A thick-gauge, economical, high-performance hot-rolled steel coil for submarine pipelines and its production method

Abstract

The present invention relates to the technical field of pipeline steel production, and more specifically to a thick, economical, high-performance hot-rolled steel coil for submarine pipelines and a production method. The production method includes smelting, continuous casting, heating, rolling, cooling, and coiling. The chemical composition of the steel billet, in percentage by mass, is as follows: C: 0.045%-0.065%, Si: 0.21%-0.30%, Mn: 1.30%-1.42%, P ≤ 0.011%, S ≤ 0.0015%, Ti: 0.010%-0.020%, Nb: 0.035%-0.045%, Cr: 0.25%-0.35%, Al: 0.020%-0.050%, N: 0.0025%-0.0050%, O ≤ 0.0030%, H ≤ 0.0002%, with the remainder being Fe and unavoidable impurities. The hot-rolled steel coil has a thickness of 15-20 mm. The present invention adopts a low-cost component design, combined with a clean steel smelting process and an appropriate controlled rolling and cooling process. The steel produced has the outstanding characteristics of cost-effectiveness, stable performance, and excellent comprehensive performance. It can be used for the manufacture of thick-walled, large-diameter spiral submerged arc welded pipes for submarine oil and gas transportation and the construction of pipelines serving in harsh environments such as low temperature, high pressure, and corrosion.

Description

Technical Field

[0001] The present invention relates to the technical field of steel smelting, and in particular to a thick-gauge, economical, high-performance hot-rolled steel coil for submarine pipelines and a production method thereof. Background Art

[0002] The seabed is rich in oil and gas resources, accounting for approximately one-third of the world's total oil and gas resources. The ever-expanding demand for energy in modern times has promoted the development of marine oil and gas resources, leading to the rapid construction and development of submarine oil and gas pipelines. In existing technologies, spiral seam submerged arc welded pipes are widely used in the laying of pipelines for transporting oil and natural gas. They are made from hot-rolled steel coils, spirally formed at room temperature, and welded using double-sided submerged arc welding. This allows the production of large-diameter steel pipes from narrower steel coils, resulting in high production efficiency. Therefore, the development of submarine oil and gas pipeline construction has led to an increasing demand for hot-rolled steel coils for submarine pipelines, and the development and application of hot-rolled steel coils for submarine pipelines has become increasingly important.

[0003] Submarine pipeline steel used in submarine oil and gas pipelines faces a more complex and harsher service environment than terrestrial pipelines. Environmental factors such as subsea water pressure, currents, and seawater corrosion require steel pipes to possess comprehensive properties, including high strength, high- and low-temperature crack arrest toughness, a low yield strength ratio, and good deformation resistance, acid corrosion resistance, and weldability. Furthermore, as offshore oil and gas pipelines continue to be laid at greater depths and travel greater distances, the pressure on the internal and external walls of the pipelines is also increasing. Pipes manufactured from hot-rolled coils are trending toward higher steel grades, larger diameters, and thicker walls, leading to a growing demand for thick, high-performance hot-rolled coils for submarine pipelines. However, in steel production, toughness typically decreases as strength increases. Furthermore, as thickness increases, organizational control becomes increasingly difficult due to limitations in production line equipment. This makes it difficult to increase the strength of thick hot-rolled coils, and their low-temperature drop weight (DWTT) performance significantly deteriorates, making it increasingly difficult to ensure low-temperature crack arrest toughness. In order to meet the use requirements of hot-rolled steel coils for submarine pipelines, the production design of such pipeline steel usually adds a large amount of precious alloy elements and adopts harsh rolling and cooling processes. The alloy cost is high, the production is difficult, the industrial mass production cost is high, and the yield rate is low.

[0004] Patent CN100359035C discloses X65 pipeline steel for acidic environments and a manufacturing method thereof, wherein precious alloying elements such as Cu, Ni, and Mo are added, and the Nb content ranges from 0.05 to 0.07%, resulting in high production costs.

[0005] Patent CN102851590B discloses an acid-resistant low-manganese X70 pipeline steel and a production method thereof, wherein the Nb content is 0.06-0.09%, and precious alloying elements such as Cu, Ni, and Mo are added, resulting in high production costs.

[0006] Patent CN105132807B discloses a pipeline steel with excellent submarine acid corrosion resistance and a production method. It adopts a Nb-V-Ti composite microalloying composition system and adds precious alloying elements such as Cu, Ni, and Mo, resulting in high production costs.

[0007] Patent CN105132833B discloses an economical high-strength submarine pipeline steel and its production method. It adopts a Nb-V-Ti composite microalloying composition system, with a cooling rate of not less than 82°C / s. A rate that is too high makes production control difficult. The coiling temperature is 200-350°C, which is too low, placing high demands on the coiler and making production difficult. The specific thickness is not specified, the performance of different batches fluctuates greatly, and the acid corrosion resistance is not mentioned.

[0008] Patent CN110273109A discloses a 450MPa-grade submarine pipeline steel coil and its preparation method. No Cr element is added, the final rolling temperature and coiling temperature are both high, and the laminar cooling method is rear-stage laminar cooling. The drop hammer performance and corrosion resistance are not mentioned, and only 12.7mm thin specifications are involved.

[0009] Patent CN107988562A discloses an X65-grade low-cost submarine pipeline steel and its manufacturing method. The steel has a high content of elements such as C, P, and S, a wide process range, a low cooling rate, and large performance fluctuations. It does not mention corrosion resistance and is only suitable for ordinary submarine pipeline steel not used for transporting oil or natural gas.

[0010] Patent CN111118399A discloses a 15-22 mm thick corrosion-resistant and high-crack arrest submarine pipeline steel X65MO and a production method thereof, which adds rare earth element Ce and precious alloy element Mo, resulting in high alloy cost.

[0011] Patent CN103834874B discloses a thick-walled, high-DWTT performance X65-70 submarine pipeline steel and its manufacturing method. It contains relatively high amounts of precious alloying elements such as Cu and Ni, resulting in high alloy costs, low finishing rolling start temperatures, and high mill loads. It belongs to the field of medium and thick plate production, has a wall thickness greater than 25.4 mm, and is suitable for the manufacture of straight seam welded pipes for submarine natural gas transportation.

[0012] Patent CN112239836A discloses a Class B acid-resistant pipeline steel and its preparation method. The preparation method belongs to the field of medium and thick plate production. The steel plate has a ferrite + pearlite microstructure and a low strength level, which makes it difficult to meet the service requirements of submarine pipelines.

[0013] As can be seen, existing technologies typically add high amounts of precious alloying elements such as Cu, Ni, and Mo to thick-gauge submarine pipeline steel to achieve high performance. This results in high alloy costs, stringent requirements for rolling and cooling processes, and significant production control challenges. Furthermore, the subsequent production of medium and thick plate products involves pipemaking via a longitudinal submerged arc welding process, resulting in lower production efficiency than hot-rolled coils. Therefore, developing thick, economical, high-performance hot-rolled coils for submarine pipelines and achieving stable, large-scale production are crucial for addressing the complex marine service environment and promoting the development of offshore oil and gas resources. Summary of the Invention

[0014] In response to the technical problems in the prior art that submarine pipeline steel contains a large amount of precious alloy elements, has high raw material costs, has high process requirements, is difficult to produce, is difficult to achieve large-scale stable production, and has poor low-temperature crack arrestability of the finished steel coil, the present invention provides a thick-gauge, economical, high-performance hot-rolled steel coil for submarine pipelines and a production method. The steel adopts a low-cost component design, combined with a clean steel smelting process and an appropriate controlled rolling and controlled cooling process. The produced steel has the outstanding characteristics of cost economy, stable performance, and excellent comprehensive performance. It can be used for the manufacture of thick-walled, large-diameter spiral submerged arc welded pipes for submarine oil and gas transportation serving in harsh environments such as low temperature, high pressure, and corrosion, as well as the construction of pipelines.

[0015] In a first aspect, the present invention provides a method for producing a thick-gauge, economical, high-performance hot-rolled steel coil for submarine pipelines, comprising smelting, continuous casting, heating, rolling, cooling, and coiling steps. The chemical composition and mass percentage of the steel billet in the smelting step are as follows: C: 0.045%-0.065%, Si: 0.21%-0.30%, Mn: 1.30%-1.42%, P≤0.011%, S≤0.0015%, Ti: 0.010%-0.020%, Nb: 0.035%-0.045%, Cr: 0.25%-0.35%, Al: 0.020%-0.050%, N: 0.0025%-0.0050%, O≤0.0030%, H≤0.0002%, and the remainder is Fe and unavoidable impurities.

[0016] Chemical composition has an important impact on product cost and performance. The present invention imposes restrictions on the chemical composition of the steel billet, as explained below.

[0017] C: Carbon is the primary solid solution strengthening element in steel, significantly increasing its strength. It also forms nano-carbides with the microalloying elements niobium and titanium, contributing to grain refinement and precipitation strengthening. Therefore, the carbon content cannot be too low. However, excessive carbon content can have a direct adverse effect on the steel's toughness, weldability, and acid corrosion resistance. The carbon content in this invention is controlled within a range of 0.045% to 0.065%.

[0018] Si: Silicon is an important solid solution strengthening element and has a deoxidizing effect, but too high a silicon content will lead to a decrease in the welding performance and toughness of the steel. In the present invention, the silicon content is controlled at 0.21%-0.30%.

[0019] Mn: Manganese has a significant solid solution strengthening effect, can also increase the hardenability of steel, reduce the phase transition temperature of steel, and refine the steel structure. It can be used to compensate for the strength loss caused by low carbon content. However, Mn is also a segregating element. If the content is too high, it can easily have an adverse effect on the toughness, welding performance and corrosion resistance of the steel. In the present invention, the manganese content is controlled at 1.30%-1.42%.

[0020] P: Phosphorus increases the cold brittleness of steel, causes banded structure and central segregation, and deteriorates the toughness, weldability and corrosion resistance of steel. It is a harmful element and its content should be reduced as much as possible. In the present invention, the phosphorus content is controlled to ≤0.011%.

[0021] S: Sulfur is also a harmful element that increases the hot brittleness of steel and reduces its ductility and toughness. Furthermore, S easily combines with Mn to form strip-shaped MnS inclusions, which are very detrimental to acid resistance. Therefore, the sulfur content in steel should be strictly controlled. In the present invention, the sulfur content is controlled at ≤0.0015%.

[0022] Ti: Titanium is a microalloying element that can combine with nitrogen at high temperatures to form nano-scale nitrides, which refine austenite grains and help improve the toughness of the heat-affected zone of welding. However, the titanium content should not be too high, otherwise it will easily form micron-sized titanium nitride, which will deteriorate the low-temperature toughness of steel, especially thick-gauge products. The titanium content of the present invention is controlled at 0.010%-0.020%.

[0023] Nb: Niobium is a microalloying element that can increase the austenite recrystallization temperature of steel, expand the rolling range in the unrecrystallized zone, and facilitate rolling in the unrecrystallized zone. Its carbides precipitate at defects such as dislocation boundaries, inhibiting austenite recrystallization and grain growth, thereby achieving grain refinement. It also promotes the formation of acicular ferrite, significantly improving the overall performance of the steel. However, due to the limitations of the carbon content, excessive niobium content not only hinders its full performance but also leads to coarse precipitates, which are detrimental to the toughness of the steel. Furthermore, niobium-iron alloys are relatively expensive. Considering cost considerations, the niobium content in this invention is controlled to 0.035%-0.045%.

[0024] Cr: Chromium has a strong solid solution strengthening effect and can also improve the hardenability of steel. It can effectively improve the uniformity of the steel's structure in the thickness direction, promote the formation of acicular ferrite, and increase the steel's strength and toughness. It also forms a passivation film to prevent hydrogen from invading the steel matrix, thereby improving acid and corrosion resistance. However, excessive chromium content increases weld crack sensitivity and reduces the steel's weldability. In the present invention, the chromium content is controlled to 0.25%-0.35%.

[0025] Al: Aluminum is an important deoxidizing element and can also refine the structure. However, its content should not be too high, otherwise it will easily cause cracks in the casting. In the present invention, the aluminum content is controlled at 0.020%-0.050%.

[0026] N: Nitrogen is a gaseous impurity element that reduces the purity of molten steel. When the nitrogen content is too high, it easily forms micron-sized TiN precipitation phases with titanium during solidification of the ingot, which will deteriorate the toughness of the steel. At the same time, it is an important element that plays a role in microalloying. It combines with Ti and Nb to form nanoscale precipitation phases, which can refine austenite grains. The nitrogen content of the present invention can be controlled within 0.0025-0.0050%.

[0027] O and H: Both are gaseous impurity elements that reduce the purity of steel and form inclusions, which are detrimental to the toughness and acid resistance of steel. They should be controlled at as low a level as possible. The upper limit of the O content in the present invention is 0.0030%, and the upper limit of the H content is 0.0002%.

[0028] Furthermore, the smelting process adopts converter smelting and LF+RH furnace double refining process, with vacuum pressure holding time ≥12min, pure degassing time ≥9min, and soft blowing time ≥13min, which can fully ensure the purity of molten steel.

[0029] Furthermore, argon protection is used during the entire continuous casting process to prevent secondary oxidation of molten steel; low superheat and constant pulling speed control are adopted, with an superheat of 15-24°C and a billet pulling speed of 1.0-1.2m / min. Constant pulling speed control can ensure stable billet quality, and limiting superheat can fully ensure billet quality and avoid or reduce segregation; billet stacks are slowly cooled for more than 36 hours to allow hydrogen to overflow from the inside of the billet, thereby improving acid corrosion resistance; billet thickness is 228-233mm.

[0030] Furthermore, in the heating process, the temperature of the ingot heating soaking section is 1190-1240°C, the soaking time is 35-60min, and the temperature out of the heating furnace is 1190-1230°C; the total heating time in the furnace is 200-260min. Based on the above-mentioned billet composition design, the above-mentioned heating temperature and time can make the billet completely austenitized and the alloy elements fully dissolved, while avoiding the coarsening of the austenite grains.

[0031] Furthermore, the rolling process is divided into rough rolling and finishing rolling. The rough rolling adopts the 1+5 mode, and the rough rolling final rolling temperature (R2DT) is 990-1030℃. Based on the research on the influence of the content of elements such as niobium, carbon, and nitrogen on the austenite recrystallization temperature, the austenite recrystallization temperature of the steel billet designed with the above composition is above 974℃. Limiting the above-mentioned final rolling temperature range can ensure that the rough rolling process is completely carried out in the recrystallization zone. At the same time, through multiple round-trip rolling, the austenite is repeatedly recrystallized to refine the grains; the thickness of the intermediate billet is 56-67mm, ensuring that there is sufficient reduction in both rough rolling and finishing rolling; the rolling rhythm is controlled before the finishing mill to ensure that the finishing rolling entrance temperature (FET) is 920-960℃, and the finishing rolling final temperature (FD The temperature of the finishing rolling mill is 790-830°C. The above-mentioned finishing rolling entry and final rolling temperatures are limited to ensure that the finishing rolling process is carried out in the non-recrystallization zone, preventing the occurrence of mixed crystals. The finishing rolling is carried out using seven continuous stands, of which the F3 and F5 rolling mills have no passes to increase the reduction ratio of each pass, especially to ensure that the reduction ratio of the two passes after the finishing rolling stage is ≥27%. The no passes and reduction ratio of the finishing rolling are limited to fully break up the austenite grains of the thick-gauge pipeline steel during the rolling process, achieving austenite grain refinement and homogenization. At the same time, a large number of deformation structures such as subgrain boundaries and dislocations, as well as deformation-induced precipitation of niobium carbonitride, are increased, creating conditions for the subsequent formation and refinement of acicular ferrite structure. The thickness of the steel strip after finishing rolling is 15-20 mm.

[0032] Furthermore, the cooling process adopts a segmented cooling process. In the first stage, the four groups of front ultra-fast cooling headers are fully opened, with 7 pipes in each group. The water pressure is controlled at 0.45-0.48MPa, the cooling rate is 36-44℃ / s, and the cooling outlet temperature (MT1) is 550-610℃. By rapidly cooling the steel strip to a certain temperature, the supercooling degree is increased, the phase transformation temperature is reduced, and the formation of fine acicular ferrite is promoted. The acicular ferrite structure has a high density of dislocations entangled with each other, which can effectively prevent the expansion of cracks and is extremely beneficial to improving toughness and acid resistance. If the cooling rate is too low at this stage, not only will the ideal acicular ferrite structure of pipeline steel fail to form, but it will also lead to insufficient cooling penetration in the thickness direction of the steel, resulting in uneven structure, especially thickness. Pearlite structure appears in the center with coarse grains, making it difficult to meet the required strength and low-temperature drop hammer performance. At the same time, the cooling rate should not be too high, otherwise it will easily cause sickle camber at the head of the steel coil, which will also increase the difficulty of control and lead to unstable process, microstructure and performance. The second stage adopts laminar sparse cooling with a cooling rate of 16-25℃ / s. Since the process in the first stage is very fast, the cooling is insufficient and the phase transformation is incomplete. The cooling in this stage is to avoid the temperature rise caused by the reheating of thick-gauge pipeline steel, so that the steel strip temperature is always maintained in the lower temperature range of 540-600℃, while promoting the further transformation of the untransformed austenite structure. The third stage is cooled in air for 4-5s to ensure sufficient phase transformation and promote the precipitation of fine niobium carbides.

[0033] Furthermore, during the coiling process, the steel sheet is coiled into a coil at a temperature of 490-530°C. The period during and after coiling provides the necessary temperature and time conditions for the precipitation of niobium carbide, increasing the amount of precipitated phases, improving their distribution uniformity, and enhancing the strength and stability of the steel. If the coiling temperature is too high, pearlite formation can easily occur, which is detrimental to the toughness and acid resistance of the steel. If the coiling temperature is too low, it not only increases the coiler load, leading to production difficulties, but also inhibits the precipitation of niobium carbide, resulting in insufficient strength. If bainite or martensite structures are formed, this is detrimental to toughness.

[0034] In a second aspect, the present invention provides a thick-gauge, economical, high-performance hot-rolled steel coil for submarine pipelines, wherein the thickness of the steel coil is 15-20 mm.

[0035] Furthermore, the microstructure of the steel coil is mainly composed of fine and uniform acicular ferrite, and the grain size is 12-13.

[0036] Furthermore, the steel coil has a non-metallic inclusion content of ≤ 0.5, a 30° yield strength (Rt0.5) of 515-535 MPa, a tensile strength of 610-630 MPa, an elongation of ≥35%, a yield strength ratio of ≤0.85, a -30°C impact energy of ≥300 J, an impact fiber fraction of 100%, a -20°C drop weight shear area of ​​≥85%, a hardness of HV10 ≤220, and qualified HIC and SSC resistance, with no cracks. The coil's performance indicators fully meet and exceed those of API SPEC 5L-2018, GB / T14164-2013, and YBT4841-2020 standards, and also exceed the performance requirements for X70-grade pipeline steel in industry standards.

[0037] The beneficial effects of the present invention are:

[0038] The present invention provides a thick-gauge, economical, high-performance hot-rolled steel coil for submarine pipelines and a production method. The invention adopts a low-cost composition design system of C-Si-Mn-Nb-Cr-Ti, strictly controls the contents of elements such as P, S, N, O, and H, does not add precious alloying elements such as Cu, Ni, and Mo, and reduces the amount of Nb added. In combination with a clean steel smelting process and an appropriate controlled rolling and controlled cooling process, the produced submarine pipeline steel hot-rolled steel coil has the characteristics of uniform structure, stable performance, high strength, low yield strength ratio, excellent low-temperature toughness, strong crack arrest ability, and good acid corrosion resistance. The hot-rolled steel coil is suitable for the manufacture of thick-walled, large-diameter spiral submerged arc welded pipes for submarine oil and gas transportation and the construction of pipelines thereof. The production cost is low, the cost-effectiveness is high, the process is simple and easy, and it is suitable for large-scale production. BRIEF DESCRIPTION OF THE DRAWINGS

[0039] In order to more clearly illustrate the embodiments of the present invention or the technical solutions in the prior art, the following briefly introduces the drawings required for use in the embodiments or the description of the prior art. Obviously, for ordinary technicians in this field, other drawings can be obtained based on these drawings without any creative work.

[0040] Figure 1 This is a microstructure diagram of the hot-rolled steel coil produced in Example 1 near the surface.

[0041] Figure 2 1 is a microstructure diagram of the thickness center of the hot-rolled steel coil produced in Example 1.

[0042] Figure 3 This is a microstructure diagram of the hot-rolled steel coil produced in Example 2 near the surface.

[0043] Figure 4 This is a microstructure diagram of the thickness center of the hot-rolled steel coil produced in Comparative Example 2. DETAILED DESCRIPTION

[0044] In order to enable those skilled in the art to better understand the technical solutions of the present invention, the technical solutions in the embodiments of the present invention will be clearly and completely described below in conjunction with the accompanying drawings of the embodiments of the present invention. Obviously, the described embodiments are only part of the embodiments of the present invention, not all of the embodiments. Based on the embodiments of the present invention, all other embodiments obtained by ordinary technicians in this field without making creative efforts should fall within the scope of protection of the present invention.

[0045] Example 1

[0046] A thick, economical, high-performance submarine pipeline steel is produced. The raw material composition is shown in Table 1. The production process includes smelting, continuous casting, heating, rolling, cooling, and coiling. Refining utilizes LF+RH double refining. The vacuum holding time during refining is 12 minutes, the vacuum pure degassing time is 9 minutes, and the soft blowing time is 13 minutes. Continuous casting utilizes argon protection throughout the entire process, with a tundish superheat of 21°C. The casting speed is constant at 1.1 m / min, and the continuous casting thickness is 230 mm. The stacked strands are slowly cooled for 48 hours to release hydrogen. The ingot is then heated to 1230°C, the soaking time is 42 minutes, the furnace temperature is 1210°C, and the heating time in the furnace is 216 minutes; after being taken out of the furnace, two-stage controlled rolling is carried out, the recrystallization zone rolling is carried out in the rough rolling stage, the rough rolling outlet temperature (R2DT) is controlled at 1021°C, the intermediate billet thickness is 56.1mm, the non-recrystallization zone rolling is carried out in the finishing rolling stage, the rolling rhythm is controlled before the finishing mill, the finishing rolling entrance temperature (FET) is 958°C, and the final rolling temperature (FDT) is 812°C. The F3 and F5 finishing mills are overhead, and the two passes after the finishing rolling stage are The primary reduction rate is 29.4%, and it is quickly rolled to a thickness of 15.18mm through the other 5 stands. After rolling, a segmented cooling strategy is adopted for cooling. In the first stage, the four groups of pre-ultra-fast cooling headers are fully opened, with 7 pipes in each group, the water pressure is controlled at 0.45MPa, the cooling rate is 41℃ / s, the cooling outlet temperature (MT1) is 567℃, and the ultra-fast cooling rate is 36-44℃ / s; the second stage adopts laminar sparse cooling with a cooling rate of 21℃ / s; the third stage is cooled in air with an air cooling time of 4s; after cooling, it is coiled with a coiling temperature of 517℃.

[0047] Examples 2-6

[0048] A thick-gauge, economical, high-performance submarine pipeline steel is prepared. The raw material composition is shown in Table 1. The production steps are the same as those in Example 1. The specific parameters of each process in the production steps that differ from those in Example 1 are shown in Tables 2 and 3.

[0049] Comparative Examples 1-5

[0050] A steel material, the raw material composition is shown in Table 1, the production steps are the same as those in Example 1, and the specific parameters of each process in the production steps that differ from those in Example 1 are shown in Tables 2 and 3.

[0051] Among them, the content of C, Mn, P, S, and Cr, the process parameters of degassing time, soft blowing time, R2DT, FET, reduction rate, MT1, cooling rate, water pressure, and CT of Comparative Example 1 are different from those of the technical solution of the present invention;

[0052] The content of C, Si, Mn, S, Cr, and Nb, the process parameters of soft blowing time, superheat, billet stacking time, heating time, FET, reduction ratio, MT1, cooling rate, water pressure, and CT in Comparative Example 2 are different from those of the technical solution of the present invention;

[0053] The composition content of each element in Comparative Example 3 is within the scope of the technical solution of the present invention, but the process parameters of thickness specification, holding time, soft blowing time, superheat, billet stacking time, R2DT, FET, reduction rate, MT1, cooling rate, water pressure, and CT are different from those of the technical solution of the present invention;

[0054] The contents of C, Si, Mn, P, S, Cr, Nb, N, and O in Comparative Example 4 are different from those in the technical solution of the present invention, and the process parameters are within the scope of the technical solution of the present invention;

[0055] In Comparative Example 5, the additional addition of precious alloy elements (Cu+Ni+Mo), the content of components such as C, Si, Mn, P, S, and Cr, and the process parameters such as reduction rate, cooling strategy, MT1, cooling rate, water pressure, and coiling temperature are different from those of the technical solution of the present invention.

[0056] The chemical composition and main process parameters of the steel billets of the embodiments and comparative examples are shown in Tables 1 to 3.

[0057] Table 1 Chemical composition of steel billets of various embodiments and comparative examples (wt.%)

[0058]

[0059]

[0060] Table 2 Main process parameters of each step in each embodiment and comparative example

[0061]

[0062] Table 3 Main process parameters of hot rolling in various embodiments and comparative examples

[0063]

[0064]

[0065] The mechanical property test data, acid resistance test data and metallographic test data of each embodiment and comparative example are shown in Table 4 to Table 6.

[0066] Table 4 Mechanical properties test data of each embodiment and comparative example

[0067]

[0068] Table 5 Acid resistance test data of each embodiment and comparative example

[0069]

[0070] Table 6 Metallographic detection data of each embodiment and comparative example

[0071]

[0072]

[0073] Comparison of Examples 1-6 and Comparative Examples 1-5 reveals that the 15-20 mm thick hot-rolled pipeline steel coils of Examples 1-6, which do not contain the high-cost alloys Cu, Ni, and Mo, exhibit a 30° yield strength (Rt0.5) of 516-531 MPa, a tensile strength of 613-629 MPa, an elongation ≥38%, a yield strength ratio ≤0.85, a -30°C impact energy ≥310 J, an impact fiber section rate of 100%, a -20°C drop weight shear area ≥85%, a hardness HV10 ≤220, acceptable HIC and SSC resistance, and a crack-free state. These coils exhibit cost-effectiveness and excellent performance, making them suitable for the manufacture of thick-walled, large-diameter spiral submerged arc welded pipes for submarine oil and gas transportation, and for pipeline construction, designed to operate in harsh environments such as low temperature, high pressure, and corrosion. However, due to differences in composition and processing, the steel produced in Comparative Examples 1-5 differs from that of Examples 1-6 in microstructure, resulting in suboptimal strength, low-temperature drop weight performance, and corrosion resistance, making it difficult to meet the requirements for submarine pipeline steel.

[0074] Although the present invention has been described in detail with reference to the accompanying drawings and in conjunction with preferred embodiments, the present invention is not limited thereto. Without departing from the spirit and essence of the present invention, persons of ordinary skill in the art may make various equivalent modifications or substitutions to the embodiments of the present invention, and such modifications or substitutions shall be within the scope of the present invention. Any changes or substitutions that can be easily conceived by persons skilled in the art within the technical scope disclosed in the present invention shall be within the scope of protection of the present invention.

Claims

1. A method for producing thick-gauge, economical, high-performance hot-rolled steel coils for submarine pipelines, comprising the steps of smelting, continuous casting, heating, rolling, cooling, and coiling, characterized in that: The chemical composition and mass percentage of the steel billet in the smelting process are C: 0.045%-0.065%, Si: 0.21%-0.30%, Mn: 1.30%-1.42%, P≤0.011%, S≤0.0015%, Ti: 0.010%-0.020%, Nb: 0.035%-0.045%, Cr: 0.25%-0.35%, Al: 0.020%-0.050%, N: 0.0025%-0.0050%, O≤0.0030%, H≤0.0002%, and the rest are Fe and unavoidable impurities; The rolling process is divided into roughing and finishing. The roughing and finishing temperature is 990-1030℃, the intermediate billet thickness is 56-67mm, the finishing inlet temperature is 920-960℃, and the finishing temperature is 790-830℃. Finishing rolling adopts 7-pass continuous rolling, of which F3 and F5 are idle. The reduction rate of the two passes after the finishing stage is ≥27%; The cooling process adopts a staged cooling process. In the first stage, the four groups of front ultra-fast cooling headers are fully opened, with 7 pipes in each group. The water pressure is controlled at 0.45-0.48MPa, the cooling rate is 36-44℃ / s, and the cooling outlet temperature is 550-610℃. The second stage adopts laminar sparse cooling with a cooling rate of 16-25℃ / s. In the third stage, the steel is cooled in air for 4-5s. In the coiling process, the steel plate is coiled into a coil at a coiling temperature of 490-530°C; The thickness of the steel coil is 15-20mm. The microstructure of the steel coil is mainly composed of fine and uniform acicular ferrite, and the grain size is 12-13. The non-metallic inclusions of the steel coil are ≤ grade 0.5, the yield strength Rt0.5 in the 30° direction is 515-535MPa, the tensile strength is 610-630MPa, the elongation is ≥35%, the yield strength ratio is ≤0.85, the impact energy at -30℃ is ≥300J, the impact fiber section rate is 100%, the drop hammer shear area at -20℃ is ≥85%, the hardness HV10 is ≤220, the HIC resistance and SSC resistance are qualified, and there is no crack.

2. The production method according to claim 1, wherein The smelting process adopts converter smelting and LF+RH furnace double refining process, with vacuum pressure holding time ≥12min, pure degassing time ≥9min, and soft blowing time ≥13min.

3. The production method according to claim 1, wherein The continuous casting process is carried out under argon protection, with low superheat and constant casting speed control. The superheat is 15-24℃, the casting speed is 1.0-1.2m / min, the casting stack is slowly cooled for more than 36 hours, and the casting thickness is 228-233mm.

4. The production method according to claim 1, wherein During the heating process, the soaking section temperature is 1190-1240°C, the soaking time is 35-60 minutes, the temperature out of the heating furnace is 1190-1230°C; the total heating time in the furnace is 200-260 minutes.

Images