A large thickness submarine pipeline steel plate and a production method thereof

Abstract

The present application relates to a kind of large thickness submarine pipeline steel plate and its production method, by the optimization design steel plate component of low-carbon low-alloy component, production method is with the special controlled rolling process of five-stage heating+recrystallization zone low temperature large reduction rolling+intermediate billet instant cooling+non-recrystallization zone high temperature large reduction rolling, and the multi-stage controlled cooling process of high temperature zone air cooling redistribution+ferrite zone cooling+waiting temperature+bainite zone cooling+air cooling self tempering cooling.The product steel plate of the present application has higher acicular ferrite proportion, and the average grain size of ferrite is finer, 3~6 μm, so that the steel plate has good strength and toughness match, the steel plate has higher strength, yield strength ≥500MPa, tensile strength ≥630MPa, and impact energy KV2 At-60 ℃, ≥250J.

Description

TECHNICAL FIELD

[0001] The present application relates to a large-thickness submarine pipeline steel plate and a production method thereof, and belongs to the technical field of cast steel. BACKGROUND

[0002] At present, the global onshore oil and gas fields have gradually entered the middle and late stages of exploitation, and the difficulty of exploration and development has increased, and the cost has increased. However, the potential of marine oil and gas resources is huge, and the prospect of exploration is good. The demand for submarine oil and gas pipelines is increasing. As the most efficient, most economical and most effective transportation mode for marine oil and gas transportation, submarine oil and gas pipelines are an indispensable lifeline project for offshore oil and gas field development and production, and will have a huge demand for submarine pipeline steel.

[0003] Because submarine pipelines are used in complex marine environments for a long time, they are subjected to environmental loads such as wave flow outside the pipe, submarine scouring and deposition, sliding changes, and fluid pressure and corrosion inside the pipe. Once the submarine pipeline is ruptured, it will cause catastrophic accidents such as economic loss, environmental pollution, and ecological damage. Therefore, submarine pipeline steel is required to have high strength, low yield ratio, excellent low-temperature toughness, corrosion resistance, and the like.

[0004] A large-thickness submarine pipeline hot-rolled steel plate and a manufacturing method thereof are provided in patent publication CN117165870A. The weight percentage of the chemical composition of the hot-rolled steel plate is: C: 0.03% to 0.05%, Si: 0.16% to 0.24%, Mn: 1.60% to 1.70%, P≤0.015%, S≤0.004%, Cr: 0.14% to 0.18%, Ni: 0.11% to 0.15%, Mo: 0.09% to 0.13%, Nb: 0.05% to 0.065%, Ti: 0.01% to 0.02%, Al: 0.02% to 0.05%, P cm ≤0.19%, and the rest is Fe and impurity elements. Through the KR desulfurization-BOF converter-LF refining-RH degassing-CCM continuous casting-slab reheating-coarse rolling-intermediate blank temperature cooling-fine rolling-accelerated cooling-heat straightening-natural cooling-warm straightening and cold straightening-outline stack cooling and ultrasonic flaw detection-finishing process, the submarine pipeline X70 grade and thickness specification above 30mm can be met.

[0005] A patent publication No. CN104357766A, a kind of for ultra-deep sea's special thick gauge high strength high toughness submarine pipeline steel, provide the chemical composition of line steel weight percentage 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%, the rest is Fe. Manufacturing method is as follows: 1) casting blank;2) rough rolling;3) finish rolling;4) cooling;5) relaxation. Special thick gauge pipeline steel thickness ≥35mm. Control finish rolling opening rolling temperature is 820~900℃, finish rolling cumulative reduction is 60~75%;Final rolling temperature is 760~840℃;After rolling, the steel plate is rolled after using two-stage accelerated cooling, I stage cooling rate 5~10℃ / s, final cooling temperature is 720~750℃, II stage cooling rate 20~40℃ / s, final cooling temperature is 100~300℃;The yield strength (R t0.5） of steel plate ≥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] A low-cost thick-gauge submarine pipeline steel plate and a manufacturing method thereof are provided in patent publication CN103451536B. The steel plate has the following chemical composition by weight percentage: 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%, and the rest is iron and unavoidable impurities. The steel plate has a dual-phase microstructure, high strength, low yield ratio, and good toughness by adopting technologies such as continuous casting billet transverse material forming, continuous casting billet low-temperature heating, low-temperature controlled rolling, and relaxation and slow cooling control. The final three passes of rolling have a reduction of 20%~30% when the finishing temperature is controlled at 990~1020℃ in the recrystallization zone, and the total reduction is 65%~75%. The total reduction is 60%~70% when the finishing temperature is controlled at 750~810℃ in the non-recrystallization zone. The steel plate is relaxed and slowly cooled after rolling, and then subjected to online quenching by MULPIC, with a cooling speed of 20~30℃ / s and a final cooling temperature of 350~450℃. The thickness of the steel plate is 28.6~31.8mm.

[0007] A large-wall-thickness submarine pipeline steel plate and a production method thereof are disclosed in patent publication CN102676925A. 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%, and the rest is Fe and unavoidable impurities. The large-wall-thickness submarine pipeline steel plate has a thickness of 25~30.2mm and is rolled by a two-stage rolling process in the recrystallization zone and the non-recrystallization zone. The first-stage rolling temperature is 950~1100℃, the single-pass reduction is 10%~20%, and the cumulative reduction is 30%~50%. The second-stage rolling temperature is 840~880℃, and the cumulative reduction is 30%~50%. The water entry temperature after rolling is 740~780℃, and the red temperature is 450~500℃. The steel plate has the following mechanical properties: yield strength between 470~570MPa, tensile strength between 535~760MPa, elongation >30%, impact energy at -20℃ >200J, DWTT shear area fraction >75%, and yield strength ratio <0.90.

[0008] Currently, there are many patents for submarine pipeline steel.

[0009] In terms of ingredients, some submarine pipeline steels use V, Cu and other alloying elements, such as patents CN111979497A, CN109234487B, CN111607747A, CN110863145A, CN108504931B, CN105132833B, CN105132807B, CN104357766A, CN103993240A, CN101701315B, CN111748741B, etc. All of them add 0.015%~0.060% V and / or 0.10%~0.30% Cu, which has high alloy content and leads to increased production cost.

[0010] In terms of process, some submarine pipeline steels are produced by coiled plate production line to obtain pipeline steel coil or steel strip. The rough rolling is 1~2 stands, the finishing rolling is 5~7 stands, and the rolling needs to be coiled after rolling, such as patents CN111979497A, CN113278880A, CN110578091B, CN111607747A, CN108504931B, etc. The submarine pipeline steel coils or steel strips produced by them have small thickness, generally 3~20mm, which is difficult to meet the needs of submarine harsh environment and high pressure pipeline transportation. Some submarine pipeline steels are rolled by medium plate or wide and thick plate rolling mill, and the steel plate thickness is large, such as patents CN103993240A, CN106566991B, etc. The thickness is 20~48mm. After rolling into steel plate, it needs to be stacked and slowly cooled, and the stacking time is 12~48 hours, which is long and affects the production efficiency.

[0011] In terms of performance, the strength of some pipeline steels in the patents is low, such as patents CN109234487B, CN107988562A, etc. They are X65 grade submarine pipeline steels with low strength, and the yield strength is ≥450MPa. The toughness of some patents is low, such as patents CN105132807B, CN103834874B, etc. The impact energy at-20℃ is ≥250J, and there is no requirement for impact energy at-40℃. The yield strength ratio of some patents is high, such as patents CN104357766A, CN103993240A, CN109234487B, CN111607747A, etc. The yield strength ratio is ≤0.90, and most of them are between 0.85~0.90.

[0012] In existing submarine pipeline steel patents, there are problems such as high alloy content, small thickness, long stacking time, low strength, low toughness, high yield strength ratio, etc. These problems lead to high production cost, slow production rhythm, long cycle, low safety, etc. of existing submarine pipeline steels, which restricts the development of submarine pipeline engineering industry. SUMMARY

[0013] In order to solve the above-mentioned problems, the present application discloses a large-thickness submarine pipeline steel plate and a production method thereof, and the specific technical scheme is as follows:

[0014] A large-thickness submarine pipeline steel plate, the chemical composition includes, in terms of 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: 0.011-0.019%, Alt: 0.021-0.049%, N: 0.0031-0.0051%, and the rest is iron and inevitable impurities, the chemical composition of the steel plate also satisfies that the carbon equivalent CEV=C+Mn / 6+(Cr+Mo+V) / 5+(Cu+Ni) / 15 is 0.350-0.427, and Pcm=C+Si / 30+(Mn+Cu+Cr) / 20+Ni / 60+Mo / 15+V / 10+5B is 0.127-0.174. The elements in the calculation formula of CEV and Pcm are the mass percentage values in the steel plate, without percentage sign.

[0015] Further, the continuous casting blank is sequentially subjected to a special controlled rolling process of five-stage heating, recrystallization zone low-temperature large reduction rolling, intermediate blank cooling, non-recrystallization zone high-temperature large reduction rolling, and a multi-stage controlled cooling process of post-rolling high-temperature zone air cooling redistribution, ferrite zone cooling, waiting, bainite zone cooling, and air cooling self-tempering cooling, to form the submarine pipeline steel plate.

[0016] Further, the process of the five-stage heating is specifically as follows: the continuous casting blank is placed in a walking beam heating furnace for heating, the heating furnace has a length of 55 m, and adopts a five-stage heating process of a heat recovery section, a preheating section, a first heating section, a second heating section and a soaking section, wherein the temperature of the heat recovery section is ≤850℃, the heating time of the heat recovery section is 0.37-0.41 min / mm, the temperature of the preheating section is ≤950℃, the heating time of the preheating section is 0.32-0.35 min / mm, the temperature of the first heating section is 1050±15℃, the heating time of the first heating section is 0.22-0.25 min / mm, the temperature of the second heating section is 1150±15℃, the heating time of the second heating section is 0.17-0.20 min / mm, the temperature of the soaking section is T1, T NbN ≤T NbN ≤T1≤T NbN+ 25℃, the heating time of the soaking section is 0.07-0.14 min / mm, and the total heating time is 1.15-1.35 min / mm, wherein, lg(Nb x N)=3.7-10800 / (T NbN +273.15).

[0017] Further, the process of the recrystallization zone low-temperature large reduction rolling specifically includes: the rough rolling temperature is T2-10℃ ~ T2+30℃, and the finish rolling temperature is T2-80℃ ~ T2-40℃; in the low-temperature section within the recrystallization zone temperature range, the rolling process with large reduction is adopted, the pass reduction is ≥31 mm, the total rough rolling passes are 4-6 passes, the temperature difference between adjacent passes is 12℃ ~ 18℃, 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 are the reductions of the nth pass and the n+1th pass respectively; the thickness t z of the intermediate billet after rolling satisfies 3.4t≤t z ≤4.4t, t is the thickness of the finished steel plate, wherein, T2 is the recrystallization temperature, T2=887+464C+6445Nb-644 +890Ti+363Al-357Si.

[0018] Further, the intermediate billet cooling includes: immediately after the rolling in the austenite recrystallization zone, the intermediate billet is cooled by the intermediate billet cooling device, the cooling header water pressure is 0.4-0.6 MPa, the upper header water flow rate is 50-150 m³ / h, the lower header water flow rate is 100-300 m³ / h, the water flow rate ratio of the lower header to the upper header is ≥2, the cooling water temperature is 10-30℃, the intermediate billet passes through the cooling device back and forth for cooling, the cooling device roller speed is 0.4-6 m / s, and the intermediate billet is cooled to the non-recrystallization rough rolling temperature range, i.e., T2-240℃ ~ T2-220℃.

[0019] Further, the non-recrystallization zone high-temperature large reduction rolling includes: the rough rolling temperature is T2-240℃ ~ T2-220℃, and the finish rolling temperature is T3 ~ T3+25℃; in the high-temperature section within the non-recrystallization zone temperature range, the rolling process with large reduction is adopted, the high-temperature section is the temperature range of T2-280℃ ~ T2-240℃, the number of rolling passes in the high-temperature zone is 4-6 passes, the rolling speed is 3-5 m / s, the pass reduction is ≥21 mm, 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+1respectively, the thickness of the blank after rolling is t+3mm ~ t+12mm; the rolling process with small reduction in the low temperature section in the non-recrystallization zone temperature range, the low temperature section is the temperature of T3+5℃ ~ T3+30℃, the rolling pass number in the low temperature section is 1 ~ 2, the rolling speed is 4 ~ 6m / s, the pass reduction is ≤6mm, and the thickness of the finished steel plate after rolling is t, t≥35mm; in the rolling process in the non-recrystallization zone, the intermediate blank cooling device is used for water cooling cooling between passes, wherein, T3 is the temperature at which the austenite starts to transform into ferrite, T3=910-310C-80Mn-15Cr-80Mo.

[0020] Further, the air cooling redistribution in the high temperature zone after rolling: after rolling, the steel plate is air cooled on the conveying roller, the length of the conveying roller is 60m, 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.

[0021] Further, the ferrite zone cooling: the steel plate enters the ultra-fast cooling system from the ultra-fast cooling inlet, the ultra-fast cooling system has 24 groups of cooling headers, and the cooling length of each group of headers is 1m; 1 ~ 24 groups of headers are opened for cooling, the cooling speed is 10 ~ 20℃ / s, the water pressure is 0.15 ~ 0.20MPa, the cooling water temperature is 10 ~ 30℃, the water flow of the upper header and the lower header is 60 ~ 200m³ / h, the water flow ratio of the lower header to the upper header is 1.1 ~ 1.5, the cooling roller speed of the ultra-fast cooling system is 1.8 ~ 2.2m / s, and the cooling is to T4+5℃ ~ T4+25℃, wherein, T4=830-270C-90Mn-37Ni-70Cr-83Mo.

[0022] Further, the specific waiting temperature is: after cooling to the intermediate temperature T4+5℃ ~ T4+25℃, the upper and lower cooling water headers are closed, the steel plate exits the ultra-fast cooling system from the ultra-fast cooling inlet, and swings on the ultra-fast cooling inlet roller for waiting, that is, swings forward and backward on the roller at a speed of 0.5 ~ 1.5m / s, the forward and backward moving distance is 5 ~ 10m, and the waiting time is 8 ~ 15 seconds.

[0023] Further, the bainite zone cooling is specifically as follows: the steel plate re-enters the ultra-fast cooling system from the ultra-fast cooling inlet, and a total of 24 groups of cooling headers are provided, and each group of headers has a cooling length of 1 m; the 1-24 groups of headers are opened to cool, the water pressure is 0.30-0.50 MPa, the cooling speed is 8-20 ℃ / s, the cooling water temperature is 10-26 ℃, the water flow of the upper header and the lower header is 100-400 m³ / h, the water flow ratio of the lower header to the upper header is 1.2-1.4, the cooling roller speed of the ultra-fast cooling system is 0.6-1.4 m / s, and the steel plate is cooled to T4-285 ℃-T4-265 ℃.

[0024] Further, the air cooling self-tempering cooling is specifically as follows: after being cooled to T4-285 ℃-T4-265 ℃, the steel plate is taken out of the ultra-fast cooling system from the ultra-fast cooling system outlet; then the steel plate is straightened and air-cooled and self-tempered on the cooling bed until the steel plate is cooled to room temperature; the air cooling self-tempering condition is that the cooling bed is not ventilated, the hot steel plate needs to be fully laid on the cooling bed, the spacing between the steel plates is 1-2 m, and the temperature of the steel plate is T4-305 ℃-T4-105 ℃.

[0025] Further, the continuous casting billet with a thickness specification of ≤320 mm is used, the center segregation is not higher than 0.5 level, the A, B, C and D type inclusions are all not higher than 1 level, the steel plate with a thickness specification of ≥35 mm is prepared, and the compression ratio is ≤9.14.

[0026] The application also protects the large-thickness submarine pipeline steel plate prepared by the production method of the large-thickness submarine pipeline steel plate.

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

[0028] The application has the following beneficial effects:

[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 structure diagram of an embodiment of the present application. DETAILED DESCRIPTION

[0035] The present application will be further illustrated below in conjunction with the specific embodiments. It should be understood that the following specific embodiments are only used to illustrate but not to limit the scope of the present application.

[0036] The large-thickness submarine pipeline steel plate and a production method thereof, the chemical composition of the steel plate includes, in percentage by mass: 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: 0.011-0.019%, Alt: 0.021-0.049%, N: 0.0031-0.0051%, and the rest is iron and inevitable impurities, the chemical composition of the steel plate also satisfies that carbon equivalent CEV=C+Mn / 6+(Cr+Mo+V) / 5+(Cu+Ni) / 15 is 0.350-0.427, and Pcm=C+Si / 30+(Mn+Cu+Cr) / 20+Ni / 60+Mo / 15+V / 10+5B is 0.127-0.174.

[0037] The main role of each chemical component in the present application and the selection of the amount thereof are specifically analyzed and explained as follows:

[0038] Carbon (C): Carbon is the most economical strengthening element in steel, which has a solid solution strengthening effect, and forms carbides with Nb, Ti, Cr, Mo, etc., thereby having a precipitation strengthening effect. The increase of the carbon content has a significant effect on improving the strength and hardness of the pipeline steel, but too high carbon will result in poor low-temperature toughness and welding performance, and reduce the low-temperature drop hammer performance of the pipeline steel, therefore, the carbon content is selected to be 0.031%-0.061%.

[0039] Silicon (Si): Silicon has a solid solution strengthening effect in steel, but too much silicon is easy to 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%-0.16%.

[0040] Manganese (Mn): Manganese has a solid solution strengthening effect in steel, which improves the strength and hardness of the steel, and a reasonable manganese content can ensure the strength of the pipeline steel at a low cost, which is the most economical strengthening element except carbon, but too much manganese will result in center segregation of the cast slab, which has a great harm to the toughness of the thick plate, and is also not conducive to the control of banded structure, therefore, the manganese content is selected to be 1.64%-1.72%.

[0041] Phosphorus (P): Phosphorus is an impurity element in steel, and too high phosphorus is easy to produce segregation, which makes the plasticity and toughness of steel decrease significantly, especially under low temperature conditions, but too low phosphorus will significantly increase the cost of steelmaking, therefore, the content of phosphorus is selected as 0.0090%~0.0140%.

[0042] Sulfur (S): Sulfur is an impurity element in steel, which not only increases the hot brittleness of steel, but also easily forms MnS inclusions with manganese, thereby reducing the low temperature toughness of steel, but too low sulfur will significantly increase the cost of steelmaking, therefore, the content of sulfur is selected as 0.0015%~0.0038%.

[0043] Chromium (Cr): Chromium has a solid solution strengthening effect in steel, and at the same time, chromium as a ferrite forming element can obtain more acicular ferrite structure in high niobium steel, but when the content of chromium is too high, the microhardness of the pipeline steel will increase and the low temperature toughness will decrease. Therefore, the content of chromium is selected as 0.12%~0.20%.

[0044] Nickel (Ni): Nickel has a solid solution strengthening effect in steel, which improves the strength of the steel without significantly increasing the hardness of the steel, and at the same time, nickel is beneficial to low temperature toughness, but too high nickel content increases the cost of alloy. Therefore, the content of nickel is selected as 0.12%~0.20%.

[0045] Molybdenum (Mo): Molybdenum can significantly improve the hardenability of steel, delay ferrite transformation, and obtain acicular ferrite structure, which is beneficial to improve the strength and toughness of pipeline steel, but molybdenum leads to an increase in the number of MA components, which is not conducive to improving toughness, and too high molybdenum content significantly increases the cost of alloy. Therefore, the content of molybdenum is selected as 0.07%~0.13%.

[0046] Niobium (Nb): Niobium is an important grain refining element in steel. In the hot rolling process, niobium strongly suppresses austenite recrystallization and its precipitation in austenite, pins austenite grain boundaries, and refines recrystallized grains. During cooling, the solid-soluble niobium can continue to precipitate in the form of niobium carbonitride, which significantly refines the structure of the material after phase transformation, further improving the strength and toughness of the steel. Adding too much niobium will first increase the cost of alloy; secondly, niobium cannot be completely solid-soluble in normal heating process; thirdly, the precipitation strengthening of niobium is not conducive to the yield ratio. Therefore, the content of niobium is selected as 0.054%~0.064%.

[0047] Titanium (Ti): Titanium is a solid nitrogen element in steel, which can form dispersed titanium nitride particles to inhibit austenite grain coarsening during billet heating and rolling. When the addition amount is too high, coarse carbon / nitride precipitates are easily formed in the center of the cast billet, which affects the low temperature toughness of the steel plate. Therefore, the content of titanium is selected as 0.011%~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, t is the thickness of the finished steel plate. Among them, T2 is the recrystallization temperature, T2 = 887 + 464C + 6445Nb - 644 + 890Ti + 363Al - 357Si.

[0054] (3) Billet cooling: immediately after the end of austenite recrystallization zone rolling, water cooling is carried out by using a billet cooling device, and the cooling is carried out to a non-recrystallization rolling temperature interval, that is, to T2-240℃ ~ T2-220℃.

[0055] (4) High temperature large reduction rolling in non-recrystallization zone: the 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 temperature range, a large reduction rolling process is adopted, and the reduction of one pass is ≥21mm, and the reduction of the previous pass is greater than or equal to the reduction of the next pass, that is, R m ≥ R m+1 , R m and R m+1 are the reductions of the mth pass and the m+1th pass, respectively; in the low temperature section within the non-recrystallization zone temperature range, a small reduction rolling process is adopted, and the reduction of one pass is ≤6mm; during the rolling process in the non-recrystallization zone, the billet cooling device is used for water cooling between passes. Among them, T3 is the temperature at which the austenite starts to transform into ferrite during cooling, T3 = 910-310C-80Mn-15Cr-80Mo.

[0056] (5) High temperature zone air cooling redistribution after rolling: after rolling, the steel plate is air cooled on the conveying roller, and the air cooling is carried out to T3-50℃ ~ T3-30℃, and then enters the ultra-fast cooling system.

[0057] (6) Ferrite zone cooling: the steel plate enters the ultra-fast cooling system, and there are 24 groups of cooling headers, and the cooling length of each group of headers is 1m; 1~24 groups of headers are opened for cooling, and the cooling is carried out to T4+5℃ ~ T4+25℃, the cooling speed is 10~20℃ / s, the water pressure is 0.15~0.20MPa, and the ultra-fast cooling system cooling roller speed is 1.8~2.2m / s. Among them, T4 = 830-270C-90Mn-37Ni-70Cr-83Mo.

[0058] (7) Waiting for temperature: after cooling to 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 for waiting for temperature, and the waiting time is 8~15 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+1 The reduction amounts for the nth and (n+1)th passes, respectively; the intermediate billet thickness t after rolling.z , meet 3.4t≤t z ≤4.4t, t is the thickness of the finished steel plate. The purpose is to use large reduction rolling in the low temperature section of the recrystallization zone to fully break the continuous casting state organization and obtain refined recrystallized grains. The larger the total reduction and the single pass reduction, the more obvious the effect of refining austenite grains. Because the temperature is relatively low, the grains will not grow too much. In actual production, because of the limitations of the billet, rolling mill and the like, the reduction cannot be infinite. According to the results of the present application, the pass reduction is ≥31mm, and R n ≤R n+1 , which can achieve the purpose of effectively refining the grains.

[0064] In the intermediate billet cooling, after the rolling in the austenite recrystallization zone is finished, the intermediate billet cooling device is used for water cooling immediately, and the cooling is performed to the non-recrystallization opening rolling temperature interval, that is, to T2-240℃ ~ T2-220℃. The purpose is that after the rolling in the recrystallization zone, fine recrystallized grains are obtained. During the growth of the grains, the rapid cooling below the recrystallization temperature can avoid the growth of the grains and reduce the waiting time and improve the rolling efficiency.

[0065] In the high temperature large reduction rolling in the non-recrystallization zone, the opening rolling temperature is T2-240℃ ~ T2-220℃, and the finish rolling temperature is T3 ~ T3 + 25℃. In the high temperature section in the temperature range of the non-recrystallization zone, the large reduction rolling process is used, and the one-pass reduction is ≥21mm, and the reduction of the previous pass is greater than or equal to the reduction of the subsequent pass, that is, R m ≥R m+1 . The purpose is that in the high temperature stage in the non-recrystallization zone, the temperature is relatively high, and the large reduction rolling can be realized, the recrystallized equiaxed grains can be deformed and flattened, and more deformation bands and more energy can be accumulated, which prepares for the subsequent cooling and phase change. In the low temperature section in the temperature range of the non-recrystallization zone, the small reduction rolling process is used, and the pass reduction is ≤6mm. The purpose is that the rolling temperature is relatively low, and the low temperature rolling process is not conducive to the shape of the steel plate, so small deformation rolling is used in the low temperature section to ensure the shape of the steel plate after rolling. In the rolling process in the non-recrystallization zone, the intermediate billet cooling device is used for water cooling between passes. The purpose is that on the one hand, the temperature interval in the non-recrystallization zone is large, and cooling is needed to match the high temperature rolling and low temperature rolling in the non-recrystallization zone, and on the other hand, the recovery time of the deformed austenite after rolling is reduced through the non-recrystallization rapid cooling and rolling, and more energy is accumulated for the subsequent cooling and phase change.

[0066] In the cooling process, the multi-stage controlled cooling process of air cooling redistribution in the high temperature zone after rolling + ferrite zone cooling + waiting + bainite zone cooling + air cooling self-tempering cooling is used.

[0067] In the air cooling redistribution in the high temperature zone after rolling, the steel plate is air cooled on the conveying roller after rolling, and air cooled to T3 - 50℃ ~ T3 - 30℃, and then enters the ultra-fast cooling system. The purpose is to redistribute carbon during the air cooling of the steel plate after rolling, and when the temperature is T3 - 50℃ ~ T3 - 30℃, it is in the two-phase region, and a carbon-rich austenite and polygonal ferrite structure is obtained.

[0068] In the ferrite zone cooling, the steel plate enters the ultra-fast cooling system and is cooled to T4 + 5℃ ~ T4 + 25℃, the cooling speed is 10 ~ 20℃ / s, the water pressure is 0.15 ~ 0.20MPa, and the ultra-fast cooling system cooling roller speed is 1.8 ~ 2.2m / s. The purpose is to quickly reduce the temperature of the steel plate to the lower part of the ferrite phase transition interval, to obtain fine acicular ferrite structure, and to ensure the yield strength and good low temperature toughness of the steel plate.

[0069] In the waiting process, after cooling to 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 for waiting, and the waiting time is 8 ~ 15 seconds. The purpose is that when the temperature of the steel plate is in the lower part of the ferrite phase transition interval, the acicular ferrite phase transition begins, and in order to obtain more fine ferrite structure, the steel plate needs to stay in this temperature interval for a long time to fully phase transition. According to the results of the present application, when the waiting time is 8 ~ 15 seconds, a certain proportion of fine acicular ferrite can be obtained, which ensures the good strength and toughness matching of the large thickness submarine pipeline steel plate.

[0070] In the bainite zone cooling, the cooling is to T4 - 285℃ ~ T4 - 265℃, the water pressure is 0.30 ~ 0.50MPa, the cooling speed is 8 ~ 20℃ / s, and the ultra-fast cooling system cooling roller speed is 0.6 ~ 1.4m / s. The purpose is that when the ferrite phase transition reaches a certain proportion, the steel plate is quickly cooled (the cooling speed is 8 ~ 20℃ / s) to make it undergo bainite phase transition, so as to ensure that the steel plate has high tensile strength, thereby reducing the yield strength ratio of the steel plate.

[0071] In the air cooling self-tempering cooling, after cooling to T4-285℃ ~ T4-265℃, the steel plate is out of the ultra-fast cooling system from the ultra-fast cooling outlet, then straightened, and air-cooled self-tempering on the cooling bed until cooled to room temperature. The purpose is, on the one hand, to eliminate or weaken the internal stress of the large-thickness steel plate during the self-tempering cooling process; on the other hand, during the bainite phase transformation process, MA hard phase is produced, which is not conducive to the toughness of the steel plate and easily leads to higher local hardness of the steel plate. The air-cooled self-tempering cooling can make the MA part decompose, improve the toughness and local hard spots. The air-cooled self-tempering conditions are that the cooling bed is permeable without wind, the hot steel plates are fully laid on the cooling bed, and the spacing between the steel plates is 1-2 m, and the temperature of the steel plates is T4-305℃ ~ T4-105℃. The purpose is to control the steel plate flat-laying density on the cooling bed, the cooling bed area temperature and the air-cooled cooling speed of the steel plate on the cooling bed through batch production, so that the pipeline steel plate on the cooling bed can produce air-cooled self-tempering effect.

[0072] By optimizing the design of low-carbon low-alloy components, and supplemented by the special controlled rolling process of five-stage heating + recrystallization zone low-temperature large reduction + intermediate billet instant cooling + non-recrystallization zone high-temperature large reduction, and the multi-stage controlled cooling process of high-temperature zone air cooling redistribution + ferrite zone cooling + waiting + bainite zone cooling + air-cooled self-tempering cooling, using ≤320mm thickness specifications of continuous casting billet, ≥35mm thickness specifications of steel plate is prepared, the compression ratio is ≤9.14, the microstructure is quasi-polygonal ferrite + acicular ferrite + bainite + tempered bainite complex phase structure, the MA self-tempering decomposition ratio in bainite is ≤20%, the average grain size of ferrite is 3-6μm, the proportion of quasi-polygonal ferrite structure is 5%-15%, the proportion of acicular ferrite structure is 25%-40%, the proportion of bainite structure is 25%-65%, the yield strength of the steel plate is ≥500MPa, the tensile strength is ≥630MPa, the elongation is ≥35%, the yield strength 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 falling shear area fraction at-10℃ is 100%, the DWTT falling shear area fraction at-20℃ is ≥90%, and the DWTT ductile-brittle transition temperature is lower than-20℃.

[0073] The large-thickness submarine pipeline steel suitable for the application is X70 grade, and the chemical composition of the steel plate includes, in terms of mass percentage: 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%, and the rest is iron and inevitable impurities, the carbon equivalent CEV of the steel plate is 0.388, and the Pcm is 0.150. The continuous casting billet with a thickness of 320 mm is obtained by smelting according to the above chemical composition and through light pressing at the end of continuous casting, the center segregation is 0.5 level, and the A, B, C and D types are all 0.5 level.

[0074] The specific process steps are as follows:

[0075] (1) five-stage heating process: a five-stage heating process of heat recovery stage, preheating stage, first heating stage, second heating stage and soaking stage is adopted, wherein 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, T1≥T NbN , and T NbN ≤T1≤T NbN + 25℃, through calculation, T NbN =1204℃. Therefore, 1204℃≤T1≤1229℃.

[0076] (2) low-temperature large-rolling reduction in the recrystallization zone: through calculation, T2=1121℃. The starting rolling temperature is T2-10℃~T2+30℃, and the final rolling temperature is T2-80℃~T2-40℃, that is, the starting rolling temperature is 1111℃~1151℃, and the final rolling temperature is 1041℃~1081℃. The large-rolling reduction process is adopted, the rolling reduction of one pass is ≥31mm, and the rolling reduction of the previous pass is less than or equal to the rolling reduction of the next pass, R n ≤R n+1 , that is, a total of 5 passes are rolled, and the rolling reduction of each pass is 31mm, 33mm, 37mm, 40mm and 44mm respectively, the thickness of the intermediate billet after rolling is t z , 3.4t≤t z ≤4.4t, 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 t is 35mm.

[0077] (3) intermediate billet cooling: after the rolling in the recrystallization zone is completed, the intermediate billet cooling device is immediately used for water cooling, and the cooling is performed to T2-240℃~T2-220℃, that is, 881℃~901℃.

[0078] (4) High temperature and large reduction in non-recrystallization zone: through calculation, T2 is 1121℃, and T3=755℃. The starting rolling temperature is T2-240℃ ~ T2-220℃, and the finishing rolling temperature is T3 ~ T3 + 25℃, i.e. the starting rolling temperature is 881℃ ~ 901℃, and the finishing rolling temperature is 755℃ ~ 780℃. In the rolling process in the non-recrystallization zone, the intermediate blank cooling device is used for water cooling cooling between passes; in the high temperature section within the temperature range of the non-recrystallization zone, the pass reduction is ≥21mm, and a total of 4 passes are rolled, and the pass reduction of each pass is 21mm, 22mm, 25mm and 26mm respectively; in the low temperature section within the temperature range of the non-recrystallization zone, the pass reduction is 6mm, and the thickness of the finished steel plate is 35mm.

[0079] (5) Air cooling redistribution in high temperature zone after rolling: after rolling, the steel plate is air cooled on the conveying roller way to T3 - 50℃ ~ T3 - 30℃, i.e. 705℃ ~ 725℃, and then enters the ultra-fast cooling system.

[0080] (6) Cooling in ferrite zone: the steel plate enters the ultra-fast cooling system and is cooled to T4 + 5℃ ~ T4 + 25℃, through calculation, T4=641℃, i.e. cooled to 646℃ ~ 666℃, the cooling speed is 18℃ / s, the water pressure is 0.20MPa, and the roller way speed of the ultra-fast cooling system is 2.0m / s.

[0081] (7) Waiting for temperature: after being cooled to the intermediate temperature 646℃ ~ 666℃, the steel plate exits the ultra-fast cooling system from the entrance of the ultra-fast cooling system and swings on the roller way at the entrance of the ultra-fast cooling system to wait for temperature, and the waiting time is 10 seconds.

[0082] (8) Cooling in bainite zone: the steel plate enters the ultra-fast cooling system again and is cooled to T4 - 285℃ ~ T4 - 265℃, through calculation, T4=641℃, i.e. cooled to 356℃ ~ 376℃. The water pressure is 0.40MPa, the cooling speed is 8~20℃ / s, and the roller way speed of the ultra-fast cooling system is 0.6~1.4m / s.

[0083] (9) Air cooling self tempering cooling: after being cooled to the temperature 356℃ ~ 376℃, the steel plate exits the ultra-fast cooling system from the exit of the ultra-fast cooling system, is then straightened, and is air cooled and self tempered on the cooling bed until being cooled to room temperature; the air cooling self tempering condition is that the cooling bed is permeable without wind, the hot steel plate needs to be fully laid on the cooling bed, the spacing between the steel plates is 1.5m, and the temperature of the steel plate is T4 - 305℃ ~ T4 - 105℃, i.e. 336℃ ~ 536℃.

[0084] The microstructure is a complex phase structure of quasi-polygonal ferrite + acicular ferrite + bainite + tempered bainite, the proportion of MA self-tempering decomposition in the bainite is ≤20%, the average grain size of ferrite is 3-6 μm, the proportion of quasi-polygonal ferrite structure is 5%-15%, the proportion of acicular ferrite structure is 25%-40%, the proportion of bainite structure is 25%-65%, the proportion of tempered bainite is 5%-20%, the yield strength of the steel plate is ≥500 MPa, the tensile strength is ≥630 MPa, the elongation is ≥35%, the yield strength ratio is ≤0.80, the impact energy KV2 at-40 ℃ is ≥300 J, the impact energy KV2 at-60 ℃ is ≥250 J, the hardness is ≤208 HV10, the DWTT falling shear area fraction at-10 ℃ is 100%, the DWTT falling shear area fraction at-20 ℃ is ≥90%, and the ductile-brittle transition temperature of DWTT is lower than-20 ℃. The metallographic structure is as shown in Figure 1 .

[0085] Based on the above ideal embodiments according to the present application, through the above description, relevant personnel can make various changes and modifications without deviating from the technical idea of the present application. The technical scope of the present application is not limited to the content in the specification, and must be determined according to the scope of claims.

Claims

1. A method for producing thick submarine pipeline steel plates, characterized in that, A special controlled rolling process was used to sequentially process the continuously cast billet through five stages of heating, low-temperature high-reduction rolling in the recrystallization zone, intermediate billet cooling, and high-temperature high-reduction rolling in the non-recrystallization zone. A multi-stage controlled cooling process followed by air cooling redistribution in the high-temperature zone, cooling in the ferrite zone, waiting to reach a warm temperature, cooling in the bainite zone, and air-cooled self-tempering cooling was then employed to produce a submarine pipeline steel plate with a thickness of t, ≥ 35 mm, and a compression ratio ≤ 9.

14. The chemical composition of the submarine pipeline steel plate, by mass percentage, includes: C: 0.031–0.061%, Si: 0.08–0.16%, Mn: 1.64–1.72%, P… The chemical composition of the steel plate is as follows: 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: 0.011~0.019%, Alt: 0.021~0.049%, N: 0.0031~0.0051%, with the remainder being iron and unavoidable impurities. The steel plate's chemical composition also satisfies the carbon equivalent CEV = C + Mn / 6 + (Cr + Mo + V) / 5 + (Cu + Ni) / 15. The value is 0.350~0.427, and Pcm=C+Si / 30+(Mn+Cu+Cr) / 20+Ni / 60+Mo / 15+V / 10+5B is 0.127~0.174; The specific process steps are as follows: (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, where lg(Nb×N)=3.7-10800 / (T) NbN +273.15); (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 thickness of the intermediate billet after rolling is t 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; (3) Intermediate billet cooling: After the austenite recrystallization zone is rolled, the intermediate billet is immediately cooled by water using an intermediate billet cooling device until it is cooled to the non-recrystallization rolling temperature range, i.e., T2-240℃ ~ T2-220℃. (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; In the low-temperature section within the temperature range of the non-recrystallization zone, a rolling process with small reduction is adopted, with a reduction of ≤6mm per pass; During the rolling process in the non-recrystallization zone, water cooling is performed by an intermediate billet cooling device between passes, and T3 is the temperature at which the transformation of austenite to ferrite begins during cooling, T3=910-310C-80Mn-15Cr-80Mo; (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℃, and then enters the ultra-fast cooling system; (6) Ferrite region cooling: The steel plate enters the ultra-fast cooling system and is cooled to T4+5℃ ~ T4+25℃. The cooling rate is 10~20℃ / s, and T4=830-270C-90Mn-37Ni-70Cr-83Mo; (7) Waiting for the temperature to reach the intermediate temperature T4+5℃ ~ T4+25℃, the steel plate is taken out of 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. (8) Bainite region cooling: The steel plate re-enters the ultra-fast cooling system from the ultra-fast cooling inlet and is cooled to T4 - 285℃ ~ T4 -265℃, with a cooling rate of 8~20℃ / s; (9) Air-cooled self-tempering cooling: After cooling to T4 - 285℃ ~ T4 - 265℃, the steel plate exits the ultra-fast cooling system from the ultra-fast cooling outlet.

2. The method for producing thick submarine pipeline steel plates according to claim 1, 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. The heating time in the heat recovery stage is 0.37-0.41 min / mm, the heating time in the preheating stage is 0.32-0.35 min / mm, the heating time in the first heating stage is 0.22-0.25 min / mm, and the heating time in the second heating stage is 0.17-0.20 min / mm.

3. The method for producing thick submarine pipeline steel plates according to claim 1, characterized in that, The specific process of low-temperature high-reduction rolling in the recrystallization zone is as follows: In the low-temperature section within the temperature range of the recrystallization zone, the total number of roughing passes is 4 to 6, the temperature difference between adjacent passes is 12℃ to 18℃, 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 These are the reduction amounts for the nth and (n+1)th passes, respectively.

4. The method for producing thick submarine pipeline steel plates according to claim 1, characterized in that, The intermediate billet cooling process involves the following steps: the cooling manifold water pressure is 0.4~0.6MPa, the upper manifold water flow rate is 50~150m³ / h, the lower manifold water flow rate is 100~300m³ / h, the ratio of the lower manifold water flow rate to the upper manifold water flow rate is ≥2, 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.

5. The method for producing thick submarine pipeline steel plates according to claim 1, characterized in that, The high-temperature, high-reduction rolling in the non-recrystallization zone: The high-temperature section, i.e., the temperature T2-280℃ ~ T2-240℃, involves 4 to 6 rolling passes at a speed of 3 to 5 m / s, with the reduction in each pass being greater than or equal to the reduction in 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, resulting in a billet thickness of t+3mm ~ t+12mm after rolling. The low-temperature section, with temperatures ranging from T3 + 5℃ to T3 + 30℃, involves 1 to 2 rolling passes at a speed of 4 to 6 m / s.

6. The method for producing thick submarine pipeline steel plates according to claim 1, characterized in that, The high-temperature zone after rolling is air-cooled and redistributed: the conveyor roller length is 60m, the roller speed is 2-5m / s, and the indoor temperature is 5-39℃.

7. The method for producing thick submarine pipeline steel plates according to claim 1, characterized in that, The ferrite region cooling system consists of 24 sets of cooling manifolds, each with a cooling length of 1m. Manifolds 1 through 24 are opened for cooling. The water pressure is 0.15–0.20 MPa, the cooling water temperature is 10–30℃, the water flow rate of the upper and lower manifolds is 60–200 m³ / h, the water flow rate ratio of the lower to upper manifolds is 1.1–1.5, and the cooling roller speed of the ultra-fast cooling system is 1.8–2.2 m / s.

8. The method for producing thick submarine pipeline steel plates according to claim 1, characterized in that, The specific process of waiting for the temperature to reach the intermediate temperature is as follows: after cooling to the intermediate temperature, the upper and lower cooling water manifolds are closed, and the steel plate swings back and forth on the roller conveyor at a speed of 0.5 to 1.5 m / s, with a forward and backward movement distance of 5 to 10 m.

9. The method for producing thick submarine pipeline steel plates according to claim 1, characterized in that, The bainitic region cooling is specifically as follows: the ultra-fast cooling system has 24 sets of cooling manifolds, each with 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 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.

10. The method for producing thick submarine pipeline steel plates according to claim 1, characterized in that, The air-cooled self-tempering cooling process specifically involves: straightening the steel plate after it exits the ultra-fast cooling system; the air-cooled self-tempering conditions are: the cooling bed is permeable and windless, the cooling bed is covered with hot steel plates, and the spacing between the steel plates is 1 to 2 meters.

11. The method for producing thick submarine pipeline steel plates according to claim 1, characterized in that, Continuous casting billets with a thickness of ≤320mm are used, with center segregation not exceeding grade 0.5 and inclusions of types A, B, C, and D not exceeding grade 1.

12. A thick subsea pipeline steel plate produced by the production method of any one of claims 1-11.

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