High-toughness low-temperature-resistant hot-rolled h-shaped steel for floating storage and production vessel and preparation method thereof

Abstract

This invention belongs to the field of metallurgy and rolling technology, specifically relating to a high-strength, high-toughness, low-temperature resistant hot-rolled H-beam for floating oil storage and transportation vessels and its preparation method. Its chemical composition by weight percentage is: C: 0.07–0.11%, Si ≤ 0.2%, Mn: 1.5–1.9%, V: 0.06–0.10%, Nb: 0.02–0.05%, Ni: 0.1–0.4%, B: 0.001–0.002%, Alt: 0.025–0.045%, P ≤ 0.015%, S ≤ 0.006%, N: 0.01–0.015%, O ≤ 0.003%, with the remainder being Fe and unavoidable impurities. This invention's H-beam is based on green and environmentally friendly principles, emphasizing lightweight technology. It reduces the overall transportation cost of floating vessels, improves material utilization efficiency, and achieves a 15–30% weight reduction target for the floating oil storage and transportation vessel structure. It provides product and technical support for steel structures used in some hull modules.

Description

Technical Field

[0001] This invention belongs to the field of metallurgy and rolling technology. Specifically, this invention relates to a high-strength, high-toughness, low-temperature resistant hot-rolled H-beam for floating oil storage vessels and its preparation method. Background Technology

[0002] Floating storage and offloading (FSO) vessels are high-end, large-scale marine engineering equipment used for the extraction of deep-sea oil and gas resources. They enable the extraction, processing, storage, and transportation of oil, natural gas, and other energy sources at sea, and are often referred to as "offshore oil refineries." Unlike relatively fixed offshore oil platforms, FSO vessels not only extract oil and store and transport crude oil products, but also serve as large-scale offshore oil production bases integrating personnel living quarters and production command centers. They can quickly reach designated locations for oil and gas development and possess unique advantages such as short construction cycles and high mobility, making them suitable for oil and gas development operations in multiple sea areas worldwide.

[0003] As oil and gas resource exploitation expands from ordinary sea areas to complex regions such as deep cold and extremely cold areas, the production, storage, unloading, and transportation of floating oil storage and offloading (Floating Storage and Offloading) vessels face increasingly complex operating conditions. Therefore, the construction of large oil storage and offloading vessels places more stringent demands on material upgrades, requiring the adoption of new materials and technologies to ensure 30 years or more of operation without dry-docking maintenance, minimizing maintenance costs. The construction of the topside modules requires a large amount of hot-rolled H-beams. With increasing requirements, there is an urgent need for high-performance hot-rolled H-beams to replace the current low-grade H-beams, achieving lightweighting while maintaining high stability and reliability. To adapt to the service environment in extremely cold and complex regions, higher requirements are placed on the low-temperature impact toughness of the steel. The required H-beams need to have a strength of 420 MPa or higher and a low ductile-brittle transition temperature, with low-temperature resistance reaching at least Grade E, or even Grade F. Therefore, with the increase in the number of floating oil storage and offloading vessels being built, the demand for high-strength hot-rolled H-beams with low-temperature resistance (above 420 MPa) is also rising.

[0004] Currently, Chinese hot-rolled H-beam manufacturers have successively developed E-grade low-temperature resistant H-beams with a yield strength of over 355 MPa and have achieved commercial development and promotion. Different production lines employ different production methods, generally using composite micro-alloying with the addition of Nb, V, and Ti combined with thermomechanical rolling. The differences in preparation methods among different companies and production lines are reflected in patent applications.

[0005] Patent application CN201510788520.8 discloses a 420MPa grade high-strength, low-yield-strength-ratio H-beam and its preparation method. The chemical composition of the H-beam, by weight percentage, is: C 0.11–0.15%, Si 0.20–0.35%, Mn 1.35–1.50%, P ≤0.035%, S ≤0.025%, Cu 0.25–0.30%, Cr 0.40–0.45%, Ni 0.20–0.30%, Nb 0.20–0.30%, with the remainder being iron and trace impurities. This invention achieves a yield strength greater than 427MPa, a tensile strength greater than 641MPa, and a yield-strength-ratio of 0.64–0.67 for the low-yield-strength-ratio H-beam. This patent adds elements such as Ni, Nb, and Cu, and is intended for use in specific high corrosion-resistant environments. The preparation process is affected by Cu, which increases the probability of cracks in the legs of H-beams under high final rolling temperatures, requiring surface grinding before use and resulting in a low yield. At the same time, the addition of Cr and Cu elements can easily lead to abnormal structures, and the material does not have obvious yield, making it difficult to ensure safety performance.

[0006] Patent application number CN201510498771.2 discloses a 420MPa grade hot-rolled H-beam with excellent low-temperature toughness and its production method. The composition is: C 0.06~0.12%, Si 0.20~0.40%, Mn 1.20~1.60%, P≤0.015%, S≤0.010%, V 0.050~0.070%, Ni 0.10~0.20%, N 0.0050~0.0100%, with the remainder being Fe and unavoidable impurities. Compared with existing technologies, this invention, through reasonable V, Ni, and N composition design and controlled rolling and cooling processes, has developed a 420MPa hot-rolled H-beam with excellent comprehensive performance; its yield strength ReH is 440–520MPa; tensile strength Rm is 550–650MPa; elongation A is A≥22%; and low-temperature impact toughness KV2≥100J at -40℃. This patented controlled cooling employs a two-stage process: rapid cooling in the first stage followed by air cooling in the second stage, requiring a special cooling device to meet the above preparation requirements. The produced 420MPa-grade low-temperature toughness hot-rolled H-beam requires a Z-axis performance requirement of 40–65%, a process design only suitable for large-diameter steel products with irregular billets exceeding 15mm, and not suitable for the preparation of small- to medium-sized thin-flange H-beam products.

[0007] Patent application number 202110818559.5 authorizes an invention patent that provides a 420MPa grade hot-rolled low-temperature resistant H-beam and its preparation method. The chemical composition of the H-beam, by weight percentage, is: C: 0.08%–0.10%, Si≤0.2%, Mn: 1.25%–1.45%, V: 0.03%–0.045%, Ti: 0.015%–0.025%, Cr: 0.15%–0.30%, Als: 0.02%–0.04%, N: 0.007%–0.01%, P≤0.008%, S≤0.005%, O≤0.004%, with the remainder being Fe and unavoidable impurities. This invention, taking into account the thin flanges of small-diameter H-beams rolled from rectangular billets, employs a low C content suitable for normalizing rolling combined with V microalloying composition design, and adds an appropriate amount of Cr to control the cooling rate, avoiding the formation of abnormal structures such as Widmanstätten, which would worsen the low-temperature performance of the steel. This invention relates to a production process design for small-to-medium-sized H-beams that achieves E-grade low-temperature toughness under controlled low phosphorus conditions in molten iron. Cost control and precise operation are required for successful implementation, presenting certain challenges. The addition of a certain amount of Ti can easily lead to TiN precipitation, affecting impact toughness.

[0008] The aforementioned patents involve 420MPa grade H-beams and their preparation technology, which require high standards for molten iron control and post-rolling cooling equipment. Therefore, to meet the demand for H-beams in floating vessels, for small-to-medium-sized H-beams rolled on short-process compact universal mills, normalizing is employed at higher final rolling temperatures. Achieving a yield strength above 420MPa while further reducing the ductile-brittle transition temperature requires compositional and phase transformation microstructure design. Simultaneously, with the overall increase in yield strength, the uniformity and stability of the microstructure in the upper and lower flanges and the bow and stern need to be improved; the strength difference between the web and flanges needs to be controlled within a suitable range to avoid affecting the overall fatigue performance of the components. Furthermore, since floating vessels frequently operate in various complex environments and polar regions, the structural steel used on board needs to have a low low-temperature ductile-brittle transition temperature to ensure good low-temperature impact toughness and prevent premature failure of hull steel structural components due to low-temperature brittle fracture. This would increase maintenance costs and even significantly shorten the service life of floating vessels, resulting in serious waste of energy and resources. Summary of the Invention

[0009] To meet the demand for high-strength, high- and low-temperature toughness steel for floating oil storage and offloading (FSO) vessels operating in complex environments, a high-strength, high-toughness, and low-temperature resistant hot-rolled H-beam and its manufacturing method were designed and invented based on the structural characteristics of the hull. This H-beam is specifically designed for the fabrication of hull structural components for floating vessels. Based on green and environmentally friendly principles, this H-beam emphasizes lightweight technology, resulting in overall reduced transportation costs for floating vessels and improved material utilization efficiency. The amount of H-beams used per vessel is reduced by more than 500 tons, achieving a 15-30% weight reduction target for the FSO structure. This provides product and technical support for steel structures used in some hull modules.

[0010] The technical solution of the present invention is as follows:

[0011] A high-strength, high-toughness, low-temperature resistant hot-rolled H-beam for floating oil storage vessels and its preparation method are disclosed. The chemical composition, by weight percentage (%), is as follows: C: 0.07-0.11, Si≤0.2, Mn: 1.5-1.9, V: 0.06-0.10, Nb: 0.02-0.05, Ni: 0.1-0.4, B: 0.001-0.002, Alt: 0.025-0.045, P≤0.015, S≤0.006, N: 0.01-0.015, O≤0.003, with the remainder being Fe and unavoidable impurities.

[0012] H-beams prepared with chemical compositions that meet these requirements can better meet the impact toughness requirements at polar temperatures.

[0013] This invention is particularly suitable for manufacturing small and medium-sized H-beam products with flange thickness of less than 15mm, but is not limited to the above-mentioned specifications.

[0014] The design principle of the main chemical elements in the high-strength hot-rolled H-beams described in this invention is as follows:

[0015] Carbon: To achieve a strength level of 420 MPa and meet the low-temperature resistance requirement of -40℃ for hot-rolled H-beams used in floating vessels, the prototype steel has a microstructure of ultrafine pearlite and ferrite, with a grain size rating of at least grade 9. Carbon also participates in the formation of a certain amount of nano-sized vanadium-containing carbonitrides. For smaller H-beams with a high compression ratio, a higher carbon content can easily lead to abnormal microstructures and a large amount of pearlite. Therefore, the carbon content is controlled within the range of 0.07% to 0.11%.

[0016] Manganese (Mn): In hot-rolled H-beams, Mn can stabilize the austenitic structure, increase the hardenability of the steel, and improve its strength. However, Mn is also prone to segregation. In useful steel profiles with complex cross-sections, variations in deformation and microstructure transformation in different areas can lead to significant performance differences. To ensure strength while minimizing segregation, it is preferable to set the Mn content at 1.5% or higher; however, excessively high Mn content can easily produce abnormal microstructures. Taking all factors into consideration, the Mn content in the steel of this invention is controlled at 1.5% to 2.0%.

[0017] Sulfur: During the preparation of H-beams, sulfur reacts with manganese (Mn) to form MnS. The intermittent inclusions introduced during the preparation process severely deteriorate the low-temperature toughness of the steel. The ratio of Mn to S affects the mechanical properties of the steel and the occurrence of surface defects. Excessive sulfur content easily leads to the formation of a large number of MnS inclusions, which worsens the processing performance. The S content is preferably limited to below 0.006%.

[0018] Aluminum (Al) is used in the preparation process as a strong deoxidizing element and to form AlN, thus refining the grain size. This is done to ensure the oxygen content in the steel is as low as possible, reducing the probability of spherical inclusions; additionally, the AlN precipitates formed by aluminum and nitrogen in the steel can refine the grain size, thereby improving the strength of the steel. Therefore, in this invention, the aluminum content is controlled at 0.025–0.045%.

[0019] Niobium (Nb) can form second-phase particles such as NbC or NbN in steel. On one hand, during recrystallization rolling, NbC and NbN significantly increase recrystallization time due to their pinning effect on dislocations and inhibition of grain growth. Niobium in steel is characterized by raising the recrystallization temperature of austenite, expanding the temperature range of the non-recrystallized region, and promoting the accumulation of austenite grain deformation and defects, ultimately refining the ferrite grains. On the other hand, the precipitated Nb(C,N) inhibits austenite grain growth during heating, thus refining the microstructure of the final austenite refinement transformation. Therefore, for low-carbon rolled small-diameter steel sections, grain refinement can be achieved by adding a small amount of niobium in controlled rolling on ordinary steel mills, with Nb controlled at 0.02–0.05%.

[0020] Vanadium (V) is a strong carbonitride-forming element, and its carbonitrides play a precipitation strengthening role. VN alloys can act as nucleation sites for ferrite and pearlite structures, contributing to grain refinement. Vitamin C (VC) is produced in the later stages of rolling and during cooling, generally existing in steel at the nanoscale, playing a precipitation strengthening role. Simultaneously, vanadium-containing steel precipitates in the later stages of rolling, resulting in low resistance to rolling deformation and reducing rolling load. For low-temperature resistant steel with a yield strength of 420 MPa, the V content is controlled at 0.06–0.10% to achieve grain refinement and nano-reinforcing precipitation.

[0021] Nickel: Ni can improve the impact toughness of steel, especially in mass production, where it can significantly expand the process window. Considering factors such as cost, the content of Ni is controlled within the range of 0.1% to 0.4%.

[0022] Nitrogen: The nitrogen element in steel forms VN alloy with v, which has a precipitation strengthening effect and improves strength; it can also form NbN with Nb at high temperatures to inhibit grain growth. However, too high a nitrogen content can easily induce surface quality defects in the cast billet, such as transverse cracks; too low a nitrogen content results in poor precipitation of VN particles. Therefore, this invention requires a nitrogen content of 0.01-0.015%.

[0023] Boron: The main function of boron is to improve the hardenability of steel; adding a small amount of boron can improve the hardenability of steel. The indirect effect of boron delaying γ-α changes is to increase the solubility of alloying elements; it also reduces the chemical inhomogeneity of continuously cast billets; boron has a strong affinity for nitrogen and oxygen, and adding an appropriate amount of boron can eliminate the aging phenomenon in steel. In this invention, a certain amount of boron is added to the steel, and the cooling rate of the steel is appropriately controlled to avoid abnormal microstructures; the addition range is 0.001% to 0.002%.

[0024] Oxygen: High oxygen content easily forms large oxide particles with strong oxidizing elements, thereby reducing the toughness and plasticity of the steel. This invention requires an oxygen content of ≤0.003%.

[0025] The hot-rolled H-beams for floating ships have a yield strength ≥440MPa, tensile strength ≥520MPa, elongation ≥18%, and impact energy ≥150J at -60℃.

[0026] The invention also provides a method for preparing the above-mentioned H-beam with a yield strength of 420MPa. The preparation method includes the following steps: hot metal pretreatment → 50t electric furnace smelting → ladle argon blowing → LF refining → VD degassing → rectangular continuous casting billet → continuous casting billet slow cooling pit for slow cooling → heating furnace heating → BD rough rolling → section steel line compact TM semi-continuous rolling → inter-stand cooling + post-rolling lower flange water cooling → dense slow cooling on cooling bed.

[0027] During the rolling process, the heating furnace soaking temperature is 1190–1220℃, and the billet time in the furnace is 180–240 min. The roughing process typically involves 7–9 passes of reciprocating rolling. The finishing process involves 5 passes of continuous rolling, with an initial finishing temperature of 960–1000℃, full activation of water cooling between the finishing mill stands, and a final finishing temperature of 780–850℃. A turning machine is used between the roughing and finishing mills to turn the steel, and a 20–50 s waiting period is maintained based on temperature monitoring results to ensure the inlet temperature drops to a suitable range. To ensure final rolling temperature control, the compression ratio of the final finishing pass is 8%–15%; sufficient cooling after finishing facilitates the precipitation of V carbonitrides, which exert a precipitation strengthening effect.

[0028] This invention achieves the industrial production of 420MPa high-strength and high-toughness H-beams for floating ships under different compression ratios by designing a micro-alloying process with the addition of Nb and V, combined with the characteristics of semi-continuous rolling of structural steel.

[0029] For any steps not mentioned in this invention, existing technologies may be used.

[0030] To ensure the consistency and uniformity of the microstructure and properties of the H-beams produced, the present invention incorporates targeted design in the process to meet the performance requirements.

[0031] Considering the characteristics of the short-process semi-continuous rolling mill equipment, the distance between the roughing and finishing mills is approximately 60 meters. This short distance cannot meet the rapid cooling requirements of the intermediate billet, and the temperature difference between the upper and lower flanges is significant, reaching up to 70°C. The rolling process needs optimization and improvement. Based on the characteristics of heating and rolling process control, a steel-turning operation is required between the roughing and finishing mills, where the upper and lower flanges of the H-beam are swapped before finishing rolling. However, because the length of the billet increases significantly after roughing, the intermediate billet temperature can reach 1050°C. Uneven steel-turning can easily cause the rolled piece to twist, increasing bite difficulties and causing jamming during finishing rolling. Therefore, to meet the steel-turning requirement, a rapid steel-turning operation for the intermediate billet needs to be implemented between the finishing and roughing mills within a short time.

[0032] To improve the yield strength of H-steel for floating ships to over 420 MPa on both the upper and lower flanges, a composite microalloying process is employed. This invention primarily uses VN alloying to enhance strength. Furthermore, considering the high cleanliness requirements of the steel, LF+VD degassing is used during the 50-ton electric furnace smelting process. However, VD degassing significantly reduces the nitrogen content, necessitating nitrogen enrichment based on the actual N content. Nitrogen enrichment is achieved by adding a certain amount of manganese nitride cored wire, adjusted according to the N content.

[0033] The advantages of the technical solution of this invention are:

[0034] 1. Considering the thin flanges of medium-sized H-beams with different compression ratios produced from rectangular billets, a low C content composition suitable for normalizing rolling, combined with Nb and V microalloying, is designed. Simultaneously, the N content is controlled during the refining process to enhance VN precipitation, meeting the strength and toughness requirements of different product specifications. This results in consistently high-strength and high-toughness hot-rolled H-beams with a strength exceeding 420 MPa, achieved on hot-rolled H-beam mills.

[0035] 2. The refining process uses LF+VD to control the cleanliness of molten steel. After VD degassing, the nitrogen content is lost. In order to improve the precipitation strengthening effect of VN, nitrogen enrichment treatment is carried out according to the N content. The nitrogen enrichment method is to add 20 to 50 meters of manganese nitride cored wire to increase the nitrogen content of the steel grade and keep it in the range of 0.01 to 0.015%.

[0036] 3. During the rolling process, a special steel-turning device is used between the roughing and finishing mills to significantly reduce the temperature difference between the upper and lower flanges, reducing it to below 25℃. Based on the intermediate temperature detection results, a roller table waiting operation is appropriately adopted. After maintaining the temperature for 20-50 seconds, the temperature of the rolled piece is controlled within the range of 960-1000℃ before entering the finishing mill for five consecutive rolling passes. This improves the low-temperature toughness of the steel, ensuring that it still maintains a high level of impact energy greater than 150J at -60℃.

[0037] 4. Turn on all cooling water between stands and after rolling to ensure that the final rolling temperature is controlled between 780 and 850°C, the temperature difference between the upper and lower flanges is reduced to less than 20°C, and the yield strength difference between the upper and lower flanges is controlled within less than 15MPa, so that the performance is uniform.

[0038] 5. The H-beam steel matrix consists of refined pearlite and proeutectoid ferrite, with grain size controlled within the range of 9 to 11. The stable precipitated second-phase particles are mainly V (C, N), resulting in high microstructure uniformity and good control of age hardening effect.

[0039] 6. By refining the matrix structure and using a precipitation strengthening mechanism mainly based on VN precipitation, the yield strength of hot-rolled H-beams for floating ships reaches 420MPa, while also achieving a good effect of obtaining a low-temperature impact energy of more than 150J at -60℃.

[0040] 7. The steel profile involved in this invention has a low carbon equivalent, which ensures high strength and toughness while also possessing excellent weldability.

[0041] 8. According to the present invention, while improving the overall strength of H-beams, the steel plates can also have good fatigue resistance, so that the steel plates according to the present invention can meet the requirements of users of ships and floating oil storage vessels. Attached Figure Description

[0042] Figure 1 This is a metallographic image (×100) of the high-toughness, low-temperature resistant H-beam with a yield strength of 420 MPa prepared according to the present invention.

[0043] Figure 2 This is a metallographic image (×200) of the high-toughness, low-temperature resistant H-beam with a yield strength of 420 MPa prepared according to the present invention.

[0044] Figure 3 This is a metallographic image (×500) of the high-toughness, low-temperature resistant H-beam with a yield strength of 420 MPa prepared according to the present invention.

[0045] Figure 4 The ductile-brittle transition curves of HE200B steel at different temperatures. Detailed Implementation

[0046] The continuously cast billets in the following examples were prepared according to the following process: Based on the set chemical composition range, blast furnace hot metal was used as raw material, with a certain amount of scrap steel added. The mixture was then smelted in an electric furnace, refined using LF+VD, and the contents of C, Si, Mn, S, P, etc., were adjusted and micro-alloyed. After the composition reached the target value, continuous casting was performed, and the billets were directly heated or homogenized. The preparation steps for Examples 1-4 are as follows:

[0047] Hot metal pretreatment → 50t electric arc furnace smelting → ladle argon blowing → LF refining → VD degassing → rectangular continuous casting billet → slow cooling in the continuous casting billet slow cooling pit → heating in the heating furnace → BD rough rolling → compact TM semi-continuous rolling of section steel line → inter-stand cooling + water cooling of the lower flange after rolling → intensive slow cooling on the cooling bed. The section steel line rolling process includes two stages: rough rolling and finish rolling. The hot rolling process focuses on temperature control; the final rolling temperature is measured at the middle of the upper and lower legs on the outer side of the flange. After rolling, the rolled material is naturally stacked and cooled on the cooling bed.

[0048] Example 1:

[0049] The chemical composition of the selected high-strength and high-toughness steel, by weight percentage (%), is: C: 0.07, Si: 0.15, Mn: 1.90, V: 0.10, Nb: 0.025, Ni: 0.11, B: 0.0011, Alt: 0.025, P: 0.013, S: 0.005, N: 0.014, O: 0.0025, with the remainder being iron (Fe); Cev: 0.41. The prepared raw materials were smelted in a 50-ton electric furnace, and then subjected to nitrogen enrichment treatment by adding 21 meters of manganese nitride cored wire during the degassing process in a VD furnace. The resulting product was then continuously cast into a rectangular continuous casting billet of 240mm × 375mm × 5800mm. In the rolling process, the continuously cast billet is heated to 1210℃ and held for 190 minutes. After exiting the BD (Bottom Belt), it undergoes an up-and-down steel flipping operation. After waiting 30 seconds, it enters the finishing continuous rolling mill for rolling. The initial rolling temperature of the finishing mill is controlled at 1000℃, and water cooling is used between the mill stands to control the temperature difference. The final rolling temperature of the finishing mill is controlled at 815℃ for the upper flange and 830℃ for the lower flange. The rolling specification is the European standard HE200B. Finally, high-strength and high-toughness H-beams are obtained and straightened.

[0050] The product's mechanical performance test data are as follows: ReH: 443MPa, Rm: 530MPa, δ: 20%, Akv at -60℃: average 203J.

[0051] Example 2:

[0052] The chemical composition of the selected high-strength and high-toughness steel, by weight percentage (%), is: C: 0.08, Si: 0.19, Mn: 1.85, V: 0.08, Nb: 0.021, Ni: 0.18, B: 0.0013, Alt: 0.035, P: 0.014, S: 0.003, N: 0.013, O: 0.003, with the remainder being iron (Fe); Cev: 0.41. The prepared raw materials were smelted in a 50-ton electric furnace, and then subjected to nitrogen enrichment treatment by adding 19 meters of manganese nitride cored wire during the degassing process in a VD furnace. The resulting product was then continuously cast into a rectangular continuous casting billet of 240mm × 375mm × 5800mm. In the rolling process, the continuously cast billet is heated to 1200℃ and held for 200 minutes. After exiting the BD (Bottom Belt) section, it is flipped over by an automatic steel-turning machine. After waiting 25 seconds, it enters the finishing continuous rolling mill for rolling. The initial rolling temperature of the finishing mill is controlled at 990℃, and water cooling is used between the mill stands to control the temperature difference. The final rolling temperature of the finishing mill is controlled at 810℃ for the upper flange and 826℃ for the lower flange. The rolling specification is the European standard HE160B. Finally, high-strength and high-toughness H-beams are obtained and straightened.

[0053] The product's mechanical properties test results are as follows: ReH: 452MPa, Rm: 542MPa, δ: 21%, -60℃ Akv impact energy: average 221J.

[0054] Example 3:

[0055] The chemical composition of the selected high-strength and high-toughness steel, by weight percentage (%), is: C: 0.10, Si: 0.20, Mn: 1.65, V: 0.09, Nb: 0.023, Ni: 0.28, B: 0.0016, Alt: 0.03, P: 0.013, S: 0.003, N: 0.012, O: 0.003, with the remainder being iron (Fe); Cev: 0.40. The prepared raw materials were smelted in a 50-ton electric furnace, and then subjected to nitrogen enrichment treatment by adding 23 meters of manganese nitride cored wire during the degassing process in a VD furnace. The resulting product was then continuously cast into a rectangular continuous casting billet of 240mm × 375mm × 5800mm. In the rolling process, the continuously cast billet is heated to 1205℃ and held for 220 minutes. After exiting the BD (Bottom Belt) section, the upper and lower flanges are flipped using an automatic turning machine. After waiting 26 seconds at the correct temperature, it enters the finishing continuous rolling mill for rolling. The initial rolling temperature of the finishing mill is controlled at 985℃, and water cooling is used between the mill stands to control the temperature difference. The final rolling temperature of the finishing mill is controlled at 805℃ for the upper flange and 816℃ for the lower flange. The rolling specification is the European standard HE220A. Finally, high-strength and high-toughness H-beams are obtained and straightened.

[0056] The product's mechanical properties test results are as follows: ReH: 462MPa, Rm: 553MPa, δ: 19%, -60℃ Akv impact energy: average 189J.

[0057] Example 4:

[0058] The chemical composition of the selected high-strength and high-toughness steel, by weight percentage (%), is: C: 0.10, Si: 0.20, Mn: 1.51, V: 0.07, Nb: 0.020, Ni: 0.37, B: 0.0012, Alt: 0.03, P: 0.012, S: 0.003, N: 0.011, O: 0.003, with the remainder being iron (Fe); Cev: 0.38. The prepared raw materials were smelted in a 50-ton electric furnace, and then subjected to nitrogen enrichment treatment by adding 18 meters of manganese nitride cored wire during the degassing process in a VD furnace. The resulting product was then continuously cast into a rectangular continuous casting billet of 240mm × 375mm × 5800mm. In the rolling process, the continuously cast billet is heated to 1190℃ and held for 240 minutes. After exiting the BD (Bottom Belt) section, the upper and lower flanges are flipped using an automatic turning machine. After waiting 36 seconds, it enters the finishing continuous rolling mill for rolling. The initial rolling temperature of the finishing mill is controlled at 980℃, and water cooling is used between the mill stands to control the temperature difference. The final rolling temperature of the finishing mill is controlled at 795℃ for the upper flange and 805℃ for the lower flange. The rolling specification is the European standard HE180A. Finally, high-strength and high-toughness H-beams are obtained and straightened.

[0059] The product's mechanical properties test results are as follows: ReH: 456MPa, Rm: 543MPa, δ: 23%, -60℃ Akv impact energy: average 225J.

[0060] The steel sections obtained in Examples 1 to 4 were tested on a PLG-100C high-frequency fatigue testing machine. The loading method was axial dynamic load, and the material was measured at N=10 using the lifting method. 7 The conditional fatigue limit at a frequency of approximately 92 Hz was determined. The results show that the conditional fatigue limits for the web and flange of the H-beam in Example 1 are 595 MPa and 545 MPa, respectively; for Example 2, they are 575 MPa and 543 MPa; for Example 3, they are 602 MPa and 558 MPa; and for Example 4, they are 585 MPa and 546 MPa.

[0061] The test methods for yield strength, tensile strength, and elongation are based on BS EN ISO 377-1997 "Metallic materials – Test specimens for mechanical properties – Sampling location and preparation"; the test methods for yield strength, tensile strength, and elongation are based on ISO 6892-1-2009 "Metallic materials – Tensile testing at room temperature"; the impact energy test method is based on ISO 148-1 "Metallic materials – Charpy impact test". Examples 1-4 of this invention maintain a yield strength above 440 MPa, exhibit good elongation, and have high impact energy at -60℃, achieving an overall F-level performance. Figure 4As shown, the ductile-brittle transition temperature is extremely low, making it suitable for floating vessels to operate in various marine environments and significantly improving the fatigue life of the material. It can meet the usage conditions of ship and marine engineering components in extremely low-temperature environments, and is suitable for manufacturing support structures for floating oil storage and offloading vessels, polar offshore oil platforms, and ocean-going transport vessels, which have high requirements for low-temperature toughness.

[0062] from Figure 1 , Figure 2 and Figure 3 It can be seen that the microstructure of this invention is an ultrafine pearlite + ferrite structure, and the temperature difference between the upper and lower flanges is controlled within 15°C, ensuring that the strength of the upper and lower flanges is controlled below 15MPa. The ultrafine grain structure meets the requirements for preparing H-beams with a strength of 420MPa, and while meeting the strength index, the low-temperature toughness is improved.

[0063] The upper and lower limits of the process parameters (such as temperature, time, etc.) and the range values ​​of the present invention can all achieve the method, and examples are not listed here.

[0064] All aspects not described in detail in this invention can be covered using conventional technical knowledge in the field.

[0065] Finally, it should be noted that the above embodiments are only used to illustrate the technical solutions of the present invention and are not intended to limit it. Although the present invention has been described in detail with reference to the embodiments, those skilled in the art should understand that modifications or equivalent substitutions to the technical solutions of the present invention do not depart from the spirit and scope of the technical solutions of the present invention, and all such modifications or substitutions should be covered within the scope of the claims of the present invention.

Claims

1. A high-strength, high-toughness, low-temperature resistant hot-rolled H-beam for floating oil storage vessels, characterized in that, Its chemical composition by weight percentage is as follows: C: 0.07–0.11%, Si ≤ 0.2%, Mn: 1.5–1.9%, V: 0.06–0.10%, Nb: 0.02–0.05%, Ni: 0.1–0.4%, B: 0.001–0.002%, Alt: 0.025–0.045%, P ≤ 0.015%, S ≤ 0.006%, N: 0.01–0.015%, O ≤ 0.003%, with the remainder being Fe and unavoidable impurities; The flange thickness of the H-beam is less than 15mm; The method for preparing the H-beam includes the following steps: Hot metal pretreatment → electric furnace smelting → ladle argon blowing → LF refining → VD degassing → rectangular continuous casting billet → continuous casting billet slow cooling pit for slow cooling → heating furnace heating → BD rough rolling → section steel line compact TM semi-continuous rolling → inter-stand cooling + post-rolling lower flange water cooling → dense slow cooling on cooling bed. A turning machine is used to turn the steel between the roughing and finishing mills. After waiting for 20 to 50 seconds based on the temperature detection results, the temperature of the rolled piece is controlled within the range of 960 to 1000℃ before entering the finishing mill for continuous rolling. The compression ratio of the final pass in finishing rolling is 8% to 15%; sufficient cooling after finishing rolling is conducive to the precipitation of V carbonitrides to exert precipitation strengthening effect; The refining process uses LF+VD to control the cleanliness of molten steel. After VD degassing, the nitrogen content is lost. In order to improve the precipitation strengthening effect of VN, nitrogen enrichment treatment is carried out according to the N content. The nitrogen enrichment method is to add 20 to 50 meters of manganese nitride cored wire to increase the nitrogen content of the steel grade to maintain it in the range of 0.01 to 0.015%.

2. The high-strength, high-toughness, low-temperature resistant hot-rolled H-beam for floating oil storage vessels according to claim 1, characterized in that, The high-strength, tough, and low-temperature resistant hot-rolled H-beam has a yield strength ≥440MPa, tensile strength ≥520MPa, elongation ≥18%, and impact energy at -60℃ ≥150J.

3. The high-strength, high-toughness, low-temperature resistant hot-rolled H-beam for floating oil storage vessels according to claim 1, characterized in that, The microstructure of H-beams consists of refined pearlite and proeutectoid ferrite, with grain size controlled within the range of 9 to 11.

4. The high-strength, high-toughness, low-temperature resistant hot-rolled H-beam for floating oil storage vessels according to claim 1, characterized in that, During the rolling process, the heating furnace temperature is 1190-1220℃, and the billet stays in the furnace for 180-240 minutes; the rough rolling process uses 7-9 reciprocating rolling passes.

5. The high-strength, high-toughness, low-temperature resistant hot-rolled H-beam for floating oil storage vessels according to claim 1, characterized in that, The finishing mill adopts continuous rolling, with a starting temperature of 960-1000℃, and all water cooling between the finishing mill stands is turned on. The finishing mill's final rolling temperature is 780-850℃.

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