Low-cost, high-strength and high-toughness 770MPa-grade seamless steel tube and manufacturing method thereof

Abstract

The invention relates to a low-cost, high-strength and high-toughness 770MPa-grade seamless steel tube and a manufacturing method thereof, in particular to a low-cost, high-strength and high-toughness 770MPa-grade seamless steel tube and a manufacturing method thereof. The steel pipe comprises the following components: C, Si, Mn, Cr, Mo, Ni, V, Nb, Ti, Al, P, S, N and the balance of Fe and impurity elements. The method comprises the following steps: smelting after component design; tapping in a converter and deoxidizing and alloying in a steel ladle; refining and tapping; continuously casting after soft blowing; performing segmentation; slowly cooling; and after finishing, blank segmented feeding is carried out, and then perforation rolling and heat treatment are carried out. On the premise of reducing the usage amount of precious alloy, the comprehensive performance of the product is improved by adopting a new alloy design principle, matching with a smelting process, a hot rolling process and a hardening and tempering process and formulating different production processes for seamless steel pipes with different wall thicknesses. The method is applied to the technical field of metallurgical processes.

Description

Technical Field

[0001] The invention belongs to the technical field of metallurgical process, and specifically relates to a low-cost, high-strength and high-toughness 770MPa grade seamless steel pipe and a manufacturing method thereof. Background Art

[0002] The combined boom of hydraulic crawler crane is the key core component of the crane. The boom is welded and assembled from 3m-12m thick-walled seamless steel pipes, with a maximum lifting capacity of 1,200 tons, a lifting height of 226m, and a maximum working radius of 164m. Therefore, the requirements for steel pipes are very high, with high strength, high and low temperature toughness, and good welding performance.

[0003] There are two significant characteristics of pipes for high-end lifting equipment, which are also the focus and difficulty of their development. The first is the continuous requirement for lightweight structure. Under the premise of meeting structural performance and safety, high-strength steel with high strength and high toughness is developed to achieve the purpose of reducing the design section, reducing the deadweight and achieving the same bearing strength, thereby reducing steel consumption and additional load on components. The second is to develop steel welding performance boom pipes to improve the safety requirements of welded structures. The boom of crawler cranes is a spatial truss structure welded by four main chords and webs. When the crane is working, the truss arm is subjected to complex forces, and the boom rods are compressed as a whole. However, the main web chord is subjected to the combined load of tension and compression. The welding performance and quality of its node welds play a decisive role in the safety of the crane. At present, the 770MPa boom pipe in the boom pipe standard GB / T30584-2014 stipulates the addition of alloy and micro-alloy elements Mo, W, Nb, V, Ti, Al to achieve the purpose of precipitation strengthening and fine grain strengthening, and the addition of Mn, Cr, Ni elements to meet the requirements of quenching and tempering process and low temperature performance. The standard also stipulates that the low-temperature impact energy is based on -20℃, and cannot meet the actual low-temperature toughness index requirements of -40℃ and below. At present, customers require the low-temperature impact energy to be based on -40℃, while meeting the welding requirements of different strengths, which results in high consumption of precious alloys and increased costs. Summary of the invention

[0004] The purpose of the present invention is to solve the problem that in order to improve the comprehensive performance of the existing 770MPa grade steel pipe for the boom of a crawler crane, a high amount of precious alloys is used, resulting in increased costs, and to provide a low-cost, high-strength and high-toughness 770MPa grade seamless steel pipe and a manufacturing method thereof.

[0005] The invention discloses a low-cost, high-strength and high-toughness 770MPa grade seamless steel pipe, which is composed of the following weight percentages: 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 total amount of microalloys is ≤1.75%, and the balance is Fe and unavoidable impurities.

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

[0007] 1. 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%, the balance is Fe and unavoidable impurities to design the composition and obtain the raw material;

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

[0009] 3. Steel tapping from converter and deoxidation and alloying in ladle: During the steel tapping process, argon is blown from the bottom, with an argon flow rate of 0.5NL / min / t steel. Alloy and refined slag are added when each furnace is tapped. When the alloy is added, lime is added with the steel flow to complete the steel tapping. The steel is then transported to the LF refining station, and ferromolybdenum, ferroniobium and nickel plates are added for alloying. When the temperature reaches 1580-1600℃, silicon-barium alloy is added for deep deoxidation. After 2 minutes, the LF refining station is powered off for steel tapping, and ferrotitanium is added for micro-titanium treatment.

[0010] 4. After the above-mentioned micro-titanium treatment, the steel is refined and then a ladle covering agent is added, and the ladle covering agent is evenly covered on the entire slag surface. The argon gas is adjusted for soft blowing. The argon gas flow rate during soft blowing is 0.2-0.5NL / min / t steel, and the soft blowing time is ≥15min;

[0011] 5. After the soft blowing is completed, the molten steel is transferred to the continuous casting station. The continuous casting process adopts full-process protection pouring to continuously cast the molten steel into continuous casting billets, which are then segmented and transferred to the slow cooling pit for slow cooling. The continuous casting billets that have been slowly cooled out of the pit are subjected to surface inspection and finishing. The qualified continuous casting billets are transferred to the steel rolling workshop for tube billet sawing, which is then heated and pierced for rolling. The billets are then subjected to micro-tension diameter reduction or sizing, air cooling on the cooling bed, online temperature monitoring during air cooling of the cooling bed, and then sawing, sampling, inspection, and non-destructive testing.

[0012] 6. Heat treatment: Use critical quenching process for quenching, and then tempering;

[0013] 7. Subsequent inspection and warehousing: After heat treatment, straightening, non-destructive testing, sampling inspection, surface inspection, spray 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.

[0014] The present invention develops a production technology of multi-element alloy + micro-alloy strengthening + controlled rolling air cooling process + critical quenching and tempering process for low-alloy high-strength 770MPa grade high-strength seamless steel pipe, thereby achieving the goal of economical high-strength, high-toughness and easy welding.

[0015] Beneficial effects of the invention: The invention provides a method for preparing a 770MPa-grade low-cost, high-strength, high-toughness, easy-to-weld seamless steel pipe for a crawler crane boom. Under the premise of reducing the use of precious alloys, the invention adopts new alloy design principles, cooperates with smelting processes, hot rolling processes, quenching and tempering processes, and formulates different production processes for seamless steel pipes with different wall thicknesses to improve the comprehensive performance of the product.

[0016] In terms of chemical composition, the multi-element alloy + micro-alloy strengthening mechanism and the narrow composition control mechanism of alloy elements are realized; the key temperature control in the rolling process realizes the stability of the seamless steel pipe structure, and the key temperature and cooling rate control in the tempering process realizes the performance stability of the seamless steel pipe.

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

[0018] Special boom pipes are required to have high strength, plasticity, toughness, and good welding performance. According to the calculation of the carbon equivalent and welding crack sensitivity coefficient of the International Welding Association, reducing the carbon content in the alloy can effectively optimize the welding performance, improve the impact toughness and low temperature resistance, and generally should not exceed 0.30%. In order to achieve the goal of lightweight design, the strength of the material is improved by using Cr and Mo elements to achieve solid solution strengthening. The added V or Nb or Ti or a combination of the three forms stable carbides in the steel, which plays a role in dispersion strengthening, controls the N content + structural transformation temperature in the steel to reduce the precipitation of nitrides, and can refine the grains of the parent material and the weld to improve and stabilize the welding performance. For equipment serving in cold areas, consider adding Ni to improve low temperature toughness. Through the pure steel smelting process, reduce the content of harmful elements and gases such as S and P to improve the comprehensive performance of the material, and eliminate the precious metal W element to reduce the alloy cost. BRIEF DESCRIPTION OF THE DRAWINGS

[0019] Figure 1 This is the BJ770 steel grade 159*10mm welding morphology and structure inspection report;

[0020] Figure 2 BJ770 steel grade 159*10mm bending performance test report;

[0021] Figure 3 The test report of BJ770 steel grade 159*10mm mother pipe;

[0022] Figure 4 This is the inspection report for BJ770 steel grade 159*10mm welding process. DETAILED DESCRIPTION

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

[0024] Specific implementation method one: In this implementation method, a low-cost, high-strength and high-toughness 770MPa grade seamless steel pipe is composed of the following weight percentages: 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%, and the balance is Fe and unavoidable impurities.

[0025] Specific embodiment 2: This embodiment is different from the specific embodiment 1 in that it is composed by weight percentage: 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. Other steps and parameters are the same as those of the specific embodiment 1.

[0026] Specific embodiment 3: This embodiment is different from the specific embodiment 1 in that it is composed by weight percentage: C is 0.18%, Si is 0.30%, Mn is 1.12%, Cr is 0.56%, Mo is 0.35%, Ni is 0.42%, Nb is 0.020%, V is 0.040%, Ti is 0.015%, Al is 0.018%, P is 0.006%, S is 0.004%, N is 0.0059%, and the balance is Fe and unavoidable impurities. Other steps and parameters are the same as those of the specific embodiment 1.

[0027] Specific embodiment 4: This embodiment is different from the specific embodiment 1 in that it is composed by weight percentage: C is 0.15%, Si is 0.25%, Mn is 1.25%, Cr is 0.51%, Mo is 0.40%, Ni is 0.54%, Nb is 0.022%, V is 0.050%, Ti is 0.015%, Al is 0.022%, P is 0.006%, S is 0.004%, N is 0.0058%, and the balance is Fe and unavoidable impurities. Other steps and parameters are the same as those of the specific embodiment 1.

[0028] Specific implementation method 5: This implementation method is a method for manufacturing a low-cost, high-strength, high-toughness 770MPa grade seamless steel pipe, which is implemented by the following steps:

[0029] 1. 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%, the balance is Fe and unavoidable impurities to design the composition and obtain the raw material;

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

[0031] 3. Steel tapping from converter and deoxidation and alloying in ladle: During the steel tapping process, argon is blown from the bottom, with an argon flow rate of 0.5NL / min / t steel. Alloy and refined slag are added when each furnace is tapped. When the alloy is added, lime is added with the steel flow to complete the steel tapping. The steel is then transported to the LF refining station, and ferromolybdenum, ferroniobium and nickel plates are added for alloying. When the temperature reaches 1580-1600℃, silicon-barium alloy is added for deep deoxidation. After 2 minutes, the LF refining station is powered off for steel tapping, and ferrotitanium is added for micro-titanium treatment.

[0032] 4. After the above-mentioned micro-titanium treatment, the steel is refined and then a ladle covering agent is added, and the ladle covering agent is evenly covered on the entire slag surface. The argon gas is adjusted for soft blowing. The argon gas flow rate during soft blowing is 0.2-0.5NL / min / t steel, and the soft blowing time is ≥15min;

[0033] 5. After the soft blowing is completed, the molten steel is transferred to the continuous casting station. The continuous casting process adopts full-process protection pouring to continuously cast the molten steel into continuous casting billets, which are then segmented and transferred to the slow cooling pit for slow cooling. The continuous casting billets that have been slowly cooled out of the pit are subjected to surface inspection and finishing. The qualified continuous casting billets are transferred to the steel rolling workshop for tube billet sawing, which is then heated and pierced for rolling. The billets are then subjected to micro-tension diameter reduction or sizing, air cooling on the cooling bed, online temperature monitoring during air cooling of the cooling bed, and then sawing, sampling, inspection, and non-destructive testing.

[0034] 6. Heat treatment: Use critical quenching process for quenching, and then tempering;

[0035] 7. Subsequent inspection and warehousing: After heat treatment, straightening, non-destructive testing, sampling inspection, surface inspection, spray 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.

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

[0037] The amounts of molten iron and scrap steel used in step 2 of this embodiment can be obtained by those skilled in the art through routine calculations based on the component design in step 1.

[0038] Specific embodiment 6: This embodiment is different from specific embodiment 5 in that the alloy in step 3: 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 refined slag in step 3 is composed of CaO: 52%-58%, 8%≤SiO2≤10%, MgO: 5%-8%, Al2O3: 20%-25%, FeO≤0.7% and the remainder of impurities in terms of mass percentage. The other steps and parameters are the same as those in specific embodiment 5.

[0039] Specific implementation method 7: This implementation method is different from specific implementation method 5 in that the amount of refined slag added in step 3 is 200kg / t steel; the amount of lime added is 300kg / t steel; the amount of silicon-barium alloy added is 1kg / t steel; the amount of ferrotitanium added is 0.5kg / t steel. The other steps and parameters are the same as those in specific implementation method 5.

[0040] Specific embodiment eight: This embodiment differs from specific embodiment five in that the temperature of the slow cooling pit in step five is ≥400°C when the continuous casting billet enters the slow cooling pit, and the continuous casting billet exits the pit when the temperature of the slow cooling pit drops below 200°C. Other steps and parameters are the same as those in specific embodiment five.

[0041] Specific embodiment 9: This embodiment is different from specific embodiment 5 in that the rolling in step 5: the hot temperature is 1170-1240°C, the soaking temperature is 1230-1260°C, the start rolling temperature is 1230-1250°C, the cooling is controlled by graded cooling after rolling, the cooling water pressure is 0.8-1.2MPa, and the final rolling temperature is 850-880°C. The other steps and parameters are the same as those in specific embodiment 5.

[0042] Specific embodiment ten: This embodiment is different from specific embodiment five in that the critical quenching process in step six is ​​external spraying and internal spraying + rapid semi-hardening, the heating temperature of the quenching furnace is 740-880°C, the quenching furnace soaking temperature is 760-880°C, and the critical quenching process cooling rate is ≥20°C / s; the tempering process is: the tempering furnace heating temperature is 520-620°C, the tempering furnace soaking temperature is 540-600°C, and the secondary tempering process is supplemented according to the wall thickness, and the tempering soaking temperature is 580-620°C. Other steps and parameters are the same as those in specific embodiment five.

[0043] The beneficial effects of the present invention are verified by the following examples:

[0044] Embodiment 1:

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

[0046] 1. 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, weigh the raw materials;

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

[0048] 3. Steel tapping from converter and deoxidation and alloying in ladle: During the steel tapping process, argon is blown from the bottom, with an argon flow rate of 0.5NL / min / t steel. Alloy and refined slag are added when each furnace is tapped. When the alloy is added, lime is added with the steel flow to complete the steel tapping. The steel is then transported to the LF refining station, and ferromolybdenum, ferroniobium and nickel plates are added for alloying. When the temperature reaches 1580-1600℃, silicon-barium alloy is added for deep deoxidation. After 2 minutes, the LF refining station is powered off for steel tapping, and ferrotitanium is added for micro-titanium treatment.

[0049] 4. After the above-mentioned micro-titanium treatment, the steel is refined and then a ladle covering agent is added, and the ladle covering agent is evenly covered on the entire slag surface. The argon gas is adjusted for soft blowing. The argon gas flow rate during soft blowing is 0.2-0.5NL / min / t steel, and the soft blowing time is ≥15min;

[0050] 5. After the soft blowing is completed, the molten steel is transferred to the continuous casting station. The continuous casting process adopts full-process protection pouring to continuously cast the molten steel into continuous casting billets, which are then segmented and transferred to the slow cooling pit for slow cooling. The continuous casting billets that have been slowly cooled out of the pit are subjected to surface inspection and finishing. The qualified continuous casting billets are transferred to the steel rolling workshop for tube billet sawing, which is then heated and pierced for rolling. The billets are then subjected to micro-tension diameter reduction or sizing, air cooling on the cooling bed, online temperature monitoring during air cooling of the cooling bed, and then sawing, sampling, inspection, and non-destructive testing.

[0051] 6. Heat treatment: Use critical quenching process for quenching, and then tempering;

[0052] 7. Subsequent inspection and warehousing: After heat treatment, straightening, non-destructive testing, sampling inspection, surface inspection, spray 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.

[0053] In step 5 of this embodiment, the continuous casting process adopts full-process protective pouring to avoid secondary oxidation of the molten steel due to air absorption.

[0054] The alloys described in step three of this embodiment: the alloy types and amounts 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.

[0055] The refined slag in step 3 is composed of CaO: 52% to 58%, 8%≤SiO2≤10%, MgO: 5% to 8%, Al2O3: 20% to 25%, FeO≤0.7% and the remainder of impurities in terms of mass percentage.

[0056] In step 3 of this embodiment, the amount of refined slag added is 200kg / t steel; the amount of lime added is 300kg / t steel; the amount of silicon-barium alloy added is 1kg / t steel; and the amount of ferrotitanium added is 0.5kg / t steel.

[0057] The slow cooling pit described in step five of this embodiment: the temperature of the continuous casting billet entering the slow cooling pit is ≥400°C, and the continuous casting billet exits the pit when the temperature of the slow cooling pit drops below 200°C.

[0058] The rolling described in step five: the hot temperature is 1170-1240°C, the soaking temperature is 1230-1260°C, the start rolling temperature is 1230-1250°C, the cooling is controlled by grades after rolling, the cooling water pressure is 0.8-1.2MPa, and the final rolling temperature is 850-880°C.

[0059] In step six, the critical quenching process is external shower and internal spray + rapid semi-quenching, the quenching furnace heating temperature is 740-880°C, the quenching furnace average temperature is 760-880°C, and the critical quenching process cooling rate is ≥20°C / s; the tempering process is: the tempering furnace heating temperature is 520-620°C, the tempering furnace average temperature is 540-600°C, and the secondary tempering process is supplemented according to the wall thickness, and the tempering average temperature is 580-620°C.

[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: yield strength 885-897MPa, tensile strength 920-938MPa, elongation after fracture 17.5%, -20℃ longitudinal impact energy 120-128J, -40℃ longitudinal impact energy 90-99J, grain size 9, and tempered troostite structure.

[0061] Embodiment 2:

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

[0063] The steel sample analysis results in this embodiment meet the composition design requirements.

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

[0065] Embodiment 3:

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

[0067] The steel sample analysis results in this embodiment 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: yield strength 860-871MPa, tensile strength 910-923MPa, elongation after fracture 17%, -20℃ longitudinal impact energy 111-118J, -40℃ longitudinal impact energy 87-93J, grain size 9, and tempered troostite structure.

[0069] The above content is a further detailed description of the present invention in combination with specific implementation methods, and it cannot be determined that the present invention is limited to the above specific implementation. Without departing from the overall concept of the present invention and the protection of the claims, several simple deductions or substitutions can be made, which should be regarded as falling within the protection scope of the present invention.

Claims

1. A low-cost, high-strength, high-toughness 770MPa grade seamless steel pipe, characterized in that The composition of the invention is as follows according to 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%, the balance is Fe and unavoidable impurities.

2. A low-cost, high-strength, high-toughness 770MPa grade seamless steel pipe according to claim 1, characterized in that It is composed by weight percentage: 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 inevitable impurities.

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

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

5. A method for manufacturing a low-cost, high-strength, high-toughness 770MPa grade seamless steel pipe as claimed in claim 1, characterized in that It is implemented as follows:

1. 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%, the remainder being Fe and inevitable impurities to carry out composition design to obtain a raw material; 2. Load the raw materials into the converter for smelting. The converter endpoint control meets the following requirements: C≤0.04%; P≤0.006%; steel tapping temperature≥1610℃; 3. Steel tapping from converter and deoxidation and alloying in ladle: During the steel tapping process, argon is blown from the bottom, with an argon flow rate of 0.5NL / min / t steel. Alloy and refined slag are added when each furnace is tapped. When the alloy is added, lime is added with the steel flow to complete the steel tapping. The steel is then transported to the LF refining station, and ferromolybdenum, ferroniobium and nickel plates are added for alloying. When the temperature reaches 1580-1600℃, silicon-barium alloy is added for deep deoxidation. After 2 minutes, the LF refining station is powered off for steel tapping, and ferrotitanium is added for micro-titanium treatment.

4. After the above-mentioned micro-titanium treatment, the steel is refined and then a ladle covering agent is added, and the ladle covering agent is evenly covered on the entire slag surface. The argon gas is adjusted for soft blowing. The argon gas flow rate during soft blowing is 0.2-0.5NL / min / t steel, and the soft blowing time is ≥15min; 5. After the soft blowing is completed, the molten steel is transferred to the continuous casting station. The continuous casting process adopts full-process protection pouring to continuously cast the molten steel into continuous casting billets, which are then segmented and transferred to the slow cooling pit for slow cooling. The continuous casting billets that have been slowly cooled out of the pit are subjected to surface inspection and finishing. The qualified continuous casting billets are transferred to the steel rolling workshop for tube billet sawing, which is then heated and pierced for rolling. The billets are then subjected to micro-tension diameter reduction or sizing, air cooling on the cooling bed, online temperature monitoring during air cooling of the cooling bed, and then sawing, sampling, inspection, and non-destructive testing.

6. Heat treatment: Use critical quenching process for quenching, and then tempering; 7. Subsequent inspection and warehousing: After heat treatment, straightening, non-destructive testing, sampling inspection, surface inspection, spray 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.

6. The method for manufacturing a low-cost, high-strength, high-toughness 770MPa grade seamless steel pipe according to claim 5, characterized in that The alloy described in step 3: the alloy type and dosage 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 refined slag described in step 3 is composed of CaO: 52%-58%, 8%≤SiO2≤10%, MgO: 5%-8%, Al2O3: 20%-25%, FeO≤0.7% and the remainder of impurities in terms of mass percentage.

7. The method for manufacturing a low-cost, high-strength, high-toughness 770MPa grade seamless steel pipe according to claim 5, characterized in that In step 3, the amount of refined slag added is 200kg / t steel; the amount of lime added is 300kg / t steel; the amount of silicon-barium alloy added is 1kg / t steel; and the amount of ferrotitanium added is 0.5kg / t steel.

8. The method for manufacturing a low-cost, high-strength, high-toughness 770MPa grade seamless steel pipe according to claim 5, characterized in that The slow cooling pit described in step 5: the temperature of the continuous casting billet entering the slow cooling pit is ≥ 400°C, and the continuous casting billet exits the pit when the temperature of the slow cooling pit drops below 200°C.

9. The method for manufacturing a low-cost, high-strength, high-toughness 770MPa grade seamless steel pipe according to claim 5, characterized in that The rolling described in step five: the hot temperature is 1170-1240°C, the soaking temperature is 1230-1260°C, the start rolling temperature is 1230-1250°C, the cooling is controlled by grades after rolling, the cooling water pressure is 0.8-1.2MPa, and the final rolling temperature is 850-880°C.

10. The method for manufacturing a low-cost, high-strength, high-toughness 770MPa grade seamless steel pipe according to claim 5, characterized in that In step six, the critical quenching process is external shower and internal spray + rapid semi-quenching, the quenching furnace heating temperature is 740-880°C, the quenching furnace average temperature is 760-880°C, and the critical quenching process cooling rate is ≥20°C / s; the tempering process is: the tempering furnace heating temperature is 520-620°C, the tempering furnace average temperature is 540-600°C, and the secondary tempering process is supplemented according to the wall thickness, and the tempering average temperature is 580-620°C.

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