Preparation method of ultra-large 9Cr series ferrite heat-resistant seamless steel pipe and product thereof

Abstract

The invention belongs to the technical field of seamless steel pipe manufacturing, and particularly relates to a preparation method of an ultra-large-specification 9Cr series ferrite heat-resistant seamless steel pipe and a product thereof. The preparation method of the ultra-large 9Cr series ferrite heat-resistant seamless steel pipe sequentially comprises the following working procedures of continuous casting, forging and pipe manufacturing. According to the continuous casting process, blast furnace molten iron and scrap steel serve as raw materials, and a continuous casting pipe blank is produced through an electric furnace, LF refining, VD vacuum treatment and round blank continuous casting machine pouring; the forging process comprises the steps of blanking, forging, annealing, perforating and the like to produce a pipe blank; the seamless steel pipe is produced through the procedures of heating, perforating, pipe rolling, pipe expanding, heat treatment, turning and the like in the pipe manufacturing procedure. And the prepared 9Cr series seamless steel tube is uniform in component and high in purity, so that the 9Cr series seamless steel tube has relatively high high-temperature strength, and the problems of unqualified flaw detection, non-uniform wall thickness, eccentricity and the like caused by alloy segregation in the prior art are solved.

Description

Technical Field

[0001] The present invention belongs to the technical field of the production and manufacturing of seamless steel pipes, and particularly relates to a preparation method for an extra-large-sized 9Cr series ferritic heat-resistant seamless steel pipe and its product. Background Art

[0002] In recent years, 9Cr series ferritic heat-resistant steels have been widely used in key components such as superheaters and reheaters of ultra-supercritical power station boilers due to their excellent high-temperature strength, oxidation resistance, and creep resistance. With the development of power station parameters towards higher temperatures and pressures, higher requirements have been put forward for the specifications and properties of heat-resistant steel pipes, especially the increasing demand for extra-large-sized seamless steel pipes with large diameters and thick walls.

[0003] However, traditional preparation methods are difficult to meet the production requirements of extra-large-sized 9Cr series ferritic heat-resistant seamless steel pipes, and there are mainly the following problems: (1) It is difficult to guarantee the quality of the continuous casting billet: Defects such as central porosity and segregation are likely to occur in extra-large-sized continuous casting billets, affecting the quality of the steel pipe. In particular, the enrichment of alloying elements caused by segregation is likely to generate cracks at the radius of 1 / 2 of the wall thickness of the steel pipe, resulting in unqualified flaw detection.

[0004] (2) The piercing is difficult: When piercing extra-large-sized tube billets, problems such as incomplete piercing and eccentricity are likely to occur.

[0005] (3) The rolling deformation is uneven: It is difficult for traditional rolling processes to guarantee the wall thickness uniformity of extra-large-sized steel pipes.

[0006] (4) The reduction ratio is insufficient: When using large-sized continuous casting round tube billets, the reduction ratio needs to be ≥3.0.

[0007] (5) The heat treatment process is complex: During the heat treatment of extra-large-sized steel pipes, problems such as uneven microstructure and unqualified properties are likely to occur.

[0008] The patent with the publication number CN116083781A, published on May 9, 2023, discloses a manufacturing method for a large-sized continuous casting round tube billet of P92 heat-resistant steel without a high-temperature ferritic structure. Its technical solution is to control the superheat of the tundish at 25 - 35°C, the drawing speed of the continuous casting round tube billet at 0.22 - 0.26 m / min, the cooling water volume of the mold at 3600 - 3800 L / min, the specific water volume of secondary cooling water at 0.11 - 0.13 L / kg, and adopt three-stage composite electromagnetic stirring. Through composition control and continuous casting process parameter control, it is ensured that the high-temperature ferritic structure is completely eliminated in the large-sized continuous casting round tube billet of P92 heat-resistant steel, but the involved specification is limited to Ф690mm and it cannot be applied to the production of larger specifications.

[0009] The patent with publication number CN115044823A, published on September 13, 2022, discloses a production process for continuously cast large round billets of ultra-supercritical high-pressure boiler steel P92. The molten steel undergoes continuous casting, heating at a rate of ≤80°C / h to ≥550°C, and slow cooling annealing processes to obtain continuously cast large round billets with a hardness of less than 230 HBW after annealing. During the continuous casting process, the superheat is between 30 - 45°C, and two-stage water cooling is combined with mold electromagnetic stirring, strand stirring, and end electromagnetic stirring to control center cracks. However, the maximum specification can only reach Ф700mm, and the upper limit of the center crack length is 90mm, which cannot be applied to the production of larger specifications and cannot meet higher requirements.

[0010] The patent with publication number CN118926493A, published on November 12, 2024, discloses a casting method for ultra-large specification ultra-supercritical P92 round steel. Through the control of casting process, straightening, and annealing processes, the produced ultra-large specification P92 round steel 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 specification of P92 round steel is small under the existing technical conditions and cannot meet the growing market demand for large specification P92 round steel with a diameter of Ф800 - Ф1000mm. However, this patent does not involve the means and effects of controlling chemical element segregation, and the center crack is relatively large. The maximum specification can only reach Φ1000mm, which cannot meet the process of preparing large specification seamless steel pipes, especially a process of piercing, rolling, and sizing.

[0011] In view of the above problems, there is an urgent need to develop a preparation method for ultra-large specification 9Cr series ferritic heat-resistant seamless steel pipes to solve the problems of unqualified flaw detection, uneven wall thickness, and eccentricity caused by component enrichment in the existing technology, which affect the properties such as high-temperature strength of the steel pipes, and at the same time improve the process universality to meet the preparation of large specification seamless steel pipes. Summary of the Invention

[0012] The object of the present invention is to provide a preparation method and its product for ultra-large specification 9Cr series ferritic heat-resistant seamless steel pipes. By selecting raw materials and precisely controlling each process step, 9Cr series seamless steel pipes with uniform composition and high purity are prepared, which have high high-temperature strength and solve the problems of unqualified flaw detection, uneven wall thickness, eccentricity, etc. caused by component enrichment in the existing technology.

[0013] To achieve the above object, the present invention provides the following technical solutions: The preparation method for the ultra-large specification 9Cr series ferritic heat-resistant seamless steel pipe successively includes the following processes: continuous casting process, forging process, pipe making process; the specific flow chart of seamless steel pipe preparation is shown in Figure 1 .

[0014] The continuous casting process specifically includes the following steps: A1. Electric furnace smelting: After adding hot metal and scrap steel into the electric furnace, quicklime is added to make steel slag while supplying oxygen. When the temperature is lower than 1570 °C, lime is added in batches to control the basicity of the steel slag between 2 and 4, and oxygen is blown at the steel slag interface for rapid stirring; when the temperature reaches 1630 °C, samples are taken to detect the chemical composition of the molten steel to meet the tapping condition 1. The molten steel is poured into the ladle. When the molten steel in the ladle reaches 25 - 50 tons, Al, Mn, Cr alloys and slag-making materials are added and mixed evenly to obtain the smelted molten steel; A2. LF refining: Transfer the smelted molten steel into the LF refining furnace, heat it up by electrifying while introducing argon gas, control the argon gas flow rate at 200 - 400 NL / min. When the molten steel temperature ≥ 1650 °C, adjust the argon gas flow rate to 120 - 250 NL / min, add alloys, supplement quicklime or synthetic slag according to the fluidity of the slag, control the system basicity at 2.5 - 4.0, adjust the argon gas flow rate to 60 - 150 NL / min, blow argon to remove impurities. After 10 minutes, samples are taken to detect the Al and S contents in the molten steel to meet the tapping condition 2, and the refined molten steel is obtained; A3. VD vacuum refining: Transfer the refined molten steel to the VD vacuum furnace, evacuate the vacuum to below 67 Pa within 8 minutes, maintain it for 15 - 20 minutes, detect that the hydrogen content is lower than 1.0 ppm. After breaking the vacuum, increase nitrogen according to the composition requirements, and then feed in the calcium wire to obtain the casting molten steel; A4. Continuous casting round tube billet: Lift the casting molten steel to the continuous casting station. The molten steel flows from the ladle into the tundish. When the weight of the casting molten steel in the tundish reaches 20 tons, add carbon-free covering agent and carbonized rice husk; when the weight of the casting molten steel in the tundish reaches 30 tons, pour the molten steel into the mold, start the liquid level automatic control system, electromagnetic stirring and secondary cooling water to enter the pouring mode to obtain the continuous casting round tube billet; A5. Annealing: Heat the continuous casting round tube billet to the annealing furnace for annealing. The annealing temperature is 780 °C, and the holding time is 27 - 43 hours to obtain the annealed continuous casting round tube billet.

[0015] Preferably, the addition amount of the hot metal is 85% of the total mass of the hot metal and scrap steel.

[0016] By weight percentage, the chemical composition of the hot metal includes: 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 is Fe and unavoidable impurities.

[0017] Preferably, the temperature of the hot metal is 1300 - 1380 °C.

[0018] The scrap steel is self-produced scrap steel. By weight percentage, the chemical composition of the scrap steel includes: C, 0.10% - 0.60%; Mn, ≤2.0%; Si, ≤1.0%; P, ≤0.15%; S, ≤0.025%; the balance is Fe and inevitable impurities.

[0019] Preferably, in step A1, the addition amount of quicklime is 30 - 60 kg / ton of hot metal.

[0020] Preferably, in step A1, the Al, Mn, Cr alloys include ferrosilicon, metallic manganese, micro-carbon ferrochrome and low-carbon ferrochrome.

[0021] In some preferred embodiments, during the electric furnace smelting process, no electricity is supplied, and the heat released by the chemical reaction of oxygen supplied by the oxygen lance with carbon is used as the heat source to provide the heat required for steelmaking.

[0022] Preferably, in step A1, the slag-making materials include quicklime and synthetic slag, and the addition amounts are 8.9 - 9.1 kg / ton of molten steel and 1.9 - 2.1 kg / ton of molten steel respectively.

[0023] Preferably, in step A1, the tapping condition 1 is: C ≤ 0.04%, P ≤ 0.005%, S ≤ 0.015% in the molten steel.

[0024] Preferably, in step A2, by weight percentage, the chemical composition of the synthetic slag includes: CaO, 45.0 - 55.0%; Al 2 O 3 , 27.0 - 35.0%; SiO 2 , ≤6.0%; MgO, ≤8.0%; Fe 2 O 3 , ≤2.0%; TiO 2 , ≤0.03%; H 2 O, ≤0.5%.

[0025] Preferably, in step A2, the quicklime or synthetic slag is supplemented according to the slag fluidity. 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 slag fluidity; when the binary basicity of the slag is less than 2.5, an appropriate amount of quicklime is added to improve the slag's ability to absorb inclusions.

[0026] Preferably, in step A2, the tapping condition 2 is: the Al content in the molten steel is 0.008 - 0.010%, S ≤ 0.0030%.

[0027] Preferably, in step A3, the nitrogen flow rate before breaking vacuum is 400 NL / min; the nitrogen flow rate after breaking vacuum is 80 - 100 NL / min.

[0028] Preferably, the addition amounts of the carbon-free covering agent and the carbonized rice husk are 500 kg and 200 kg respectively.

[0029] By using hot metal from blast furnace as the main raw material and adopting LF refining and VD vacuum refining to control residual elements such as P, S, As, and Bi in the steel, the purity of the molten steel is improved from the source, significantly reducing the precipitation of brittle phases at grain boundaries. This not only enhances the high-temperature service performance but also reduces the damage of inclusions to plasticity, improving the uniform deformation ability of the material. Combining with the optimization of the continuous casting process, continuous casting round tube billets with high purity, uniform composition, and small central defects are produced, thereby improving the elongation and creep strength of the steel pipe. This may be because during the electric furnace smelting process, the carbon content in the hot metal can be removed through oxidation reactions. Quicklime is added in an oxidizing atmosphere, and the phosphorus content in the steel can be removed through interfacial reactions. By adding the slag-making material quicklime, the S content in the molten steel can be removed and inclusions can be adsorbed. During the LF refining process, 2 bottom tuyeres are used to blow argon gas throughout the process to ensure the floating of inclusions, the uniformity of temperature and composition, and prevent secondary oxidation of the molten steel. The control of alkalinity (2.5 - 4.0) helps to form a low-melting-point slag to adsorb impurities. At the same time, the flow rate of argon gas is controlled during LF refining. In the early stage, the argon gas stirring is appropriately increased to promote deoxidation and alloying; during the middle stage of refining, medium argon gas intensity is maintained except during the alloy addition stage; in the later stage of refining, the argon gas flow rate is reduced to avoid large-scale oxidation of the molten steel. In addition, an appropriate amount of synthetic slag is added during the LF refining process to ensure good fluidity and adsorbability of the slag while maintaining high alkalinity. In the VD vacuum refining stage, the risk of hydrogen-induced cracks is reduced through vacuum degassing, and the Al content is controlled, 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 hydrogen content in the steel is removed to less than 1.0 ppm during the VD vacuum treatment, and then a Ca wire is fed into the steel to promote the modification of inclusions, improving the purity of the steel and meeting the basic conditions required for smooth casting production.

[0030] Preferably, the electromagnetic stirring adopts three-stage electromagnetic stirring of M-EMS (mold electromagnetic stirring), S-EMS (strand electromagnetic stirring), and F-EMS (final electromagnetic stirring).

[0031] In some preferred solutions, electromagnetic stirrers of different specifications are selected for continuous casting round tube billets of different specifications. For continuous casting round tube billets of the same specification, the sizes and specifications of the electromagnetic stirrers used for S-EMS and F-EMS are the same. The specifications of electromagnetic stirrers of different specifications are as follows: when the required continuous casting round tube billet is of Φ900 specification, the specification of the electromagnetic stirrer is Φ1665×Φ1200×1100 mm; when the required continuous casting round tube billet is of Φ1000 specification, the specification of the electromagnetic stirrer is Φ1765×Φ1300×1100 mm; when the required continuous casting round tube billet is of Φ1100 specification, the specification of the electromagnetic stirrer is Φ1865×Φ1400×1100 mm.

[0032] In some preferred embodiments, the molten casting steel is poured into a mold, and the superheat of the molten steel is controlled at 20 - 30 °C, and continuous casting is carried out at a constant casting speed, and the casting speed is 0.10 - 0.40 m / min.

[0033] In some preferred embodiments, when the required diameter of the continuously cast round tube blank is Φ900 mm, the casting speed is 0.15 - 0.17 m / min; when the required diameter of the continuously cast round tube blank is Φ1000 mm, the casting speed is 0.13 - 0.16 mm / min; when the required diameter of the continuously cast round tube blank is Φ1100 mm, the casting speed is 0.11 - 0.14 m / min.

[0034] In step A4, the dummy bar in the mold adopts a chain - type integral dummy bar. When the distance from the red billet (the billet just pulled out from the continuous casting machine mold and still with a relatively high temperature) is 1.0 - 2.0 meters, a large reduction is started, and the upper limit of the pressure is set to 350 - 400 tons. When the head of the red billet passes through the straightening machine, it continues for 1.0 - 1.5 meters, and then gradually changes to the pressure of the hot billet, and the pressure is designed to be 50 - 90 tons. When the casting is finished, when the distance from the tail is about 6 - 8 meters, the upper limit of the pressure returns to 350 - 400 tons.

[0035] By selecting different sizes of electromagnetic stirrers for continuously cast round tube blanks of different specifications, combined with continuous casting at a constant speed and pouring with low superheat, the solidification structure of the continuously cast round tube blank is optimized, ensuring the uniformity of the chemical composition on the cross - section of the continuously cast round tube blank, reducing element segregation, allowing for higher reduction ratio processing, and reducing the forging cracking rate. This is because on the one hand, through three - stage electromagnetic stirring, M - EMS breaks the initial grains, S - EMS inhibits dendrite growth, and F - EMS refines equiaxed grains, thereby reducing central segregation. On the other hand, continuously cast round tube blanks of different specifications correspond to different casting speeds and electromagnetic stirring, ensuring the best effect of electromagnetic stirring, prolonging the residence time of the molten steel in the mold, and promoting component diffusion. Through the action of electromagnetic force, the particles in the molten steel generate rotational motion, thereby breaking the composition segregation and temperature gradient in the molten steel and promoting the uniform distribution of components. At the same time, electromagnetic stirring can also improve the fluidity of the molten steel and the solidification process, reducing the generation of internal defects. By making the composition in the steel uniform through low superheat, reasonable casting speed, electromagnetic stirring, etc., high - quality continuously cast round tube blanks with small central porosity, central cracks, and small segregation can be obtained.

[0036] In some preferred embodiments, for the straightening of the head billets of continuously cast round tube blanks with Φ900 and Φ1100 specifications, see Figure 2 For the macro - etching photograph of Φ1000 - specification P92 continuously cast billet, see Figure 3 For the macro - etching photograph of the longitudinal section of Φ900 - specification P92 continuously cast billet, see Figure 4 .

[0037] Preferably, in step A5, the temperature of the annealing furnace during hot delivery of the continuously cast round tube blank is 530 °C.

[0038] The forging process specifically includes the following steps: B1. Forging: After cutting, heating, and forging the annealed continuous casting round tube billet, a tube billet is obtained. B2. Annealing; B3. Through-hole.

[0039] Preferably, in step B1, the total heating time ≥ 30 hours, where the temperature in the high-temperature section is 1220 - 1250 °C and the holding time ≥ 10 hours.

[0040] In some preferred embodiments, the specific steps of the heating are as follows: The temperature when entering the furnace is lower than 200 °C. After holding for 2 hours, it is heated at a rate ≤ 50 °C / h to 500 °C and held for 5 hours, then heated at a rate ≤ 70 °C / h to 850 °C and held for 4 hours, then heated at a rate ≤ 80 °C / h to 1100 °C and held for 2 - 3 hours, and then heated at a rate ≤ 100 °C / h to 1200 °C and held for 8 - 15 hours. The total heating time is 36.5 - 42.5 hours.

[0041] Preferably, in step B1, the specific steps of the forging are as follows: The surface temperature of the continuous casting round tube billet after leaving the heating furnace is 1160 - 1190 °C. After one-pass drawing to the target diameter, the surface temperature ≥ 850 °C.

[0042] Preferably, the specific steps of the one-pass drawing are as follows: The heated continuous casting round tube billet is first gently tapped, then pressed down with a large reduction from one end. Starting from the second anvil, each anvil makes a gradual progress on the basis of the previous anvil. When forging to the middle position, it is turned around, and the principle of first gently tapping and then large pressing is continued, and finally it is rounded; during the forging process, compressed air is used to blow off the surface scale.

[0043] Through the forging process, not only can the residual stress of the steel during forging be minimized to the greatest extent, but also the eccentricity problem of the continuous casting round tube billet can be avoided, making the structure of the steel more dense, providing good conditions for subsequent pipe making. This is because the heating rate is controlled during the forging heating process, and the slow heating reduces the thermal stress, and the subsequent high-temperature section promotes the diffusion of alloying elements; the one-pass drawing process dynamically recrystallizes and refines the grains, and the compressed air purges the scale to avoid pressing-in defects; annealing eliminates the forging residual stress, and the porosity approaches zero, thereby optimizing the residual stress and tissue density, and further increasing the high-temperature fracture time.

[0044] In some preferred embodiments, the schematic diagram of the forging process is shown in Figure 5 .

[0045] Preferably, in step B2, the specific annealing conditions are as follows: the furnace temperature before charging 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.

[0046] In some preferred embodiments, the specific operating steps of step B3 are as follows: saw cut one end of the tube blank, with the saw cut surface perpendicular, and at the same time turn and polish the end of the saw cut surface, and mark the center point on the end face; turn and polish both ends of the tube blank in the length direction to ensure that the diameters of the turned and polished parts are the same; lift the tube blank onto the lathe, place the turned and polished part on the support roller, and drill a hole with the geometric center point as the center on the end face; the drilled hole diameters for different specifications of steel pipes are different.

[0047] Through the through-hole process, the accuracy of the drilling positioning is ensured, and the center crack is ensured to be removed. The center point can be accurately positioned on the saw cut surface to ensure that the center crack is completely removed within the drilling range. The precise drilling process can ensure that the steel pipe will not have problems such as stress concentration and crack propagation when bearing internal and external pressures during subsequent processing and use, thereby increasing the fracture time and elongation rate. At the same time, by removing defects such as center cracks, the overall quality and safety of the steel pipe can be improved.

[0048] In some preferred embodiments, the schematic diagram of the tube blank drilling equipment is shown in Figure 6 .

[0049] The pipe manufacturing process specifically includes the following steps: C1. Heating; C2. Pipe manufacturing: Pass the heated tube blank through piercing, rolling, sizing, and preliminary heat treatment to make a rough pipe; C3. Heat treatment, finishing, and inspection: After heat-treating the rough pipe, turn the inner and outer surfaces, take samples for inspection, and perform UT and MT flaw detections. After passing the inspection, it is stored in the warehouse to obtain the finished steel pipe.

[0050] Preferably, in step C1, the heating includes a heating section and a high-temperature section, and the total heating time is 35 - 43 hours.

[0051] 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. In this stage, to reduce thermal stress and tissue stress, the heating rate is relatively small, controlled at 50 - 80 °C / h.

[0052] Preferably, the high-temperature section is divided into two zones. The soaking zone 1 has a temperature of 1200 - 1230 °C and a holding time of ≥10 hours. The soaking zone 2 has a temperature of 1240 - 1270 °C and a holding time of ≥15 hours. In this stage, the heating rate from zone 1 to zone 2 is 100 °C / h.

[0053] By designing the heating process in the pipe manufacturing process, the enrichment of alloying elements, especially Cr, W, and Mo, is reduced, and the equipment load during piercing, tube rolling, and sizing is decreased, thus solving the risk of internal cracking of the steel pipe during processing. The purpose of the heating section is to slowly heat the billet in the low-temperature section, reduce thermal stress and tissue stress, and minimize the cracking risk. The purpose of the high-temperature section is to make the internal temperature and surface temperature of the billet consistent, fully dissolve the alloying elements, and ensure uniform diffusion. In addition, the heating rate is high and the holding time is long in the high-temperature section, which helps the round billet tube to be fully austenitized and enables strong carbide elements such as Cr, W, and Mo to be fully dissolved and evenly distributed, facilitating subsequent processes such as piercing and rolling.

[0054] Preferably, in step C2, the specific conditions for piercing, tube rolling, and sizing are as follows: the temperature before piercing is greater than 1150 °C, two-roll skew rolling and one mandrel are used in both cases, the rotational speed of the piercing rolls is 5 - 6 r / min, the surface temperature after piercing is 1020 - 1150 °C, the temperature after tube rolling is 1020 - 1100 °C, and the temperature after sizing is 950 - 1000 °C.

[0055] In some preferred embodiments, when the surface temperature after piercing or tube rolling drops below 850 °C, it is promptly returned to the furnace for temperature compensation. The temperature compensation time is 1 - 2 hours, and the upper temperature limit is 1200 °C.

[0056] By controlling the temperature drop during the three major deformation processes of piercing, rolling, and sizing of the steel pipe, the thermal stress and tissue stress cracking caused by excessive temperature drop are ensured to the greatest extent, thereby increasing the fracture time and high-temperature elongation. The two-roll skew rolling combined with the mandrel process, while controlling the rotational speed, can reduce the deformation heat loss. Controlling the temperature ≥950 °C after sizing retains the dynamic recrystallization ability and eliminates work hardening. When the temperature is below 850 °C, promptly returning to the furnace for temperature compensation can restore the austenite uniformity. By precisely controlling the temperature to reduce the processing temperature drop, the performance of the steel pipe is improved.

[0057] Preferably, in step C2, the specific conditions for the preliminary heat treatment are as follows: the temperature of the mandrel before loading into the furnace is (550 ± 20) °C, the furnace temperature is 550 °C, and after the temperature is raised to (930 ± 10) °C, it is held for 6 - 9 hours and then normalized.

[0058] Preferably, in step C3, the specific steps of the heat treatment are as follows: the mandrel is normalized by holding at 1040 - 1080 °C for 5 - 7 hours, and then air-cooled and tempered by holding at 750 - 790 °C for 7 - 8 hours.

[0059] Through the control of the heat treatment process, the tissue uniformity is promoted, the precipitation of harmful phases is avoided, and thus the high-temperature strength and elongation of the steel pipe are improved. In the high-temperature normalizing stage, complete austenitization occurs to dissolve carbides, and then medium-temperature tempering promotes a uniform lath martensite structure, greatly reducing the retained austenite content and refining the grains, thereby improving the creep resistance of the steel pipe, reducing the creep rate and the content of harmful phases, and thus enhancing the high-temperature strength of the steel pipe.

[0060] The seamless steel pipes involved in the present invention include P91 steel grade, P92 steel grade, and P9 steel grade.

[0061] By weight percentage, the chemical composition of the P91 steel grade includes: 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 balance is Fe and unavoidable impurities.

[0062] By weight percentage, the chemical composition of the P92 steel grade includes: 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.

[0063] By weight percentage, the chemical composition of the P9 steel grade includes: 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 balance is Fe and unavoidable impurities.

[0064] The second aspect of the present invention provides a product prepared by the preparation method of the ultra-large specification 9Cr series ferritic heat-resistant seamless steel pipe.

[0065] Compared with the prior art, the advantages and beneficial effects of the present invention are as follows: 1. The present invention provides a preparation method of an ultra-large specification 9Cr series ferritic heat-resistant seamless steel pipe. By selecting specific Fe materials and various supplementary materials in the middle, and combining the continuous casting process, forging process, and pipe-making process, a 9Cr series seamless steel pipe with uniform composition and high purity is prepared, which has high high-temperature strength. At the same time, when preparing large-specification seamless steel pipes, the reduction ratio still meets the requirements.

[0066] 2. The present invention uses blast furnace hot metal 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. Starting from the source, the purity of the molten steel is improved, significantly reducing the precipitation of grain boundary brittle phases. This not only improves the long-term high-temperature service performance but also reduces the damage of inclusions to plasticity and enhances the uniform deformation ability of the material. Combining with the optimization of the continuous casting process, a continuous casting round tube blank with high purity, uniform composition, and small central defects is produced, thereby improving the elongation and creep strength of the steel pipe.

[0067] 3. The present invention selects electromagnetic stirrers of different sizes for continuous casting round tube blanks of different specifications, combines constant casting speed and low superheat pouring, optimizes the solidification structure of the continuous casting round tube blank, ensures the uniformity of chemical composition on the cross-section of the continuous casting round tube blank, reduces carbon segregation, allows higher reduction ratio processing, and reduces the rolling cracking rate.

[0068] 4. Through the forging process, the present invention can not only minimize the residual stress of the steel during forging but also avoid the eccentricity problem of the continuous casting round tube blank, making the structure of the steel more dense and providing good conditions for subsequent pipe making.

[0069] 5. The present invention ensures the accuracy of punching positioning through the through-hole process, and ensures that the central crack is removed.

[0070] 6. By designing the heating process in the pipe manufacturing process, the present invention reduces the enrichment of alloying elements, especially Cr, W, and Mo, and reduces the equipment load during piercing, rolling, and sizing, thereby solving the risk of internal cracking of the steel pipe during processing and improving the high-temperature strength.

[0071] 7. By controlling the temperature drop during the three major deformation processes of piercing, rolling, and sizing of the steel pipe, the present invention maximally ensures the cracking of thermal stress and tissue stress caused by excessive temperature drop, thereby increasing the fracture time and high-temperature elongation.

[0072] 8. By controlling the heat treatment process, the present invention promotes the tissue uniformity, avoids the precipitation of harmful phases, and further improves the high-temperature strength and elongation of the steel pipe. Description of the Drawings

[0073] In order to more clearly illustrate the technical solutions in the embodiments of the present invention or the prior art, the following will briefly introduce the drawings required for use in the description of the embodiments or the prior art.

[0074] Figure 1 It is the specific flow chart for the preparation of seamless steel pipes of the present invention; Figure 2 It is the straightening condition of the head billets of continuous casting round tube blanks with specifications of Φ900 (left) and Φ1100 (right); Figure 3 It is the macrostructure photo of P92 with a specification of Φ1000; Figure 4 It is the longitudinal section sample photo of P92 with a specification of Φ900; Figure 5 It is the schematic diagram of the forging process; Figure 6 It is the schematic diagram of the pipe blank punching equipment; Figure 7 It is the tissue photo of the steel pipe after heat treatment in Example 1; Figure 8 It is the tissue photo of the steel pipe after heat treatment in Example 2.

[0075] Wherein: 1 - pliers; 2 - blank; 3 - anvil; 4 - forging machine; 5 - second anvil; 6 - first anvil; 7 - punching equipment; 8 - drill pipe; 9 - support roll 1; 10 - roller table; 11 - support roll 2; 12 - pipe blank; 13 - support roll 3; 14 - fixing equipment; 15 - geometric center. Detailed Embodiments

[0076] The technical solutions in the embodiments of the present invention will be clearly and completely described below. Obviously, the described embodiments are only a part of the embodiments of the present invention, rather than all embodiments. All other embodiments obtained by those of ordinary skill in the art based on the embodiments of the present invention without creative efforts shall fall within the protection scope of the present invention.

[0077] The test materials or reagents used in the following embodiments, unless otherwise specified, are all existing and can be obtained from commercial channels. For those not specified in the embodiments for specific technologies or conditions, they can all be carried out according to the conventional technologies or conditions disclosed in the art.

[0078] By weight percentage, the chemical composition of the molten iron is: 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 is Fe and unavoidable impurities.

[0079] The scrap steel is self-produced scrap steel. By weight percentage, the chemical composition of the scrap steel is: C, 0.10% - 0.60%; Mn, ≤2.0%; Si, ≤1.0%; P, ≤0.15%; S, ≤0.025%; the balance is Fe and unavoidable impurities.

[0080] Example 1 (preparing SA335 P92 steel pipe, with specifications of ID933×8500×44mm, and the continuous casting round billet is of Φ1000mm specification) This example provides a preparation method for an ultra-large specification 9Cr series ferritic heat-resistant seamless steel pipe, which successively includes the following processes: continuous casting process, forging process, and pipe making process.

[0081] The continuous casting process specifically includes the following steps: A1. Electric furnace smelting: First, add scrap steel to the electric furnace, then pour in molten iron at 1350 - 1380°C, and at the same time add quicklime to make steel slag and supply oxygen. When the temperature is lower than 1570°C, add lime in batches to control the basicity of the steel slag between 2 - 4, and blow oxygen at the steel slag interface for rapid stirring; when the temperature reaches 1630°C, take a sample to detect that C≤0.04%, P≤0.005%, and S≤0.015% in the molten steel, and then pour the molten steel into the ladle; when the molten steel reaches 25 tons, add Al, Mn, Cr alloys and slag-making materials and mix them evenly to obtain the smelted molten steel; A2, LF Refining: Transfer the molten steel from smelting into the LF refining furnace. While heating up by power supply, introduce argon gas. Control the argon gas flow rate at 300 NL / min. When the molten steel temperature ≥ 1650 °C, adjust the argon gas flow rate to 200 NL / min, add alloys, supplement quicklime or synthetic slag according to the slag fluidity, control the system alkalinity at 2.5 - 4.0, adjust the argon gas flow rate to 100 NL / min, and blow argon to remove impurities. No alloys and auxiliary materials shall be added within 10 minutes before the LF ladle is lifted. Control the Al content in the ladle between 0.008% - 0.010%, and S in the molten steel ≤ 0.0030% to obtain refined molten steel; A3, VD Vacuum Refining: Transfer the refined molten steel to the VD vacuum furnace, evacuate the vacuum to below 67 Pa within 8 minutes, and keep it for 20 minutes. After that, the hydrogen content is lower than 1.0 ppm. Inject nitrogen to increase nitrogen content before breaking the vacuum, add ferroboron after breaking the vacuum, and then feed in calcium wire to obtain casting molten steel; A4, Continuous Casting of Round Tube Blanks: Lift the casting molten steel to the continuous casting station and let it flow from the ladle into the tundish. When the weight of the casting molten steel in the tundish reaches 20 tons, add carbon-free covering agent and carbonized rice husk; when the weight of the casting molten steel in the tundish reaches 30 tons, pour the molten steel into the mold, start the automatic liquid level control system, electromagnetic stirring, and the secondary cooling to start the casting mode. The specifications of the casting stream and the end electromagnetic stirrer are Φ1765×Φ1300×1100 mm; during normal casting, control the molten steel superheat between 20 - 30 °C, cast with a constant casting speed, and the casting speed is 0.13 m / min; adopt a chain-type integral dummy bar, start large reduction at a distance of 1.0 m from the red billet, set the upper limit of the pressure at 350 tons, and continue for 1.0 - 1.5 m after the billet head of the red billet passes the straightener, and then gradually turn to the pressure of the hot billet, with the designed pressure of 80 tons; when the casting is about to end, at a distance of about 6 - 8 m from the tail, restore the upper limit of the pressure to 380 tons; after casting, obtain a continuous casting round tube blank with a Φ1000 specification; A5, Annealing: Hot transfer the continuous casting round tube blank to an annealing furnace at 530 °C for annealing. The annealing temperature is 780 °C, and the holding time is 43 hours to obtain the annealed continuous casting round tube blank.

[0082] The addition amount of the hot metal is 85% of the total mass of the hot metal and scrap steel.

[0083] In the step A1, the addition amount of quicklime is 55 kg / ton of hot metal.

[0084] In the step A1, the Al, Mn, Cr alloys include ferrotitanium, metallic manganese, micro-carbon ferrochrome, and low-carbon ferrochrome.

[0085] The addition amounts of the ferrotitanium, metallic manganese, micro-carbon ferrochrome, and low-carbon ferrochrome are 1.6 kg / t, 3.5 kg / t, 95.8 kg / , and 6.01 kg / t respectively.

[0086] In the step A1, the oxygen supply amount during oxygen supply is 58 Nm 3 / t.

[0087] In the step A1, the slag-making materials are quicklime and synthetic slag, and the addition amounts are 9 kg / ton of molten steel and 2 kg / ton of molten steel respectively.

[0088] In the step A2, the specific operation method for supplementing quicklime or synthetic slag according to the slag fluidity is as follows: when the binary basicity of the slag is greater than 4.0, an appropriate amount of synthetic slag is added to increase the slag fluidity; when the binary basicity of the slag is less than 2.5, an appropriate amount of quicklime is added to improve the slag's ability to absorb inclusions.

[0089] In the step A2, the alloys are 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 silicomanganese 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 beans, 60.12 kg / t of micro-carbon ferrochrome, and 23.1 kg / t of ferrotungsten.

[0090] In the step A3, the ferroboron alloy is 0.15 kg / t.

[0091] In the step A3, the nitrogen flow rate before breaking vacuum is 400 NL / min; the nitrogen flow rate after breaking vacuum is 90 NL / min.

[0092] The addition amounts of the carbon-free covering agent and carbonized rice husk are 500 kg and 200 kg respectively.

[0093] The electromagnetic stirring adopts three-stage electromagnetic stirring of M-EMS, S-EMS, and F-EMS.

[0094] The forging process, the specific steps are as follows: B1. Forging: Cut the annealed continuous casting round tube billet with a Φ1000mm specification into single lengths of 2.2m, and heat it in a trolley furnace. The temperature when entering the furnace is lower than 200°C. After holding for 2 hours, it is heated to 500°C at a rate of 50°C / h and held for 5 hours, then heated to 850°C at a rate of 70°C / h and held for 4 hours, then heated to 1100°C at a rate of 80°C / h and held for 3 hours, and then heated to 1200°C at a rate of 100°C / h and held for 10 hours. The total heating time is not less than 39.5 hours. After heating, it is directly drawn out to the target diameter of Φ770±2mm in one heat. First, gently tap the heated continuous casting round tube billet to release residual stress and remove the scale. Then, press down with a large reduction from one end. Starting from the second anvil, each anvil gradually progresses on the basis of the previous anvil. When forging to the middle position, turn it around, and continue to follow the principle of first gently tapping and then large reduction, and finally swing it into a round shape. During the forging process, use compressed air to blow off the surface scale. The surface temperature of the continuous casting round tube billet before forging is 1170°C, and the surface temperature at the end of large-pressure forging is 900°C. After forging, a tube billet is obtained. B2. Annealing: Load the tube billet into the furnace for annealing. The furnace temperature before loading is 550°C, the heating rate is 70°C / h, the annealing holding temperature is 780°C, and the holding time is 45h. B3. Through-hole making: Turn and finish the 1 / 3, 2 / 3 positions along the length direction of the tube billet and about 150mm in length at the end face, and draw vertical lines in the vertical and horizontal directions at the end face. The intersection point is the geometric center point of the tube billet. Lift the tube billet to the lathe, place the 1 / 3 and 2 / 3 positions on the support rollers, and drill holes at the end face with the geometric center point as the center. The drilling diameter for the Φ770 specification is Φ180mm.

[0095] The specific steps of the tube manufacturing process are as follows: 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 first soaking zone has a temperature of 1230°C and a holding time of 10 hours. The second soaking zone has a temperature of 1270°C and a holding time of 15 hours. The total heating time is 37 hours. C2. Tube manufacturing: Pass the heated tube billet through piercing, tube rolling, sizing, and preliminary heat treatment to make a rough tube. C3. Heat treatment, finishing, and inspection: After heat-treating the rough tube, turn the inner and outer surfaces, take samples for inspection, and perform UT and MT flaw detections. After passing the inspection, it is stored in the warehouse, and the finished steel pipe is obtained.

[0096] In the said step C2, the specific conditions for piercing, tube rolling, and sizing are as follows: the temperature before piercing is greater than 1150 °C, two-roll skew rolling and a mandrel are both adopted, the rotational speed of the piercing rolls 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 sizing is 970 °C.

[0097] In the said step C2, the specific conditions for preliminary heat treatment are as follows: the temperature of the capillary tube before charging into the furnace is 550 ± 20 °C, the furnace temperature is set at 550 °C, and it is heated to 930 °C and held for 6 hours for normalizing.

[0098] The target dimensions after piercing are Φ780×170 mm, the target dimensions after tube rolling are Φ920×100 mm, the target dimensions after sizing are Φ1020×78.5 mm, and the target dimensions after tube expanding are Φ1055×76 mm. Among them, the inner diameter of the steel pipe is 933 mm, the wall thickness is 44 mm, the machining allowance for the inner wall is 15 mm respectively, and the machining allowance for the outer wall is 17 mm respectively.

[0099] In the said step C3, the specific steps for heat treatment are as follows: the capillary tube is held at 1070 °C for 5.5 hours for normalizing, and then held at 770 °C for 7.5 hours for air-cooled tempering.

[0100] By weight percentage, the chemical composition of the said P92 steel grade includes: 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.

[0101] The steel pipe ID933×44 mm represents an inner diameter pipe, that is, the inner hole diameter is 933 mm, the wall thickness is 44 mm. The compression ratio from the continuous casting round tube billet to the forging bar is 1.69, and the compression ratio from the forging bar to the steel pipe is 1.88, with a total of 3.57, meeting the standard requirements of a compression ratio of more than 3.

[0102] Example 2 (preparing a P91 steel pipe with a specification of ID885×4500×43.5 mm, and the continuous casting round tube billet is of Φ900 mm specification) The differences between this embodiment and Embodiment 1 are as follows: A4. Continuous casting round billet: Lift the molten steel for casting to the continuous casting station and let it flow from the ladle into the tundish. When the weight of the molten steel in the tundish reaches 20 tons, add carbon-free covering agent and carbonized rice husk. When the weight of the molten steel in the tundish reaches 30 tons, pour it into the mold, start the automatic liquid level control system, electromagnetic stirring, and the secondary cooling water to start the pouring mode. The specifications of the casting stream and the end electromagnetic stirrer are Φ1665×Φ1200×1100mm. During normal pouring, the superheat of the molten steel is controlled between 20 - 30°C, and continuous casting is carried out at a constant casting speed of 0.15 m / min. Use a chain-type integral dummy bar. Start large reduction when the red billet is 1.0 meter away, and set the upper limit of the pressure to 350 tons. After the head of the red billet passes the straightening machine, continue for 1.0 - 1.5 meters, and then gradually change to the pressure for the hot billet, with the pressure designed to be 80 tons. When the pouring is about to end, when it is about 6 - 8 meters away from the tail, the upper limit of the pressure is restored to 380 tons. After the pouring is completed, a continuous casting round billet with a Φ900 specification is obtained. A5. Annealing: Hot transfer the continuous casting round billet to an annealing furnace at 530°C for annealing. The annealing temperature is 780°C, and the holding time is 27 hours to obtain the annealed continuous casting round billet.

[0103] The specific steps of the forging process are as follows: B1. Forging: Cut the annealed continuous casting round billet with a Φ900mm specification into single-length billets with a length of 1.2m and heat them in a trolley furnace. The temperature when entering the furnace is lower than 200°C. After holding for 2 hours, heat it at a rate of 50°C / h to 500°C and hold for 5 hours, then heat it at a rate of 70°C / h to 850°C and hold for 4 hours, then heat it at a rate of 80°C / h to 1100°C and hold for 2 hours, and then heat it at a rate of 100°C / h to 1200°C and hold for 8 hours. The total heating time is not less than 36.5 hours. After heating, directly draw out the billet to the target diameter of Φ730±2mm in one heat. First, gently tap the heated continuous casting round billet blank to release the residual stress and remove the scale. Then, press down with a large reduction from one end. Starting from the second anvil, each anvil progresses step by step on the basis of the previous anvil. When forging to the middle position, turn it around and continue to follow the principle of first gently tapping and then large reduction, and finally swing it into a circle. During the forging process, use compressed air to blow off the surface scale. The surface temperature of the continuous casting round billet before forging is 1170°C, and the surface temperature at the end of the large-pressure forging is 900°C. After forging, a billet is obtained. B2. Annealing: Load the billet into the furnace for annealing. The furnace temperature before loading is 550°C, the heating rate is 70°C / h, the annealing holding temperature is 780°C, and the holding time is 43h. B3. Through-holes: On the tube blank, turn and finish the surfaces at the 1 / 3 and 2 / 3 positions along the length direction and about 150 mm length on the end face. Draw vertical lines in the vertical and horizontal directions on the end face, and the intersection point is the geometric center point of the tube blank. Lift the tube blank onto the lathe, place the 1 / 3 and 2 / 3 positions on the support rollers, and drill holes with the geometric center point on the end face as the center. The drilling diameter for the Φ730 specification is Φ150 mm.

[0104] The specific steps of the tube manufacturing process are as follows: C1. Heating: Place the tube blank in the 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 first soaking zone has a temperature of 1230 °C and a holding time of 8 hours. The second soaking zone has a temperature of 1250 °C and a holding time of 15 hours. The total heating time is 35 hours. C2. Tube manufacturing: Pass the heated tube blank through piercing, tube rolling, sizing, and preliminary heat treatment to make a rough tube. C3. Heat treatment, finishing, and inspection: After heat-treating the rough tube, turn the inner and outer surfaces, take samples for inspection, and perform UT and MT flaw detections. After passing the inspection, store it in the warehouse to obtain the finished steel pipe.

[0105] In the step C2, the specific conditions for piercing, tube rolling, and sizing are as follows: The temperature before piercing is greater than 1150 °C. Two-roll skew rolling and one mandrel are used. The rotational speed of the piercing rolls 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 sizing is 970 °C.

[0106] In the step C2, the specific conditions for preliminary heat treatment are as follows: The temperature of the rough tube before loading into the furnace is 550 ± 20 °C, the furnace temperature is 550 °C, and it is heated to 930 °C and held for 6 hours for normalizing.

[0107] The target dimensions after piercing are Φ750×160 mm, the target dimensions after tube rolling are Φ870×100 mm, the target dimensions after sizing are Φ960×78 mm, and the target dimensions after tube expanding are Φ1006×75.5 mm. The inner diameter of the steel pipe is 885 mm, the wall thickness is 43.5 mm, the inner wall turning allowance is 15 mm, and the outer wall turning allowance is 17 mm.

[0108] In the step C3, the specific steps of heat treatment are as follows: Normalize the rough tube at 1050 °C for 6.5 hours, and then perform air-cooled tempering at 750 °C for 7.5 hours.

[0109] By weight percentage, the chemical composition of the P91 steel grade includes: 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 balance is Fe and unavoidable impurities.

[0110] The steel pipe ID885 represents an inner diameter pipe. The compression ratio from the continuous casting round billet to the forging bar is 1.52, and the compression ratio from the forging bar to the steel pipe is 1.82, with a total of 3.34, meeting the standard requirement of a compression ratio of 3 or more.

[0111] Example 3 (preparing a P9 steel pipe with a specification of φ965×5500×95mm, and the continuous casting round billet is of Φ1100mm specification) The difference between this example and Example 1 is as follows: A3. VD vacuum refining: Transfer the refined molten steel to the VD vacuum furnace, evacuate to below 67 Pa within 8 minutes, keep it for 20 minutes, then detect that the hydrogen content is lower than 1.0 ppm. Blow in argon before breaking the vacuum, and feed in the calcium wire after breaking the vacuum to obtain the casting molten steel. A4. Continuous casting round billet: Lift the casting molten steel to the continuous casting station and let it flow from the ladle into the tundish. When the weight of the casting molten steel in the tundish reaches 20 tons, add a carbon-free covering agent and carbonized rice husk; when the weight of the casting molten steel in the tundish reaches 30 tons, pour it into the mold, start the automatic liquid level control system, electromagnetic stirring, and the secondary cooling water to start the pouring mode. The specifications of the casting stream and the end electromagnetic stirrer are Φ1865×Φ1400×1100mm; during normal pouring, the superheat of the molten steel is controlled between 20 - 30°C, and continuous casting is carried out at a constant casting speed of 0.11 m / min; use a chain-type integral dummy bar, start large reduction at a distance of 1.0 meter from the red billet, set the upper limit of the pressure to 380 tons, and continue for 1.0 - 1.5 meters after the billet head of the red billet passes the straightener, and then gradually turn to the hot billet pressure, with the pressure designed to be 90 tons; when the pouring is about to end, at a distance of about 6 - 8 meters from the tail, the upper limit of the pressure is restored to 400 tons; after pouring, a continuous casting round billet of Φ1100mm specification is obtained. A5. Annealing: Heat the continuous casting round billet to the annealing furnace at 530°C for annealing. The annealing temperature is 780°C, and the holding time is 48 hours to obtain the annealed continuous casting round billet.

[0112] The forging process has the following specific steps: B1. Forging: Cut the annealed continuous casting round tube billet with a specification of Φ1100mm into single lengths of 2.5m, and heat it in a trolley furnace. When entering the furnace, the temperature is lower than 200°C. After holding for 2 hours, it is heated to 500°C at a rate of 50°C / h and held for 5 hours, then heated to 850°C at a rate of 70°C / h and held for 4 hours, then heated to 1100°C at a rate of 80°C / h and held for 2 hours, and then heated to 1200°C at a rate of 100°C / h and held for 15 hours. The total heating time is 42.5 hours. After heating, it is directly drawn out to the target diameter of Φ770±2mm in one heat. First, gently tap the heated continuous casting round tube billet to release residual stress and remove the scale. Then, press down with a large reduction from one end. Starting from the second anvil, each anvil makes a step-by-step progress on the basis of the previous anvil. When forging to the middle position, turn it around and continue to follow the principle of first gently tapping and then large pressing, and finally swing it round. During the forging process, use compressed air to blow off the surface scale. The surface temperature of the continuous casting round tube billet before forging is 1170°C, and the surface temperature at the end of large-pressure forging is 900°C. After forging, a tube billet is obtained. B2. Annealing: Load the tube billet into the furnace for annealing. The furnace temperature before loading is 550°C, the heating rate is 70°C / h, the annealing holding temperature is 780°C, and the holding time is 46h. B3. Through-hole making: Turn and finish the 1 / 3, 2 / 3 positions in the length direction of the tube billet and about 150mm in length on the end face, and draw vertical lines in the vertical and horizontal directions on the end face. The intersection point is the geometric center point of the tube billet. Lift the tube billet to the lathe, place the 1 / 3, 2 / 3 positions on the support rollers, and drill a hole at the geometric center point on the end face. The drilling diameter for the Φ770 specification is Φ150mm.

[0113] The tube manufacturing process has the following specific 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 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 has a temperature of 1230°C and a holding time of 10 hours. The soaking zone 2 has a temperature of 1240°C and a holding time of 20 hours. The total heating time is 42 hours. C2. Tube manufacturing: Pass the heated tube billet through piercing, tube rolling, sizing, and preliminary heat treatment to make a rough tube. C3. Heat treatment, finishing, and inspection: After heat-treating the rough tube, turn the inner and outer surfaces, take samples for inspection, and perform UT and MT flaw detections. After passing the inspection, it is warehoused to obtain the finished steel pipe.

[0114] In the step C2, the specific conditions for piercing, tube rolling, and sizing are as follows: the temperature before piercing is greater than 1150°C. Two-roll skew rolling and a mandrel are used in all processes. The rotational speed of the piercing rolls 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 sizing is 970°C.

[0115] In the step C2, the specific conditions for preliminary heat treatment are as follows: the temperature of the capillary before charging into the furnace is (550 ± 20)°C, the furnace temperature is 550°C, and it is heated to 930°C and held for 8.5 hours for normalizing.

[0116] In the step C3, the specific steps for heat treatment are as follows: the capillary is held at 1040°C for 5.5 hours for normalizing, and then held at 740°C for 7.5 hours for air-cooled tempering.

[0117] The target dimensions after piercing are Φ790×185 mm, the target dimensions after tube rolling are Φ860×130 mm, the target dimensions after sizing are Φ940×120 mm, and the target dimensions after tube expanding are Φ975×105 mm. The outer diameter of the finished steel pipe is Φ965 mm, the wall thickness is 95 mm, and the grinding allowances for the inner and outer walls are 5 mm respectively.

[0118] By weight percentage, the chemical composition of the P9 steel grade includes: 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 balance is Fe and unavoidable impurities.

[0119] The steel pipe Φ965×95 mm represents an inner diameter pipe, that is, the outer diameter is 965 mm and the wall thickness is 95 mm. The compression ratio from continuous casting round billet to forging bar is 2.04, and the compression ratio from forging bar to steel pipe is 1.56, with a total of 3.60, meeting the standard requirements of a compression ratio of more than 3.

[0120] Comparative Example 1 The difference between this comparative example and Example 1 is that: the addition amount of hot metal is 50% of the total mass of hot metal and scrap steel. During the smelting process, the residual elements in the molten steel are difficult to control, resulting in relatively high contents of some residual elements. For example, Cu: 0.25%, P: 0.018%, As: 0.03%, Sb: 0.015%. These low-melting-point elements are enriched at the grain boundaries, affecting the high-temperature strength of the steel.

[0121] Comparative Example 2 The difference between this comparative example and Example 1 is that: A3, VD vacuum refining: The refined molten steel is transferred to a VD vacuum furnace, and the vacuum is pumped to below 67 Pa within 8 minutes. After maintaining for 5 minutes, the hydrogen content in the molten steel is 2.5 ppm. After breaking the vacuum, ferrochromium nitride cored wire is added to increase nitrogen, and then calcium wire is fed to obtain the casting molten steel. Due to the high alloy content and large product specifications of the steel, hydrogen-induced white spots are easily formed. After breaking the vacuum, the method of feeding wire is used to increase nitrogen, resulting in serious secondary oxidation, reducing the purity of the steel, and affecting the high-temperature strength of the product.

[0122] Comparative Example 3 The difference between this comparative example and Example 1 is that: continuous casting electromagnetic stirring and final electromagnetic stirring are not adopted. During the continuous casting process, the liquid metal is rapidly cooled in the mold to form a layer of fine equiaxed crystal zone. After entering the secondary cooling strong convection heat conduction zone, a columnar crystal growing radially towards the center will be formed on the surface after being cooled. When the columnar crystal grows to the center area of the round billet, the temperature distribution of the remaining liquid metal tends to be uniform, the heat dissipation directivity weakens, and an isotropic thermal environment is formed, forming an equiaxed crystal zone. During the growth of the columnar crystal, the contents of carbon element and alloy elements continuously increase, but the contents of various elements in the equiaxed crystal zone tend to be stable until the content increases again at the solidification end. Therefore, the chemical compositions at the junction of the columnar crystal and the equiaxed crystal (CET zone) and at the solidification end are relatively high. The continuous casting electromagnetic stirring and final electromagnetic stirring adopted in Example 1 are to weaken the segregation of chemical compositions at CET and the solidification end. In the comparative example, continuous casting electromagnetic stirring and final electromagnetic stirring are not adopted during the production process, resulting in relatively large segregation of chemical compositions at these two places, affecting the high-temperature strength of the material.

[0123] Comparative Example 4 The difference between this comparative example and Example 1 is that: in step A4, when producing a φ900 specification continuous casting round billet, a φ1100 specification electromagnetic stirrer (Φ1865×Φ1400×1100 mm) is used for the continuous casting stirrer and the final electromagnetic stirrer. When the electromagnetic stirrer is too large, the magnetic leakage rate will increase significantly, affecting the stirring effect, resulting in the enrichment of strong carbide elements such as W, Mo, Cr, etc., forming serious composition segregation, and ultimately affecting the high-temperature strength of the material.

[0124] Comparative Example 5 The difference between this comparative example and Example 1 is that in step B1, directly pressing down with great force will cause internal stress concentration and failure to weld during the subsequent pipe manufacturing process, resulting in a significant reduction in the high-temperature strength of the finished pipe.

[0125] Comparative Example 6 The difference between this comparative example and Example 1 is that in step B3, no positioning drilling is performed. Wall thickness unevenness occurs during the pipe manufacturing process, and some central cracks remain on the inner wall of the steel pipe, leading to premature failure of the finished pipe.

[0126] Comparative Example 7 The difference between this comparative example and Example 1 is 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 the preheating zone with a temperature of 850 °C and a holding time of 2 hours. The second zone is the heating zone with a temperature of 1000 °C and a holding time of 3 hours. The high-temperature section is divided into two zones. The first soaking zone has a temperature of 1100 °C and a holding time of 5 hours. The second soaking zone has a temperature of 1200 °C and a holding time of 10 hours. The total heating time is 20 hours.

[0127] Comparative Example 8 The difference between this comparative example and Example 1 is that in step C2, after the mandrel tube is produced, it is randomly placed at 300 °C without performing preliminary heat treatment, forming unbalanced structures such as ferrite, pearlite, and bainite, resulting in uneven original grain sizes.

[0128] Performance Test Referring to GB / T 2039-2012, GB / T 5310-2006, and ASME BPVC-II-Part D, the elongation of seamless steel pipes under different test stresses at a test temperature of 625 °C was tested, and the high-temperature tensile strength and high-temperature yield strength at 100,000 hours under this condition were also tested. The results are shown in Table 1.

[0129] Table 1 Measurement Results

[0130] According to statistics, for the seamless steel pipes prepared in Examples 1 to 3 of the present invention, which are based on three different components and different diameters, the fracture time is long, and both the elongation rate and high-temperature strength are relatively high. Taking Example 1 with the largest specification as the control group, in Comparative Example 1, smelting was not carried out with an 85% hot metal ratio, resulting in excessive enrichment of residual elements such as As, Sb, and P at the grain boundaries. In Comparative Example 2, the hydrogen content was not controlled below 1.0 ppm, and hydrogen-induced white spots appeared in the finished product. In Comparative Example 3, casting stream electromagnetic stirring and final electromagnetic stirring were not used, resulting in relatively large composition segregation. In Comparative Example 4, the electromagnetic stirrer was not distinguished according to the specification, resulting in relatively large composition segregation. In Comparative Example 5, forging was not carried out as required, resulting in the formation of fine cracks inside the product. In Comparative Example 6, inaccurate positioning led to an out-of-tolerance wall thickness of the steel pipe, affecting the high-temperature strength of the steel pipe. In Comparative Example 7, the holding temperature and holding time were insufficient, and fine cracks were generated in the steel pipe during the piercing process. In Comparative Example 8, preliminary heat treatment was not carried out, and the steel pipe had a mixed crystal phenomenon.

[0131] After taking longitudinal section samples of the steel pipes prepared in Example 1 and Example 2 and corroding them with ferric chloride or picric acid, tissue photos were taken, as shown in Figure 7 and Figure 8 . It can be seen from Figure 7 and Figure 8 that the tissue is all dispersed tempered sorbite, and no high-temperature ferrite is found.

[0132] Therefore, the 9Cr series seamless steel pipe prepared by the method described in the present application has a uniform composition and high purity, and thus has a relatively high high-temperature strength. The steel pipe described in Example 1 is mainly used for the main steam pipeline and reheater steam pipeline of an ultra-supercritical unit with a service temperature not exceeding 625 °C. The steel pipe described in Example 2 is mainly used for the steam pipeline of a supercritical power station boiler with a service temperature not exceeding 593 °C. The steel pipe described in Example 3 is mainly used for petrochemical equipment and high-temperature pressure vessels with a service temperature not exceeding 550 °C.

[0133] The above is the preferred embodiment of the present invention. It should be noted that for those of ordinary skill in the art, without departing from the principle described in the present invention, several improvements and refinements can be made, and these improvements and refinements should also be regarded as the protection scope 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 specifically comprises the following steps: A1. Electric furnace smelting: After adding molten iron and scrap steel into the electric furnace, quicklime is added to make slag while supplying oxygen. When the temperature and composition meet the design requirements and the conditions for tapping are met, the molten steel is poured into the ladle and Al, Mn, Cr alloy and slag-making materials are added and mixed to obtain smelting molten steel; A2. LF refining: transfer the molten steel into the LF refining furnace, and introduce argon gas while heating it up. When the temperature of the molten steel is ≥1650℃, add alloy, add quicklime or synthetic slag according to the fluidity of the slag, blow argon to remove impurities, and obtain refined molten steel; A3. VD vacuum refining: transfer the refined molten steel to a VD vacuum furnace, evacuate to below 67Pa and maintain 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 that needs nitrogen addition according to the chemical composition requirements, and argon is blown into the steel that does not need nitrogen addition. After breaking the vacuum, calcium wire is fed to obtain casting 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 poured into the crystallizer. The liquid level automatic control system, electromagnetic stirring and secondary cooling water are started to start the casting mode to obtain the continuous casting round tube billet; A5. Annealing: The continuous casting round tube billet is hot sent to an annealing furnace for annealing to obtain the annealed continuous casting round tube billet; Different specifications of electromagnetic stirrers are selected for continuous casting round tubes of different specifications; when the required continuous casting round tube is Φ900, the specifications of the electromagnetic stirrer are Φ1665×Φ1200×1100mm; when the required continuous casting round tube is Φ1000, the specifications of the electromagnetic stirrer are Φ1765×Φ1300×1100mm; when the required continuous casting round tube is Φ1100, the specifications of the electromagnetic stirrer are Φ1865×Φ1400×1100mm; during pouring, the superheat of molten steel is controlled at 20-30℃, and the pouring is performed at a constant pulling speed of 0.10-0.40m / min.

2. The method for preparing the super-large size 9Cr ferrite heat-resistant seamless steel pipe according to claim 1, characterized in that: In the 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.

3. The method for preparing the super-large size 9Cr ferrite heat-resistant seamless steel pipe according to claim 1, characterized in that: In the step A3, if nitrogen addition is required, nitrogen gas is blown in to break the air, and if nitrogen addition is not required, argon gas is blown in to break the air. The flow rate of the gas before breaking the air is 400 NL / min; the flow rate of the gas after breaking the air is 80-100 NL / min.

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

5. The method for preparing the super-large size 9Cr ferrite heat-resistant seamless steel pipe according to claim 1, characterized in that: The forging process specifically includes the following steps: B1. Forging: Cut, heat and forge the annealed continuous casting round tube to obtain a tube blank; B2, annealing; B3, through hole.

6. The method for preparing the super-large size 9Cr ferrite heat-resistant seamless steel pipe according to claim 5, characterized in that: In the step B1, the total heating time is ≥30 hours, wherein the temperature in the high temperature section is 1220-1250° C., and the insulation time is ≥10 hours.

7. The method for preparing the super-large size 9Cr ferrite heat-resistant seamless steel pipe according to claim 6, characterized in that: 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 speed of ≤50°C / h and kept warm for 5 hours, and then the temperature is increased to 850°C at a speed of ≤70°C / h and kept warm for 4 hours, and then the temperature is increased to 1100°C at a speed of ≤80°C / h and kept warm for 2-3 hours, and then the temperature is increased to 1200°C at a speed of ≤100°C / h and kept warm for 8-15 hours. The total heating time is 36.5-42.5 hours.

8. The method for preparing the super-large size 9Cr ferrite heat-resistant seamless steel pipe according to claim 1, characterized in that: The pipe making process specifically includes the following steps: C1. Heating; C2. Pipe 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, sampled and tested, and UT and MT flaw detection are carried out. After passing the test, it can be put into storage to obtain the finished steel tube.

9. The method for preparing the super-large size 9Cr ferrite heat-resistant seamless steel pipe according to claim 8, characterized in that: The heating includes a heating section and a high temperature section, and the total heating time is 35-43 hours; the heating section is divided into two zones, the first zone is a preheating zone, the temperature is 550-700°C, the insulation time is ≥3 hours, and the second zone is a heating zone, the temperature is 850-1100°C, and the insulation time is ≥8 hours; The high temperature section is divided into two zones, the temperature of the first aquaponics zone is 1200-1230°C, the insulation time is ≥10 hours, and the temperature of the second aquaponics zone is 1240-1270°C, the insulation time is ≥15 hours.

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

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