Production method for efficiently and high-quality producing X70 petroleum pipeline steel plate with length being 3 times of length

Abstract

The invention aims to provide a method for producing X70 pipeline steel by using a TMCP type plate assembly with the length of 3 times of length. The method comprises the following steps: adopting low C and low S as components, ensuring that the outbound hydrogen content of refined molten steel is less than 1.5 ppm, performing hot charging and hot delivery on a 260mm continuous casting blank, performing plate assembly on a mother plate with the length of 3 times of length, cooling after rolling, changing the water quantity and the roller speed to control cooling compensation, and performing temperature pile cold hydrogen evolution after edge cutting, so as to obtain the X70 pipeline steel with the TMCP type length of 3 times of length. The thickness specification of 12-25 mm is achieved, and the width meets the pipe diameter phi of 1016 mm. The process route comprises the steps of converter BOF, refining, continuous casting, plate blank hot charging and hot delivery (cooling to below 550 DEG C through a water cooling device before charging), TMCP, finishing (rapid stack cooling and fixed-length marking after trimming) and finished product obtaining, the structure mainly comprises bainite and acicular ferrite, the strength difference of the head, the middle and the tail is within 50 MPa, the steel plate performance meets the steel grade requirement, and the production cost is low. The flaw detection of the steel plate can meet the level that 10dB is added on the basis of the ISO10893-9-2011-A1-2020 (detection of layered defects in a strip plate for manufacturing the ninth part of welded steel pipes in nondestructive testing of steel pipes) standard.

Description

Technical Field

[0001] The present invention belongs to the technical field of wide and thick plate metallurgy, and specifically relates to a production method for producing a high-strength and high-toughness X70 oil pipeline steel plate with a thickness specification of 12-25 mm (the maximum width meets the pipe diameter of φ1016 mm) without heat treatment (TMCP process), by hot charging and hot conveying of 260 mm cross-section continuous casting billets, assembling plates with a length three times that of a mother plate, and cold hydrogen precipitation with warm stacking after trimming. Background Art

[0002] X70 steel plate is widely used in oil and gas pipeline projects. Long-distance oil and gas pipelines are in high pressure, high corrosion, and temperature-variable service environments, which have strict requirements on materials: the internal quality of the steel plate must meet high-level flaw detection requirements, and the physical properties must have high strength, high toughness, and DWTT (drop weight). At the same time, the plate shape must be flat and the surface quality must be excellent. In order to meet the above technical quality requirements, the medium and thick plate production industry generally adopts double-length assembly (the industry generally uses a single length of 12-13m, and the double-length mother plate length is 26-27m) to ensure that the mother plate is square and straight. In order to meet the process window, it is difficult to further improve the rolling efficiency and yield rate. In order to avoid surface defects of steel plates caused by abnormal slab quality and solve the problem of unqualified flaw detection caused by hydrogen-induced cracks inside the steel plates, the slabs are often piled up for surface inspection and treatment before the cold slabs are loaded into the furnace, which wastes energy seriously. Directly piling up the steel plates after rolling can solve the internal hydrogen evolution problem that is not solved by the hot charging and hot delivery mode of the slabs. However, the uneven stress release of the mother plate during the piling up and cooling process will cause serious deformation of the steel plate, affecting the plate shape and surface quality, and at the same time resulting in a long process flow and high process cost.

[0003] There are inherent technical difficulties in assembling high-grade TMCP pipeline steel with a length of 3 times (the mother plate is nearly 40m long). Limited by the cooling capacity of the equipment, the cooling roller speed of the X70-level TMCP pipeline steel produced in the industry is usually within 2m / s. For every 12m increase in the mother plate length, the tail cooling time will be extended by more than 6s. Depending on the ambient temperature, the natural temperature drop at the tail will reach 20-30℃, and the difference in the cooling temperature of the whole plate will reach 50-80℃, which exceeds the process control window of the X70 steel grade. Too low water entry speed will cause a large amount of soft-phase ferrite to be generated, making it difficult for the tail yield strength to meet the steel grade requirements; if the tail water entry temperature is given priority, the head and middle of the steel plate will enter the water. The temperature will be too high, causing the front end of the mother plate to drop hammer worse; at the same time, the large temperature difference in the length direction of the mother plate will cause uneven cooling of the steel plate, further worsening the overcooling at the head and tail ends, resulting in strength inconsistency. In order to avoid the performance inconsistency caused by such end overcooling, the industry often adopts the method of increasing the head and tail overcooling zone removal. When TMCP type X70 grade pipeline steel is assembled in double length, the cumulative head and tail removal amount is generally 1-2m, which is equivalent to the overcooling zone removal amount of each sub-plate "shared" close to 1m, affecting the improvement of pipeline steel yield rate. At present, there are few cases in the industry where TMCP process is used in large quantities to produce 3x X70. After searching, no technical description of assembling 3x length plates of X70 grade pipelines was found.

[0004] By optimizing and developing a new post-rolling cooling compensation mode (head-to-tail variable water volume control and cooling roller speed control), uniform cooling of the 3x-size mother plate within the process range can be achieved, creating the technical basis for 3x-size plate assembly. Compared with double-size plate assembly, it can not only directly increase the unit hourly output by nearly 50% (without increasing the slab heating time and rolling passes), but also effectively reduce the cutting loss of the head-to-tail overcooling zone of the unit sub-plate (equivalent to the increase of the head-to-tail overcooling zone removal amount from every 2 sub-plates to every 3 sub-plates). At the same time, the cooling compensation function can improve the overcooling of the head-to-tail ends themselves, further reduce the head-to-tail overcooling zone removal amount, and improve the comprehensive yield rate by more than Over 1%, through reasonable component design and optimization of stack cooling process path, for 20-25mm thickness specifications, the burrs on the head, tail and edges are removed while warm before hydrogen-induced cracks are formed, and then quickly hoisted to the open space next to the roller for stack cooling, which can effectively release hydrogen and eliminate the hydrogen release problem of slabs caused by hot charging and hot transportation of slabs. The flaw detection pass rate of pipeline steel is increased to more than 99.6% (for thickness below 19mm, due to the thin thickness of the steel plate, from the actual situation on site, stack cooling is not required to ensure a good hydrogen release effect and avoid hydrogen-induced cracks). After the burrs with the largest internal stress on the edges and ends are removed, the plate shape after stack cooling is greatly improved, and the cold straightening rate of the steel plate after shearing to size can be reduced to less than 5%.

[0005] Chinese patent CN112246881A published "A method for controlling the shape of X70 thin and wide plates suitable for ultra-fast cooling". The initial roller speed of this method is only 0.8m / s, and the acceleration is set at 0.012~0.015 / s 2 , even at the maximum acceleration of 0.015 / s 2 From the start of cooling at the head, the rear sub-board cannot be put into water for more than 30 seconds (calculated based on a 40m long motherboard). The natural temperature drop at the rear is too large and cannot meet the 3x length production requirement at all.

[0006] Chinese patent CN113862446A discloses "a method for producing X70 pipeline steel with high heating temperature" which is only applicable to double-length / single-length production, and does not mention cooling compensation under triple-length conditions and subsequent methods to ensure hydrogen evolution.

[0007] Chinese patent CN112680659A discloses "a low compression ratio economical X70 pipeline steel and its production method", whose production specifications are limited to 15-19mm, and the slabs are not hot loaded or hot delivered, and there is no mention of cooling compensation under the condition of 3 times the length and the subsequent method to ensure hydrogen evolution. Summary of the invention

[0008] The purpose of the present invention is to provide a method for producing X70 pipeline steel by TMCP type three-fold plate assembly, which adopts low C and low S components, smelting fixed hydrogen <1.5ppm, adopts 260mm continuous casting billet hot charging and hot delivery, mother plate length three-fold plate assembly, variable water volume for cooling after rolling, variable roller speed control cooling compensation, and hydrogen precipitation by warm stack cooling after trimming, so as to achieve production of 12-25mm thickness specification and φ1016mm width. Process path: converter BOF - refining - continuous casting - hot charging and hot delivery of slab (cooled to below 550℃ by water cooling device before loading) - TMCP - finishing (fast stack cooling after trimming, fixed length marking) - finished product.

[0009] By applying this technology, the hourly output per unit is increased by nearly 50% and the yield rate is increased by >1% compared with double-size plate assembly. The structure is mainly bainite + acicular ferrite, with a yield strength of 485-630MPa, a tensile strength of 570-720MPa, a head-middle-tail strength difference within 50MPa, an elongation of ≥25%, a yield strength ratio of <0.9, an impact energy of -30℃ ≥300J, and a drop hammer section ratio of -25℃ >85%. The flaw detection of steel plates can meet the ISO10893-9-2011-A1-2020 (non-destructive testing of steel pipes Part 9 Detection of laminar defects in strip plates for the manufacture of welded steel pipes) standard, which is 10dB stricter.

[0010] To achieve the above object, the present invention provides the following technical solution: an efficient and high-quality production of 3-times-length X70 oil pipeline steel plate, comprising steel, characterized in that the chemical composition content of the steel is C=0.05%-0.07%, Si=0.20%-0.27%, Mn=1.50%-1.60%, P≤0.015%, S≤0.003%, Al=0.02%-0.05%, Ti=0.01%-0.02%, Mo≤0.10%, Nb=0.040%-0.050%, Cr=0.16%-0.25%, CE Pcm≤0.19, and the balance is Fe and unavoidable impurity elements.

[0011] A method for efficiently and high-quality production of 3-times-length X70 oil pipeline steel plates, the steps are as follows:

[0012] (1) Smelting: converter endpoint control: C ≤ 0.04%, P ≤ 0.010%; LF total station time ≥ 50 min, bottom blowing argon throughout the process, argon blowing is started when the molten steel enters the VD (RH) furnace, vacuum target ≤ 0.5 tor, vacuum maintenance time ≥ 15 min, soft blowing before the molten steel leaves the VD (RH) furnace ≥ 12 min, hydrogen [H] ≤ 1.5 ppm when the molten steel leaves the station;

[0013] The continuous casting process is protected, the continuous casting ladle tonnage is ≥25 tons, full ladle operation, the crystallizer liquid level fluctuation is ≤±5mm, the 260 section pulling speed is 0.8~0.85m / min, the continuous casting steel retention is 5~6t, and the slag of the large ladle is strictly prohibited;

[0014] (2) TMCP rolling, using 3 times the length of the plate assembly, hot loading and hot delivery of the slab, by water cooling to below 550 ° C before loading into the furnace, and then hot loading the slab into the furnace;

[0015] The slab heating temperature is ≤1250℃, the soaking time is ≥40min, the slab tapping temperature is 1180-1220℃, the rough rolling and final rolling temperature is ≥1140℃, the intermediate billet is greater than 3 times the finished product thickness (set to 70-80mm), the finishing rolling is 7-9 passes, the starting rolling temperature is ≤980℃, the final rolling temperature is 790-850℃, and the finishing rolling descaling adopts the cross setting of the entrance and exit;

[0016] (3) After rolling, cooling compensation control is performed by changing the water volume and roller speed. After the motherboard passes the highest cooling temperature point, the motherboard length is 40m as a reference, and the highest cooling temperature point is in the 12-20m area. The cooling roller motor starts an acceleration (acceleration setting 0.002-0.05m / s) based on the original uniform speed mode. 2 After all the steel plates enter the water cooling device, the acceleration returns to zero and the plates are cooled at a uniform speed at the current roller speed until they exit the water cooling device.

[0017] After the acceleration is turned on, the solenoid valve opens synchronously to increase the cooling water ratio (the increase is 5% to 30%, and the opening value can be set arbitrarily within this range) to increase the cooling water volume in the middle and tail parts, thereby achieving uniform cooling in the length direction. The cooling temperature is 770-820℃, the target return temperature is 480-540℃, and the cooling speed is 15-20m / s;

[0018] (4) After rolling, the strip is sheared at temperature. The head and tail (single-side cutting amount is not less than 400 mm) and burrs (single-side cutting amount is not less than 50 mm) of the mother plate with the largest and uneven internal stress are cut off before stress release. After the 19-25 mm thick steel plate is trimmed, it is quickly hoisted from the roller to the open space beside the roller. The stacking temperature is greater than 90 ° C, the stack cooling time is greater than 8 hours, and when the steel plate temperature is less than 50 ° C, the stack is removed and put on line UT, and samples are taken for sizing and labeling, and finally the finished products are put into storage.

[0019] Compared with the prior art, the present invention has the following beneficial effects:

[0020] (1) The components of the present invention are designed with low carbon, low sulfur and low hydrogen. Through steelmaking process control (including water injection in front of the billet heating furnace), it is ensured that the slab has no steel defects and is hot charged and hot transported, thereby reducing the energy waste caused by cold charging of the billet.

[0021] (2) The present invention adopts a three-times-size plate assembly, which significantly improves the production efficiency and yield rate of the rolling line, thereby achieving efficiency improvement and cost reduction.

[0022] (3) The present invention creatively uses variable water volume and variable roller speed control in the post-rolling controlled cooling process to ensure that when the mother plate reaches 40m in length, the entire plate can still be controlled within the process window, effectively alleviating the natural temperature drop caused by the delay in tail cooling due to the length, and improving the cooling uniformity of the entire plate; although it is difficult to achieve precise matching of theoretical uniformity by performing variable roller speed and variable water cooling control during the cooling process, since the input variables are only the roller acceleration and the valve opening ratio, and the production specifications of pipeline steel are generally unified (the industry has standardized pipe diameters), in actual large-scale industrial production, the cooling effect can be quickly set and checked through the approximation method, and only a few steel plates need to be tested to find out the appropriate parameter combination setting for the specification. In subsequent production, only fine-tuning on this basis can achieve a more ideal state, and it has strong on-site operability and adaptability.

[0023] (4) The present invention creatively develops a process path (20-25mm thickness) of low-speed stack cooling after warm shearing and trimming. It can not only effectively ensure the hydrogen evolution effect before hydrogen-induced cracks are formed, thereby improving the flaw detection pass rate, but also greatly improve the stack cooling plate shape by cutting off the head and tail and steel plate burrs (four sides of the mother plate) with the largest internal stress under warm conditions. After the steel plate is stacked and cut to size, the plate shape can meet the standard requirements, reducing the process processing time and cost caused by cold straightening and improving the surface quality.

[0024] (5) The X70 pipeline steel with a thickness of 12-25 mm and a maximum diameter of φ1016 mm produced by the present invention can achieve a primary performance qualification rate of more than 99%, a primary flaw detection qualification rate of more than 99.6%, a yield rate of more than 1% (maximum ≥91%), and a cold straightening rate of less than 5%. At the same time, the difficult problem of high-grade pipeline steel production efficiency, production cost and process quality being difficult to achieve at the same time is solved.

[0025] (6) The present invention can be applied to the production of higher-grade steels such as TMCP type X80 with only a small adjustment, and has wide process scalability and adaptability. BRIEF DESCRIPTION OF THE DRAWINGS

[0026] The present invention is further described below in conjunction with the accompanying drawings and embodiments:

[0027] Figure 1 This is a schematic diagram of the temperature uniformity of the entire steel plate after cooling according to Example 1 of the present invention;

[0028] Figure 2 This is a schematic diagram of the temperature uniformity when the present invention is not used (the black area on the plate surface is the non-uniform cooling area). DETAILED DESCRIPTION

[0029] This section will describe in detail the specific embodiments of the present invention. The preferred embodiments of the present invention are shown in the accompanying drawings. The purpose of the accompanying drawings is to supplement the description of the text part of the specification with graphics, so that people can intuitively and vividly understand each technical feature and the overall technical solution of the present invention, but it cannot be understood as a limitation on the scope of protection of the present invention.

[0030] See also Figure 1-2 , the present invention provides a technical solution:

[0031] Embodiment 1:

[0032] A method for producing X70 oil pipeline steel plates with high efficiency and quality,

[0033] Select 16mmX70 pipeline steel (mother plate size 16mm*2550mm*40m)

[0034] The chemical composition of the steel is C=0.06%, Si=0.23%, Mn=1.59%, P=0.012%, S=0.0012%, Al=0.020%, Ti=0.012%, Nb=0.045%, Cr=0.22%, Mo=0.01%, CE Pcm=0.16, and the balance is Fe and unavoidable impurity elements; the key process steps include:

[0035] S1. Smelting: The molten steel is refined by LF and vacuum treated by VD. The hydrogen [H] of the molten steel is set to 1.2ppm when it leaves the station. Then it is cast into continuous casting billets with a cross section of 260mm*2280mm. The billets are cooled to 535℃ by the cooling device after the second cutting and then sent to the steel rolling heating furnace by rollers.

[0036] S2. Heating: During the heating process, the highest furnace temperature is 1250°C, the soaking temperature is 1220°C, the soaking time is 44 minutes, and the core temperature of the slab is 1212°C.

[0037] S3, TMCP rolling: rough rolling and final rolling temperature is 1156℃, intermediate billet is 75mm, finishing rolling temperature is 947℃, rolling is 7 passes, final rolling temperature is 834℃, finishing descaling is 4 passes, the first and seventh passes are descaling at the mill entrance, and the third and fifth passes are descaling at the mill exit.

[0038] S4, cooling after rolling is controlled by variable water volume and variable roller speed. The cooling roller speed is 1.5m / s, and the initial cooling water volume is 360L / s and 380L / s respectively (water temperature 27.5℃). The counter is set to start at 0.02m / s after the steel plate head enters the rapid cooling for 24m. 2 Roller acceleration, solenoid valve synchronously open cooling water ratio plus 11% water (cooling water volume is 400L / s and 420L / s respectively), normal use head and tail shielding settings, cooling temperature 810℃, return to red 480-535℃.

[0039] S5. After rolling, the products are directly sheared and UTed off the line, and samples are taken for sizing and labeling, and the finished products are put into storage.

[0040] Embodiment 2:

[0041] Select 21mmX70 pipeline steel (mother plate size 21mm*3180mm*40m);

[0042] The chemical composition of the steel is C=0.06%, Si=0.23%, Mn=1.54%, P=0.01%, S=0.0013%, Al=0.026%, Ti=0.012%, Nb=0.045%, Cr=0.18%, Mo=0.08%, CE Pcm=0.16, and the balance is Fe and unavoidable impurity elements; the key process steps include:

[0043] S1. Smelting: The molten steel is refined by LF and vacuum treated by VD. The hydrogen [H] of the molten steel is set to 1.16ppm when it leaves the station. Then it is cast into continuous casting billets with a cross section of 260mm*2280mm. The billets are cooled to 530℃ by the cooling device after the second cutting and then sent to the steel rolling heating furnace by rollers.

[0044] S2. Heating: During the heating process, the highest furnace temperature is 1240°C, the soaking temperature is 1220°C, the soaking time is 44 minutes, and the slab core temperature is 1213°C.

[0045] S3, TMCP rolling: rough rolling and final rolling temperature 1155℃, intermediate billet 80mm, finishing rolling temperature 960℃, 9 rolling passes, final rolling temperature 814℃, 4 finishing descaling passes, the first and ninth passes are descaling at the mill entrance, and the third and sixth passes are descaling at the mill exit.

[0046] S4, cooling after rolling is controlled by variable water volume and variable roller speed. The cooling roller speed is 1.4m / s, and the initial cooling water volume is 440L / s and 480L / s respectively (water temperature 29℃). The counter is set to start at 0.02m / s after the head of the steel plate enters the rapid cooling for 20m. 2 Roller acceleration, solenoid valve synchronously open cooling water ratio plus 14% water (cooling water volume is 500L / s and 550L / s respectively), normal use head and tail shielding settings, cooling temperature 800℃, return to red 460-510℃.

[0047] S5. The cooling bed on the steel plate completes the online cutting of the head and tail and the edge cutting. The time interval is 75 minutes. The stacking temperature after edge cutting is 102℃, the stack cooling time is 8h, the surface temperature is 44℃, and the stacking is removed and put on line.

[0048] The mechanical properties of the steel plates are shown in Table 1.

[0049] Embodiment three:

[0050] Select 24mmX70 pipeline steel (mother plate size 24mm*2870mm*40m);

[0051] The chemical composition of the steel is C=0.07%, Si=0.21%, Mn=1.57%, P=0.009%, S=0.0017%, Al=0.021%, Ti=0.012%, Nb=0.049%, Cr=0.20%, Mo=0.01%, CE Pcm=0.17, and the balance is Fe and unavoidable impurity elements; the key process steps include:

[0052] S1. Smelting: The molten steel is refined by LF and vacuum treated by VD. The hydrogen [H] of the molten steel is set to 1.13ppm when it leaves the station. Then it is cast into continuous casting billets with a cross section of 260mm*2280mm. The billets are cooled to 545℃ by the cooling device after the second cutting and then sent to the steel rolling heating furnace by rollers.

[0053] S2. Heating: During the heating process, the highest furnace temperature is 1240°C, the soaking temperature is 1210°C, the soaking time is 47 minutes, and the slab core temperature is 1202°C.

[0054] S3, TMCP rolling: rough rolling and final rolling temperature is 1160℃, intermediate billet is 80mm, finishing rolling temperature is 910℃, rolling is repeated 7 times, final rolling temperature is 814℃, and finishing descaling is repeated 4 times. The first and seventh passes are descaling at the mill entrance, and the third and fifth passes are descaling at the mill exit.

[0055] S4, cooling after rolling is controlled by variable water volume and variable roller speed. The cooling roller speed is 1.3m / s, and the initial cooling water volume is 420L / s and 460L / s respectively (water temperature 31℃). The counter is set to start at 0.02m / s after the head of the steel plate enters the rapid cooling for 20m. 2 Roller acceleration, solenoid valve synchronously open cooling water ratio plus 15% water (cooling water volume is 480L / s and 530L / s respectively), normal use head and tail shielding settings, cooling temperature 800℃, return to red 460-510℃.

[0056] S5. The cooling bed on the steel plate completes the online cutting of the head and tail and the edge cutting. The time interval is 80 minutes. The stacking temperature after edge cutting is 110℃, the stack cooling time is 9h, the surface temperature is 42.5℃, and the stacking is unpacked and put on line.

[0057] The mechanical properties of the steel plates are shown in Table 1.

[0058] Table 1. Mechanical properties of 3x X70 pipeline steel.

[0059]

[0060]

[0061] The embodiments of the present invention are described in detail above with reference to the accompanying drawings. However, the present invention is not limited to the above embodiments. Various changes can be made within the knowledge scope of ordinary technicians in the technical field without departing from the purpose of the present invention.

Claims

1. An efficient and high-quality production of 3-times-length X70 oil pipeline steel plate, comprising steel, characterized by: The chemical composition of the steel is C=0.05%~0.07%, Si=0.20%~0.27%, Mn=1.50%~1.60%, P≤0.015%, S≤0.003%, Al=0.02%~0.05%, Ti=0.01%~0.02%, Mo≤0.10%, Nb=0.040%~0.050%, Cr=0.16%~0.25%, CE Pcm≤0.19, and the balance is Fe and unavoidable impurity elements.

2. According to claim 1, a method for efficiently and high-quality production of 3 times length X70 oil pipeline steel plate, the steps are as follows: S1. Smelting: converter endpoint control: C≤0.04%, P≤0.010%; LF total station time ≥50min, bottom blowing argon throughout the whole process, argon blowing is started when molten steel enters VD (RH) furnace, vacuum target ≤0.5tor, vacuum maintenance time ≥15min, soft blowing before molten steel leaves VD (RH) furnace ≥12min, hydrogen [H] of molten steel leaving station ≤1.5ppm; The continuous casting process is protected, the continuous casting ladle tonnage is ≥25 tons, full ladle operation, the crystallizer liquid level fluctuation is ≤±5mm, the 260 section pulling speed is 0.8~0.85m / min, the continuous casting steel retention is 5~6t, and the slag of the large ladle is strictly prohibited; S2, TMCP rolling, using 3 times the length of the plate assembly, hot loading and hot delivery of the slab, through the water cooling to below 550 ℃ before loading into the furnace, then the hot slab is loaded into the furnace; The slab heating temperature is ≤1250℃, the soaking time is ≥40min, the slab tapping temperature is 1180-1220℃, the rough rolling and final rolling temperature is ≥1140℃, the intermediate billet is greater than 3 times the finished product thickness (set to 70-80mm), the finishing rolling is 7-9 passes, the starting rolling temperature is ≤980℃, the final rolling temperature is 790-850℃, and the finishing rolling descaling adopts the cross setting of the entrance and exit; S3. After rolling, cooling compensation control is performed by changing the water volume and roller speed. After the motherboard passes the highest cooling temperature point, the motherboard length is 40m as a reference, and the highest cooling temperature point is in the 12-20m area. The cooling roller motor starts an acceleration (acceleration setting 0.002-0.05m / s 2 After all the steel plates enter the water cooling device, the acceleration returns to zero and the plates are cooled at a uniform speed at the current roller speed until they exit the water cooling device. After the acceleration is turned on, the solenoid valve opens synchronously to increase the cooling water ratio (the increase is 5% to 30%, and the opening value can be set arbitrarily within this range) to increase the cooling water volume in the middle and tail parts, thereby achieving uniform cooling in the length direction. The cooling temperature is 770-820℃, the target return temperature is 480-540℃, and the cooling speed is 15-20m / s; S4. After rolling, the strip is sheared at temperature to remove the head and tail (single-side cutting amount is not less than 400mm) and burrs (single-side cutting amount is not less than 50mm) of the mother plate with the largest and uneven internal stress before stress release; after cutting the edges of the steel plates with a thickness of 19 to 25mm, they are quickly hoisted from the roller table to the open space beside the roller table. The stacking temperature is greater than 90℃, the stack cooling time is greater than 8 hours, and when the steel plate temperature is less than 50℃, the stack is removed and put on line for UT, and samples are taken for sizing and labeling, and finally the finished products are put into storage.

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