A method for forming a high-strength and high-toughness duplex stainless steel seamless pipe

Abstract

The application relates to a forming method of high-strength and high-toughness duplex stainless steel seamless pipes, and steps of the method comprise the following: (1) blank preparation: arc furnace+AOD furnace refining+LF furnace refining+electroslag remelting smelting out of a cylindrical electroslag ingot; (2) electroslag ingot rough machining; (3) electroslag ingot heat treatment; (4) electroslag ingot finish machining; (5) hot expanding, expanding with a 2500-ton vertical expander; (6) hot extrusion: extruding the steel pipe with a 6300-ton horizontal extruder; (7) steel pipe heat treatment: heating to 1070 DEG C to 1120 DEG C, and water cooling to below 40 DEG C; (8) straightening and cutting; (9) pickling; (10) steel pipe grinding; (11) cold working: total reducing amount of the steel pipe is 15mm to 30mm; (12) finishing treatment: oil removal-inspection-spray mark-bundling. The duplex stainless steel seamless pipe is produced by combining the optimized composition, the electroslag ingot, the hot expanding, the hot extrusion and the cold working, the purity of the steel, the metal yield and the production efficiency are improved, and the product has high strength, high toughness and good corrosion resistance.

Description

Technical Field

[0001] This invention belongs to the field of industrial equipment manufacturing technology, and relates to a method for forming high-strength and high-toughness duplex stainless steel seamless pipes. Background Technology

[0002] With the development of industries such as petroleum, natural gas, and chemicals, the demand for pipelines is increasing. In particular, the oil extraction sector places higher demands on the resistance of stainless steel pipes to sulfide and chloride corrosion. This requires the pipe manufacturing industry to be market-oriented, developing and utilizing moderately priced, high-performance stainless steel pipes to meet the ever-growing market demand and establish its own brand within the industry. Nickel-based alloy pipes are widely used in the petrochemical field, but their high price deters many users. To reduce pipe costs, the development of high-strength duplex stainless steel pipes has emerged. Duplex stainless steel seamless pipes are currently the most economical alloy on the market for resisting sulfide and chloride corrosion. Many leading steel-producing countries have begun to develop high-grade duplex stainless steel products; however, due to the need for cold-worked delivery, the surface quality, dimensional accuracy, and performance stability of these products are relatively poor.

[0003] Currently, the main production method in China is to produce high-grade steel by cold-drawing bars followed by hole-cutting. This method results in low material utilization, unstable cross-sectional properties, and extreme difficulty in producing high-grade steel. Summary of the Invention

[0004] The purpose of this invention is to provide a method for forming high-strength and high-toughness duplex stainless steel seamless pipes, which enables the pipes to have high strength and good toughness, good surface quality, high dimensional accuracy, and good corrosion resistance, and can be used in industries such as petroleum, natural gas, and chemical industry.

[0005] The technical solution of this invention is: a method for forming high-strength and high-toughness duplex stainless steel seamless pipes, comprising the following steps:

[0006] (1) Billet preparation: Refining in an electric arc furnace + AOD furnace + LF furnace + electroslag remelting to produce cylindrical electroslag ingots.

[0007] (2) Rough machining of electroslag ingots: electroslag ingots are cut and the inner and outer circles are machined; 4mm to 8mm is machined off on one side of the outer circle, and a through hole with a diameter of 35mm to 120mm is machined in the center to remove the surface oxide scale.

[0008] (3) Heat treatment of electroslag ingots: heating the electroslag ingots to 1040℃~1080℃ and water cooling to below 40℃ helps to reduce the content of intermetallic phases, nitrides and carbides, so that the volume fraction of ferrite in the electroslag ingot structure is 40%~60%.

[0009] (4) Finishing of electroslag ingots: The inner hole of the head is machined into a flared opening, and the outer circle of the tail is machined with chamfers and rounded corners. After machining, the α angle of the flared opening at the head is 46°, and the M value is 10mm to 20mm larger than the diameter of the working section of the reaming head. The L value is calculated based on the α angle using trigonometric functions. A chamfer with a β angle of 35° is machined at the outer circle of the tail. A rounded corner with a radius of R30mm is machined at the connection between the chamfer and the ingot body. The inner and outer surfaces are ground and polished to remove scratches, tool marks, and other defects. The surface roughness Ra≤3.2μm, diameter deviation±1.0mm, length deviation±5mm, and end face perpendicularity≤1mm.

[0010] (5) Hot expansion: The finely processed electroslag ingot is placed in the expansion barrel of a 2500-ton vertical expansion machine for expansion.

[0011] (6) Hot extrusion: The expanded electroslag ingot is placed in the extrusion barrel of a 6300-ton horizontal extruder for extrusion.

[0012] (7) Steel pipe heat treatment: The steel pipe is heated to 340℃~350℃ and placed in a heating furnace. The temperature is increased to 900℃ at a rate of 250~300℃ / h and held for 1.5h. Then, the temperature is increased to 1070℃~1120℃ at a rate of 200℃ / h and held for 1.0~2.5h before being removed from the furnace. Cooling treatment: The steel pipe is quickly transferred to rotating rollers in a pool filled with circulating water. The rollers rotate at 35rpm~55rpm, with a spacing of 1.0m~1.6m between each pair of rotating rollers. The steel pipe rotates under the drive of the rotating towing wheel, and the inner nozzle switch located at one end of the steel pipe is opened. The inner diameter of the inner nozzle is 25mm to 35mm smaller than the inner diameter of the steel pipe. Water is sprayed into the inner hole of the steel pipe along the axial direction. The time from exiting the furnace to the start of water cooling is ≤120s. After 5s to 15s of internal water cooling, the rotating towing wheel device sinks as a whole, so that the steel pipe is immersed in the water surface 350mm below the diameter. The steel pipe is cooled to below 40℃ under the simultaneous action of internal spraying and immersion in water, and the heat treatment is completed.

[0013] (8) Straightening and head cutting: Straightening is performed using pressure straightening, and the head is cut using a band saw.

[0014] (9) Pickling: The treated steel pipe is washed in a mixed acid solution to remove oxide scale and glass powder. The mass ratio of the mixed acid is hydrofluoric acid: nitric acid: water = 3%: 17%: 80%, and the temperature of the mixed acid solution is 30℃~50℃.

[0015] (10) Steel pipe grinding: Grinding the steel pipe wall to remove defects such as cracks and pits on the inner and outer surfaces;

[0016] (11) Cold working: Control the total reduction in diameter of the steel pipe to 15mm-30mm. During cold working, the wall thickness may increase by 1mm-3mm. Subsequent processes are machining, and the wall thickness is not controlled during the reduction process. Diameter reduction can be achieved gradually through a multi-stand reducing mill, with each stand reducing the diameter by 0.5mm-3.0mm; diameter reduction can also be achieved through rolling with rolls of gradually changing dimensions on the working surface, achieving the target size in one or two cold rolling passes; diameter reduction can also be achieved through forging with a radial high-speed forging mill of not less than 1100T, with each forging reduction of 1.0mm-6.0mm.

[0017] 12 Finishing: Degrease the steel pipes processed in step 11, inspect them, spray them with a mark, and bundle them.

[0018] The composition of steel used for seamless steel pipes, by mass percentage, is as follows: C: ≤0.03, Si: 0.20~0.70, Mn: 0.35~1.00, P: ≤0.030, S: ≤0.010, Cr: 24.5~27.5, Ni: 6.2~8.5, Mo: 2.8~5.5, N: 0.24~0.32, rare earth Ce: 0.005~0.05, V: 0.02~0.10, Al: 0.005~0.04, Cu: ≤0.8, W: ≤1.0, O: ≤0.0060, H: ≤0.0008, with the remainder being Fe and unavoidable impurities. PRE = %Cr + 3.3(%Mo + 0.5%W) + 16%N, with a PRE range of 41~46.

[0019] The billet preparation steps are as follows: ① Electric arc furnace smelting: using scrap steel, molten iron, and alloys as raw materials, the steel is first smelted in an electric arc furnace to obtain molten steel. The tapping temperature of the electric arc furnace smelting is ≥1620℃; ② AOD furnace refining: alloys are added according to the composition of the molten steel, along with lime, fluorite, and aluminum ingots. The pure Ar blowing time is ≥3min, the lime is baked at >500℃, and the fluorite and calcium silicate powder need to be baked at a low temperature of <200℃. The tapping temperature of the AOD refining furnace is ≥1580℃; ③ LF furnace refining: to ensure a reducing atmosphere in the ladle, calcium silicate wire and rare earth are added, and the composition of the molten steel is adjusted to the finished product range. The white slag holding time is ≥20min; ④ Electroslag remelting: the electrode billet is cast and then electroslag remelted to produce electroslag ingots.

[0020] The hot expansion process includes: ① The outer diameter of the finished electroslag ingot is 7mm to 15mm smaller than the inner diameter of the expansion barrel, and the expansion barrel is preheated to 100℃ to 350℃. ② Before expansion, the finished electroslag ingot is electromagnetically induction heated to 1185℃ to 1210℃. ③ Before expansion, the inner and outer surfaces of the finished electroslag ingot are coated with glass powder lubricant, placed inside the expansion barrel with the flared end facing upwards, and a pre-prepared glass powder lubricating bowl is placed at the flared end. The ingot is left to stand for 1 minute to 2.5 minutes, then the expansion head is placed above the glass powder lubricating bowl to expand the hole. The expansion speed is 150mm / s to 250mm / s, and the ratio of the cross-sectional area before expansion to the cross-sectional area after expansion is 1.02 to 1.41.

[0021] During the hot expansion stage, the electroslag ingot is heated by electromagnetic induction three times before expansion. The first heating power is 300KW~350KW, the temperature reaches 900℃~950℃, and the heating is uniform for 2~5 minutes. The second heating power is 400KW~500KW, the temperature reaches 1050℃~1100℃, and the heating is uniform for 2~5 minutes. The third heating power is 550KW~600KW, the temperature reaches 1185℃~1210℃, and the heating is not uniform.

[0022] The hot extrusion process includes: ① Preheating the extrusion barrel to 250℃~350℃, preheating the extrusion die and mandrel to 300℃~400℃, and coating the inner wall of the extrusion barrel and the surface of the mandrel with graphite emulsion. ② Placing the expanded electroslag ingot on the conveyor rollers and turning it around so that the flared end faces backward. ③ Performing electromagnetic induction heating on the electroslag ingot before extrusion. ④ Coating the inner and outer surfaces of the expanded electroslag ingot with glass powder lubricant, then placing it in the extrusion barrel, placing a glass lubricating pad at the entrance of the extrusion die, and extruding the steel pipe. A tail pad is prepared before extrusion. The tail pad is made of ordinary carbon steel, with an outer diameter 3mm~7mm smaller than the inner diameter of the expanding barrel, an inner diameter 3mm~5mm larger than the inner diameter of the expanding head, a length of 60mm~80mm, and a heating temperature of 950℃~1050℃. It is attached to the back of the electroslag ingot before extrusion and extruded together, automatically detaching from the pipe body after extrusion. The inner diameter of the extrusion barrel is 4mm to 10mm larger than that of the expansion barrel. The extrusion speed is 200mm / s to 350mm / s. The ratio of the cross-sectional area before extrusion to the cross-sectional area after extrusion is 4.0 to 15.0. When the temperature of the steel pipe after extrusion is not lower than 980℃, it is water-cooled to below 40℃ and then air-cooled.

[0023] During the hot extrusion stage, the electroslag ingot, after being expanded and turned, undergoes two rounds of electromagnetic induction heating before extrusion. To ensure uniform material temperature, compensating pads are placed at both ends of the ingot before heating. The first heating power is 500KW~600KW, with the temperature reaching 1100℃~1125℃, and the heating is allowed to reach uniform temperature for 2~4 minutes. The second heating power is 600KW~800KW, with the temperature reaching 1130℃~1180℃. This heating is not uniform, and the ingot is then descaled using high-pressure water after exiting the furnace.

[0024] The properties of seamless steel pipes are as follows: tensile strength ≥900MPa, yield strength Rp0.2 862~1034MPa, elongation ≥20%, hardness 29~36HRC, and transverse Charpy V-notch full-size impact energy ≥41J at -10℃.

[0025] During the hot expansion and hot extrusion stages, the glass powder lubricant consists of SiO2, Al2O3, CaO, MgO, TiO2, K2O, Na2O, and B2O3. The composition by mass percentage is: SiO2: 50%–70%, Al2O3: 1%–8%, CaO: 2%–12%, MgO: 2%–8%, TiO2: 0.1%–3%, K2O: 0.2%–3%, Na2O: 5%–20%, and B2O3: 0.5%–10%. Based on the characteristics of the extrusion process, a suitable ratio of glass powder composition is selected. The particle size of both the internal and external lubricating powders is 0.2 mm–0.3 mm, the viscosity is 20 Pa·s–80 Pa·s, and the coating thickness of both internal and external lubricating powders is 0.1 mm–0.4 mm. The particle size of the glass lubrication bowl and glass lubrication pad powder is 0.2mm to 1.5mm, the viscosity is 70pa·s to 100pa·s, the shape of the glass pad is determined by the working surface of the extrusion die and the shape of the front end of the blank, the inner hole of the glass pad is 15mm to 35mm larger than the diameter of the steel pipe, and the thickness of the glass pad is 20mm to 25mm.

[0026] This invention employs a combined process route of optimized composition, electroslag ingot, hot expansion, hot extrusion, and cold working to produce high-strength, high-toughness duplex stainless steel seamless pipes, improving steel purity, metal yield, and production efficiency. Direct extrusion of the electroslag billet reduces the electroslag ingot forging process. Extrusion molding under triaxial compressive stress facilitates the development of material plasticity, improving yield, uniform cooling during heat treatment, and straightness of the pipe body. The extruded pipe then undergoes a rotational cooling heat treatment, optimizing the microstructure and straightness. The duplex stainless steel seamless pipes produced by this invention exhibit high dimensional accuracy, good surface quality, fine internal grains, stable mechanical properties, high strength, high toughness, and good corrosion resistance. Attached Figure Description

[0027] Figure 1 Schematic diagram of the billet structure;

[0028] Figure 2 A schematic diagram of the process flow for forming high-strength and high-toughness duplex stainless steel seamless pipes.

[0029] Figure 3 A schematic diagram of the process for forming high-strength, high-toughness duplex stainless steel seamless pipes.

[0030] Figure 4 This is a schematic diagram of a vertical reamer.

[0031] Figure 5 This is a schematic diagram of a horizontal extrusion press.

[0032] Figure 6 This is a schematic diagram of the heat treatment cooling system.

[0033] Among them: 1—EAF electric arc furnace, 2—AOD refining furnace, 3—LF refining furnace, 4—ESR electroslag furnace, 5—bulk processing equipment, 6—bulk heat treatment equipment, 7—electromagnetic induction furnace before reaming, 8—lubrication equipment, 9—reamer, 10—electromagnetic induction furnace before extrusion, 11—extrusion press, 12—tube heating furnace, 13—tube cooling system, 14—pressure straightening machine, 15—degreasing and pickling tank, 16—cold working equipment, 17—ejection mechanism, 18—reaming 19—Shearing ring support; 20—Shearing ring; 21—Expanding barrel; 22—Expanding rod; 23—Expanding head; 24—Die base; 25—Extrusion die; 26—Die support; 27—Extrusion cylinder liner; 28—Extrusion cylinder intermediate layer; 29—Extrusion cylinder outer sleeve; 30—Mandrel; 31—Extrusion pad; 32—Extrusion rod; 33—Cooling water tank; 34—Inner nozzle; 35—Rotating support roller; 36—Agitator; 37—Cooling water; 38—Burnt material; 39—Steel pipe. Detailed Implementation

[0034] The present invention will now be described in detail with reference to the embodiments and accompanying drawings. The scope of protection of the present invention is not limited to the embodiments, and any modifications made by those skilled in the art within the scope defined by the claims also fall within the scope of protection of the present invention.

[0035] High-strength and high-toughness duplex stainless steel seamless pipe forming equipment, such as Figure 3 As shown, the equipment includes an EAF electric arc furnace 1, an AOD refining furnace 2, an LF refining furnace 3, an ESR electroslag furnace 4, billet processing equipment 5, billet heat treatment equipment 6, a pre-expansion electromagnetic induction furnace 7, lubrication equipment 8, an reamer 9, a pre-extrusion electromagnetic induction furnace 10, an extruder 11, a tube heating furnace 12, a tube cooling system 13, a pressure straightening machine 14, a degreasing and pickling tank 15, and cold working equipment 16. The EAF electric arc furnace, AOD refining furnace, LF refining furnace, ESR electroslag furnace, billet processing equipment, billet heat treatment equipment, pre-expansion electromagnetic induction furnace, lubrication equipment, reamer, pre-extrusion electromagnetic induction furnace, extruder, tube heating furnace, tube cooling system, pressure straightening machine, degreasing and pickling tank, and cold working equipment are arranged sequentially.

[0036] Figure 4The diagram shows a 2500T vertical reaming machine, equipped with an ejector mechanism 17, a reaming barrel base 18, a shearing ring support 19, a shearing ring 20, a reaming barrel 21, a reaming pin 22, and a reaming head 23. The diameter of the reaming pin is 5mm to 10mm smaller than the working section diameter of the reaming head, and the inner diameter of the shearing ring is 1mm to 2mm larger than the working section diameter of the reaming head. The reaming pin's deviation from the centerline of the reaming barrel must not exceed 1mm. The reaming barrel has a taper of 0.4mm to 1mm along its inner length. The reaming barrel base and the reaming barrel are fixedly connected by high-strength bolts. The reaming pin is connected to a force transmission rod via a threaded connector. The reaming barrel base supports the shearing ring support, which in turn supports both the shearing ring and the billet. The shearing ring supports the billet and removes excess material. The force transmission rod applies reaming force to the reaming head via the reaming pin. The outer diameter of the billet is 7mm to 15mm smaller than the inner diameter of the expanding barrel. The expanding barrel is preheated to 100℃ to 350℃. With the flared end of the billet facing upwards, the expanding head is placed at the flared end of the billet to expand the hole. The expanding speed is 150mm / s to 220mm / s, and the expanding ratio is 1.02 to 1.41.

[0037] Figure 5 The image shows a 6300T horizontal extrusion press, equipped with a die holder 24, an extrusion die 25, a die support 26, an extrusion barrel liner 27, an extrusion barrel intermediate layer 28, an extrusion barrel outer layer 29, a mandrel 30, an extrusion pad 31, and an extrusion rod 32. The die holder and die support are fixed by bottom slots. The front end structure of the extrusion barrel is designed to correspond to the die holder structure. The extrusion barrel has a taper of 0.3mm to 1mm along its inner length. The cavity of the die support is 15mm to 30mm larger than the cavity of the extrusion die. The extrusion rod deviates from the centerline of the extrusion barrel by no more than 0.5mm. The gap between the outer circle of the extrusion pad and the inner hole of the extrusion barrel is 1.0mm to 1.5mm. The gap between the inner hole of the extrusion pad and the mandrel is 1.5mm to 2.5mm. The length of the extrusion pad is 200mm to 210mm. The front end of the extrusion pad is the billet, and the rear end is the extrusion rod. The extrusion pad serves to transmit the thrust of the extrusion rod. The extrusion die consists of an inlet transition zone, an intermediate sizing zone, and an outlet reverse cone, with a thickness of 30mm to 38mm. During operation, the extrusion rod, mandrel, and extrusion barrel are powered by hydraulic cylinders. The extrusion barrel moves back and forth via a bottom slide, with a stroke of 2400mm to 2800mm. The mandrel's stroke relative to the extrusion rod is 1500mm, and the extrusion rod's main cylinder stroke is 2900mm to 3200mm.

[0038] Figure 6 The diagram shows a cooling system for heat treatment, comprising a cooling water tank 33, an inner nozzle 34, a rotating support roller 35, and a stirring device 36. The rotating support roller is used to horizontally position the steel pipe during cooling, and the inner nozzle is used to spray circulating water into the inner bore of the steel pipe along the axial direction. The cooling water tank is filled with circulating water and has an outlet connected to the inner nozzle via a water pump. The stirring device is used to agitate the water, promoting its flow within the cooling water tank.

[0039] Example 1

[0040] High-strength and high-toughness duplex stainless steel seamless pipes are available in two sizes: extruded pipes with an outer diameter of φ190mm and a wall thickness of 31mm, and cold-worked pipes with an outer diameter of φ172mm and a wall thickness of 32mm. The chemical composition is shown in Table 1.1.

[0041] Table 1.1 Chemical composition (mass percentage) of φ172mm×32mm duplex stainless steel seamless pipe

[0042]

[0043] Note: The remainder consists of Fe and unavoidable impurities.

[0044] The present invention discloses a method for forming high-strength, high-toughness duplex stainless steel seamless pipes, comprising the following steps:

[0045] (1) Billet Preparation: Cylindrical electroslag ingots with a diameter of φ370mm are produced by electro-arc furnace and electroslag remelting. The preparation steps are as follows: ① Electro-arc furnace smelting: using scrap steel, molten iron and alloys as raw materials, molten steel is first obtained by electro-arc furnace smelting. The steel tapping temperature of the electro-arc furnace is ≥1620℃; ② AOD furnace refining: alloys are added according to the composition of the molten steel, along with lime, fluorite and aluminum ingots. The pure Ar blowing time is ≥3min, the lime is baked at >500℃, and the fluorite and calcium silicate powder need to be baked at a low temperature <200℃. The steel tapping temperature of the AOD refining furnace is ≥1580℃; ③ LF furnace refining: to ensure a reducing atmosphere in the ladle, calcium silicate wire and rare earth are added, and the composition of the molten steel is adjusted to the finished product range. The white slag holding time is ≥20min; ④ Electroslag remelting: the electrode billet is cast and then electroslag remelted to produce electroslag ingots.

[0046] (2) Rough machining of electroslag ingots: Based on the finished duplex stainless steel seamless pipe, the cylindrical electroslag ingots are cut into sections, and then the inner and outer circles are machined to produce a billet with an outer diameter of φ362mm / inner diameter of φ40mm and a length of 890mm.

[0047] (3) Heat treatment of electroslag ingots: Heat the rough-processed electroslag ingots to 1040℃~1080℃, cool them with water to below 40℃, and then dry them.

[0048] (4) Finishing of electroslag ingots: The inner hole of the head is machined into a flared opening, and the outer circle of the tail is machined with chamfers and rounded corners. After machining, the α angle of the flared opening at the head is 46°, and the M value is 10mm to 20mm larger than the diameter of the working section of the reaming head. A chamfer with a β angle of 35° is machined at the outer circle of the tail. A rounded corner with a radius of R30mm is machined at the connection between the chamfer and the ingot body. The inner and outer surfaces are ground and polished to remove scratches, tool marks and other defects. The surface roughness Ra≤3.2μm, diameter deviation±1.0mm, length deviation±5mm, and end face perpendicularity≤1mm are achieved. A billet with an outer diameter of φ358mm / inner diameter of φ45mm × length of 890mm is produced.

[0049] (5) Hot Enlargement: The finely machined electroslag ingot is placed in the enlargement barrel of a 2500-ton vertical enlarger for enlargement. The steps include: ① Preheating the enlargement barrel to 100℃~350℃; ② Electromagnetic induction heating three times before enlargement of the finely machined electroslag ingot, heating to 1185℃~1210℃; ③ Applying glass powder lubricant to the inner and outer surfaces of the finely machined electroslag ingot before enlargement, placing it in the enlargement barrel with the flared end facing upwards, placing the pre-prepared glass powder lubricating bowl at the flared end, letting it stand for 1min~2.5min, and then placing the enlargement head above the glass powder lubricating bowl for enlargement. The enlargement speed is 150mm / s~250mm / s, the diameter of the enlargement head is φ138mm, and the inner diameter of the enlargement barrel is φ369mm.

[0050] During the hot expansion stage, the electroslag ingot undergoes three rounds of electromagnetic induction heating before expansion. The first heating round uses a power of 300-350 kW to reach a temperature of 900-950°C, with uniform heating for 2-5 minutes. The second heating round uses a power of 400-500 kW to reach a temperature of 1050-1100°C, with uniform heating for 2-5 minutes. The third heating round uses a power of 550-600 kW to reach a temperature of 1185-1210°C, but without uniform heating.

[0051] (6) Hot Extrusion: The expanded electroslag ingot is placed in the extrusion barrel of a 6300-ton horizontal extruder for extrusion. The steps include: ① Preheating the extrusion barrel to 250℃~350℃, preheating the extrusion die and mandrel to 300℃~400℃, and coating the inner wall of the extrusion barrel and the surface of the mandrel with graphite emulsion. ② Placing the expanded electroslag ingot on the conveyor rollers and turning it around so that the flared end faces backward. ③ Performing electromagnetic induction heating of the electroslag ingot before extrusion. ④ Coating the inner and outer surfaces of the expanded electroslag ingot with glass powder lubricant, then placing it in the extrusion barrel, placing a glass lubricating pad at the entrance of the extrusion die, and extruding the steel pipe. Before extrusion, prepare a tail pad made of ordinary carbon steel. The outer diameter of the tail pad is 3mm-7mm smaller than the inner diameter of the expanding barrel, and the inner diameter of the tail pad is 3mm-5mm larger than the inner diameter of the expanding head. The tail pad is 60mm-80mm long and heated to 950℃-1050℃. Before extrusion, attach the tail pad to the back of the electroslag ingot and extrude together. After extrusion, it automatically detaches from the tube body. After extrusion, when the steel tube temperature is not lower than 980℃, immerse it in water to cool to below 40℃ and then air cool. Preheat the extrusion barrel to 250℃-350℃ and the mandrel to 350℃-400℃. The extrusion speed is 200mm / s-350mm / s. The inner diameter of the extrusion barrel is φ375mm, the mandrel diameter is φ130mm, the inner diameter of the extrusion die is φ193.2mm, and the cooled extruded tube is a φ190mm×31mm steel tube.

[0052] During the hot extrusion stage, the electroslag ingot, after being expanded and turned, undergoes two rounds of electromagnetic induction heating before extrusion. To ensure uniform material temperature, compensating pads are placed at both ends of the ingot before heating. The first heating power is 500KW~600KW, with the temperature reaching 1100℃~1125℃, and the heating is allowed to reach uniform temperature for 2~4 minutes. The second heating power is 600KW~800KW, with the temperature reaching 1130℃~1180℃. This heating is not uniform, and the ingot is then descaled using high-pressure water after exiting the furnace.

[0053] (7) Steel pipe heat treatment: The steel pipe is heated to 340℃~350℃ and placed in a heating furnace. The temperature is increased to 900℃ at a rate of 250~300℃ / h and held for 1.5h. Then, the temperature is increased to 1070℃~1120℃ at a rate of 200℃ / h and held for 1.0~2.5h before being removed from the furnace. Cooling treatment: The steel pipe is quickly transferred to a rotating support roller in a water tank filled with circulating water. The rotation speed of the support roller is 35rpm~55rpm, and the distance between each pair of rotating support rollers is 1.0m~1.6m. The steel pipe rotates under the drive of the rotating support roller. The inner nozzle switch located at one end of the steel pipe is opened. The inner diameter of the inner nozzle is 25mm~35mm smaller than the inner diameter of the steel pipe. The water is sprayed axially into the inner hole of the steel pipe. The time from removal from the furnace to the start of water cooling is ≤120s. After internal water spraying for 5 to 15 seconds, the rotating support roller device is lowered as a whole, so that the steel pipe is immersed in the water surface at a depth of 350 mm or less in the diameter direction. The steel pipe is cooled to below 40°C under the combined action of internal spraying and immersion in water, and the heat treatment is completed.

[0054] (8) Straightening and head cutting: Straightening is performed using pressure straightening, and the head is cut using a band saw.

[0055] (9) Pickling: The steel pipe treated in step (8) is washed in a mixed acid solution to remove oxide scale and glass powder. The mass ratio of the mixed acid is hydrofluoric acid: nitric acid: water = 3%: 17%: 80%. The temperature of the mixed acid solution is 30℃~50℃. After pickling, rinse with clean water.

[0056] (10) Steel pipe grinding: Grinding the steel pipe wall to remove defects such as cracks and pits on the inner and outer surfaces.

[0057] 11. Cold working: The total diameter reduction of the steel pipe is 18mm, and the wall thickness is not controlled; the diameter reduction is achieved gradually through a multi-stand diameter reduction machine, with each stand reducing the diameter by 0.5mm to 3.0mm.

[0058] 12 Finishing process: Degrease the treated steel pipes, inspect them, spray them with a mark, and bundle them.

[0059] The high-strength and high-toughness duplex stainless steel seamless pipes produced were inspected, and the inspection data are shown in Table 1.2.

[0060] Table 1.2. Properties of φ172mm×32mm duplex stainless steel seamless pipe

[0061]

[0062] Example 2

[0063] High-strength, high-toughness duplex stainless steel seamless pipes are available in two sizes: extruded pipes with an outer diameter of φ230mm and a wall thickness of 51mm, and cold-worked pipes with an outer diameter of φ215mm and a wall thickness of 52mm. The chemical composition is shown in Table 2.1.

[0064] Table 2.1 Chemical composition (mass percentage) of φ215mm×52mm duplex stainless steel seamless pipe

[0065]

[0066] Note: The remainder consists of Fe and unavoidable impurities.

[0067] The method for forming high-strength and high-toughness duplex stainless steel seamless pipes includes the following steps:

[0068] (1) Billet Preparation: Cylindrical electroslag ingots with a diameter of φ370mm are produced by electro-arc furnace and electroslag remelting. The preparation steps are as follows: ① Electro-arc furnace smelting: using scrap steel, molten iron and alloys as raw materials, molten steel is first obtained by electro-arc furnace smelting. The tapping temperature of the electro-arc furnace is ≥1620℃; ② AOD furnace refining: alloys are added according to the composition of the molten steel, along with lime, fluorite and aluminum ingots. The pure Ar blowing time is ≥3min, the lime is baked at >500℃, and the fluorite and calcium silicate powder need to be baked at a low temperature <200℃. The tapping temperature of the AOD refining furnace is ≥1580℃; ③ LF furnace refining: the molten steel is sent to ensure a reducing atmosphere in the ladle, and calcium silicate wire and rare earth are added to adjust the composition of the molten steel to the finished product range. The white slag holding time is ≥20min; ④ Electroslag remelting: electrode billets are cast, and then electroslag ingots are produced by electroslag remelting.

[0069] (2) Rough machining of electroslag ingots: Based on the finished duplex stainless steel seamless pipe, the cylindrical electroslag ingots are cut into sections, and then the inner and outer circles are machined to produce a billet with an outer diameter of φ419mm / inner diameter of φ40mm and a length of 760mm.

[0070] (3) Heat treatment of electroslag ingots: Heat the rough-processed electroslag ingots to 1040℃~1080℃, cool them with water to below 40℃, and then dry them.

[0071] (4) Finishing of electroslag ingots: The inner hole of the head is machined into a flared opening, and the outer circle of the tail is machined with chamfers and fillets. After machining, the α angle of the flared opening at the head is 46°, and the M value is 10mm-20mm larger than the diameter of the working section of the reaming head. A β angle of 35° is machined at the outer circle of the tail. A fillet with a radius R30mm is machined at the connection between the chamfer and the ingot body. The inner and outer surfaces are ground and polished to remove scratches, tool marks, and other defects. The surface roughness Ra≤3.2μm, diameter deviation ±1.0mm, length deviation ±5mm, and end face perpendicularity ≤1mm. A billet with an outer diameter of φ415mm / inner diameter of φ45mm × length of 760mm is produced.

[0072] (5) Hot Enlargement: The finely finished electroslag ingot is placed in the enlargement barrel of a 2500-ton vertical enlarger for enlargement. The steps include: ① Preheating the enlargement barrel to 100℃~350℃. ② Electromagnetically induction heating is applied three times before enlargement of the finely finished electroslag ingot, with the heating temperature reaching 1185℃~1210℃. ③ Before enlargement, glass powder lubricant is applied to the inner and outer surfaces of the finely finished electroslag ingot. It is placed in the enlargement barrel with the flared end facing upwards. A pre-prepared glass powder lubricating bowl is placed at the flared end, and the ingot is left to stand for 1min~2.5min. Then, the enlargement head is placed above the glass powder lubricating bowl for enlargement. The enlargement speed is 150mm / s~250mm / s, the diameter of the enlargement head is φ140mm, and the inner diameter of the enlargement barrel is φ428mm.

[0073] (6) Hot Extrusion: The expanded electroslag ingot is placed in the extrusion barrel of a 6300-ton horizontal extruder for extrusion. The steps include: ① Preheating the extrusion barrel to 250℃~350℃, preheating the extrusion die and mandrel to 300℃~400℃, and coating the inner wall of the extrusion barrel and the surface of the mandrel with graphite emulsion. ② Placing the expanded electroslag ingot on the conveyor rollers and turning it around so that the flared end faces backward. ③ Performing electromagnetic induction heating of the electroslag ingot before extrusion. ④ Coating the inner and outer surfaces of the expanded electroslag ingot with glass powder lubricant, then placing it in the extrusion barrel, placing a glass lubricating pad at the entrance of the extrusion die, and extruding the steel pipe. Before extrusion, prepare a tail pad made of ordinary carbon steel. The outer diameter of the tail pad is 3mm-7mm smaller than the inner diameter of the expanding barrel, and the inner diameter of the tail pad is 3mm-5mm larger than the inner diameter of the expanding head. The tail pad is 60mm-80mm long and heated to 950℃-1050℃. Before extrusion, attach the tail pad to the back of the electroslag ingot and extrude it together. After extrusion, it automatically detaches from the tube body. After extrusion, when the steel tube temperature is not lower than 980℃, immerse it in water to cool to below 40℃ and then air cool. Preheat the extrusion barrel to 250℃-350℃ and the mandrel to 350℃-400℃. The extrusion speed is 200mm / s-350mm / s. The inner diameter of the extrusion barrel is φ435mm, the mandrel diameter is φ129.5mm, the inner diameter of the extrusion die is φ233.9mm, and the dimensions of the cooled extruded tube are φ230mm×51mm steel pipe.

[0074] (7) Steel pipe heat treatment: The steel pipe is heated to 340℃~350℃ and placed in a heating furnace. The temperature is increased to 900℃ at a rate of 250~300℃ / h and held for 1.5h. Then, the temperature is increased to 1070℃~1120℃ at a rate of 200℃ / h and held for 1.0~2.5h before being removed from the furnace. Cooling treatment: The steel pipe is quickly transferred to a rotating support roller in a water tank filled with circulating water. The rotation speed of the support roller is 35rpm~55rpm, and the distance between each pair of rotating support rollers is 1.0m~1.6m. The steel pipe rotates under the drive of the rotating support roller. The inner nozzle switch located at one end of the steel pipe is opened. The inner diameter of the inner nozzle is 25mm~35mm smaller than the inner diameter of the steel pipe. The water is sprayed axially into the inner hole of the steel pipe. The time from removal from the furnace to the start of water cooling is ≤120s. After internal water spraying for 5 to 15 seconds, the rotating support roller device is lowered as a whole, so that the steel pipe is immersed in the water surface at a depth of 350 mm or less in the diameter direction. The steel pipe is cooled to below 40°C under the combined action of internal spraying and immersion in water, and the heat treatment is completed.

[0075] (8) Straightening and head cutting: Straightening is performed using pressure straightening, and the head is cut using a band saw.

[0076] (9) Pickling: The treated steel pipe is washed in a mixed acid solution to remove oxide scale and glass powder. The mass ratio of the mixed acid is hydrofluoric acid: nitric acid: water = 3%: 17%: 80%. The temperature of the mixed acid solution is 30℃~50℃. After pickling, rinse with clean water.

[0077] (10) Steel pipe grinding: Grinding the steel pipe wall to remove defects such as cracks and pits on the inner and outer surfaces.

[0078] 11. Cold working: The total diameter reduction of the steel pipe is 15mm, and the wall thickness is not controlled; the diameter reduction is achieved by cold rolling with rolls of gradually changing size on the working surface, and the target size is achieved in one cold rolling pass.

[0079] 12 Finishing process: Degrease the treated steel pipes, inspect them, spray them with a mark, and bundle them.

[0080] The high-strength and high-toughness duplex stainless steel seamless pipes produced were inspected, and the inspection data are shown in Table 2.2.

[0081] Table 2.2. Properties of φ215mm×52mm duplex stainless steel seamless pipe

[0082]

Claims

1. A method of forming a high-strength high-ductility duplex stainless steel seamless pipe, characterized by: The forming method comprises the following steps:

1. blank preparation: arc furnace+AOD furnace refining+LF furnace refining+electroslag remelting to produce cylindrical electroslag ingot; 2. electroslag ingot rough machining: electroslag ingot cutting, processing inner and outer circles; outer circle single edge processing 4mm-8mm, center processing 35mm-120mm diameter through hole, removing surface oxide skin; 3. electroslag ingot heat treatment: electroslag ingot heating to 1040-1080℃, water cooling to below 40℃; 4. electroslag ingot finishing machining: processing horn mouth in head inner hole, processing bevel and round angle in tail outer circle; after processing, head horn mouth α angle is 46°, M value is 10mm-20mm larger than the working section diameter of hole expanding head, L value is calculated according to α angle by trigonometric function, tail outer circle is processed with β angle of 35°, bevel and round angle with radius R of 30mm is processed at the connection between bevel and ingot body, inner and outer surface grinding and polishing, removing scratch and tool mark defects, surface roughness Ra≤3.2μm, diameter deviation ±1.0mm, length deviation ±5mm, end surface perpendicularity ≤1mm; 5. hot hole expansion: placing the finished electroslag ingot in the hole expansion barrel of 2500 ton vertical hole expander for hole expansion; 6. hot extrusion: placing the electroslag ingot after hole expansion in the extrusion barrel of 6300 ton horizontal extruder for extrusion; 7. steel pipe heat treatment: heating the steel pipe to 340-350℃, loading into heating furnace, heating to 900℃ at 250-300℃ / h, holding for 1.5h, heating to 1070-1120℃ at 200℃ / h, holding for 1.0-2.5h, discharging; cooling treatment: quickly transferring the steel pipe to the rotating support wheel in the water pool filled with circulating water, rotating speed of support wheel 35-55rpm, distance between each pair of rotating support wheel 1.0-1.6m, rotating the steel pipe under the driving of rotating support wheel, opening the inner nozzle switch arranged at one end of the steel pipe, inner nozzle inner diameter is 25-35mm smaller than the inner diameter of the steel pipe, inner nozzle water is sprayed into the inner hole of the steel pipe along the axial direction, time from discharging to starting water cooling ≤120s; after 5-15s of inner nozzle water cooling, the whole rotating support wheel device sinks, so that the steel pipe is immersed below the water surface in the diameter direction by 350mm, the steel pipe is cooled to below 40℃ under the action of inner nozzle water and immersion water, and the heat treatment is finished; 8. straightening and cutting head: straightening by pressure straightening, cutting head by band saw; 9. pickling: removing oxide skin and glass powder of the treated steel pipe in mixed acid solution, mass ratio of mixed acid is hydrofluoric acid:nitric acid:water=3%:17%:80%, temperature of mixed acid solution 30-50℃; 10. steel pipe grinding: grinding the wall of the steel pipe, removing the inner and outer surface cracks and pit defects; 11. cold machining: controlling the total reduction of the steel pipe 15-30mm, the wall thickness is thickened by 1-3mm during cold machining, the subsequent process is machining, and the wall thickness is not controlled during reduction; 12. finishing treatment: oil removal, inspection, marking and bundling of the steel pipe treated in step 11. Wherein: α is the horn mouth angle of the blank head; β is the processing angle of the blank tail outer circle; M value is the opening diameter of the horn mouth of the blank head; L value is the height of the horn mouth of the blank head.

2. The method of forming a high-strength high-ductility duplex stainless steel seamless pipe according to claim 1, characterized by: The seamless steel tube steel has the following components by mass percentage: C: ≤0.03, Si: 0.20-0.70, Mn: 0.35-1.00, P: ≤0.030, S: ≤0.010, Cr: 24.5-27.5, Ni: 6.2-8.5, Mo: 2.8-5.5, N: 0.24-0.32, rare earth Ce: 0.005-0.05, V: 0.02-0.10, Al: 0.005-0.04, Cu: ≤0.8, W: ≤1.0, O: ≤0.0060, H: ≤0.0008, and the rest is Fe and inevitable impurities.

3. The method of forming a high-strength high-ductility duplex stainless steel seamless pipe according to claim 1, characterized by: The blank preparation step is: ① arc furnace smelting, taking scrap steel, molten iron and alloy as raw materials, first smelting in an arc furnace to obtain molten steel, the arc furnace smelting temperature is ≥1620℃; ② AOD furnace refining, feeding into an AOD refining furnace, adding alloy according to the composition content of the molten steel, adding lime, fluorite and aluminum ingot, pure Ar blowing time ≥3min, lime baking >500℃, fluorite and silicon calcium powder need low temperature baking <200℃, AOD refining furnace tapping temperature ≥1580℃; ③ LF furnace refining, the molten steel is sent to the LF furnace for refining, ensuring a reducing atmosphere in the ladle, adding silicon calcium wire and rare earth, adjusting the composition of the molten steel to the product range, and the white slag retention time is ≥20min; ④ electroslag remelting, pouring out the electrode blank after electroslag remelting, and smelting the electroslag ingot.

4. The method of forming a high-strength high-ductility duplex stainless steel seamless pipe according to claim 1, characterized by: The hot expanding step includes: ① the outer diameter of the finished electroslag ingot is 7mm-15mm smaller than the inner diameter of the expanding barrel, and the expanding barrel is preheated to a temperature of 100℃-350℃; ② electromagnetic induction heating before expanding the electroslag ingot, heating temperature to 1185℃-1210℃; ③ smearing glass powder lubricant on the inner and outer surfaces of the electroslag ingot before expanding, placing the expanding barrel with the horn mouth end upward, placing the prepared glass powder lubricating bowl at the horn mouth, standing for 1min-2.5min, and then placing the expanding head above the glass powder lubricating bowl for expanding, the expanding speed is 150mm / s-250mm / s, and the ratio of the cross-sectional area before expanding to the cross-sectional area after expanding is 1.02-1.

41.

5. The method of forming a high-strength high-ductility duplex stainless steel seamless pipe according to claim 4, characterized by: In the hot expanding stage, the electroslag ingot is heated by electromagnetic induction for a total of 3 times before expanding; the first heating power is 300KW-350KW, the temperature is 900℃-950℃, and the soaking time is 2-5min; the second heating power is 400KW-500KW, the temperature is 1050℃-1100℃, and the soaking time is 2-5min; the third heating power is 550KW-600KW, the temperature is 1185℃-1210℃, and there is no soaking.

6. The method of forming a high strength high toughness duplex stainless steel seamless pipe according to claim 1, characterized by: The hot extrusion step comprises: ①preheating the extrusion barrel to a temperature of 250-350 DEG C, preheating the extrusion die and the core rod to a temperature of 300-400 DEG C, and brushing the inner wall of the extrusion barrel and the surface of the core rod with graphite milk; ②placing the reamed electroslag ingot on a conveying roller, turning the reamed electroslag ingot, and making the flared end face backward; ③electromagnetic induction heating before the electroslag ingot is extruded; ④applying glass powder lubricant to the inner and outer surfaces of the reamed electroslag ingot, then placing the reamed electroslag ingot into the extrusion barrel, placing a glass lubricating pad at the entrance of the extrusion die, and extruding the steel pipe; preparing a tail pad in advance before extrusion, the tail pad being made of plain carbon steel, the outer diameter of the tail pad being 3-7 mm smaller than the inner diameter of the reaming barrel, the inner diameter of the tail pad being 3-5 mm larger than the inner diameter of the reaming head, the length of the tail pad being 60-80 mm, and the heating temperature of the tail pad being 950-1050 DEG C; the tail pad is automatically separated from the pipe body after extrusion.

7. The method of forming a high-strength high-ductility duplex stainless steel seamless pipe according to claim 6, characterized by: In the hot extrusion stage, the reamed electroslag ingot is heated by electromagnetic induction for two times before extrusion, in order to realize uniform overall material temperature, the head and tail of the electroslag ingot are respectively placed with compensation pads before heating; the first heating power is 500-600 KW, the temperature is 1100-1125 DEG C, and the temperature is kept for 2-4 min; the second heating power is 600-800 KW, the temperature is 1130-1180 DEG C, and the temperature is not kept, and high-pressure water descaling is performed after the furnace is discharged.

8. The method of forming a high-strength high-ductility duplex stainless steel seamless pipe according to claim 1, characterized by: The properties of the seamless steel pipe are as follows: tensile strength ≥ 900 MPa, Rp0.2 yield strength 862-1034 MPa, elongation ≥ 20%, hardness 29-36 HRC, and transverse Charpy V-notch full-size impact energy at -10 DEG C ≥ 41 J.

9. The method of forming a high-strength high-ductility duplex stainless steel seamless pipe according to claim 1, characterized by: In the hot reaming and hot extrusion stage, the glass powder lubricant is composed of SiO2, Al2O3, CaO, MgO, TiO2, K2O, Na2O and B2O3; the components are as follows in terms of mass percentage: SiO2: 50-70%, Al2O3: 1-8%, CaO: 2-12%, MgO: 2-8%, TiO2: 0.1-3%, K2O: 0.2-3%, Na2O: 5-20%, and B2O3: 0.5-10%.

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