High-toughness 35crni3mov forged thick-wall seamless pipe and manufacturing method thereof

Abstract

The present application relates to alloy steel manufacturing technical field, especially to a kind of high strength and toughness 35CrNi3MoV forged thick-walled seamless pipe and manufacturing method thereof.In one aspect, the composition of the conventional 35CrNi3MoV steel is optimized, and on the other hand, when upsetting and drawing forging is carried out, the total upsetting amount of the upsetting and drawing forging is controlled to be greater than 70%, which is generally controlled to be less than 50% in the prior art, which is beneficial to break the as-cast structure of the core of the steel ingot and break the coarse grains.In addition, the temperature of each forging and drawing is controlled in a reasonable range and the short reheat time can avoid grain growth again and improve the uniformity of the structure.Finally, the traditional quenching uses oil cooling, and the quenching of the present application uses water-air alternating cooling, which is beneficial to obtain complete martensite structure.The thick-walled seamless pipe manufactured by the present application has uniform structure and high strength and toughness.

Description

Technical Field

[0001] This invention relates to the field of alloy steel manufacturing technology, and in particular to a high-strength and high-toughness 35CrNi3MoV forged thick-walled seamless tube and its manufacturing method. Background Technology

[0002] 35CrNi3MoV steel is a high-strength alloy structural steel containing various alloying elements such as chromium, nickel, molybdenum, and vanadium. It possesses excellent comprehensive mechanical properties and has extremely broad application prospects in the industrial field. This steel can be used to manufacture various key components subjected to high loads and complex working conditions. Specifically, in the petrochemical industry, it can be used to manufacture core forgings for equipment such as hydrogenation reactors, coal liquefaction reactors, and ultra-high pressure artificial crystal reactors; in the nuclear power industry, it can be used for large forgings such as main flanges, nozzle sections, and heads of reactor pressure vessels, meeting design life requirements of over 60 years and radiation resistance; in the military industry, it can be used to manufacture key components such as artillery barrels, breechblocks, and breechblocks, capable of withstanding high temperatures, high pressures, and high impact loads; in the energy equipment industry, it can be used to manufacture rotors in thermal power, nuclear power, and large metallurgical equipment, as well as load-bearing components such as connecting rods, crankshafts, and high-load gears in heavy machinery; and in the deep-sea equipment industry, it can also be used to manufacture pressure hulls for full-ocean-depth manned submersibles and ultra-high pressure experimental chambers.

[0003] With the development of industrial equipment towards larger scale and longer service life, higher requirements are being placed on the specifications and performance of 35CrNi3MoV steel products, especially large-section 35CrNi3MoV forged thick-walled seamless tubes with a finished weight exceeding 20 tons, whose application demand is increasing. However, the manufacturing process of such large thick-walled seamless tubes faces many technical challenges: due to the large size and thick cross-section of the base material, problems such as incomplete forging, uneven microstructure, and coarse grains are prone to occur during forging. Moreover, the microstructure of 35CrNi3MoV steel after forging has a very strong heritability. If the forging ratio is insufficient or the final heat treatment is performed directly after forging, the original coarse-grained state will be retained due to microstructure inheritance, further leading to uneven overall mechanical properties of the product.

[0004] Currently, the performance of large 35CrNi3MoV forged thick-walled seamless tubes produced using traditional manufacturing methods is approaching the inherent limits of the material, making it difficult to simultaneously meet the dual requirements of high strength and high toughness. For example, a company's trial production of this type of product using traditional processes resulted in a yield strength Rp of... 0.1 The strength is approximately 1100MPa, while the impact absorption energy at -40℃ is only 5~15J. The toughness is seriously insufficient, which cannot meet the stringent requirements for impact resistance and fracture resistance of components in fields such as nuclear power, deep-sea equipment, and high-end military industry. This limits the application of 35CrNi3MoV steel in large-scale high-end equipment. Summary of the Invention

[0005] In view of this, the purpose of this invention is to provide a high-strength and high-toughness 35CrNi3MoV forged thick-walled seamless tube and its manufacturing method. The thick-walled seamless tube manufactured by this invention has a uniform microstructure and possesses both high strength and high toughness.

[0006] To achieve the above-mentioned objectives, the present invention provides the following technical solution: This invention provides a method for manufacturing a high-strength and high-toughness 35CrNi3MoV forged thick-walled seamless tube, comprising the following steps: The steel ingot is heated and then upset forged to obtain the first billet; the total upsetting amount of the upset forging is >70%; the steel ingot, by mass percentage, comprises the following elements: C: 0.36~0.39%, Mn: 0.45~0.65%, Si: 0.25~0.35%, Cr: 0.90~1.10%, Ni: 2.90~3.20%, Mo: 0.50~0.60%, V: 0.18~0.23%, P≤0.008%, S≤0.003%, Cu≤0.20%, Al≤0.010%, N≤60ppm, O≤30ppm, H≤1.5ppm and the balance Fe; After the first billet is reheated in the furnace, it is forged and drawn to obtain the second billet. The second billet is reheated in the furnace for the second time and then forged and drawn again to obtain the third billet. The third billet is reheated in the furnace for the third time and then forged and drawn for the third time to obtain the fourth billet. The fourth billet is subjected to post-forging heat treatment to obtain the fifth billet; The fifth blank is subjected to external turning and boring to obtain a tube blank; The tube blank is subjected to normalizing, quenching, first tempering and second tempering in sequence to obtain the high strength and toughness 35CrNi3MoV forged thick-walled seamless tube; the quenching is cooled by alternating water cooling and air cooling. The temperatures for the first and second forging drawing are 1160±10℃~900±10℃; the temperature for the third forging drawing is 1120~850℃. The holding time for the first reheating, the second reheating, and the third reheating is 2-4 hours independently; The high-strength and tough 35CrNi3MoV forged thick-walled seamless tube has a wall thickness of 150~500mm, an inner diameter of ≤350mm, and a length greater than 10m.

[0007] Preferably, the initial forging temperature of the upsetting and drawing forging is 1160±10℃, and the final forging temperature is 900±10℃.

[0008] Preferably, the upsetting and drawing forging is a multi-pass upsetting and drawing forging; except for the first upsetting and drawing forging, each subsequent upsetting and drawing forging is followed by a reheating and heat preservation process, with each reheating and heat preservation process taking 4 to 6 hours independently.

[0009] Preferably, the post-forging heat treatment includes: air-cooling the fourth billet to a surface temperature of 600~700℃ and holding it in a furnace for at least 5 hours; then furnace-cooling it to 250~300℃ and holding it for 16~18 hours; then heating it to 870℃ at a heating rate of ≤80℃ / h and holding it for at least 18 hours; then cooling it to 570℃ and holding it for at least 15 hours; then heating it to 650℃ at a heating rate of ≤50℃ / h and holding it for at least 150 hours; then cooling it to 400~450℃ at a cooling rate of ≤40℃ / h; and finally cooling it to ≤150℃ at a cooling rate of ≤20℃ / h before removing it from the furnace and air-cooling it.

[0010] Preferably, the normalizing is carried out in a pit furnace with 17 zones. From top to bottom, the temperature of zones 1 to 7 is 880±10℃, and the temperature of zones 8 to 17 is 890±10℃. The rate of heating to the normalizing temperature is ≤100℃ / h, and the furnace is air-cooled for ≥9h after exiting the furnace.

[0011] Preferably, the quenching temperature is 860+10℃, and the rate of heating to the quenching temperature is ≤100℃ / h.

[0012] Preferably, the first tempering is carried out in a 12-zone pit furnace, with the temperature of zones 1-3 being 580+5℃, zones 4-6 being 585+5℃, and zones 8-12 being 590+5℃ from top to bottom, the heating rate being ≤100℃ / h, and the temperature being cooled to room temperature.

[0013] Preferably, the secondary tempering temperature is 520+10℃, the heating rate is ≤100℃ / h, and the temperature is air-cooled to room temperature.

[0014] Preferably, the third forging and drawing is a stacking process, with the overlapping area being 20-30% of the area of ​​the previous strike.

[0015] This invention provides a high-strength and high-toughness 35CrNi3MoV forged thick-walled seamless tube manufactured by the method described above, with a yield strength Rp 0.1 ≥1104MPa; Elongation after fracture A≥40%; Reduction of area Z≥12%; AKV impact energy at -40℃≥40J.

[0016] This invention provides a method for manufacturing high-strength and toughness 35CrNi3MoV forged thick-walled seamless tubes. On one hand, this invention optimizes the composition of traditional 35CrNi3MoV steel. Specifically, C, Si, Cr, Ni, Mo, and V are selected from the upper limit range based on the traditional broad range, while the five harmful elements and P, S, O, N, and H are removed to the maximum extent. Compared to the traditional middle limit range, this refines the grains, increases the upper limit of the material's strength and toughness, and slightly improves hardenability. On the other hand, during upset forging, this invention controls the total upset amount to >70% (existing technologies generally control it below 50%), which is beneficial for breaking the as-cast structure of the ingot core and crushing coarse grains. Furthermore, this invention controls the temperature of each forging and drawing stage within a reasonable range and the reheating time in the furnace to prevent grain regrowth and improve the uniformity of the microstructure. This creates better conditions for subsequent heat treatment, ensuring the uniformity of the overall material properties (reflected in the uniformity of surface hardness) after heat treatment while significantly improving the material's strength and toughness. Finally, traditional quenching uses oil cooling, which cannot obtain a complete martensitic structure. However, the quenching of this invention uses alternating water cooling and air cooling, which can obtain a complete martensitic structure, further improving the strength and toughness of the material. It also effectively avoids the drawbacks of quenching 35CrNi3MoV forged thick-walled seamless tubes, such as difficulty in quenching through, excessive cooling intensity leading to cracking, and easy deformation.

[0017] The results of the embodiments show that the yield strength Rp of the 35CrNi3MoV forged thick-walled seamless tube manufactured by the present invention is [missing information]. 0.1 ≥1104MPa; Elongation after fracture A≥40%; Reduction of area Z≥12%; AKV impact energy at -40℃≥40J. Attached Figure Description

[0018] Figure 1 This is a flowchart illustrating the manufacturing method of the high-strength and high-toughness 35CrNi3MoV forged thick-walled seamless tube in the embodiment. Detailed Implementation

[0019] This invention provides a method for manufacturing a high-strength and high-toughness 35CrNi3MoV forged thick-walled seamless tube, comprising the following steps: The steel ingot is heated and then upset forged to obtain the first billet; the total upsetting amount of the upset forging is >70%; the steel ingot, by mass percentage, comprises the following elements: C: 0.36~0.39%, Mn: 0.45~0.65%, Si: 0.25~0.35%, Cr: 0.90~1.10%, Ni: 2.90~3.20%, Mo: 0.50~0.60%, V: 0.18~0.23%, P≤0.008%, S≤0.003%, Cu≤0.20%, Al≤0.010%, N≤60ppm, O≤30ppm, H≤1.5ppm and the balance Fe; After the first billet is reheated in the furnace, it is forged and drawn to obtain the second billet. The second billet is reheated in the furnace for the second time and then forged and drawn again to obtain the third billet. The third billet is reheated in the furnace for the third time and then forged and drawn for the third time to obtain the fourth billet. The fourth billet is subjected to post-forging heat treatment to obtain the fifth billet; The fifth blank is subjected to external turning and boring to obtain a tube blank; The tube blank is subjected to normalizing, quenching, first tempering and second tempering in sequence to obtain the high strength and toughness 35CrNi3MoV forged thick-walled seamless tube.

[0020] The present invention involves heating a steel ingot and then upsetting and forging it to obtain a first billet.

[0021] In this invention, the steel ingot comprises, by mass percentage, the following elements: C: 0.36~0.39%, Mn: 0.45~0.65%, Si: 0.25~0.35%, Cr: 0.90~1.10%, Ni: 2.90~3.20%, Mo: 0.50~0.60%, V: 0.18~0.23%, P≤0.008%, S≤0.003%, Cu≤0.20%, Al≤0.010%, N≤60ppm, O≤30ppm, H≤1.5ppm, and the balance Fe.

[0022] In a specific embodiment, the C content in the steel ingot can be 0.36%, 0.37%, 0.38%, or 0.39%; the Mn content can be 0.45%, 0.50%, 0.55%, 0.60%, or 0.65%; the Si content can be 0.25%, 0.28%, 0.30%, 0.32%, or 0.35%; the Cr content can be 0.90%, 0.95%, 1.00%, 1.05%, or 1.10%; the Ni content can be 2.90%, 2.95%, 3.00%, 3.05%, 3.10%, 3.15%, or 3.20%; the Mo content can be 0.50%, 0.52%, 0.54%, 0.55%, 0.56%, 0.58%, or 0.60%; and the V content can be 0.18%, 0.19%, 0.20%, 0.21%, 0.22%, or 0.23%. In related technologies, when preparing 35CrNi3MoV seamless steel pipes, the content of each element is generally controlled within the middle range of the national standard range for 35CrNi3MoV steel. However, this application increases the content of C, Si, Cr, Ni, Mo, and V (i.e., adopts the upper limit range), while lowering the limits of the five harmful elements P, S, O, N, and H. This is beneficial to improving the strength of 35CrNi3MoV steel, refining the grain (improving toughness), and also improving the hardenability of the steel pipe.

[0023] In this invention, the steel ingot is preferably an electroslag ingot. Before heating, the electroslag ingot is preferably subjected to compositional testing and appearance quality control. Specifically, the present invention preferably removes 150mm from the ingot's lead plate and tail section, and samples are taken from both ends for compositional testing. Only ingots meeting the aforementioned compositional requirements can be put into use. Furthermore, the surface of the electroslag ingot must be free of defects such as flow channels and slag grooves; any normal residual defects on the surface of the electroslag remelted ingot must be thoroughly cleaned. This is well-known in the art and will not be elaborated upon here.

[0024] In this invention, the heating preferably includes: first heating the steel ingot to 650±10℃ and holding it at that temperature for at least 5 hours, then heating it to 850±10℃ and holding it at that temperature for at least 6 hours, then heating it to 1230±10℃ and holding it at that temperature for at least 36 hours, and finally cooling it to 1180±10℃ and holding it at that temperature for at least 6~8 hours.

[0025] In this invention, the rates of the first heating, the second heating, the third heating, and the cooling are preferably ≤100℃ / h. Large steel ingots have large internal and external temperature differences. This invention employs segmented heating to avoid inconsistent internal and external temperature rises during heating, which can generate significant stress and lead to thermal stress cracking. The purpose of holding at 1230±10℃ is for homogenization, while 1180±10℃ is the forging temperature after removal from the furnace.

[0026] In this invention, the initial forging temperature of the upset forging is preferably 1160±10℃, and the final forging temperature is preferably 900±10℃; the upset forging is preferably a multi-pass upset forging; except for the first upset forging, each subsequent upset forging is followed by a reheating and heat preservation process, and the reheating and heat preservation time for each process is independently 4~6 hours, which can be 4, 5 or 6 hours in specific embodiments.

[0027] In this invention, the upsetting and drawing forging is more preferably performed in two passes. The upsetting amount in the first upsetting and drawing forging is preferably 30-40%, and in specific embodiments it can be 30%, 32%, 35%, 37%, or 40%. The upsetting amount in the second upsetting and drawing forging is preferably 50-70%, and in specific embodiments it can be 50%, 53%, 55%, 58%, 60%, 62%, 65%, 68%, or 70%. In this invention, after each upsetting, the billet is drawn back to near its original height, which is well known in the art. After the final drawing, the billet is preferably turned into an octagon to obtain the first billet. In this invention, the total upsetting amount of the upsetting and drawing forging is >70%, preferably >70% and ≤110%, and in specific embodiments it can be 75%, 80%, 85%, 90%, 95%, 100%, 105%, or 110%. Prior to this application, those skilled in the art were unaware that the plasticity of 35CrNi3MoV steel billets could withstand deformation exceeding 70%. Therefore, related technologies generally controlled the total upsetting amount in upsetting and drawing forging to below 50%. However, this invention, by creatively controlling the total upsetting amount in upsetting and drawing forging to >70%, can break the as-cast structure of the steel ingot core, break coarse grains, and thus improve the uniformity of the structure.

[0028] After obtaining the first billet, the present invention reheats the first billet in the furnace and then performs a first forging and drawing process to obtain the second billet.

[0029] In this invention, the preferred temperature for the first reheating is 1160±10℃, and the preferred holding time is 2~4h, which can be 2, 2.5, 3, 3.5 or 4h in specific embodiments. In this invention, the preferred temperature for the first forging drawing is 1160±10℃~900±10℃; the preferred reduction amount for the first forging drawing is 20~30%, which can be 20%, 22%, 24%, 26%, 28% or 30% in specific embodiments. This invention controls the reduction amount of the first forging drawing within the above range to avoid forging cracks.

[0030] After obtaining the second billet, the present invention reheats the second billet in the furnace for a second time and then performs a second forging and drawing process to obtain the third billet.

[0031] In this invention, the preferred temperature for the second reheating is 1160±10℃, and the preferred holding time is 2~4h, which can be 2, 2.5, 3, 3.5 or 4h in specific embodiments. In this invention, the preferred temperature for the second forging drawing is 1160±10℃~900±10℃; the preferred reduction amount for the second forging drawing is 20~30%, which can be 20%, 22%, 24%, 26%, 28% or 30% in specific embodiments. Preferably, a 16-angle beveling is performed during the second forging drawing process to ensure subsequent forming.

[0032] After obtaining the third billet, the present invention reheats the third billet in the furnace for a third time and then performs a third forging and drawing process to obtain the fourth billet.

[0033] In this invention, the preferred temperature for the third reheating is 1140±10℃, and the preferred reheating time is 2~4 hours, which can be 2, 3, or 4 hours in specific embodiments. In this invention, the preferred temperature for the third forging and drawing is 1120~850℃. This invention does not impose a special limitation on the deformation amount of the third forging and drawing; drawing to the target outer diameter is sufficient, and the reduction is generally 5~15%.

[0034] In this invention, the third forging drawing is preferably performed in a V-shaped die. The third forging drawing is preferably performed using a stacking process, with the overlapping area preferably being 20-30% of the area of ​​the previous strike; in specific embodiments, this can be 20%, 22%, 24%, 26%, 28%, or 30%. This stacking process avoids defects such as folding and improves the dimensional accuracy of the forging billet. In this invention, when forging the end section, light hammering and continuous repositioning are required to prevent rapid temperature drop at the forging billet end, which could lead to rapid deformation and cracking due to low temperature. Alternatively, the ends can be hammered preferentially to maintain the forging billet within a temperature range of 1120-850°C.

[0035] After obtaining the fourth billet, the present invention performs post-forging heat treatment on the fourth billet to obtain the fifth billet.

[0036] In this invention, the post-forging heat treatment preferably includes: air-cooling the fourth billet to a surface temperature of 600-700°C and holding it in a furnace for at least 5 hours; then furnace-cooling it to 250-300°C and holding it for 16-18 hours; next, heating it to 870°C at a rate of ≤80°C / h and holding it for at least 18 hours; then cooling it to 570°C and holding it for at least 15 hours; then heating it to 650°C at a rate of ≤50°C / h and holding it for at least 150 hours; then cooling it to 400-450°C at a rate of ≤40°C / h; and finally, cooling it to ≤150°C at a rate of ≤20°C / h before removing it from the furnace and air-cooling it. This invention, through post-forging heat treatment, homogenizes the billet's microstructure, eliminates thermal stress, and reduces hardness and strength through annealing, facilitating subsequent machining.

[0037] After obtaining the fifth blank, the present invention performs external turning and boring on the fifth blank to obtain a tube blank.

[0038] The present invention does not have any special requirements for the method of external turning and hole machining; well-known machining methods in the art can be used.

[0039] After obtaining the tube blank, the present invention performs normalizing, quenching, first tempering and second tempering on the tube blank in sequence to obtain the high strength and toughness 35CrNi3MoV forged thick-walled seamless tube.

[0040] In this invention, the normalizing is preferably carried out in a 17-zone pit furnace. From top to bottom, the temperature of zones 1 to 7 is preferably 880±10℃, and the temperature of zones 8 to 17 is preferably 890±10℃. The rate of heating to the normalizing temperature is preferably ≤100℃ / h, and the tube is air-cooled for ≥9h after exiting the furnace. In this invention, the bottom end of the tube blank can be located in any temperature zone from 8 to 17, and the top end can be located in any temperature zone from 1 to 7, mainly related to the length of the tube blank. In this invention, the holding time for normalizing is related to the wall thickness. The holding time for normalizing is = a×k×D, where a is the heating coefficient (min / mm); k is the correction coefficient (1.2~1.4); and D is the tube blank wall thickness (mm). For the high-strength and high-toughness 35CrNi3MoV forged thick-walled seamless tube of this invention, the holding time for normalizing is = (1.2~1.4)×(1.2~1.4)×D.

[0041] Since the tube blank is longer than 10m and has extremely high strength, it is easy to deform when laid flat, and the deformation cannot be corrected. The present invention can avoid the deformation problem by normalizing in a pit furnace. At the same time, controlling the temperature of each zone within the above range is beneficial to maintaining the temperature uniformity of different positions of the tube blank. Normalizing refines the grains and achieves the homogenization of the microstructure.

[0042] In this invention, the quenching temperature is preferably 860 ± 10°C, and the heating rate to the quenching temperature is preferably ≤100°C / h. In this invention, the holding time for quenching is related to the wall thickness, and the holding time for quenching is = a × k × D, where a is the heating coefficient (min / mm); k is a correction coefficient (1.2~1.4); and D is the wall thickness of the tube blank (mm). For the high-strength and high-toughness 35CrNi3MoV forged thick-walled seamless tube of this invention, the holding time for quenching is = (1.2~1.4) × (1.2~1.4) × D. In this invention, the cooling method for quenching is preferably alternating between water cooling and air cooling. After the holding time is completed, this invention preferably involves vertical suspension and air cooling for 5~6 minutes, followed by water cooling for 3~4 minutes, alternating cooling to room temperature.

[0043] In this field, conventional quenching is oil cooling, which cannot obtain a fully martensitic structure. This invention employs alternating water and air cooling to obtain a fully martensitic structure. A fully martensitic structure provides excellent conditions for achieving higher uniformity in the tempered structure and properties. If the tempered martensite is incomplete, i.e., there is residual austenite, the austenite cannot transform into sorbite + bainite during tempering and will slowly transform into martensite later, generating internal stress and potentially leading to cracking. Even trace amounts of austenite can cause uneven hardness, even without cracking.

[0044] In this invention, the primary tempering is preferably carried out in a 12-zone pit furnace. From top to bottom, the preferred temperatures for zones 1-3 are 580 ± 5°C, for zones 4-6 are 585 ± 5°C, and for zones 8-12 are 590 ± 5°C. The rate of heating to the first tempering temperature is preferably ≤100°C / h, followed by cooling to room temperature. In this invention, the holding time for the primary tempering is related to the wall thickness. The holding time for the primary tempering is calculated as a × k × D, where a is the heating coefficient (min / mm), k is a correction coefficient (1.2-1.4), and D is the billet wall thickness (mm). For the high-strength and high-toughness 35CrNi3MoV forged thick-walled seamless tube of this invention, the holding time for the primary tempering is (1.2-1.4) × (1.2-1.4) × D. This invention obtains a tempered microstructure of sorbite + bainite, and a small amount of retained martensite or fine retained austenite through primary tempering, achieving an excellent balance of strength and toughness.

[0045] In this invention, the preferred temperature for the secondary tempering is 520 ± 10°C, and the rate of heating to the second tempering temperature is preferably ≤100°C / h, followed by air cooling to room temperature. In this invention, the holding time for the secondary tempering is related to the wall thickness, and the holding time for the secondary tempering is = a × k × D, where a is the heating coefficient (min / mm); k is a correction coefficient, preferably 1.2~1.3; and D is the wall thickness of the tube blank (mm). For the high-strength and high-toughness 35CrNi3MoV forged thick-walled seamless tube of this invention, the holding time for the secondary tempering is = (1.2~1.3) × (1.2~1.3) × D. This invention further stabilizes the sorbite + bainite structure and reduces internal stress through secondary tempering, improving the overall uniformity and impact toughness of the material, and further optimizing the balance between strength and toughness.

[0046] In this invention, the wall thickness of the high-strength and toughness 35CrNi3MoV forged thick-walled seamless tube is preferably 150~500mm, and in specific embodiments it can be 150, 200, 300, 400 or 500mm; the inner diameter is ≤350mm, preferably 120~350mm, and in specific embodiments it can be 120, 150, 170, 200, 250, 300 or 350mm; the length is greater than 10m, and in specific embodiments it can be 10, 11, 12, 13, 14 or 15m.

[0047] This invention provides a high-strength and high-toughness 35CrNi3MoV forged thick-walled seamless tube manufactured by the manufacturing method described above, characterized in that the yield strength Rp 0.1 ≥1104MPa; Elongation after fracture A≥40%; Reduction of area Z≥12%; AKV impact energy at -40℃≥40J.

[0048] The following detailed description, in conjunction with embodiments, illustrates the high-strength and high-toughness 35CrNi3MoV forged thick-walled seamless tube and its manufacturing method provided by the present invention. However, these descriptions should not be construed as limiting the scope of protection of the present invention.

[0049] Example 1 according to Figure 1 Manufacture high-strength and high-toughness 35CrNi3MoV forged thick-walled seamless tubes.

[0050] (1) Raw material inspection Inspection of alloy steel electroslag ingots includes control of composition and appearance quality. After removing 150mm from the ingot sprue and tail end, the composition of samples taken from both ends is shown in Table 1. The surface of the electroslag ingot must be free of defects such as flow spots and slag grooves. Normal residual defects on the surface of the electroslag remelted ingot must be cleaned thoroughly.

[0051] (2) Feeding The hot conveying of steel ingots adopts end-receiving heat removal.

[0052] (3) Heating The steel ingot is heated, and the processing steps are as follows: 1) When the steel ingot is heated to 650℃, it is held at 650±10℃ for 6 hours, with a heating rate ≤100℃ / h; 2) Then slowly heat the temperature at a rate of ≤100℃ / h to 850±10℃ and hold for 6 hours; 3) Then, increase the temperature at ≤100℃ / h, and hold the steel ingot at 1230±10℃ for 48 hours. 4) Then, when the temperature drops to 1180±10℃ at a rate of ≤100℃ / h, it is kept warm for 7 hours.

[0053] (4) Upsetting and drawing (opening) The heated steel ingots are subjected to two upsetting and drawing forging processes. The first upsetting is 35% and then the ingots are drawn to their original height. The second upsetting is 60% and the ingots are drawn to their original height and then drawn back to their original height. The initial forging temperature is 1160℃±10℃ and the final forging temperature is 900℃±10℃. The intermediate reheating time is 5 hours.

[0054] (5) Forging and drawing Heat to 1160±10℃ and hold for 3 hours, then perform the first forging drawing with a forging reduction of 25%, a starting forging temperature of 1160±10℃, and a final forging temperature of 900±10℃. After reheating in the furnace for 3 hours, the second forging and drawing process is carried out. The forging reduction is 25%, the initial forging temperature is 1160±10℃, and the final forging temperature is 900±10℃. The forging is also carried out at 16-corner within the range of 1160±10℃ to 900±10℃ to ensure subsequent forming. The blank is reheated in the furnace for 3 hours at a temperature of 1140±10℃. Then, the third forging and drawing process is carried out. The V-shaped die is changed, and the drawing process is used to forge each section of the blank. Each time, the blank is overlapped by 25% to forge the blank to the set forging size. When forging the end section, it is required to hit it lightly with a hammer and change its position continuously to avoid the blank end temperature dropping too quickly and causing cracks due to low temperature. The blank is kept in the temperature range of 1120~850℃.

[0055] (6) Post-forging heat treatment After forging, the surface temperature is air-cooled to 600~700℃ and held in the furnace for 6 hours. Then, it is furnace-cooled to 250~300℃ and held for 18 hours. Next, the temperature is increased to 870℃ at a rate of ≤80℃ / h and held for 20 hours. Then, it is cooled to 570℃ and held for 16 hours. Then, it is heated to 650℃ at a rate of ≤50℃ / h and held for 160 hours. Then, it is cooled to 400~450℃ at a rate of ≤40℃ / h. Finally, it is cooled to ≤150℃ at a rate of ≤20℃ / h before being removed from the furnace and air-cooled.

[0056] (7) Rough processing The forging billet is machined by turning the outer diameter and boring the inner hole according to the dimensions specified in the roughing drawing. The inner diameter is 370mm, the wall thickness is 350mm, and the length is 13.7m.

[0057] (8) Performance heat treatment The heat treatment is carried out in a 17-zone pit furnace, with normalizing, pit quenching, first tempering and second tempering performed in sequence. Normalizing: Zones 1-7 880±10℃, Zones 8-17 890±10℃, hold for 10h, heating rate ≤100℃ / h, air cool for ≥9h after removing from the furnace; quenching can be performed when the surface temperature is consistent with room temperature. Well-type quenching: heating temperature 860+10℃, holding temperature for 8h, heating rate ≤100℃ / h; vertically suspended air cooling for 5 minutes, water cooling for 3 minutes, continue air cooling for 5 minutes and water cooling for 3 minutes, and so on to cool to room temperature; Tempering: Zones 1-3 580+5℃, Zones 4-6 585+5℃, Zones 8-12 590+5℃, hold for 13 hours, heating rate ≤100℃ / h, air cooling, cool to room temperature.

[0058] Secondary tempering: 520+10℃, hold for 9 hours, heating rate ≤100℃ / h, cooling conditions: air cooling to room temperature.

[0059] (9) Mechanical property testing According to the requirements of the rough machining drawing, samples were cut from the end face of the forging for mechanical property testing. The test indicators included: yield strength, tensile strength, elongation, (-40℃, V-notch) impact energy, and 180-degree generatrix surface hardness. The test results are shown in Tables 2 and 3. The average grain size was 6.5 grade, and the grade difference was 1.0.

[0060] (10) Ultrasonic flaw detection Forgings that have passed mechanical property tests shall be subjected to ultrasonic testing. Ultrasonic testing shall be carried out in accordance with the requirements of NB / T 47013 that no defects larger than Φ2 equivalent are allowed for quality grade.

[0061] (11) Product inspection Once the inspection is passed, the product is complete.

[0062] Table 1 shows the composition (%) of the head and tail of the electroslag steel ingot and the finished product in Example 1. Note: A total of 4 seamless tubes were produced in Example 1. Table 1 shows the test results of 2 of the samples. The smelting composition is the composition of the electroslag steel ingot, which is the average of the beginning and end compositions. The composition of the finished product is the composition of the seamless tube.

[0063] Table 2 Surface Hardness (180-degree Linear Distance Test)

[0064] As shown in Table 2, the seamless tubes manufactured by this invention have uniform surface hardness, indicating that the microstructure is uniform.

[0065] Table 3 Mechanical property test results

[0066] As shown in Table 3, the seamless tube manufactured by this invention has high strength and a yield strength Rp. 0.1 ≥1150MPa, tensile strength R m ≥1254MPa; excellent plasticity, elongation after fracture ≥49%, reduction of area Z≥15%; also has good toughness, AKV8 impact energy ≥47J at -40℃.

[0067] Comparative Example 1 Typical data for a high-strength forging, using traditional manufacturing process: forging (second upsetting deformation not exceeding 50%) + post-forging heat treatment + normalizing + quenching (oil cooling) + two temperings, its chemical composition is shown in Table 4.

[0068] Table 4 Chemical composition (wt%) of seamless tube finished product of Comparative Example 1

[0069] Mechanical properties: 401 HB, hardness range 31 HB; tensile strength 1268 MPa, yield strength Rp 0.1 1140 MPa; impact energy 9 J. Grain size 3~6.

[0070] As can be seen from the above data, the large forged thick-walled seamless tubes produced by the component system, forging method and heat treatment conditions designed in this invention have a higher overall strength and toughness limit than those produced by traditional methods, and the quenching process has strong quality stability and reliability, reducing the risk of cracking and deformation.

[0071] The above description is only a preferred embodiment of the present invention. It should be noted that for those skilled in the art, several improvements and modifications can be made without departing from the principle of the present invention, and these improvements and modifications should also be considered within the scope of protection of the present invention.

Claims

1. A method for manufacturing a high-strength and high-toughness 35CrNi3MoV forged thick-walled seamless tube, characterized in that, Includes the following steps: The steel ingot is heated and then upset forged to obtain the first billet; the total upsetting amount of the upset forging is >70%; the steel ingot, by mass percentage, comprises the following elements: C: 0.36~0.39%, Mn: 0.45~0.65%, Si: 0.25~0.35%, Cr: 0.90~1.10%, Ni: 2.90~3.20%, Mo: 0.50~0.60%, V: 0.18~0.23%, P≤0.008%, S≤0.003%, Cu≤0.20%, Al≤0.010%, N≤60ppm, O≤30ppm, H≤1.5ppm and the balance Fe; After the first billet is reheated in the furnace, it is forged and drawn to obtain the second billet. The second billet is reheated in the furnace for the second time and then forged and drawn again to obtain the third billet. The third billet is reheated in the furnace for the third time and then forged and drawn for the third time to obtain the fourth billet. The fourth billet is subjected to post-forging heat treatment to obtain the fifth billet; The fifth blank is subjected to external turning and boring to obtain a tube blank; The tube blank is subjected to normalizing, quenching, first tempering and second tempering in sequence to obtain the high strength and toughness 35CrNi3MoV forged thick-walled seamless tube; the quenching is cooled by alternating water cooling and air cooling. The temperatures for the first and second forging drawing are 1160±10℃~900±10℃; the temperature for the third forging drawing is 1120~850℃. The holding time for the first reheating, the second reheating, and the third reheating is 2-4 hours independently; The high-strength and tough 35CrNi3MoV forged thick-walled seamless tube has a wall thickness of 150~500mm, an inner diameter of ≤350mm, and a length greater than 10m.

2. The manufacturing method according to claim 1, characterized in that, The initial forging temperature of the upsetting and drawing forging is 1160±10℃, and the final forging temperature is 900±10℃.

3. The manufacturing method according to claim 1 or 2, characterized in that, The upsetting and drawing forging process is a multi-pass upsetting and drawing forging process; except for the first upsetting and drawing forging, each subsequent upsetting and drawing forging process is followed by a reheating and heat preservation process, and the reheating and heat preservation time for each process is 4 to 6 hours independently.

4. The manufacturing method according to claim 1, characterized in that, The post-forging heat treatment includes: air-cooling the fourth billet to a surface temperature of 600~700℃ and holding it in a furnace for no less than 5 hours; then furnace-cooling it to 250~300℃ and holding it for 16~18 hours; then heating it to 870℃ at a heating rate of ≤80℃ / h and holding it for no less than 18 hours; then cooling it to 570℃ and holding it for no less than 15 hours; then heating it to 650℃ at a heating rate of ≤50℃ / h and holding it for no less than 150 hours; then cooling it to 400~450℃ at a cooling rate of ≤40℃ / h; and finally cooling it to ≤150℃ at a cooling rate of ≤20℃ / h before removing it from the furnace and air-cooling it.

5. The manufacturing method according to claim 1, characterized in that, The normalizing process is carried out in a pit furnace with 17 zones. From top to bottom, the temperature in zones 1 to 7 is 880±10℃, and the temperature in zones 8 to 17 is 890±10℃. The rate of heating to the normalizing temperature is ≤100℃ / h, and the furnace is air-cooled for ≥9h after being taken out of the furnace.

6. The manufacturing method according to claim 1, characterized in that, The quenching temperature is 860+10℃, and the rate of heating to the quenching temperature is ≤100℃ / h.

7. The manufacturing method according to claim 1, characterized in that, The first tempering is carried out in a 12-zone pit furnace. From top to bottom, the temperature of zones 1 to 3 is 580 ± 5℃, the temperature of zones 4 to 6 is 585 ± 5℃, and the temperature of zones 8 to 12 is 590 ± 5℃. The heating rate is ≤100℃ / h, and the furnace is cooled to room temperature.

8. The manufacturing method according to claim 1, characterized in that, The secondary tempering temperature is 520+10℃, the heating rate is ≤100℃ / h, and it is air-cooled to room temperature.

9. The manufacturing method according to claim 1, characterized in that, The third forging and drawing process is a stacking and forming process, with the overlapping area being 20-30% of the area of ​​the previous strike.

10. A high-strength and high-toughness 35CrNi3MoV forged thick-walled seamless tube manufactured by the manufacturing method according to any one of claims 1 to 9, characterized in that, Yield strength Rp 0.1 ≥1104MPa; Elongation after fracture A≥40%; Reduction of area Z≥12%; AKV impact energy at -40℃≥40J.

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