Low-cost high-strength high-toughness 770mpa grade seamless steel pipe and method for manufacturing the same

Abstract

The application relates to a low-cost high-strength high-toughness 770MPa-grade seamless steel pipe and a manufacturing method thereof. The steel pipe is composed of C, Si, Mn, Cr, Mo, Ni, V, Nb, Ti, Al, P, S, N and the balance of Fe and impurities. The method comprises the following steps: smelting after component design; converter tapping and ladle deoxidization alloying; refining tapping; soft blowing and continuous casting; segmentation; slow cooling; and after finishing, blank segmentation feeding, then piercing rolling and heat treatment. Under the premise of reducing the use amount of valuable alloy, the application adopts new alloy design principles, cooperates with smelting processes, hot rolling processes, quenching and tempering processes and different production processes for different wall thickness seamless steel pipes to improve the comprehensive performance of products. The application is applied to the metallurgical process technical field.

Description

Technical Field

[0001] This invention belongs to the field of metallurgical process technology, specifically relating to a low-cost, high-strength, high-toughness 770MPa seamless steel pipe and its manufacturing method. Background Technology

[0002] The boom of a hydraulic crawler crane is a key core component. The boom is welded and assembled from 3m-12m thick-walled seamless steel pipes, with a maximum lifting capacity of 1200 tons, a lifting height of 226m, and a maximum working radius of 164m. Therefore, the requirements for the steel pipes are extremely high, demanding high strength, high and low temperature toughness, and excellent weldability.

[0003] High-end lifting equipment tubing exhibits two significant characteristics, which also represent the key focus and challenges in its development. First, there is the ongoing requirement for lightweight structures. Under the premise of meeting structural performance and safety requirements, high-strength steel with high strength and high toughness is developed to reduce the design cross-section, lower the self-weight, and achieve the same load-bearing strength, thereby reducing steel consumption and additional loads on components. Second, there is the development of weldable boom tubing to improve the safety requirements of welded structures. The boom of a crawler crane is a spatial truss structure welded from four main chords and web members. During crane operation, the truss boom experiences complex forces; the boom members as a whole are under compression, but the main web chords bear a combined tensile and compressive load. The welding performance and quality of the joint welds at these points play a crucial role in the crane's safety. Currently, the boom tubing standard GB / T30584-2014 specifies that 770MPa boom tubing requires the addition of alloying and microalloying elements Mo, W, Nb, V, Ti, and Al to achieve precipitation strengthening and grain refinement, while adding Mn, Cr, and Ni elements to meet the requirements of tempering processes and low-temperature performance. The standard also specifies that the low-temperature impact energy is based on -20°C conditions, and cannot meet the low-temperature toughness requirements at -40°C and below. Currently, customers require the low-temperature impact energy to be based on -40°C conditions, while also meeting welding requirements for different strengths, which results in a high amount of expensive alloys being used, leading to increased costs. Summary of the Invention

[0004] The purpose of this invention is to solve the problem that the use of expensive alloys in existing 770MPa grade steel pipes for crawler crane booms increases costs in order to improve overall performance. The invention provides a low-cost, high-strength, and high-toughness 770MPa grade seamless steel pipe and its manufacturing method.

[0005] A low-cost, high-strength, and high-toughness 770MPa grade seamless steel pipe has the following composition by weight percentage: C: 0.10-0.18%; Si: 0.20-0.35%; Mn: 1.10-1.70%; Cr: 0.40-0.60%; Mo: 0.20-0.40%; Ni: 0.30-0.60%; V: 0.030-0.070%; Nb: 0.015-0.025%; Ti: 0.010-0.025%; Al≤0.03%; P≤0.012%; S≤0.008%; N≤0.007%; total microalloying ≤1.75%, with the balance being Fe and unavoidable impurities.

[0006] The above-mentioned method for manufacturing a low-cost, high-strength, and high-toughness 770MPa grade seamless steel pipe is carried out according to the following steps:

[0007] I. The raw material was designed according to the weight percentage composition of low-cost, high-strength, and high-toughness 770MPa grade seamless steel pipe: C: 0.10-0.18%; Si: 0.20-0.35%; Mn: 1.10-1.70%; Cr: 0.40-0.60%; Mo: 0.20-0.40%; Ni: 0.30-0.60%; V: 0.030-0.070%; Nb: 0.015-0.025%; Ti: 0.010-0.025%; Al≤0.03%; P≤0.012%; S≤0.008%; N≤0.007%, with the balance being Fe and unavoidable impurities.

[0008] 2. The raw materials are loaded into the converter for smelting. The converter endpoint control meets the following requirements: C≤0.04%; P≤0.006%; tapping temperature≥1610℃.

[0009] III. Converter tapping and ladle deoxidation and alloying: During the tapping process, bottom blowing argon gas is used at a flow rate of 0.5 NL / min / t steel. Alloy and refining slag are added during each tapping. After the alloy is added, lime is added with the steel stream to complete the tapping. The steel is then transferred to the LF refining station, where ferromolybdenum, ferroniobium and nickel plates are added for alloying. When the temperature reaches 1580-1600℃, silicon-barium alloy is added for deep deoxidation. Two minutes later, the LF refining station is powered off and the steel is tapped. Ferrotitanium is added for micro-titanium treatment.

[0010] IV. After the above micro-titanium treatment, the steel is refined and then a ladle covering agent is added to evenly cover the entire slag surface. Argon gas is adjusted for soft blowing. The argon gas flow rate during soft blowing is 0.2-0.5 NL / min / t steel, and the soft blowing time is ≥15min.

[0011] 5. After soft blowing, the molten steel is transferred to the continuous casting station. The continuous casting process adopts full-process protective pouring to continuously cast the molten steel into continuous casting billets. Then, the continuous casting billets are segmented and transferred to the slow cooling pit for slow cooling. The continuous casting billets that are slowly cooled out of the pit are subjected to surface inspection and finishing. Qualified continuous casting billets are transferred to the steel rolling workshop for tube sawing. After sawing, they are heated, pierced, and rolled. Then, they are subjected to micro-tension reduction or sizing, and air cooling on a cooling bed. The temperature is monitored online during air cooling on the cooling bed. After that, they are sawed, and samples are taken for inspection and non-destructive testing.

[0012] VI. Heat treatment: Quenching is performed using a critical quenching process, followed by tempering;

[0013] VII. Subsequent Inspection and Warehousing: After heat treatment, straightening, non-destructive testing, sampling inspection, surface inspection, and marking and warehousing are carried out in sequence to obtain low-cost, high-strength, and high-toughness 770MPa grade seamless steel pipes, thus completing the manufacturing method described above.

[0014] This invention develops a production technology for low-alloy high-strength 770MPa grade seamless steel pipes, which combines multi-alloy reinforcement, micro-alloying, controlled rolling and air cooling, and critical quenching and tempering processes to achieve the goal of economical, high-strength, high-toughness, and easy-to-weld steel pipes.

[0015] The beneficial effects of this invention: This invention provides a method for preparing 770MPa grade, low-cost, high-strength, high-toughness, and easily weldable seamless steel pipes for crawler crane booms. While reducing the amount of precious alloys used, it employs new alloy design principles, combined with smelting processes, hot rolling processes, tempering processes, and different production processes tailored to different wall thicknesses of seamless steel pipes, thereby improving the overall performance of the product.

[0016] In terms of chemical composition, a multi-element alloy + micro-alloy strengthening mechanism and a narrow composition control mechanism for alloying elements were achieved; in the rolling process, key temperature control was achieved to ensure the stability of the seamless steel pipe structure; and in the quenching and tempering process, key temperature and cooling rate control were achieved to ensure the performance stability of the seamless steel pipe.

[0017] Key performance indicators: yield strength ≥770MPa, tensile strength 820~1000MPa, elongation after fracture ≥15%, longitudinal impact energy at -20℃ ≥70J, longitudinal impact energy at -40℃ ≥60J, tempered microstructure, grain size grade 9.0, no cracks detected by 180° bending test.

[0018] Specialized boom tubing requires high strength, ductility, and toughness, along with good weldability. Based on calculations using the International Welding Association's carbon equivalent and weld crack sensitivity coefficient, reducing the carbon content in the alloy can effectively optimize weldability, improve impact toughness, and low-temperature resistance; generally, it should not exceed 0.30%. To achieve lightweight design goals, solid solution strengthening is achieved using Cr and Mo elements to enhance material strength. Added V, Nb, Ti, or a combination of these elements form stable carbides in the steel, providing dispersion strengthening. Controlling the N content and microstructure transformation temperature reduces nitride precipitation and refines the grain size of the base metal and weld, improving and stabilizing weldability. For equipment operating in cold regions, Ni is considered to improve low-temperature toughness. Reducing the content of harmful elements and gases such as S and P through pure steel smelting processes improves the overall material performance, while eliminating the precious metal W reduces alloy costs. Attached Figure Description

[0019] Figure 1 Inspection report on the weld morphology and microstructure of 159*10mm steel grade BJ770;

[0020] Figure 2 Test report on the bending performance of BJ770 steel grade 159*10mm;

[0021] Figure 3 Inspection report for BJ770 grade 159*10mm main pipe;

[0022] Figure 4 This is an inspection report on the welding process of BJ770 steel grade 159*10mm. Detailed Implementation

[0023] The technical solution of the present invention is not limited to the specific embodiments listed below, but also includes any combination of the specific embodiments.

[0024] Specific Implementation Method 1: This implementation method provides a low-cost, high-strength, and high-toughness 770MPa grade seamless steel pipe with the following composition by weight percentage: C: 0.10-0.18%; Si: 0.20-0.35%; Mn: 1.10-1.70%; Cr: 0.40-0.60%; Mo: 0.20-0.40%; Ni: 0.30-0.60%; V: 0.030-0.070%; Nb: 0.015-0.025%; Ti: 0.010-0.025%; Al≤0.03%; P≤0.012%; S≤0.008%; N≤0.007%, with the balance being Fe and unavoidable impurities.

[0025] Specific Implementation Method Two: This implementation method differs from Specific Implementation Method One in that its composition by weight percentage is as follows: C 0.11%, Si 0.22%, Mn 1.35%, Cr 0.43%, Mo 0.25%, Ni 0.35%, Nb 0.018%, V 0.065%, Ti 0.014%, Al 0.021%, P 0.006%, S 0.004%, N 0.0063%, with the balance being Fe and unavoidable impurities. Other steps and parameters are the same as in Specific Implementation Method One.

[0026] Specific Implementation Method Three: This implementation method differs from Specific Implementation Method One in that its composition by weight percentage is as follows: C 0.18%, Si 0.30%, Mn 1.12%, Cr 0.56%, Mo 0.35%, Ni 0.42%, Nb 0.020%, V 0.040%, Ti 0.015%, Al 0.018%, P 0.006%, S 0.004%, N 0.0059%, with the balance being Fe and unavoidable impurities. Other steps and parameters are the same as in Specific Implementation Method One.

[0027] Specific Implementation Method Four: This implementation method differs from Specific Implementation Method One in that its composition by weight percentage is as follows: C 0.15%, Si 0.25%, Mn 1.25%, Cr 0.51%, Mo 0.40%, Ni 0.54%, Nb 0.022%, V 0.050%, Ti 0.015%, Al 0.022%, P 0.006%, S 0.004%, N 0.0058%, with the balance being Fe and unavoidable impurities. Other steps and parameters are the same as in Specific Implementation Method One.

[0028] Specific Implementation Method 5: This implementation method provides a low-cost, high-strength, and high-toughness 770MPa grade seamless steel pipe manufacturing method, which is achieved through the following steps:

[0029] I. The raw material was designed according to the weight percentage composition of low-cost, high-strength, and high-toughness 770MPa grade seamless steel pipe: C: 0.10-0.18%; Si: 0.20-0.35%; Mn: 1.10-1.70%; Cr: 0.40-0.60%; Mo: 0.20-0.40%; Ni: 0.30-0.60%; V: 0.030-0.070%; Nb: 0.015-0.025%; Ti: 0.010-0.025%; Al≤0.03%; P≤0.012%; S≤0.008%; N≤0.007%, with the balance being Fe and unavoidable impurities.

[0030] 2. The raw materials are loaded into the converter for smelting. The converter endpoint control meets the following requirements: C≤0.04%; P≤0.006%; tapping temperature≥1610℃.

[0031] III. Converter tapping and ladle deoxidation and alloying: During the tapping process, bottom blowing argon gas is used at a flow rate of 0.5 NL / min / t steel. Alloy and refining slag are added during each tapping. After the alloy is added, lime is added with the steel stream to complete the tapping. The steel is then transferred to the LF refining station, where ferromolybdenum, ferroniobium and nickel plates are added for alloying. When the temperature reaches 1580-1600℃, silicon-barium alloy is added for deep deoxidation. Two minutes later, the LF refining station is powered off and the steel is tapped. Ferrotitanium is added for micro-titanium treatment.

[0032] IV. After the above micro-titanium treatment, the steel is refined and then a ladle covering agent is added to evenly cover the entire slag surface. Argon gas is adjusted for soft blowing. The argon gas flow rate during soft blowing is 0.2-0.5 NL / min / t steel, and the soft blowing time is ≥15min.

[0033] 5. After soft blowing, the molten steel is transferred to the continuous casting station. The continuous casting process adopts full-process protective pouring to continuously cast the molten steel into continuous casting billets. Then, the continuous casting billets are segmented and transferred to the slow cooling pit for slow cooling. The continuous casting billets that are slowly cooled out of the pit are subjected to surface inspection and finishing. Qualified continuous casting billets are transferred to the steel rolling workshop for tube sawing. After sawing, they are heated, pierced, and rolled. Then, they are subjected to micro-tension reduction or sizing, and air cooling on a cooling bed. The temperature is monitored online during air cooling on the cooling bed. After that, they are sawed, and samples are taken for inspection and non-destructive testing.

[0034] VI. Heat treatment: Quenching is performed using a critical quenching process, followed by tempering;

[0035] VII. Subsequent Inspection and Warehousing: After heat treatment, straightening, non-destructive testing, sampling inspection, surface inspection, and marking and warehousing are carried out in sequence to obtain low-cost, high-strength, and high-toughness 770MPa grade seamless steel pipes, thus completing the manufacturing method described above.

[0036] In this embodiment, the total amount of nickel, molybdenum, chromium, vanadium, niobium and titanium is ≤1.75%, and the precious metal W element is eliminated, which reduces the cost of 770MPa grade seamless steel pipe.

[0037] The amounts of molten iron and scrap steel used in step two of this embodiment can be obtained by those skilled in the art through conventional calculations based on the composition design in step one.

[0038] Specific Implementation Method Six: This implementation method differs from Specific Implementation Method Five in that the alloys mentioned in step three are: ferrosilicon alloy 3 kg / t steel, aluminum ingot 1.5–2 kg / t steel, medium-carbon ferromanganese 17 kg / t steel, low-carbon ferrochrome 9 kg / t steel, and ferrovanadium 1.0 kg / t steel; the refining slag mentioned in step three consists of CaO: 52%–58%, 8% ≤ SiO2 ≤ 10%, MgO: 5%–8%, Al2O3: 20%–25%, FeO ≤ 0.7%, and the balance being impurities. Other steps and parameters are the same as in Specific Implementation Method Five.

[0039] Specific Implementation Method Seven: This implementation method differs from Specific Implementation Method Five in that the amount of refining slag added in step three is 200 kg / t steel; the amount of lime added is 300 kg / t steel; the amount of silicon-barium alloy added is 1 kg / t steel; and the amount of ferrotitanium added is 0.5 kg / t steel. Other steps and parameters are the same as in Specific Implementation Method Five.

[0040] Specific Implementation Method Eight: This implementation method differs from Specific Implementation Method Five in that, in step five, the slow cooling pit is used when the continuous casting billet enters the slow cooling pit at a temperature ≥400℃, and it exits the pit when the temperature drops below 200℃. Other steps and parameters are the same as in Specific Implementation Method Five.

[0041] Specific Implementation Method Nine: This implementation method differs from Specific Implementation Method Five in that, in step five, the rolling process involves: a heating temperature of 1170-1240℃, a soaking temperature of 1230-1260℃, an initial rolling temperature of 1230-1250℃, graded controlled cooling after rolling, a cooling water pressure of 0.8-1.2 MPa, and a final rolling temperature of 850-880℃. Other steps and parameters are the same as in Specific Implementation Method Five.

[0042] Specific Implementation Method Ten: This implementation method differs from Specific Implementation Method Five in that, in step six, the critical quenching process is an external spraying and internal spraying combined with a rapid semi-blanking process. The quenching furnace heating temperature is 740-880℃, the quenching furnace uniform heating temperature is 760-880℃, and the critical quenching process cooling rate is ≥20℃ / s. The tempering process is as follows: the tempering furnace heating temperature is 520-620℃, the tempering furnace uniform heating temperature is 540-600℃, and a secondary tempering process is added according to the wall thickness, with a tempering uniform heating temperature of 580-620℃. Other steps and parameters are the same as in Specific Implementation Method Five.

[0043] The beneficial effects of the present invention are verified through the following embodiments:

[0044] Example 1:

[0045] A method for manufacturing a low-cost, high-strength, and high-toughness 770MPa grade seamless steel pipe is implemented according to the following steps:

[0046] I. According to the weight percentage composition of low-cost, high-strength, and high-toughness 770MPa grade seamless steel pipe, C is 0.11%, Si is 0.22%, Mn is 1.35%, Cr is 0.43%, Mo is 0.25%, Ni is 0.35%, Nb is 0.018%, V is 0.065%, Ti is 0.014%, Al is 0.021%, P is 0.006%, S is 0.004%, N is 0.0063%, and the balance is Fe and unavoidable impurities, the raw materials are weighed.

[0047] 2. The raw materials are loaded into the converter for smelting. The converter endpoint control meets the following requirements: C≤0.04%; P≤0.006%; tapping temperature≥1610℃.

[0048] III. Converter tapping and ladle deoxidation and alloying: During the tapping process, bottom blowing argon gas is used at a flow rate of 0.5 NL / min / t steel. Alloy and refining slag are added during each tapping. After the alloy is added, lime is added with the steel stream to complete the tapping. The steel is then transferred to the LF refining station, where ferromolybdenum, ferroniobium and nickel plates are added for alloying. When the temperature reaches 1580-1600℃, silicon-barium alloy is added for deep deoxidation. Two minutes later, the LF refining station is powered off and the steel is tapped. Ferrotitanium is added for micro-titanium treatment.

[0049] IV. After the above micro-titanium treatment, the steel is refined and then a ladle covering agent is added to evenly cover the entire slag surface. Argon gas is adjusted for soft blowing. The argon gas flow rate during soft blowing is 0.2-0.5 NL / min / t steel, and the soft blowing time is ≥15min.

[0050] 5. After soft blowing, the molten steel is transferred to the continuous casting station. The continuous casting process adopts full-process protective pouring to continuously cast the molten steel into continuous casting billets. Then, the continuous casting billets are segmented and transferred to the slow cooling pit for slow cooling. The continuous casting billets that are slowly cooled out of the pit are subjected to surface inspection and finishing. Qualified continuous casting billets are transferred to the steel rolling workshop for tube sawing. After sawing, they are heated, pierced, and rolled. Then, they are subjected to micro-tension reduction or sizing, and air cooling on a cooling bed. The temperature is monitored online during air cooling on the cooling bed. After that, they are sawed, and samples are taken for inspection and non-destructive testing.

[0051] VI. Heat treatment: Quenching is performed using a critical quenching process, followed by tempering;

[0052] VII. Subsequent Inspection and Warehousing: After heat treatment, straightening, non-destructive testing, sampling inspection, surface inspection, and marking and warehousing are carried out in sequence to obtain low-cost, high-strength, and high-toughness 770MPa grade seamless steel pipes, thus completing the manufacturing method described above.

[0053] In step five of this embodiment, the continuous casting process employs full-process protective pouring to prevent the molten steel from absorbing air and undergoing secondary oxidation.

[0054] The alloys mentioned in step three of this embodiment are: ferrosilicon alloy 3 kg / t steel, aluminum ingot 1.5-2 kg / t steel, medium carbon ferromanganese 17 kg / t steel, low carbon ferrochrome 9 kg / t steel, and ferrovanadium 1.0 kg / t steel.

[0055] The refining slag mentioned in step three consists of CaO: 52%–58%, 8% ≤ SiO2 ≤ 10%, MgO: 5%–8%, Al2O3: 20%–25%, FeO ≤ 0.7%, and the balance being impurities, by mass percentage.

[0056] In step three of this embodiment, the amount of refining slag added is 200 kg / t steel; the amount of lime added is 300 kg / t steel; the amount of silicon-barium alloy added is 1 kg / t steel; and the amount of ferrotitanium added is 0.5 kg / t steel.

[0057] The slow cooling pit described in step five of this embodiment: the continuous casting billet enters the slow cooling pit at a temperature ≥400℃, and exits the pit when the temperature of the continuous casting billet in the slow cooling pit drops to below 200℃.

[0058] The rolling process described in step five is as follows: the initial rolling temperature is 1170-1240℃, the soaking temperature is 1230-1260℃, the initial rolling temperature is 1230-1250℃, the rolling is followed by graded controlled cooling with a cooling water pressure of 0.8-1.2MPa, and the final rolling temperature is 850-880℃.

[0059] In step six, the critical quenching process is external spraying and internal spraying combined with rapid semi-blanking. The quenching furnace heating temperature is 740-880℃, the quenching furnace uniform heating temperature is 760-880℃, and the critical quenching process cooling rate is ≥20℃ / s. The tempering process is as follows: the tempering furnace heating temperature is 520-620℃, the tempering furnace uniform heating temperature is 540-600℃, and a secondary tempering process is added according to the wall thickness, with a tempering uniform heating temperature of 580-620℃.

[0060] The low-cost, high-strength, and high-toughness 770MPa grade seamless steel pipe manufactured in this embodiment has a production specification of φ88.9*6.3mm. After testing, the performance indicators are as follows: yield strength 885-897MPa, tensile strength 920-938MPa, elongation after fracture 17.5%, longitudinal impact energy at -20℃ 120-128J, longitudinal impact energy at -40℃ 90-99J, grain size grade 9, and tempered sorbite microstructure.

[0061] Example 2:

[0062] The difference between this embodiment and Embodiment 1 is that in step one, the composition of the low-cost, high-strength, high-toughness 770MPa grade seamless steel pipe is designed according to the following weight percentage: C 0.18%, Si 0.30%, Mn 1.12%, Cr 0.56%, Mo 0.35%, Ni 0.42%, Nb 0.020%, V 0.040%, Ti 0.015%, Al 0.018%, P 0.006%, S 0.004%, N 0.0059%, with the balance being Fe and unavoidable impurities; the rest is the same as in Embodiment 1.

[0063] In this embodiment, the steel sample analysis results meet the composition design requirements.

[0064] The low-cost, high-strength, and high-toughness 770MPa grade seamless steel pipe manufactured in this embodiment has a production specification of φ159*10mm. The welding morphology and bending inspection reports issued by the National Quality Inspection Center and the Steel Research Institute Nake Testing Center show the results. Figure 1 and Figure 2 After passing the inspection, the sample was sent to the end customer, Sany Group Zhejiang Sany Equipment Co., Ltd., and further tested by the steel laboratory of Zhejiang Jiulit Materials Technology Co., Ltd. The test results met the customer's requirements; specific data is as follows: Figure 3 and 4 The test report in the document.

[0065] Example 3:

[0066] The difference between this embodiment and Embodiment 1 is that in step one, the weight percentage composition of the low-cost, high-strength, and high-toughness 770MPa grade seamless steel pipe is as follows: C 0.15%, Si 0.25%, Mn 1.25%, Cr 0.51%, Mo 0.40%, Ni 0.54%, Nb 0.022%, V 0.050%, Ti 0.015%, Al 0.022%, P 0.006%, S 0.004%, N 0.0058%, with the balance being Fe and unavoidable impurities; the rest is the same as in Embodiment 1.

[0067] In this embodiment, the steel sample analysis results meet the composition design requirements.

[0068] The low-cost, high-strength, and high-toughness 770MPa grade seamless steel pipe manufactured in this embodiment has a production specification of φ139.7*15mm. After testing, the performance indicators are as follows: yield strength 860-871MPa, tensile strength 910-923MPa, elongation after fracture 17%, longitudinal impact energy at -20℃ 111-118J, longitudinal impact energy at -40℃ 87-93J, grain size grade 9, and tempered sorbite microstructure.

[0069] The above description, in conjunction with specific embodiments, provides a further detailed explanation of the present invention and should not be construed as limiting the invention to the specific implementations described above. Without departing from the overall concept and protection of the claims, several simple deductions or substitutions can be made, all of which should be considered within the scope of protection of the present invention.

Claims

1. A low-cost high-strength high-toughness 770 MPa grade seamless steel pipe, characterized by It is composed of: C: 0.10~0.18%; Si: 0.20~0.35%; Mn: 1.10~1.70%; Cr: 0.40~0.60%; Mo: 0.20~0.40%; Ni: 0.30~0.60% by weight percentage; V: 0.030~0.070%; Nb: 0.015~0.025%; Ti: 0.010~0.025%; Al≤0.03%; P≤0.012%; S≤0.008%; N≤0.007%, the balance is Fe and inevitable impurities; The manufacturing method of the low-cost high-strength high-toughness 770MPa grade seamless steel pipe is realized according to the following steps: One, according to the low-cost high-strength high-toughness 770MPa grade seamless steel pipe, the weight percentage composition C: 0.10~0.18%; Si: 0.20~0.35%; Mn: 1.10~1.70%; Cr: 0.40~0.60%; Mo: 0.20~0.40%; Ni: 0.30~0.60%; V: 0.030~0.070%; Nb: 0.015~0.025%; Ti: 0.010~0.025%; Al≤0.03%; P≤0.012%; S≤0.008%; N≤0.007%, the balance is Fe and inevitable impurities, the composition is designed to obtain raw materials; Two, the raw materials are loaded into the converter for smelting, and the converter endpoint control meets: C≤0.04%; P≤0.006%; the tapping temperature is≥1610℃; Three, the converter tapping and ladle deoxidization and alloying: argon blowing during tapping, argon flow rate 0.5NL / min / t steel, alloy and refining slag are added during each tapping, when the alloy is added, lime is added with the steel flow, the tapping is completed, then it is transferred to the LF refining station, then molybdenum iron, niobium iron and nickel plate are added for alloying, when the temperature reaches 1580~1600℃, silicon barium alloy is added for deep deoxidization, 2 minutes later, the LF refining station is powered off and the steel is tapped, titanium iron is added for micro-titanium treatment; Four, after the above micro-titanium treatment, the refined steel is tapped, then the ladle covering agent is added and evenly covers the whole slag surface, adjust the argon for soft blowing, the argon flow rate is 0.2~0.5 NL / min / t steel, the soft blowing time is≥15min; Five, after the soft blowing is finished, the molten steel is transferred to the continuous casting station, the whole process is protected casting, the molten steel is continuously cast into continuous casting billet, then the continuous casting billet is segmented, then it is transferred to the slow cooling pit for slow cooling, the continuous casting billet in the slow cooling pit is surface inspected and finished, the qualified continuous casting billet is transferred to the rolling workshop, the pipe billet is sawn, then it is heated, pierced, and rolled; then it is micro-tension reduced or sized, cold bed air cooling, online temperature monitoring during cold bed air cooling, then it is sawn, sampled and inspected, and non-destructive testing; Six, heat treatment: quenching treatment is carried out by using critical quenching process, and then tempering is carried out; wherein the critical quenching process is external spraying and internal spraying + rapid semi-continuous, the quenching furnace heating temperature is 740-880℃, the quenching furnace soaking temperature is 760-880℃, the critical quenching process cooling speed is ≥20℃ / s; the tempering process is: the tempering furnace heating temperature is 520-620℃, the tempering furnace soaking temperature is 540-600℃, according to the wall thickness, the second tempering process is supplemented, the tempering furnace soaking temperature is 580-620℃; Seven, subsequent inspection and storage: after heat treatment, straightening, nondestructive testing, sampling inspection, surface inspection, spraying and storage are sequentially carried out, and a low-cost high-strength high-toughness 770MPa grade seamless steel pipe is obtained, that is, the manufacturing method is completed.

2. The low-cost, high-strength, high-ductility, 770 MPa grade seamless steel pipe according to claim 1, characterized by It is composed of 0.11% of C, 0.22% of Si, 1.35% of Mn, 0.43% of Cr, 0.25% of Mo, 0.35% of Ni, 0.018% of Nb, 0.065% of V, 0.014% of Ti, 0.021% of Al, 0.006% of P, 0.004% of S, 0.0063% of N, and the balance of Fe and inevitable impurities.

3. The low-cost, high-strength, high-ductility, 770 MPa grade seamless steel pipe according to claim 1, characterized by It is composed of 0.18% of C, 0.30% of Si, 1.12% of Mn, 0.56% of Cr, 0.35% of Mo, 0.42% of Ni, 0.020% of Nb, 0.040% of V, 0.015% of Ti, 0.018% of Al, 0.006% of P, 0.004% of S, 0.0059% of N, and the balance of Fe and inevitable impurities.

4. The low-cost, high-strength, high-ductility, 770 MPa grade seamless steel pipe according to claim 1, characterized by It is composed of 0.15% of C, 0.25% of Si, 1.25% of Mn, 0.51% of Cr, 0.40% of Mo, 0.54% of Ni, 0.022% of Nb, 0.050% of V, 0.015% of Ti, 0.022% of Al, 0.006% of P, 0.004% of S, 0.0058% of N, and the balance of Fe and inevitable impurities.

5. The low cost, high strength, high toughness, 770 MPa grade seamless steel pipe of claim 1 wherein The alloy in step three: the alloy type and amount are 3kg / t steel of ferrosilicon alloy, 1.5-2kg / t steel of aluminum ingot, 17kg / t steel of medium-carbon ferromanganese, 9kg / t steel of low-carbon ferrochrome and 1.0kg / t steel of ferrovanadium; the refining slag in step three is composed of CaO: 52%-58%, 8%≤SiO2≤10%, MgO: 5%-8%, Al2O3: 20%-25%, FeO≤0.7% and the balance of impurities.

6. The low cost, high strength, high toughness, 770 MPa grade seamless steel pipe of claim 1 wherein The adding amount of the refining slag in step three is 200kg / t steel; the adding amount of the lime is 300kg / t steel; the adding amount of the silicon-barium alloy is 1kg / t steel; the adding amount of the ferrotitanium is 0.5kg / t steel.

7. The low cost, high strength, high toughness, 770 MPa grade seamless steel pipe of claim 1 wherein The slow cooling pit in step five: the continuous casting blank enters the slow cooling pit at a temperature of ≥400℃, and the continuous casting blank is taken out of the pit when the temperature is reduced to ≤200℃.

8. The low cost, high strength, high toughness, 770 MPa grade seamless steel pipe of claim 1 wherein The rolling in step five: hot temperature is 1170-1240℃, soaking temperature is 1230-1260℃, open rolling temperature is 1230-1250℃, grading control cooling after rolling, cooling water pressure is 0.8-1.2MPa, finish rolling temperature is 850-880℃.

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