An extreme grade x100 pipeline steel and method of production thereof

Abstract

The present application relates to a kind of limit specification X100 pipeline steel and its production method, the chemical composition of steel is C:0.03%~0.05%, Si:0.20%~0.30%, Mn:1.80%~1.95%, P≤0.010%, S≤0.001%, N≤0.0030%, O≤0.0010%, Cr:0.50%~0.80%, Cu:0.25%~0.40%, Zr:0.10%~0.20%, B:0.0025%~0.0040%, Ti:0.015%~0.03%, Nb:0.10%~0.15%, Al:0.015%~0.055%, Mg:0.0020%~0.0060%, the balance is Fe and impurity.By reasonable component design, the mechanical properties, welding performance, dynamic tear resistance and low temperature impact toughness of steel are improved;Pure smelting and strong cold process are used, so that finished steel plate has excellent resistance to large deformation, excellent low temperature drop hammer performance and good shape.

Description

TECHNICAL FIELD

[0001] The present application relates to the technical field of pipeline steel production, and particularly relates to an extreme specification (25.6mm thick x 1550mm wide) X100 pipeline steel and a production method thereof. BACKGROUND

[0002] With the continuous development of science and technology, the demand for energy such as oil and natural gas is gradually increasing; in order to improve the long-distance transportation efficiency of oil and natural gas and reduce the construction and operation cost of pipelines, the pipelines for transporting oil and natural gas gradually develop in the direction of large wall thickness, high pressure resistance and large diameter. Based on the concept of "large deformation design", the pipeline steel is required to not only have high strain capacity (high stress ratio, high uniform elongation and low yield ratio), but also have good low-temperature drop hammer performance to resist external impact. It has become a development trend to manufacture oil and natural gas pipelines by using hot-rolled pipeline steel with high strength, high toughness, thick specification and excellent weldability.

[0003] The extreme specification X100 pipeline steel product with a thickness of 25.6mm and a width of 1550mm is suitable for pipeline laying in earthquake zones, permafrost zones, mud-rock flow zones and seabed, and the performance of the pipeline steel product mainly depends on the following aspects: (1) the alloy elements added in the component design and the content ratio; (2) the proportion of hard phase and soft phase in the microstructure; (3) the morphology and size of the grains after the steel plate is rolled.

[0004] Chinese patent application CN102560284A discloses a "high-strength and high-toughness X100 pipeline steel hot-rolled strip and a manufacturing method thereof", which has the following mass percentage of chemical elements: C: 0.015-0.090%, Si: 0.1-0.5%, Mn: 1.50-1.79%, P≤0.015%, S≤0.003%, Cr: 0.10-0.40%, Nb: 0.03-0.10%, Zr: 0.001-0.100%, Ti: 0.01-0.035%, Mo: 0.31-0.60%, Cu: 0.10-0.40%, Ni: 0.10-0.50%, Ca: 0.0010-0.0050%, Al: 0.02-0.045%, N≤0.010%, O≤0.008%, and the balance of Fe and other inevitable inclusions. The microstructure of the high-strength and high-toughness X100 pipeline steel hot-rolled strip is lower bainite; the manufacturing method comprises the following steps: smelting, continuous casting, slab reheating, rough rolling, finish rolling, controlled cooling, and coiling; in the controlled cooling step, the cooling speed is 41-70℃ / s. Due to the addition of expensive Mo, Ni and Nb elements in the component design, and the Mo content is 0.31-0.60% and the Ni content is 0.10-0.50%, the alloy cost is high; in addition, the controlled cooling speed is 41-70℃ / s, which is not easy to realize on site.

[0005] Chinese patent application CN102851587A discloses an "anti-deformation X80-X100 pipeline steel plate and a production method thereof", which comprises the following components in percentage by weight: C=0.04-0.09%, Si=0.10-0.50%, Mn=1.0-2.0%, P≤0.015%, S≤0.005%, Nb=0.05-0.11%, Ti=0.010-0.025%, Mo≤0.30%, Cu≤0.40%, Ni≤0.50%, Cr≤0.40%, and the balance comprising Fe. The preparation method of the steel plate comprises the following steps: material preparation, hot metal pre-desulphurization, converter smelting, LF refining, RH treatment, slab continuous casting, slab reheating, temperature control rolling, relaxation air cooling, pre-straightening, accelerated cooling, hot straightening, cold bed cooling, UT flaw detection, shearing, and warehousing. It relates to plate rolling technology, and the process is relatively complex, the production rhythm is slow, and the yield is affected.

[0006] Chinese patent application CN103233185A discloses "a kind of X100 pipeline steel and its production method", the component and weight percentage content of steel are as follows: C: 0.035~0.065%, Si: 0.15~0.40%, Mn: 1.90~2.30%, P: ≤0.012%, S: ≤0.0015%, Nb: 0.030~0.060%, V: 0.030~0.055%, Ti: 0.010~0.025%, Cu: 0.15~0.35%, Cr: 0.30~0.60%, Ni: 0.15~0.30%, Al: 0.020%~0.050%, B: 0.0008~0.0025%, N: ≤0.0080%, Ni / Cu≥0.5;steps: smelting and continuous casting into billet;heating to cast billet;rough rolling;finish rolling;rapid cooling;straightening.It adds nickel in the composition, and the alloy cost is higher, and its composition only restricts N, and does not limit O content, which is not conducive to realizing large-scale production.

[0007] At present, the production of limit specification X100 pipeline steel with thickness of 25.6 mm and width of 1550 mm has the following problems: (1) the X100 steel coil with thickness of 25.6 mm has high requirement on cooling capacity of a hot rolling production line, and the existing hot continuous rolling mill group is difficult to meet the required cooling capacity, especially in summer, the cooling water temperature is higher, and the problem is more prominent. (2) due to high strength grade, the X100 pipeline steel with thickness of 25.6 mm has great impact on equipment, and often causes equipment damage. (3) when the X100 pipeline steel with thickness of 25.6 mm is produced by using conventional process, the finished steel plate often has low yield strength and yield strength ratio exceeding the standard. (4) when the X100 pipeline steel with thickness of 25.6 mm is produced by using conventional process, the production efficiency and yield rate are low. SUMMARY

[0008] The present application provides a limit specification X100 pipeline steel and a production method thereof, which overcomes the shortcomings of low production efficiency, high production cost and difficult precise control of the existing pipeline steel, improves the mechanical properties, welding properties, dynamic tear resistance and low temperature impact toughness of the steel through reasonable component design, does not add expensive metals in the steel, reduces the alloy cost, and uses pure smelting and strong cooling process to make the finished steel plate have excellent large deformation resistance (including high stress ratio, high uniform elongation and low yield ratio), excellent low temperature drop hammer performance and good plate shape.

[0009] In order to achieve the above purpose, the present application adopts the following technical scheme:

[0010] An extreme specification X100 pipeline steel, the chemical composition of the steel is as follows in percentage by mass: C: 0.03%~0.05%, Si: 0.20%~0.30%, Mn: 1.80%~1.95%, P≤0.010%, S≤0.001%, N≤0.0030%, O≤0.0010%, Cr: 0.50%~0.80%, Cu: 0.25%~0.40%, Zr: 0.10%~0.20%, B: 0.0025%~0.0040%, Ti: 0.015%~0.03%, Nb: 0.10%~0.15%, Al: 0.015%~0.055%, Mg: 0.0020%~0.0060%, and the balance is Fe and impurities; the size specification of the finished steel plate is 25.6mm thick x 1550mm wide.

[0011] Further, the microstructure of the finished steel plate is as follows: quasi-polygonal ferrite + bainite + MA component + nanometer precipitated phase, wherein the volume fraction of the quasi-polygonal ferrite is≥35%.

[0012] Further, the performance of the finished steel plate is as follows: the yield strength Rp 0.2 is 750~800MPa, the tensile strength Rm is 920~960MPa, the elongation A 50 after fracture is 35%~45%, the yield strength ratio is≤0.85; the stress ratio is Rt1.5 / Rt0.5≥1.25, Rt2.0 / Rt1.0≥1.15; the uniform elongation UEL is≥12.0%; the average value of the Charpy impact energy at-40℃ is≥320J, and the average value of the falling weight tear shear area at-40℃ is≥95%.

[0013] A production method of an extreme specification X100 pipeline steel, comprising the following steps: top and bottom combined blowing converter smelting, secondary refining, large slab continuous casting, slab reheating, rough rolling, finish rolling, controlled cooling, coiling and slow cooling processes; the following processes are controlled:

[0014] (1) slab reheating: the heating temperature is 1160~1200℃, and the heating time is 1.5h~2h;

[0015] (2) rough rolling: the rough rolling rolling temperature is 950~1140℃, and the rough rolling is carried out in a walking waiting mode, and when the outlet temperature reaches the finish rolling opening temperature, the finish rolling mill group is entered;

[0016] (3) finish rolling: the finish rolling rolling temperature is 820~950℃;

[0017] (4) controlled cooling: the laminar flow cooling mode is adopted, and the cooling speed is 30~40℃ / s;

[0018] (5) Coiling: the coiling temperature is determined according to the cooling control factor Q and the cooling speed in the cooling process of step (4), Q = coiling temperature / cooling speed, 8s ≤ Q ≤ 10s;

[0019] (6) Slow cooling: after the steel coil is discharged from the laminar cooling unit, the steel coil is naturally cooled in a slow cooling pit for more than 48h.

[0020] Compared with the prior art, the present application has the following beneficial effects:

[0021] (1) The present application adopts a low-carbon micro-alloying component design, and the carbon equivalent is low, which improves the welding performance of the steel pipe forming and welding and on-site girth welding; by adding a higher content of Cr element, the bainite phase transition is promoted, and the mechanical properties of the steel are improved; the middle Mn component design is adopted to improve the segregation, and the dynamic tear resistance and low-temperature impact toughness of the steel are improved;

[0022] (2) The X100 pipeline steel of the present application does not add expensive metals such as Mo, V and Ni, and the production cost is reduced while the performance of the steel plate is ensured;

[0023] (3) The X100 pipeline steel of the present application adopts a pure smelting method during production, and the contents of O, N, P and S in the steel are strictly controlled, and a certain amount of Zr and Mg elements are added to improve the inclusion shape and size, refine the grain, and further improve the drop weight tear resistance and low-temperature impact toughness of the steel plate;

[0024] (4) The X100 pipeline steel of the present application adopts a strong cooling process during production, and by controlling the cooling factor Q and the cooling speed, the microstructure of the steel plate is effectively controlled, and the effective grain size is less than 4µm, so that the steel plate has good strength and toughness matching, and is especially suitable for "anti-large deformation design" of oil and gas pipeline engineering. BRIEF DESCRIPTION OF DRAWINGS

[0025] Figure 1 is a metallographic structure photo of the X100 pipeline steel produced by the embodiment 1 of the present application. DETAILED DESCRIPTION

[0026] The limit specification X100 pipeline steel provided by the application has the following chemical components in percentage by mass: C: 0.03%-0.05%, Si: 0.20%-0.30%, Mn: 1.80%-1.95%, P≤0.010%, S≤0.001%, N≤0.0030%, O≤0.0010%, Cr: 0.50%-0.80%, Cu: 0.25%-0.40%, Zr: 0.10%-0.20%, B: 0.0025%-0.0040%, Ti: 0.015%-0.03%, Nb: 0.10%-0.15%, Al: 0.015%-0.055%, Mg: 0.0020%-0.0060%, and the rest is Fe and impurities; and the size specification of the finished steel plate is 25.6mm thick*1550mm wide.

[0027] Further, the microstructure of the finished steel plate is: quasi-polygonal ferrite + bainite + MA component + nanometer precipitated phase, wherein the volume fraction of the quasi-polygonal ferrite is≥35%.

[0028] Further, the performance of the finished steel plate is as follows: the yield strength Rp 0.2 is 750-800MPa, the tensile strength Rm is 920-960MPa, the elongation A 50 after fracture is 35%-45%, the yield strength ratio is≤0.85; the stress ratio is Rt1.5 / Rt0.5≥1.25 and Rt2.0 / Rt1.0≥1.15; the uniform elongation UEL is≥12.0%; the average Charpy impact energy at-40℃ is≥320J, and the average falling weight tear shear area at-40℃ is≥95%.

[0029] The production method of the limit specification X100 pipeline steel provided by the application comprises the following steps: top and bottom combined blowing converter smelting, secondary refining, large slab continuous casting, slab reheating, rough rolling, finish rolling, controlled cooling, coiling and slow cooling processes; and the following processes are controlled:

[0030] (1) slab reheating: the heating temperature is 1160-1200℃, and the heating time is 1.5h-2h;

[0031] (2) rough rolling: the rough rolling rolling temperature is 950-1140℃, and the rough rolling is carried out in a moving waiting mode, and when the outlet temperature reaches the finish rolling opening temperature, the rough rolling enters the finish rolling unit;

[0032] (3) finish rolling: the finish rolling rolling temperature is 820-950℃;

[0033] (4) controlled cooling: the laminar flow cooling mode is adopted, and the cooling speed is 30-40℃ / s;

[0034] (5) Coiling: the coiling temperature is determined according to the cooling control factor Q in the cooling and the cooling speed, Q = coiling temperature / cooling speed, 8s ≤ Q ≤ 10s, according to step (4);

[0035] (6) Slow cooling: after the steel coil is out of the laminar cooling unit, the steel coil is naturally cooled in the slow cooling pit for more than 48h.

[0036] The design principle of the chemical composition of the limit specification X100 pipeline steel is as follows:

[0037] Carbon (C): C is the main element affecting the toughness, hardness, strength and welding performance of the pipeline steel, and plays a role in the pipeline steel through precipitation strengthening and solid solution strengthening; the strength of the steel is obviously improved with the increase of the content of C, but the toughness, ductility and welding performance of the steel will decrease with the increase of the content of C. According to the Graville curve of the relationship between the content of C in the steel, carbon equivalent and the weldability of the steel, when C < 0.11%, the pipeline steel has good weldability. Therefore, generally, C < 0.11% in the pipeline steel, and the content of C in the steel is controlled to be 0.03% to 0.05% in the present application.

[0038] Manganese (Mn): Mn is an element that expands the γ phase region, effectively reduces the γ→α phase transition temperature, improves the strength of the steel through solid solution strengthening, obtains fine phase transition products, thereby improving the toughness of the steel, and reducing the ductile-brittle transition temperature. When the content of Mn is relatively high, segregation is prone to occur at the center of the plate thickness, and hard phase martensite organization is generated, thereby reducing the dynamic tear resistance of the steel plate. Therefore, the content of Mn in the steel is limited to 1.80% to 1.95% in the present application.

[0039] Silicon (Si): Si is a strong deoxidizing element in the steel, which plays a solid solution strengthening role, but excessive Si content is prone to generate iron oxide red defects, which is not conducive to the removal of the iron oxide skin of the steel plate and deteriorates the welding performance. Therefore, the content of Si in the steel is controlled to be 0.20% to 0.30% in the present application.

[0040] Chromium (Cr): Cr is a carbide forming element, which mainly improves the hardness of the steel plate through precipitation strengthening, and can significantly improve the strength of the steel; Cr can replace Mo, V and other alloys to improve the corrosion resistance and hydrogen-induced cracking resistance of the steel. However, excessive Cr will reduce the elongation of the steel plate, cause the formation of low-melting-point Cr-Mn composite oxides, form surface cracks during hot working, and have an adverse effect on the welding performance; therefore, the content of Cr in the steel is controlled to be 0.50% to 0.80% in the present application.

[0041] Copper (Cu): Cu plays a solid solution strengthening role in the steel, improves the strength of the steel plate and the welding heat affected zone, and also improves the fatigue resistance and corrosion resistance of the steel; however, Cu is a low-melting-point metal, which is prone to cause "copper brittleness", and excessive Cu is not conducive to the low-temperature toughness of the steel; therefore, the content of Cu in the steel is controlled to be 0.25% to 0.40% in the present application.

[0042] Boron (B): the right amount of B can make the bainite transformation curve smooth, inhibit the nucleation of ferrite on the austenite grain boundary; in order to improve the strength of the pipeline steel, under the condition of low carbon, it must have a fast cooling speed to get bainite structure, but too much B will make the steel brittle; therefore, the B content in the steel is controlled to be 0.0025%-0.0040%.

[0043] Aluminum (Al): a strong deoxidizing element in steel, which can significantly reduce the oxygen content of the steel; the aluminum in the steel combines with nitrogen to form AlN, which can refine the grain and improve the strength and toughness; but too high aluminum content will lead to a significant increase in Al2O3 inclusions, which will destroy the cleanliness of the molten steel; therefore, the Al content in the steel is controlled to be 0.015%-0.055%.

[0044] Zirconium (Zr): has strong binding ability with N and O, which can make the inclusions spheroidized to form oxide metallurgy, that is, the oxide and nitride of zirconium as the core, other inclusions attached to it, improve the size and shape of the inclusions, and get more dense and fine inclusions, which is beneficial to improve the low temperature toughness of the steel; the Zr content in the steel is controlled to be 0.10%-0.20%.

[0045] Magnesium (Mg): the present invention controls the morphology and size of sulfides by adding Mg to improve the low temperature toughness and effectively improve the anisotropy of the steel plate; in order to ensure the best effect, the Mg content in the steel is controlled to be 0.0020%-0.0060%.

[0046] Titanium (Ti): forms TiN particles with nitrogen, which is a strong carbonitride forming element, usually requires Ti / N≥3.42, inhibits the growth of austenite grains during heating, plays a role in refining grains and precipitation strengthening, and improves the low temperature toughness of the steel; in addition, Ti and Nb composite strengthening can improve the thermal stability of (NbTi)(CN), improve the toughness of the steel plate, and improve the welding performance of the steel plate, which has a good inhibitory effect on the growth of austenite grains during heating and the coarsening of grains in the heat affected zone, the titanium content in the steel is controlled to be 0.015%-0.03%.

[0047] Niobium (Nb): can inhibit the excessive growth of austenite grains, delay the γ→α phase transformation, refine the ferrite grains, improve the toughness of the steel, and get fine structure. In the process of hot rolling of the steel plate, niobium carbonitride can delay recrystallization and grain growth, and retain more dislocation density in the matrix by pinning dislocations, thereby improving the strength and toughness of the steel. The present invention adds Nb to replace V and Mo to achieve precipitation strengthening and solid solution strengthening, which reduces the metallurgical cost; the Nb content in the steel is controlled to be 0.10%-0.15%.

[0048] Phosphorus (P), sulfur (S): the lower the content in the steel, the better; the application controls the sulfur content in the steel (≤0.0010%) and magnesium treatment, modifies the inclusions, controls the phosphorus content in the steel (≤0.010%), and significantly improves the performance of the steel plate against HIC and SSCC, and improves the low-temperature impact toughness of the pipeline steel.

[0049] Oxygen (O), nitrogen (N): nitrogen and titanium form high-melting-point TiN particles, which can inhibit excessive grain growth during heating and improve the strength and toughness of the steel. However, when the N content is too high, high-concentration free N atoms cause failure, increase the yield strength by pinning dislocations, but reduce the toughness. For the oxygen content in the steel, deoxidation treatment is required at the end of smelting to reduce oxide inclusions and bubbles, improve the internal quality of the steel, and improve the dynamic tear resistance and low-temperature impact toughness of the pipeline steel. Therefore, the application controls the O content in the steel to be ≤0.0010%, and the N content to be ≤0.0030%.

[0050] The production method of the extreme specification X100 pipeline steel includes top and bottom combined blowing converter smelting, secondary refining, large slab continuous casting, slab reheating, rough rolling, finish rolling, controlled cooling, coiling and slow cooling processes. In the controlled cooling step, the steel plate is cooled by a front section rapid cooling process after finish rolling, and the forced cooling system is opened from the first group. To ensure the uniformity of the X100 pipeline steel, the temperature of the laminar cooling water is ≤20℃, and in summer high temperature, the temperature of the laminar cooling water can be quickly reduced to below 20℃ by increasing new water, and the cooling rate of the steel plate is ensured to be 30-40℃ / S. In the coiling step, the coiling temperature is determined according to the cooling control factor Q and the cooling speed in the rapid cooling process, Q=coiling temperature / cooling speed, 8s≤Q≤10s.

[0051] In order to more intuitively embody the application, the embodiments of the application are further described in combination with examples. The following examples are only preferred specific embodiments of the application, but the protection scope of the application is not limited thereto, and any skilled person in the art can obviously obtain technical solutions within the technical range disclosed by the application, including simple changes or equivalent replacements, which are within the protection scope of the application.

[0052] Example:

[0053] Examples 1-5 all produce X100 pipeline steel hot-rolled steel plates of extreme specification (25.6mm thick x 1550mm wide) according to the following steps:

[0054] (1) 180-ton top and bottom combined blowing converter is used to smelt raw materials to obtain molten steel meeting the composition requirements;

[0055] (2) the molten steel is obtained by continuous casting to obtain a continuous casting billet, and the thickness of the slab is 250mm;

[0056] (3) reheating the continuous casting billet, the heating temperature being 1160-1200 °C, and the heating time being 90-120 min;

[0057] (4) rough rolling: the rough rolling temperature being 950-1000 °C, in order to avoid two-phase zone rolling, rough rolling is carried out with temperature waiting, and when the outlet temperature reaches the roughing opening temperature, it enters the finishing rolling unit;

[0058] (5) finishing rolling: the finishing rolling temperature being 820-950 °C;

[0059] (6) controlled cooling: after the steel plate exits the finishing rolling, a front-end rapid cooling process is adopted, the forced cooling system is opened from the first group, and the cooling speed is 30-40 °C / s;

[0060] (7) coiling: the coiling temperature is determined according to the cooling control factor Q and the cooling speed in the controlled cooling step, Q = coiling temperature / cooling speed, 8s ≤ Q ≤ 10s;

[0061] (8) slow cooling: after the steel coil exits the laminar cooling unit, it enters the slow cooling pit for natural cooling for more than 48h.

[0062] The chemical composition of the steel plates produced in Examples 1-5 is shown in Table 1, the production process parameters of Examples 1-5 are shown in Table 2, and the mechanical properties of the finished steel plates of Examples 1-5 are shown in Table 3 (tested according to the standard of API-Spec-5L).

[0063] Table 1 Chemical composition of steel plate (wt%)

[0064]

[0065] Table 2 Production process parameters

[0066]

[0067] Table 3 Mechanical properties of finished steel plate

[0068]

[0069] In the table: Rp 0.2 / Rm is the yield ratio, KV2 is the -40 °C Charpy impact energy, FA is the -40 °C drop weight tear shear area, and SA is the -40 °C impact shear area.

[0070] The test results show that the X100 pipeline steel plate obtained by using the chemical composition and the production process has excellent comprehensive mechanical properties, good toughness, good large deformation resistance (high stress ratio Rt1.5 / Rt0.5≥1.25, Rt2.0 / Rt1.0≥1.15, high uniform elongation UEL≥12.0%, and low yield strength ratio ≤0.85), and has a flat hot-rolled plate shape, and is especially suitable for petroleum and natural gas pipeline engineering project construction based on "large deformation design".

[0071] The above merely describes preferred specific embodiments of the present application, but the protection scope of the present application is not limited thereto, and any person skilled in the art can make equivalent replacements or changes to the technical solutions and the inventive concept of the present application within the technical scope disclosed by the present application, which should be covered within the protection scope of the present application.

Claims

1. A method for producing pipeline steel of the extreme specification X100, characterized in that, The chemical composition of the steel, by mass percentage, is: C: 0.03%–0.05%, Si: 0.20%–0.30%, Mn: 1.80%–1.95%, P≤0.010%, S≤0.001%, N≤0.0030%, O≤0.0010%, Cr: 0.50%–0.80%, Cu: 0.25%–0.40%, Zr: 0.11%–0.20%, B: 0.0025%–0.0040%, Ti: 0.015%–0.03%, Nb: 0.12%–0.15%, Al: 0.015%–0.055%, Mg: 0.0053%–0.0060%, with the balance being Fe and impurities; the finished steel plate has dimensions of 25.6 mm thick × 1550 mm wide. The production method for the X100 pipeline steel of the specified limit specification includes top and bottom blowing converter smelting, ladle refining, large slab continuous casting, slab reheating, rough rolling, finish rolling, controlled cooling, coiling and slow cooling processes; the following processes are controlled: (1) Reheating of slab: The heating temperature is 1160~1200℃ and the heating time is 1.5h~2h; (2) Rough rolling: The rough rolling temperature is 950~1140℃. The rough rolling is carried out in a floating waiting temperature. When the exit temperature reaches the finishing rolling start temperature, it enters the finishing mill. (3) Finish rolling: The finishing rolling temperature is 820~950℃; (4) Controlled cooling: Laminar flow cooling is adopted, with a cooling rate of 30-40℃ / s; (5) Winding: Determine the winding temperature according to the cooling control factor Q and cooling rate in step (4), Q = winding temperature / cooling rate, 8s ≤ Q ≤ 10s; (6) Slow cooling: After the steel coil exits the laminar flow cooling unit, it enters the slow cooling pit for natural cooling for more than 48 hours; The microstructure of the finished steel plate is: quasi-polygonal ferrite + bainite + MA component + nano-precipitates, wherein the volume fraction of quasi-polygonal ferrite is ≥35%; The properties of the finished steel plate are as follows: yield strength Rp 0.2 Its tensile strength is 750–800 MPa, its tensile strength Rm is 920–960 MPa, and its elongation after fracture is A. 50 The yield strength is 35%–45%, the yield strength ratio is ≤0.85; the stress ratio is Rt1.5 / Rt0.5≥1.25, Rt2.0 / Rt1.0≥1.15; the uniform elongation UEL is ≥12.0%. The average Charpy impact energy at -40℃ is ≥320J, and the average drop hammer tear shear area at -40℃ is ≥95%.

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