Process method for producing x80 pipeline steel having extreme specifications in high water temperature seasons

Abstract

A process method for producing X80 pipeline steel having extreme specifications in high water temperature seasons. Ideal mechanical properties and microstructure are obtained by means of controlling composition, heating, rolling, cooling, laminar cooling water temperature and other processes and technologies. The metallographic structure of the steel is composed of acicular ferrite + granular bainite + M-A; in particular, the content of the M-A component is controlled to be 4-8%, and the average short axis size of austenite grains having large angle grain boundaries is 7.4 um. The steel has a yield strength of 605-635 MPa, a tensile strength of 720-750 MPa, an elongation after fracture A50 of 35-40%, a yield-to-strength ratio of ≤0.85, a low-temperature impact energy (-60°C) of 400-430 J, an impact shear area of ​​100%, and a drop hammer shear area of ​​95-100% at -40°C, which fully meet the standard requirements of X80 pipeline steel. The steel can be used for pipeline projects prone to large plastic deformation, such as liquefaction settlement, ground sliding deformation, and submarine pipeline laying.

Description

Process for producing extreme specification X80 pipeline steel in high water temperature season Technical Field

[0001] The invention belongs to the technical field of iron and steel metallurgy, and particularly relates to a process for producing extreme specification X80 pipeline steel in high water temperature seasons. Background Art

[0002] With the sustainable development of the national economy, domestic energy supply is in short supply. This shortfall in demand for oil and natural gas has significantly stimulated the development of exploration. To meet this growing domestic energy demand, the search for new oil and gas fields is underway. Natural gas resources, in particular, have seen significant breakthroughs in recent years, and the introduction of oil and gas resources from neighboring countries has also become a priority. The transportation of oil, gas, and natural gas is now concentrated in pipeline construction.

[0003] Pipeline transportation is a key method for long-distance transport of oil and natural gas. To improve operational efficiency and reduce costs, pipeline transportation is trending toward larger diameters, thicker walls, and higher pressures. The use of thick-gauge, high-strength, high-toughness, and excellent weldability hot-rolled steel coils for the production of oil and natural gas pipelines has become a future trend. Currently, the China-Russia natural gas pipeline is effectively alleviating natural gas shortages in Northeast China and will also optimize the energy structure of the Beijing-Tianjin-Hebei region, improving air pollution in the region and promoting energy restructuring in the Yangtze River Delta. This will have a positive and far-reaching impact.

[0004] Generally speaking, producing extreme-specification X80 pipeline steel during high-water-temperature seasons presents the following difficulties: 25.6mm thick X80 coils place extremely high demands on the cooling capacity of the hot rolling line. With existing equipment and technical conditions, it is difficult for hot rolling mills to achieve this cooling capacity, especially during high-water-temperature seasons (summer), when the cooling water temperature cannot reach this cooling capacity. Due to its high strength level, 25.6mm thick X80 pipeline steel exerts significant impact on equipment, causing serious damage. Existing technologies for producing thick X80 sometimes result in low yield strength and excessive yield-to-strength ratio. Currently, the production efficiency and yield rate of extreme-specification X80 are low. The performance of this extreme-specification X80 pipeline steel primarily depends on: 1. The alloying elements added in the composition design and their content ratios; 2. The proportion of hard and soft phases in the microstructure; and 3. The morphology and size of the grains after rolling.

[0005] In the prior art, patent CN114058978A discloses X80 pipeline steel with low-temperature toughness. Expensive elements such as Mo, Ni, and Nb are added to the composition design. The Mo content is 0.20-0.35%, the Ni content is 0.30-0.80%, and the Nb content is 0.04-0.08%. The alloy cost is relatively high. In addition, the conventional control method is to control the cooling rate at 15-30°C / s and cool the steel plate to 450-550°C. Patent CN111910126A discloses a thick, high-toughness X80 pipeline steel and its production method. Its chemical composition is C: 0.04-0.08%, Cr: 0.15-0.25%, Nb: 0.07-0.08%, and Ti: 0.015-0.025%. The total cumulative reduction is 56.7-60.0%. The steel plate is rapidly cooled at a cooling rate of 60-80°C / s, a relatively fast cooling rate that is difficult to achieve in production. Patent CN111961957A discloses an X80-grade pipeline steel plate that is both resistant to seawater corrosion and large deformation, and its production method. This involves the addition of high chromium, nickel, and molybdenum to the steel plate, resulting in high costs. The steel plate is formed using a two-stage controlled rolling process: after hot rolling, DQ rapid cooling and ACC slow cooling are sequentially used, making it difficult to achieve in production.

[0006] Summary of the Invention

[0007] In order to overcome the defects of the above-mentioned prior art, the present invention significantly reduces the alloy cost, and by regulating the process and controlling the cooling water temperature, obtains extreme specification X80 pipeline steel with good strength and toughness matching, excellent surface quality and good welding performance, and significantly improves the production efficiency and yield rate.

[0008] To achieve the above-mentioned object, the present invention provides a process for producing X80 pipeline steel of extreme specifications in high-water-temperature seasons, comprising the steps of top-bottom combined-blowing converter smelting, off-furnace refining, large slab continuous casting, heating, controlled rolling, and controlled cooling. The process includes the following technical solutions:

[0009] ① The continuous casting billet drawing speed is 1.0-1.1 m / min, and the continuous casting billet thickness is 250 mm.

[0010] ② The continuous casting billet adopts a low-temperature heating process to avoid excessive growth of austenite. The heating temperature is 1140-1160℃ and the heating time is 1.5-2h.

[0011] ③ Rough rolling: Relax and wait for temperature in the rough rolling unit, and the outlet temperature of the rough rolling unit shall be ≤945℃; the last pass R7 of rough rolling shall be used as the first pass of finishing rolling.

[0012] The most significant innovation in roughing is the use of the final roughing pass, R7, as the first pass in finishing. This utilizes the high deformation of this pass, allowing F5 to be dropped, and increasing the reduction ratios in the other finishing mills. This allows the austenite grains to be fully flattened and gradually elongated along the rolling direction. This creates numerous deformation bands and dislocations within the deformed austenite grains, providing more nucleation sites for ferrite transformation. Furthermore, the elongated austenite grains hinder the growth of ferrite grains, resulting in a finer ferrite structure after phase transformation. Therefore, controlling the reduction ratio in finishing can significantly improve steel strength. By keeping the total number of passes in the finishing mill constant and shifting the reduction ratio to a certain extent, this helps refine grains and improves performance.

[0013] In addition, a "0+7" control mode is adopted, that is, R1 is not passed, and R2 adopts a 7-pass rolling control mode. The deformation amount in the first two passes of the rough rolling is increased by 35-38mm, and the thickness of the intermediate bar is controlled to 68-70mm. By increasing the thickness of the intermediate bar, it is beneficial to increase the reduction of the finishing rolling unit, so that more rolling deformation remains in the unrecrystallized zone of austenite, achieving the purpose of grain refinement.

[0014] Taking the intermediate billet thickness of 70mm as an example, the reduction distribution of each rough rolling pass is:

[0015] 250mm→215mm→180mm→165mm→130mm→110mm→90mm→70mm.

[0016] ④ Finish rolling

[0017] Breaking the inherent rolling mode, changing the distribution mode of the reduction rate from F1 to F7 during conventional rolling from large to small, leaving F5 empty, and significantly shifting the reduction rate back, the mode is adjusted to high reduction rates in the middle rolling mills F3, F4, and F6, and low reduction rates in the two end rolling mills F1, F2, and F7. This effectively increases the cumulative deformation below Tnr, making the structure of X80 pipeline steel fine and uniform, achieving the purpose of refining grains and increasing toughness, especially improving the low-temperature drop hammer tearing performance.

[0018] Ultra-high pressure water is used for descaling before finishing rolling. The descaling water pressure is 38MPa, which can remove the iron oxide scale on the surface of the steel plate.

[0019] The finishing rolling inlet temperature is ≤940℃, below the Tnr temperature to avoid mixed crystal phenomenon.

[0020] Finishing rolling reduction distribution:

[0021] 70mm→66.5mm→61.3mm→50.8mm→41.7mm→41.7mm→31.6mm→25.6mm

[0022] ⑤Laminar cooling

[0023] In order to enhance the cooling effect, the water temperature of the laminar cooling water is limited and the water volume of the cooling water is increased. When producing in high water temperature seasons, liquid nitrogen or canal water from a fire truck is added through the laminar cooling circulation system to lower the water temperature, increase the cooling rate, and ensure that the laminar cooling water temperature is ≤20℃.

[0024] ⑥ Final rolling and coiling of extreme specification X80 pipeline steel: The final rolling temperature is set at 800℃, and the coiling temperature is 360-380℃. The final structure is acicular ferrite + bainite + MA structure, which meets the requirements of low-temperature impact toughness (-60℃) and drop weight tear toughness, and obtains extreme specification X80 pipeline steel with good strength and toughness matching, excellent surface quality and good welding performance.

[0025] Calculation of cooling rate: Cooling rate = layer cooling temperature drop / (cooling section length / speed)

[0026] For X80 pipeline steel, the cooling rate is controlled between 30 and 35°C / s to ensure a good match between strength and toughness.

[0027] The chemical composition of the extreme specification X80 pipeline steel produced by the above process is as follows: C: 0.03-0.04%, Si: 0.10-0.20%, Mn: 1.6-1.70%, P≤0.010%, S≤0.001%, Alt: 0.025-0.035%, N≤0.0030%, O≤0.0010%. In addition, the above steel composition also contains Cr: 1.0-1.2%, Ti: 0.10-0.15%, V: 0.035-0.055%, and the balance is iron and unavoidable impurities.

[0028] By appropriately increasing the V and Cr contents, precipitation strengthening and precipitation strengthening are achieved. Vanadium mainly precipitates as V (C, N) in acicular ferrite as a precipitation strengthening phase at low temperature to improve the strength of the steel. Chromium makes the grains sufficiently fine, promotes the cross-generation of slip bands during the transformation process, and reduces the resistance of transformation dislocations. During the heating process, the continuous casting billet is heated at low temperature to effectively control the size of the original austenite grains and prevent excessive growth. Through rough rolling, low-temperature slow rolling and large reduction are adopted to strengthen the deformation of the high-temperature austenite recrystallization zone and make the austenite grains sufficiently fine. Then, the accelerated cooling process is used to exert the effect of water cooling, refine the phase transformation structure, and obtain a uniform and consistent acicular ferrite + bainite + Mahō Island (MA) structure, thereby improving the low-temperature impact resistance and drop hammer tearing resistance of the extreme specification X80 pipeline steel.

[0029] Furthermore, the limit specification of the X80 pipeline steel is 25.6mm×2000mm, the yield strength is 605-635MPa, the tensile strength is 720-750MPa, and the elongation after fracture is A 50The strength is 35-40%, the yield strength ratio is ≤0.85, the low temperature impact energy (-60℃) is 400-430J, the impact shear area is 100%, and the -40℃ drop hammer shear area is 95-100%, which fully meets the requirements of the X80 pipeline steel standard.

[0030] One application of the aforementioned extreme specification X80 pipeline steel is in pipeline projects prone to large plastic deformation. For example, it is widely used in pipeline projects prone to large plastic deformation, such as liquefaction settlement, ground sliding deformation, and submarine pipeline laying. It is mainly used in pipelines in China and Russia, Pakistan, and India. It can also be used in the construction of natural gas and oil pipelines in harsh environments such as seismically active zones, permafrost, and mountain bottom landslides.

[0031] Compared with the prior art, the present invention has the following beneficial effects:

[0032] ① The present invention provides the composition and process technology formula of X80 pipeline steel with the production limit specification in high water temperature seasons. By controlling the composition, heating, rolling, cooling and other processes and technologies, ideal mechanical properties and microstructure are obtained, especially low-temperature impact resistance and drop weight tear resistance.

[0033] ②The metallographic structure of the steel consists of acicular ferrite + granular bainite + MA, the MA component content is 4-8%, and the average minor axis size of the austenite grains is 7.4μm.

[0034] ③ The limit specification X80 pipeline steel has a yield strength of 605-635 MPa, a tensile strength of 720-750 MPa, and an elongation after fracture of A 50 The strength is 35-40%, the yield strength ratio is ≤0.85, the low temperature impact energy (-60℃) is 400-430J, the impact shear area is 100%, and the -40℃ drop hammer shear area is 95-100%, which fully meets the requirements of the X80 pipeline steel standard. BRIEF DESCRIPTION OF THE DRAWINGS

[0035] FIG1 is a CCT curve of X80 pipeline steel of the present invention;

[0036] FIG2 is a metallographic structure diagram of the X80 pipeline steel of Example 1;

[0037] FIG3 is a microstructure diagram of X80 austenite grains of Example 1;

[0038] FIG4 is a metallographic structure diagram of the comparative example X80 pipeline steel. DETAILED DESCRIPTION

[0039] The present invention will be further described below with reference to specific examples, but the present invention is not limited in any way. To avoid redundancy, the raw materials in the following examples are all commercially available products unless otherwise specified, and the methods used are all conventional methods unless otherwise specified.

[0040] According to the present invention, the process for producing X80 pipeline steel, a critical specification for high-temperature seasons, involves smelting in a 180-ton top-and-bottom combined-blowing converter and controlled rolling and cooling in a 2300mm hot rolling mill. The process flow is as follows: 250mm continuous casting slab → heating → rough rolling (0+7) (relaxation and warming) → 7-stand finishing rolling (controlled rolling) → ultra-rapid cooling → coiling (controlled phase transformation) → finished product. The chemical composition of the steel plates in this example is shown in Table 1. Figure 1 shows the CCT curve of the X80 pipeline steel.

[0041] Table 1 Chemical composition (wt%) of the extreme specification X80 pipeline steel plate in the embodiment

[0042] Example 1

[0043] The process for producing X80 pipeline steel with extreme specifications in high water temperature seasons (X80 pipeline steel plate specifications 25.6mm×2000mm) includes top and bottom combined blowing converter smelting, off-furnace refining, large slab continuous casting, heating, controlled rolling, and controlled cooling. The specific technical solution is as follows:

[0044] ① The continuous casting billet drawing speed is 1.0m / min, and the continuous casting billet thickness is 250mm.

[0045] ② The continuous casting billet adopts hot delivery and hot charging process, and the continuous casting billet adopts low temperature heating process, the heating temperature is 1142℃, and the heating time is 1.5h.

[0046] ③ Rough rolling: Relax and wait for temperature in the rough rolling unit, the outlet temperature of the rough rolling unit is 945℃; the last pass R7 of rough rolling is used as the first pass of finishing rolling.

[0047] The most significant innovation in roughing is the use of the final roughing pass, R7, as the first pass in finishing. This utilizes the high deformation in this pass, allowing F5 to be dropped from the stand and increasing the reduction ratios in the other finishing stands. This allows the austenite grains to be fully flattened and gradually elongated along the rolling direction. This also creates numerous deformation bands and dislocations within the deformed austenite grains, providing more nucleation sites for ferrite transformation. Furthermore, the elongated austenite grains hinder the growth of ferrite grains, resulting in a finer ferrite structure after the phase transformation. Therefore, controlling the reduction ratio in finishing can significantly improve steel strength. By keeping the total number of passes in the finishing section constant and shifting the reduction ratio to a certain extent, this contributes to grain refinement and improved performance.

[0048] In addition, a "0+7" control mode is adopted, that is, R1 is not passed, and R2 adopts a 7-pass rolling control mode. The deformation amount in the first two passes of the rough rolling is increased by 35-38mm, and the thickness of the intermediate bar is controlled to 68-70mm. By increasing the thickness of the intermediate bar, it is beneficial to increase the reduction of the finishing rolling unit, so that more rolling deformation remains in the unrecrystallized zone of austenite, achieving the purpose of grain refinement.

[0049] The distribution of reduction in each pass of rough rolling is:

[0050] 250mm→215mm→180mm→165mm→130mm→110mm→90mm→70mm.

[0051] ④ Finish rolling

[0052] Breaking the inherent rolling mode, changing the distribution mode of the reduction rate from F1 to F7 during conventional rolling from large to small, leaving F5 empty, and significantly shifting the reduction rate back, the mode is adjusted to high reduction rates in the middle rolling mills F3, F4, and F6, and low reduction rates in the two-end rolling mills F1, F2, and F7. This effectively increases the cumulative deformation below Tnr, making the structure of X80 pipeline steel fine and uniform, achieving the purpose of refining grains and increasing toughness, especially improving the drop weight tearing performance.

[0053] Ultra-high pressure water is used for descaling before finishing rolling. The descaling water pressure is 38MPa, which can remove the iron oxide scale on the surface of the steel plate.

[0054] The finishing rolling inlet temperature is 940℃, which is below the Tnr temperature to avoid mixed crystal phenomenon.

[0055] Finishing rolling reduction distribution:

[0056] 70mm→66.5mm→61.3mm→50.8mm→41.7mm→41.7mm→31.6mm→25.6mm

[0057] ⑤Laminar cooling

[0058] In order to enhance the cooling effect, the water temperature of the laminar cooling water is limited and the water volume of the cooling water is increased. When producing in the high water temperature season, liquid nitrogen is added through the laminar cooling circulation system or new water from the Taizi River is shipped by fire trucks to lower the water temperature, increase the cooling rate, and ensure that the laminar cooling water temperature is 19°C.

[0059] ⑥ Final rolling and coiling of extreme specification X80 pipeline steel: The final rolling temperature is set at 800℃, and the coiling temperature is 380℃. Low-temperature final rolling and low-temperature coiling processes are adopted to obtain a final structure of acicular ferrite + bainite + MA structure, which meets the requirements of low-temperature impact toughness and drop weight tear toughness, and obtains extreme specification X80 pipeline steel with good strength and toughness matching, excellent surface quality and good weldability.

[0060] Calculation of cooling rate: Cooling rate = layer cooling temperature drop / (cooling section length / speed)

[0061] For X80 pipeline steel, the cooling rate is controlled at 30℃ / s to ensure a good match between strength and toughness.

[0062] The metallographic structure of the X80 pipeline steel of Example 1 is shown in Figure 2, which shows a structure consisting of acicular ferrite, granular bainite, and MA, with the MA content controlled to 5%. The microstructure of the X80 austenite grains of Example 1 is shown in Figure 3, with an average minor axis size of 7.4 μm for the high-angle grain boundaries. The comprehensive mechanical properties of the steel plate of Example 1 are shown in Table 2.

[0063] Example 2

[0064] The process for producing X80 pipeline steel with extreme specifications in high water temperature seasons (X80 pipeline steel plate specifications 25.6mm×2000mm) includes top and bottom combined blowing converter smelting, off-furnace refining, large slab continuous casting, heating, controlled rolling, and controlled cooling. The specific technical solution is as follows:

[0065] ① The continuous casting billet drawing speed is 1.05m / min, and the continuous casting billet thickness is 250mm.

[0066] ② The continuous casting billet adopts hot delivery and hot charging process, and the continuous casting billet adopts low temperature heating process, the heating temperature is 1151℃, and the heating time is 1.7h.

[0067] ③ Rough rolling: Relax and wait for temperature in the rough rolling mill, the outlet temperature of the rough rolling mill is 940℃;

[0068] R1 adopts the blank pass, and R2 adopts the 7-pass rolling control mode. The deformation of the first two passes is 37mm, and the thickness of the intermediate billet is controlled to be 68mm.

[0069] The distribution of reduction in each pass of rough rolling is:

[0070] 250mm→213mm→176mm→164mm→132mm→108mm→92mm→68mm.

[0071] ④ Finish rolling

[0072] F5 is passed, and the reduction rate distribution mode from F1 to F7 is changed from large to small, and the reduction rate is significantly shifted back, and adjusted to a mode in which the reduction rate of the middle rolling mills F3, F4, and F6 is high, and the reduction rate of the two end rolling mills F1, F2 and F7 is low.

[0073] Ultra-high pressure water is used for descaling before finishing rolling, with a water pressure of 38 MPa. The finishing rolling inlet temperature is 936°C.

[0074] Finishing rolling reduction distribution:

[0075] 70mm→65.7mm→62.8mm→48.5mm→40.7mm→40.7mm→30.5mm→25.6mm

[0076] ⑤Laminar cooling: laminar cooling water temperature is 18℃.

[0077] ⑥ Final rolling and coiling of the extreme specification X80 pipeline steel: The final rolling temperature was set at 800°C, the coiling temperature was set at 370°C, and the cooling rate was controlled at 33°C / s. The comprehensive mechanical properties of the steel plate of Example 2 are shown in Table 2.

[0078] Example 3

[0079] The process for producing X80 pipeline steel with extreme specifications in high water temperature seasons (X80 pipeline steel plate specifications 25.6mm×2000mm) includes top and bottom combined blowing converter smelting, off-furnace refining, large slab continuous casting, heating, controlled rolling, and controlled cooling. The specific technical solution is as follows:

[0080] ① The continuous casting billet drawing speed is 1.1m / min, and the continuous casting billet thickness is 250mm.

[0081] ② The continuous casting billet adopts hot delivery and hot charging process, and the continuous casting billet adopts low temperature heating process, the heating temperature is 1159℃, and the heating time is 2.0h.

[0082] ③ Rough rolling: Relax and wait for temperature in the rough rolling mill, the outlet temperature of the rough rolling mill is 938℃;

[0083] R1 adopts the blank pass, and R2 adopts the 7-pass rolling control mode. The deformation of the first two passes is 38mm, and the thickness of the intermediate billet is controlled to be 67mm.

[0084] The distribution of reduction in each pass of rough rolling is:

[0085] 250m→212mm→174mm→163mm→130mm→105mm→90mm→67mm.

[0086] ④ Finish rolling

[0087] F5 is passed empty, and the mode of high reduction rate of the middle rolling mills F3, F4, and F6 and low reduction rate of the two end rolling mills F1, F2 and F7 is adopted.

[0088] Ultra-high pressure water is used for descaling before finishing rolling, with a water pressure of 38 MPa. The finishing rolling inlet temperature is 935°C.

[0089] Finishing rolling reduction distribution:

[0090] 67mm→63.5mm→60.5mm→45.2mm→38.7mm→38.7mm→29.5mm→25.6mm

[0091] ⑤Laminar cooling: laminar cooling water temperature is 18℃.

[0092] ⑥ Final rolling and coiling of the extreme specification X80 pipeline steel: The final rolling temperature was set at 800°C, the coiling temperature was 360°C, and the cooling rate was controlled at 35°C / s. The comprehensive mechanical properties of the steel plate of Example 3 are shown in Table 2.

[0093] Comparative Example

[0094] Using conventional X80 pipeline steel as a comparative example, this existing X80 pipeline steel has a yield strength of no less than 555 MPa, a tensile strength of no less than 625 MPa, a Charpy impact energy value of 300 J at -20°C, and a shear area of ​​85% at -15°C. The comprehensive mechanical properties of the steel plate in this comparative example are shown in Table 2.

[0095] The metallographic structure of the existing X80 pipeline steel (comparative example) is shown in FIG4 , which is polygonal ferrite + bainite + MA, with an MA content of 14.1%. Excessive MA content is not conducive to increasing low-temperature impact toughness.

[0096] Table 2 Mechanical properties of steel plates in various embodiments and comparative examples

[0097] Compared with the existing X80 pipeline steel, the product of the embodiment of the present invention can still have a high Charpy impact energy value and impact shear area at a temperature of -60°C or lower, and still have a good drop weight tear area (DWTT) at -40°C, and a low yield strength ratio, indicating that the product of the present invention has good low-temperature toughness, and its yield strength and tensile strength indicators are higher than those of the existing X80 pipeline steel.

[0098] Anyone skilled in the art will be able to utilize the above-disclosed technical content to make many possible changes and modifications to the technical solution of the present invention, or to modify it into equivalent embodiments with equivalent changes, without departing from the scope of the technical solution of the present invention. Therefore, any simple modifications, equivalent changes, and modifications made to the above embodiments based on the technical essence of the present invention that do not depart from the content of the technical solution of the present invention shall still fall within the scope of protection of the technical solution of the present invention.

Claims

1. A process for producing X80 pipeline steel with extreme specifications in high water temperature seasons, comprising top and bottom combined blowing converter smelting, refining outside the furnace, large slab continuous casting, heating, controlled rolling, and controlled cooling steps, characterized in that: The process method includes the following technical solutions: ① The continuous casting billet drawing speed is 1.0-1.1 m / min, and the continuous casting billet thickness is 250 mm; ② The continuous casting billet adopts low temperature heating process, the heating temperature is 1140~1160℃, and the heating time is 1.5~2h; ③ Rough rolling: Relax and wait for temperature in the rough rolling unit, the outlet temperature of the rough rolling unit is ≤945℃; the last pass R7 of rough rolling is used as the first pass of finishing rolling; ④ Finish rolling: Adjust the rolling mode to a mode where the middle mills F3, F4, and F6 have high reduction rates, F5 is empty, and the two end mills F1, F2, and F7 have low reduction rates, to increase the cumulative deformation below Tnr; ⑤Laminar cooling: laminar cooling water temperature ≤20℃; ⑥ Final rolling and coiling of extreme specification X80 pipeline steel: the final rolling temperature is set at 800℃, the coiling temperature is 360~380℃; the cooling rate is controlled at 30~35℃ / s.

2. The process according to claim 1, characterized in that: In step ③, the rough rolling adopts the "0+7" control mode, that is, R1 is passed without any control, R2 adopts the rolling 7-pass control mode, the last pass of R2 is used as the first pass of finishing rolling, and the large reduction makes the grains after rough rolling fully refined and uniform. The first two passes of rough rolling increase the deformation by 35-38mm, and the thickness of the intermediate billet is controlled to be 68-70mm.

3. The process according to claim 1, characterized in that: In step ④, ultra-high pressure water is used for descaling before finishing rolling, and the descaling water pressure is 38 MPa; the finishing rolling inlet temperature is ≤940°C.

4. The process according to claim 1, characterized in that: The calculation formula of the cooling rate in step ⑥ is: cooling rate = layer cooling temperature drop / (cooling section length / speed).

5. An extreme specification X80 pipeline steel produced according to the process method of claim 1, characterized in that: The chemical composition of the X80 pipeline steel is C: 0.03-0.04%, Si: 0.10-0.20%, Mn: 1.6-1.70%, P≤0.010%, S≤0.001%, Alt: 0.025-0.035%, N≤0.0030%, O≤0.0010% in percentage by mass; in addition, the steel chemical composition also contains Cr: 1.0-1.2%, Ti: 0.10-0.15%, V: 0.035-0.055%, and the balance is iron and unavoidable impurities.

6. The extreme specification X80 pipeline steel according to claim 5, characterized in that: The X80 pipeline steel has a limit specification of 25.6 mm × 2000 mm, a yield strength of 605-635 MPa, a tensile strength of 720-750 MPa, and an elongation of A 50 The yield strength ratio is 35-40%, the yield strength ratio is ≤0.85, the low-temperature impact energy of -60℃ is 400-430J, the impact shear area is 100%, and the -40℃ drop hammer shear area is 95-100%.

7. Use of the extreme specification X80 pipeline steel as claimed in claim 5 in pipeline projects prone to large plastic deformation.

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