Rack steel plate with thickness up to 160 mm for offshore wind power installation platform and manufacturing method therefor

Abstract

The present invention relates to a rack steel plate with a thickness up to 160 mm for an offshore wind power installation platform and a manufacturing method therefor. The steel plate comprises the following elemental components in percentages by mass: C: 0.30-0.40%, Si: 0.15-0.35%, Mn: 0.85-1.20%, P≤0.010%, S≤0.002%, Cr: 0.45-0.75%, Mo: 0.45-0.65%, Ni: 1.3-1.8%, Al: 0.06-0.10%, V: 0.02-0.05%, Nb: 0.01-0.04%, Ti≤0.006%, N≤0.007%, B: 0.001-0.002%, and the balance being iron and inevitable elemental impurities. The steel plate has a manufacturing thickness of less than or equal to 160 mm, a yield strength of greater than or equal to 1000 MPa, a tensile strength of greater than or equal to 1100 MPa, and an elongation of greater than or equal to 12%; and the lateral Charpy impact energy of the steel plate at -40ºC at a quarter-thickness position of the steel plate and the lateral Charpy impact energy thereof at -30ºC at a mid-thickness position of the steel plate are both greater than or equal to 46 J. The structure near the surface is tempered troostite; the structure at the quarter-thickness position of the steel plate is tempered troostite and tempered lower bainite; and the structure at the mid-thickness position of the steel plate is tempered troostite, tempered lower bainite and a small amount of tempered granular bainite. The rack steel plate has high strength, high hardness, hardness uniformity, good plasticity, high low-temperature toughness and good weldability.

Description

Rack steel plate with a thickness of 160 mm for offshore wind power installation platform and manufacturing method thereof Technical Field

[0001] The invention belongs to the technical field of iron-based alloys, and in particular relates to a rack steel plate and a manufacturing method thereof. Background Art

[0002] There are two main types of jack-up offshore wind turbine installation platforms: the hydraulic latch type and the rack-and-pinion type. The leg racks of the latter are typically manufactured from 690MPa-grade rack steel plate with a yield strength of ≥690MPa, a tensile strength of 770-940MPa, and a Brinell hardness of 240-290 HBW at 1 / 4 thickness. This material, derived from the leg racks of jack-up drilling platforms used in offshore oil and gas production, meets the requirements of offshore wind turbine installation platforms to a certain extent. However, due to the increasing weight of installed wind turbine equipment and the more frequent lifting and lowering of installation platforms compared to offshore oil and gas platforms, the platforms are becoming larger and larger, resulting in increased wear on the leg racks, a significant reduction in the platform's service life, and significant limitations on the weight of installed wind turbine equipment. To address these issues, offshore wind turbine installation platform manufacturers are seeking to manufacture leg racks using extra-thick steel plates with higher strength, higher hardness, and improved cross-sectional hardness uniformity across the plate thickness. Furthermore, this new material must maintain the high toughness and good weldability of existing 690MPa-grade rack steel plates. Specifically, a new rack steel plate with a thickness of 160mm must be developed, with a yield strength ≥1000MPa, a tensile strength ≥1100MPa, an elongation ≥12%, a transverse Charpy impact energy ≥46J at low temperatures (-40°C at 1 / 4 thickness and -30°C at 1 / 2 thickness), a Brinell hardness ≥320HBW across the full thickness of the plate, and good weldability to meet the construction requirements of larger load-bearing offshore installation platforms.

[0003] Summary of the Invention

[0004] The purpose of this invention is to develop a rack steel plate with a thickness of up to 160 mm for offshore wind turbine installation platforms and its manufacturing method. The steel plate has a target yield strength of 1000 MPa or greater, a tensile strength of 1100 MPa or greater, and an elongation of 12% or greater. The transverse Charpy impact energy at both the quarter-thickness (-40°C) and half-thickness (-30°C) points should be 46 J or greater. The full-thickness Brinell hardness of the steel plate should be 320 HBW or greater, demonstrating excellent weldability.

[0005] The technical solution of the present invention is: a rack steel plate, in particular a rack steel plate with a thickness of up to 160 mm that can be used for an offshore wind power installation platform, wherein the chemical composition of the steel plate is, by mass percentage, C: 0.30-0.40%, Si: 0.15-0.35%, Mn: 0.85-1.20%, P: ≤0.010%, S: ≤0.002%, Cr: 0.45-0.75%, Mo: 0.45-0.65%, Ni: 1.3-1.8%, Al: 0.06-0.10%, V: 0.02-0.05%, Nb: 0.01-0.04%, Ti: ≤0.006%, N: ≤0.007%, B: 0.001-0.002%, and the balance is iron and unavoidable impurity elements.

[0006] The rack steel plate is produced in a thickness of up to 160mm. Its yield strength is ≥1000MPa, its tensile strength is ≥1100MPa, its elongation is ≥12%, and its transverse Charpy impact energy at both 1 / 4 thickness (-40°C) and 1 / 2 thickness (-30°C) is ≥46J. Its Brinell hardness across the full thickness section is ≥320HBW, and it exhibits excellent weldability.

[0007] The basis for setting the element composition and content of the rack steel plate:

[0008] Carbon (C) is an essential element for ensuring the strength and hardness of steel plates. Increasing the carbon content in steel increases its martensite transformation capacity and the squareness of the martensite, thereby improving the steel's strength and hardness. However, excessive C content is detrimental to the steel's ductility, toughness, and weldability. The present invention limits this content to 0.30-0.40%.

[0009] Si: A deoxidizing element in steel, it increases the strength and hardness of steel through solid solution strengthening. When Si content is below 0.10%, the deoxidizing effect is poor, while higher Si content reduces toughness. In the present invention, the Si content is controlled to 0.15-0.35%.

[0010] Mn: An element that improves the hardenability of steel and provides solid solution strengthening, compensating for the loss of strength and hardness caused by a reduction in carbon content. When the Mn content in steel is less than 0.8%, the steel's strength and hardness cannot be fully realized. However, when the Mn content is too high, it can easily lead to temper brittleness and impair the material's weldability. Therefore, the Mn content in the present invention is controlled to 0.85-1.20%.

[0011] Nickel (Ni) is an element that enhances the hardenability of steel sheets and significantly improves their low-temperature toughness. It also has a beneficial effect on impact toughness and the ductile-brittle transition temperature. However, excessive Ni content can lead to the formation of sticky iron oxide scale on the steel surface, making it difficult to remove and affecting the surface quality of the steel sheet. Furthermore, Ni is a precious metal, and excessive Ni content can increase costs. Therefore, the present invention controls Ni content within a range of 1.3-1.8%, which helps achieve optimal cost-performance.

[0012] Cr: This element improves the hardenability of steel, contributing to increased strength and hardness. Adding an appropriate amount of Cr ensures the steel plate achieves the required strength and hardness. However, excessive addition reduces the material's toughness and weldability. The present invention controls its content to 0.45-0.75%.

[0013] Mo: Mo significantly improves the hardenability of steel, thereby increasing its strength and hardness. Adding a small amount of Mo to steel can also reduce or eliminate temper brittleness, improving the steel's heat treatability. However, Mo is a precious metal, and excessive Mo content increases costs and reduces the material's weldability. In the present invention, the Mo content is controlled within a range of 0.45-0.65%.

[0014] V: This element causes V (C, N) to precipitate, significantly increasing the strength and hardness of steel through dispersed precipitation. However, excessive addition can reduce the toughness and weldability of the steel plate. The present invention limits its content to 0.02-0.05%.

[0015] Nb: An element that significantly contributes to grain refinement during rolling and heat treatment. During the steel plate rolling and heat treatment stages, it retards austenite grain growth, thereby refining the grains. This provides the foundation for thicker steel plates to retain a fine microstructure after quenching and tempering, thereby improving the steel's strength, hardness, and toughness. However, excessive Nb levels cannot dissolve, rendering them ineffective and increasing costs. Furthermore, excessive Nb levels can negatively impact weldability. The present invention limits its content to 0.01-0.04%.

[0016] Al: It primarily acts as a nitrogen fixator and deoxidizer. AlN, formed by the combination of Al and N, effectively refines grains and protects the addition of element B to the steel. However, excessive Al content can impair the steel's toughness. The present invention limits Al content to 0.06-0.10%.

[0017] B is the element that most significantly improves the hardenability of steel. Both too low and too high a content are detrimental to hardenability. Excessive B content increases weld crack sensitivity, thereby reducing the weldability of the steel plate. The present invention controls its content to 0.0010-0.0020%.

[0018] S and P are harmful impurity elements in steel, easily causing defects such as segregation and inclusions. As impurities, they can adversely affect the toughness of the steel plate (especially the toughness of the core) and the toughness near the weld, so their content should be minimized. The present invention controls P to ≤ 0.010% and S to ≤ 0.002%. Furthermore, Ca treatment is performed during molten steel smelting to spheroidize and evenly distribute inclusions, minimizing their impact on plasticity and toughness.

[0019] The present application also provides a method for manufacturing the above-mentioned rack steel plate, the steps comprising:

[0020] Molten steel smelting: Select high-quality pig iron and steel plate scraps to prepare smelting raw materials according to chemical composition, and then go through electric furnace smelting, LF refining, VD refining, and die casting in sequence to produce molten steel with S≤0.002%, P≤0.010%, H≤0.00015%, O≤0.0015%, and N≤0.007%. It is preferred to use silicon calcium wire to carry out calcium treatment after VD breaking to denature the inclusions.

[0021] Casting: Molten steel is preferably cast into flat ingots using a mold casting method. The ingot mold and base are preferably preheated to 60-130°C to ensure sufficient drying. During the pouring process, a low superheat of 30-40°C, preferably under argon protection, is preferably used. After the ingot is uncapped, it is placed in a slow cooling pit with the mold and slowly cooled for 48 hours before demolding.

[0022] Ingot rolling: The ingot (e.g., after demolding) is hot-charged into a soaking furnace and heated to 1220-1260°C and fully insulated. Thicker ingots can be insulated for 10-20 hours. After exiting the furnace, they are descaled with high-pressure water and then rolled to obtain intermediate ingots. The intermediate ingots are air-cooled on a cooling bed until the temperature is controlled to 300-40°C. They are then removed from the cooling bed and subjected to hydrogen expansion heat treatment. Preferably, the intermediate ingot is heated to within 650±20°C in the furnace, held at this temperature for 48 hours, and then cooled with the furnace. After cooling to ~300°C, they are removed from the furnace and air-cooled. The hydrogen expansion heat treatment fully reduces the hydrogen content in the ingot, ensuring that the core properties of the finished steel plate meet the requirements.

[0023] Rolling: Hot rolling is used. The intermediate billet is first heated, preferably to a surface temperature ≥150°C. It is then placed in a continuous furnace and heated to 1220-1260°C. This temperature is then held for 2.5-4 hours to achieve full solution of alloying elements in the steel, ensuring uniform composition and performance of the final product. After exiting the furnace, the billet undergoes high-pressure water descaling and is then rolled at temperatures above 1000°C with a high reduction ratio. The maximum single-pass reduction is ≥20%. The rolled steel plate is then air-cooled on a cooling bed.

[0024] After the steel plate is air-cooled to 500-600℃ on the cooling bed, it is removed from the cooling bed and slowly cooled with a cover. When the surface temperature of the steel plate is slowly cooled to 100-200℃, the cover is removed and air-cooled to room temperature.

[0025] Tempering: The steel plate after slow cooling is subjected to tempering to obtain the finished rack steel plate. Preferably, the quenching heating during the tempering stage is performed in a continuous furnace. The quenching heating temperature is 850-900°C, the steel plate is in the furnace for 1.8-2.2 min / mm, and the steel plate is water quenched to ≤100°C using a roller quenching machine, then air-cooled to room temperature. Tempering is also performed in a continuous furnace. The tempering heating temperature is 580-620°C, the steel plate is in the furnace for 3.5-5.5 min / mm, and air-cooled to room temperature after exiting the furnace.

[0026] This invention addresses the current demand for rack steel plates for offshore wind turbine installation platforms in the marine equipment manufacturing industry. Using die-cast flat ingots with optimized composition, high-purity molten steel, and low-superheat casting using argon protection throughout the entire process, this material is then cast. Through subsequent cogging, rolling, and tempering heat treatment, and by fully reducing the hydrogen content throughout the entire production process, the resulting rack steel plates exhibit high strength, high hardness and uniform hardness, good ductility, high low-temperature toughness, and excellent weldability. These plates are suitable for use in offshore wind turbine installation platform leg manufacturing. The resulting steel plates can reach a thickness of 160 mm.

[0027] The rack steel plate not only has excellent tensile properties (yield strength ≥1000MPa, tensile strength ≥1100MPa, elongation ≥12%), but also has a Brinell hardness ≥320HBW on the full-thickness section of the steel plate, a Charpy transverse impact energy ≥46J at low temperatures (-40℃ at 1 / 4 thickness and -30℃ at 1 / 2 thickness), and good weldability. BRIEF DESCRIPTION OF THE DRAWINGS

[0028] Figure 1 shows the optical metallographic microstructure and scanning transmission electron microscopy microstructure of the steel plate near the surface (a) and (b), 1 / 4 thickness (c) and (d), and 1 / 2 thickness (e) and (f);

[0029] Figure 2 shows the hardness variation along the thickness direction of the steel plate;

[0030] Figure 3 shows the cold crack sensitivity curves measured at 1 / 4 and 1 / 2 thickness of the steel plate. DETAILED DESCRIPTION

[0031] The present invention is further described in detail below with reference to the examples. The examples are illustrative and intended to explain the present invention, but are not to be construed as limiting the present invention.

[0032] Example 1

[0033] The rack steel plate involved in this embodiment has a thickness of 160 mm, and contains the following components and their mass percentages: C: 0.35%, Si: 0.23%, Mn: 0.93%, P: 0.005%, S: 0.001%, Cr: 0.57%, Mo: 0.58%, Ni: 1.48%, Al: 0.072%, V: 0.03%, Nb: 0.02%, Ti: 0.002%, N: 0.005%, B: 0.0013%, and the remainder is iron and unavoidable impurity elements.

[0034] The production process of the rack steel plate is as follows:

[0035] According to the chemical composition of the above rack steel plate, the smelting raw materials are configured and carried out in sequence: electric furnace smelting - LF refining - VD refining - die casting - slow cooling in a slow cooling pit - ingot heating - billet rolling - H expansion treatment - (intermediate billet) billet heating - high-pressure water descaling - rolling - hooded slow cooling - tempering treatment.

[0036] Specifically, the manufacturing process involving slab rolling, billet rolling, H-expansion treatment, hooded slow cooling, and cooling is as follows: the demoulded ingot is directly heated to 1240-1260°C in a soaking furnace, held at this temperature for 18 hours, and then removed from the furnace. After high-pressure water descaling, slab rolling is performed to form a 450mm thick billet. The billet is then air-cooled to ~350°C on a cooling bed and subjected to H-expansion annealing (heated to 650°C, held at this temperature for 48 hours, and then cooled in the furnace). After the billet is slowly cooled to ~300°C, it is removed from the furnace, air-cooled, and subjected to warm cleaning (150-300°C).

[0037] After slab rolling, the steel slabs are preheated to a surface temperature of ~350°C, then placed in a continuous-speed furnace and heated to 1230°C for three hours. After descaling with high-pressure water, they are rolled at a temperature of ≥1030°C using high-reduction technology. The maximum single-pass reduction is 22.5%. The finished steel plates are air-cooled on a cooling bed, achieving a final thickness of 160mm. After air-cooling to ~520°C, the plates are removed from the bed and slowly cooled under a hood. When the surface temperature reaches ~180°C, the hood is removed and air-cooled to room temperature.

[0038] After slow cooling, the steel plates are quenched in a continuous furnace at 870°C for 2.0 min / min. Using a roller quencher, the steel plates are water quenched to a surface temperature of ~70°C before being air-cooled to room temperature on a cooling bed. The quenched steel plates are tempered in a continuous furnace at 600°C for 4.5 min / min. After exiting the furnace, they are air-cooled to room temperature.

[0039] Figure 1 shows the microstructure of the finished steel plate manufactured using the aforementioned process at different locations throughout its thickness. Near the surface, the plate exhibits tempered troostite. At 1 / 4 of the plate's thickness, the structure is a mixture of tempered troostite and tempered lower bainite. In the core, at 1 / 2 of the plate's thickness, the structure is a mixture of tempered troostite, tempered lower bainite, and a small amount of tempered granular bainite. Precipitated phases are relatively fine throughout the entire thickness of the plate. This ensures that the finished steel plate not only possesses high strength, good ductility, and high low-temperature toughness, but also high hardness and uniform hardness. The maximum hardness variation through the thickness is less than 30 HBW. The finished steel plate exhibits excellent overall performance, as shown in Table 1 and Figure 2.

[0040] The weldability of the finished steel plates manufactured using the aforementioned composition and process was evaluated using a cold crack sensitivity test on an inclined Y-groove. The test was conducted in accordance with the national standard GB-T 32260.2-2015, "Metallic Materials - Destructive Testing of Welds - Arc Welding Methods for Cold Crack Testing of Welds." The results are shown in Figure 3. If calculated based on a 20% crack rate threshold, the preheating temperature at half the plate thickness is approximately 120°C, and at quarter the plate thickness is approximately 135°C. If calculated based on a zero crack rate, the preheating temperature at half the plate thickness is 190°C, and at quarter the plate thickness is 175°C. As can be seen, even using the most stringent evaluation method, assuming a zero crack rate, the steel plates manufactured in this manner exhibit excellent weldability.

[0041] Table 1. Mechanical properties of thick rack steel plates manufactured in the examples

[0042] In addition to the above embodiments, the present invention also includes other implementation methods. Any technical solutions formed by equivalent transformation or equivalent replacement should fall within the scope of protection of the claims of the present invention.

Claims

1. A rack steel plate, characterized in that: The element composition is calculated by mass percentage as follows: C: 0.30-0.40%, Si: 0.15-0.35%, Mn: 0.85-1.20%, P: ≤0.010%, S: ≤0.002%, Cr: 0.45-0.75%, Mo: 0.45-0.65%, Ni: 1.3-1.8%, Al: 0.06-0.10%, V: 0.02-0.05%, Nb: 0.01-0.04%, Ti: ≤0.006%, N: ≤0.007%, B: 0.001-0.002%, and the balance is iron and unavoidable impurity elements.

2. The rack steel plate according to claim 1, characterized in that: The production thickness of the steel plate is ≤160mm, the yield strength is ≥1000MPa, the tensile strength is ≥1100MPa, the elongation is ≥12%, and the transverse Charpy impact energy at -40℃ at 1 / 4 thickness of the steel plate and at -30℃ at 1 / 2 thickness of the steel plate is ≥46J.

3. The rack steel plate according to claim 1, characterized in that: The structure near the surface of the steel plate is tempered troostite, the structure at 1 / 4 of the thickness of the steel plate is tempered troostite + tempered lower bainite, and the structure at 1 / 2 of the thickness of the steel plate is tempered troostite + tempered lower bainite + a small amount of tempered granular bainite.

4. A method for manufacturing the rack steel plate according to claim 1, characterized in that: The steps include, 1. Molten steel smelting: smelting molten steel according to composition design; 2. Casting and slow cooling: Molten steel is cast into billets, which are then slowly cooled; 3. Billeting: Reheat the billet to 1220-1260℃ and keep it warm to make the elements dissolved and the structure homogenized. Then, billet rolling is carried out to obtain the intermediate billet, which is then slowly cooled to 300-400℃.

4. Hydrogen expansion treatment: reheat the intermediate billet to induce hydrogen diffusion; 5. Rolling: Reheat the intermediate billet after hydrogen expansion treatment to 1220-1260℃ and keep it warm to make the elements solid solution and the structure homogenized. After taking out of the furnace, descale the billet and start rolling at above 1000℃. Use large reduction rolling, and the maximum single-pass rolling reduction rate is ≥20%. Roll the steel plate of target thickness. After rolling, the steel plate is air-cooled on the cooling bed. After air cooling to 500-600℃, it is placed on the cooling bed and slowly cooled with a cover. After slowly cooling until the surface temperature reaches 100-200℃, the cover is removed and air-cooled to room temperature.

6. Quenching and tempering treatment: quenching and tempering of steel plates.

5. The method according to claim 4, characterized in that: Step 1: Select high-quality pig iron and steel plate scraps to prepare smelting raw materials according to elemental composition, and then carry out electric furnace smelting, LF refining, VD refining in sequence. After VD refining, calcium treatment is carried out to produce molten steel with S≤0.002%, P≤0.010%, H≤0.00015%, O≤0.0015%, and N≤0.007%.

6. The method according to claim 4, characterized in that: Step 2: Use mold casting to cast the molten steel into a flat steel ingot with a pouring superheat of 30-40°C. Use argon gas to protect the casting process and isolate it from the air. After the ingot is uncapped, it is placed in the pit with the mold and slowly cooled. After slow cooling, it is demolded.

7. The method according to claim 6, characterized in that: Before pouring, preheat the ingot mold and chassis to 60-130℃ to fully dry them.

8. The method according to claim 4, wherein: Step 4: The steel billet is reheated to 650±(0-20)℃ and kept warm, cooled with the furnace to nearly 300℃, and then air-cooled.

9. The method according to claim 4, wherein: Step six, the quenching heating temperature is 850-900℃, the time in the furnace is 1.8-2.2min / mm, water quenching to ≤100℃ and then air cooling; the tempering heating temperature is 580-620℃, the time in the furnace is 3.5-5.5min / mm, and air cooling to room temperature after being taken out of the furnace.

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