Low-cost thick-specification high-toughness low-temperature-resistant hot-rolled h-shaped steel for ship structure, and preparation method for h-shaped steel

Abstract

The present invention relates to the technical field of metallurgy, and in particular to low-cost thick-specification high-toughness low-temperature-resistant hot-rolled H-shaped steel for a ship structure, and a preparation method for the H-shaped steel. The H-shaped steel comprises the following chemical components in percentage by weight: C: 0.05%-0.07%; Si: ≤0.25%; Mn: 1.20%-1.50%; V: 0.11%-0.15%: A1t: 0.035%-0.055%; Cr: 0.3%-0.5%; P: ≤0.015%; S: ≤0.005%; N: ≤0.010%-0.015%; O: ≤0.004%; and the balance being Fe and inevitable impurities. The preparation method comprises the following steps: pretreatment of a liquid iron, smelting in a 120 t converter, argon blowing of ladle, refining in an LF, casting of a special-shaped continuous casting blank, slow cooling of the continuous casting blank in a slow cooling pit, heating by a heating furnace, rough rolling by a BD, a steel turning operation of an intermediate billet, reversible rolling by a TM during finish rolling, inter-rack cooling, water cooling of rolled flanges in a phase change area, dense slow cooling on a cooling bed, straightening by a straightening machine, and a finishing treatment of a finished product. According to the present invention, high-toughness hot-rolled H-shaped steel having stable performance and having a 420 MPa grade or above is obtained on a universal hot-rolled H-shaped steel rolling mill.

Description

A low-cost, thick-gauge, high-strength, and low-temperature-resistant hot-rolled H-shaped steel for ship structures and its preparation method

[0001] Related applications

[0002] This application claims priority to Chinese patent application No. 2023113493932, entitled “A thick-gauge, high-strength, and low-temperature-resistant hot-rolled H-shaped steel for low-cost ship structures and its preparation method,” filed on October 18, 2023, which is hereby incorporated by reference. Technical Field

[0003] The present invention belongs to the technical field of metallurgy, and in particular, relates to a low-cost, thick-gauge, high-strength, tough, and low-temperature-resistant hot-rolled H-shaped steel for ship structures and a preparation method thereof. Background Art

[0004] Steel is a key raw material in shipbuilding structures, accounting for 20-30% of hull construction costs. Hull construction consumes approximately 60% of the total ship mass, with steel sections accounting for over 10%. With advancements in shipbuilding processes and technologies, the number of specialized vessels is increasing, including bulk carriers, very large crude oil tankers, floating production storage and offloading vessels (FPSOs), container ships, LNG carriers, and jack-up drilling platforms. With increasing demand for larger, lighter, and faster ships, the strength and toughness of hot-rolled H-beam steel required for these specialized vessels is steadily increasing. Yield strength has increased from 355 MPa to over 390 / 420 MPa, while toughness has also increased from room-temperature impact toughness under normal operating conditions to -40°C or even -60°C, reaching F-grade. This meets environmental adaptability requirements for rapid arrival at designated locations, facilitating vessel development and operation in all oceans worldwide.

[0005] Therefore, different companies have successively developed high-grade, high-strength and high-toughness H-beams to achieve product upgrades. In particular, the preparation of large-size, thick-flange hot-rolled H-beams can be found in the published patents. The differences in preparation methods among different companies and production lines can be found.

[0006] Patent application number CN201510498771.2 describes a 420MPa hot-rolled H-beam with excellent low-temperature toughness and its production method. The chemical 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. Through rational design of the V, Ni, and N components and controlled rolling and cooling processes, the present invention develops a 420MPa hot-rolled H-beam with excellent overall performance. The steel features a yield strength (ReH) of 440-520MPa, a tensile strength (Rm) of 550-650MPa, an elongation (A) of ≥ 22%, and a low-temperature impact toughness (KV2) of ≥ 100J at -40°C. This patented controlled cooling adopts a two-stage method: rapid cooling in the first stage and air cooling in the second stage. Special cooling equipment is required to meet the above-mentioned preparation requirements. At the same time, the later cooling is prone to produce abnormal tissues, affecting the stability of strength and toughness.

[0007] Application number CN201610096590.1 relates to a particularly thick, low-ductile-brittle transition temperature hot-rolled H-beam and its production method. The chemical composition of the invented hot-rolled H-beam, by mass percentage, includes: C 0.08-0.15%, Si 0.10-0.40%, Mn 1.0-1.5%, P ≤ 0.015%, S ≤ 0.008%, V 0.020-0.070%, Ti 0.005-0.025%, N 0.006-0.015%, Ni 0.10-0.50%, with the remainder being iron and residual trace impurities. The H-beam has a flange thickness between 25mm and 36mm, a web height between 700mm and 1000mm, a rolling reduction ratio of less than 3.5, and is produced using a hot rolling process (without water cooling, ultra-rapid cooling, or heat treatment). The final product has a yield strength exceeding 420 MPa, a tensile strength exceeding 500 MPa, a ductile-brittle transition temperature below -65°C, and excellent Charpy impact resistance at low temperatures of -45 to -65°C. The aforementioned patented finish rolling temperature is controlled between 860 and 900°C, making stable control difficult to achieve through natural cooling alone. This also results in significant strength variations between the upper and lower flanges of the H-beam. Furthermore, the carbon content in the examples is 0.11 to 0.14%, but excessively high carbon content can negatively impact strength, toughness, and stability.

[0008] 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 Ni, Cr, and Cu corrosion-resistant elements; the addition of a certain amount of Cu element significantly increases the probability of cracks forming on the legs of H-shaped steel ingots and finished products, requiring surface grinding, resulting in a low yield; the lack of effective cooling control measures makes the addition of Cr and Cu elements prone to abnormal tissue.

[0009] The aforementioned patent involves thick, 420MPa-grade H-beam and its manufacturing technology, but it does not provide effective control measures for equipment and cooling conditions. Furthermore, the addition of nickel to the composition design significantly increases manufacturing costs. As the yield strength of marine, low-temperature-resistant H-beam increases to above 420MPa, the uniformity and stability of the upper and lower flanges and the bow and stern structures need to be improved. At the same time, marine structural steel requires a low ductile-brittle transition temperature to ensure good low-temperature impact toughness and avoid fatigue failure of hull steel components due to low-temperature brittle fracture, which shortens the service life and cycle.

[0010] In order to improve the consistency and uniformity of the upper and lower flange heights of the microstructure and performance of hot-rolled H-shaped steel for ships, the present invention has carried out targeted design in terms of process to meet the requirements of integrated improvement of microstructure and performance while significantly reducing costs.

[0011] Therefore, in order to meet the preparation needs of low-temperature resistant H-shaped steel for ships, on the domestically commonly used XH rolling method for steel sections, by designing special steel turning equipment between rough rolling and finishing rolling, combined with corresponding low-cost chemical composition design and organizational design, the yield strength of thick-gauge H-shaped steel is achieved to reach 420MPa level through fine grain strengthening and precipitation strengthening, while further reducing the ductile-brittle transition temperature.

[0012] Summary of the Invention

[0013] In order to meet the demand for the preparation of high-strength, toughness, and low-temperature-resistant steel for ship structures in complex and changing environments, a low-cost high-strength, toughness, and low-temperature-resistant hot-rolled H-shaped steel for ship structures and a preparation method were designed and invented according to the characteristics of the hull superstructure, which are specifically used for the preparation of hull superstructure parts.

[0014] To achieve the above object, the technical solution of the present invention is as follows:

[0015] A low-cost, thick-gauge, high-strength, low-temperature-resistant H-beam for ship structures. Its chemical composition, by weight, is as follows: C: 0.05-0.07%, Si: ≤0.25%, Mn: 1.20-1.50%, V: 0.11-0.15%, Al2O3: 0.035-0.055%, Cr: 0.3-0.5%, P ≤0.015%, S ≤0.005%, N 0.010-0.015%, O ≤0.004%, with the remainder being Fe and unavoidable impurities. H-beams produced with this compositional requirement can better meet impact toughness requirements at different ambient temperatures.

[0016] The invention is particularly suitable for preparing steel products with flange thickness of 20mm to 40mm, but is not limited to the above-mentioned products, and is also applicable to other related products.

[0017] The design principles of the main chemical elements in the high-strength hot-rolled H-beam of the present invention are as follows:

[0018] Carbon: To meet the 420MPa strength level of hot-rolled H-beam for marine applications while ensuring low-temperature impact toughness at -60°C, ultrafine ferrite and a small amount of pearlite are achieved, with a grain size of 9 or above. For thick H-beams, where the compression ratio is low and a high carbon content is prone to abnormal microstructure, carbon segregation can easily lead to coarse pearlite. Therefore, based on the comprehensive strength and toughness requirements, the carbon content is controlled within the range of 0.05-0.07%.

[0019] Manganese: Mn stabilizes the austenite structure in steel, increases hardenability, and improves strength through solid solution strengthening. Deformation and structural transformation vary across different parts of H-beams. Excessive Mn, a segregating element, can lead to significant performance variations. To ensure the strength of marine steel while minimizing segregation, the upper limit of Mn content should not exceed 1.5%. Based on these considerations, the present invention limits the Mn content to 1.20-1.50%.

[0020] Aluminum: As a strong deoxidizing element, aluminum enhances steel cleanliness. It also forms a large amount of AlN with nitrogen, refining the grain size. This ensures the oxygen content in the steel is kept as low as possible, reducing the likelihood of large oxide inclusions. Furthermore, aluminum reacts with nitrogen in the steel to form AlN precipitates, which refines the grain size and improves the steel's strength. Therefore, in the present invention, the aluminum content is controlled between 0.035% and 0.055%.

[0021] Chromium: Cr significantly improves the hardenability of steel, stabilizes the austenite structure, and enhances pearlite transformation. Excessively high or low Cr content negatively impacts the steel's hardenability and delayed fracture resistance, and can easily lead to localized defects. Cr additions should be limited to avoid localized segregation caused by high hardenability and the appearance of abnormal structures such as large amounts of widmanstattenite and variant pearlite. For comprehensive considerations, the Cr content should be controlled between 0.30% and 0.50%.

[0022] Vanadium: As a strong carbonitride-forming element, V (C, N) contributes to precipitation strengthening. Furthermore, V (C, N) nanoparticles serve as nucleation sites for ferrite and pearlite, contributing to grain refinement and austenite refinement. Due to the low C content and high N content in the present invention, V precipitation is minimal. Therefore, V primarily exists as VN during rolling and post-rolling cooling, enhancing precipitation strengthening. To achieve a yield strength of 420 MPa for low-temperature-resistant sections, the V content is controlled between 0.11 and 0.15%.

[0023] Nitrogen: A certain concentration of nitrogen in steel combines with vanadium to form VN second-phase particles, which exert a precipitation strengthening effect and significantly improve strength. However, too high a nitrogen content can easily induce surface defects in the cast billet, such as transverse cracks and corner cracks on the surface of the beam billet. Low nitrogen content results in fewer VN particles and a poor precipitation strengthening effect. Therefore, the present invention requires a nitrogen content of 0.011-0.015%.

[0024] Oxygen: A higher oxygen content easily forms large oxide inclusions with elements such as Al, Mg, and Ca. Controlling the oxygen content to a lower level will lead to increased costs. Therefore, in order to ensure strength while improving the toughness and plasticity of marine hot-rolled steel, the present invention requires an oxygen content of ≤0.004%.

[0025] The marine hot-rolled H-shaped steel has an upper yield strength of ≥430 MPa, a tensile strength of ≥530 MPa, an elongation of ≥19%, and an impact energy of ≥180 J at -60°C.

[0026] The present invention provides a method for preparing the above-mentioned hot-rolled H-shaped steel for ships with a yield strength of 420 MPa. The preparation method comprises the following steps: molten iron pretreatment → smelting in a 120-ton converter → argon blowing in a ladle → LF refining → casting of special-shaped continuous casting billets → slow cooling in a slow cooling pit for the continuous casting billets → heating in a heating furnace → BD rough rolling → intermediate billet turning operation → finishing rolling (TM) reversible rolling → cooling between stands → water cooling in the flange phase change zone after rolling → intensive slow cooling on a cooling bed → straightening in a straightener → finishing treatment of the finished product.

[0027] During the rolling process, the heating furnace soaking temperature is 1230-1250°C, and the ingot remains in the furnace for 210-300 minutes. During the rough rolling process, BD rough rolling is performed using reversible rolling in 7-9 passes. The overall reduction ratio is controlled above 40%. A high reduction ratio is maintained during the rough rolling stage, and the intermediate billet undergoes sufficient recrystallization to increase the austenite nucleation rate and refine the austenite structure. Finishing rolling is performed using reciprocating reversible rolling in 5-7 passes. The starting temperature of finishing rolling is 960-980°C, and the inter-stand cooling water is fully activated. The finishing temperature is 760-830°C. Due to the heavy weight of the billets used for thick sections, a specially designed steel turning machine is used between roughing and finishing to minimize the temperature difference between the upper and lower flanges. To ensure the final rolling temperature control, the reduction ratio of the final finishing rolling pass is 8% to 12%, and the rolling mill is controlled at less than 2m / s for reduced speed rolling; the final finishing rolling TM pass is fully cooled to facilitate the subsequent precipitation of vanadium-containing carbonitrides to exert precipitation strengthening effects.

[0028] In order to realize the BD rough rolling and steel turning process operation, an automatic steel turning equipment is used, as shown in Figure 1, to realize the flipping operation of the upper and lower flanges. The specific implementation process is as follows: After BD rolling, the intermediate billet is quickly moved to the middle position of the steel turning machine, and the steel turning machine is started after it stops, and the three driving gears 1-3 are used to drive the rotating mechanism 4 to realize rotation. The upper roller 6 and the lower roller 8 are driven by design. The upper and lower rollers clamp the steel billet 7 under the action of the upper hydraulic cylinder 5 and the lower hydraulic cylinder 9, and the rotating mechanism drives the steel billet to rotate under the drive of the driving gear. The upper and lower rollers of the device are driven, so that sliding friction is formed between the steel billet and the rollers to avoid scratches on the steel billet. The device can realize the rotation of the steel billet at any angle, including 45°, 90°, and 180° steel turning. Due to the unique H-shaped shape of H-beams, the upper flange naturally cools faster than the lower flange during rolling. This results in the upper flange temperature of the intermediate billet being lower than the lower flange temperature after rough rolling. Using this device, the upper and lower flanges are flipped, and the "lower flange with higher temperature" becomes the "upper flange with higher temperature." After flipping, the upper flange temperature of the intermediate billet is higher than the lower flange temperature. During subsequent finishing rolling, the upper flange cools faster than the lower flange, and the upper and lower flange temperatures gradually converge, thus achieving the technical effect of reducing the temperature difference between the upper and lower flanges of the rolled material after final rolling to below 20°C. This device can also be placed at the rolling inlet side to clamp the billet entering the rolling mill and, in conjunction with the drive roller, achieve pinch feeding. The device can also be placed at the rolling outlet side to support the billet exiting the rolling mill, preventing instability such as twisting and buckling. Three such devices can be placed along the length of the billet at the head, middle, and tail of the transport roller conveyor. When rotating the billet, the three devices are controlled in a coordinated manner to achieve the above four effects.

[0029] The present invention realizes the industrial production of thick-gauge 420MPa-level high-strength and tough H-shaped steel products for ships through micro-alloying process design combined with the characteristics of reciprocating rolling of thick-gauge steel sections.

[0030] The advantages of the technical solution of the present invention are:

[0031] 1. Low carbon + VN alloying + recrystallization zone controlled rolling is adopted to achieve the refinement of matrix grain size to above level 9. Compared with other patents, the Ni element is removed from the composition design to meet the requirements of low-cost composition design. Vanadium-nitrogen alloy and ferrovanadium are added in a proportion. On the basis of ensuring that the nitrogen content meets the requirements, the addition amount is changed according to the price of the two alloys to meet the cost control requirements.

[0032] 2. Taking into account the characteristics of rolling 20-40mm thick H-beams with different compression ratios from special-shaped billets, a low C content suitable for temperature-controlled rolling is used in combination with a VN micro-alloying composition design. At the same time, the N content is controlled during the LF refining process to increase the amount and size of VN precipitation, meeting the -60°C strength and toughness requirements of products of different specifications. As a result, high-strength and toughness hot-rolled H-beams with stable performance above 420MPa are obtained on a general hot-rolled H-beam mill.

[0033] 3. During the refining process, due to the addition of VN alloy, the nitrogen content increases significantly while the vanadium content increases. In order to control the nitrogen content within a reasonable range and control the crack defects of the ingot, VN alloy and ferrovanadium are added according to a certain ratio. During the refining process, a certain amount of manganese nitride cored wire is added according to the nitrogen recovery rate to ensure that the nitrogen content in the steel is kept within the range of 0.011-0.015%, so as to avoid cracks in the flange corners and web caused by excessive nitrogen content.

[0034] 4. During the rolling process, a specially designed steel turning device is used to perform steel turning operations between rough rolling and finishing rolling, which significantly reduces the temperature difference between the upper and lower flanges. The temperature difference between the upper and lower flanges is reduced to below 15°C. Before the rolled piece enters the TM finishing mill, the automatic steel turning device is used to turn the steel and then the roller table is properly kept warm. Depending on the stability, the billet can be turned multiple times. After maintaining the temperature for 60 to 120 seconds, the rolled piece is controlled within the range of 960 to 980°C before entering the finishing mill for multiple reciprocating rolling, which has the effect of refining the structure.

[0035] 5. The cooling water between the stands and after rolling is all turned on. The cooling water after rolling is mainly used to cool the lower flange. The gap between the upper and lower flanges is reduced to below 20°C, ensuring that the final rolling temperature is controlled between 760 and 830°C. The difference in yield strength between the upper and lower legs is controlled within the range of less than 15MPa to meet the requirements of uniform performance.

[0036] 6. After rolling, the matrix structure of the H-shaped steel is fine lamellar pearlite + ultrafine proeutectoid ferrite, and the grain size is controlled in the range of 9 to 11 levels; the second phase particles stably precipitated in the steel are mainly VN, and the precipitated particles with a size of less than 50nm account for more than 60%, with high organizational uniformity and improved strength and toughness.

[0037] 7. The steel section involved in the present invention has a low carbon equivalent and excellent welding performance, and the carbon equivalent index is controlled below 0.42. BRIEF DESCRIPTION OF THE DRAWINGS

[0038] FIG1 is a schematic diagram of the steel turning equipment designed by the present invention;

[0039] FIG2 is a microstructure diagram (×100) of a 420 MPa grade hot-rolled low-temperature resistant H-beam according to Example 2 of the present invention;

[0040] FIG3 is a microstructure diagram (×200) of the 420 MPa grade hot-rolled low-temperature resistant H-beam according to Example 2 of the present invention.

[0041] Reference numerals: 1-3, driving gear; 4, rotating mechanism; 5, upper hydraulic cylinder; 6, upper roller; 7, special-shaped billet; 8, lower roller; 9, lower hydraulic cylinder. DETAILED DESCRIPTION

[0042] The technical solution of the present invention is described in detail below with reference to the accompanying drawings and embodiments.

[0043] The special-shaped continuous casting billets in the following embodiments are all prepared according to the following process flow: according to the set chemical composition range, blast furnace molten iron is used as raw material and a certain amount of scrap steel is added. After converter smelting and LF refining, the contents of C, Si, Mn, S, P, etc. are adjusted and V micro-alloying is performed. After the composition reaches the target value, continuous casting and direct heating or soaking of the billet are carried out.

[0044] The preparation steps of Examples 1-5 are as follows:

[0045] Hot metal pretreatment → 150t converter smelting → ladle argon blowing → LF refining → special-shaped continuous casting → slow cooling in the continuous casting cooling pit → heating in the reheating furnace → multi-pass rough rolling → automatic transfer of the intermediate slab → reversible rolling in the section steel line → inter-stand cooling + water cooling of the lower flange after finishing rolling → intensive slow cooling on the cooling bed. Section steel line rolling consists of two stages: rough rolling and finishing rolling. The hot rolling process focuses on temperature control, and the rolled product is naturally stacked and cooled on the cooling bed after rolling.

[0046] Example 1:

[0047] The chemical composition of the selected ship structural steel, by weight, is: C: 0.05%, Si: 0.20%, Mn: 1.25%, V: 0.11%, Al2O3: 0.05%, Cr: 0.35%, P: 0.013%, S: 0.005%, N: 0.011%, O: 0.003%, with the remainder being iron (Fe); Cev: 0.35. The prepared raw materials are smelted in a 150-ton converter. The ratio of vanadium-nitrogen alloy to ferrovanadium (VF) is 2:1, calculated based on vanadium content. Specifically, 1.1 kg of vanadium-nitrogen alloy with a 77% vanadium content and 0.55 kg of ferrovanadium alloy with a 50% vanadium content are added per ton of steel. The steel is then continuously cast into special-shaped ingots. During the final LF refining stage, 100 meters of manganese nitride cored wire is added for fine-tuning based on nitrogen content. The steel is then continuously cast into special-shaped ingots. In the rolling process, the continuous casting billet is heated to 1230℃ and kept warm for 280min. After BD, the steel is turned up and down by the steel turning device. After waiting for 70s, it enters the finishing continuous rolling mill for rolling. The starting temperature of the finishing rolling is controlled at 980℃. Water cooling is used to control the temperature difference between the finishing rolling stands. The finishing rolling temperature of the upper flange is controlled at 790℃, and the lower flange is controlled at 803℃. The rolling specifications are the national standard specifications H700*300*17*24mm, and the steel enters the straightening machine for straightening.

[0048] The product's grain size rating is 9.5, and its mechanical properties test data are: ReH: 456MPa, Rm: 551MPa, δ: 23%, -50℃ Akv: average 233J.

[0049] Example 2:

[0050] The chemical composition of the selected ship structural steel, by weight, is: C: 0.06%, Si: 0.18%, Mn: 1.35%, V: 0.13%, Al2O3: 0.04%, Cr: 0.3%, P: 0.013%, S: 0.003%, N: 0.012%, O: 0.003%, with the remainder being iron (Fe); Cev: 0.39. The prepared raw materials are smelted in a 150-ton converter. The ratio of vanadium-nitrogen alloy to ferrovanadium, calculated based on vanadium content, is 2:1. Specifically, 1.32 kg of vanadium-nitrogen alloy with a 77% vanadium content and 0.66 kg of ferrovanadium alloy with a 50% vanadium content are added per ton of steel. During the final LF refining phase, 80 meters of manganese nitride cored wire is added for fine-tuning based on testing results. The steel is then continuously cast into special-shaped billets. In the rolling process, the continuous casting billet is heated to 1240℃ and kept warm for 300min. After BD, the steel turning device is used to turn the steel up and down. After waiting for 80s, it enters the finishing continuous rolling mill for rolling. The starting temperature of the finishing rolling is controlled at 990℃. Water cooling is used to control the temperature difference between the finishing rolling stands. The finishing temperature of the upper flange is controlled at 800℃, and the lower flange is controlled at 814℃. The rolling specifications are the national standard specifications H900*300*16*28mm, and the billet enters the straightening machine for straightening.

[0051] The product's grain size rating is level 10, and the mechanical properties test results are: ReH: 446MPa, Rm: 542MPa, δ: 22%, and Akv impact energy at -60°C: an average of 224J.

[0052] Example 3:

[0053] The chemical composition of the selected ship structural steel, by weight, is: C: 0.065%, Si: 0.21%, Mn: 1.42%, V: 0.14%, Al2O3: 0.035%, Cr: 0.38%, P: 0.014%, S: 0.004%, N: 0.013%, O: 0.003%, with the remainder being iron (Fe); Cev: 0.40. The prepared raw materials are smelted in a 150-ton converter. The ratio of vanadium-nitrogen alloy to ferrovanadium, calculated based on vanadium content, is 2:1. Specifically, 1.4 kg of vanadium-nitrogen alloy with a 77% vanadium content and 0.7 kg of ferrovanadium alloy with a 50% vanadium content are added per ton of steel. During the final LF refining phase, 50 meters of manganese nitride cored wire is added for fine-tuning based on testing results. The steel is then continuously cast into special-shaped billets. In the rolling process, the continuous casting billet is heated to 1230℃ and kept warm for 300min. After BD, the steel turning device is used to turn the steel up and down. After waiting for 100s, it enters the finishing continuous rolling mill for rolling. The starting temperature of the finishing rolling is controlled at 980℃. Water cooling is used to control the temperature difference between the finishing rolling stands. The finishing temperature of the upper flange is controlled at 810℃, and the lower flange is controlled at 823℃. The rolling specifications are the national standard specifications H1000*300*19*36mm, and the billet enters the straightening machine for straightening.

[0054] The product's grain size rating is level 9, and the mechanical properties test results are: ReH: 436MPa, Rm: 539MPa, δ: 21%, -60℃ Akv impact energy: average 205J.

[0055] Example 4:

[0056] The chemical composition of the selected ship structural steel, by weight, is: C: 0.07%, Si: 0.18%, Mn: 1.41%, V: 0.13%, Al2O3: 0.04%, Cr: 0.40%, P: 0.012%, S: 0.003%, N: 0.012%, O: 0.004%, with the remainder being iron (Fe); Cev: 0.41. The prepared raw materials were smelted in a 150-ton converter. The ratio of vanadium-nitrogen alloy to ferrovanadium (VF) was 2:1, calculated based on vanadium content. Specifically, 1.32 kg of vanadium-nitrogen alloy with a 77% vanadium content and 0.66 kg of ferrovanadium alloy with a 50% vanadium content were added per ton of steel. During the final LF refining phase, 80 meters of manganese nitride cored wire was added for fine-tuning based on testing results. The steel was then continuously cast into special-shaped billets. In the rolling process, the continuous casting billet is heated to 1245℃ and kept warm for 270min. After BD, the steel turning device is used to turn the steel up and down. After waiting for 90s, it enters the finishing continuous rolling mill for rolling. The starting temperature of the finishing rolling is controlled at 1000℃. Water cooling is used to control the temperature difference between the finishing rolling stands. The finishing rolling temperature of the upper flange is controlled at 820℃, and the lower flange is controlled at 835℃. The rolling specifications are national standard H-shaped steel H850*300*16*27|mm, and the steel enters the straightening machine for straightening.

[0057] The product's grain size rating is 10.5; its mechanical properties test results are: ReH: 452MPa, Rm: 549MPa, δ: 25%, -60℃ Akv impact energy: average 245J.

[0058] Example 5:

[0059] The chemical composition of the selected ship structural steel, by weight (%), is: C: 0.07%, Si: 0.21%, Mn: 1.25%, V: 0.11%, Al2O3: 0.05%, Cr: 0.35%, P: 0.013%, S: 0.005%, N: 0.011%, O: 0.003%, with the remainder being iron (Fe); Cev: 0.40. The prepared raw materials are smelted in a 150-ton converter. The ratio of vanadium-nitrogen alloy to ferrovanadium (FV) is 3:1, calculated based on vanadium content. Specifically, 1.1 kg of vanadium-nitrogen alloy with a 77% vanadium content and 0.55 kg of FV-Ferrovanadium alloy with a 50% vanadium content are added per ton of steel. The steel is then continuously cast into special-shaped ingots. During the final LF refining stage, 100 meters of manganese nitride cored wire is added for fine-tuning based on nitrogen content. The steel is then continuously cast into special-shaped ingots. In the rolling process, the continuous casting billet is heated to 1235℃ and kept warm for 300min. After BD, the steel is turned up and down by the steel turning device. After waiting for 95s, it enters the finishing continuous rolling mill for rolling. The starting temperature of the finishing rolling is controlled at 980℃. Water cooling is used to control the temperature difference between the finishing rolling stands. The finishing temperature of the upper flange is controlled at 815℃, and the lower flange is controlled at 825℃. The rolling specifications are the national standard specifications H1000*300*19*36mm, and the steel enters the straightening machine for straightening.

[0060] Sampling was carried out in accordance with the standard GB / T 2975-2018 "Steel and Steel Products Mechanical Properties Test Sampling Location and Sample Preparation". The test methods for yield strength, tensile strength, and elongation refer to the standard GB / T228-2021 "Metallic Materials Room Temperature Tensile Test Method"; the impact energy test method refers to the standard GB / T 229-2020 "Metallic Materials Charpy Pendulum Impact Test Method". The yield strength of Examples 1-5 of the present invention remains above 440 MPa, and has good elongation performance. Its -60°C impact energy is relatively high, reaching the F-level as a whole. The ductile-brittle transition temperature is extremely low, making it suitable for floating ships to operate in different sea environments. 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 and offshore oil platforms in extremely cold areas.

[0061] Figures 2 and 3 show that the microstructure of the present invention is an ultrafine pearlite + ferrite structure. 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. This 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.

[0062] 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.

[0063] Any content not described in detail in the present invention can be based on conventional technical knowledge in the art.

[0064] 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 low-cost, thick-gauge, high-strength, and low-temperature-resistant hot-rolled H-beam for ship structures, characterized in that: The chemical composition of the H-shaped steel is as follows by weight percentage: C: 0.05-0.07%, Si: ≤0.25%, Mn: 1.20-1.50%, V: 0.11-0.15%, Alt: 0.035-0.055%, Cr: 0.3-0.5%, P≤0.015%, S≤0.005%, N: 0.010-0.015%, O≤0.004%, and the rest is Fe and unavoidable impurities.

2. The low-cost thick-gauge high-strength and high-toughness low-temperature-resistant hot-rolled H-beam for ship structure according to claim 1 is characterized in that: The flange thickness of the H-shaped steel is 20 mm to 40 mm, and the carbon equivalent index is controlled below 0.

42.

3. The low-cost thick-gauge high-toughness low-temperature-resistant H-shaped steel for ship structures according to claim 1 is characterized in that: The yield strength of the H-shaped steel is ≥430MPa, the tensile strength is ≥530MPa, the elongation is ≥19%, and the impact energy at -60°C is ≥180J.

4. The low-cost thick-gauge high-strength and high-toughness low-temperature-resistant hot-rolled H-beam for ship structure according to claim 1 is characterized in that: After rolling, the matrix structure of the H-beam is fine lamellar pearlite + ultrafine proeutectoid ferrite, and the grain size is controlled in the range of 9 to 11 levels; the second phase particles stably precipitated in the steel are mainly VN, and the precipitated particles with a size of less than 50nm account for more than 60%.

5. A method for preparing a low-cost, thick-gauge, high-toughness, low-temperature-resistant hot-rolled H-shaped steel for ship structures, the method comprising the following steps: Hot metal pretreatment → smelting in 120-ton converter → argon blowing in ladle → LF refining → casting of special-shaped continuous casting billets → slow cooling in the slow cooling pit for continuous casting billets → heating in the heating furnace → BD rough rolling → intermediate billet turning operation → finishing rolling TM reversible rolling → cooling between stands → water cooling in the phase change zone of the flange after rolling → intensive slow cooling on the cooling bed → straightening by the straightener → finishing treatment of the finished product.

6. The method for preparing the thick-gauge, high-strength, and low-temperature-resistant hot-rolled H-beam for low-cost ship structures according to claim 5, characterized in that: During the rolling process, the heating furnace has an average heating temperature of 1230-1250°C and the casting time in the furnace is 210-300 minutes.

7. The method for preparing thick-gauge, high-strength, and low-temperature-resistant hot-rolled H-beam for low-cost ship structures according to claim 5, characterized in that: The BD rough rolling process adopts reversible rolling for 7 to 9 passes, and the overall compression ratio of the rough rolling is controlled at more than 40%. A large compression ratio is ensured in the rough rolling stage, and the intermediate billet is fully recrystallized to increase the austenite nucleation rate and refine the austenite structure.

8. The method for preparing the low-cost thick-gauge high-strength and high-toughness low-temperature-resistant hot-rolled H-beam for ship structure according to claim 5, characterized in that: The finishing rolling adopts reciprocating reversible rolling for 5 to 7 passes, the starting temperature of the finishing rolling is 960 to 980°C, the cooling water between the finishing rolling stands is all turned on, and the final rolling temperature of the finishing rolling is 760 to 830°C.

9. According to the method for preparing the thick-gauge high-toughness low-temperature-resistant hot-rolled H-shaped steel for ship structure at low cost in claim 5, since the steel billet used for the thick-gauge steel is heavy, in order to reduce the temperature difference between the upper and lower flanges, a steel turning machine is used to turn the steel between rough rolling and finish rolling, specifically: After BD rolling, the intermediate billet is quickly moved to the middle position of the steel turning machine. After it stops, the steel turning machine is operated. The three driving gears (1), (2) and (3) are used to drive the rotating mechanism (4) to rotate. The upper roller (6) and the lower roller (8) are driven. The upper and lower rollers clamp the steel billet (7) under the action of the upper hydraulic cylinder (5) and the lower hydraulic cylinder (9). The rotating mechanism drives the steel billet to rotate under the drive of the driving gear.

10. The method for preparing the low-cost thick-gauge high-strength and high-toughness low-temperature-resistant hot-rolled H-beam for ship structures according to claim 5, characterized in that: In order to ensure the final rolling temperature control, the compression ratio of the last pass of finishing rolling is 8% to 12%, and the rolling mill is controlled below 2m / s for reduced speed rolling; the last pass of finishing rolling TM is fully cooled to facilitate the subsequent precipitation of vanadium-containing carbonitrides to play a precipitation strengthening role.

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