High-strength thin-gauge X80 pipeline steel and construction method thereof

Abstract

The invention discloses high-strength thin X80 pipeline steel and a construction method thereof, and belongs to the technical field of metallurgical production processes. According to the high-strength thin-gauge X80 pipeline steel and the construction method thereof, the production cost is relatively low, and the strength surplus can be effectively improved. The high-strength thin X80 pipeline steel is a hot rolled steel strip with the thickness smaller than 16 mm and comprises, by mass, 0.040%-0.070% of C, 0.18%-0.25% of Si, 1.70%-1.80% of Mn, smaller than or equal to 0.015% of P, smaller than or equal to 0.005% of S, 0.010%-0.050% of Als, 0.060%-0.070% of Nb, 0.010%-0.020% of Ti, 0.22%-0.26% of Cr, 0.17%-0.20% of Mo, smaller than or equal to 0.0050% of N and the balance Fe and inevitable impurities. The construction method comprises the steps of molten steel smelting, plate blank preparation, plate blank reheating, plate blank rolling, cooling and coiling, during plate blank preparation, molten steel which is smelted to be qualified is directly and continuously cast on a continuous casting machine to form a plate blank meeting the requirement, the continuously cast plate blank is slowly cooled for more than 48 hours and then fed into a heating furnace to be heated in the furnace for 180-240 minutes, rough rolling and finish rolling are carried out after the continuous cast plate blank is taken out of the furnace, and the plate blank is obtained. And after laminar cooling, coiling to obtain the high-strength thin-gauge X80 pipeline steel.

Description

Technical Field

[0001] The present invention relates to an X80 pipeline steel, in particular to a high-strength and thin-gauge X80 pipeline steel, belonging to the technical field of metallurgical production technology. The present invention also relates to a construction method for preparing the high-strength and thin-gauge X80 pipeline steel. Background Art

[0002] As the most economical and efficient way to transport oil and natural gas, pipeline transportation has the advantages of being less affected by external environment such as climate and geology, and having low construction and operation costs. With the gradual completion of long-distance pipeline construction, the market has gradually shifted to branch line and urban pipeline network construction. With the improvement of pipeline steel production technology, requirements have been put forward to reduce costs and increase strength margins.

[0003] Patent document CN103397260B "A thin-gauge high-toughness X80 hot-rolled steel plate and its production method" discloses a method of using 250-300mm thick continuous casting slabs to produce X80 wide and thick plates with a thickness of 8-10mm and a width of 2-3m, solving the problem of controlling the strength and toughness of thin-gauge and wide steel plates X80. Its chemical composition: 0.05-0.09% C, 0.15-0.40% Si, 1.60-1.90% Mn, P≤0.015%, S≤0.005%, 0.010-0.060% Alt, 0.025-0.055% Nb, 0.02-0.05% V, 0.005-0.025% Ti, N≤0.008%, H≤0.0002%, 0.21-0.40% Cr, 0.00-0.50% Ni, 0.00-0.30% Cu, and the rest is Fe and unavoidable impurities. The second heating and second rolling process is adopted. After the first rolling, the plate is air-cooled to room temperature, and the final rolling thickness is 140-150mm. After the second rolling, the final rolling temperature is 900-950℃, and then ultra-fast cooling is performed. After cooling to 500-560℃, the plate is naturally cooled to room temperature. This patent solves the problem of rolling thin-gauge X80 wide and thick plates, and has no reference value for hot continuous rolling coil manufacturing.

[0004] Patent document CN104264070B "A thin gauge X80 pipeline steel and its manufacturing method" discloses a method for manufacturing a thin gauge X80 pipeline steel with simple composition, excellent low temperature performance and a thickness of 10-20 mm by hot continuous rolling. Its chemical composition is 0.05-0.08% C, 0.10-0.30% Si, 1.30-1.70% Mn, P≤0.010%, S≤0.005%, 0.020-0.060% Nb, 0.006-0.020% Ti, 0.10-0.30% Ni, 0.10-0.30% Cr, 0.010-0.050% Al, and the rest is Fe and unavoidable impurities. The process includes: KR hot metal pretreatment - converter smelting - LF refining - RH vacuum degassing - continuous casting - hot rolling, rolling 150-180mm continuous casting billets into 10-20mm coils, and obtaining X80 pipeline steel with a yield strength of 580-670MPa, a tensile strength of 660-750MPa, an elongation of 31-36%, a yield strength ratio of ≤0.90, a -40℃ Charpy impact energy of >190J, and a -40℃ drop hammer tearing area of ​​100%. The hot rolling method described in the patent is to accumulate strain energy in austenite through low-temperature and large-pressure deformation, increase the instability of austenite, and this method easily leads to the nucleation of ferrite at a higher temperature, reducing the proportion of acicular ferrite and reducing strength and toughness.

[0005] Patent document CN114058978B "X80 pipeline steel with low temperature toughness" discloses a chemical composition of 0.05-0.07% C, 1.60-1.85% Mn, S≤0.002%, P≤0.010%, 0.1-0.3% Si, 0.04-0.08% Nb, 0.01-0.03% Ti, 0.30-0.35% Mo, 0.10-0.25% Cu, 0.50-0.80% Ni, 0.15-0.35% Cr, B≤0.0005%, Ca≤0.009%, N<0.008%, O<0.005%, and the rest is Fe and unavoidable impurities. The 300 mm thick continuous casting slab is heated-rough rolled-finished rolled-fast cooled-coiled to produce X80 pipeline steel with Charpy impact energy ≥300 J and DWTT shear area ≥85% at -45°C. The alloy cost of this method is relatively high, and the strength margin of the listed examples is relatively low. Summary of the invention

[0006] The technical problem to be solved by the present invention is to provide a high-strength thin-gauge X80 pipeline steel with relatively low production cost and effectively improved strength margin, as well as a construction method for preparing the high-strength thin-gauge X80 pipeline steel.

[0007] The technical solution adopted to solve the above technical problems is: a high-strength thin-gauge X80 pipeline steel, wherein the high-strength thin-gauge X80 pipeline steel is a hot-rolled steel strip containing the following mass percentage components and having a thickness of less than 16 mm,

[0008] The mass percentage components are 0.040-0.070% C, 0.18-0.25% Si, 1.70-1.80% Mn, P≤0.015%, S≤0.005%, 0.010-0.050% Als, 0.060-0.070% Nb, 0.010-0.020% Ti, 0.22-0.26% Cr, 0.17-0.20% Mo, N≤0.0050%, and the rest are Fe and unavoidable impurities.

[0009] Furthermore, the microstructure of high-strength thin-gauge X80 hot-rolled steel is acicular ferrite + a small amount of bainite.

[0010] The preferred embodiment of the above scheme is that the oblique yield strength of the finished high-strength thin-gauge X80 hot-rolled steel strip is ≥555MPa, the tensile strength is ≥645MPa, the yield strength ratio is ≤0.90; the oblique full-size impact energy at -25°C is ≥180J, the ductile-brittle transition temperature is lower than -60°C; the drop hammer tear area ratio at -20°C is ≥85%.

[0011] A construction method for preparing the high-strength thin-gauge X80 pipeline steel, the construction method comprising the steps of molten steel smelting, slab preparation, slab reheating, slab rolling, cooling and coiling.

[0012] When preparing the slab, qualified molten steel is directly cast on the continuous casting machine into slabs that meet the requirements. After the continuous cast slab is slowly cooled for more than 48 hours, it is sent to the heating furnace and heated in the furnace for 180-240 minutes. After it is taken out of the furnace, it is rough rolled, finish rolled, and coiled after laminar cooling to obtain high-strength thin-gauge X80 pipeline steel.

[0013] Furthermore, the slab reheating furnace temperature is 1180-1200°C, high pressure descaling is performed before rough rolling, the rough rolling passes are 5, the deformation of a single pass is ≥18%, and the thickness ratio of the intermediate slab to the finished product is in the range of 3.4-3.8.

[0014] The preferred embodiment of the above scheme is that the intermediate billet after rough rolling is cooled to 980-1000°C before finishing rolling. The finishing rolling adopts 7-stand continuous rolling. The F1-F2 stand turns on the inter-stand cooling water to cool the strip. The final rolling temperature range is 800-830°C.

[0015] Furthermore, the compression of the finishing rolling is controlled between 3.2-3.4.

[0016] The preferred embodiment of the above scheme is that laminar cooling is carried out in a two-stage cooling mode. During cooling, the steel strip runs at a uniform speed of 2.42-2.45 m / s. The first stage is cooled by a sparse front-stage + dense rear-stage boiling water mode. The second stage is cooled by a dense cooling mode with 8 rows of headers opened at the same time.

[0017] Furthermore, during the first stage of cooling, the layer cooling water temperature is less than 32°C, the layer cooling water pressure is 0.30-0.33 bar, and the strip is cooled to 480-550°C at a cooling rate of 13-17°C / s.

[0018] The preferred embodiment of the above scheme is that, during the second stage of cooling, the first 8 rows of headers are densely opened, followed by air cooling to 370-420°C before coiling.

[0019] The beneficial effects of the present invention are as follows: the mass percentage components of the high-strength thin-gauge X80 pipeline steel provided by the present application are 0.040-0.070% C, 0.18-0.25% Si, 1.70-1.80% Mn, P≤0.015%, S≤0.005%, 0.010-0.050% Als, 0.060-0.070% Nb, 0.010-0.020% Ti, 0.22-0.26% Cr, 0.17-0.20% Mo, N≤0.0050%, and the rest are Fe and unavoidable impurities. The thickness of the rolled hot-rolled steel strip is less than 16 mm, and the steel is prepared according to the steps of molten steel smelting, slab preparation, slab reheating, slab rolling, cooling and coiling. Since the present application strictly controls the chemical composition, reheats the prepared slab, and then performs rough rolling, finish rolling and laminar cooling, and finally air-cooling coiling, a high-strength thin-gauge X80 pipeline steel with a strength margin of ≥30MPa, a full-size Charpy impact energy of ≥240J at -25°C, a ductile-brittle transition temperature of less than -60°C, and a drop hammer tearing area ratio of ≥85% at -20°C is obtained. This not only significantly reduces the production cost, but also can effectively improve the strength margin of the finished thin-gauge X80 pipeline steel. BRIEF DESCRIPTION OF THE DRAWINGS

[0020] Figure 1 This is the surface microstructure diagram of X80 pipeline steel of the present invention;

[0021] Figure 2 This is the microstructure diagram of the core of the X80 pipeline steel of the present invention. DETAILED DESCRIPTION

[0022] like Figure 1 , Figure 2The present invention provides a high-strength thin-gauge X80 pipeline steel with relatively low production cost and effectively improved strength margin, and a construction method for preparing the high-strength thin-gauge X80 pipeline steel. The high-strength thin-gauge X80 pipeline steel is a hot-rolled steel strip with a thickness of less than 16 mm, which contains the following mass percentage components: 0.040-0.070% C, 0.18-0.25% Si, 1.70-1.80% Mn, P≤0.015%, S≤0.005%, 0.010-0.050% Als, 0.060-0.070% Nb, 0.010-0.020% Ti, 0.22-0.26% Cr, 0.17-0.20% Mo, N≤0.0050%, and the rest is Fe and unavoidable impurities. The construction method includes several steps of molten steel smelting, slab preparation, slab reheating, slab rolling, cooling and coiling. When preparing the slab, the smelted qualified molten steel is directly continuously cast on the continuous casting machine into a slab that meets the requirements. After the continuous casting slab is slowly cooled for more than 48 hours, it is sent to the heating furnace and heated in the furnace for 180-240 minutes. After being taken out of the furnace, it is rough rolled, finished rolled, and laminar cooled and coiled to obtain high-strength thin grid specification X80 pipeline steel. Since the present application strictly controls the chemical composition, and reheats the prepared slab, and then performs rough rolling, finish rolling and laminar cooling, and finally air-cooled coiling, a high-strength thin specification X80 pipeline steel with a strength margin of ≥30MPa, a full-size Charpy impact energy of ≥240J at -25°C, a ductile-brittle transition temperature below -60°C, and a drop hammer tearing area ratio of ≥85% at -20°C is obtained, which not only significantly reduces the production cost, but also can effectively improve the strength margin of the finished thin specification X80 pipeline steel.

[0023] Correspondingly, the microstructure of the high-strength thin-gauge X80 hot-rolled steel prepared by controlling the chemical composition and according to the above parameters and steps is acicular ferrite + a small amount of bainite, the oblique yield strength of the hot-rolled steel strip is ≥555MPa, the tensile strength is ≥645MPa, the yield strength ratio is ≤0.90; the oblique full-size impact energy at -25°C is ≥180J, the ductile-brittle transition temperature is lower than -60°C; the drop hammer tearing area ratio at -20°C is ≥85%.

[0024] Furthermore, in order to maximize the control of the physical and chemical properties of the finished high-strength thin-gauge X80 pipeline steel, the reheating temperature of the slab in this application is 1180-1200°C, and it is first subjected to high-pressure descaling before rough rolling. The number of rough rolling passes is 5 passes, the deformation of a single pass is ≥18%, and the thickness ratio of the intermediate slab to the finished product ranges from 3.4 to 3.8. After the intermediate slab is cooled to 980-1000°C after rough rolling, it is then finished. The finishing rolling adopts 7-stand continuous rolling. The F1-F2 stand opens the inter-stand cooling water to cool the strip steel, and the final rolling temperature range is 800-830°C. The compression of the finishing rolling is controlled between 3.2-3.4. At the same time, laminar cooling adopts a two-stage cooling method. During cooling, the steel strip runs at a uniform speed of 2.42-2.45m / s. The first stage adopts the sparse front section + dense back section boiling water mode for cooling, and the second stage adopts the dense cooling mode with 8 rows of headers opened at the same time. In the first stage of cooling, the layer cooling water temperature is less than 32°C, the layer cooling water pressure is 0.30-0.33 bar, and the strip is cooled to 480-550°C at a cooling rate of 13-17°C / s. In the second stage of cooling, the first 8 rows of headers are densely opened, and then air-cooled to 370-420°C before coiling.

[0025] The technical solution of the present application is further described below through specific embodiments:

[0026] This application aims to improve the strength margin by designing an innovative hot rolling process to solve the problem of reduced performance margin caused by reduced alloy cost.

[0027] The present application provides an X80 pipeline steel with a thickness of less than 16 mm, which comprises, by mass percentage, 0.040-0.070% C, 0.18-0.25% Si, 1.70-1.80% Mn, P≤0.015%, S≤0.005%, 0.010-0.050% Als, 0.060-0.070% Nb, 0.010-0.020% Ti, 0.22-0.26% Cr, 0.17-0.20% Mo, N≤0.0050%, and the rest is Fe and unavoidable impurities.

[0028] The microstructure of the X80 pipeline steel is acicular ferrite + a small amount of bainite.

[0029] The thickness specification of the rolled plate is <16mm.

[0030] The rolled plate has an oblique yield strength of ≥555MPa, a tensile strength of ≥645MPa, and a yield strength ratio of ≤0.90.

[0031] The oblique full-size impact energy of the rolled plate at -25°C is ≥180J, and the ductile-brittle transition temperature is lower than -60°C.

[0032] The drop hammer tearing area ratio of the coil at -20°C is ≥85%.

[0033] The present application also provides a hot rolling method for improving the yield strength of the above-mentioned X80 pipeline steel, the process flow includes: slab reheating → rough rolling → finishing rolling → cooling → coiling, the specific steps are as follows:

[0034] (1) After being slowly cooled for more than 48 hours, the continuous casting slab is sent to the heating furnace, with a furnace temperature of 1180-1200°C and a furnace time of 180-240 minutes;

[0035] (2) After being heated, the slab is descaled under high pressure and then enters the rough rolling process, with 5 passes, the deformation of each pass is ≥18%, and the thickness ratio of the intermediate slab to the finished product is in the range of 3.4-3.8;

[0036] (3) After the intermediate billet is cooled to 980-1000°C, it enters the finishing rolling process, using 7 stands for continuous rolling. The F1-F2 stands use inter-stand cooling water to cool the strip steel, and the final rolling temperature ranges from 800-830°C.

[0037] (4) After finishing, the strip enters the laminar cooling section, and the strip runs at a uniform speed of 2.42-2.45 m / s. The laminar cooling adopts a two-stage cooling method. In the first stage, the "sparse front section + dense back section" water mode is used to cool the strip to 480-550°C at a cooling rate of 13-17°C / s. In the second stage, 8 rows of headers are densely opened, the laminar cooling water temperature is <32°C, and the laminar cooling water pressure is 0.30-0.33 bar. Then, it is air-cooled to 370-420°C before coiling.

[0038] The thin-gauge X80 pipeline steel prepared through the above steps has a strength margin of ≥30MPa, a full-size Charpy impact energy of ≥240J at -25°C, a ductile-brittle transition temperature below -60°C, and a drop hammer tearing area ratio of ≥85% at -20°C.

[0039] Embodiment 1

[0040] The chemical composition restrictions of the hot-rolled coil described in the present invention are described below:

[0041] Carbon (C): The main function of carbon is to produce solid solution strengthening and form strong carbides with microalloying elements such as niobium, molybdenum, and titanium. It prevents abnormal grain growth at various stages of slab heating and rolling and has a certain precipitation strengthening effect. Too high a carbon content will lead to deterioration of plastic toughness under the low-temperature coiling process and affect welding performance. Therefore, the present invention controls the C content to 0.04-0.07%.

[0042] Silicon (Si): Silicon acts as a reducing agent and deoxidizer during the smelting process of pipeline steel, and also provides a certain solid solution strengthening effect. However, too high a silicon content will reduce the plasticity, toughness and welding performance of the steel. Therefore, the designed addition amount of Si is 0.18-0.25%.

[0043] Manganese (Mn): The main function of manganese is solid solution strengthening, which is used to compensate for the decrease in strength caused by the reduction in carbon content. At the same time, manganese can reduce the austenite phase transition temperature, stabilize the austenite phase, promote low-temperature phase transition, refine the phase transition structure, and reduce the toughness-brittle transition temperature. A high manganese content can easily cause center segregation, reduce the impact toughness of steel, and cause anisotropy of mechanical properties. The present invention controls the Mn content to 1.70%-1.80%.

[0044] Niobium (Nb) and titanium (Ti): Niobium and titanium are the main microalloying elements. The carbonitrides precipitate at different stages of slab heating and hot rolling, which play a role in grain refinement and precipitation strengthening. Niobium can refine austenite grains and increase the austenite recrystallization temperature. Titanium can precipitate carbonitrides at higher temperatures to pin grain boundaries and refine weld grains. Considering the effects and alloy costs, the present invention controls the Nb content to 0.060-0.070% and the Ti content to 0.010-0.020%. In order to increase the effective titanium content, the nitrogen content should be limited to less than 0.005%.

[0045] Chromium (Cr): Chromium can increase the bainite phase transition temperature, promote the formation of acicular ferrite or bainite, and improve the corrosion resistance of pipeline steel to a certain extent. However, too high a content is not conducive to surface quality and microstructure control. Therefore, the Cr content is limited to 0.22-0.26%.

[0046] Molybdenum (Mo): Mo can separate the pearlite phase transformation zone and the bainite phase transformation zone, inhibit pearlite transformation, promote bainite transformation, and increase the proportion of acicular structure. It is the most important alloy element in high-grade pipeline steel. Considering the cost and performance comprehensively, the present invention limits the Mo content to 0.17-0.20%.

[0047] The reasons for the manufacturing process limitations of the hot-rolled coil described in the present invention are explained below:

[0048] Slab reheating: The main function is to dissolve the alloy elements and make them homogenized. If the slab heating temperature is too low, the microalloying elements niobium and titanium will not be effectively dissolved, thus affecting their subsequent precipitation and making it impossible to effectively refine the grains; if the slab heating temperature is too high, it may cause coarse grains and reduce the strength and toughness of the material. Therefore, the present invention sets the heating temperature at 1180-1200℃.

[0049] Rough rolling: The main function is to refine the austenite grains by controlling the dynamic recrystallization of austenite. The deformation of a single rough rolling pass must be greater than the critical deformation, otherwise it will easily lead to insufficient deformation at the thickness center. Therefore, the deformation of a single rough rolling pass should not be less than 20%, in order to fully recrystallize the austenite grains.

[0050] Finish rolling: Its main function is to provide nucleation energy and nucleation particles for ferrite phase transformation by controlling the deformation of unrecrystallized austenite grains. If the cumulative deformation is insufficient, the austenite deformation degree and substructure are small, and the grain refinement effect is weak; if the cumulative deformation is too large, it will cause austenite instability, high-temperature transformation, reduce the proportion of needle-shaped structure, and reduce strength and toughness. The present invention limits the finishing rolling compression ratio to the range of 3.2-3.4, and limits the final rolling temperature to between 800-830℃, which can avoid austenite instability.

[0051] Laminar cooling: Form the supercooling required for phase transformation and control the temperature range of phase transformation. Combined with the characteristics of thin-gauge X80, a suitable boiling water mode combination is adopted, and the front section is cooled to the range of 480-550℃ at a cooling rate of 13-17℃ / s to avoid the formation of too much equiaxed ferrite or bainite, thereby obtaining more acicular ferrite, increasing the strength margin and optimizing the impact performance.

[0052] The key points of the process control provided by the present invention are to adopt a suitable finishing compression ratio in the finishing stage and to ensure a suitable phase change temperature so as to avoid the instability of the austenite phase while allowing the austenite to fully deform. In addition, a suitable cooling rate is adopted in the laminar cooling stage to avoid the generation of excessive equiaxed ferrite or bainite, thereby increasing the proportion of acicular ferrite to obtain a good strength-toughness match.

[0053] The specific implementation of the present invention is further described below in conjunction with the examples, but the present invention is not limited to the scope of the examples. Table 1 shows the components of the examples of the present invention and the comparative examples, Table 2 shows the hot rolling process parameters of the examples of the present invention and the comparative examples, and Table 3 shows the performance of the examples of the present invention and the comparative examples.

[0054] Table 1 Example composition (mass percentage, wt-%)

[0055] C Si Mn P S Nb Ti Cr Mo Als Example 1 0.049 0.20 1.75 0.010 0.003 0.064 0.016 0.24 0.17 0.027 Example 2 0.053 0.23 1.75 0.010 0.002 0.067 0.017 0.25 0.19 0.029 Comparative Example 1 0.050 0.22 1.73 0.008 0.003 0.065 0.017 0.24 0.17 0.024 Comparative Example 2 0.050 0.21 1.72 0.007 0.002 0.060 0.018 0.23 0.18 0.028 ;

[0056] Table 2 Key hot rolling parameters of the embodiment

[0057]

[0058] Table 3 Performance of the embodiment

[0059] <![CDATA[R t0.5 / MPa]]> <![CDATA[R m / MPa]]> Elongation / % -25℃ impact energy / J -20℃DWTT / % Yield-to-strength ratio Example 1 608 722 33 372 100 0.84 Example 2 580 700 35.5 352 100 0.83 Comparative Example 1 558 683 34 292 100 0.82 Comparative Example 2 560 687 36.5 295 100 0.82 .

Claims

1. High-strength thin-gauge X80 pipeline steel, characterized by: The high-strength thin-gauge X80 pipeline steel is a hot-rolled steel strip with a thickness of less than 16 mm and containing the following components in percentage by mass: The mass percentage components are 0.040-0.070% C, 0.18-0.25% Si, 1.70-1.80% Mn, P≤0.015%, S≤0.005%, 0.010-0.050% Als, 0.060-0.070% Nb, 0.010-0.020% Ti, 0.22-0.26% Cr, 0.17-0.20% Mo, N≤0.0050%, and the rest are Fe and unavoidable impurities.

2. The high-strength thin-gauge X80 pipeline steel according to claim 1, characterized in that: The microstructure of high-strength and thin-gauge X80 hot-rolled steel is acicular ferrite + a small amount of bainite.

3. The high-strength thin-gauge X80 pipeline steel according to claim 2, characterized in that: The finished high-strength thin-gauge X80 hot-rolled steel strip has an oblique yield strength of ≥555MPa, a tensile strength of ≥645MPa, and a yield strength ratio of ≤0.90; the oblique full-size impact energy at -25°C is ≥180J, and the ductile-brittle transition temperature is lower than -60°C; the drop hammer tearing area ratio at -20°C is ≥85%.

4. A construction method for preparing the high-strength thin-gauge X80 pipeline steel according to claim 1, 2 or 3, characterized in that: The construction method includes the steps of molten steel smelting, slab preparation, slab reheating, slab rolling, cooling and coiling. When preparing the slab, qualified molten steel is directly cast on the continuous casting machine into slabs that meet the requirements. After the continuous cast slab is slowly cooled for more than 48 hours, it is sent to the heating furnace and heated in the furnace for 180-240 minutes. After it is taken out of the furnace, it is rough rolled, finish rolled, and coiled after laminar cooling to obtain high-strength thin-gauge X80 pipeline steel.

5. The construction method according to claim 4, characterized in that: The furnace temperature of the reheated slab is 1180-1200℃. It is descaled under high pressure before rough rolling. The number of rough rolling passes is 5, the deformation of a single pass is ≥18%, and the thickness ratio of the intermediate slab to the finished product is in the range of 3.4-3.

8.

6. The construction method according to claim 5, characterized in that: After rough rolling, the intermediate billet is cooled to 980-1000℃ before finishing rolling. Finishing rolling adopts 7-stand continuous rolling. The F1-F2 stand turns on the cooling water between the stands to cool the strip. The final rolling temperature range is 800-830℃.

7. The construction method according to claim 6, characterized in that: The compression of finishing rolling is controlled between 3.2-3.

4.

8. The construction method according to claim 7, characterized in that: Laminar cooling adopts a two-stage cooling method. During cooling, the steel belt runs at a uniform speed of 2.42-2.45m / s. The first stage adopts a sparse front-stage + dense back-stage boiling water mode for cooling. The second stage adopts a dense cooling mode with 8 rows of headers opened at the same time.

9. The construction method according to claim 8, characterized in that: In the first stage of cooling, the layer cooling water temperature is less than 32°C, the layer cooling water pressure is 0.30-0.33 bar, and the strip is cooled to 480-550°C at a cooling rate of 13-17°C / s.

10. The construction method according to claim 8, characterized in that: In the second stage of cooling, the first 8 rows of headers are opened densely, and then air-cooled to 370-420℃ before coiling.

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