A casting method for ultra-large ultra-supercritical P92 round steel

Abstract

This invention provides a method for casting ultra-large ultra-supercritical P92 round steel. By controlling the casting, straightening, and annealing processes, the method produces ultra-large P92 round steel with no noticeable surface cracks and internal cracks meeting performance requirements of less than 2.5. This method addresses the problem that existing P92 round steel production methods are limited in size and cannot meet the growing market demand for large-size P92 round steel (Ø800-Ø1000 mm).

Description

Technical Field

[0001] The invention belongs to the field of steel production, and in particular relates to a method for casting ultra-large-size ultra-supercritical P92 round steel. Background Art

[0002] P92 is a steel used for ultra-supercritical high-pressure boiler tube billets. It has excellent properties such as high strength, high temperature resistance, oxidation resistance and corrosion resistance. It is mainly used in various major fields such as petroleum, power generation, ocean, and chemical industry.

[0003] In recent years, market demand for high-quality, large-section continuously cast billets has been growing, especially for high-value-added products like P92, which have increasingly stringent specifications. Large-size continuously cast round billets are one of the raw materials for seamless steel pipe production. This high-alloy steel, especially P92, places high demands on continuous casting process control technology and is difficult to manufacture. The large solidification shrinkage of the molten steel makes it prone to quality defects such as surface and center cracks during the straightening process. When producing ultra-large P92 round steel (Ø800-Ø1000mm), the large cross-section of the continuously cast billets leads to slow cooling and rapid surface temperature recovery. Improper cooling and straightening control can result in center cracks of Grade 2.5 or higher and surface straightening cracks, failing to meet customer requirements for subsequent rolled tube production.

[0004] Patent publication number CN 116083781 A, published on May 9, 2023, discloses a method for producing large-scale continuous casting round billets of P92 heat-resistant steel without high-temperature ferrite. The technical solution involves controlling the tundish superheat at 25-35°C, the casting speed at 0.22-0.26 m / min, the mold cooling water flow at 3600-3800 L / min, the secondary cooling water ratio at 0.11-0.13 L / kg, and the use of three-stage composite electromagnetic stirring. By controlling the composition and continuous casting process parameters, the method ensures that high-temperature ferrite is completely eliminated in large-scale continuous casting round billets of P92 heat-resistant steel. However, the method is limited to a diameter of 690 mm and is not suitable for larger sizes.

[0005] The patent with publication number CN 115044823 A published on September 13, 2022 discloses a production process for continuous casting large round billets of ultra-supercritical high-pressure boiler steel P92. The molten steel is continuously cast, heated to ≥550℃ at a rate of ≤80℃ / h, and slowly cooled and annealed to obtain a continuously cast large round billet with a hardness of less than 230HBW after annealing. The superheat in the continuous casting process is between 30-45℃. Two-stage water cooling is used in combination with crystallizer electromagnetic stirring, casting strand stirring and end electromagnetic stirring to control the central crack. However, the maximum specification can only reach 700mm, and the upper limit of the central crack length is 90mm. It is not suitable for larger specification production and cannot meet higher requirements. Summary of the Invention

[0006] The purpose of the present invention is to provide a casting method for ultra-large-sized ultra-supercritical P92 round steel, so as to solve the problem that the production specifications of P92 round steel under existing technical conditions are small and cannot meet the market's growing demand for large-sized P92 round steel of Ф800-Ф1000mm. The present invention can effectively avoid quality defects such as surface cracks and internal cracks in Ф800-Ф1000mm round steel, and meet the various performance requirements of customers.

[0007] The specific technical solutions of the present invention are as follows:

[0008] A method for casting ultra-large ultra-supercritical P92 round steel comprises the following steps:

[0009] 1) Protective casting is used throughout the process;

[0010] 2) Straightening;

[0011] 3) Annealing.

[0012] The diameter of the super-large ultra-supercritical P92 round steel is Ф800-Ф1000mm;

[0013] Step 1) specifically includes: argon sealing protection of the ladle slide, argon flow rate ≥ 15NL / min; argon blowing protection of the ladle long nozzle, argon flow rate ≥ 60NL / min; argon blowing protection of the tundish, argon flow rate ≥

[0014] 1500NL / min, normal pouring argon flow rate ≥1000NL / min;

[0015] Step 1) also includes: covering agent and protective slag need to be added in time to ensure black slag operation;

[0016] The mass percentages and indicators of the main chemical components of the mold slag used are: 0% ≤ H2O ≤ 0.5%, 0.99 ≤ R ≤ 1.19, 1040°C ≤ RD (melting point) ≤ 1120°C, 0.05 ≤ ND (viscosity) ≤ 0.25, 13% ≤ TC ≤ 18%, 25% ≤ SiO2 ≤ 33.0%, 23% ≤ CaO ≤ 31%;

[0017] In step 1), the first cooling water volume is controlled at 5500L / min to 6800L / min; the second cooling adopts mist cooling, the water volume is 5 to 45L / min, and the gas volume is 0 to 130Nm 3 / h;

[0018] In step 1), the pulling speed is controlled to be 0.14 m / min<0.18 m / min;

[0019] In step 1), the superheat is 30-55°C for single furnace opening and 20-50°C for continuous casting;

[0020] In step 1), a three-stage electromagnetic stirring is adopted, wherein the M-EMS has a frequency of 1.5 Hz, a current of 300-450 A, and is continuous; the S-EMS has a frequency of 4.0-6.0 Hz, a current of 180-240 A, and is alternating, with an interval time of 30s-5s-30s; and the F-EMS has a frequency of 4.0-6.0 Hz, a current of 1000-1600 A, and is continuous;

[0021] In step 2), the straightening and straightening machine frame pressure distribution is distributed according to the set straightening range and straightening force, the head and tail billet status is monitored in real time, and online adjustments are made according to the on-site conditions; the specific control is as follows:

[0022] The drawing speed for producing Ф800~Ф1000mmP92 round steel is 0.14m / min<drawing speed≤0.18m / min, and the corresponding hot billet pressure range is 24~95t;

[0023] The head billet undergoes two straightening intervals. The length of the first interval is 0 to 700 mm, corresponding to a straightening coefficient of 2.0 to 10.0, and the length of the second interval is 700 mm to 1800 mm, corresponding to a straightening coefficient of 1.0 to 5.0.

[0024] When the billet exceeds the length of the second section, it begins to enter the normal hot billet pressure (24-95t). There are four straightening sections for tail billet straightening. Calculated based on the distance between the tail billet position and the frame group position, the first section is 4000mm-6000mm, with a straightening coefficient of 1.0-2.5; the second section is 2800mm-4000mm, with a straightening coefficient of 1.0-3.5; the third section is 2300mm-2800mm, with a straightening coefficient of 3.0-5.5; and the fourth section is 1500mm-2300mm, with a straightening coefficient of 5.0-7.5.

[0025] Furthermore, when producing ultra-large ultra-supercritical P92 round steel, the flame cutting machine needs to be equipped with iron powder cutting to ensure smooth cutting to length;

[0026] In step 3), the annealing is specifically as follows: the ultra-large size ultra-supercritical P92 round steel is hot, with a surface temperature of ≥500°C, and is placed in a slow cooling pit with a heating function, and the slow cooling pit is maintained at a pit temperature of ≥400°C before the billet is placed in the pit; the specific control is as follows:

[0027] For round billets with a diameter of 800mm, the pit temperature is ≥400℃, the heating rate is ≤40℃ / h, the temperature is raised to 640±10℃, and the temperature is kept at this temperature for 13.0h; then the temperature is raised to 780±10℃ at a heating rate ≤80℃ / h, and the temperature is kept at this temperature for 68h, and the billet is cooled in the pit at a cooling rate ≤30℃ / h; the billet is removed from the furnace when the temperature is lower than 200℃ during the cooling process;

[0028] For round billets with a diameter of 900mm, the pit temperature is ≥400℃, the heating rate is ≤40℃ / h, the temperature is raised to 640±10℃, and the temperature is kept at this temperature for 14.5h; then the temperature is raised to 780±10℃ at a heating rate ≤80℃ / h, and the temperature is kept at this temperature for 72h, and the billet is cooled in the pit at a cooling rate ≤30℃ / h; the billet is removed from the furnace when the temperature is lower than 200℃ during the cooling process;

[0029] For round billets with a diameter of 1000mm, the pit temperature is ≥400℃, the heating rate is ≤40℃ / h, the temperature is raised to 640±10℃, and kept warm for 16.0h; then the temperature is raised to 780±10℃ at a heating rate ≤80℃ / h, kept warm for 76h, and cooled in the pit at a cooling rate ≤30℃ / h; it is removed from the furnace when the temperature is below 200℃ while cooling in the pit.

[0030] At present, the design specifications of large-size round billets in the market are mostly Ф600, 700, 800, 900, 1000 and other sizes. The present invention designs annealing process parameters for large sizes of Ф800, 900 and 1000.

[0031] The ultra-large size ultra-supercritical P92 round steel comprises the following components by mass percentage: C: 0.07-0.13%, Si: 0.20-0.40%, Mn: 0.30-0.50%, P≤0.015%, S: ≤0.005%, Cr: 8.50-8.9%, V: 0.15-0.25%, Al: ≤0.015%, Ni: 0.15-0.18%, Nb: 0.04-0.09%, Mo: 0.30-0.60%, W: 1.55-1.75%, Cu: ≤0.10%, Ti:

[0032] ≤0.008%, B: 0.001~0.006%, H: ≤2.0ppm, O: ≤40ppm, N: 0.040~0.070%, and the balance is Fe and impurity elements.

[0033] P92 has a high N content of 400-700 ppm, and its alloy content is 8.5%-8.9% Cr and 1.55%-1.75% W. Therefore, its hardness is very high. In particular, large-sized P92 round steel with a diameter of 700 mm or more can be straightened smoothly during the casting process without causing quality defects such as straightening cracks and center cracks. This invention solves the problem. The specific innovative points are described as follows:

[0034] First, regarding hot billet pressure, multi-stand arc continuous casting machines must consider the number and location of straightening units. For P92 round steel with diameters of 800 to 1000 mm, the drawing speed should be 0.14 m / min < 0.18 m / min, corresponding to a hot billet pressure range of 24 to 95 tons. Due to the rapid cooling and solidification rates and large shrinkage ratios of the head and tail billets, as well as the extra-large, high-alloy P92 steel sizes of 800 to 1000 mm, straightening these billets is more challenging. Therefore, the straightening ranges and straightening pressure coefficients for the head and tail billets need to be adjusted as follows: the head billet undergoes two straightening sections: the first section is 0 to 700 mm long, corresponding to a straightening coefficient of 2.0 to 10.0; the second section is 700 to 1800 mm long, corresponding to a straightening coefficient of 1.0 to 5.0. Once the billet exceeds the second section length, normal hot billet pressure is applied. The tail billet straightening system has four straightening zones. Calculated based on the distance between the tail billet and the stand assembly, the first zone is 4000mm-6000mm with a straightening coefficient of 1.0-2.5; the second zone is 2800mm-4000mm with a straightening coefficient of 1.0-3.5; the third zone is 2300mm-2800mm with a straightening coefficient of 3.0-5.5; and the fourth zone is 1500mm-2300mm with a straightening coefficient of 5.0-7.5. An annealing process for ultra-large P92 round steel has also been developed to reduce thermal stress in the structure and effectively reduce the length of the central crack in the billet, meeting the performance requirements of P92 steel for ultra-supercritical boiler tubes.

[0035] Compared with the prior art, the present invention controls the casting process, annealing and straightening process, and produces super-large-size P92 round steel with no obvious surface cracks, and internal cracks that meet performance requirements below level 2.5. BRIEF DESCRIPTION OF THE DRAWINGS

[0036] Figure 1 The surface of the round billet of Example 1;

[0037] Figure 2 The surface of the round billet of Example 2;

[0038] Figure 3 The surface of the round billet of Example 3;

[0039] Figure 4 The surface of the round billet of Comparative Example 1;

[0040] Figure 5 The surface of the round billet of Comparative Example 2;

[0041] Figure 6 This is the surface of the round billet of Comparative Example 3;

[0042] Figure 7 This is the surface of the round billet of comparative example 4. DETAILED DESCRIPTION

[0043] To make the objectives, technical solutions, and advantages of the embodiments of the present invention more clear, the technical solutions in the embodiments of the present invention will be clearly and completely described below in conjunction with the embodiments of the present invention. Obviously, the described embodiments are only part of the embodiments of the present invention, not all of the embodiments. All other embodiments obtained by ordinary technicians in this field based on the embodiments of the present invention without making any creative efforts shall fall within the scope of protection of the present invention.

[0044] The inventors found that the difficulties in continuous casting of P92 round steel with a specification of 800mm or above mainly include:

[0045] 1. Straightening of the head and tail billets during the casting process. For the arc-shaped continuous casting machines, the mainstream continuous casting models at home and abroad, the molten steel needs to go through the process of vibration, cooling, and straightening from the arc section to the horizontal section to become a billet. Since the cooling and solidification speed of the head and tail billets is fast, their hardness is higher than that of the intermediate billets, and the required straightening force is large. In particular, steel grades like P92 contain high amounts of alloys such as Cr, Mo, Ni, and W, and a high nitrogen content, resulting in extremely high hardness. For arc-shaped continuous casting machines, the smooth straightening of the head and tail billets is one of the key factors for the stable production of P92. Moreover, as the specifications of the continuous casting billets increase, the required straightening force also increases, which can easily lead to quality defects such as surface cracks and center cracks during the straightening process. Currently, the maximum specification of P92 round steel produced is only 800mm. To produce larger specifications of P92 round steel, the torque balance between the straightening machines is crucial. A reasonable straightening model must be developed so that large-size P92 round steel can be straightened smoothly at the head and tail billets. At the same time, the large straightening force will not cause compression cracks or even surface defects.

[0046] 2. Annealing Process Exploration: The presence of a temperature gradient on the billet surface (core) generates thermal stress within the billet. High-alloy steels experience very high core thermal stress at high temperatures. If the annealing process is not properly designed, this thermal stress cannot be properly released, potentially leading to surface cracks along the surface or center cracks and intermediate cracks along the core. This is especially true for large-sized, high-alloy steels like P92 round steel (over 800mm), which experience slow cooling rates, high core temperatures, and excessive localized temperature recovery. Similar annealing processes have not been reported, and there are no reference cases for data such as the temperature drop before and after annealing, or the maximum temperature difference. If the annealing process is not properly selected, the billet thermal stress may exceed the critical strength, resulting in crack defects in the product. Therefore, a reasonable and effective annealing plan must be developed.

[0047] 3. Selection of combined electromagnetic stirring parameters: Single electromagnetic stirring, such as mold stirring, strand stirring, and end stirring, can effectively improve the quality of one or more surfaces and the center of the continuous casting product, but three-stage combined electromagnetic stirring can solve the overall performance of P92 round steel. There are no reference cases for the frequency, current, and other parameters of three-stage electromagnetic stirring for specifications above 800mm. Reasonable parameter settings can effectively increase the proportion of equiaxed crystal areas and improve center segregation and center cracks. If the parameter settings are not consistent, negative effects such as white bright bands and V-shaped segregation will occur at low magnification in the casting, seriously affecting the quality of the casting product. Due to the wide range of equipment parameters designed by electromagnetic stirring manufacturers at home and abroad, it takes a long time to explore the appropriate combined electric stirring parameters.

[0048] Molten steel superheat, drawing speed, cooling intensity, electric stirring, straightening stress, protective casting, annealing time, etc. will affect the quality of continuous casting billets and are complementary to product quality. Therefore, it is necessary to reasonably set the casting process parameters.

[0049] The present invention controls the superheat to be 30-55°C for single furnace firing and 20-50°C for continuous casting; if it is too high, the shell of the mold blank will be thin and steel will be easily leaked; the center will be loose and shrinkage will be serious; the refractory material will be seriously corroded and the inclusions in the steel will increase; if it is too low, the central equiaxed crystal zone will be wide, segregation will be reduced, and the floating of inclusions in the molten steel will be affected, and the freezing of the water outlet of the tundish will be affected.

[0050] The present invention controls the pulling speed, 0.14m / min<pulling speed≤0.18m / min. If the pulling speed is too high, the looseness and shrinkage will increase; the center segregation will be serious; the inclusions of steel grades will increase; if the pulling speed is too low, the center mass will increase; the production efficiency will be low; the straightening force will be greater, which may cause straightening cracks;

[0051] The present invention controls the cooling intensity. The first cooling water volume is 5500L / min~6800L / min. The second cooling adopts mist cooling with a water volume of 5~45L / min and a gas volume of 0~130Nm 3 / h; excessive cooling of the crystallizer will cause longitudinal cracks on the surface of the ingot; high cooling intensity in the secondary cooling zone will cause crack propagation and even the risk of steel leakage; weak cooling of the crystallizer can improve cracks, but excessive cooling may also cause copper tube burnout, forcing casting to be stopped.

[0052] The present invention controls straightening stress, adjusting the straightening intervals and straightening pressure coefficients for the lead and tail billets as follows: The lead billet undergoes two straightening intervals: the first interval is 0 to 700 mm long, with a corresponding straightening coefficient of 2.0 to 10.0; the second interval is 700 mm to 1800 mm long, with a corresponding straightening coefficient of 1.0 to 5.0. Once the billet exceeds the second interval, it transitions to normal hot billet pressure. There are 4 straightening intervals for tail billet straightening. Calculated based on the distance between the tail billet position and the frame group position, the first interval is 4000mm-6000mm, with a straightening coefficient of 1.0-2.5; the second interval is 2800mm-4000mm, with a straightening coefficient of 1.0-3.5; the third interval is 2300mm-2800mm, with a straightening coefficient of 3.0-5.5; the fourth interval is 1500mm-2300mm, with a straightening coefficient of 5.0-7.5; if the stress is too high, straightening will produce surface or core cracks; if the stress is too low, the straightening effect cannot be achieved, and the head and tail billets cannot be smoothly pulled out of the straightening machine, so the casting process is hindered and normal production cannot be achieved.

[0053] The present invention controls and protects casting: argon sealing protection of the ladle slide (argon flow rate ≥15NL / min), argon blowing protection of the ladle long nozzle (argon flow rate ≥60NL / min), and argon blowing protection of the tundish (argon flow rate ≥1500NL / min before pouring, argon flow rate ≥1000NL / min during normal pouring); prevents the molten steel from being secondary oxidized, reduces non-metallic inclusions, and improves the cleanliness of the molten steel; improper control leads to the increase of new oxide inclusions due to secondary oxidation, which pollutes the molten steel.

[0054] The present invention controls the annealing time. For a round billet with a diameter of 800 mm, the heating rate is ≤40°C / h, the temperature is raised to 640±10°C, and the temperature is kept for 13.0 hours; then the heating rate is ≤80°C / h, the temperature is raised to 780±10°C, and the temperature is kept for 68 hours; for a round billet with a diameter of 900 mm, the heating rate is ≤40°C / h, the temperature is raised to 640±10°C, and the temperature is kept for 14.5 hours; then the heating rate is ≤80°C / h, the temperature is raised to 780±10°C, and the temperature is kept for 72 hours; for a round billet with a diameter of 1000 mm, the heating rate is

[0055] ≤40℃ / h, heat to 640±10℃, keep warm for 16.0h; then heat to 780±10℃ at a heating rate of ≤80℃ / h, keep warm for 76h; too long a time will increase the cost; too low a time will result in the ineffective removal of thermal stress in the core of the ingot, and the increase of the central crack.

[0056] Specific embodiments of the present invention are as follows:

[0057] Example 1

[0058] A method for casting ultra-large ultra-supercritical P92 round steel comprises the following steps:

[0059] 1) The specification of the cast P92 round steel is 1000mm, and the chemical composition is recorded as follows by mass fraction: C: 0.09%, Si: 0.31%, Mn: 0.38%, P: 0.01%, S: 0.002%, Cr: 8.75%, V: 0.16%, Al: 0.01%, Ni: 0.16%, Nb: 0.05%, Mo: 0.34%, W: 1.60%, Cu: 0.016%, Ti: 0.001%, B: 0.003%, H: 0.9ppm, O: 30ppm, N: 0.062%, and the balance is Fe and unavoidable impurities.

[0060] 2) The casting process adopts protective casting: argon sealing protection of the ladle slide (argon flow rate 15NL / min), argon blowing protection of the ladle long nozzle (argon flow rate 100NL / min), and argon blowing protection of the tundish (argon flow rate 1000NL / min). The covering agent and protective slag need to be added in time to ensure black slag operation; the mass percentage and indicators of the main chemical components of the protective slag are: H2O0.29%, R 1.09, RD (melting point) 1075℃; (viscosity) 0.164, TC 13.17%, SiO229.17%, CaO 30.89%; pulling speed 0.16m / min, using three-stage electromagnetic stirring, of which M-EMS frequency is 1.5Hz, current 300~450A, continuous; S-EMS frequency is 4.0~6.0Hz, current 180~240A, alternating, interval time is 30s-5s-30s; F-EMS frequency is 4.0~6.0Hz, current 1000~1600A, continuous; specific parameters are shown in Table 1, crystallizer cooling water volume 6500L / min, secondary cooling water flow 8.8L / min, secondary cooling air flow 100Nm 3 / h;

[0061] 3) Straightening. Specific straightening parameters include: the length of the first section is 600mm, corresponding to a straightening coefficient of 5.0; the length of the second section is 1500mm, corresponding to a straightening coefficient of 3.0. When the billet exceeds the length of the second section, it begins to enter the normal hot billet pressure (75t). The tail billet straightening is set with 4 straightening sections. Calculated based on the distance between the tail billet position and the frame group position, the first section is 4500mm, with a straightening coefficient of 2.2; the second section is 3500mm, with a straightening coefficient of 3.0; the third section is 2500mm, with a straightening coefficient of 4.5; and the fourth section is 2000mm, with a straightening coefficient of 6.5.

[0062] 4) The ingot is annealed at 550℃ in the pit, with the pit temperature at 400℃ and a heating rate of 35℃ / h to 640℃, and kept warm for 16.0h; then the ingot is heated at a heating rate of 65℃ / h to 785℃, kept warm for 76h, cooled in the pit at a cooling rate of 25℃ / h, and taken out of the pit when the temperature is below 200℃.

[0063] Table 1 Main parameters of continuous casting of Example 1-Example 3

[0064]

[0065]

[0066] Example 2

[0067] A method for casting ultra-large ultra-supercritical P92 round steel comprises the following steps:

[0068] 1) The specification of the cast P92 round steel is 900mm, and the chemical composition is recorded as follows by mass fraction: C: 0.11%, Si: 0.28%, Mn: 0.38%, P: 0.01%, S: 0.002%, Cr: 8.6%, V: 0.18%, Al: 0.010%, Ni: 0.16%, Nb: 0.06%, Mo: 0.4%, W: 1.62%, Cu: 0.017%, Ti: 0.001%, B: 0.002%, H: 0.9ppm, O: 32ppm, N: 0.061%, and the balance is Fe and unavoidable impurities.

[0069] 2) Protective casting was used during the casting process: argon sealing was used for the ladle slide (argon flow rate 20 NL / min), argon blowing was used for the ladle long nozzle (argon flow rate 120 NL / min), and argon blowing was used for the tundish (argon flow rate 1100 NL / min). Covering agent and protective slag were added promptly to ensure black slag operation. The protective slag consisted of 0.28% H2O, R 1.12, RD (melting point) 1079°C, viscosity 0.163, TC 14.27%, SiO2 28.61%, and CaO 30.95%. The casting speed was 0.17 m / min, the mold electric stirrer was 1.5 Hz, 400 A; the casting strand electric stirrer was 6.0 Hz, 200 A; the end electric stirrer was 4.0 Hz, 1200 A; the mold cooling water volume was 6200 L / min, the secondary cooling water flow rate was 10.2 L / min, and the secondary cooling air flow rate was 113 Nm. 3 / h;

[0070] 3) Specific straightening parameters include: the length of the first section is 500mm, corresponding to a straightening coefficient of 5.5; the length of the second section is 1400mm, corresponding to a straightening coefficient of 2.8. When the billet exceeds the length of the second section, it begins to transfer to normal hot billet pressure (58t). The tail billet straightening is set with four straightening sections. Calculated based on the distance between the tail billet position and the frame group position, the first section is 4000mm, with a straightening coefficient of 2.0; the second section is 3000mm, with a straightening coefficient of 2.8; the third section is 2500mm, with a straightening coefficient of 4.0; and the fourth section is 2000mm, with a straightening coefficient of 5.5.

[0071] 4) The ingot is annealed at 520℃ in the pit, with the pit temperature at 400℃ and a heating rate of 34℃ / h to 643℃, and kept at this temperature for 14.5h; then the ingot is heated at a heating rate of 70℃ / h to 785℃, kept at this temperature for 72h, and cooled in the pit at a cooling rate of 25℃ / h; and the ingot is removed from the furnace when it cools to below 200℃.

[0072] Example 3

[0073] A method for casting ultra-large ultra-supercritical P92 round steel comprises the following steps:

[0074] 1) The specification of the cast P92 round steel is 1000mm, and the chemical composition is recorded as follows by mass fraction: C: 0.10%, Si: 0.26%, Mn: 0.35%, P: 0.010%, S: 0.003%, Cr: 8.71%, V: 0.159%, Al: 0.007%, Ni: 0.159%, Nb: 0.05%, Mo: 0.34%, W: 1.58%, Cu: 0.015%, Ti: 0.001%, B: 0.002%, H: 0.8ppm, O: 26ppm, N: 0.059%, and the balance is Fe and unavoidable impurities.

[0075] 2) Protective casting is adopted in the casting process: argon sealing protection of the large ladle slide (argon flow rate 18NL / min), argon blowing protection of the large ladle long nozzle (argon flow rate 105NL / min), and argon blowing protection of the tundish (argon flow rate 1200NL / min). The covering agent and protective slag need to be added in time to ensure black slag operation; the mass percentage and indicators of the main chemical components of the protective slag components used are: H2O 0.3%, R 1.03, RD (melting point) 1080℃, ND (viscosity) 0.2, TC 16%, SiO229%, CaO 30%; pulling speed 0.17m / min, crystallizer electric stirring 1.5Hz, 400A; casting stream electric stirring 6.0Hz, 200A; end electric stirring 4.0Hz, 1200A, crystallizer cooling water volume 6500L / min, secondary cooling water flow 14.5L / min, secondary cooling air flow 115Nm 3 / h;

[0076] 3) Specific straightening parameters include: the length of the first section is 400mm, corresponding to a straightening coefficient of 5.0; the length of the second section is 1200mm, corresponding to a straightening coefficient of 2.5. When the billet exceeds the length of the second section, it begins to transfer to normal hot billet pressure (78t). The tail billet straightening is set with four straightening sections. Calculated based on the distance between the tail billet position and the frame group position, the first section is 3500mm, with a straightening coefficient of 2.0; the second section is 3000mm, with a straightening coefficient of 2.5; the third section is 2500mm, with a straightening coefficient of 3.5; and the fourth section is 2000mm, with a straightening coefficient of 5.0.

[0077] 4) The ingot is annealed at 515℃ in the pit, with the pit temperature at 400℃ and a heating rate of 35℃ / h to 645℃, and kept warm for 16.0h; then the ingot is heated at a heating rate of 70℃ / h to 780℃, kept warm for 76h, cooled in the pit at a cooling rate of 27℃ / h, and taken out of the pit when the temperature is below 200℃.

[0078] Comparative Example 1

[0079] A method for casting ultra-large ultra-supercritical P92 round steel comprises the following steps:

[0080] Comparative Example 1 was carried out in the same manner as Example 2, except that the end stirring current of Comparative Example 1 was higher, 1650 A, as shown in Table 2. This resulted in the formation of a bright white band in the ingot, which could not be eliminated during the subsequent heating and forging process and would have an adverse effect on the final product.

[0081] Table 2 Main continuous casting parameters of comparative example 1

[0082] Superheat / ℃ Pulling speed Crystallizer electric stirring Casting fluid electric stirring End electric stirring Comparative Example 1 25 0.17m / min 1.5Hz, 400A 6.0Hz, 200A 4.0Hz, 1650A

[0083] Comparative Example 2

[0084] A method for casting ultra-large ultra-supercritical P92 round steel comprises the following steps:

[0085] The process was carried out in accordance with Example 1, except that the casting speed in Comparative Example 2 was too low, which resulted in the expansion of the crack in the center of the ingot.

[0086] Table 3 Main continuous casting parameters of comparative example 2

[0087] Superheat / ℃ Pulling speed Crystallizer electric stirring Casting fluid electric stirring End electric stirring Comparative Example 2 30 0.14m / min 1.5Hz, 400A 6.0Hz, 200A 4.0Hz, 1200A

[0088] Comparative Example 3

[0089] A method for casting ultra-large ultra-supercritical P92 round steel comprises the following steps:

[0090] The process was carried out in accordance with Example 1, except that the superheat in Comparative Example 3 was too high, resulting in severe shrinkage holes in the center of the ingot.

[0091] Table 4 Main continuous casting parameters of comparative example 3

[0092] Superheat / ℃ Pulling speed Crystallizer electric stirring Casting fluid electric stirring End electric stirring Comparative Example 3 60 0.16m / min 1.5Hz, 400A 6.0Hz, 200A 4.0Hz, 1200A

[0093] Comparative Example 4

[0094] A method for casting ultra-large ultra-supercritical P92 round steel comprises the following steps:

[0095] The process was carried out according to Example 1, except that the annealing time of Comparative Example 4 was insufficient, resulting in the expansion of the central crack.

[0096] Table 4 Main annealing parameters of comparative example 4

[0097]

[0098] The central crack of Example 1 is level 1.5, and the longest part is 82mm; the central crack of Example 2 is level 1.0, and the longest part is 70mm; the central crack of Example 3 is level 1.5, and the longest part is 85mm; the central crack of Comparative Example 1 is level 2.5, with a white bright band defect; the central crack of Comparative Example 2 is level 3.0, and the longest part is 130mm; the central crack of Comparative Example 3 is level 3.0, and the longest part is 135mm; the central crack of Comparative Example 4 is level 3.0, and the longest part is 138mm.

[0099] The above embodiments are described to facilitate understanding and use of the invention by those skilled in the art. It will be apparent that those skilled in the art can readily make various modifications to these embodiments and apply the general principles described herein to other embodiments without requiring inventive effort. Therefore, the present invention is not limited to the above embodiments. Improvements and modifications made by those skilled in the art based on the disclosure of the present invention without departing from the scope of the present invention should be within the scope of protection of the present invention.

Claims

1. A method for casting ultra-large size ultra-supercritical P92 round steel, characterized in that: The casting method comprises the following steps: 1) Protective casting is used throughout the entire process, and the casting speed is controlled at 0.14 m / min<0.18 m / min; 2) Straightening; 3) Annealing; The diameter of the super-large ultra-supercritical P92 round steel is Ф800-Ф1000mm; In step 1), the superheat is 30-55°C for single-fired furnaces and 20-50°C for continuous casting furnaces; a three-stage electromagnetic stirring is adopted, wherein the M-EMS has a frequency of 1.5 Hz, a current of 300-450 A, and is continuous; the S-EMS has a frequency of 4.0-6.0 Hz, a current of 180-240 A, and is alternating with an interval of 30s-5s-30s; and the F-EMS has a frequency of 4.0-6.0 Hz, a current of 1000-1600 A, and is continuous; In the straightening described in step 2), the head billet undergoes two straightening intervals, the length of the first interval is 0~700mm, the corresponding straightening coefficient is 2.0~10.0, the length of the second interval is 700mm~1800mm, and the corresponding straightening coefficient is 1.0~5.0; when the billet exceeds the length of the second interval, it begins to enter the 24~95t hot billet pressure, and the tail billet straightening is set to 4 straightening intervals, calculated based on the distance between the tail billet position and the frame group position, the first interval is 4000mm-6000mm, the straightening coefficient is 1.0~2.5, the second interval is 2800mm-4000mm, the straightening coefficient is 1.0~3.5, the third interval is 2300mm-2800mm, the straightening coefficient is 3.0~5.5, and the fourth interval is 1500mm-2300mm, with a straightening coefficient of 5.0~7.5; In step 3), the annealing is specifically as follows: the ultra-large size ultra-supercritical P92 round steel is hot and placed in a slow cooling pit with a heating function with a surface temperature of ≥500°C, and the slow cooling pit maintains a pit temperature of ≥400°C before the billet is placed in the pit.

2. The casting method according to claim 1, characterized in that In step 1), the first cooling water volume is controlled at 5500L / min~6800L / min; the second cooling adopts mist cooling, the water volume is 5~45 L / min, and the air volume is 0~130 Nm 3 / h.

3. The casting method according to claim 1, characterized in that In step 1), the ladle slide is sealed with argon, and the argon flow rate is ≥15NL / min; the ladle long nozzle is blown with argon for protection, and the argon flow rate is ≥60NL / min; the tundish is blown with argon for protection, and the argon flow rate before pouring is ≥1500NL / min, and the argon flow rate during normal pouring is ≥1000NL / min.

4. The casting method according to claim 1, characterized in that For round billets with a diameter of 800mm, the pit temperature is ≥400℃, the heating rate is ≤40℃ / h, the temperature is raised to 640±10℃, and the temperature is kept at this temperature for 13.0h; then the temperature is raised to 780±10℃ at a heating rate ≤80℃ / h, and the temperature is kept at this temperature for 68h, and the billet is cooled in the pit at a cooling rate ≤30℃ / h; the billet is removed from the furnace when the temperature is lower than 200℃ during the cooling process; For round billets with a diameter of 900mm, the pit temperature is ≥400℃, the heating rate is ≤40℃ / h, the temperature is raised to 640±10℃, and the temperature is kept at this temperature for 14.5h; then the temperature is raised to 780±10℃ at a heating rate ≤80℃ / h, and the temperature is kept at this temperature for 72h, and the billet is cooled in the pit at a cooling rate ≤30℃ / h; the billet is removed from the furnace when the temperature is lower than 200℃ during the cooling process; For round billets with a diameter of 1000mm, the pit temperature is ≥400℃, the heating rate is ≤40℃ / h, the temperature is raised to 640±10℃, and kept warm for 16.0h; then the temperature is raised to 780±10℃ at a heating rate ≤80℃ / h, kept warm for 76h, and cooled in the pit at a cooling rate ≤30℃ / h; it is removed from the furnace when the temperature is below 200℃ while cooling in the pit.

5. The casting method according to claim 1, characterized in that The ultra-large specification ultra-supercritical P92 round steel includes the following components in mass percentage: C: 0.07-0.13%, Si: 0.20-0.40%, Mn: 0.30-0.50%, P≤0.015%, S: ≤0.005%, Cr: 8.50-8.9%, V: 0.15-0.25%, Al: ≤0.015%, Ni: 0.15-0.18%, Nb: 0.04-0.09%, Mo: 0.30-0.60%, W: 1.55-1.75%, Cu: ≤0.10%, Ti: ≤0.008%, B: 0.001-0.006%, H: ≤2.0ppm, O: ≤40ppm, N: 0.040-0.070%, and the balance is Fe and impurity elements.

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