rolling in recrystallization zone + intermediate billet cooling + high temperature large reduction rolling in non-recrystallization zone is adopted. Each rolling stage is precisely controlled. The continuous casting billet with a thickness of ≤320mm is used to produce steel plates with a thickness of ≥35mm. The compression ratio is ≤9.14, the microstructure is fine, the toughness is excellent, the average ferrite grain size is 3~6μm, the impact energy KV2 at -60℃ is ≥250J, and the DWTT ductile-brittle transition temperature is below -20℃.

[0031] (3) This invention employs a multi-stage controlled cooling process, adjusting and controlling the type and proportion of the finished product's microstructure according to the cooling parameters of each stage to obtain a multiphase microstructure. Quasi-polygonal ferrite is formed during the first stage of air cooling after rolling and before ultra-rapid cooling; acicular ferrite is formed during the second stage of cooling and the third stage of waiting to reach a warm temperature; bainite is formed during the fourth stage of cooling; and tempered bainite is formed during the fifth stage of cooling. This multiphase microstructure ensures a good match between various mechanical properties of the steel plate, such as strength, low-temperature toughness, yield strength ratio, hardness, and drop hammer performance.

[0032] (4) Under the reasonable matching of chemical composition with five-stage heating, special controlled rolling process and multi-stage controlled cooling process, the microstructure of this invention is a multiphase microstructure of quasi-polygonal ferrite + acicular ferrite + bainite + tempered bainite. The self-tempering decomposition ratio of MA in bainite is ≤20%, the proportion of quasi-polygonal ferrite is 5%~15%, the proportion of acicular ferrite is 25%~40%, the proportion of bainite is 25%~65%, the proportion of tempered bainite is 5%~20%, the yield strength of the steel plate is ≥500MPa, the tensile strength is ≥630MPa, the elongation is ≥35%, the yield ratio is ≤0.80, the impact energy KV2 at -40℃ is ≥300J, the hardness is ≤208HV10, the DWTT drop shear area fraction at -10℃ is 100%, and the DWTT drop shear area fraction at -20℃ is ≥90%.

[0033] (5) The steel plate of the present invention has a high proportion of acicular ferrite and a fine average grain size of ferrite, which is 3~6μm, so that the steel plate has a good strength and toughness match, and the steel plate has high strength, yield strength ≥500MPa, tensile strength ≥630MPa, and impact energy KV2 ≥250J at -60℃. Attached Figure Description

[0034] Figure 1 is a metallographic diagram of an embodiment of the present invention. Detailed Implementation

[0035] The present invention will be further illustrated below with reference to specific embodiments. It should be understood that the following specific embodiments are for illustrative purposes only and are not intended to limit the scope of the invention.

[0036] Thick-walled subsea pipeline steel plates and their production method. The chemical composition of the steel plates, by mass percentage, includes: C: 0.031–0.061%, Si: 0.08–0.16%, Mn: 1.64–1.72%, P: 0.0090–0.0140%, S: 0.0015–0.0038%, Cr: 0.12–0.20%, Ni: 0.12–0.20%, Mo: 0.07–0.13%, Nb: 0.054–0.064%, Ti: 0. 0.011~0.019%, Alt: 0.021~0.049%, N: 0.0031~0.0051%, the remainder being iron and unavoidable impurities. The chemical composition of the steel plate also meets the following requirements: carbon equivalent CEV=C+Mn / 6+(Cr+Mo+V) / 5+(Cu+Ni) / 15 is 0.350~0.427, Pcm=C+Si / 30+(Mn+Cu+Cr) / 20+Ni / 60+Mo / 15+V / 10+5B is 0.127~0.174.

[0037] The following is a detailed analysis and explanation of the main functions of each chemical component in this invention and the selection of their dosage:

[0038] Carbon (C): Carbon is the most economical strengthening element in steel, providing solid solution strengthening. It also forms carbides with Nb, Ti, Cr, Mo, etc., resulting in precipitation strengthening. Increasing the carbon content significantly improves the strength and hardness of pipeline steel; however, excessive carbon content leads to poor low-temperature toughness and weldability, reducing the low-temperature drop hammer test performance of pipeline steel. Therefore, the carbon content is selected to be 0.031%–0.061%.

[0039] Silicon (Si): Silicon plays a solid solution strengthening role in steel, but too much silicon can easily produce Fe2SiO4 on the surface of the slab, which is not conducive to the control of the surface quality of the steel plate. Therefore, the silicon content is selected to be 0.08% to 0.16%.

[0040] Manganese (Mn): Manganese plays a solid solution strengthening role in steel, improving the strength and hardness of steel. A reasonable manganese content can ensure the strength of pipeline steel at low cost. It is the most economical strengthening element besides carbon. However, too much manganese will cause segregation in the center of the billet, which will greatly damage the toughness of thick plates. At the same time, it is not conducive to the control of banded structure. Therefore, the manganese content is selected as 1.64% to 1.72%.

[0041] Phosphorus (P): Phosphorus is an impurity element in steel. Excessive phosphorus content can easily lead to segregation, which can significantly reduce the plasticity and toughness of steel, especially under low temperature conditions. However, excessively low phosphorus content can significantly increase the cost of steelmaking. Therefore, the phosphorus content is selected to be 0.0090%~0.0140%.

[0042] Sulfur (S): Sulfur is an impurity element in steel. It not only increases the hot brittleness of steel, but also easily forms MnS inclusions with manganese, which reduces the low-temperature toughness of steel. However, too low sulfur content will significantly increase the cost of steelmaking. Therefore, the sulfur content is selected to be 0.0015% to 0.0038%.

[0043] Chromium (Cr): Chromium plays a role in solid solution strengthening in steel. As a ferrite-forming element, chromium can produce more acicular ferrite in high-niobium steel. However, excessive chromium content can increase the microhardness of pipeline steel and reduce its low-temperature toughness. Therefore, the chromium content is selected to be 0.12%~0.20%.

[0044] Nickel (Ni): Nickel plays a solid solution strengthening role in steel, increasing its strength without significantly increasing its hardness. It also benefits low-temperature toughness. However, excessively high nickel content increases alloy costs. Therefore, the nickel content is typically chosen to be 0.12%~0.20%.

[0045] Molybdenum (Mo): Molybdenum can significantly improve the hardenability of steel, delay the ferrite transformation, and obtain acicular ferrite structure, which is beneficial for improving the strength and toughness of pipeline steel. However, molybdenum leads to an increase in the number of MA components, which is detrimental to improving toughness. Moreover, when the molybdenum content is too high, the alloy cost increases significantly. Therefore, the molybdenum content is selected to be 0.07% to 0.13%.

[0046] Niobium (Nb): Niobium is an important grain-refining element in steel. During hot rolling, niobium strongly inhibits austenite recrystallization and precipitation within austenite, pinning austenite grain boundaries and refining recrystallized grains. During cooling, dissolved niobium can continue to precipitate as niobium carbonitrides, significantly refining the microstructure after phase transformation and further improving the strength and toughness of the steel. Adding excessive niobium firstly increases alloy costs; secondly, niobium cannot be completely dissolved during normal heating; and thirdly, precipitation strengthening of niobium is detrimental to the yield strength ratio. Therefore, the niobium content is selected to be 0.054%–0.064%.

[0047] Titanium (Ti): Titanium is a nitrogen-fixing element in steel. It can form dispersed titanium nitride particles, which inhibit the coarsening of austenite grains during billet heating and rolling. However, if the amount added is too high, large carbide / nitride precipitates can easily form in the core of the cast billet, affecting the low-temperature toughness of the steel plate. Therefore, the titanium content is selected to be 0.011% to 0.019%.

[0048] Aluminum (Al): Aluminum is a deoxidizing element in steel. Excessive aluminum can increase the number of Al2O3 inclusions in steel, affecting its low-temperature toughness. Therefore, the aluminum content should be minimized while ensuring effective deoxidation. Thus, the aluminum content is selected as 0.021%–0.049%.

[0049] Nitrogen (N): Nitrogen is an impurity element in steel, which reduces the plasticity and toughness of steel plates. Too low a nitrogen content will increase the cost of steelmaking. Therefore, the nitrogen content is selected to be 0.0031% to 0.0051%.

[0050] Smelting is carried out according to the above chemical composition, and a continuous casting billet with a thickness of ≤320mm is obtained by lightly pressing at the end of continuous casting. The center segregation is no higher than level 0.5, and the inclusions of type A, B, C, and D are all no higher than level 1.

[0051] The continuously cast billet undergoes a special controlled rolling process involving five stages of heating, low-temperature rolling with large reduction in the recrystallization zone, intermediate billet cooling, and high-temperature rolling with large reduction in the non-recrystallization zone. This, combined with a multi-stage controlled cooling process—including air cooling and redistribution in the high-temperature zone after rolling, cooling in the ferrite zone, waiting to reach a suitable temperature, cooling in the bainite zone, and air-cooled self-tempering—to produce subsea pipeline steel plates. The specific process steps are as follows:

[0052] (1) Five-stage heating process: The heating process consists of a heat recovery stage, a preheating stage, a first heating stage, a second heating stage, and a soaking stage. The temperature of the heat recovery stage is ≤850℃, the temperature of the preheating stage is ≤950℃, the temperature of the first heating stage is 1050±15℃, the temperature of the second heating stage is 1150±15℃, and the temperature of the soaking stage is T1, where T1≥T NbN And T NbN ≤T1≤T NbN At +25℃, the heating time in the soaking zone is 0.07–0.14 min / mm, and the total heating time is 1.15–1.35 min / mm. Wherein, lg(Nb×N) = 3.7-10800 / (T) NbN +273.15).

[0053] (2) Low-temperature rolling with large reduction in the recrystallization zone: The initial rolling temperature is T2 - 10℃ ~ T2 + 30℃, and the final rolling temperature is T2 - 80℃ ~ T2 -40℃; in the low-temperature section within the recrystallization zone temperature range, a rolling process with large reduction is adopted, with a reduction of ≥31mm per pass, and the reduction of the previous pass is less than or equal to the reduction of the next pass, i.e., R n ≤R n+1 R n and R n+1 The reduction amounts for the nth and (n+1)th passes, respectively; the intermediate billet thickness t after rolling. z , satisfying 3.4t≤t z≤4.4t, where t is the thickness of the finished steel plate. T2 is the recrystallization temperature, T2 = 887 + 464°C + 6445Nb - 644°C. +890Ti+363Al-357Si.

[0054] (3) Intermediate billet cooling: After the austenite recrystallization zone is rolled, the intermediate billet cooling device is immediately used for water cooling to cool it to the non-recrystallization rolling temperature range, that is, to T2-240℃ ~ T2-220℃.

[0055] (4) High-temperature rolling with large reduction in the non-recrystallization zone: The initial rolling temperature is T2-240℃ ~ T2-220℃, and the final rolling temperature is T3 ~ T3 + 25℃; in the high-temperature section within the temperature range of the non-recrystallization zone, a rolling process with large reduction is adopted, with a reduction of ≥21mm per pass, and the reduction of the previous pass is greater than or equal to the reduction of the next pass, i.e., R m ≥R m+1 R m and R m+1 These represent the reduction amounts for the m-th and m+1-th passes, respectively. In the low-temperature section within the non-recrystallization zone, a small reduction rolling process is used, with a pass reduction ≤ 6 mm. During rolling in the non-recrystallization zone, water cooling is performed between passes using an intermediate billet cooling device. T3 is the temperature at which the austenite-ferrite transformation begins during cooling, T3 = 910 - 310°C - 80Mn - 15Cr - 80Mo.

[0056] (5) Air cooling and redistribution in the high-temperature zone after rolling: After rolling, the steel plate is air cooled on the conveyor rollers to T3 - 50℃ ~ T3 - 30℃, and then enters the ultra-fast cooling system.

[0057] (6) Ferrite region cooling: The steel plate enters the ultra-fast cooling system, which has 24 sets of cooling manifolds, each with a cooling length of 1m. Manifolds 1 to 24 are opened for cooling to T4 + 5℃ ~ T4 + 25℃, with a cooling rate of 10~20℃ / s, a water pressure of 0.15~0.20MPa, and a cooling roller speed of 1.8~2.2m / s. Wherein, T4 = 830-270C-90Mn-37Ni-70Cr-83Mo.

[0058] (7) Waiting for the temperature to reach the intermediate temperature T4+5℃ ~ T4+25℃, the steel plate exits the ultra-fast cooling system from the ultra-fast cooling inlet and swings on the ultra-fast cooling inlet roller conveyor to wait for the temperature to reach the intermediate temperature for 8 to 15 seconds.

[0059] (8) Bainite region cooling: Turn on 1~24 sets of manifolds for cooling, cool to T4 - 285℃ ~ T4 - 265℃, water pressure is 0.30~0.50MPa, cooling speed is 8~20℃ / s; ultra-fast cooling system cooling roller speed is 0.6~1.4m / s.

[0060] (9) Air-cooled self-tempering: After cooling to T4 - 285℃ ~ T4 - 265℃, the steel plate exits the ultra-fast cooling system from the ultra-fast cooling outlet, is then straightened, and air-cooled self-tempering is performed on the cooling bed until it is cooled to room temperature; the air-cooled self-tempering conditions are that the cooling bed is open and windless, the cooling bed is covered with hot steel plates, the spacing between the steel plates is 1~2m, and the temperature of the steel plates is T4 - 305℃ ~ T4 - 105℃.

[0061] The heating, rolling, and cooling processes in this invention are described in detail below:

[0062] The heating process employs a five-stage heating system: a heat recovery stage, a preheating stage, a primary heating stage, a secondary heating stage, and a soaking stage. The heat recovery stage temperature is ≤850℃, the preheating stage temperature is ≤950℃, the primary heating stage temperature is 1050±15℃, the secondary heating stage temperature is 1150±15℃, and the soaking stage temperature is T1, where T1 ≥ T NbN And T NbN ≤T1≤T NbN The heating temperature is +25℃, and the soaking time is 0.07~0.13min / mm, with a total heating time of 1.15~1.35min / mm. The purpose is to ensure that the billet is heated uniformly in the heating furnace. Secondly, by rationally selecting the heating temperature and heating time during billet heating, it is ensured that the Nb precipitates in the steel are completely dissolved and that the austenite grains do not grow excessively, thus preparing for precipitation during the subsequent recrystallization rolling process.

[0063] In the rolling process, a special controlled rolling process is adopted, consisting of low-temperature, high-reduction rolling in the recrystallization zone, intermediate billet cooling, and high-temperature, high-reduction rolling in the non-recrystallization zone. The first stage is low-temperature, high-reduction rolling in the recrystallization zone, with an initial rolling temperature of T2 - 10℃ ~ T2 + 30℃ and a final rolling temperature of T2 - 80℃ ~ T2 - 40℃. The purpose is to ensure that the billet can be rolled in the recrystallization zone, avoiding mixed crystals, and simultaneously preventing the growth of recrystallized grains by the precipitation of Nb compounds during rolling, thus refining the recrystallized grains. In the low-temperature section within the recrystallization zone temperature range, a high-reduction rolling process is used, with a reduction of ≥31mm per pass, and the reduction of the previous pass being less than or equal to the reduction of the next pass, i.e., R... n ≤R n+1 R n and R n+1The reduction amounts for the nth and (n+1)th passes, respectively; the intermediate billet thickness t after rolling. z , satisfying 3.4t≤t z ≤4.4t, where t is the thickness of the finished steel plate. The purpose is to fully break down the continuously cast microstructure by using a large reduction in the low-temperature section of the recrystallization zone, resulting in refined recrystallized grains. The larger the total reduction and the larger the reduction per pass, the more significant the refinement effect on austenite grains, and because the temperature is low, the grains will not grow excessively. In actual production, due to limitations of billet and rolling mill, the reduction cannot be infinitely large. According to the results of this invention, the reduction per pass is ≥31mm, and R... n ≤R n+1 This can effectively refine the grain size.

[0064] During intermediate billet cooling, immediately after rolling in the austenite recrystallization zone, water cooling is performed using an intermediate billet cooling device to cool the billet to the non-recrystallization starting rolling temperature range, i.e., T2-240℃ ~ T2-220℃. The purpose is to obtain fine recrystallized grains after rolling in the recrystallization zone. During grain growth, rapid cooling to below the recrystallization temperature avoids grain growth and reduces waiting time, thereby improving rolling efficiency.

[0065] In high-temperature, high-reduction rolling in the non-recrystallization zone, the initial rolling temperature is T2-240℃ ~ T2-220℃, and the final rolling temperature is T3 ~ T3 + 25℃. In the high-temperature section within the non-recrystallization zone, a high-reduction rolling process is used, with a single-pass reduction ≥ 21mm, and the reduction of the preceding pass being greater than or equal to the reduction of the following pass, i.e., R... m ≥R m+1 The purpose is to achieve high-reduction rolling in the high-temperature stage of the non-recrystallization region, which allows for the deformation and flattening of recrystallized equiaxed grains, forming more deformation bands and accumulating more energy to prepare for subsequent cooling phase transformation. In the low-temperature section within the non-recrystallization region, a low-reduction rolling process is used, with a reduction per pass ≤6mm. The purpose is that the rolling temperature is relatively low, and low-temperature rolling processes are detrimental to the steel plate shape; therefore, a small deformation amount is used in the low-temperature section to ensure the final plate shape. During the rolling process in the non-recrystallization region, water cooling is performed between passes using an intermediate billet cooling device. The purpose is twofold: firstly, the non-recrystallization region has a large temperature range, requiring cooling to simultaneously match the high-temperature and low-temperature rolling in the non-recrystallization region; secondly, rapid cooling and rolling in the non-recrystallization region reduces the recovery time of the deformed austenite after rolling, accumulating more energy for subsequent cooling phase transformation.

[0066] In the cooling process, a multi-stage controlled cooling process is adopted, which includes air cooling redistribution in the high-temperature zone after rolling, cooling in the ferrite zone, waiting to reach the desired temperature, cooling in the bainite zone, and air cooling self-tempering.

[0067] In the post-rolling high-temperature air-cooling redistribution process, after rolling, the steel plate is air-cooled on the conveyor rollers to a temperature of T3 - 50℃ ~ T3 - 30℃, and then enters the ultra-fast cooling system. The purpose is to redistribute carbon during the air-cooling process of the rolled steel plate, so that when the temperature is T3 - 50℃ ~ T3 - 30℃, it is in the two-phase region, resulting in a carbon-rich austenite and polygonal ferrite microstructure.

[0068] During the ferrite cooling process, the steel plate enters an ultra-rapid cooling system and is cooled to T4+5℃ ~ T4+25℃ at a cooling rate of 10~20℃ / s, with a water pressure of 0.15~0.20MPa and a cooling roller speed of 1.8~2.2m / s. The purpose is to rapidly reduce the steel plate temperature to the lowermost part of the ferrite phase transformation region, obtaining a fine acicular ferrite structure, thus ensuring the steel plate's yield strength and good low-temperature toughness.

[0069] In the waiting-warming process, after cooling to an intermediate temperature of T4 + 5℃ ~ T4 + 25℃, the steel plate exits the ultra-rapid cooling system from the ultra-rapid cooling inlet and oscillates on the ultra-rapid cooling inlet roller conveyor for 8 to 15 seconds to wait for warming. The purpose is to ensure that when the steel plate temperature is at the lower end of the ferrite phase transformation range, the acicular ferrite phase transformation begins. In order to obtain a larger amount of fine ferrite structure, the steel plate needs to remain in this temperature range for a longer period of time to allow for sufficient phase transformation. According to the results of this invention, when the waiting-warming time is 8 to 15 seconds, a certain proportion of fine acicular ferrite can be obtained, ensuring a good strength and toughness match for thick submarine pipeline steel plates.

[0070] In the bainitic cooling process, the temperature is reduced to T4 - 285℃ ~ T4 - 265℃, with a water pressure of 0.30~0.50MPa and a cooling rate of 8~20℃ / s. The ultra-fast cooling system has a cooling roller speed of 0.6~1.4m / s. The purpose is to rapidly cool the steel plate (cooling rate of 8~20℃ / s) when the ferrite phase transformation reaches a certain proportion, causing the bainitic phase transformation to occur, ensuring that the steel plate has high tensile strength, thereby reducing the yield strength ratio of the steel plate.

[0071] In air-cooled self-tempering, after cooling to T4 -285℃ ~ T4 -265℃, the steel plate exits the ultra-rapid cooling system through the ultra-rapid cooling outlet, is then straightened, and subjected to air-cooled self-tempering on a cooling bed until it cools to room temperature. The purpose is twofold: firstly, to eliminate or reduce internal stress during the self-tempering cooling process of thick steel plates; secondly, the bainitic phase transformation generates a hard MA phase, which is detrimental to the toughness of the steel plate and easily leads to locally high hardness. Air-cooled self-tempering allows for partial decomposition of MA, improving toughness and reducing localized hard spots. The air-cooled self-tempering conditions are: the cooling bed is completely open and windless; the cooling bed must be fully covered with hot steel plates with a spacing of 1-2 meters; and the steel plate temperature is T4 -305℃ ~ T4 -105℃. The aim is to control the steel plate density on the cooling bed, the temperature of the cooling bed area, and the air-cooling rate of the steel plates on the cooling bed through mass production, so that the pipeline steel plates on the cooling bed can achieve the desired air-cooled self-tempering effect.

[0072] Through optimized design of low-carbon and low-alloy components, coupled with a specially controlled rolling process that includes five-stage heating, low-temperature high-reduction rolling in the recrystallization zone, immediate cooling of intermediate billets, and high-temperature high-reduction rolling in the non-recrystallization zone, and a multi-stage controlled cooling process involving air-cooled redistribution in the high-temperature zone, cooling in the ferrite zone, waiting to reach the desired temperature, cooling in the bainite zone, and air-cooled self-tempering, steel plates with a thickness of ≥35mm are produced from continuously cast billets with a thickness of ≤320mm. The compression ratio is ≤9.14, and the microstructure consists of quasi-polygonal ferrite + acicular ferrite + bainite + tempered bainite. The steel exhibits a multiphase microstructure, with a bainite MA self-temper decomposition ratio ≤20%, an average ferrite grain size of 3~6μm, a quasi-polygonal ferrite microstructure ratio of 5%~15%, a acicular ferrite microstructure ratio of 25%~40%, and a bainite microstructure ratio of 25%~65%. The steel plate has a yield strength ≥500MPa, tensile strength ≥630MPa, elongation ≥35%, yield strength ratio ≤0.80, impact energy KV2 ≥300J at -40℃, impact energy KV2 ≥250J at -60℃, hardness ≤208HV10, DWTT drop shear area fraction 100% at -10℃, DWTT drop shear area fraction ≥90% at -20℃, and a DWTT ductile-brittle transition temperature below -20℃.

[0073] This invention applies to thick-walled subsea pipeline steel of grade X70. The chemical composition of the steel plate, by mass percentage, includes: C: 0.045%, Si: 0.12%, Mn: 1.68%, P: 0.0120%, S: 0.0022%, Cr: 0.16%, Ni: 0.16%, Mo: 0.10%, Nb: 0.060%, Ti: 0.016%, Alt: 0.035%, N: 0.0041%, with the remainder being iron and unavoidable impurities. The carbon equivalent (CEV) of the steel plate is 0.388, and the Pcm is 0.150. Smelting is carried out according to the above chemical composition, and a 320mm thick continuously cast billet is obtained by lightly reducing the thickness at the end of continuous casting. The center segregation is grade 0.5, and the segregation grades for types A, B, C, and D are all grade 0.5.

[0074] A specific embodiment of the present invention is given below.

[0075] The specific process steps are as follows:

[0076] (1) Five-stage heating process: The heating process consists of a heat recovery stage, a preheating stage, a first heating stage, a second heating stage, and a soaking stage. The temperature of the heat recovery stage is 840℃, the temperature of the preheating stage is 950℃, the temperature of the first heating stage is 1060℃, the temperature of the second heating stage is 1155℃, and the temperature of the soaking stage is T1, where T1 ≥ T NbN And T NbN ≤T1≤T NbN +25℃, through calculation, T NbN =1204℃. Therefore, 1204℃≤T1≤1229℃.

[0077] (2) Low-temperature high-reduction rolling in the recrystallization zone: Calculations show T2 = 1121℃. The initial rolling temperature is T2 - 10℃ ~ T2 + 30℃, and the final rolling temperature is T2 - 80℃ ~ T2 - 40℃, i.e., the initial rolling temperature is 1111℃ ~ 1151℃, and the final rolling temperature is 1041℃ ~ 1081℃. A high-reduction rolling process is adopted, with a reduction of ≥31mm per pass, and the reduction of the previous pass being less than or equal to the reduction of the next pass. R n ≤R n+1 That is, a total of 5 rolling passes are performed, with reductions of 31mm, 33mm, 37mm, 40mm and 44mm per pass, respectively. The thickness t of the intermediate billet after rolling is... z 3.4t≤t z ≤4.4t, where t is the thickness of the steel plate, that is, the thickness of the intermediate billet is 135mm, and the thickness of the finished steel plate is 35mm.

[0078] (3) Intermediate billet cooling: After the recrystallization zone rolling is completed, the intermediate billet cooling device is immediately used for water cooling to T2-240℃ ~ T2-220℃, which is 881℃ ~ 901℃.

[0079] (4) High-temperature rolling with large reduction in the non-recrystallization zone: Through calculation, T2 is 1121℃ and T3 is 755℃. The initial rolling temperature is T2-240℃ ~ T2-220℃, and the final rolling temperature is T3 ~ T3 + 25℃, that is, the initial rolling temperature is 881℃ ~ 901℃ and the final rolling temperature is 755℃ ~ 780℃. During the rolling process in the non-recrystallization zone, water cooling is carried out by the intermediate billet cooling device between passes. In the high-temperature section within the temperature range of the non-recrystallization zone, the reduction per pass is ≥21mm, and a total of 4 passes are rolled, with reductions of 21mm, 22mm, 25mm and 26mm per pass, respectively. In the low-temperature section within the temperature range of the non-recrystallization zone, the reduction per pass is 6mm, and the thickness of the finished steel plate is 35mm.

[0080] (5) Air cooling and redistribution in the high-temperature zone after rolling: After rolling, the steel plate is air cooled on the conveyor roller table to T3 - 50℃ ~ T3 - 30℃, that is, 705℃ ~ 725℃, and then enters the ultra-fast cooling system.

[0081] (6) Ferrite region cooling: The steel plate enters the ultra-fast cooling system and is cooled to T4 + 5℃ ~ T4 + 25℃. Through calculation, T4 = 641℃, that is, it is cooled to 646℃ ~ 666℃. The cooling rate is 18℃ / s, the water pressure is 0.20MPa, and the speed of the cooling roller conveyor of the ultra-fast cooling system is 2.0m / s.

[0082] (7) Waiting for the temperature to reach the intermediate temperature of 646℃ ~ 666℃, the steel plate exits the ultra-fast cooling system from the ultra-fast cooling inlet and swings on the ultra-fast cooling inlet roller conveyor to wait for the temperature to reach the intermediate temperature for 10 seconds.

[0083] (8) Bainite region cooling: The steel plate re-enters the ultra-fast cooling system and is cooled to T4 - 285℃ ~ T4 - 265℃. By calculation, T4 = 641℃, which means it is cooled to 356℃ ~ 376℃. The water pressure is 0.40MPa, the cooling rate is 8~20℃ / s, and the speed of the cooling roller conveyor in the ultra-fast cooling system is 0.6~1.4m / s.

[0084] (9) Air-cooled self-tempering: After cooling to a temperature of 356℃ ~ 376℃, the steel plate exits the ultra-fast cooling system from the ultra-fast cooling outlet, is then straightened, and air-cooled self-tempering is performed on the cooling bed until it is cooled to room temperature; the air-cooled self-tempering conditions are that the cooling bed is open and windless, the cooling bed is covered with hot steel plates, the spacing between the steel plates is 1.5m, and the temperature of the steel plates is T4 - 305℃ ~ T4 - 105℃, which is 336℃ ~ 536℃.

[0085] The microstructure is a multiphase structure consisting of quasi-polygonal ferrite, acicular ferrite, bainite, and tempered bainite. The proportion of MA (magnesium oxide) in the bainite after self-tempering is ≤20%. The average grain size of ferrite is 3~6 μm. The proportions of quasi-polygonal ferrite (5%~15%), acicular ferrite (25%~40%), bainite (25%~65%), and tempered bainite (5%~20%) are as follows: yield strength ≥500 MPa, tensile strength ≥630 MPa, elongation ≥35%, yield ratio ≤0.80, impact energy (KV2) ≥300 J at -40℃, impact energy (KV2) ≥250 J at -60℃, hardness ≤208 HV10, DWTT (drop weight shear fraction) ≥100% at -10℃, DWTT ≥90% at -20℃, and DWTT ductile-brittle transition temperature below -20℃. The metallographic structure is shown in Figure 1.

[0086] Based on the above-described preferred embodiments of the present invention, and through the foregoing description, those skilled in the art can make various changes and modifications without departing from the inventive concept. The technical scope of this invention is not limited to the contents of the specification, but must be determined according to the scope of the claims.

Claims

A large thickness submarine pipeline steel plate characterized in that, The chemical composition, by mass percentage, includes: C: 0.031–0.061%, Si: 0.08–0.16%, Mn: 1.64–1.72%, P: 0.0090–0.0140%, S: 0.0015–0.0038%, Cr: 0.12–0.20%, Ni: 0.12–0.20%, Mo: 0.07–0.13%, Nb: 0.054–0.064%, Ti: 0.011–0.01%. 9%, Alt: 0.021~0.049%, N: 0.0031~0.0051%, the remainder being iron and unavoidable impurities. The chemical composition of the steel plate also meets the following requirements: carbon equivalent CEV=C+Mn / 6+(Cr+Mo+V) / 5+(Cu+Ni) / 15 is 0.350~0.427, Pcm=C+Si / 30+(Mn+Cu+Cr) / 20+Ni / 60+Mo / 15+V / 10+5B is 0.127~0.

174. The microstructure of the thick submarine pipeline steel plate is a multiphase structure consisting of quasi-polygonal ferrite, acicular ferrite, bainite, and tempered bainite. The proportion of MA self-tempering decomposition in the bainite is ≤20%. The average grain size of the ferrite is 3~6μm. The proportion of quasi-polygonal ferrite is 5%~15%, acicular ferrite is 25%~40%, bainite is 25%~65%, and tempered bainite is 5%~20%. The steel plate has a yield strength ≥500MPa, tensile strength ≥630MPa, elongation ≥35%, yield ratio ≤0.80, impact energy KV2 ≥300J at -40℃, impact energy KV2 ≥250J at -60℃, hardness ≤208HV10, DWTT drop shear area fraction 100% at -10℃, DWTT drop shear area fraction ≥90% at -20℃, and DWTT ductile-brittle transition temperature below -20℃. The method for producing thick submarine pipeline steel plates according to claim 1 is characterized in that, The continuous casting billet is subjected to a special controlled rolling process, which involves five stages of heating, low-temperature rolling with large reduction in the recrystallization zone, intermediate billet cooling, and high-temperature rolling with large reduction in the non-recrystallization zone. It is also subjected to a multi-stage controlled cooling process, which involves air cooling and redistribution in the high-temperature zone after rolling, cooling in the ferrite zone, waiting to reach a certain temperature, cooling in the bainite zone, and air cooling and self-tempering. This process produces submarine pipeline steel plates. The method for producing thick submarine pipeline steel plates according to claim 2 is characterized in that, The five-stage heating process is as follows: the continuously cast billet is placed in a walking beam furnace for heating. The furnace is 55m long and employs a five-stage heating process consisting of a heat recovery stage, a preheating stage, a first heating stage, a second heating stage, and a soaking stage. Specifically, the heat recovery stage temperature is ≤850℃, and the heating time is 0.37–0.41 min / mm; the preheating stage temperature is ≤950℃, and the heating time is 0.32–0.35 min / mm; the first heating stage temperature is 1050±15℃, and the heating time is 0.22–0.25 min / mm; the second heating stage temperature is 1150±15℃, and the heating time is 0.17–0.20 min / mm; and the soaking stage temperature is T1, where T1 ≥ T... NbN And T NbN ≤T1≤T NbN At +25℃, the heating time in the soaking zone is 0.07–0.14 min / mm, and the total heating time is 1.15–1.35 min / mm, where lg(Nb×N)=3.7-10800 / (T) NbN +273.15). The method for producing thick submarine pipeline steel plates according to claim 2 is characterized in that, The specific process of low-temperature high-reduction rolling in the recrystallization zone is as follows: the initial rolling temperature is T2 - 10℃ ~ T2 + 30℃, and the final rolling temperature is T2 - 80℃ ~ T2 - 40℃; in the low-temperature section within the recrystallization zone temperature range, a high-reduction rolling process is adopted, with a reduction per pass ≥ 31mm, a total of 4 to 6 roughing passes, a temperature difference of 12℃ to 18℃ between adjacent passes, and the reduction of the previous pass is less than or equal to the reduction of the next pass, i.e., R n ≤R n+1 R n and R n+1 The reduction amounts for the nth and (n+1)th passes, respectively; the intermediate billet thickness t after rolling. z , satisfying 3.4t≤t z ≤4.4t, where t is the thickness of the finished steel plate, and T2 is the recrystallization temperature, T2=887+464C+6445Nb-644 +890Ti+363Al-357Si. The method for producing thick submarine pipeline steel plates according to claim 3 is characterized in that, Intermediate billet cooling: After rolling in the austenite recrystallization zone, the intermediate billet is immediately cooled by water using an intermediate billet cooling device. The water pressure in the cooling manifold is 0.4~0.6MPa, the water flow rate in the upper manifold is 50~150m³ / h, the water flow rate in the lower manifold is 100~300m³ / h, the water flow rate ratio between the lower and upper manifolds is ≥2, and the cooling water temperature is 10~30℃. The intermediate billet passes back and forth through the cooling device for cooling. The roller speed of the cooling device is 0.4~6m / s. The billet is cooled to the non-recrystallization rolling temperature range, i.e., T2-240℃ ~ T2-220℃. The method for producing thick submarine pipeline steel plates according to claim 3 is characterized in that, The non-recrystallization zone high temperature large reduction rolling: the starting rolling temperature is T2-240℃ ~ T2-220℃, and the final rolling temperature is T3 ~ T3 + 25℃; the high temperature section in the non-recrystallization zone temperature range adopts a large reduction rolling process, the high temperature section is T2-280℃ ~ T2-240℃, the rolling pass number in the high temperature zone is 4-6 passes, the rolling speed is 3-5 m / s, the pass reduction is greater than or equal to 21 mm, and the pass reduction of the previous pass is greater than or equal to the pass reduction of the next pass, i.e. R m ≥ R m+1 , R m and R m+1 are the reductions of the mth pass and the m+1th pass respectively, and the thickness of the rolled blank is t+3mm ~ t+12mm; the low temperature section in the non-recrystallization zone temperature range adopts a small reduction rolling process, the low temperature section is T3 + 5℃ ~ T3 + 30℃, the rolling pass number in the low temperature section is 1-2 passes, the rolling speed is 4-6 m / s, the pass reduction is less than or equal to 6 mm, and the thickness of the finished steel plate after rolling is t, t≥35mm; in the non-recrystallization zone rolling process, the intermediate blank cooling device is used for water cooling between passes, wherein T3 is the temperature at which the austenite starts to transform into ferrite during cooling, T3 =910-310C-80Mn-15Cr-80Mo. The method for producing thick submarine pipeline steel plates according to claim 6 is characterized in that, The high-temperature zone air cooling redistribution after rolling: After rolling, the steel plate is air cooled on the conveyor rollers. The conveyor rollers are 60m long and the roller speed is 2-5m / s. The indoor temperature is 5-39℃. The steel plate is air cooled to T3 - 50℃ ~ T3 - 30℃, and then enters the ultra-fast cooling system. The method for producing thick submarine pipeline steel plates according to claim 2 is characterized in that, The ferrite region cooling process involves the steel plate entering the ultra-fast cooling system through an ultra-fast cooling inlet. The ultra-fast cooling system has 24 sets of cooling manifolds, each with a cooling length of 1m. Manifolds 1 to 24 are opened for cooling at a rate of 10-20℃ / s, a water pressure of 0.15-0.20MPa, a cooling water temperature of 10-30℃, a water flow rate of 60-200m³ / h for the upper and lower manifolds, a water flow rate ratio of 1.1-1.5 between the lower and upper manifolds, and a cooling roller speed of 1.8-2.2m / s. The cooling temperature is reduced to T4 + 5℃ ~ T4 + 25℃, where T4 = 830-270C-90Mn-37Ni-70Cr-83Mo. The method for producing thick submarine pipeline steel plates according to claim 8 is characterized in that, The specific process of waiting for the temperature to be reached is as follows: after cooling to an intermediate temperature of T4 + 5℃ ~ T4 + 25℃, the upper and lower cooling water manifolds are closed, the steel plate is exited from the ultra-fast cooling system at the inlet of the ultra-fast cooling system, and it swings on the ultra-fast cooling inlet roller conveyor to wait for the temperature to be reached. That is, it swings back and forth on the roller conveyor at a speed of 0.5 to 1.5 m / s, the back and forth movement distance is 5 to 10 m, and the waiting time is 8 to 15 seconds. The method for producing thick submarine pipeline steel plates according to claim 8 is characterized in that, The bainitic cooling process is as follows: the steel plate re-enters the ultra-fast cooling system through the ultra-fast cooling inlet. There are 24 sets of cooling manifolds, each with a cooling length of 1m. Manifolds 1 to 24 are opened for cooling. The water pressure is 0.30 to 0.50 MPa, the cooling rate is 8 to 20℃ / s, the cooling water temperature is 10 to 26℃, the water flow rate of the upper and lower manifolds is 100 to 400 m³ / h, the water flow rate ratio of the lower to upper manifolds is 1.2 to 1.4, the cooling roller speed of the ultra-fast cooling system is 0.6 to 1.4 m / s, and the temperature is cooled to T4 - 285℃ ~ T4 - 265℃. The method for producing thick submarine pipeline steel plates according to claim 8 is characterized in that, The air-cooled self-tempering process specifically involves: cooling to T4 - 285℃ ~ T4 - 265℃, then exiting the ultra-fast cooling system from the outlet; followed by straightening and air-cooled self-tempering on a cooling bed until cooled to room temperature; the air-cooled self-tempering conditions are: the cooling bed is open and windless, the cooling bed is covered with hot steel plates with a spacing of 1-2m, and the steel plate temperature is T4 - 305℃ ~ T4 - 105℃. The method for producing thick submarine pipeline steel plates according to claim 2 is characterized in that, Using continuously cast billets with a thickness of ≤320mm, center segregation not higher than grade 0.5, and inclusions of types A, B, C, and D not higher than grade 1, steel plates with a thickness of ≥35mm and a compression ratio of ≤9.14 are produced. Thick submarine pipeline steel plate produced by the production method of thick submarine pipeline steel plate according to any one of claims 2-12.

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