A kind of extra-thick high strength and toughness rack steel for offshore platform and its production method

Abstract

The application discloses a kind of extra-thick high strength and toughness rack steel for offshore platform and a production method thereof.The chemical composition of the rack steel is as follows: C 0.08% to 0.12%, Si 0.25% to 0.40%, Mn 0.90% to 1.20%, P≤0.006%, S≤0.005%, Ni 1.7% to 2.3%, Cr 0.6% to 1.0%, Cu 0.15% to 0.40%, Mo 0.2% to 0.5%, Alt 0.030% to 0.060%, Ti 0.007% to 0.015%, B 0.0010% to 0.0015%, Ce 0.005% to 0.020%, N≤0.005%, and the balance is Fe and unavoidable impurities; and the production method comprises electric furnace smelting→LF→VD→mold casting→heating→rolling→heat treatment.The extra-thick rack steel provided by the application has good strength and toughness.

Description

Technical Field

[0001] This invention belongs to the field of metallurgical technology, specifically relating to an extra-thick, high-strength, and high-toughness rack steel for offshore platforms and its production method. Background Technology

[0002] As a core load-bearing component of offshore oil and gas exploration and development equipment, the rack of a self-elevating offshore platform endures harsh conditions such as complex alternating loads and seawater corrosion over long periods, placing extremely high demands on the thickness, strength, low-temperature toughness, and microstructure uniformity of the steel. With the increasing scale and depth of marine engineering equipment, the demand for rack steel thickness is constantly rising. Conventional thicknesses of rack steel can no longer meet the requirements, while extra-thick rack steel is prone to technical challenges during production, such as uneven core microstructure, severe segregation, insufficient low-temperature toughness, and difficulty in matching strength and toughness.

[0003] Existing technologies for producing extra-thick rack steel often employ high-alloy designs to enhance hardenability and toughness, but these methods suffer from high alloy costs, complex production processes, and low production efficiency. Furthermore, conventional rolling and heat treatment processes struggle to achieve uniform microstructure and properties between the core and surface layers of extra-thick steel plates, resulting in significant fluctuations in impact toughness at -60℃, which fails to meet the requirements for rack steel applications. Additionally, some processes utilize a double quenching and tempering process, leading to low productivity.

[0004] Therefore, developing an extra-thick rack steel with low alloy cost, high production efficiency, good uniformity in the thickness direction, and high strength and excellent impact toughness at -60℃ has become an urgent technical problem to be solved in the field of marine engineering steel. Summary of the Invention

[0005] The purpose of this invention is to provide an extra-thick, high-strength and high-toughness rack steel for marine platforms and its production method. It adopts an innovative alloy composition system, combining rare earth Ce alloying at the end of refining with rare earth addition process of rare earth rods suspended in the mold, so that the yield strength of the steel plate is ≥690MPa, the tensile strength is ≥810MPa, and the single value of transverse Charpy impact energy at -60℃ at 1 / 2 thickness is ≥100J.

[0006] To achieve the above objectives, the present invention adopts the following technical solution: A type of extra-thick, high-strength, and tough toothed steel for offshore platforms has the following chemical composition and mass percentage: C 0.08%–0.12%, Si 0.25%–0.40%, Mn 0.90%–1.20%, P≤0.006%, S≤0.005%, Ni 1.7%–2.3%, Cr 0.6%–1.0%, Cu 0.15%–0.40%, Mo 0.2%–0.5%, Alt 0.030%–0.060%, Ti 0.007%–0.015%, B 0.0010%–0.0015%, Ce 0.005%–0.020%, N≤0.005%, with the balance being Fe and unavoidable impurities.

[0007] The thickness of the rack steel described in this invention is 177.8-256 mm.

[0008] The toothed steel plate of the present invention has a yield strength of ≥690MPa and a tensile strength of ≥810MPa throughout its thickness, and a transverse Charpy impact energy of ≥100J at -60℃ at 1 / 2 thickness of the steel plate.

[0009] The production method of extra-thick high-strength and high-toughness rack steel for offshore platforms according to the present invention includes electric furnace smelting → LF refining → VD vacuum treatment → ingot casting → steel ingot heating → rolling → heat treatment.

[0010] The electric furnace smelting of the present invention requires: a steel output of ≥90 tons, the use of a normal turnover ladle within 4-8 hours, a 40-60mm initial nozzle, and a flat ladle edge; a single furnace feed of ≥60t of molten iron, no material added during tapping, a ladle temperature of ≥1590℃, and a ladle P ≤0.004%.

[0011] The LF refining method described in this invention employs external dephosphorization.

[0012] The VD vacuum treatment described in this invention has a vacuum degree ≤66Pa, a holding time ≥20min, and calcium treatment is performed immediately after vacuum destruction. The amount of calcium added is 15-30ppm. After 1-3min, rare earth iron-cerium master alloy is added, and the amount of rare earth Ce added is 20-50ppm. The soft blowing time at normal pressure is 8-15min.

[0013] The ingot casting method described in this invention utilizes two high-quality ingot molds, each with a steel molten capacity of 45-55 tons. Rare earth rods are suspended within the molds, with a diameter of 8-15mm and a length ≥1m. The rare earth rods contain 99.4-99.9% Ce by mass, and the distance between the suspended rods and the bottom of the ingot mold is ≤100mm. The rare earth content is 30-150ppm. During casting, the superheat is controlled between 30-50℃, and bottom pouring is employed with argon protection. The argon flow rate is 20-40L / min, and the steel molten flow rate is 3.0-3.5t / min. The time from casting completion to demolding is ≥36 hours, and the demolding temperature is ≤200℃.

[0014] The rolling process described in this invention is as follows: After the steel ingot is peeled and cleaned, it is placed in a slow cooling pit and held at 200-300℃ for ≥4 hours. It is then placed in a heating furnace with a heating rate ≤150℃ / h and a holding temperature of 1240~1260℃. The total time the steel ingot is in the furnace is ≥12min / cm. After exiting the furnace, the steel ingot is promptly placed in a high-pressure water descaling box to remove scale, minimizing the amount of primary iron oxide scale on the upper and lower surfaces.

[0015] The rolling process described in this invention employs a two-stage rolling process. The roughing rolling begins at a temperature of 1150–1200℃, with the first two passes involving a large reduction of 40–50 mm per pass, resulting in a thickness of 450–500 mm. The finishing rolling begins at a temperature of 900–930℃, with the first two passes also involving a large reduction of 40–50 mm per pass, and a final rolling temperature of 830–880℃. After rolling, online water cooling is performed, with a reheat temperature ≤300℃.

[0016] The heat treatment process described in this invention involves quenching and tempering the rolled steel plate in a roller hearth furnace. The quenching temperature is 890–930℃, and the holding time is 2.2–2.6 min / mm. The tempering temperature is 610–640℃, and the holding time is 3.5–4.5 min / mm. After exiting the tempering furnace, the plate is flame-cut to length while still warm. The surface and flame-cut surface quality are then inspected before the plate is stored in the warehouse.

[0017] Design concept of this invention: This invention addresses industry challenges such as insufficient hardenability of the core of ultra-thick steel plates, poor strength and toughness matching across the entire cross-section of thick plates, uneven low-temperature impact performance, high cost of high-nickel-chromium alloy ratios, and easy segregation of microstructure during large-thickness rolling. It innovatively proposes a composition system of rare earth Ce microalloying and trace amount of boron synergistic strengthening, coupled with a full-process coupled control technology including precise impurity control in steelmaking, dual-mode rare earth gradient addition, two-stage differentiated high-reduction rolling, and precise temperature field heat treatment. This enables low-cost, high-stability, and high-performance industrial mass production of ultra-thick rack steel, effectively balancing the hardenability, low-temperature strength and toughness of ultra-thick steel plates with production costs, and breaking through the technical bottlenecks of traditional ultra-thick high-strength marine engineering steel.

[0018] In terms of the core design of the composition system, this invention abandons the traditional design approach of relying on high-content Ni, Cr, and Mo precious alloys to improve hardenability in extra-thick high-strength steel. Instead, it adopts an alloy design scheme with low alloy ratio and rare earth boron composite modification. The content of basic matrix elements C, Si, and Mn is precisely limited to ensure the basic strength and weldability of the steel. Harmful impurity elements such as P, S, and N are strictly controlled to minimize the damage of sulfide and nitride inclusions to the continuity of the matrix and low-temperature impact toughness. Building upon this foundation, a performance leap is achieved through a dual coupling mechanism of trace boron (B) element segregation at grain boundaries and rare-earth cerium (Ce) lattice modification: B atoms readily accumulate at austenite grain boundaries, filling grain boundary vacancies and inhibiting carbon atom diffusion, effectively hindering the preferential nucleation of softening structures such as ferrite and pearlite at grain boundaries, thus initially improving austenite stability and steel hardenability; while the radius of rare-earth cerium atoms is much larger than that of Fe, C, and B matrix atoms, and after solid dissolution into the steel matrix, it induces significant lattice distortion, greatly increasing the activation energy for carbon atom migration and diffusion, and significantly reducing the bulk diffusion rate of C atoms and the grain boundary diffusion rate, thus inhibiting the high-temperature decomposition of supercooled austenite at the microscopic level. This composite strengthening mechanism can significantly improve the hardenability and toughness of ultra-thick steel plates, reducing the amount of high-cost alloys while ensuring the hardenability and toughness of ultra-thick steel plates, resulting in a significant reduction in alloy costs.

[0019] In terms of metallurgical purity and precise control of rare earth elements, this invention innovatively adopts a dual-gradient rare earth addition process, combining alloying at the end of refining with in-mold hanging replenishment. This solves the problems of low yield, uneven distribution, insufficient micro-alloying effect, and incomplete inclusion modification associated with single rare earth addition methods. The steelmaking process utilizes high-proportion molten iron input into the electric furnace, low-phosphorus tapping control, precise dephosphorization outside the furnace (LF), and long-term high-vacuum pressure holding treatment (VD) to deeply remove harmful impurities such as oxygen, hydrogen, phosphorus, and sulfur from the molten steel, purifying the steel matrix and laying a pure metallurgical foundation for the stable existence and full utilization of rare earth elements. After breaking the vacuum in VD, calcium treatment is first performed to modify inclusions, breaking down long strip-shaped sulfide and oxide inclusions. Subsequently, rare earth iron-cerium alloy and in-mold rare earth rods are added at the end of refining. The early refining of rare earth elements achieves overall purification of the molten steel and modification of fine inclusions. This research employs a composite process combining in-mold suspended rare earth rods and full-process argon gas sealing protection. Solid rare earth rods are pre-placed inside the mold in a suspended manner to prevent premature contact between rare earths and molten steel, which could cause a sudden and violent reaction and burn-off. Utilizing the inert gas properties of argon, a sealed atmosphere is created to isolate the mold cavity and the entire solidification process of the molten steel, constructing a low-oxygen, low-nitrogen protective environment. The suspended rare earth rods are slowly melted by the weight of the molten steel, achieving uniform, stable, and homogeneous dissolution of rare earth elements into the molten steel. This replaces the traditional one-time large-scale feeding method, precisely controlling the amount of rare earth added and the reaction rate. This addresses technical challenges such as rare earth burn-off, component segregation, and inclusion deterioration from the source, achieving precise refinement of as-cast grains and eliminating central porosity and segregation defects in thick steel ingots. This improves the microstructure uniformity and internal quality of extra-thick steel plates from the source of the billet. Simultaneously, strict control of in-mold superheat, casting rate, demolding temperature, and settling time effectively avoids internal cracks and component segregation defects that occur during the solidification of thick steel ingots.

[0020] In terms of rolling technology, this invention specifically designs a steel ingot homogenization pretreatment + two-stage differentiated high-reduction rolling process. After peeling, the steel ingot is slowly cooled and held at low temperature to eliminate as-cast internal stress and homogenize the initial microstructure. This is combined with a low-speed heating and high-temperature long-term homogenization heating regime to thoroughly dissolve residual carbonitrides in the as-cast state and eliminate compositional inhomogeneities, ensuring a uniform and consistent overall microstructure of the steel ingot. The rolling process is divided into two temperature-controlled zones for precise rolling control: the high-temperature roughing stage uses an ultra-large reduction to quickly break up coarse as-cast columnar crystals and compact the loose and porous defects in the core of the steel ingot, achieving initial densification of the steel microstructure; the medium-temperature finishing zone precisely controls the opening and closing rolling temperatures, again using high-reduction rolling to continuously refine the austenite grains and accumulate a large amount of dislocation energy, providing a microstructure basis for subsequent heat treatment phase transformation and microstructure refinement. After rolling, rapid water cooling and temperature control quickly lock in the refined microstructure, inhibit high-temperature grain growth, and further reduce the differences in microstructure and properties between the surface and core of the steel plate.

[0021] In terms of heat treatment, relying on the synergistic strengthening effect of rare earth Ce and trace amounts of B at grain boundaries, the hardenability of the steel is significantly improved. A single quenching process is sufficient to achieve full austenitization and uniform solid solution of alloying elements across the entire thickness of the extra-thick steel plate, completely eliminating residual rolling stress and differences in cross-sectional microstructure, and achieving precise control of microstructure homogeneity across the entire plate thickness. Compared to traditional multi-quenching processes, this invention eliminates one quenching step, significantly simplifying the heat treatment process, shortening the production cycle, and significantly improving the production efficiency and batch production stability of extra-thick high-strength and high-toughness rack steel. It achieves an integrated match of high strength, high toughness, and high microstructure homogeneity in extra-thick rack steel, meeting the stringent requirements for long-term safe service of heavy-duty racks on offshore platforms.

[0022] Compared with the prior art, the beneficial effects of the present invention are as follows: 1) The alloy system is economically superior, significantly reducing overall production costs. This invention abandons the traditional high-cost alloy design approach of high-nickel, high-chromium, high-molybdenum, and niobium-vanadium composites for ultra-thick high-strength steel. Instead, it relies on the microscopic composite strengthening mechanism of trace rare earth elements and trace boron to achieve a leap in hardenability and toughness, effectively reducing the proportion of precious alloys added. While ensuring ultra-high strength and toughness in ultra-thick sections, it significantly reduces alloy costs.

[0023] 2) Achieving integrated matching of ultra-high strength and toughness in extra-thick steel plates, significantly improving the uniformity of performance across the entire cross-section. This invention utilizes the synergistic mechanism of rare-earth Ce lattice distortion strengthening and Bo element grain boundary stabilization to significantly improve the stability of supercooled austenite, greatly enhancing the hardenability of ultra-thick steel plates. High strength and high toughness matching across a full plate thickness of 177.8–256 mm can be achieved without significantly increasing the proportion of expensive alloys. The overall yield strength of the steel plate is ≥690 MPa, and the tensile strength is ≥810 MPa. Furthermore, the single-value longitudinal and transverse low-temperature impact energy at -60℃ in the core of the steel plate at 1 / 2 thickness is consistently ≥100 J, completely solving the problems of insufficient hardenability of the core, large microstructure gradient, and poor low-temperature toughness in traditional extra-thick rack steel.

[0024] 3) High steel purity and strong controllability of inclusions significantly optimize the internal quality of the steel. This invention strictly controls the content of harmful impurities such as P, S, and N through high-proportion electric furnace smelting, deep LF dephosphorization, and VD high-vacuum long-term purification processes. At the same time, it adopts a dual-gradient rare earth addition process of feeding rare earth alloy at the end of refining and hanging rare earth rods in the mold, which effectively improves the rare earth yield and distribution uniformity, gives full play to the deoxidation, desulfurization, inclusion spheroidization modification and grain refinement effects of rare earth, eliminates the central segregation, porosity and banded structure defects in the solidification process of thick steel ingots, and greatly improves the internal density and structure uniformity of steel plates, thereby improving the mechanical properties of extra-thick toothed steel from the metallurgical source.

[0025] 4) Simplified heat treatment process, significantly improved production efficiency, and effective reduction in energy costs. Traditional extra-thick high-strength and high-toughness marine steels of the same grade generally adopt a two-stage quenching + high-temperature tempering process to ensure full-thickness hardening and uniform microstructure. This process is lengthy, energy-intensive, has a long production cycle, and low yield. This invention relies on the rare-earth Ce-boron composite hardening-enhancing mechanism to achieve complete austenitization of the entire thickness of the steel plate, full alloy solution, and complete release of microstructural stress through a single quenching + tempering process, resulting in a stable, uniform, and refined tempered microstructure. While maintaining or even surpassing the performance of multiple quenching processes, it reduces one high-temperature quenching step, significantly shortens the heat treatment production cycle, reduces heating energy consumption and oxidation loss, greatly improves production line efficiency and batch production stability, and is suitable for large-scale industrial mass production.

[0026] 5) The production method for extra-thick, high-strength, and tough rack steel for marine platforms provided by this invention has strong process adaptability, good production stability, and high industrialization value. Production is completed using conventional steelmaking, ingot casting, rolling, and heat treatment equipment. The process parameters have a wide window, strong controllability, simple operation, and good repeatability. It avoids high-threshold production conditions such as special processes, extreme temperature control, and precise micro-control. It can stably achieve batch production of ultra-thick high-strength and tough rack steel with specifications of 177.8–256 mm, with good product quality consistency and sufficient performance margin, possessing extremely strong industrial applicability and market promotion value. Attached Figure Description

[0027] Figure 1 This is a schematic diagram of the rare earth rod casting process suspended inside the mold; in the diagram, 1 is the vertical flow channel, 2 is the horizontal flow channel, 3 is the ingot mold, 4 is the rare earth rod, and 5 is the argon gas pipeline. Detailed Implementation

[0028] The technical solution of the present invention will be further described in detail below with reference to the embodiments.

[0029] The in-mold hanging rare earth rod casting process in each embodiment is as follows: Figure 1 As shown, the details are as follows: Molten steel injection channel: Open the ladle nozzle, and molten steel flows into the vertical channel 1 through the nozzle brick, and then is distributed to each ingot mold 3 through the horizontal channel 2.

[0030] In-mold rare earth addition: After the molten steel enters the ingot mold 3, it is immersed in the pre-suspended rare earth rods (REM) 4. The rare earth rods 4 slowly dissolve in the high-temperature molten steel, realizing the rare earth micro-alloying and inclusion modification of the molten steel.

[0031] Argon protection: Argon gas is continuously introduced into the ingot mold 3 through argon gas pipeline 5 to form an inert protective gas layer on the surface of the molten steel, which isolates the air and prevents the molten steel from secondary oxidation and gas absorption, while inhibiting the volatilization and burning loss of rare earth elements.

[0032] Molten steel gradually solidifies within the ingot mold, with rare earth elements evenly distributed throughout the billet, forming a clean and high-performance rare earth steel ingot. After pouring, the ladle nozzle is closed, argon gas supply is stopped, and the ingot is demolded after complete solidification, completing the entire pouring process. Example 1

[0033] In this embodiment, the extra-thick high-strength and tough rack steel used in the offshore platform has a thickness of 177.8 mm. Its chemical composition and mass percentage are shown in Table 1.

[0034] The production method of extra-thick, high-strength, and high-toughness rack steel for offshore platforms in this embodiment includes electric furnace smelting → LF refining → VD vacuum treatment → ingot casting → ingot heating → rolling → heat treatment; the specific process control is as follows: (1) Steelmaking process: Electric furnace smelting: 90 tons of steel were produced, using a normal turnover ladle with a 4-inch ladle, a 50mm initial nozzle, and a flat ladle edge; the single furnace feed of molten iron was 60t, no material was added during tapping, the ladle temperature was 1590℃, and the ladle P was 0.004%. LF refining adopted external dephosphorization, VD vacuum degree was 65Pa, holding time was 20min, and calcium treatment was carried out immediately after vacuum was broken, with calcium added at 15ppm. After 1min, rare earth iron-cerium intermediate alloy was added, rare earth Ce added at 20ppm, and atmospheric pressure soft blowing time was 8min.

[0035] (2) Ingot casting process: Two high-quality ingot molds are used, each with a steel molten capacity of 45 tons. Rare earth rods are suspended inside the molds. The rare earth rods are 8mm in diameter and 1m in length. The rare earth Ce mass fraction in the rare earth rods is 99.4%. The rare earth rods are suspended 100mm from the bottom of the steel ingot mold. The rare earth addition amount is 30ppm. The superheat is controlled at 30℃ during casting. Bottom pouring is used for casting, and argon gas protection is used. The argon gas flow rate is 20L / min, and the steel molten flow rate is 3.0t / min. The time from casting to demolding is 36 hours, and the demolding temperature is 200℃.

[0036] (3) Rolling process: After the steel ingot is peeled and cleaned, it is placed in a slow cooling pit and kept at 200℃ for 4 hours. Then it is put into a heating furnace with a heating rate of 150℃ / h and a holding temperature of 1240℃. The total time the steel ingot is in the furnace is 12min / cm. After the steel ingot is taken out of the furnace, it is promptly placed in a high-pressure water descaling box to remove scale, minimizing the amount of primary iron oxide scale on the upper and lower surfaces. Two-stage rolling is adopted. The roughing rolling starts at 1150℃, with a large reduction of 40mm per pass in the first two passes, and the thickness is rolled to 450mm. The finishing rolling starts at 900℃, with a large reduction of 40mm per pass in the first two passes, and the final rolling temperature is 830℃. After rolling, it is water-cooled online, and the reddening temperature is 300℃.

[0037] (4) Heat treatment process: After rolling, the steel plate is quenched and tempered in a roller hearth furnace. The quenching temperature is 890℃ and the holding time is 2.2min / mm. The tempering temperature is 610℃ and the holding time is 3.5min / mm. After the steel plate comes out of the tempering furnace, it is fire-cut to length while still warm. After checking the surface and fire-cut surface quality, it is put into storage.

[0038] The extra-thick high-strength and high-toughness rack steel for offshore platforms produced in this embodiment has a microstructure of tempered martensite + tempered bainite + ferrite. The strength and impact properties of the steel plate are shown in Table 2. Example 2

[0039] In this embodiment, the extra-thick high-strength and tough rack steel used in the offshore platform has a thickness of 190mm. Its chemical composition and mass percentage are shown in Table 1.

[0040] The production method of extra-thick, high-strength, and high-toughness rack steel for offshore platforms in this embodiment includes electric furnace smelting → LF refining → VD vacuum treatment → ingot casting → ingot heating → rolling → heat treatment; the specific process control is as follows: (1) Steelmaking process: Electric furnace smelting: 94 tons of steel were produced, and a normal turnover ladle was used within 5 hours. The first nozzle was 50mm and the ladle edge was flat. The single furnace feed of molten iron was 65t. No material was added when the steel was tapped. The ladle temperature was 1600℃ and the ladle P was 0.004%. LF refining adopted dephosphorization outside the furnace. The VD vacuum degree was 60Pa and the holding time was 21min. After the vacuum was broken, calcium treatment was carried out immediately. The calcium addition amount was 20ppm. After 2min, rare earth iron-cerium master alloy was added. The rare earth Ce addition amount was 30ppm. The atmospheric pressure soft blowing time was 10min.

[0041] (2) Ingot casting process: Two high-quality ingot molds are used, each with a steel liquid capacity of 47 tons. Rare earth rods are suspended inside the molds. The rare earth rods are 12 mm in diameter and 1.1 m in length. The rare earth Ce mass fraction in the rare earth rods is 99.6%. The rare earth rods are suspended 80 mm from the bottom of the steel ingot molds. The rare earth addition amount is 50 ppm. The superheat is controlled at 35℃ during casting. Bottom pouring is used for casting, and argon gas protection is used. The argon gas flow rate is 25 L / min, and the steel liquid flow rate is 3.1 t / min. The time from casting to demolding is 38 hours, and the demolding temperature is 190℃.

[0042] (3) Rolling process: After the steel ingot is peeled and cleaned, it is placed in a slow cooling pit and kept at 220℃ for 4.5 hours. Then it is put into a heating furnace with a heating rate of 140℃ / h and a holding temperature of 1245℃. The total time the steel ingot is in the furnace is 13 min / cm. After the steel ingot is taken out of the furnace, it is promptly placed in a high-pressure water descaling box to remove scale, minimizing the amount of primary iron oxide scale on the upper and lower surfaces. Two-stage rolling is adopted. The roughing rolling starts at 1160℃, with a large reduction of 45mm per pass in the first two passes, and the thickness is rolled to 460mm. The finishing rolling starts at 910℃, with a large reduction of 42mm per pass in the first two passes, and the final rolling temperature is 840℃. After rolling, it is water-cooled online, and the reddening temperature is 280℃.

[0043] (4) Heat treatment process: After rolling, the steel plate is quenched and tempered in a roller hearth furnace. The quenching temperature is 900℃ and the holding time is 2.3min / mm. The tempering temperature is 620℃ and the holding time is 3.8min / mm. After the steel plate comes out of the tempering furnace, it is fire-cut to length while still warm. After checking the surface and fire-cut surface quality, it is put into storage.

[0044] The extra-thick high-strength and high-toughness rack steel for offshore platforms produced in this embodiment has a microstructure of tempered martensite + tempered bainite + ferrite. The strength and impact properties of the steel plate are shown in Table 2. Example 3

[0045] In this embodiment, the extra-thick high-strength and tough rack steel used in the offshore platform has a thickness of 210mm. Its chemical composition and mass percentage are shown in Table 1.

[0046] The production method of extra-thick, high-strength, and high-toughness rack steel for offshore platforms in this embodiment includes electric furnace smelting → LF refining → VD vacuum treatment → ingot casting → ingot heating → rolling → heat treatment; the specific process control is as follows: (1) Steelmaking process: Electric furnace smelting: 100 tons of steel output, normal turnover ladle used within 6 hours, 50mm initial nozzle, ladle edge flat; single furnace molten iron intake 65t, no material added during tapping, ladle temperature 1600℃, ladle P 0.003%. LF refining adopts external dephosphorization, VD vacuum degree 62Pa, holding time 22min, calcium treatment is carried out immediately after vacuum is broken, calcium addition amount is 23ppm, rare earth iron cerium intermediate alloy is added after 3min, rare earth Ce addition amount is 40ppm, atmospheric pressure soft blowing time is 12min.

[0047] (2) Ingot casting process: Two high-quality ingot molds are used, each with a steel liquid capacity of 50 tons. Rare earth rods are suspended inside the molds. The rare earth rods are 13 mm in diameter and 1.2 m in length. The rare earth Ce mass fraction in the rare earth rods is 99.7%. The rare earth rods are suspended 60 mm from the bottom of the steel ingot molds. The rare earth addition amount is 60 ppm. The superheat is controlled at 40℃ during casting. Bottom pouring is used for casting, and argon gas protection is used. The argon gas flow rate is 30 L / min, and the steel liquid flow rate is 3.2 t / min. The time from casting to demolding is 38 hours, and the demolding temperature is 180℃.

[0048] (3) Rolling process: After the steel ingot is peeled and cleaned, it is placed in a slow cooling pit and kept at 240℃ for 6 hours. Then it is put into a heating furnace with a heating rate of 120℃ / h and a holding temperature of 1250℃. The total time the steel ingot is in the furnace is 12min / cm. After the steel ingot is taken out of the furnace, it is promptly placed in a high-pressure water descaling box to remove scale, minimizing the amount of primary iron oxide scale on the upper and lower surfaces. Two-stage rolling is adopted. The roughing rolling starts at 1170℃, with a large reduction of 46mm per pass in the first two passes, and the thickness is rolled to 470mm. The finishing rolling starts at 920℃, with a large reduction of 46mm per pass in the first two passes, and the final rolling temperature is 855℃. After rolling, it is water-cooled online, and the reddening temperature is 260℃.

[0049] (4) Heat treatment process: After rolling, the steel plate is quenched and tempered in a roller hearth furnace. The quenching temperature is 910℃ and the holding time is 2.4min / mm. The tempering temperature is 630℃ and the holding time is 4.0min / mm. After the tempering furnace is removed, the steel plate is fire-cut to length while still warm. After checking the surface and fire-cut surface quality, it is put into storage.

[0050] The microstructure of the extra-thick high-strength and high-toughness rack steel for offshore platforms produced in this embodiment is tempered martensite + tempered bainite + ferrite. The strength and impact properties of the steel plate are shown in Table 2. Example 4

[0051] In this embodiment, the extra-thick high-strength and tough rack steel used in the offshore platform has a thickness of 230mm. Its chemical composition and mass percentage are shown in Table 1.

[0052] The production method of extra-thick, high-strength, and high-toughness rack steel for offshore platforms in this embodiment includes electric furnace smelting → LF refining → VD vacuum treatment → ingot casting → ingot heating → rolling → heat treatment; the specific process control is as follows: (1) Steelmaking process: Electric furnace smelting: 106 tons of steel output, normal turnover ladle used within 7 hours, 50mm initial nozzle, ladle edge flat; single furnace molten iron intake 70t, no material added during tapping, ladle temperature 1595℃, ladle P 0.003%. LF refining adopts external dephosphorization, VD vacuum degree ≤60Pa, holding time 23min, calcium treatment is carried out immediately after vacuum is broken, calcium addition amount is 25ppm, rare earth iron cerium intermediate alloy is added 2min later, rare earth Ce addition amount is 45ppm, atmospheric pressure soft blowing time is 14min.

[0053] (2) Ingot casting process: Two high-quality ingot molds are used, each with a steel liquid capacity of 53 tons. Rare earth rods are suspended inside the molds. The rare earth rods are 14 mm in diameter and 1 m in length. The rare earth Ce mass fraction in the rare earth rods is 99.8%. The rare earth rods are suspended 50 mm from the bottom of the steel ingot molds. The rare earth addition amount is 105 ppm. The superheat is controlled at 45℃ during casting. Bottom pouring is used for casting, and argon gas protection is used. The argon gas flow rate is 30 L / min, and the steel liquid flow rate is 3.3 t / min. The time from casting to demolding is 40 hours, and the demolding temperature is 170℃.

[0054] (3) Rolling process: After the steel ingots are peeled and cleaned, they are placed in a slow cooling pit and kept at 270℃ for 5 hours. Then they are put into a heating furnace with a heating rate of 130℃ / h and a holding temperature of 1250℃. The total time the steel ingots are in the furnace is 13min / cm. After the steel ingots are taken out of the furnace, they are promptly placed in a high-pressure water descaling box to remove scale, minimizing the amount of primary iron oxide scale on the upper and lower surfaces. A two-stage rolling process is adopted. The roughing rolling starts at 1190℃, with a large reduction of 48mm per pass in the first two passes, resulting in a thickness of 480mm. The finishing rolling starts at 925℃, with a large reduction of 48mm per pass in the first two passes, and a final rolling temperature of 865℃. After rolling, the ingots are water-cooled online, with a reddening temperature of 250℃.

[0055] (4) Heat treatment process: After rolling, the steel plate is quenched and tempered in a roller hearth furnace. The quenching temperature is 920℃ and the holding time is 2.6min / mm. The tempering temperature is 635℃ and the holding time is 4.2min / mm. After the steel plate comes out of the tempering furnace, it is fire-cut to length while still warm. After checking the surface and fire-cut surface quality, it is put into storage.

[0056] The extra-thick high-strength and high-toughness rack steel for offshore platforms produced in this embodiment has a microstructure of tempered martensite + tempered bainite + ferrite. The strength and impact properties of the steel plate are shown in Table 2. Example 5

[0057] In this embodiment, the extra-thick high-strength and tough rack steel used in the offshore platform has a thickness of 256mm. Its chemical composition and mass percentage are shown in Table 1.

[0058] The production method of the extra-thick, high-strength, and high-toughness rack steel for offshore platforms in this embodiment includes electric furnace smelting → LF refining → VD vacuum treatment → ingot casting → ingot heating → rolling → heat treatment; the specific process control is as follows: (1) Steelmaking process: Electric furnace smelting: 110 tons of steel were produced, and a normal turnover ladle was used within 8 hours. The first nozzle was 50mm and the ladle edge was flat. The single furnace feed of molten iron was 75t. No material was added when the steel was tapped. The ladle temperature was 1595℃ and the ladle P was 0.003%. LF refining adopted dephosphorization outside the furnace, VD vacuum degree was 60Pa, holding time was 23min, and calcium treatment was carried out immediately after the vacuum was broken. The calcium addition amount was 30ppm. After 2min, rare earth iron cerium master alloy was added. The rare earth Ce addition amount was 50ppm. The atmospheric pressure soft blowing time was 15min.

[0059] (2) Ingot casting process: Two high-quality ingot molds are used, each with a steel molten capacity of 55 tons. Rare earth rods are suspended inside the molds. The rare earth rods are 15 mm in diameter and 1.1 m in length. The rare earth Ce mass fraction in the rare earth rods is 99.9%. The rare earth rods are suspended 40 mm from the bottom of the steel ingot molds. The rare earth addition amount is 150 ppm. The superheat is controlled at 50℃ during casting. Bottom pouring is used for casting, and argon gas protection is used. The argon gas flow rate is 40 L / min, and the steel molten flow rate is 3.5 t / min. The demolding time after casting is 42 hours, and the demolding temperature is 160℃.

[0060] (3) Rolling process: After the steel ingots are peeled and cleaned, they are placed in a slow cooling pit and kept at 300℃ for 5 hours. Then they are put into a heating furnace with a heating rate of 140℃ / h and a holding temperature of 1255℃. The total time the steel ingots are in the furnace is 13min / cm. After the steel ingots are taken out of the furnace, they are promptly placed in a high-pressure water descaling box to remove scale, minimizing the amount of primary iron oxide scale on the upper and lower surfaces. Two-stage rolling is adopted. The roughing rolling starts at 1200℃, with a large reduction of 50mm per pass in the first two passes, and the thickness is rolled to 500mm. The finishing rolling starts at 920℃, with a large reduction of 50mm per pass in the first two passes, and the final rolling temperature is 880℃. After rolling, the ingots are water-cooled online, and the reddening temperature is 250℃.

[0061] (4) Heat treatment process: After rolling, the steel plate is quenched and tempered in a roller hearth furnace. The quenching temperature is 930℃ and the holding time is 2.6min / mm. The tempering temperature is 640℃ and the holding time is 4.5min / mm. After the steel plate comes out of the tempering furnace, it is fire-cut to length while still warm. After checking the surface and fire-cut surface quality, it is put into storage.

[0062] The extra-thick high-strength and high-toughness rack steel for offshore platforms produced in this embodiment has a microstructure of tempered martensite + tempered bainite + ferrite. The strength and impact properties of the steel plate are shown in Table 2. Comparative Example 1

[0063] The extra-thick, high-strength, and tough rack steel used in this comparative offshore platform is 210 mm thick. Its chemical composition and mass percentage are shown in Table 1.

[0064] The production method of extra-thick, high-strength, and high-toughness rack steel for offshore platforms, as shown in this comparative example, includes electric furnace smelting → LF refining → VD vacuum treatment → ingot casting → ingot heating → rolling → heat treatment; the specific process control is as follows: (1) Steelmaking process: Electric furnace smelting: 100 tons of steel output, normal turnover ladle used within 6 hours, 50mm initial nozzle, ladle edge flat; single furnace molten iron intake 65t, no material added during tapping, ladle temperature 1600℃, ladle P 0.003%. LF refining adopts external dephosphorization, VD vacuum degree 62Pa, holding time 22min, calcium treatment is carried out immediately after vacuum is broken, calcium addition amount is 23ppm, atmospheric pressure soft blowing time 12min.

[0065] (2) Molding process: Two high-quality ingot molds are used, each with a steel liquid capacity of 50 tons. The superheat is controlled at 40℃ during casting. Bottom pouring is used for casting, and argon gas is used for protection. The argon gas flow rate is 30L / min, the steel liquid flow rate is 3.2t / min, and the time from casting to demolding is 38 hours. The demolding temperature is 180℃.

[0066] (3) Rolling process: After the steel ingots are peeled and cleaned, they are placed in a slow cooling pit and kept at 240℃ for 6 hours. Then they are put into a heating furnace with a heating rate of 120℃ / h and a holding temperature of 1250℃. The total time the steel ingots are in the furnace is 12min / cm. After the steel ingots are taken out of the furnace, they are promptly placed in a high-pressure water descaling box to remove scale, minimizing the amount of primary iron oxide scale on the upper and lower surfaces. Two-stage rolling is adopted. The roughing rolling starts at 1170℃, with a large reduction of 45mm per pass in the first two passes, and the thickness is rolled to 450mm. The finishing rolling starts at 920℃, with a large reduction of 46mm per pass in the first two passes, and the final rolling temperature is 855℃. After rolling, the ingots are water-cooled online, and the reddening temperature is 260℃.

[0067] (4) Heat treatment process: After rolling, the steel plate is quenched and tempered in a roller hearth furnace. The quenching temperature is 910℃ and the holding time is 2.4min / mm. The tempering temperature is 630℃ and the holding time is 4.0min / mm. After the steel plate comes out of the tempering furnace, it is fire-cut to length while still warm. After checking the surface and fire-cut surface quality, it is put into storage.

[0068] The extra-thick high-strength and high-toughness rack steel for offshore platforms produced in this embodiment has a microstructure of tempered martensite + tempered bainite + ferrite. The strength and impact properties of the steel plate are shown in Table 2. Comparative Example 2

[0069] The extra-thick, high-strength, and tough rack steel used in this comparative offshore platform is 210 mm thick. Its chemical composition and mass percentage are shown in Table 1.

[0070] The production method of extra-thick, high-strength, and high-toughness rack steel for offshore platforms, as shown in this comparative example, includes electric furnace smelting → LF refining → VD vacuum treatment → ingot casting → ingot heating → rolling → heat treatment; the specific process control is as follows: (1) Steelmaking process: Electric furnace smelting: 100 tons of steel output, normal turnover ladle used within 6 hours, 50mm initial nozzle, ladle edge flat; single furnace molten iron intake 65t, no material added during tapping, ladle temperature 1600℃, ladle P 0.003%. LF refining adopts external dephosphorization, VD vacuum degree 62Pa, holding time 22min, calcium treatment is carried out immediately after vacuum is broken, calcium addition amount is 23ppm, rare earth iron cerium intermediate alloy is added after 3min, rare earth Ce addition amount is 40ppm, atmospheric pressure soft blowing time is 12min.

[0071] (2) Ingot casting process: Two high-quality ingot molds are used, each with a steel liquid capacity of 50 tons. Rare earth rods are suspended inside the molds. The rare earth rods are 13 mm in diameter and 1.2 m in length. The rare earth Ce mass fraction in the rare earth rods is 99.7%. The rare earth rods are suspended 60 mm from the bottom of the steel ingot molds. The rare earth addition amount is 60 ppm. The superheat is controlled at 40℃ during casting. Bottom pouring is used for casting, and argon gas protection is used. The argon gas flow rate is 30 L / min, and the steel liquid flow rate is 3.2 t / min. The time from casting to demolding is 38 hours, and the demolding temperature is 180℃.

[0072] (3) Rolling process: After the steel ingots are peeled and cleaned, they are placed in a slow cooling pit and kept at 240℃ for 6 hours. Then they are put into a heating furnace with a heating rate of 120℃ / h and a holding temperature of 1250℃. The total time the steel ingots are in the furnace is 12min / cm. After the steel ingots are taken out of the furnace, they are promptly placed in a high-pressure water descaling box to remove scale, minimizing the amount of primary iron oxide scale on the upper and lower surfaces. Two-stage rolling is adopted. The roughing rolling starts at 1170℃, with a large reduction of 45mm per pass in the first two passes, and the thickness is rolled to 450mm. The finishing rolling starts at 920℃, with a large reduction of 46mm per pass in the first two passes, and the final rolling temperature is 855℃. After rolling, the ingots are water-cooled online, and the reddening temperature is 260℃.

[0073] (4) Heat treatment process: After rolling, the steel plate is quenched and tempered in a roller hearth furnace. The quenching temperature is 910℃ and the holding time is 2.4min / mm. The tempering temperature is 630℃ and the holding time is 4.0min / mm. After the steel plate comes out of the tempering furnace, it is fire-cut to length while still warm. After checking the surface and fire-cut surface quality, it is put into storage.

[0074] The microstructure of the extra-thick high-strength and high-toughness rack steel for offshore platforms produced in this comparative model is tempered martensite + tempered bainite + ferrite. The strength and impact properties of the steel plate are shown in Table 2. Comparative Example 3

[0075] The extra-thick, high-strength, and tough rack steel used in this comparative offshore platform is 210 mm thick. Its chemical composition and mass percentage are shown in Table 1.

[0076] The production method of extra-thick, high-strength, and high-toughness rack steel for offshore platforms, as shown in this comparative example, includes electric furnace smelting → LF refining → VD vacuum treatment → ingot casting → ingot heating → rolling → heat treatment; the specific process control is as follows: (1) Steelmaking process: Electric furnace smelting: 100 tons of steel output, normal turnover ladle used within 6 hours, 50mm initial nozzle, ladle edge flat; single furnace molten iron intake 65t, no material added during tapping, ladle temperature 1600℃, ladle P 0.003%. LF refining adopts external dephosphorization, VD vacuum degree 62Pa, holding time 22min, calcium treatment is carried out immediately after vacuum is broken, calcium addition amount is 23ppm, rare earth iron cerium intermediate alloy is added after 3min, rare earth Ce addition amount is 100ppm, atmospheric pressure soft blowing time is 12min.

[0077] (2) Molding process: Two high-quality ingot molds are used, each with a steel liquid capacity of 50 tons. The superheat is controlled at 40℃ during casting. Bottom pouring is used for casting, and argon gas is used for protection. The argon gas flow rate is 30L / min, the steel liquid flow rate is 3.2t / min, and the time from casting to demolding is 38 hours. The demolding temperature is 180℃.

[0078] (3) Rolling process: After the steel ingots are peeled and cleaned, they are placed in a slow cooling pit and kept at 240℃ for 6 hours. Then they are put into a heating furnace with a heating rate of 120℃ / h and a holding temperature of 1250℃. The total time the steel ingots are in the furnace is 12min / cm. After the steel ingots are taken out of the furnace, they are promptly placed in a high-pressure water descaling box to remove scale, minimizing the amount of primary iron oxide scale on the upper and lower surfaces. Two-stage rolling is adopted. The roughing rolling starts at 1170℃, with a large reduction of 40mm per pass in the first two passes, and the thickness is rolled to 450mm. The finishing rolling starts at 920℃, with a large reduction of 46mm per pass in the first two passes, and the final rolling temperature is 855℃. After rolling, the ingots are water-cooled online, and the reddening temperature is 260℃.

[0079] (4) Heat treatment process: After rolling, the steel plate is quenched and tempered in a roller hearth furnace. The quenching temperature is 910℃ and the holding time is 2.4min / mm. The tempering temperature is 630℃ and the holding time is 4.0min / mm. After the tempering furnace is removed, the steel plate is fire-cut to length while still warm. After checking the surface and fire-cut surface quality, it is put into storage.

[0080] The microstructure of the extra-thick high-strength and high-toughness rack steel for offshore platforms produced in this comparative model is tempered martensite + tempered bainite + ferrite. The strength and impact properties of the steel plate are shown in Table 2. Comparative Example 4

[0081] The extra-thick, high-strength, and tough rack steel used in this comparative offshore platform is 210 mm thick. Its chemical composition and mass percentage are shown in Table 1.

[0082] The production method of extra-thick, high-strength, and high-toughness rack steel for offshore platforms, as shown in this comparative example, includes electric furnace smelting → LF refining → VD vacuum treatment → ingot casting → ingot heating → rolling → heat treatment; the specific process control is as follows: (1) Steelmaking process: Electric furnace smelting: 100 tons of steel output, normal turnover ladle used within 6 hours, 50mm initial nozzle, ladle edge flat; single furnace molten iron intake 65t, no material added during tapping, ladle temperature 1600℃, ladle P 0.003%. LF refining adopts external dephosphorization, VD vacuum degree 62Pa, holding time 22min, calcium treatment is carried out immediately after vacuum is broken, calcium addition amount is 23ppm, atmospheric pressure soft blowing time 12min.

[0083] (2) Ingot casting process: Two high-quality ingot molds are used, each with a steel liquid capacity of 50 tons. Rare earth rods are suspended inside the molds. The rare earth rods are 13 mm in diameter and 1.2 m in length. The rare earth Ce mass fraction in the rare earth rods is 99.7%. The rare earth rods are suspended 60 mm from the bottom of the steel ingot molds. The rare earth addition amount is 100 ppm. The superheat is controlled at 40℃ during casting. Bottom pouring is used for casting, and argon gas protection is used. The argon gas flow rate is 30 L / min, and the steel liquid flow rate is 3.2 t / min. The time from casting to demolding is 38 hours, and the demolding temperature is 180℃.

[0084] (3) Rolling process: After the steel ingots are peeled and cleaned, they are placed in a slow cooling pit and kept at 240℃ for 6 hours. Then they are put into a heating furnace with a heating rate of 120℃ / h and a holding temperature of 1250℃. The total time the steel ingots are in the furnace is 12min / cm. After the steel ingots are taken out of the furnace, they are promptly placed in a high-pressure water descaling box to remove scale, minimizing the amount of primary iron oxide scale on the upper and lower surfaces. Two-stage rolling is adopted. The roughing rolling starts at 1170℃, with a large reduction of 45mm per pass in the first two passes, and the thickness is rolled to 450mm. The finishing rolling starts at 920℃, with a large reduction of 46mm per pass in the first two passes, and the final rolling temperature is 855℃. After rolling, the ingots are water-cooled online, and the reddening temperature is 260℃.

[0085] (4) Heat treatment process: After rolling, the steel plate is quenched and tempered in a roller hearth furnace. The quenching temperature is 910℃ and the holding time is 2.4min / mm. The tempering temperature is 630℃ and the holding time is 4.0min / mm. After the steel plate comes out of the tempering furnace, it is fire-cut to length while still warm. After checking the surface and fire-cut surface quality, it is put into storage.

[0086] The microstructure of the extra-thick high-strength and high-toughness rack steel for offshore platforms produced in this comparative model is tempered martensite + tempered bainite + ferrite. The strength and impact properties of the steel plate are shown in Table 2.

[0087] Table 1. Chemical composition and percentage content (wt%) of the rack steel in each embodiment and comparative example.

[0088] The balance in Table 1 is Fe and unavoidable impurities.

[0089] Table 2. Strength and transverse impact energy at -60°C of rack steel in each embodiment and comparative example.

[0090] Compared to Example 3, Comparative Example 1 did not add rare earth element Ce; Comparative Example 2 did not add element B. From Table 2, the strength and transverse impact energy at -60℃ of the rack steel show little difference in yield strength and tensile strength at 1 / 4 thickness for the samples of each example and comparative example. However, the strength indicators of the examples at the 1 / 2 thickness position are significantly better than those of Comparative Examples 1 and 2. For example, the yield strength at 1 / 2 thickness in Example 3 reaches 729 MPa, while that in Comparative Examples 1 and 2 is only 650 MPa and 644 MPa, respectively. This indicates that the synergistic distribution of Ce and B can effectively improve the cross-sectional strength uniformity of thick rack steel and avoid the problem of reduced core strength.

[0091] Compared to Example 3, Comparative Example 3 only added rare earth iron-cerium alloy at the end of refining without hanging rare earth rods in the mold, while Comparative Example 4 only added rare earth iron-cerium alloy by hanging rare earth rods in the mold without adding rare earth iron-cerium alloy at the end of refining. As shown in Table 2, compared to either a single refining process with Ce addition or a single in-mold rod hanging process, the combined addition method of adding rare earth Ce alloy at the end of refining and hanging rare earth rods in the mold significantly improves the strength at half the steel thickness.

[0092] Low-temperature toughness performance: The transverse impact energy of the rack steel in each embodiment at -60℃ is significantly better than that of all comparative examples. Taking Example 3 as an example, the average transverse impact energy at -60℃ is approximately 193 J for 1 / 4 thickness and approximately 176 J for 1 / 2 thickness; while the average impact energy of Comparative Example 1 (without Ce) is only about 64 J for 1 / 4 thickness and only about 44 J for 1 / 2 thickness, and the average impact energy of Comparative Example 2 (without B) is only about 71 J for 1 / 2 thickness. This indicates that the synergistic addition of B and Ce is the key to ensuring the high toughness of rack steel in the ultra-low temperature environment of -60℃, which can significantly improve the low-temperature impact resistance of large-thickness sections and meet the safety requirements for use under extreme conditions.

[0093] The number of rare earth inclusions with different area ratios in the continuously cast billets obtained in each embodiment and comparative example is shown in Table 3. The total scanned area is 20 mm × 6 mm.

[0094] Table 3. SEM statistical analysis of rare earth inclusion area in each embodiment and comparative example.

[0095] As shown in Table 3, the number of inclusions in Comparative Example 3 and the number of inclusions of different areas differ significantly from those in Example 3, especially the number of large-area inclusions, which has increased significantly. This indicates that adding rare earth only at the end of refining causes the local rare earth concentration in the molten steel to exceed the standard instantaneously, and the reaction to be too violent. A large amount of rare earth deoxidation and desulfurization products are generated in a short period of time, greatly increasing the probability of collision, adsorption, and aggregation of fine new inclusions. This makes it impossible to achieve a uniformly dispersed distribution, and they quickly agglomerate to form large-size rare earth composite inclusions, replacing the original fine spherical inclusions, resulting in a significant increase in the number of large-size inclusions in the steel. The number of inclusions in Comparative Example 4 is similar to that in Example 3, but the number of large-size inclusions in Comparative Example 4 is significantly increased. This is because the rare earth is introduced too late, and the inclusions in the early stage have no opportunity for modification. The rare earth is only slowly dissolved in during the solidification stage of the molten steel in the mold. The oxide and sulfide inclusions in the early stage of refining and casting have already fully formed and grown. The formed coarse inclusions cannot be fully spheroidized and refined, and a large number of large-size inclusions remain directly.

Claims

1. A type of extra-thick, high-strength, and high-toughness rack steel for offshore platforms, characterized in that: The chemical composition and mass percentage of the rack steel are as follows: C 0.08%~0.12%, Si 0.25%~0.40%, Mn 0.90%~1.20%, P≤0.006%, S≤0.005%, Ni 1.7%~2.3%, Cr 0.6%~1.0%, Cu 0.15%~0.40%, Mo 0.2%~0.5%, Alt 0.030%~0.060%, Ti 0.007%~0.015%, B 0.0010%~0.0015%, Ce 0.005%~0.020%, N≤0.005%, with the balance being Fe and unavoidable impurities.

2. The extra-thick, high-strength, and tough rack steel for offshore platforms as described in claim 1, characterized in that, The thickness of the rack steel plate is 177.8-256 mm.

3. The extra-thick, high-strength, and tough rack steel for offshore platforms according to claim 1, characterized in that, The toothed steel plate has a yield strength of ≥690MPa and a tensile strength of ≥810MPa throughout its thickness. The transverse Charpy impact energy at -60℃ at 1 / 2 of the plate thickness is ≥100J.

4. A method for producing extra-thick, high-strength, and tough rack steel for offshore platforms according to any one of claims 1-3, characterized in that: The process includes electric furnace smelting → LF refining → VD vacuum treatment → ingot casting → heating → rolling → heat treatment.

5. The method for producing extra-thick, high-strength, and tough rack steel for offshore platforms according to claim 4, characterized in that, The electric furnace smelting process requires: a steel output of ≥90 tons, the use of a normal turnover ladle within 4-8 hours, a 40-60mm initial nozzle, and a flat ladle edge. The single furnace feed rate is ≥60t, no material is added during tapping, the ladle temperature is ≥1590℃, and the ladle P is ≤0.004%.

6. The method for producing extra-thick, high-strength, and tough rack steel for offshore platforms according to claim 4, characterized in that, The VD vacuum treatment process is as follows: vacuum degree ≤ 66 Pa, pressure holding time ≥ 20 min, calcium treatment is performed immediately after vacuum destruction, calcium addition amount is 15-30 ppm, rare earth iron cerium master alloy is added 1-3 min, rare earth Ce addition amount is 20-50 ppm, and atmospheric pressure soft blowing time is 8-15 min.

7. The method for producing extra-thick, high-strength, and tough rack steel for offshore platforms according to claim 4, characterized in that, The casting process involves suspending rare earth rods inside the mold. The rare earth rods contain 99.4-99.9% rare earth Ce by mass. The distance between the suspended rare earth rods and the bottom of the steel ingot mold is ≤100mm. The amount of rare earth added is 30-150ppm.

8. The method for producing extra-thick, high-strength, and tough rack steel for offshore platforms according to claim 4, characterized in that, The rolling process is as follows: After the steel ingot is peeled and cleaned, it is placed in a slow cooling pit and kept at 200-300℃ for ≥4h. Then it is put into a heating furnace with a heating rate ≤150℃ / h and a holding temperature of 1240~1260℃. The total time the steel ingot is in the furnace is ≥12min / cm.

9. The method for producing extra-thick, high-strength, and tough rack steel for offshore platforms according to claim 4, characterized in that, The rolling process is as follows: a two-stage rolling process is adopted. The roughing rolling temperature is 1150-1200℃, with a large reduction in the first two passes, and a reduction of 40-50mm per pass, resulting in a thickness of 450-500mm. The finishing rolling temperature is 900-930℃, with a large reduction in the first two passes, and a reduction of 40-50mm per pass, resulting in a final rolling temperature of 830-880℃. After rolling, online water cooling is performed, and the temperature at which the rolling temperature returns to its original level is ≤300℃.

10. The method for producing extra-thick, high-strength, and tough rack steel for offshore platforms according to claim 4, characterized in that, The heat treatment process is as follows: the rolled steel plate is quenched and tempered in a roller hearth furnace; the quenching temperature is 890~930℃, the holding time is 2.2~2.6min / mm, the tempering temperature is 610~640℃, and the holding time is 3.5~4.5min / mm.

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