High-strength, high-toughness, low-temperature-resistant hot-rolled h-shaped steel for floating oil storage ship and preparation method therefor

Abstract

The present invention belongs to the technical field of metallurgy and rolling, and specifically relates to high-strength, high-toughness, low-temperature-resistant hot-rolled H-shaped steel for a floating oil storage ship and a preparation method therefor. The high-strength, high-toughness, low-temperature-resistant hot-rolled H-shaped steel comprises the following chemical components in percentages by weight: 0.07-0.11% of C, less than or equal to 0.2% of Si, 1.5-1.9% of Mn, 0.06-0.10% of V, 0.02-0.05% of Nb, 0.1-0.4% of Ni, 0.001-0.002% of B, 0.025-0.045% of Alt, less than or equal to 0.015% of P, less than or equal to 0.006% of S, 0.01-0.015% of N, less than or equal to 0.003% of O, and the balance being Fe and inevitable impurities. The H-shaped steel of the present invention is based on a green and environmentally friendly concept, has an outstanding lightweight technology, reduces the transportation cost of the floating ship as a whole, improves the utilization efficiency of the material, achieves the aim of weight reduction of 15-30% for the lightweight structure of the floating oil storage ship, and provides the product and technical support for the steel structure used in the hull module.

Description

A high-strength, tough, low-temperature-resistant hot-rolled H-shaped steel for floating oil storage vessels and its preparation method

[0001] This application claims priority to Chinese patent application No. 2023112308667, entitled “A high-strength, tough, and low-temperature-resistant hot-rolled H-shaped steel for a floating oil storage vessel and its preparation method,” filed on September 22, 2023, and this application is included herein by reference. Technical Field

[0002] The present invention belongs to the technical field of metallurgy and rolling, and in particular, relates to a high-strength, tough, and low-temperature-resistant hot-rolled H-shaped steel for a floating oil storage vessel and a preparation method thereof. Background Art

[0003] Floating oil storage and offloading vessels (FOSOs) are high-end, large-scale marine engineering equipment used to exploit deep-sea oil and gas resources. They enable the extraction, processing, storage, and transportation of offshore oil, natural gas, and other energy resources, earning them the nickname "offshore oil processing plants." Unlike relatively fixed offshore oil platforms, FOSOs not only extract oil, store, and transport crude oil products, but also serve as large-scale offshore oil production bases, integrating accommodation and production control. They can quickly reach designated locations for oil and gas development, and boast unique advantages such as a short construction cycle and high mobility, making them suitable for oil and gas development operations in multiple ocean regions around the world.

[0004] As oil and gas resource extraction expands from ordinary sea areas to complex regions like deep and extreme cold, the production, storage, unloading, and transportation of floating oil storage vessels (FSOVs) face increasingly complex operating conditions. Consequently, the construction of large-scale FSOVs places increasingly stringent demands on material upgrades, necessitating the use of new materials and technologies to ensure 30 years or longer without requiring dry docking, minimizing maintenance costs. The construction of topside modules requires a significant amount of hot-rolled H-beam steel. With these increasing requirements, there is an urgent need for hot-rolled H-beams with higher comprehensive performance to replace the current low-grade H-beams, achieving lightweighting while maintaining high stability and reliability. To withstand service in complex environments like extreme cold, higher requirements are placed on steel's low-temperature impact toughness. The required H-beams must achieve strength exceeding 420 MPa, a lower ductile-brittle transition temperature, and low-temperature resistance of at least E or even F grade. Therefore, as the number of FSOVs increases, demand for hot-rolled H-beams with high strength grades exceeding 420 MPa and low-temperature resistance is also increasing.

[0005] Currently, Chinese hot-rolled H-beam manufacturers have developed and commercialized E-grade, low-temperature-resistant H-beams with a yield strength exceeding 355 MPa. Production methods vary by production line, generally employing a composite microalloying process with the addition of Nb, V, and Ti combined with thermomechanical rolling. These differences in preparation methods are reflected in patent applications filed by different companies and production lines.

[0006] Patent application number CN201510788520.8 discloses a 420MPa-grade high-strength, low-yield ratio H-beam and its preparation method. The chemical composition of the H-beam, calculated by weight, is as follows: 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. The invention achieves a yield strength greater than 427MPa, a tensile strength greater than 641MPa, and a yield ratio of 0.64-0.67 for the low-yield ratio H-beam. This patent adds elements such as Ni, Nb, and Cu for use in specific high-corrosion-resistant environments. The preparation process is affected by the Cu element, and the probability of cracks forming on the legs of the H-shaped steel increases under high final rolling temperature conditions, requiring surface grinding and resulting in a low yield rate. At the same time, the addition of Cr and Cu elements can easily lead to abnormal tissue formation, and the material has no obvious yield, making it difficult to ensure safety performance.

[0007] The invention patent application number CN201510498771.2 provides a 420MPa grade excellent low-temperature toughness hot-rolled H-shaped steel and its production method, with the following composition: 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%, and the rest is Fe and unavoidable impurities. Compared with existing technologies, this patented method, through rational design of V, Ni, and N components and controlled rolling and cooling processes, has developed 420MPa hot-rolled H-beams with excellent overall performance. These include a yield strength ReH of 440-520MPa, a tensile strength Rm of 550-650MPa, an elongation A ≥ 22%, and a -40°C low-temperature impact toughness KV2 ≥ 100J. This patented controlled cooling system utilizes a two-stage process: rapid cooling in the first stage and air cooling in the second, requiring specialized cooling equipment to meet these production requirements. The resulting 420MPa-grade, low-temperature-toughness hot-rolled H-beams meet Z-direction performance requirements of 40-65%. This process is designed only for large-sized profiles with profiles larger than 15mm, and is unsuitable for the production of small and medium-sized thin-flange H-beams.

[0008] The invention patent authorized by application number 202110818559.5 provides a 420MPa grade hot-rolled low-temperature resistant H-shaped steel and its preparation method. The chemical composition of the H-shaped steel is as follows by weight percentage: C: 0.08% to 0.10%, Si≤0.2%, Mn: 1.25% to 1.45%, V: 0.03% to 0.045%, Ti: 0.015% to 0.025%, Cr: 0.15% to 0.30%, A1s: 0.02% to 0.04%, N: 0.007% to 0.01%, P≤0.008%, S≤0.005%, O≤0.004%, and the rest are Fe and unavoidable impurities. The present invention combines the characteristics of thin flanges of small and medium-sized H-shaped steels rolled in rectangular billets, adopts a low C content suitable for normalizing rolling combined with a V micro-alloying composition design, adds an appropriate amount of Cr element to control the cooling rate, avoids the occurrence of abnormal structures such as widmanstattenite, and worsens the low temperature of the steel. This invention designs a production process for small and medium-sized H-beams that achieves E-grade low-temperature toughness while maintaining low-P content in molten iron. Cost control and process implementation require precise operation, which presents challenges. Adding a certain amount of Ti can easily lead to TiN precipitation, which can affect impact toughness.

[0009] The aforementioned patent involves 420MPa-level H-shaped steel and its preparation technology, which places high demands on molten iron control and post-rolling cooling equipment. Therefore, in order to meet the demand for H-shaped steel for floating vessels, normalizing rolling is adopted at higher final rolling temperatures for rolling small and medium-sized H-shaped steel using short-process compact universal rolling mills. The composition and phase transformation structure design is required to achieve a strength above the yield level of 420MPa while further reducing the ductile-brittle transition temperature. At the same time, with the overall increase in yield strength, the uniformity and stability of the upper and lower flanges and the head and tail structures need to be improved; the strength difference between the web and the flange needs to be controlled within an appropriate range to avoid affecting the overall fatigue performance of the component. At the same time, since floating vessels often travel back and forth in various complex environments and polar waters, 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 avoid premature failure of the hull steel structure components due to low-temperature brittle fracture, which increases the maintenance cost of the floating vessel and even greatly shortens the service life, resulting in a serious waste of energy and resources.

[0010] Summary of the Invention

[0011] To meet the demand for high-strength, high-low-temperature toughness steel for floating oil storage vessels operating in complex environments, a high-strength, toughness, and low-temperature-resistant hot-rolled H-beam and its preparation method were designed based on the structural characteristics of the hull. This H-beam, based on environmentally friendly principles and emphasizing lightweighting technology, reduces overall floating vessel transportation costs and improves material utilization efficiency. The amount of H-beam used per vessel is reduced by over 500 tons, achieving a 15-30% weight reduction for floating oil storage vessels. This provides product and technical support for the steel structures used in some hull modules.

[0012] The technical solutions of the present invention are as follows:

[0013] Disclosed are high-strength, tough, and low-temperature-resistant hot-rolled H-shaped steel for floating oil storage vessels and a preparation method thereof. The chemical composition of the steel, 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, Al2O3: 0.025-0.045, P≤0.015, S≤0.006, N: 0.01-0.015, O≤0.003, and the remainder is Fe and unavoidable impurities.

[0014] H-shaped steel produced by meeting the chemical composition requirements can better meet the impact toughness requirements under polar temperatures.

[0015] The invention is particularly suitable for preparing small and medium-sized H-shaped steel products with flange thickness below 15mm, but is not limited to the above-mentioned specifications.

[0016] The design principles of the main chemical elements in the high-strength hot-rolled H-beam of the present invention are as follows:

[0017] Carbon: To achieve a 420MPa strength level for hot-rolled H-beams used in floating vessels while also meeting low-temperature resistance requirements at -40°C, the pilot steel features an ultrafine-grained matrix of pearlite and ferrite, achieving a grain size rating of at least Grade 9 according to the standard. Carbon also contributes to the formation of a certain number of nano-scale vanadium-containing carbonitrides. Smaller H-beams experience significant compression, and higher carbon contents are prone to abnormal microstructures and the formation of large amounts of pearlite. Therefore, the carbon content is controlled within the range of 0.07% to 0.11%.

[0018] Manganese: In hot-rolled H-beams, Mn stabilizes the austenite structure, increases the steel's hardenability, and improves its strength. However, Mn is also a readily segregating element. This can lead to significant performance variations in complex cross-sections, resulting in varying deformation and structural transformations in different areas. To ensure strength while minimizing segregation, the Mn content is preferably set at 1.5% or higher. However, excessively high Mn levels can easily lead to abnormal microstructures. Taking all factors into consideration, the Mn content in the present steel is controlled to 1.5% to 2.0%.

[0019] Sulfur: During the H-beam production process, sulfur reacts with manganese to form MnS. These intermittent inclusions severely degrade the steel's low-temperature toughness. The ratio of manganese to sulfur affects the steel's mechanical properties and the occurrence of surface defects. Excessive sulfur levels can easily form large numbers of MnS inclusions, degrading processability. The sulfur content is preferably limited to 0.006% or less.

[0020] Aluminum: During the production process, aluminum acts as a strong deoxidizing element and forms AlN, which refines grains. This ensures the oxygen content in the steel is kept as low as possible, reducing the likelihood of spherical inclusions. Furthermore, the AlN precipitates formed by aluminum and nitrogen in the steel refine grains, thereby increasing steel strength. Therefore, in the present invention, the aluminum content is controlled to 0.025-0.045%.

[0021] Niobium: Nb can form second-phase particles such as NbC or NbN in steel. During recrystallization rolling, NbC and NbN pin dislocations and inhibit grain growth, significantly increasing recrystallization time. Niobium in steel increases the recrystallization temperature of austenite, expands the temperature range of the unrecrystallized zone, promotes austenite grain deformation and defect accumulation, and ultimately refines the ferrite grains. Furthermore, the precipitated Nb(C,N) inhibits austenite grain growth during heating, thereby refining the austenite structure during the final fine transformation. Therefore, for low-carbon rolled small and medium-sized sections, grain refinement can be achieved by adding a small amount of niobium through controlled rolling on conventional section mills. Nb content is controlled at 0.02-0.05%.

[0022] Vanadium: As a strong carbonitride-forming element, its carbonitrides act as precipitation strengthening. VN alloys serve as nucleation sites for ferrite and pearlite, contributing to grain refinement. VC is generated during the late stages of rolling and cooling, typically existing at the nanoscale within the steel, where it acts as precipitation strengthening. Furthermore, vanadium-containing steels precipitate in the late stages of rolling, reducing rolling deformation resistance and reducing rolling loads. For low-temperature-resistant steels with a yield strength of 420 MPa, the V content is controlled between 0.06% and 0.10% to achieve grain refinement and nano-enhancement precipitation.

[0023] Nickel: The Ni element in steel can improve the impact toughness, especially in the mass production process, it can significantly expand the process window. Considering factors such as cost, the Ni element is controlled in the range of 0.1-0.4%.

[0024] Nitrogen: Nitrogen in steel forms a VN alloy with vanadium, which enhances strength through precipitation strengthening. It also forms NbN with niobium at high temperatures, preventing grain growth. However, excessive nitrogen content can easily lead to surface defects such as transverse cracks in the ingot. Low nitrogen content results in poor precipitation of VN particles. Therefore, the present invention requires a nitrogen content of 0.01-0.015%.

[0025] Boron: Boron's primary function is to improve the hardenability of steel. Adding a small amount of boron can improve hardenability. Boron's indirect effect, by delaying the change in γ-α, is to increase the solubility of alloying elements and reduce chemical inhomogeneities in continuous castings. Boron has a strong affinity for nitrogen and oxygen, and adding an appropriate amount can eliminate aging effects in steel. A certain amount of boron is added to the steel of this invention to appropriately control the cooling rate and avoid abnormal microstructures. The addition range is 0.001 to 0.002%.

[0026] Oxygen: A higher oxygen content easily forms large oxide inclusions with strong oxidizing elements, thereby reducing the toughness and plasticity of the steel. The present invention requires an oxygen content of ≤0.003%.

[0027] The hot-rolled H-shaped steel for floating vessels has a yield strength of ≥440 MPa, a tensile strength of ≥520 MPa, an elongation of ≥18%, and an impact energy of ≥150 J at -60°C.

[0028] The invention also provides a method for preparing the above-mentioned H-shaped steel with a yield strength of 420 MPa, which comprises the following steps: molten iron pretreatment → 50t electric furnace smelting → ladle argon blowing → LF refining → VD degassing → rectangular continuous casting → continuous casting slow cooling in a slow cooling pit → heating in a heating furnace → BD rough rolling → compact TM semi-continuous rolling on a section steel line → inter-stand cooling + water cooling of the lower flange after rolling → intensive slow cooling on a cooling bed;

[0029] During the rolling process, the heating furnace soaking temperature is 1190-1220°C, and the ingot remains in the furnace for 180-240 minutes. Rough rolling typically involves 7-9 reciprocating passes. Finishing rolling is performed in five continuous passes, with a start temperature of 960-1000°C, water cooling between the stands, and a final temperature of 780-850°C. Between roughing and finishing, the steel is turned using a steel turning mill. Based on temperature monitoring, the steel is held at a temperature for 20-50 seconds to ensure the inlet temperature falls within the appropriate range. To ensure control of the final rolling temperature, the reduction ratio in the final finishing pass is 8%-15%. Sufficient cooling after finishing facilitates the precipitation of V carbonitrides, which exerts its precipitation strengthening effect.

[0030] The present invention realizes the industrial production of 420MPa high-strength and tough H-beam products for floating vessels under different compression ratios by designing a microalloying process of adding Nb and V and combining the characteristics of semi-continuous rolling of steel sections.

[0031] The processes not mentioned in the present invention can all adopt existing technologies.

[0032] In order to ensure the high consistency and uniformity of the upper and lower flanges of the produced H-shaped steel structure and performance, the present invention has carried out targeted design in terms of process to meet the performance requirements.

[0033] Due to the characteristics of the short-process semi-continuous rolling equipment, the distance between the roughing and finishing mills is approximately 60 meters. This short distance does not allow for the rapid cooling of the intermediate billet. Furthermore, the temperature difference between the upper and lower flanges is large, reaching up to 70°C. The rolling process needs to be optimized and improved. Based on the characteristics of the heating and rolling process control, a steel turning operation is required between the roughing and finishing mills. This operation allows the upper and lower flanges of the H-beam to be swapped before finishing rolling begins. However, since the billet length increases significantly after roughing, the temperature of the intermediate billet can reach 1050°C. Uneven steel turning can easily cause the rolled piece to twist, which can increase biting difficulty during finishing rolling and cause steel jamming. Therefore, to meet the steel turning requirements, a rapid steel turning operation is implemented between finishing and roughing mills within a short period of time.

[0034] To increase the yield strength of H-steel for floating vessels to above 420 MPa for both upper and lower flanges, a composite microalloying process is employed. This invention primarily utilizes VN alloying to enhance strength. Furthermore, given the high cleanliness requirements for the steel, LF+VD degassing is employed during the 50-ton electric furnace smelting process. However, VD degassing significantly reduces the nitrogen content, necessitating nitrogen enrichment based on the actual nitrogen content. This is achieved by adding a certain amount of manganese nitride cored wire, adjusted according to the nitrogen content.

[0035] The advantages of the technical solution of the present invention are:

[0036] 1. Taking into account the thin flange characteristics of medium-sized H-beams with varying compression ratios rolled from rectangular billets, a low carbon content suitable for normalizing rolling is combined with a Nb and V microalloying composition design. Simultaneously, the nitrogen content is controlled during the refining process to enhance VN precipitation, meeting the strength and toughness requirements of products of varying specifications. This results in a stable, controlled, and high-strength and toughness hot-rolled H-beam exceeding 420 MPa on the hot-rolling mill.

[0037] 2. LF+VD is used in the refining process to control the cleanliness of molten steel. Nitrogen content is lost after VD degassing. In order to enhance the precipitation strengthening effect of VN, nitrogen addition treatment is carried out according to the N content. The nitrogen addition 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%.

[0038] 3. During the rolling process, special steel turning equipment is used for steel turning operation between rough rolling and finishing rolling, which significantly reduces the temperature difference between the upper and lower flanges to below 25°C; according to the intermediate temperature detection results, appropriate roller temperature holding operation is adopted to maintain the temperature of the rolled piece within the range of 960-1000°C for 20-50s before entering the finishing mill for five-pass continuous rolling, thereby improving the low-temperature toughness of the steel and maintaining a high level of impact energy greater than 150J at -60°C.

[0039] 4. All cooling water between stands and after rolling is turned on to ensure that the final rolling temperature is controlled between 780 and 850 ° C, the gap between the upper and lower flanges is reduced to below 20 ° C, and the difference in yield strength of the upper and lower legs is controlled within the range of less than 15 MPa, so that the performance is uniform.

[0040] 5. The matrix structure of H-shaped steel is refined pearlite + proeutectoid ferrite, the grain size is controlled in the range of 9 to 11, the stably precipitated second phase particles are mainly V (C, N), the structure is highly uniform, and the aging strengthening effect is well controlled.

[0041] 6. Through the ultra-fine structure of the matrix and the precipitation strengthening mechanism dominated by VN precipitation, the yield strength of the hot-rolled H-beam for floating ships reaches 420MPa, while the low-temperature impact energy at -60℃ is greater than 150J.

[0042] 7. The steel section involved in the present invention has a low carbon equivalent, ensuring high strength and toughness while having excellent welding performance.

[0043] 8. According to the present invention, while improving the overall strength of the H-shaped steel, the steel plate can have good fatigue resistance, so that the steel plate according to the present invention can meet the use requirements of users of ships and floating oil storage vessels. BRIEF DESCRIPTION OF THE DRAWINGS

[0044] FIG1 is a metallographic structure diagram (×100) of a high-toughness, low-temperature-resistant H-beam with a yield strength of 420 MPa prepared by the present invention;

[0045] FIG2 is a metallographic structure diagram (×200) of a high-toughness, low-temperature-resistant H-beam with a yield strength of 420 MPa prepared by the present invention;

[0046] FIG3 is a metallographic structure diagram (×500) of a high-toughness, low-temperature-resistant H-beam with a yield strength of 420 MPa prepared by the present invention;

[0047] Fig. 4 Ductile-brittle transition curves of HE200B steel at different temperatures. DETAILED DESCRIPTION

[0048] The continuous casting ingots in the following examples were all prepared according to the following process flow: blast furnace hot metal was used as the raw material and a certain amount of scrap steel was added according to the set chemical composition range. The ingots were smelted in an electric furnace and refined by LF+VD. The contents of C, Si, Mn, S, P, etc. were adjusted and microalloyed. After the composition reached the target value, continuous casting was performed and the ingots were directly heated or soaked. The preparation steps of Examples 1-4 are as follows:

[0049] Hot metal pretreatment → smelting in a 50t electric furnace → argon blowing in a ladle → LF refining → VD degassing → rectangular continuous casting → slow cooling in a slow cooling pit → heating in a reheating furnace → rough rolling → compact TM semi-continuous rolling of the section line → inter-stand cooling + water cooling of the lower flange after rolling → intensive slow cooling on a cooling bed. Section line rolling consists of two stages: rough rolling and finishing rolling. The hot rolling process focuses on temperature control, with the final rolling temperature measured between the upper and lower legs outside the flange. After rolling, the rolled product is naturally stacked and cooled on a cooling bed.

[0050] Example 1:

[0051] The chemical composition of the selected high-strength and toughness steel is as follows: C: 0.07, Si: 0.15, Mn: 1.90, V: 0.10, Nb: 0.025, Ni: 0.11, B: 0.0011, Al2O3: 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, degassed in a refining VD furnace, and nitrogenized by adding 21 meters of manganese nitride cored wire. The steel was then continuously cast into rectangular ingots measuring 240 mm x 375 mm x 5800 mm. During the rolling process, the continuous casting slab is heated to 1210°C and held at this temperature for 190 minutes. After exiting the BD, the slab is turned upside down and then held at this temperature for 30 seconds before entering the finishing mill. The starting temperature for finishing rolling is controlled at 1000°C, and water cooling is used between the stands to control the temperature difference. The final finishing rolling temperature is controlled at 815°C for the upper flange and 830°C for the lower flange. The rolled product meets the European standard HE200B. The resulting high-strength and tough H-beam is then straightened.

[0052] The mechanical properties test data of the product are: ReH: 443MPa, Rm: 530MPa, δ: 20%, -60℃ Akv: average 203J.

[0053] Example 2:

[0054] The chemical composition of the selected high-strength and toughness steel is as follows: C: 0.08, Si: 0.19, Mn: 1.85, V: 0.08, Nb: 0.021, Ni: 0.18, B: 0.0013, Al2O3: 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, followed by a refining VD furnace degassing step, where 19 meters of manganese nitride cored wire was added for nitrogen addition. The steel was then continuously cast into rectangular ingots measuring 240 mm x 375 mm x 5800 mm. During the rolling process, the continuous casting slab is heated to 1200°C and held for 200 minutes. After exiting the BD, it is turned up and down by an automatic steel turning machine. After 25 seconds of holding at temperature, it enters the finishing continuous rolling mill. The starting temperature of the finishing rolling is controlled at 990°C. Water cooling is used between the finishing stands to control the temperature difference. The finishing rolling temperature is controlled at 810°C for the upper flange and 826°C for the lower flange. The rolled specifications meet the European standard HE160B. The resulting high-strength and tough H-beam is then straightened.

[0055] The mechanical properties test results of the product are: ReH: 452MPa, Rm: 542MPa, δ: 21%, -60℃ Akv impact energy: average 221J.

[0056] Example 3:

[0057] The chemical composition of the selected high-strength and toughness steel is as follows: C: 0.10, Si: 0.20, Mn: 1.65, V: 0.09, Nb: 0.023, Ni: 0.28, B: 0.0016, Al2O3: 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, degassed in a refining VD furnace, and nitrogenized by adding 23 meters of manganese nitride cored wire. The steel was then continuously cast into rectangular ingots measuring 240 mm x 375 mm x 5800 mm. During the rolling process, the continuous casting slab is heated to 1205°C and held at that temperature for 220 minutes. After exiting the BD process, the upper and lower flanges are turned over by an automatic steel turning machine. After 26 seconds of holding at temperature, it enters the finishing continuous rolling mill. The starting temperature of the finishing rolling is controlled at 985°C. Water cooling is used between the finishing stands to control the temperature difference. The final finishing rolling temperature is controlled at 805°C for the upper flange and 816°C for the lower flange. The rolled specifications meet the European standard HE220A. The resulting high-strength and tough H-beam is then straightened.

[0058] The mechanical properties test results of the product are: ReH: 462MPa, Rm: 553MPa, δ: 19%, -60℃ Akv impact energy: average 189J.

[0059] Example 4:

[0060] The chemical composition of the selected high-strength and toughness steel is as follows: C: 0.10, Si: 0.20, Mn: 1.51, V: 0.07, Nb: 0.020, Ni: 0.37, B: 0.0012, Al2O3: 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, degassed in a refining VD furnace, and nitrogenized by adding 18 meters of manganese nitride cored wire. The steel was then continuously cast into rectangular ingots measuring 240 mm x 375 mm x 5800 mm. During the rolling process, the continuous casting slab is heated to 1190°C and held at this temperature for 240 minutes. After exiting the BD process, the upper and lower flanges are turned over by an automatic steel turning machine. After 36 seconds of holding at this temperature, the slab enters the finishing mill for rolling. The starting temperature of the finishing rolling is controlled at 980°C. Water cooling is used between the finishing mill stands to control the temperature difference. The final finishing rolling temperature is controlled at 795°C for the upper flange and 805°C for the lower flange. The rolled specifications meet the European standard HE180A. The resulting high-strength and tough H-beam is then straightened.

[0061] The mechanical properties test results of the product are: ReH: 456MPa, Rm: 543MPa, δ: 23%, -60℃ Akv impact energy: average 225J.

[0062] The steel sections obtained in Examples 1 to 4 were tested on a PLG-100C high frequency fatigue testing machine. The loading mode was axial dynamic load. The lifting method was used to measure the material at N=10 7 The conditional fatigue limit of the H-beam in Example 1 is 595MPa and 545MPa, respectively, at a frequency of approximately 92Hz. The results show that the conditional fatigue limits of the web and flange of the H-beam in Example 1 are 575MPa and 543MPa, respectively; the conditional fatigue limits of the web and flange of the H-beam in Example 2 are 602MPa and 558MPa, respectively; and the conditional fatigue limits of the web and flange of the H-beam in Example 3 are 585MPa and 546MPa, respectively.

[0063] The standard is BS EN ISO 377-1997 "Sampling location and preparation of mechanical test specimens"; the test methods for yield strength, tensile strength, and elongation refer to the standard ISO6892-1-2009 "Room temperature tensile test methods for metallic materials"; the impact energy test method refers to the standard ISO 148-1 "Charpy pendulum impact test for metallic materials". The yield strength of Examples 1-4 of the present invention remains above 440MPa, with good elongation performance. Its -60℃ impact energy is relatively high, reaching the F-level as a whole. As shown in Figure 4, the ductile-brittle transition temperature is extremely low, which is suitable for floating ships to operate in different sea environments and significantly improves the fatigue life of the material. It can meet the conditions for the use of ships and marine engineering components in extremely low environments, and is suitable for the production of supporting structures with high low-temperature toughness requirements such as floating oil storage vessels, polar marine oil platforms, and ocean transport ships.

[0064] Figures 1, 2, and 3 show that the microstructure of the present invention is ultrafine pearlite + ferrite. The temperature difference between the upper and lower flanges is controlled within 15°C, ensuring that the strength of the upper and lower legs is controlled below 15 MPa. The ultrafine grain structure meets the requirements for producing H-beams with a strength of 420 MPa, meeting strength requirements while also improving low-temperature toughness.

[0065] The process parameters (such as temperature, time, etc.) of the present invention can realize the method by taking upper and lower limits and interval values, and the embodiments are not listed here one by one.

[0066] Any content not described in detail in the present invention can be based on conventional technical knowledge in the art.

[0067] Finally, it should be noted that the above embodiments are intended only to illustrate the technical solutions of the present invention and are not intended to limit the present invention. Although the present invention has been described in detail with reference to the embodiments, it should be understood by those skilled in the art 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 are intended to be encompassed by the claims of the present invention.

Claims

1. A high-strength and low-temperature-resistant hot-rolled H-beam for a floating oil storage vessel, characterized in that: 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%, A1t: 0.025-0.045%, P≤0.015%, S≤0.006%, N: 0.01-0.015%, O≤0.003%, and the rest is Fe and unavoidable impurities.

2. The high-strength and tough low-temperature-resistant hot-rolled H-shaped steel for floating oil storage vessels according to claim 1, characterized in that: The flange thickness of the H-shaped steel is less than 15 mm.

3. The high-strength and tough low-temperature-resistant hot-rolled H-beam for floating oil storage vessel according to claim 1, characterized in that: The high-strength, tough, and low-temperature-resistant hot-rolled H-shaped steel has a yield strength of ≥440MPa, a tensile strength of ≥520MPa, an elongation of ≥18%, and an impact energy of ≥150J at -60°C.

4. The high-strength and tough low-temperature-resistant hot-rolled H-beam for floating oil storage vessel according to claim 1, characterized in that: The matrix structure of H-beam steel is refined pearlite + proeutectoid ferrite, and the grain size is controlled in the range of 9 to 11.

5. A method for preparing high-strength and low-temperature-resistant hot-rolled H-shaped steel for a floating oil storage vessel, the method comprising the following steps: Hot metal pretreatment → electric furnace smelting → argon blowing in ladle → LF refining → VD degassing → rectangular continuous casting billet → slow cooling in continuous casting billet slow cooling pit → heating in heating furnace → BD rough rolling → compact TM semi-continuous rolling of steel line → cooling between stands + water cooling of lower flange after rolling → intensive slow cooling on cooling bed.

6. The preparation method according to claim 5, characterized in that: During the rolling process, the heating furnace has an average heating temperature of 1190-1220°C, and the billet is in the furnace for 180-240 minutes; the rough rolling adopts 7-9 reciprocating rolling passes.

7. The preparation method according to claim 5, characterized in that: The finishing rolling adopts continuous rolling, the starting rolling temperature of the finishing rolling is 960-1000℃, the water cooling between the finishing rolling stands is fully turned on, and the final rolling temperature of the finishing rolling is 780-850℃.

8. The preparation method according to claim 5, characterized in that: The steel turning operation is carried out by using a steel turning machine between rough rolling and finishing rolling. According to the temperature detection results, the temperature of the rolled piece is controlled within the range of 960-1000℃ after waiting for 20-50s, and then enters the finishing mill for continuous rolling.

9. The preparation method according to claim 5, characterized in that: The compression ratio of the last pass of finishing rolling is 8% to 15%; sufficient cooling after finishing rolling is conducive to the precipitation of V carbonitrides to exert precipitation strengthening effect.

10. The preparation method according to claim 5, characterized in that: The refining process uses LF+VD to control the cleanliness of molten steel. The nitrogen content is lost after VD degassing. In order to improve the precipitation strengthening effect of VN, nitrogen addition treatment is carried out according to the N content. The nitrogen addition method is to add 20 to 50 meters of manganese nitride cored wire to increase the nitrogen content of the steel to maintain it in the range of 0.01 to 0.015%.

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