A preparation method and product of an ultra-large size 9Cr ferrite heat-resistant seamless steel pipe

Abstract

The present invention belongs to the technical field of seamless steel pipe manufacturing, and specifically relates to a method for preparing an ultra-large-sized 9Cr-based ferrite heat-resistant seamless steel pipe and its product. The method for preparing the ultra-large-sized 9Cr-based ferrite heat-resistant seamless steel pipe comprises the following steps in sequence: a continuous casting step, a forging step, and a pipe making step. The continuous casting step uses blast furnace molten iron and scrap steel as raw materials, and produces continuous casting pipe billets through an electric furnace, LF refining, VD vacuum treatment, and round billet continuous casting machine casting; the forging step produces pipe billets through processes such as blanking, forging, annealing, and through-hole processing; and the pipe making step produces seamless steel pipes through processes such as heating, perforation, pipe rolling, pipe expansion, heat treatment, and lathe processing. The prepared 9Cr-based seamless steel pipe has uniform composition and high purity, thus having high high-temperature strength, solving the problems of unqualified flaw detection, uneven wall thickness, eccentricity, etc. caused by alloy segregation in the prior art.

Description

Technical Field

[0001] The present invention belongs to the technical field of production and manufacturing of seamless steel pipes, and in particular relates to a preparation method of an ultra-large specification 9Cr series ferrite heat-resistant seamless steel pipe and a product thereof. Background Art

[0002] In recent years, 9Cr-based ferritic heat-resistant steels have been widely used in key components such as superheaters and reheaters in supercritical and ultra-supercritical power plant boilers due to their excellent high-temperature strength, oxidation resistance, and creep resistance. As power plant parameters evolve toward higher temperatures and pressures, higher requirements are placed on the specifications and performance of heat-resistant steel pipes, particularly with the increasing demand for large-diameter, thick-walled, ultra-large-size seamless steel pipes.

[0003] However, the traditional preparation method is difficult to meet the production requirements of ultra-large-size 9Cr ferrite heat-resistant seamless steel pipes. The main problems are as follows:

[0004] (1) The quality of the ingot is difficult to guarantee: Oversized ingots are prone to defects such as central porosity and segregation, which affect the quality of the steel pipe. In particular, the enrichment of alloy elements caused by segregation is prone to cracks at the radius of 1 / 2 of the steel pipe wall thickness, resulting in failure in flaw detection.

[0005] (2) Difficulty in perforation: When perforating oversized tubes, problems such as incomplete penetration and eccentricity are likely to occur.

[0006] (3) Uneven rolling deformation: Traditional rolling technology makes it difficult to ensure the uniformity of the wall thickness of ultra-large steel pipes.

[0007] (4) Insufficient compression ratio: When using large-sized continuous casting round tube billets, the compression ratio must be ≥3.0.

[0008] (5) Complex heat treatment process: During the heat treatment process of ultra-large steel pipes, problems such as uneven structure and substandard performance are likely to occur.

[0009] Patent publication number CN116083781A, published on May 9, 2023, discloses a method for producing large-size continuous-cast round tubes of P92 heat-resistant steel without high-temperature ferrite. The method employs a tundish superheat controlled at 25-35°C, a continuous-cast round tube casting speed controlled at 0.22-0.26 m / min, a mold cooling water flow rate controlled at 3600-3800 L / min, a secondary cooling water ratio controlled at 0.11-0.13 L / kg, and three-stage composite electromagnetic stirring. By controlling the composition and continuous casting process parameters, the method ensures that high-temperature ferrite is completely eliminated from large-size continuous-cast round tubes of P92 heat-resistant steel. However, the method is limited to a diameter of 690 mm and is not suitable for larger sizes.

[0010] The patent with publication number CN115044823A 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 electromagnetic stirring of the crystallizer, casting strand stirring and end electromagnetic stirring to control the central crack. However, the maximum specification can only be Ф700mm, and the upper limit of the central crack length is 90mm. It is not suitable for larger specification production and cannot meet higher requirements.

[0011] The patent with publication number CN118926493A published on November 12, 2024, discloses a method for casting ultra-large-sized ultra-supercritical P92 round steel. Through the control of casting process, straightening and annealing process, the ultra-large-sized P92 round steel produced has no obvious surface cracks on the surface, and the internal cracks meet the performance requirements below level 2.5. It solves 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. However, the patent does not involve the means and effects of controlling the segregation of chemical elements, and the central crack is large, and the maximum specification can only reach Φ1000mm, which cannot meet the process of preparing large-sized seamless steel pipes, especially a process through perforation, rolling, and leveling.

[0012] In response to the above problems, it is urgent to develop a preparation method for ultra-large-sized 9Cr ferritic heat-resistant seamless steel pipes to solve the problems in the existing technology such as unqualified flaw detection, uneven wall thickness, and eccentricity caused by component enrichment, thereby affecting the high-temperature strength and other properties of the steel pipe. At the same time, the process universality is improved to meet the preparation of large-sized seamless steel pipes. Summary of the Invention

[0013] The purpose of the present invention is to provide a method for preparing ultra-large-sized 9Cr-based ferritic heat-resistant seamless steel pipes and the products thereof. By selecting raw materials and precisely controlling each step of the process, 9Cr-based seamless steel pipes with uniform composition and high purity are prepared. These pipes have high high-temperature strength and solve the problems of flaw detection failure, uneven wall thickness, eccentricity, etc. caused by component enrichment in the prior art.

[0014] In order to achieve the above object, the present invention provides the following technical solutions:

[0015] The method for preparing the super-large size 9Cr ferrite heat-resistant seamless steel pipe comprises the following steps in sequence: continuous casting, forging, and pipe making. The specific flow chart of seamless steel pipe preparation is shown in FIG. Figure 1 .

[0016] The continuous casting process specifically includes the following steps:

[0017] A1. Electric Furnace Smelting: After adding molten iron and scrap steel to the electric furnace, quicklime is added to create slag while supplying oxygen. When the temperature drops below 1570°C, lime is added in batches to control the slag alkalinity to between 2-4. Oxygen is blown into the slag interface for rapid stirring. When the temperature reaches 1630°C, samples are taken to test the chemical composition of the molten steel to ensure that it meets tapping conditions 1. The molten steel is poured into a ladle. When the molten steel in the ladle reaches 25-50 tons, Al, Mn, and Cr alloys are added and mixed with slag-forming materials to obtain molten steel.

[0018] A2. LF refining: The molten steel is transferred to the LF refining furnace, and argon is introduced while the power is turned on to increase the temperature. The argon flow rate is controlled at 200-400 NL / min. When the molten steel temperature is ≥1650°C, the argon flow rate is adjusted to 120-250 NL / min, and alloys are added. Quicklime or synthetic slag is added according to the fluidity of the slag. The basicity of the system is controlled to 2.5-4.0, and the argon flow rate is adjusted to 60-150 NL / min. Argon is blown to remove impurities. After 10 minutes, samples are taken to test the Al and S contents in the molten steel. The steel meets the tapping condition 2, and refined molten steel is obtained.

[0019] A3. VD vacuum refining: The refined molten steel is transferred to a VD vacuum furnace and vacuumed to below 67 Pa within 8 minutes, maintained for 15-20 minutes, and the hydrogen content is tested to be below 1.0 ppm. After breaking the vacuum, nitrogen is added according to the composition requirements, and then calcium wire is fed to obtain casting molten steel;

[0020] A4. Continuous Casting of Round Billets: The molten steel is hoisted to the continuous casting station. The molten steel flows from the ladle into the tundish. When the weight of the molten steel in the tundish reaches 20 tons, a carbon-free covering agent and carbonized rice husks are added. When the weight of the molten steel in the tundish reaches 30 tons, the molten steel is poured into the crystallizer. The automatic liquid level control system, electromagnetic stirring, and secondary cooling water are activated and enter the pouring mode to produce the continuously cast round billets.

[0021] A5. Annealing: The continuous casting round tube billet is sent hot to the annealing furnace for annealing at a temperature of 780°C for a holding time of 27-43 hours to obtain the annealed continuous casting round tube billet.

[0022] Preferably, the amount of molten iron added is 85% of the total mass of the molten iron and scrap steel.

[0023] The chemical composition of the molten iron includes, by weight percentage, C, ≥3.5%; Mn, ≤1.0%; Si, 0.20%-0.60%; Ni, ≤0.05%; Cu, ≤0.05%; P, ≤0.10%; S, ≤0.03%; Mo, ≤0.05%; Ti, ≤0.060%; As, ≤0.005%; Sn, ≤0.005%; Bi, ≤0.005%; Pb, ≤0.003%; Sb, ≤0.003%; the balance being Fe and unavoidable impurities.

[0024] Preferably, the temperature of the molten iron is 1300-1380°C.

[0025] The scrap steel is self-produced scrap steel, and the chemical composition of the scrap steel includes, by weight percentage: C, 0.10%-0.60%; Mn, ≤2.0%; Si, ≤1.0%; P, ≤0.15%; S, ≤0.025%; the balance is Fe and unavoidable impurities.

[0026] Preferably, in step A1, the amount of quicklime added is 30-60 kg / ton of molten iron.

[0027] Preferably, in step A1, the Al, Mn, and Cr alloys include aluminum-iron alloy, metallic manganese, low-carbon ferrochrome, and low-carbon ferrochrome.

[0028] In some preferred solutions, no electricity is supplied during the electric furnace smelting process, and the heat released by the chemical reaction between oxygen supplied by the oxygen lance and carbon is used as a heat source to provide the heat required for steelmaking.

[0029] Preferably, in step A1, the slag-making materials include quicklime and synthetic slag, and the added amounts are 8.9-9.1 kg / ton of molten steel and 1.9-2.1 kg / ton of molten steel, respectively.

[0030] Preferably, in step A1, the tapping condition 1 is: C≤0.04%, P≤0.005%, and S≤0.015% in the molten steel.

[0031] Preferably, in step A2, the chemical composition of the synthetic slag includes, by weight percentage: CaO, 45.0-55.0%; Al2O3, 27.0-35.0%; SiO2, ≤6.0%; MgO, ≤8.0%; Fe2O3, ≤2.0%; TiO2, ≤0.03%; and H2O, ≤0.5%.

[0032] Preferably, in step A2, quicklime or synthetic slag is added according to the fluidity of the slag. The specific operation method is: when the binary basicity of the slag is greater than 4.0, an appropriate amount of synthetic slag is added to increase the fluidity of the slag; when the binary basicity of the slag is less than 2.5, an appropriate amount of quicklime is added to improve the ability of the slag to absorb inclusions.

[0033] Preferably, in step A2, the tapping condition 2 is: the Al content in the molten steel is 0.008-0.010%, and S≤0.0030%.

[0034] Preferably, in step A3, the flow rate of nitrogen before breaking the air is 400 NL / min; the flow rate of nitrogen after breaking the air is 80-100 NL / min.

[0035] Preferably, the added amounts of the carbon-free covering agent and carbonized rice husk are 500 kg and 200 kg respectively.

[0036] By using blast furnace hot metal as the primary raw material and employing LF refining and VD vacuum refining to control residual elements such as P, S, As, and Bi in the steel, the steel's purity is enhanced from the source, significantly reducing the precipitation of brittle phases at grain boundaries. This not only improves high-temperature serviceability but also reduces the impact of inclusions on ductility, enhancing the material's uniform deformation capability. Combined with continuous casting process optimization, this produces continuously cast round tubes with high purity, uniform composition, and minimal central defects, thereby improving the elongation and long-lasting strength of the steel tubes. This is likely due to the fact that during the electric furnace smelting process, oxidation removes carbon from the hot metal. The addition of quicklime in the oxidizing atmosphere removes phosphorus from the steel through interfacial reactions. The addition of quicklime, a slag-forming material, removes S from the steel and adsorbs inclusions. During the LF refining process, two bottom air bricks are used to purge argon throughout the process, ensuring inclusion buoyancy, uniform temperature and composition, and preventing secondary oxidation of the steel. Controlling the basicity (2.5-4.0) helps form a low-melting-point slag to adsorb impurities. At the same time, the argon flow rate is controlled during LF refining, and the argon stirring is appropriately increased in the early stage to promote deoxidation and alloying; the argon intensity is maintained at a medium level except for the alloying stage in the middle stage of refining; the argon flow rate is reduced in the later stage of refining to avoid large-scale oxidation of the molten steel. In addition, the addition of an appropriate amount of synthetic slag during the LF refining process can ensure that the slag maintains good fluidity and adsorption while maintaining high alkalinity. In the VD vacuum refining stage, vacuum degassing is used to reduce the risk of hydrogen-induced cracking and control the Al content, which not only inhibits the precipitation of AlN, but also prevents the risk of premature failure of the product in a high-temperature, high-pressure and water vapor environment. The VD vacuum treatment removes the hydrogen content in the steel to below 1.0ppm, and then feeds Ca wire into the steel to promote the degeneration of inclusions, improve the purity of the steel, and meet the basic conditions required for smooth casting production.

[0037] Preferably, the electromagnetic stirring adopts three-stage electromagnetic stirring of M-EMS (mold electromagnetic stirring), S-EMS (strand electromagnetic stirring), and F-EMS (end electromagnetic stirring).

[0038] In some preferred embodiments, different sizes of electromagnetic stirrers are selected for continuous casting round tubes of different specifications. For continuous casting round tubes of the same specification, the electromagnetic stirrers used by S-EMS and F-EMS are of the same size and specifications. The different sizes of electromagnetic stirrers are as follows: for continuous casting round tubes of Φ900, the electromagnetic stirrer specifications are Φ1665×Φ1200×1100mm; for continuous casting round tubes of Φ1000, the electromagnetic stirrer specifications are Φ1765×Φ1300×1100mm; for continuous casting round tubes of Φ1100, the electromagnetic stirrer specifications are Φ1865×Φ1400×1100mm.

[0039] In some preferred solutions, the molten steel is poured into the crystallizer, the superheat of the molten steel is controlled at 20-30° C., and the casting is performed at a constant pulling speed of 0.10-0.40 m / min.

[0040] In some preferred schemes, when the required diameter of the continuous casting round tube billet is Φ900mm, the pulling speed is 0.15-0.17m / min; when the required diameter of the continuous casting round tube billet is Φ1000mm, the pulling speed is 0.13-0.16mm / min; when the required diameter of the continuous casting round tube billet is Φ1100mm, the pulling speed is 0.11-0.14m / min.

[0041] In step A4, the dummy bar in the mold uses a chain-type, integrated dummy bar. High-pressure reduction begins 1.0-2.0 meters from the hot billet (the billet, freshly pulled from the continuous casting mold and still relatively hot), with a maximum pressure limit of 350-400 tons. After the head of the hot billet passes the straightening machine, pressure reduction continues for another 1.0-1.5 meters before gradually transitioning to hot billet pressure at a designed pressure of 50-90 tons. At the end of the pouring process, approximately 6-8 meters from the tail, the pressure limit is restored to 350-400 tons.

[0042] By selecting different electromagnetic stirrer sizes for different sizes of continuously cast round billets, combined with constant casting speed and low superheat casting, the solidification structure of the continuously cast round billets is optimized, ensuring uniform chemical composition across the cross-section of the billet, reducing element segregation, allowing for higher compression ratios, and lowering forging cracking rates. This is achieved by, on the one hand, using three-stage electromagnetic stirring: M-EMS fragments initial grains, S-EMS inhibits dendrite growth, and F-EMS refines equiaxed grains, thereby reducing center segregation. On the other hand, different casting speeds and electromagnetic stirring are used for different sizes of continuously cast round billets, ensuring optimal electromagnetic stirring effects, extending the residence time of the molten steel in the mold and promoting chemical diffusion. The electromagnetic force induces rotational motion in the molten steel, breaking down chemical segregation and temperature gradients within the steel and promoting uniform distribution of components. Furthermore, electromagnetic stirring improves the flowability and solidification process of the molten steel, reducing the occurrence of internal defects. By combining low superheat, appropriate casting speeds, and electromagnetic stirring, the composition of the steel is uniform, resulting in high-quality continuously cast round billets with a loose center, minimal center cracks, and minimal segregation.

[0043] In some preferred solutions, the straightening of the head billet of Φ900 and Φ1100 continuous casting round tube billets is shown in Figure 2 Φ1000 specification P92 continuous casting low magnification photo, see Figure 3 Φ900 specification P92 continuous casting longitudinal section sample low magnification photo, see Figure 4 .

[0044] Preferably, in step A5, the temperature of the annealing furnace is 530°C when the continuously cast round tube billet is hot delivered.

[0045] The forging process specifically includes the following steps:

[0046] B1. Forging: The annealed continuous casting round tube is cut, heated and forged to obtain a tube blank;

[0047] B2, annealing;

[0048] B3, through hole.

[0049] Preferably, in step B1, the total heating time is ≥30 hours, wherein the temperature of the high temperature section is 1220-1250° C., and the holding time is ≥10 hours.

[0050] In some preferred schemes, the specific steps of heating are: the temperature is lower than 200°C when entering the furnace, and after keeping warm for 2 hours, the temperature is increased to 500°C at a rate of ≤50°C / h and kept warm for 5 hours, and then the temperature is increased to 850°C at a rate of ≤70°C / h and kept warm for 4 hours, and then the temperature is increased to 1100°C at a rate of ≤80°C / h and kept warm for 2-3 hours, and then the temperature is increased to 1200°C at a rate of ≤100°C / h and kept warm for 8-15 hours. The total heating time is 36.5-42.5 hours.

[0051] Preferably, in step B1, the specific steps of forging are: the surface temperature of the continuously cast round tube billet after leaving the heating furnace is 1160-1190°C, and after being drawn to the target diameter through a first fire, the surface temperature is ≥850°C.

[0052] Preferably, the specific steps of the single fire drawing are: first tap the heated continuous casting round tube billet, then press it down with a large amount of pressure from one end, and starting from the second anvil, each anvil is gradually advanced on the basis of the previous anvil. When it is forged to the middle position, turn around and continue to use the principle of first tapping, then pressing down with a large amount of pressure, and finally swinging it into a round shape; during the forging process, compressed air is used to blow away the surface iron oxide.

[0053] The forging process not only minimizes residual stress in steel during forging but also avoids eccentricity in continuous-cast round tube billets, making the steel structure denser and providing favorable conditions for subsequent tube making. This is because the heating rate is controlled during the forging heating process. The slow heating reduces thermal stress, and the subsequent high-temperature stage promotes the diffusion of alloying elements. The single-fire drawing process dynamically recrystallizes and refines the grains, and compressed air purges oxide scale to avoid indentations. Annealing eliminates forging residual stress and reduces porosity to near zero, thereby optimizing residual stress and structural density, and thus increasing high-temperature fracture time.

[0054] In some preferred embodiments, the forging process diagram is shown in FIG. Figure 5 .

[0055] Preferably, in step B2, the specific conditions for annealing are: the furnace temperature before entering the furnace is 550°C, the heating rate is ≤70°C / h, the annealing holding temperature is 780°C, and the holding time is 3.5 min / mm thickness.

[0056] In some preferred schemes, the specific operating steps of step B3 are: sawing one end with the sawing surface vertical, and polishing the end of the sawn surface, and marking the center point on the end surface; polishing two sections of the tube blank in the length direction to ensure that the diameters of the polished parts are the same; hanging the tube blank on the lathe, placing the polished part on the support roller, and punching a hole on the end surface with the geometric center point as the circle; the diameters of the holes punched are different for steel pipes of different specifications.

[0057] The through-hole process ensures accurate drilling positioning and eliminates central cracks. The center point can be precisely located on the sawn surface, ensuring that the drilling range completely eliminates the central crack. This precise drilling process prevents stress concentration and crack propagation during subsequent processing and use of the steel pipe under internal and external pressure, thereby improving break time and elongation. Furthermore, by eliminating defects such as central cracks, the overall quality and safety of the steel pipe are improved.

[0058] In some preferred embodiments, the schematic diagram of the tube blank punching equipment is shown in FIG. Figure 6 .

[0059] The pipe making process specifically includes the following steps:

[0060] C1. Heating;

[0061] C2. Tube making: The heated tube blank is subjected to perforation, tube rolling, leveling, and preliminary heat treatment to form a rough tube;

[0062] C3. Heat treatment, finishing and testing: After the rough tube is heat treated, the inner and outer surfaces are turned, samples are taken for testing, and UT and MT flaw detection are performed. After passing the test, it is put into storage to obtain the finished steel pipe.

[0063] Preferably, in step C1, the heating includes a heating section and a high temperature section, and the total heating time is 35-43 hours.

[0064] Preferably, the heating section is divided into two zones: the first zone is the preheating zone, with a temperature of 550-700°C and a holding time of ≥3 hours; the second zone is the heating zone, with a temperature of 850-1100°C and a holding time of ≥8 hours. During this stage, the heating rate is kept low, at 50-80°C / h, to reduce thermal and tissue stress.

[0065] Preferably, the high-temperature section is divided into two zones: the first zone is heated at 1200-1230°C for 10 hours or longer, and the second zone is heated at 1240-1270°C for 15 hours or longer. During this stage, the temperature rise rate from zone 1 to zone 2 is 100°C / h.

[0066] By designing the heating process during the pipe-making process, the enrichment of alloying elements, especially Cr, W, and Mo, is reduced, and the equipment load during piercing, pipe rolling, and leveling is reduced, thereby eliminating the risk of internal cracking of the steel pipe during processing. The purpose of the heating section is to slowly heat the ingot in the low-temperature section, reduce thermal stress and structural stress, and minimize the risk of cracking. The purpose of the high-temperature section is to make the internal temperature and surface temperature of the ingot consistent, so that the alloying elements are fully dissolved and evenly diffused. In addition, the high heating rate and long holding time in the high-temperature section facilitate the full austenitization of the round billet tube and the full and uniform dissolution of strong carbide elements such as Cr, W, and Mo, facilitating subsequent piercing, rolling, and other processes.

[0067] Preferably, in step C2, the specific conditions for perforation, tube rolling and leveling are: the temperature before perforation is greater than 1150°C, two-roller cross-rolling and one mandrel are used, the rotation speed of the perforation roller is 5-6 r / min, the surface temperature after perforation is 1020-1150°C, the temperature after tube rolling is 1020-1100°C, and the temperature after leveling is 950-1000°C.

[0068] In some preferred solutions, when the surface temperature after piercing or rolling drops below 850°C, the steel pipe is promptly returned to the furnace for heating for 1-2 hours, with an upper temperature limit of 1200°C.

[0069] By controlling the temperature drop during the three main steel pipe deformation processes of piercing, rolling, and leveling, we minimize thermal stress and structural stress cracking caused by excessive temperature drop, thereby improving the fracture time and high-temperature elongation. Two-roll cross-rolling combined with a mandrel process and controlled speed reduces deformation heat loss. Maintaining a temperature of ≥950°C after leveling preserves dynamic recrystallization and eliminates work hardening. When the temperature drops below 850°C, timely reheating can restore austenite uniformity. Precise temperature control reduces processing temperature drop and improves steel pipe performance.

[0070] Preferably, in step C2, the specific conditions for the preliminary heat treatment are: the capillary temperature before charging is (550±20)°C, the furnace temperature is 550°C, the temperature is raised to (930±10)°C and kept at this temperature for 6-9 hours before normalizing.

[0071] Preferably, in step C3, the specific steps of heat treatment are: normalizing the capillary at 1040-1080° C. for 5-7 hours, and then air-cooling and tempering at 750-790° C. for 7-8 hours.

[0072] Through controlled heat treatment processes, microstructure uniformity is promoted, precipitation of harmful phases is avoided, and the high-temperature strength and elongation of the steel pipe are improved. The high-temperature normalizing stage completely austenitizes and dissolves carbides. The subsequent medium-temperature tempering promotes a uniform martensitic lath structure, significantly reduces the content of retained austenite, and refines the grains, thereby improving the creep resistance of the steel pipe, reducing the creep rate and the content of harmful phases, and thus improving the high-temperature strength of the steel pipe.

[0073] The seamless steel pipes involved in the present invention include P91 steel, P92 steel and P9 steel.

[0074] The chemical composition of the P91 steel grade includes, by weight percentage, C, 0.08-0.12%; Si, 0.20-0.40%; Mn, 0.30-0.50%; P, ≤0.012%; S, ≤0.0030%; Cr, 8.20-9.00%; Ni, ≤0.40%; Mo, 0.85-1.05%; Al, ≤0.015%; V, 0.16-0.25%; Nb, 0.04-0.10%; N, 0.050-0.075%, W, ≤0.05%; B, ≤0.001%, Pb, ≤0.003%; Sn, ≤0.010%; As, ≤0.010%; Sb, ≤0.003%; Bi, ≤0.003%; Pb+Sn+As+Sb+Bi, ≤0.025%; the remainder is Fe and unavoidable impurities.

[0075] The chemical composition of the P92 steel grade includes, by weight percentage: C, 0.08-0.12%; Si, 0.20-0.40%; Mn, 0.30-0.60%; P, ≤0.012%; S, ≤0.0030%; Cr, 8.50-9.00%; Ni, ≤0.40%; Mo, 0.30-0.35%; Al, ≤0.015%; V: 0.16-0.18%; Nb , 0.04-0.09%; N, 0.050-0.075%; W, 1.55-1.75%; B: 0.0015-0.0035%; Pb, ≤0.003%; Sn, ≤0.010%; As, ≤0.010%; Sb, ≤0.003%; Bi, ≤0.003%; Pb+Sn+As+Sb+Bi, ≤0.025%; the balance is Fe and unavoidable impurities.

[0076] The chemical composition of the P9 steel grade includes, by weight percentage: C, ≤0.15%; Si: 0.25-1.00%; Mn: 0.30-0.60%; P, ≤0.012%; S, ≤0.0030%; Cr: 8.50-10.00%; Ni, ≤0.60%; Mo, 0.90-1.10%; Al, ≤0.015%; V, ≤0.020%; Nb, ≤0.020%, N, ≤0.012%, W, ≤0.050%, B, ≤0.001%, Pb, ≤0.003%; Sn, ≤0.010%; As, ≤0.010%; Sb, ≤0.003%; Bi, ≤0.003%; Pb+Sn+As+Sb+Bi, ≤0.025%; the remainder is Fe and unavoidable impurities.

[0077] The second aspect of the present invention provides a product prepared by the method for preparing the ultra-large size 9Cr-based ferrite heat-resistant seamless steel pipe.

[0078] Compared with the prior art, the advantages and beneficial effects of the present invention are:

[0079] 1. The present invention provides a method for preparing ultra-large-sized 9Cr-based ferritic heat-resistant seamless steel pipes. By selecting a specific Fe material and various intermediate supplementary materials, and combining three steps of continuous casting, forging, and pipe making, a 9Cr-based seamless steel pipe with uniform composition and high purity is prepared. The pipe has high high-temperature strength. At the same time, when preparing large-sized seamless steel pipes, the compression ratio still meets the requirements.

[0080] 2. The present invention uses blast furnace molten iron as the main raw material and adopts LF refining and VD vacuum refining to control residual elements such as P, S, As, and Bi in the steel, thereby improving the purity of the molten steel from the source and significantly reducing the precipitation of brittle phases at grain boundaries. This not only improves the high-temperature and long-term service performance, but also reduces the damage to plasticity caused by inclusions, improves the material's uniform deformation ability, and combines with continuous casting process optimization to produce continuously cast round tube billets with high purity, uniform composition, and small central defects, thereby improving the elongation and long-term strength of the steel pipe.

[0081] 3. The present invention optimizes the solidification structure of the continuous casting round tube billet by selecting electromagnetic stirrers of different sizes for continuous casting round tube billets of different specifications, combining constant casting speed and low superheat casting, ensuring the uniformity of chemical composition on the cross section of the continuous casting round tube billet, reducing carbon segregation, allowing higher compression ratio processing, and reducing rolling cracking rate.

[0082] 4. The present invention uses a forging process to not only minimize the residual stress of steel during the forging process, but also avoid the eccentricity problem of the continuous casting round tube billet, making the steel structure denser and providing good conditions for subsequent tube making.

[0083] 5. The present invention ensures the accuracy of punching positioning and the removal of center cracks through the through-hole process.

[0084] 6. The present invention reduces the enrichment of alloying elements, especially Cr, W, and Mo, by designing the heating process during the pipe manufacturing process, thereby reducing the equipment load during piercing, pipe rolling, and leveling, thereby eliminating the risk of internal cracking of the steel pipe during processing and further improving the high-temperature strength.

[0085] 7. The present invention controls the temperature drop during the three main steel pipe deformation processes of perforation, rolling and leveling, thereby minimizing thermal stress and structural stress cracking caused by excessive temperature drop, thereby improving the fracture time and high-temperature elongation.

[0086] 8. The present invention promotes microstructure uniformity and avoids the precipitation of harmful phases through the control of heat treatment process, thereby improving the high-temperature strength and elongation of the steel pipe. BRIEF DESCRIPTION OF THE DRAWINGS

[0087] In order to more clearly illustrate the embodiments of the present invention or the technical solutions in the prior art, the following briefly introduces the drawings required for use in the embodiments or the description of the prior art.

[0088] Figure 1 This is a specific flow chart for preparing the seamless steel pipe of the present invention;

[0089] Figure 2 The straightening of the head billet of Φ900 (left) and Φ1100 (right) continuous casting round tube billets;

[0090] Figure 3 This is a Φ1000 specification P92 low-magnification tissue photograph;

[0091] Figure 4 This is a photo of the longitudinal section of the Φ900 specification P92;

[0092] Figure 5 Schematic diagram of the forging process;

[0093] Figure 6 This is a schematic diagram of the tube punching equipment;

[0094] Figure 7 This is a microstructure photo of the steel pipe after heat treatment in Example 1;

[0095] Figure 8 This is a microstructure photograph of the steel pipe of Example 2 after heat treatment.

[0096] Among them: 1-pliers; 2-blank; 3-anvil; 4-forging machine; 5-second anvil; 6-first anvil; 7-punching equipment; 8-drill rod; 9-support roller 1; 10-roller table; 11-support roller 2; 12-tube blank; 13-support roller 3; 14-fixing equipment; 15-geometric center. DETAILED DESCRIPTION

[0097] The following is a clear and complete description of the technical solutions in the embodiments of the present invention. Obviously, the embodiments described are only some embodiments of the present invention, not all embodiments. Based on the embodiments of the present invention, all other embodiments obtained by ordinary technicians in this field without making any creative efforts are within the scope of protection of the present invention.

[0098] Unless otherwise specified, the experimental materials and reagents used in the following examples are all currently available and commercially available. Where specific techniques or conditions are not specified in the examples, they can be carried out according to conventional techniques or conditions disclosed in the art.

[0099] The chemical composition of the molten iron is as follows, by weight percentage: C, ≥3.5%; Mn, ≤1.0%; Si, 0.20%-0.60%; Ni, ≤0.05%; Cu, ≤0.05%; P, ≤0.10%; S, ≤0.03%; Mo, ≤0.05%; Ti, ≤0.060%; As, ≤0.005%; Sn, ≤0.005%; Bi, ≤0.005%; Pb, ≤0.003%; Sb, ≤0.003%; the balance being Fe and unavoidable impurities.

[0100] The scrap steel is self-produced scrap steel, and the chemical composition of the scrap steel is as follows, by weight percentage: C, 0.10%-0.60%; Mn, ≤2.0%; Si, ≤1.0%; P, ≤0.15%; S, ≤0.025%; the balance is Fe and unavoidable impurities.

[0101] Example 1 (Preparation of SA335 P92 steel pipe with specifications of ID933×8500×44mm and continuous casting of round pipe billets with specifications of Φ1000mm)

[0102] This embodiment provides a method for preparing an ultra-large-size 9Cr-based ferrite heat-resistant seamless steel pipe, which includes the following steps in sequence: a continuous casting step, a forging step, and a pipe making step.

[0103] The continuous casting process specifically includes the following steps:

[0104] A1. Electric furnace smelting: First, add scrap steel into the electric furnace, then add molten iron at 1350-1380℃, then add quicklime to make slag and supply oxygen at the same time. When the temperature is below 1570℃, add lime in batches to control the slag alkalinity between 2-4, and blow oxygen into the slag interface for rapid stirring. When the temperature reaches 1630℃, take samples to test the molten steel. When C≤0.04%, P≤0.005%, and S≤0.015%, pour the molten steel into the ladle. When the molten steel reaches 25 tons, add Al, Mn, Cr alloy and slag-making materials and mix them evenly to obtain smelting molten steel.

[0105] A2. LF refining: transfer the molten steel into the LF refining furnace, and introduce argon while heating it with electricity. Control the argon flow rate to 300NL / min. When the molten steel temperature is ≥1650℃, adjust the argon flow rate to 200NL / min, add alloy, and add quicklime or synthetic slag according to the slag fluidity. Control the system alkalinity to 2.5-4.0, adjust the argon flow rate to 100NL / min, blow argon to remove impurities, and do not add alloy and auxiliary materials 10 minutes before LF hanging ladle. The Al content of hanging ladle is controlled between 0.008%-0.010%, and S in molten steel is ≤0.0030% to obtain refined molten steel;

[0106] A3. VD vacuum refining: The refined molten steel is transferred to a VD vacuum furnace and vacuumed to below 67 Pa within 8 minutes. After maintaining the vacuum for 20 minutes, the hydrogen content is less than 1.0 ppm. Nitrogen is blown into the furnace before vacuuming to increase nitrogen. Boron iron is added after vacuuming, and calcium wire is fed to obtain casting molten steel.

[0107] A4. Continuous casting of round tube billets: The molten steel is hoisted to the continuous casting station and flows from the steel ladle into the tundish. When the weight of the molten steel in the tundish reaches 20 tons, a carbon-free covering agent and carbonized rice husk are added. When the weight of the molten steel in the tundish reaches 30 tons, the molten steel is poured into the crystallizer, and the liquid level automatic control system, electromagnetic stirring and secondary cooling water are started to start the pouring mode. The specifications of the casting strand and the end electromagnetic stirrer are Φ1765×Φ1300×1100mm. During normal pouring, the superheat of the molten steel is controlled at 20 The casting process was carried out at a constant pulling speed of 0.13 m / min at a temperature between -30°C and -30°C. A chain-type integral dummy bar was used, and large reduction began at a distance of 1.0 m from the red billet. The upper limit of the pressure was set to 350 tons. After the red billet head passed the straightening machine, the pressure was continued for another 1.0-1.5 m, and then gradually switched to hot billet pressure, with a designed pressure of 80 tons. When the casting was completed, about 6-8 m from the tail, the upper limit of the pressure was restored to 380 tons. After the casting was completed, a Φ1000 continuous casting round tube billet was obtained.

[0108] A5. Annealing: The continuous casting round tube billet is hot sent to a 530°C annealing furnace for annealing at a temperature of 780°C for 43 hours to obtain the annealed continuous casting round tube billet.

[0109] The amount of molten iron added is 85% of the total mass of the molten iron and scrap steel.

[0110] In step A1, the amount of quicklime added is 55 kg per ton of molten iron.

[0111] In the step A1, the Al, Mn, and Cr alloys include aluminum-iron alloys, metallic manganese, low-carbon ferrochrome, and low-carbon ferrochrome.

[0112] The addition amounts of the aluminum-iron alloy, metallic manganese, low-carbon ferrochrome and low-carbon ferrochrome are 1.6 kg / t, 3.5 kg / t, 95.8 kg / t and 6.01 kg / t respectively.

[0113] In step A1, the oxygen supply amount is 58Nm 3 / t.

[0114] In step A1, the slag-making materials are quicklime and synthetic slag, and the added amounts are 9 kg / ton of molten steel and 2 kg / ton of molten steel respectively.

[0115] In step A2, the specific operation method of adding quicklime or synthetic slag according to the fluidity of the slag is: when the binary basicity of the slag is greater than 4.0, an appropriate amount of synthetic slag is added to increase the fluidity of the slag; when the binary basicity of the slag is less than 2.5, an appropriate amount of quicklime is added to improve the ability of the slag to absorb inclusions.

[0116] In step A2, the alloy comprises 76.7 kg / t of low-carbon ferrochrome, 0.3 kg / t of electrolytic nickel, 2.7 kg / t of ferrovanadium, 0.5 kg / t of silicon manganese alloy, 1.1 kg / t of ferrosilicon alloy, 1.1 kg / t of metallic manganese, 5.5 kg / t of ferromolybdenum alloy, 0.67 kg / t of ferroniobium, 0.6 kg / t of nickel nitride, 60.12 kg / t of micro-carbon ferrochrome, and 23.1 kg / t of ferrotungsten.

[0117] In the step A3, the ferroboron alloy is 0.15 kg / t.

[0118] In step A3, the flow rate of nitrogen before breaking the air is 400 NL / min; the flow rate of nitrogen after breaking the air is 90 NL / min.

[0119] The added amounts of the carbon-free covering agent and carbonized rice husk are 500 kg and 200 kg respectively.

[0120] The electromagnetic stirring adopts three stages of electromagnetic stirring: M-EMS, S-EMS and F-EMS.

[0121] The forging process comprises the following specific steps:

[0122] B1. Forging: Cut the 1000mm Φ continuous casting round tube into single lengths of 2.2m after annealing and heat it in a trolley furnace. The temperature is kept below 200°C upon entry. After holding for 2 hours, heat the tube at a rate of 50°C / h to 500°C and hold for 5 hours. Then, heat the tube at a rate of 70°C / h to 850°C and hold for 4 hours. Then, heat the tube at a rate of 80°C / h to 1100°C and hold for 3 hours. Finally, heat the tube at a rate of 100°C / h to 1200°C and hold for 10 hours. The total heating time should be no less than 39.5 hours. After heating, the tube is directly drawn to the target diameter of 770±2mm. The heated tube is patted to release residual stress and remove scale. Then, a large amount of pressure is applied from one end. Starting from the second anvil, each anvil is gradually pressed on the basis of the previous anvil. When the forging reaches the middle position, it turns around and continues to use the principle of tapping first, then pressing down with large pressure, and finally swinging into a round shape. During the forging process, compressed air is used to blow away the surface oxide scale. The surface temperature of the continuous casting round tube billet is 1170℃ before forging, and the surface temperature is 900℃ at the end of large pressure forging. The tube billet is obtained after forging.

[0123] B2. Annealing: The tube billet is charged into the furnace for annealing. The furnace temperature before entering the furnace is 550℃, the heating rate is 70℃ / h, the annealing holding temperature is 780℃, and the holding time is 45h;

[0124] B3. Through-holes: Lathe the tube at 1 / 3 and 2 / 3 of its length and approximately 150mm on the end face. Draw vertical lines on the end face in both the vertical and horizontal directions. The intersection point is the geometric center point of the tube. Hoist the tube onto the lathe, place the 1 / 3 and 2 / 3 sections on support rollers, and drill holes on the end face with the geometric center point as the center point. The diameter of the holes for Φ770 specifications is Φ180mm.

[0125] The pipe making process comprises the following specific steps:

[0126] C1. Heating: Place the tube billet in a heating furnace for heating. The heating furnace is divided into a heating section and a high-temperature section. The heating section is divided into two zones: the first zone is the preheating zone, with a temperature of 630°C and a holding time of 3 hours; the second zone is the heating zone, with a temperature of 1000°C and a holding time of 8 hours. The high-temperature section is divided into two zones: the soaking zone 1, with a temperature of 1230°C and a holding time of 10 hours; the soaking zone 2, with a temperature of 1270°C and a holding time of 15 hours. The total heating time is 37 hours.

[0127] C2. Tube making: The heated tube blank is subjected to perforation, tube rolling, leveling, and preliminary heat treatment to form a rough tube;

[0128] C3. Heat treatment, finishing and testing: After the rough tube is heat treated, the inner and outer surfaces are turned, samples are taken for testing, and UT and MT flaw detection are performed. After passing the test, it is put into storage to obtain the finished steel pipe.

[0129] In step C2, the specific conditions for piercing, tube rolling, and leveling are as follows: the temperature before piercing is greater than 1150°C, two-roller cross-rolling and one mandrel are used, the rotation speed of the piercing roller is 5.5 r / min, the surface temperature after piercing is 1100°C, the temperature after tube rolling is 1060°C, and the temperature after leveling is 970°C.

[0130] In step C2, the specific conditions for the preliminary heat treatment are: the capillary temperature before loading the furnace is 550±20°C, the furnace temperature is set to 550°C, the temperature is raised to 930°C and kept at this temperature for 6 hours for normalizing.

[0131] The target size after piercing is Φ780×170mm, the target size after rolling is Φ920×100mm, the target size after leveling is Φ1020×78.5mm, and the target size after expanding is Φ1055×76mm. The inner diameter of the steel pipe is 933mm, the wall thickness is 44mm, the inner wall turning allowance is 15mm, and the outer wall turning allowance is 17mm.

[0132] In step C3, the specific steps of heat treatment are: normalizing the capillary at 1070° C. for 5.5 hours, and then air-cooling and tempering at 770° C. for 7.5 hours.

[0133] The chemical composition of the P92 steel grade includes, by weight percentage: C, 0.08-0.12%; Si, 0.20-0.40%; Mn, 0.30-0.60%; Cr, 8.50-9.00%; P, ≤0.012%; S, ≤0.0030%; Ni, ≤0.40%; Mo, 0.30-0.35%; Al, ≤0.015%; V: 0.16-0.18%; Nb , 0.04-0.09%; N, 0.050-0.075%; W, 1.55-1.75%; B: 0.0015-0.0035%; Pb, ≤0.003%; Sn, ≤0.010%; As, ≤0.010%; Sb, ≤0.003%; Bi, ≤0.003%; Pb+Sn+As+Sb+Bi, ≤0.025%; the balance is Fe and unavoidable impurities.

[0134] The steel pipe ID933×44mm indicates the inner diameter pipe, that is, the inner hole diameter is 933mm, the wall thickness is 44mm, the compression ratio of the continuous casting round tube billet to the forged rod is 1.69, and the compression ratio of the forged rod to the steel pipe is 1.88, totaling 3.57, which meets the standard requirement of a compression ratio of 3 or above.

[0135] Example 2 (Preparation of P91 steel pipe with specifications of ID885×4500×43.5mm and continuous casting of round tube billets with specifications of Φ900mm)

[0136] The difference between this embodiment and embodiment 1 is as follows: A4, continuous casting of round tube billets: the cast steel is hoisted to the continuous casting station and flows from the ladle into the tundish. When the weight of the cast steel in the tundish reaches 20 tons, a carbon-free covering agent and carbonized rice husk are added; when the weight of the cast steel in the tundish reaches 30 tons, it is poured into the crystallizer, the liquid level automatic control system, electromagnetic stirring and secondary cooling water are started to start the casting mode, and the specifications of the casting strand and the end electromagnetic stirrer are Φ1665×Φ1200×1100mm; during normal casting, the molten steel is overheated. The temperature is controlled between 20-30°C, and the casting is performed at a constant pulling speed of 0.15m / min. A chain-type integral dummy bar is used, and large reduction begins at a distance of 1.0m from the red billet, with the upper limit of the pressure set to 350 tons. After the red billet head passes the straightening machine, the pressure is continued for another 1.0-1.5m, and then gradually switches to hot billet pressure, with the pressure designed to be 80 tons. When the casting is completed, about 6-8m from the tail, the upper limit of the pressure is restored to 380 tons. After the casting is completed, the continuous casting round tube billet with a specification of Φ900 is obtained.

[0137] A5. Annealing: The continuous casting round tube billet is hot sent to a 530°C annealing furnace for annealing at a temperature of 780°C for 27 hours to obtain the annealed continuous casting round tube billet.

[0138] The forging process comprises the following specific steps:

[0139] B1. Forging: Cut the 900mm Φ continuous casting round tube into single lengths of 1.2m after annealing and heat it in a trolley furnace. The temperature is below 200°C upon entry. After holding for 2 hours, heat it up at a rate of 50°C / h to 500°C, then hold it for 5 hours. Then, heat it up at a rate of 70°C / h to 850°C, then hold it for 4 hours. Then, heat it up at a rate of 80°C / h to 1100°C, then hold it for 2 hours. Finally, heat it up at a rate of 100°C / h to 1200°C, then hold it for 8 hours. The total heating time should be no less than 36.5 hours. After heating, the tube is directly drawn to the target diameter of 730±2mm. The heated continuous casting round tube is first tapped to release residual stress and remove scale. Then, a large amount of pressure is applied from one end. Starting from the second anvil, each anvil is gradually pressed on the basis of the previous anvil. When the forging reaches the middle position, it turns around and continues to use the principle of tapping first, then pressing down with large pressure, and finally swinging into a round shape. During the forging process, compressed air is used to blow away the surface oxide scale. The surface temperature of the continuous casting round tube billet is 1170℃ before forging, and the surface temperature is 900℃ at the end of large pressure forging. The tube billet is obtained after forging.

[0140] B2. Annealing: The tube billet is charged into the furnace for annealing. The furnace temperature before entering the furnace is 550℃, the heating rate is 70℃ / h, the annealing holding temperature is 780℃, and the holding time is 43h;

[0141] B3. Through-holes: Lathe the tube at 1 / 3 and 2 / 3 of its length and approximately 150mm on the end face. Draw vertical lines in the vertical and horizontal directions on the end face. The intersection point is the geometric center point of the tube. Hoist the tube onto the lathe, place the 1 / 3 and 2 / 3 sections on the support rollers, and drill holes on the end face with the geometric center point as the center point. The diameter of the holes for Φ730 specifications is Φ150mm.

[0142] The pipe making process comprises the following specific steps:

[0143] C1. Heating: Place the tube billet in a heating furnace for heating. The heating furnace is divided into a heating section and a high-temperature section. The heating section is divided into two zones: the first zone is the preheating zone, with a temperature of 630°C and a holding time of 3 hours; the second zone is the heating zone, with a temperature of 1000°C and a holding time of 8 hours. The high-temperature section is divided into two zones: the soaking zone 1, with a temperature of 1230°C and a holding time of 8 hours; the soaking zone 2, with a temperature of 1250°C and a holding time of 15 hours. The total heating time is 35 hours.

[0144] C2. Tube making: The heated tube blank is subjected to perforation, tube rolling, leveling, and preliminary heat treatment to form a rough tube;

[0145] C3. Heat treatment, finishing and testing: After the rough tube is heat treated, the inner and outer surfaces are turned, samples are taken for testing, and UT and MT flaw detection are performed. After passing the test, it is put into storage to obtain the finished steel pipe.

[0146] In step C2, the specific conditions for piercing, tube rolling, and leveling are as follows: the temperature before piercing is greater than 1150°C, two-roller cross-rolling and one mandrel are used, the rotation speed of the piercing roller is 5.5 r / min, the surface temperature after piercing is 1070°C, the temperature after tube rolling is 1040°C, and the temperature after leveling is 970°C.

[0147] In step C2, the specific conditions for the preliminary heat treatment are: the capillary temperature before loading the furnace is 550±20°C, the furnace temperature is 550°C, the temperature is raised to 930°C and kept at this temperature for 6 hours for normalizing.

[0148] The target size after piercing is Φ750×160mm, the target size after rolling is Φ870×100mm, the target size after leveling is Φ960×78mm, and the target size after expanding is Φ1006×75.5mm. The inner diameter of the steel pipe is 885mm, the wall thickness is 43.5mm, the inner wall turning allowance is 15mm, and the outer wall turning allowance is 17mm.

[0149] In step C3, the specific steps of heat treatment are: normalizing the capillary at 1050° C. for 6.5 hours, and then air-cooling and tempering at 750° C. for 7.5 hours.

[0150] The chemical composition of the P91 steel grade includes, by weight percentage, C, 0.08-0.12%; Si, 0.20-0.40%; Mn, 0.30-0.50%; P, ≤0.012%; S, ≤0.0030%, Cr, 8.20-9.00%; Ni, ≤0.40%; Mo, 0.85-1.05%; Al, ≤0.015%; V, 0.16-0.25%; Nb, 0.04-0.10%; N, 0.050-0.075%, W, ≤0.05%; B, ≤0.001%, Pb, ≤0.003%; Sn, ≤0.010%; As, ≤0.010%; Sb, ≤0.003%; Bi, ≤0.003%; Pb+Sn+As+Sb+Bi, ≤0.025%; the remainder is Fe and unavoidable impurities.

[0151] Steel pipe ID885 indicates inner diameter pipe. The compression ratio from continuous casting round tube billet to forged rod is 1.52, and the compression ratio from forged rod to steel pipe is 1.82, totaling 3.34, which meets the standard requirement of compression ratio above 3.

[0152] Example 3 (Preparation of P9 steel pipe with specifications of φ965×5500×95mm, and continuous casting of round tube billets with specifications of Φ1100mm)

[0153] The difference between this embodiment and embodiment 1 is as follows: A3, VD vacuum refining: the refined molten steel is transferred to a VD vacuum furnace, evacuated to below 67 Pa within 8 minutes, maintained at this temperature for 20 minutes, and the hydrogen content is detected to be lower than 1.0 ppm. Argon is blown into the furnace before vacuum is broken, and calcium wire is fed into the furnace after vacuum is broken to obtain cast molten steel;

[0154] A4. Continuous casting of round tube billets: The molten steel is hoisted to the continuous casting station and flows from the steel ladle into the tundish. When the weight of the molten steel in the tundish reaches 20 tons, carbon-free covering agent and carbonized rice husk are added. When the weight of the molten steel in the tundish reaches 30 tons, it is poured into the crystallizer, and the liquid level automatic control system, electromagnetic stirring and secondary cooling water are started to start the casting mode. The specifications of the casting strand and the end electromagnetic stirrer are Φ1865×Φ1400×1100mm. During normal casting, the superheat of the molten steel is controlled at 20-3 The casting process was carried out at a constant pulling speed of 0.11 m / min at a temperature between 0°C and 0°C. A chain-type integral dummy bar was used, and large reduction began at a distance of 1.0 m from the red billet. The upper limit of the pressure was set to 380 tons. After the red billet head passed the straightening machine, the pressure was continued for another 1.0-1.5 m, and then gradually switched to hot billet pressure, with a designed pressure of 90 tons. When the casting was completed, about 6-8 m from the tail, the upper limit of the pressure was restored to 400 tons. After the casting was completed, a Φ1100 mm continuous casting round tube billet was obtained.

[0155] A5. Annealing: The continuous casting round tube billet is hot sent to a 530°C annealing furnace for annealing at a temperature of 780°C for 48 hours to obtain the annealed continuous casting round tube billet.

[0156] The forging process comprises the following specific steps:

[0157] B1. Forging: Annealed Φ1100mm continuous casting round tubes are cut into single lengths of 2.5m and heated in a trolley furnace. The temperature is kept below 200°C upon entry. After holding for 2 hours, the temperature is increased at a rate of 50°C / h to 500°C, held for 5 hours, then at a rate of 70°C / h to 850°C, held for 4 hours, then at a rate of 80°C / h to 1100°C, held for 2 hours, and finally at a rate of 100°C / h to 1200°C, held for 15 hours. The total heating time is 42.5 hours. After heating, the tubes are directly drawn to the target diameter of Φ770±2mm. The heated continuous casting tubes are patted to release residual stress and remove scale. Then, a large amount of pressure is applied from one end. Starting from the second anvil, each anvil is gradually pressed on the basis of the previous anvil. When the forging reaches the middle position, it turns around and continues to use the principle of tapping first, then pressing down with large pressure, and finally swinging into a round shape. During the forging process, compressed air is used to blow away the surface oxide scale. The surface temperature of the continuous casting round tube billet is 1170℃ before forging, and the surface temperature is 900℃ at the end of large pressure forging. The tube billet is obtained after forging.

[0158] B2. Annealing: The tube billet is charged into the furnace for annealing. The furnace temperature before entering the furnace is 550℃, the heating rate is 70℃ / h, the annealing holding temperature is 780℃, and the holding time is 46h;

[0159] B3. Through-holes: Lathe the tube at 1 / 3 and 2 / 3 of its length and approximately 150mm on the end face. Draw vertical lines on the end face in both the vertical and horizontal directions. The intersection is the geometric center of the tube. Hoist the tube onto the lathe, place the 1 / 3 and 2 / 3 sections on support rollers, and drill holes on the end face with the geometric center as the center point. The diameter of the holes for Φ770 specifications is Φ150mm.

[0160] The pipe making process comprises the following specific steps:

[0161] C1. Heating: Place the tube billet in a heating furnace for heating. The heating furnace is divided into a heating section and a high-temperature section. The heating section is divided into two zones: the first zone is the preheating zone, with a temperature of 630°C and a holding time of 3 hours; the second zone is the heating zone, with a temperature of 1000°C and a holding time of 8 hours. The high-temperature section is divided into two zones: the soaking zone 1, with a temperature of 1230°C and a holding time of 10 hours; the soaking zone 2, with a temperature of 1240°C and a holding time of 20 hours. The total heating time is 42 hours.

[0162] C2. Tube making: The heated tube blank is subjected to perforation, tube rolling, leveling, and preliminary heat treatment to form a rough tube;

[0163] C3. Heat treatment, finishing and testing: After the rough tube is heat treated, the inner and outer surfaces are turned, samples are taken for testing, and UT and MT flaw detection are performed. After passing the test, it is put into storage to obtain the finished steel pipe.

[0164] In step C2, the specific conditions for piercing, tube rolling, and leveling are as follows: the temperature before piercing is greater than 1150°C, two-roller cross-rolling and one mandrel are used, the rotation speed of the piercing roller is 5.5 r / min, the surface temperature after piercing is 1070°C, the temperature after tube rolling is 1050°C, and the temperature after leveling is 970°C.

[0165] In step C2, the specific conditions for the preliminary heat treatment are: the capillary temperature before charging is (550±20)°C, the furnace temperature is 550°C, the temperature is raised to 930°C and kept at this temperature for 8.5 hours for normalizing.

[0166] In step C3, the specific steps of heat treatment are: normalizing the capillary at 1040° C. for 5.5 hours, and then air-cooling and tempering at 740° C. for 7.5 hours.

[0167] The target size after piercing is Φ790×185mm, the target size after rolling is Φ860×130mm, the target size after leveling is Φ940×120mm, and the target size after expanding is Φ975×105mm. The outer diameter of the finished steel pipe is Φ965mm, the wall thickness is 95mm, and the inner and outer wall grinding allowances are 5mm respectively.

[0168] The chemical composition of the P9 steel grade includes, by weight percentage: C, ≤0.15%; Si: 0.25-1.00%; Mn: 0.30-0.60%; P: ≤0.012%; S≤0.0030%; Cr: 8.50-10.00%; Ni, ≤0.60%; Mo, 0.90-1.10%; Al, ≤0.015%; V, ≤0.020%; Nb, ≤0.020%, N, ≤0.012%, W, ≤0.050%, B, ≤0.001%, Pb, ≤0.003%; Sn, ≤0.010%; As, ≤0.010%; Sb, ≤0.003%; Bi, ≤0.003%; Pb+Sn+As+Sb+Bi, ≤0.025%; the remainder is Fe and unavoidable impurities.

[0169] The steel pipe Φ965×95mm represents the inner diameter pipe, that is, the outer diameter is 965mm and the wall thickness is 95mm. The compression ratio from the continuous casting round tube billet to the forged rod is 2.04, and the compression ratio from the forged rod to the steel pipe is 1.56, totaling 3.60, which meets the standard requirement of a compression ratio of 3 or above.

[0170] Comparative Example 1

[0171] This comparative example differs from Example 1 in that the amount of molten iron added is 50% of the total mass of the molten iron and scrap steel. Residual elements in the molten steel are difficult to control during the smelting process, resulting in high residual element contents, such as 0.25% Cu, 0.018% P, 0.03% As, and 0.015% Sb. These low-melting-point elements accumulate at grain boundaries, affecting the high-temperature strength of the steel.

[0172] Comparative Example 2

[0173] This comparative example differs from Example 1 in that: A3, VD vacuum refining: The refined molten steel was transferred to a VD vacuum furnace, where the vacuum was evacuated to below 67 Pa over 8 minutes and maintained for 5 minutes. The hydrogen content in the molten steel was 2.5 ppm. After the vacuum was broken, chromium nitride iron-cored wire was added to increase nitrogen, and calcium wire was then fed to produce cast molten steel. Due to the high alloy content and large product specifications in this steel, hydrogen white spots are easily formed. The use of wire feeding after the vacuum was broken resulted in severe secondary oxidation, which reduced the purity of the steel and affected the product's high-temperature strength.

[0174] Comparative Example 3

[0175] This comparative example differs from Example 1 in that neither strand electromagnetic stirring nor end-stage electromagnetic stirring is employed. During the continuous casting process, the liquid metal rapidly cools within the mold, forming a layer of fine equiaxed crystals. Upon entering the secondary cooling zone of intense convection heat conduction, the surface cools, resulting in columnar crystals that grow radially toward the center. As the columnar crystals grow to the center of the billet, the temperature distribution of the remaining liquid metal becomes uniform, weakening the directionality of heat dissipation and creating an isotropic thermal environment, resulting in the formation of an equiaxed crystal zone. During columnar crystal growth, the carbon and alloying element contents continuously increase, while the element contents in the equiaxed crystal zone stabilize until they rise again at the end of solidification. Consequently, the chemical composition at the junction of the columnar and equiaxed crystals (CET zone) and at the end of solidification is relatively high. The use of strand electromagnetic stirring and end-stage electromagnetic stirring in Example 1 is intended to mitigate chemical composition segregation at the CET and end of solidification. In the comparative example, neither strand electromagnetic stirring nor end-stage electromagnetic stirring was employed during production, resulting in significant chemical composition segregation at these two locations, impacting the material's high-temperature strength.

[0176] Comparative Example 4

[0177] This comparative example differs from Example 1 in that, in step A4, when producing φ900 continuous casting round billets, a φ1100 electromagnetic stirrer (φ1865 × φ1400 × 1100 mm) was used for both the strand stirrer and the end-stage electromagnetic stirrer. If the electromagnetic stirrer is too large, the magnetic leakage rate will increase significantly, affecting the stirring effect. This can lead to the enrichment of strong carbide elements such as W, Mo, and Cr, causing severe compositional segregation and ultimately compromising the high-temperature strength of the material.

[0178] Comparative Example 5

[0179] The difference between this comparative example and Example 1 is that in step B1, directly applying strong pressure to hit the pipe will cause internal stress concentration, resulting in failure to weld in the subsequent pipe making process, resulting in a significant reduction in the high-temperature strength of the finished pipe.

[0180] Comparative Example 6

[0181] The difference between this comparative example and Example 1 is that in step B3, no positioning punching is performed. As a result, uneven wall thickness occurs during the pipe making process, and some central cracks remain on the inner wall of the steel pipe, causing premature failure of the finished pipe.

[0182] Comparative Example 7

[0183] This comparative example differs from Example 1 in that, in step C1, the tube blank is placed in a heating furnace for heating. The heating furnace is divided into a heating section and a high-temperature section. The heating section is divided into two zones: the first zone is a preheating zone, with a temperature of 850°C and a holding time of 2 hours; the second zone is a heating zone, with a temperature of 1000°C and a holding time of 3 hours. The high-temperature section is divided into two zones: a soaking zone 1, with a temperature of 1100°C and a holding time of 5 hours; and a soaking zone 2, with a temperature of 1200°C and a holding time of 10 hours. The total heating time is 20 hours.

[0184] Comparative Example 8

[0185] The difference between this comparative example and Example 1 is that in step C2, the capillary is randomly placed at 300° C. after production without preliminary heat treatment, forming unbalanced structures such as ferrite, pearlite, and bainite, resulting in uneven original grain size.

[0186] Performance Testing

[0187] With reference to GB / T 2039-2012, GB / T 5310-2006, and ASME BPVC-II-Part D, the elongation of seamless steel pipes was tested at a test temperature of 625°C and different test stresses. The high-temperature tensile strength and high-temperature yield strength were also tested under these conditions for 100,000 hours. The results are shown in Table 1.

[0188] Table 1 Measurement results

[0189]

[0190] According to statistics, the seamless steel pipes based on three different compositions and different diameters prepared in Examples 1 to 3 of the present invention have long fracture times, high elongation and high-temperature strength. Taking Example 1 with the largest specification as the control group, Example 1 did not use an 85% molten iron ratio for smelting, resulting in excessive enrichment of residual elements such as As, Sb, and P at the grain boundaries. Example 2 did not control the hydrogen content below 1.0ppm, and hydrogen white spots appeared in the finished product. Example 3 did not use electromagnetic stirring of the casting strand and electromagnetic stirring at the end, resulting in large component segregation. Example 4 did not distinguish electromagnetic stirrers according to specifications, resulting in large component segregation. Example 5 was not forged as required, resulting in the formation of fine cracks inside the product. Example 6 failed to accurately position the steel pipe, resulting in excessive wall thickness, affecting the high-temperature strength of the steel pipe. Example 7 did not have sufficient holding temperature and holding time, resulting in fine cracks in the steel pipe during the perforation process. Example 8 did not undergo preliminary heat treatment, and the steel pipe had mixed crystal phenomenon.

[0191] The longitudinal cross-sections of the steel pipes prepared in Example 1 and Example 2 were sampled and corroded with ferric chloride or picric acid, and the tissue photographs were taken. Figure 7 and Figure 8 .from Figure 7 and Figure 8 It can be seen that the structure is all diffuse tempered troostite, and no high-temperature ferrite is found.

[0192] Therefore, the 9Cr seamless steel pipe produced using the method described in this application has uniform composition and high purity, resulting in high high-temperature strength. The steel pipe described in Example 1 is primarily used for the main steam pipes and reheat steam pipes of ultra-supercritical units with a service temperature not exceeding 625°C. The steel pipe described in Example 2 is primarily used for steam pipes of supercritical power plant boilers with a service temperature not exceeding 593°C. The steel pipe described in Example 3 is primarily used for petrochemical equipment and high-temperature pressure vessels with a service temperature not exceeding 550°C.

[0193] The above is a preferred embodiment of the present invention. It should be pointed out that for ordinary technicians in this technical field, several improvements and modifications can be made without departing from the principles of the present invention. These improvements and modifications should also be regarded as within the scope of protection of the present invention.

Claims

1. A method for preparing an ultra-large size 9Cr ferrite heat-resistant seamless steel pipe, characterized in that: The process includes the following steps in sequence: continuous casting process, forging process, and pipe making process; The continuous casting process includes the following steps: A1. Electric Furnace Smelting: After charging molten iron and scrap steel into the electric furnace, quicklime is added to form slag while supplying oxygen. When the temperature and composition meet the design requirements and the steel is ready for tapping, the molten steel is poured into the ladle and mixed with Al, Mn, Cr alloy and slag-forming materials to obtain molten steel. The slag-forming materials include quicklime and synthetic slag, with the addition amounts being 8.9-9.1 kg / ton of molten steel and 1.9-2.1 kg / ton of molten steel, respectively. A2. LF refining: The molten steel is transferred to the LF refining furnace, and argon is introduced while the temperature is increased. When the temperature of the molten steel is ≥1650℃, alloy is added. Quicklime or synthetic slag is added according to the fluidity of the slag. Argon is blown to remove impurities to obtain refined molten steel. A3. VD vacuum refining: The refined molten steel is transferred to a VD vacuum furnace and evacuated to below 67 Pa for 15-20 minutes until the hydrogen content in the molten steel is less than 1.0ppm. Before breaking the vacuum, nitrogen is blown into the steel for grades that require nitrogen addition, while argon is blown into the steel for grades that do not require nitrogen addition. The gas flow rate before breaking the vacuum is 400 NL / min; the gas flow rate after breaking the vacuum is 80-100 NL / min. After breaking the vacuum, calcium wire is fed to obtain cast molten steel. A4. Continuous casting of round tube billets: The molten steel is hoisted to the continuous casting station. The molten steel flows from the ladle into the tundish and then into the crystallizer. The automatic liquid level control system, electromagnetic stirring, and secondary cooling water are activated to start the casting mode to obtain the continuously cast round tube billet. A5. Annealing: The continuous casting round tube billet is sent to an annealing furnace for annealing to obtain the annealed continuous casting round tube billet; Different specifications of continuous casting round tube billets require different specifications of electromagnetic stirrers; the superheat of molten steel is controlled at 20-30℃ during pouring, and the casting speed is constant, with a casting speed of 0.10-0.40m / min; The forging process includes the following steps: B1. Cutting, heating and forging the annealed continuous casting round tube billet to obtain a tube billet; B2, annealing; B3, through hole; The pipe making process includes the following steps: C1. Heating: Place the tube billet in a heating furnace for heating. The heating furnace is divided into a heating section and a high-temperature section. The total heating time is 35-43 hours. The heating section is divided into two zones. The first zone is a preheating zone with a temperature of 550-700°C and a holding time of ≥3 hours. The second zone is a heating zone with a temperature of 850-1100°C and a holding time of ≥8 hours. The high-temperature section is divided into two zones. The temperature of the soaking zone 1 is 1200-1230°C and a holding time of ≥10 hours. The temperature of the soaking zone 2 is 1240-1270°C and a holding time of ≥15 hours. C2. Tube making: The heated tube blank is subjected to perforation, tube rolling, leveling, and preliminary heat treatment to form a rough tube; C3. Heat treatment, finishing and testing: After the rough tube is heat treated, the inner and outer surfaces are turned, samples are taken for testing, and UT and MT flaw detection are performed. After passing the test, it is put into storage to obtain the finished steel pipe.

2. The method for preparing an ultra-large size 9Cr ferrite heat-resistant seamless steel pipe according to claim 1, characterized in that: The electromagnetic stirring adopts three stages of electromagnetic stirring: M-EMS, S-EMS and F-EMS.

3. The method for preparing an ultra-large size 9Cr ferrite heat-resistant seamless steel pipe according to claim 2, characterized in that: The specific steps of heating in step B1 are: the temperature is lower than 200°C when entering the furnace, and after keeping it for 2 hours, the temperature is increased to 500°C at a rate of ≤50°C / h and kept for 5 hours, then the temperature is increased to 850°C at a rate of ≤70°C / h and kept for 4 hours, then the temperature is increased to 1100°C at a rate of ≤80°C / h and kept for 2-3 hours, and then the temperature is increased to 1200°C at a rate of ≤100°C / h and kept for 8-15 hours. The total heating time is 36.5-42.5 hours.

4. A product prepared according to the method for preparing an ultra-large size 9Cr ferrite heat-resistant seamless steel pipe according to any one of claims 1 to 3.

Images