A super-thick rack steel eq70 for offshore platform and a production method thereof

Abstract

This invention discloses an extra-thick rack steel EQ70 for offshore platforms and its production method. The steel plate has a thickness of 210-254 mm and, by weight percentage, contains: C 0.10-0.12%, Si 0.10-0.20%, Mn 1.60-1.70%, P≤0.010%, S≤0.003%, Als 0.030-0.050%, Ni 2.00-2.50%, Cr 0.40-0.50%, Mo 0.50-0.60%, Nb 0.030-0.040%, B 0.0015-0.0020%, with the remainder being Fe and unavoidable impurities. Its composition satisfies Ceq≤0.76%, Pcm≤0.32%, and I≥8.30. An innovative alloy composition system, combined with clean steel smelting, water-cooled die casting, heating and temperature control processes, high-reduction rolling and post-rolling cooling processes, high-temperature rapid quenching processes, and rapid cooling processes after tempering, ultimately yields steel plates with the following properties: ReH ≥ 690 MPa, Rm 810~890 MPa, and Charpy impact energy ≥ 120 J at 1 / 4 and 1 / 2 thicknesses at -40℃ in both longitudinal and transverse directions. Flaw detection meets GB / T 2970 Level 1. This approach results in low alloy cost, high yield, and high production efficiency, giving it strong market competitiveness.

Description

Technical Field

[0001] This invention relates to the field of extra-thick plate production technology, specifically to an extra-thick rack steel EQ70 for offshore platforms and its production method. Background Technology

[0002] With the increasing global development of deep-sea oil and gas, mineral resources, and the construction of marine engineering infrastructure such as deep-sea ports, submarine tunnels, and offshore wind power, the demand for extra-thick rack steel is constantly increasing, with the maximum thickness actually used reaching 254mm. While the thickness has increased significantly, strict performance requirements remain, specifically including: first, high strength, with its properties not decreasing with increasing thickness to meet the demands of heavy loads and stresses; second, good low-temperature impact toughness to maintain structural stability and safety in complex and variable marine environments; third, excellent resistance to seawater corrosion to withstand long-term use in harsh marine environments without easily being damaged; fourth, excellent wear resistance, fatigue resistance, and impact resistance to maintain good performance stability during dynamic operation and extend service life; and fifth, good machinability and weldability to meet the manufacturing needs of complex structures in marine engineering equipment.

[0003] Currently, there are many bottlenecks in the manufacturing of extra-thick rack steel plates. First, high-quality alloy raw materials are required, which are expensive. At the same time, due to strict quality requirements, the scrap rate and loss rate during the production process are high, which significantly increases production costs. Second, the companies that can currently produce extra-thick 254mm rack steel mainly use forging (or forging + rolling) methods, which result in low production efficiency and seriously affect the construction period of downstream customers.

[0004] Patent CN107974638B discloses a method for manufacturing a 180mm thick rack steel plate from a continuously cast billet. The chemical composition of the steel plate, by mass percentage, is: C: 0.11-0.15%, Si: 0.15-0.35%, Mn: 0.95-1.25%, P: ≤0.010%, S: ≤0.002%, Cr: 0.45-0.75%, Mo: 0.4-0.6%, Ni: 1.3-2.6%, Cu: 0.2-0.4%, Al: 0.06-0.09%, V: 0.03-0.06%, Nb: ≤0.04%, N: ≤0.006%, B: 0.001-0.002%, with the balance being iron and unavoidable impurities. A 370mm thick continuously cast billet is used as the raw material for manufacturing. The greatest advantage of this invention is that it uses a low compression ratio in continuously cast billets to produce high-performance 180mm rack steel, significantly reducing production costs and delivery time. However, the level of internal flaw detection assurance is not mentioned, and its applicable thickness is limited to ≤180mm; it is insufficient for thicknesses exceeding this range.

[0005] Patent CN112226687B discloses a low-compression-ratio rack steel plate and its manufacturing method, with a thickness of 200-260 mm. The chemical composition of the steel, by mass percentage, is: C 0.12%-0.15%, Si 0.1%-0.3%, Mn 1.0%-1.3%, P≤0.02%, S≤0.01%, Als 0.01%-0.03%, Ni 2.5%-3.0%, Cr 0.5%-0.8%, Mo 0.5%-0.8%, Cu 0.2%-0.5%, Nb 0.02%-0.04%, V 0.03%-0.06%, Ti 0.005%-0.03%, B 0.001%-0.0015%, with the balance being iron and unavoidable impurities. After electroslag remelting and forging, the initial rolling temperature is 1150–1250℃, the second-stage rolling temperature is 950–1000℃, and the final rolling temperature is 850–900℃, with a rolling compression ratio ≤2. Following this, high-temperature quenching + sub-temperature quenching + tempering treatment is employed, resulting in a Charpy impact energy of ≥90J for the core at -40℃ in the finished steel plate. This patented product uses electroslag remelting and forging, which involves complex production processes and high production costs. Furthermore, the Charpy impact energy at -40℃ in the core is unstable, and the quality margin is small.

[0006] Patent CN109234643B discloses a method for rolling ultra-high strength rack steel for offshore platforms, producing steel with a thickness of 150–210 mm. The steel contains the following chemical composition by mass percentage: C 0.14%–0.21%, Si 0.15%–0.35%, Mn 0.95%–1.30%, P ≤0.035%, S ≤0.035%, Cr 1.00%–1.50%, Ni 1.50%–2.00%, Mo 0.40%–0.60%, V 0.03%–0.08%, Cu ≤0.15%, Nb 0.020%–0.040%, with the remainder being Fe and unavoidable impurities. According to embodiments of this patent, rack steel with yield strengths of 543 MPa and 566 MPa were obtained. However, the yield strength is relatively low and does not meet the current requirement of ≥690 MPa for rack steel performance. Summary of the Invention

[0007] To address the aforementioned technical deficiencies, the present invention aims to provide an extra-thick rack steel EQ70 for offshore platforms. Overcoming the shortcomings of existing technologies, this invention produces an EQ70 rack steel plate with a thickness of 210–254 mm suitable for the marine engineering field through rolling. Its strength, low-temperature impact toughness, seawater corrosion resistance, wear resistance, fatigue and impact resistance, machinability, and weldability meet the service requirements of offshore platforms.

[0008] Another object of the present invention is to provide a method for producing extra-thick rack steel EQ70 for offshore platforms.

[0009] To achieve the above objectives, the technical solution adopted in this invention is as follows: The inventors, through composition design combining low C, high Mn with Cr, Ni, Mo, Nb, and B elements, and by optimizing clean steel smelting, water-cooled die casting, heating, rolling, and heat treatment processes, ultimately determined the alloy element ratios and production processes that meet the objectives of this invention. The specific technical solution is as follows:

[0010] An extra-thick rack steel EQ70 for offshore platforms contains, by weight percentage: C 0.10–0.12%, Si 0.10–0.20%, Mn 1.60–1.70%, P≤0.010%, S≤0.003%, Als 0.030–0.050%, Ni 2.00–2.50%, Cr 0.40–0.50%, Mo 0.50–0.60%, Nb 0.030–0.040%, B 0.0015–0.0020%, with the remainder being Fe and unavoidable impurities. Its composition satisfies Ceq≤0.76%, Pcm≤0.32%, and I≥8.30.

[0011] The rationale for designing the chemical composition of the steel is as follows:

[0012] (1) Carbon (C) can increase the stability of supercooled austenite, which is beneficial for obtaining a deeper hardened layer, thereby improving the hardenability of steel. C forms solid solution and carbide structures with iron, which significantly improves the strength and hardness of steel and significantly enhances its wear resistance. However, steel with high C content is more prone to electrochemical corrosion and oxidative corrosion in humid or corrosive environments, leading to a decrease in the corrosion resistance of steel; it is also prone to cracking and embrittlement during welding, resulting in poor weldability and reduced weldability; when the C content is >0.30%, it will lead to increased lattice distortion in steel, thereby increasing the brittleness of steel, deteriorating its plasticity and toughness, and making it prone to cold brittleness at low temperatures. In this invention, the C content is controlled at 0.10% to 0.12%.

[0013] (2) Si is a ferrite-forming element that exists in solid solution form within ferrite or austenite, exhibiting strong solid solution strengthening properties. In hypoeutectoid steel, Si can shrink the austenite phase region, inhibiting ferrite nucleation and growth, shifting the C-curve to the right, thereby improving the hardenability of the steel. During tempering, Si can inhibit the nucleation, growth, and transformation of carbides, thus improving the low-temperature tempering stability of the steel. However, excessive Si can exacerbate lattice distortion in the steel, increasing its brittleness and reducing its plasticity and toughness. Furthermore, Si has a stronger affinity for O than iron, easily generating low-melting-point silicates during welding. These silicates increase the fluidity of slag and molten metal, causing spattering and affecting welding quality. In this invention, the Si content is 0.10%–0.20%.

[0014] (3) Mn in steel mainly improves strength through solid solution strengthening and grain refinement, and can also improve toughness to a certain extent. The addition of Mn can improve the hardenability of steel, making it easier for high-strength structures such as martensite to form during quenching. Considering the effect of excessive addition on weldability, the Mn content in this invention is 1.60% to 1.70%.

[0015] (4) P and S can seriously affect the mechanical properties of steel, mainly including reducing toughness, causing segregation, promoting cold brittleness and hot brittleness, reducing impact toughness and fatigue strength, and causing welding cracks. In this invention, the P and S content is controlled to P≤0.010% and S≤0.003%.

[0016] (5) Al is a strong deoxidizing and nitrogen-fixing agent, which can effectively remove oxygen and nitrogen impurities from steel and improve the purity of steel. It can refine the intrinsic grain of steel, increase the strength and toughness of steel, and improve the mechanical properties of steel. However, steel with high Al content is prone to defects such as cracks during welding, and Al oxide and nitride inclusions are easily formed during the welding process. The Al content in this invention is 0.030% to 0.050%.

[0017] (6) Ni can dissolve infinitely in iron, significantly expanding the austenite region and increasing its stability. This stability helps maintain the austenitic structure even at slower cooling rates, thus providing the possibility for the subsequent formation of high-strength martensite. Simultaneously, Ni lowers the critical transformation temperature and critical cooling rate of steel, making it easier for steel to form high-strength structures such as martensite during quenching. This means that steel can form high-hardness, high-strength structures even at slower cooling rates, which is particularly important for extra-thick steel plates, as they often struggle to achieve uniform quenching results at faster cooling rates. Cr-free steel with a Ni content of 3.5% can be air-quenched, and Cr steel with a Ni content of 8% can also transform into martensite at very low cooling rates. Since Ni is a relatively scarce resource, the Ni content in this invention is 2.00%–2.50%.

[0018] (7) The main role of Cr in quenched and tempered steel is to improve hardenability, making it easier for the steel to form high-strength structures such as martensite during quenching, resulting in better comprehensive mechanical properties after quenching and tempering. The dense and stable passivation film formed by Cr on the steel surface can improve the corrosion resistance of the steel. When used in combination with elements such as Ni and Mn at an appropriate Cr content, it can improve the toughness of the steel by forming a stable austenitic phase. When the Cr content is too high, it will promote the precipitation of brittle phases (such as σ phase) in the steel. These brittle phases are prone to becoming crack initiation sites under stress, leading to a sharp decrease in the toughness of the steel. In addition, it may increase the temper brittleness tendency of the steel, further reducing the toughness of the steel. The Cr content in this invention is 0.40% to 0.50%.

[0019] (8) Mo can significantly improve the hardenability of steel, enabling the steel to obtain a deeper hardened layer after quenching, thereby improving its overall performance. Mo can form a protective layer on the steel surface, acting as an antioxidant and rust inhibitor, effectively preventing the corrosion of steel by corrosive media such as oxygen, chlorine, and sulfur, thereby improving the corrosion resistance of the steel. Mo can inhibit the brittleness phenomenon that occurs in steel during tempering, allowing the steel to maintain high strength while having good toughness. When the content of Mo in steel is too high, it may lead to an increase in the brittleness of the steel, thereby reducing its toughness and impact strength. Mo may promote the decarburization of steel during heating, which will affect the microstructure and properties of the steel. Since Mo is a relatively scarce resource, the Mo content in this invention is 0.50% to 0.60%.

[0020] (9) Nb plays several important roles in extra-thick quenched and tempered steel: Nb in the solid solution state segregates towards grain boundaries, generating a solute dragging effect, reducing the migration rate of grain boundaries, and thus hindering the growth of austenite grains; precipitated Nb forms fine precipitate particles (such as NbC, (Nb,Ti,V)(C,N), etc.), which can pin grain boundaries and dislocations, hindering grain growth and thus refining the grains. In addition, Nb can improve the grain boundary stability of steel, reduce the brittleness of the heat-affected zone, thereby improving the strength and toughness of welded joints; stable compounds formed with other elements in steel can form a robust protective film on the steel surface, reducing surface wear and improving the wear resistance of the steel; and stable compounds such as iron nitride formed with iron in steel enhance the corrosion resistance of the steel. The Nb content in this invention is 0.030%–0.040%.

[0021] (10) Boron can significantly improve the hardenability of steel, thus saving some of the rarer and more expensive metallic elements, such as nickel, chromium, and molybdenum. However, it should be noted that boron tends to promote temper brittleness and increase the susceptibility to welding cracks. In this invention, the boron content is controlled at 0.0015% to 0.0020%.

[0022] The production method of extra-thick rack steel EQ70 for offshore platforms includes the following steps:

[0023] 1) Clean steel smelting: To avoid slag discharge during the casting process, which could lead to defects in the inspection due to inclusions, the steel output from the converter should be 8-10 tons greater than the total ingot weight of a single furnace; when tapping steel from the converter, C ≥ 0.06% and P ≤ 0.015%; LF refining temperature ≥ 1550℃, LF refining time ≥ 90 min, and white slag holding time ≥ 25 min; slag must be removed before entering VD, and the amount of slag removed should be controlled to be more than half of the total slag volume; the vacuum degree in the VD ladle should be ≤ 45 Pa, and the VD holding time should be ≥ 18 min;

[0024] Molten steel is smelted through a converter, LF furnace, and VD furnace to reduce the content of P, S and non-metallic inclusions. The composition is adjusted in the LF stage to obtain clean molten steel with the above-mentioned components.

[0025] 2) Water-cooled mold casting: Select steel ingots with a thickness greater than twice the thickness of the finished steel plate, and use argon gas protection during casting to reduce the degree of oxidation of the molten steel. The casting temperature is 1560℃~1570℃, and the casting speed of a single steel ingot is 1.5~1.9t / min. To accelerate solidification, water cooling is applied to the bottom and surrounding cavities of the ingot mold during both casting and solidification processes. The temperature difference between the inlet and outlet water is ≤30℃, and the flow rate is ≥300m³. 3 / h; Due to the high alloy content and large internal stress of this steel, in order to avoid cracking, the total cooling time is 6 to 10 hours, the demolding ingot temperature is 450 to 600℃, and hot cleaning is adopted. After the cleaning is qualified, it is loaded into the furnace.

[0026] To address the problems of porosity, severe segregation, and coarsening of the solidification structure in steel ingots cast using conventional cast iron molds, an innovative production method was adopted: water-cooled molds are used for casting, which accelerate cooling during solidification. This accelerated cooling helps suppress the growth of dendritic crystals, allowing more molten steel to solidify as equiaxed crystals. The growth of equiaxed crystals helps fill the voids between dendrites, reducing porosity. Furthermore, rapid cooling prevents solute elements in the molten steel from diffusing and accumulating, thus reducing segregation and porosity, while also refining the as-cast grains.

[0027] 3) Heating: When loading the furnace, the steel ingot temperature is 300℃~500℃, the furnace temperature is 350℃~650℃, the initial heating rate is ≤30℃ / h, and when the ingot temperature reaches 1160℃~1180℃, the temperature is raised to 1200~1240℃ at the maximum rate. At the same time, the holding time is started when the ingot temperature reaches 1160℃, and the holding time is controlled at 6~8min / cm.

[0028] To prevent cracking of steel ingots due to excessive thermal stress during heating, a slow heating rate is adopted. By extending the low-temperature stage before heat preservation, the temperature gradient along the thickness of the ingot is reduced, preventing cracking under the combined effects of temperature stress and microstructural stress. To avoid improper heat preservation and coarsening of the internal structure, an innovative process is adopted for the heat preservation stage, which involves pre-calculating the heat preservation time and continuing to heat until the heat preservation stage is reached.

[0029] 4) Controlled rolling and cooling: The initial rolling temperature is ≥950℃, and the final rolling temperature is not limited. The reduction per pass is ≥60mm. To control the flatness of the plate, a reduction allowance for the flattening pass is reserved before the end of rolling. The number of flattening passes is ≥3, and the reduction per pass is 5mm~10mm. Online water cooling is performed after rolling. The reddening temperature is ≤450℃, and the cooling rate is not limited.

[0030] By selecting high-quality water-cooled steel ingots as raw materials and employing a high-pressure rolling and post-rolling cooling process, steel plates with low compression ratios and excellent flaw detection quality can be produced. During rolling, the high-pressure process significantly increases the deformation of extra-thick plates, promoting plastic deformation and recrystallization within the metal. This helps eliminate or reduce defects in the original ingot, such as porosity and cracks, and facilitates hydrogen diffusion and removal, reducing hydrogen-induced crack formation, thereby improving the flaw detection quality of extra-thick plates. The high-pressure process introduces greater plastic deformation in the core and quarter-thickness regions of the extra-thick plate, promoting grain refinement and increasing dislocation density in these areas. This leads to the formation of more dislocation entanglements and dislocation cells. Simultaneously, the distribution and morphology of precipitates (such as carbides and nitrides) are optimized, and the uniformity of the microstructure in the thickness direction is improved. All of these factors contribute to enhancing the strength, hardness, and corrosion resistance of extra-thick plates. Post-rolling cooling can reduce the grain size and control the phase transformation process in extra-thick plates, thereby improving their mechanical properties, such as tensile strength, yield strength, and impact toughness. The specific production methods adopted are as follows:

[0031] 5) Heat treatment: It includes two process stages: quenching and tempering. First, high-temperature rapid quenching is used, with a quenching temperature of 900-940℃, a holding time of 2.0-2.5 min / mm, a water pressure of quenching machine ≥0.8MPa, and a quenching cooling rate ≥8℃ / min. After quenching to room temperature, tempering is performed, with a tempering temperature of 600-650℃ and a holding time of 3.0-3.5 min / mm. After being taken out of the furnace, it is water-cooled in a quenching water tank to below 400℃, and then air-cooled to room temperature.

[0032] The biggest challenge in heat treatment of extra-thick quenched and tempered steel is controlling the uniformity of the microstructure across its entire thickness and achieving the desired microstructure type. To achieve this goal, this patent employs a novel heat treatment process involving high-temperature quenching, tempering, and accelerated cooling. By rationally controlling the temperature and cooling rate during high-temperature quenching, the austenite phase in the extra-thick quenched and tempered steel undergoes a phase transformation under rapid cooling, transforming into martensite. This transformation involves atomic rearrangement and lattice reconstruction, with precipitates pinning grain boundaries and dislocations, resulting in a significant increase in the material's hardness and strength. Simultaneously, the type, quantity, and distribution of precipitates also affect the material's corrosion resistance. During tempering, a certain amount of carbides and other precipitates are formed inside the steel. Accelerated cooling after tempering stabilizes these precipitates, preventing their redissolution or adverse transformations, thus maintaining the steel's performance stability. It also reduces or avoids temper brittleness, thereby preserving the steel's toughness. The specific production method is as follows:

[0033] Compared with the prior art, the beneficial effects of the present invention include:

[0034] 1) The innovative alloy composition system of this invention reduces the amount of Ni, Mo, Cr and other alloys. Combined with the key technology of water-cooled die casting, it significantly increases the Mn content in the steel, improves the hardenability of the steel, and combined with the heat treatment process, the thickness section structure is a uniform tempered sorbite and tempered bainite structure, which can guarantee the steel plate performance: ReH≥690MPa, Rm810~890MPa, and the single value of Charpy impact energy at 1 / 4 and 1 / 2 of the thickness at -40℃ in the longitudinal and transverse directions is ≥120J.

[0035] 2) By combining clean steel smelting, water-cooled ingot casting, heating and temperature control processes, high-reduction rolling and post-rolling cooling processes, high-temperature rapid quenching processes, and rapid cooling processes after tempering, the primary grain structure in the as-cast state is refined. The heating temperature is stable and uniform, the rolling process fully deforms, and micro-defects in the full-thickness section are pressed together, thus ensuring internal quality. This reduces the lower limit of the compression ratio for producing extra-thick rack steel plates, requiring only a ratio of 2 or higher. It ensures that the steel plate meets the GB / T2970 Level 1 flaw detection quality standard. Attached image description:

[0036] Figure 1 This is a schematic diagram of grain size detection at the surface, 1 / 4 thickness, and 1 / 2 thickness of the steel plate in Example 3.

[0037] Figure 2 This is a schematic diagram of sulfur mark detection on steel plates in Example 3, where no segregation or sulfide aggregation was found.

[0038] Figure 3 This is a schematic diagram of the steel plate tested by cold acid etching in Example 3. After acid etching, the steel plate has no visible defects such as cracks, white spots, or inclusions.

[0039] Implementation example:

[0040] The following embodiments are implemented based on the technical solution of the present invention, providing detailed implementation methods and specific operation processes. However, the scope of protection of the present invention is not limited to the following embodiments. Unless otherwise specified, the methods used in the following embodiments are conventional methods.

[0041] The thickness of the product produced in Example 1 of this invention is 210mm, the thickness produced in Example 2 is 230mm, and the thickness produced in Example 3 is 254mm.

[0042] All examples were produced using water-cooled steel ingots with a thickness of 510 mm.

[0043] All embodiments adopted the following process flow: converter → LF → VD → water-cooled ingot casting → heating → rolling → post-rolling water cooling → quenching → tempering, wherein

[0044] In clean steelmaking, to avoid slag discharge during casting, which could lead to defects in the inspection due to inclusions, the converter tapping volume should be 8-10 tons greater than the total ingot weight of a single furnace; when tapping from the converter, C ≥ 0.06%, P ≤ 0.015%, LF refining temperature ≥ 1550℃, LF refining time ≥ 90 min, white slag holding time ≥ 25 min, slag must be dumped before entering VD, and the amount of slag dumped should be controlled to be more than half of the total slag volume; the vacuum degree in the VD ladle should be ≤ 45 Pa, and the VD holding time should be ≥ 18 min.

[0045] In water-cooled mold casting, steel ingots with a thickness greater than twice the thickness of the finished steel plate are selected. The casting temperature is 1560℃~1570℃, and the casting speed of a single steel ingot is 1.5~1.9t / min. To accelerate solidification, water is circulated to cool the bottom and surrounding cavities of the ingot mold during both casting and solidification processes. The inlet and outlet water temperature difference is ≤30℃, and the flow rate is ≥300m³ / min. 3 / h, because this type of steel has a high alloy content and large internal stress, in order to avoid cracking, the total cooling time is 6 to 10 hours, the demolding ingot temperature is 450 to 600℃, and heated cleaning is adopted. After the cleaning is qualified, it is loaded into the furnace.

[0046] During the heating process, the ingot temperature is 300℃~500℃ when the steel is loaded into the furnace, the furnace temperature is 350℃~650℃, the initial heating rate is ≤30℃ / h, and when the ingot temperature reaches 1160℃~1180℃, the temperature is increased to 1200~1240℃ at the maximum rate. At the same time, the holding time starts when the ingot temperature reaches 1160℃, and the holding time is controlled at 6~8min / cm.

[0047] In controlled rolling and cooling, the initial rolling temperature is ≥950℃, the final rolling temperature is not limited, the reduction per pass is ≥60mm, a leveling reduction allowance is reserved before the end of rolling, the number of leveling passes is ≥3, the reduction per pass is 5mm~10mm, online water cooling is performed after rolling, and the reddening temperature is ≤450℃.

[0048] In the heat treatment process, which includes quenching and tempering, high-temperature rapid quenching is first used, with a quenching temperature of 900-940℃, a holding time of 2.0-2.5 min / mm, a water pressure of ≥0.8MPa in the quenching machine, and a quenching cooling rate of ≥8℃ / min. After quenching to room temperature, tempering is performed, with a tempering temperature of 600-650℃ and a holding time of 3.0-3.5 min / mm. After being taken out of the furnace, the furnace is cooled to below 400℃ in a quenching water bath, and then air-cooled to room temperature.

[0049] The chemical composition of the steel in this embodiment is shown in Table 1; the water-cooled die casting process of the steel ingot in this embodiment is shown in Table 2; the heating process of the steel ingot in this embodiment is shown in Table 3; the rolling and post-rolling cooling process in this embodiment is shown in Table 4; the heat treatment process of the steel plate in this embodiment is shown in Table 5; and the mechanical properties of the steel plate in this embodiment are shown in Table 6.

[0050] Table 1. Chemical composition (wt%) of the steel in the embodiments of the present invention.

[0051] Example C Si Mn P S Als Cr Ni Mo Nb B Ceq Pcm I 1 0.11 0.13 1.61 0.006 0.001 0.048 0.46 2.13 0.57 0.037 0.0017 0.726 0.300 9.00 2 0.11 0.18 1.62 0.007 0.001 0.035 0.45 2.35 0.53 0.038 0.0019 0.733 0.305 9.90 3 0.12 0.12 1.67 0.006 0.001 0.035 0.46 2.30 0.58 0.032 0.0019 0.760 0.319 9.63

[0052] Table 2. Water-cooled mold casting process of steel ingots in embodiments of the present invention.

[0053]

[0054]

[0055] Table 3 Steel ingot heating process in embodiments of the present invention

[0056]

[0057] Table 4 Rolling and post-rolling cooling processes of embodiments of the present invention

[0058]

[0059] Table 5. Heat treatment process of steel plate in embodiments of the present invention.

[0060]

[0061] Table 6 Mechanical properties of steel plates in embodiments of the present invention

[0062]

[0063]

[0064] All finished steel plates were inspected and found to meet the GB / T 2970 Class 1 standard.

Claims

1. A method for producing extra-thick rack steel EQ70 for offshore platforms, characterized in that, The steel has a thickness of 210–254 mm and, by weight percentage, contains: C 0.10–0.12%, Si 0.10–0.20%, Mn 1.60–1.70%, P≤0.010%, S≤0.003%, Als 0.030–0.050%, Ni 2.00–2.50%, Cr 0.40–0.50%, Mo 0.50–0.60%, Nb 0.030–0.040%, B 0.0015–0.0020%, with the remainder being Fe and unavoidable impurities. Its composition satisfies Ceq≤0.76%, Pcm≤0.32%, and I≥8.30 J. The steel ReH ≥690MPa Rm 810~890MPa, the single value of Charpy impact energy in the longitudinal and transverse directions at 1 / 4 and 1 / 2 thickness of the steel plate at -40℃ is ≥120J; The production method of the aforementioned extra-thick EQ70 rack steel for offshore platforms includes clean steel smelting, water-cooled mold casting, heating, controlled rolling and cooling, and heat treatment, specifically as follows: 1) Clean steel smelting: To avoid slag discharge during the casting process, which may lead to non-compliance in flaw detection due to inclusions, the steel output from the converter should be 8-10 tons greater than the total ingot weight of a single furnace; when tapping steel from the converter, C ≥ 0.06%, P ≤ 0.015%, LF refining temperature ≥ 1550℃, LF refining time ≥ 90 min, white slag holding time ≥ 25 min, slag must be dumped before entering VD, and the amount of slag dumped should be controlled to more than half of the total slag volume, the vacuum degree in the VD ladle ≤ 45 Pa, and the VD holding time ≥ 18 min; 2) Water-cooled mold casting: Select steel ingots with a thickness greater than twice the thickness of the finished steel plate, casting temperature 1560℃~1570℃, single steel ingot casting speed 1.5~1.9t / min. To accelerate solidification, water is circulated to cool the bottom and surrounding cavity of the ingot mold during casting and solidification. The temperature difference between the inlet and outlet water is ≤30℃ and the flow rate is ≥300m³ / h. Because this steel has a high alloy content and large internal stress, in order to avoid cracking, the total cooling time is 6~10 hours. The demolding ingot temperature is 450~600℃. Warm cleaning is adopted. After cleaning is qualified, it is loaded into the furnace. 3) Heating: When loading the furnace, the steel ingot temperature is 300℃~500℃, the furnace temperature is 350℃~650℃, the initial heating rate is ≤30℃ / h, and when the ingot temperature reaches 1160℃~1180℃, the temperature is raised to 1200~1240℃ at the maximum rate. At the same time, the holding time starts when the ingot temperature reaches 1160℃, and the holding time is controlled at 6~8min / cm. 4) Controlled rolling and cooling: The initial rolling temperature is ≥950℃, the final rolling temperature is not limited, the reduction per pass is ≥60mm, and a reduction allowance for the leveling pass is reserved before the end of rolling. The number of leveling passes is ≥3, the reduction per pass is 5mm~10mm, online water cooling is performed after rolling, and the reddening temperature is ≤450℃. 5) Heat treatment: including quenching and tempering. First, high-temperature rapid quenching is used, with a quenching temperature of 900-940℃ and a holding time of 2.0-2.5 min / mm. The water pressure of the quenching machine is ≥0.8MPa and the quenching cooling rate is ≥8℃ / min. After quenching to room temperature, tempering is performed at a tempering temperature of 600-650℃ and a holding time of 3.0-3.5 min / mm. After being taken out of the furnace, the furnace is cooled to below 400℃ in a quenching water tank, and then air-cooled to room temperature.

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