Thin L450M pipeline steel plate based on strain design and manufacturing method thereof

Abstract

The invention discloses a thin L450M pipeline steel plate based on strain design and a manufacturing method of the thin L450M pipeline steel plate, and belongs to the technical field of low-carbon microalloyed steel production. The steel comprises the following chemical components in percentage by weight: 0.05%-0.08% of C, 0.15%-0.25% of Si, 1.55%-1.70% of Mn, less than or equal to 0.020% of P, less than or equal to 0.015% of S, 0.02%-0.04% of Nb, 0.008%-0.020% of Ti, 0.015%-0.04% of Al, 0.15%-0.25% of Cr, 0.03%-0.06% of Mo, 0.003%-0.006% of N and the balance of Fe and inevitable impurities, and the total amount of other impurity elements is less than 0.05%. The manufacturing method comprises the steps of molten steel smelting, continuous casting, casting blank stacking and slow cooling, casting blank heating, controlled rolling, relaxation and cooling and air cooling to the room temperature. The steel plate has good toughness.

Description

Technical Field

[0001] The invention belongs to the technical field of low-carbon microalloy steel production, and in particular relates to a thin-gauge L450M pipeline steel plate based on strain design and a manufacturing method thereof. Background Art

[0002] Energy pipeline construction has rapidly expanded in recent years. To improve transmission efficiency and operational safety, the strength and toughness requirements for the steel materials used are becoming increasingly stringent. Currently, major domestic and international pipeline trunk lines primarily utilize L485M and L555M steel grades, while branch lines and urban pipelines often utilize thin-gauge (9-15mm) and small-diameter (Ø914mm or less) L450M steel grades. When pipelines operate in extremely cold regions or cross cold zones, operational stability and safety require even higher standards for raw material performance, such as low-temperature toughness and plastic deformation capacity. For example, low-temperature drop hammer and ductile-brittle transition temperature tests require lower temperatures and higher Charpy impact energy values. Furthermore, stringent requirements are placed on the longitudinal uniform elongation of steel pipes. Consequently, these projects impose increasingly specialized requirements, and simply meeting API SPEC 5L is no longer sufficient. Furthermore, facing the challenging market conditions, user demands, and manufacturing cost pressures in the steel industry, it is crucial to reduce alloy and energy costs while ensuring product quality, minimizing resource consumption in production lines. However, steel plates after alloy reduction are typically produced using a two-stage controlled rolling process. To ensure the cumulative reduction rate during the finishing rolling stage and the static recrystallization of the intermediate billet, the intermediate billet must be kept relatively thick during warming. This results in a longer intermediate billet warming time and the final rolling temperature may be controlled below 820°C or even lower, significantly increasing the difficulty of rolling. First, mill load, plate temperature uniformity, and plate shape control all face severe challenges. Second, improving steel plate performance depends primarily on the content of impurities such as phosphorus and sulfur in the steel, the content of alloying elements in the steel, and the control of key process parameters during smelting and rolling. Therefore, solving the aforementioned problems in producing thin-gauge L450M pipeline steel plates based on strain design, while reducing pipeline steel alloy costs and ensuring excellent performance indicators, is the key to the development of thin-gauge, strain-designed L450M pipeline steel.

[0003] Compared with existing technologies: To date, there have been few reports domestically or internationally on the production of thin-gauge L450M pipeline steel plate using strain design. Prior to the present invention, application number CN201010243241.0 disclosed an X65 pipeline steel and its production method. The patented composition by weight is: C 0.055% to 0.090%, Si 0.15% to 0.35%, Mn 1.50% to 1.65%, P ≤ 0.020%, S ≤ 0.005%, Nb 0.040% to 0.055%, V 0.040% to 0.070%, Ti 0.010% to 0.025%, N ≤ 0.008%, and Al 0.005% to 0.060%. This steel contains a high content of V and utilizes natural air cooling, which is inefficient and fails to fully utilize the water-substitution alloying effect, thereby reducing costs and improving strength and toughness. Furthermore, the steel does not specify the specific strength, toughness, and plastic deformation characteristics of the product. Application number CN201110179945.0 discloses an X65 pipeline steel with excellent low-temperature toughness and its manufacturing method. The patented composition by weight is: C 0.020%-0.055%, Si 0.10%-0.25%, Mn 1.50%-1.70%, Nb 0.060%-0.080%, Cr 0.20%-0.35%, V 0.020%-0.040%, Ti 0.010%-0.020%, Al 0.010%-0.040%, P ≤ 0.018%, S ≤ 0.005%, and N ≤ 0.006%. This steel contains vanadium, resulting in high alloy costs. The laminar cooling process doesn't fully utilize water-substitution to reduce costs and improve toughness. Furthermore, it doesn't meet the requirements for strain-designed pipeline steel. Application No. KR20020027013(A) discloses a steel product for stress-relief heat treatment guaranteed API-X65 grade linepipes and a method for manufacturing the same. The weight percentages of the patented components are C 0.07% to 0.09%, Si 0.2% to 0.5%, Mn 1.40% to 1.60%, P≤0.025%, S≤0.005%, Nb 0.035% to 0.045%, V 0.04% to 0.05%, Mo 0.03% to 0.07%, Cr 0.05% to 0.15%, Ti 0.005% to 0.015%, N 0.002% to 0.007%, and Als 0.015% to 0.050%. This steel contains V and Mo, which results in high cost and insufficient low-temperature toughness of the product.

[0004] While the steels disclosed in the aforementioned patent documents possess high strength and toughness, they are either rolled or have high production costs, do not meet the technical requirements for cross-strip (plastic deformation index - uniform elongation), and often employ two-stage controlled rolling. Therefore, they are not suitable for producing strain-engineered, low-cost, thin-gauge, high-flatness L450M pipeline steel plates. The technical solution provided by the present invention effectively overcomes these deficiencies, enabling the production of strain-engineered, thin-gauge L450M pipeline steel plates with a thickness of 9-15 mm and a diameter of 914 mm or less using continuously cast billets with a thickness of 135-200 mm. These steel plates exhibit high strength, excellent low-temperature toughness, and high resistance to plastic deformation. Summary of the Invention

[0005] The present invention aims to overcome the aforementioned technical problems and deficiencies by providing a strain-engineered, thin-gauge L450M pipeline steel plate and its manufacturing method. The plate is a high-strength, hot-rolled L450M steel plate for gas pipelines, with a thickness of 9-15 mm and a diameter of 914 mm or less. This steel plate addresses issues such as low rolling efficiency, high rolling resistance, uniform performance, low plastic deformation capacity, and difficult shape control. The plate offers low cost, high strength and toughness, and high resistance to plastic deformation, ensuring the safety of gas pipelines.

[0006] To achieve the above object, the technical solution of the present invention is: A strain-designed thin-gauge L450M pipeline steel plate comprises, by weight, the following chemical compositions: C 0.05% to 0.08%, Si 0.15% to 0.25%, Mn 1.55% to 1.70%, P ≤ 0.020%, S ≤ 0.015%, Nb 0.02% to 0.04%, Ti 0.008% to 0.020%, Al 0.015% to 0.04%, Cr 0.15% to 0.25%, Mo 0.03% to 0.06%, N 0.003% to 0.006%, with the balance being Fe and unavoidable impurities, with the total amount of other impurity elements being less than 0.05%. The functions of the main elements in the chemical composition of the steel plate of the present invention are as follows: C: The most economical and fundamental strengthening element in steel, it significantly increases steel strength through solid solution strengthening and precipitation strengthening. However, increasing C content negatively impacts steel's plasticity, toughness, and weldability. Therefore, the present invention sets the C content range to 0.05% to 0.08%.

[0007] Mn: It increases steel strength through solid solution strengthening, compensating for the loss of steel plate strength caused by reduced carbon content. It also lowers the γ-α transformation temperature, thereby refining ferrite grains and contributing to the production of fine low-temperature transformation products, thereby improving toughness. However, increasing the Mn content exacerbates central segregation in the continuously cast slab, hindering low-temperature toughness and ensuring uniform cross-sectional structure. Therefore, the Mn content in this invention is designed to be between 1.55% and 1.70%.

[0008] Si: It deoxidizes steel during steelmaking and improves matrix strength. Increasing Si content purifies ferrite, reduces pearlite, and helps mitigate the Bauschinger effect in the matrix material. However, excessive Si content can reduce the toughness of the weld heat-affected zone of the base material. Therefore, the Si content in this invention is set at 0.15% to 0.25%.

[0009] Nb is a common element in modern microalloyed pipeline steels, providing excellent grain refinement and precipitation strengthening effects. It also delays austenite recrystallization, but excessive Nb increases production costs and complicates continuous casting process control. The present invention selects a Nb content range of 0.02% to 0.04%, combined with appropriate heating and rolling processes, to achieve a uniform, multiphase structure dominated by bainite and polygonal ferrite, with polygonal ferrite accounting for over 65%, resulting in excellent toughness.

[0010] N: The N element in steel has no other significant function except forming fine TiN particles to refine austenite grains. Therefore, the N content needs to be kept at a relatively low level. The N content selected in the present invention is in the range of 0.003% to 0.006%.

[0011] Ti: A strong nitrogen-binding element, it exists as TiN in the continuous casting ingot. Fine TiN particles effectively inhibit austenite grain growth during reheating of the continuous casting ingot, help increase the solid solubility of Nb in austenite, and improve the impact toughness of the weld heat-affected zone. When Ti addition exceeds a certain level, the TiN particles coarsen, increasing stress concentration at the particle interface and the matrix. Therefore, the Ti content in this invention is selected to be within the range of 0.008% to 0.02%.

[0012] Al: Typically acts as a deoxidizer in steel and, if formed into AlN, also refines the steel's structure. When the Al content exceeds 0.04%, excessive aluminum oxide inclusions can reduce the steel's cleanliness. Too low an Al content results in inadequate deoxidation, and easily oxidizable elements like Ti can form oxides. Therefore, the lower limit for Al is set at 0.015%.

[0013] Cr: A major element that can effectively improve hardenability, inhibit the formation of some ferrite, and promote the formation of bainite. It plays an important role in controlling phase transformation structure, promoting the formation of polygonal ferrite, pearlite, and acicular ferrite with a large number of dislocations distributed within the crystal in the medium and low temperature regions, thereby improving the strength, plasticity, and toughness of the steel plate. The present invention selects the Cr content range to be 0.15% to 0.25%.

[0014] Mo: It can significantly improve hardenability, increase strength, promote medium and low temperature microstructure transformation, and optimize the microstructure and properties of the heat-affected zone of the steel plate and weld. However, too high a molybdenum content will increase production costs. Therefore, the present invention controls the Mo content to 0.03% to 0.06%.

[0015] P and S are unavoidable impurity elements in steel and should be as low as possible. However, due to smelting costs and process considerations, they cannot be kept indefinitely low. Therefore, the present invention sets the upper limits of P and S content at 0.020% and 0.015%, respectively.

[0016] The strain-designed high-strength L450M hot-rolled steel plate for gas pipelines of the present invention has a target thickness of 9-15 mm, is produced on a medium and heavy plate reciprocating rolling mill using continuous casting billets with a thickness of 135-200 mm, and uses water as a cooling medium.

[0017] The object of the present invention is achieved through the following technical solutions: The present invention provides a method for manufacturing thin-gauge L450M pipeline steel plates based on strain design, comprising molten steel smelting → furnace refining (LF refining), RH vacuum degassing → continuous casting → heating of the ingot → controlled rolling → relaxation + cooling → air cooling to room temperature; specifically comprising the following steps: 1) Molten steel smelting and continuous casting: smelting according to the following chemical composition, the weight percentage of which includes C 0.05%~0.08%, Si 0.15%~0.25%, Mn 1.55%~1.70%, P≤0.020%, S≤0.015%, Nb 0.02%~0.04%, Ti0.008%~0.020%, Al 0.015%~0.04%, Cr 0.15%~0.25%, Mo 0.03%~0.06%, N 0.003%~0.006%, the balance is Fe and unavoidable impurities, and the total amount of other impurity elements is less than 0.05%. The molten iron undergoes pretreatment, converter smelting, LF refining, RH vacuum degassing, and continuous casting; the continuous casting superheat is 9-15°C, and the continuous casting billet drawing rate is 0.7-1.0m / min; in the fan-shaped section, strong cooling is adopted, the total cooling water volume in the first half is 1000-1200L / min, and the total cooling water volume in the second half is 950-1100L / min. At the same time, light reduction is applied at the end of solidification, and the reduction of the continuous casting billet is 8-15mm (strong cooling is adopted to ensure the temperature gradient in the thickness direction of the billet, and light reduction is applied at the same time to promote the crushing of the core grains, thereby ensuring the strength and toughness of the subsequent steel plate); after the billet comes off the line, it is stacked and slowly cooled, the stacking temperature is not lower than 650°C, and the slow cooling time is not less than 48h (to promote the diffusion of Mn and Cr elements and reduce the influence of their composition segregation on the structure and performance).

[0018] 2) Ingot heating: The ingot is fed into a walking beam furnace for heating. The ingot passes through the preheating section, heating section, and soaking section in sequence before being discharged from the furnace. The temperature range of the preheating section is 950-1100°C (to promote the rapid and complete solid solution of Nb, Cr, and Ti carbides and nitrides in the matrix and their sufficient diffusion), the temperature range of the heating section is 1210-1230°C, and the temperature range of the soaking section is 1100-1125°C. The total furnace time of the heating and soaking sections is controlled at 2.5-3.5 hours to ensure that the temperature difference between the upper and lower surfaces of the ingot is within 15°C. The heating section provides high temperature heating to ensure temperature uniformity among all parts of the ingot, further promote the diffusion of alloying elements Mn, Cr, and C, reduce the impact of their composition segregation on the microstructure and properties, and improve the uniformity of metal flow in the horizontal and vertical directions on the steel plate surface. In combination with the ingot composition, by reducing the heating temperature in the soaking section, energy consumption is reduced, and the effect of the coarsening of the original austenite grains on the DWTT properties is also suppressed. 3) Controlled rolling: The casting adopts the rolling mode of transverse and longitudinal rolling. In the transverse rolling stage, the reduction rate of each of the first two passes of rolling is greater than 20%, and the first pass of rolling is sprayed with water for descaling, the time is 1-1.5min, and the pressure is 20-25MPa. (The first two passes of rolling try to give full play to the capacity of the rolling mill, adopt a large reduction rate, promote the dynamic crystallization of austenite, refine the original austenite grains, and use high-pressure water descaling in the rolling mill to make the structure from the surface to the core of the steel plate During the longitudinal rolling stage, the reduction rate of each of the first three passes is greater than 25%, and the reduction rate of the final pass is less than 5%. The final rolling temperature of the steel plate is 780-810°C (the first three passes of the longitudinal rolling stage use a large reduction rate to further refine the austenite grains and improve the core structure. In the later stage of rolling, due to the temperature drop of the steel plate, the deformation resistance increases. The final pass uses a small reduction rate to flatten the plate shape and reduce the internal stress of the steel plate). 4) Relaxation + Cooling: After rolling, the steel plate is relaxed and kept warm, and pre-straightened (to ensure the initial plate shape before entering the water, and to control the water temperature of the steel plate to ensure that the ferrite ratio of the steel plate before entering the water is not less than 65%), followed by laminar cooling. The starting cooling temperature range is 610-630℃, the final cooling temperature range is 370-400℃, and the cooling rate is controlled at 10-15℃ / s. (A slow cooling rate is adopted to ensure that the bainite ratio is below 30%, while reducing the formation of pearlite, inhibiting the formation of wide banded structure, and ensuring the low-temperature DWTT performance and plasticity of the steel plate). After the steel plate is cooled, the side spraying and wind purge are turned on, and the side spraying pressure and water volume are 5-10MPa and 35-50m 3 / h, the wind blowing pressure is 5~10MPa, and then hot straightening is carried out.

[0019] 5) Air cool to room temperature.

[0020] Furthermore, in step 1), the raw material is pretreated with KR molten iron to control the S content to be less than 0.015%, and then enters the converter after slag removal; during converter smelting, the P content is controlled to be ≤0.02%, and the C content is controlled to be 0.05-0.08% at the end of converter smelting. Argon is blown for 15-20 minutes during tapping; then LF refining and RH vacuum degassing are carried out, and the RH vacuum is maintained for more than 30 minutes; and then continuous casting is carried out.

[0021] Furthermore, during the converter smelting process, a double slag method is used to remove phosphorus.

[0022] Furthermore, during the continuous casting process, the first half is sections 1-4, and the second half is sections 5-8.

[0023] Furthermore, in step 3), the cast slab is descaled for 1 to 2 minutes using high-pressure water before rolling, and the descaling machine pressure is 15 to 25 MPa.

[0024] Furthermore, in step 4), the number of cooling headers opened is 3 to 6, and the water volume of a single header is 120 to 150 m 3 / h.

[0025] Furthermore, in step 4), hot straightening is performed in three passes, with the lead-in roller position ranging from -1.5mm to -2.6mm, the lead-out roller position ranging from -3.3mm to -4.1mm, and the straightening force ranging from 2500KN to 3000KN (by setting appropriate roller gap and straightening force, the straightened steel plate is ensured to have a good shape).

[0026] By adopting the above-mentioned composition, steelmaking, continuous casting, controlled rolling and controlled cooling scheme, the shortcomings of the existing technology are overcome, and the production and application of L450M pipeline steel plate for rolling thin specifications (9-15mm) with a pipe diameter of less than Φ914mm based on strain design is realized. The structure of the steel plate is a complex phase structure mainly composed of bainite and polygonal ferrite, of which polygonal ferrite accounts for more than 65%.

[0027] Beneficial effects of the present invention: 1. KR hot metal pretreatment and deep desulfurization are followed by clean slag removal. The converter utilizes a double-slag method for P removal, resulting in low P and S content in the ingot. Controlled argon blowing time and a consistent RH vacuum degassing time mitigate defects such as central segregation, inclusions, and excessive H and O content caused by high Mn, Cr, and C content, thereby improving the ductility and toughness of pipeline steel. Reducing superheat and continuous casting speeds improves macrosegregation in the ingot and reduces the spacing between secondary dendrite arms in the solidified structure, helping to minimize segregation and internal structural defects. Strong cooling is employed in the fan-shaped sections to maintain a temperature gradient through the ingot's thickness. Simultaneously, light reduction is applied to promote core grain breakage, ensuring the subsequent strength and toughness of the steel plate. Furthermore, the ingots are stacked for slow cooling after leaving the production line, with a time limit for stacking temperatures below the slow cooling time. This promotes the diffusion of Mn and Cr, mitigating the impact of compositional segregation on the structure and properties.

[0028] 2. Heating of the ingot: The temperature and time of the ingot in the preheating section, heating section, and soaking section are limited to ensure that the carbides and nitrides of Nb, Cr, and Ti are quickly and fully dissolved in the matrix and fully diffused. At the same time, high-temperature heating is provided in the heating section to ensure temperature uniformity in all parts of the ingot, further promote the diffusion of alloying elements Mn, Cr, and C, reduce the impact of their component segregation on the structure and performance, and improve the uniformity of metal flow in the horizontal and vertical directions on the surface of the steel plate. In combination with the ingot composition, by lowering the heating temperature in the soaking section of the ingot, energy consumption is reduced, and the impact of the coarsening of the original austenite grains on the DWTT performance is also suppressed. 3. A transverse and longitudinal rolling mode is adopted. The mill capacity is maximized in the first few passes of the transverse and longitudinal rolling phase. High reduction ratios are used to promote dynamic re-crystallization of austenite and refine the original austenite grains. High-pressure water descaling is used in the mill to achieve uniform microstructure distribution from the surface to the core of the steel plate, improving its strength and toughness. Furthermore, this rolling process does not utilize the traditional TMCP process, eliminating the intermediate billet warming process, which also improves rolling efficiency. The water entry temperature of the rolled steel plate is controlled to ensure that the ferrite content is no less than 65% before the steel plate enters the water. A slow cooling rate is adopted to keep the bainite content below 30%, while simultaneously reducing pearlite formation and suppressing the formation of wide banded structure, ensuring the steel plate's low-temperature DWTT performance and ductility. Side spraying, air purging, and hot straightening are used to facilitate plate shape control, improve performance uniformity, reduce the likelihood of buckling and buckling problems, and save on subsequent cold straightening equipment investment.

[0029] 4. The present invention reduces alloy costs through simple composition design, and achieves the production and application of a thin-gauge L450M pipeline steel plate based on strain design by regulating the steelmaking, continuous casting, heating, controlled rolling and cooling, and hot straightening processes. The thickness of the steel plate is 9-15mm, the pipe diameter is less than Φ914mm, and the structure of the steel plate is a complex phase structure mainly composed of bainite and polygonal ferrite, of which polygonal ferrite accounts for more than 65%. The steel plate has good low-temperature toughness and plasticity. The specific performance indicators are: Transverse: tensile yield strength performance is between 480 and 520MPa, tensile strength is between 570 and 630MPa, yield strength ratio is less than 0.85, and elongation is ≥25%. -40℃ transverse Charpy impact energy is ≥150J, -20℃ DWTT transverse shear area SA is not less than 90%, longitudinal: uniform elongation is ≥10%. DETAILED DESCRIPTION

[0030] The following examples are used to specifically illustrate the present invention. These examples are only general descriptions of the present invention and do not limit the present invention.

[0031] A strain-designed thin-gauge L450M pipeline steel plate has a chemical composition by weight of: C 0.05% to 0.08%, Si 0.15% to 0.25%, Mn 1.55% to 1.70%, P ≤ 0.020%, S ≤ 0.015%, Nb 0.02% to 0.04%, Ti 0.008% to 0.020%, Al 0.015% to 0.04%, Cr 0.15% to 0.25%, Mo 0.03% to 0.06%, and N 0.003% to 0.006%. The balance is Fe and unavoidable impurities, with the total content of other impurity elements being less than 0.05%. The steel plate is manufactured in a thickness of 9-15 mm using continuous casting slabs with a thickness of less than 250 mm on a medium and heavy plate reciprocating mill, using water as the cooling medium.

[0032] The manufacturing method of the thin-gauge L450M pipeline steel plate based on strain design includes molten steel smelting → furnace refining (LF refining), RH vacuum degassing → continuous casting → heating of the ingot → controlled rolling → relaxation + cooling → air cooling to room temperature; specifically, the steps include: 1) Molten steel smelting and continuous casting: smelting according to the above composition. The raw materials are pretreated with KR molten iron to control the S content to be lower than 0.015%, and then enter the converter after slag removal; during converter smelting, the P content is controlled to be ≤0.02%, and the C content is controlled to be 0.05-0.08% at the end of converter smelting. Argon is blown for 15-20 minutes when tapping; then LF refining and RH vacuum degassing are carried out, and the RH vacuum is maintained for more than 30 minutes; thereafter, continuous casting is carried out, the continuous casting superheat is 9-15°C, and the continuous casting billet drawing rate is 0.7-1.0m / min; in the fan-shaped section, strong cooling is adopted, the total cooling water volume of the 1st to 4th sections is 1000-1200L / min, and the total cooling water volume of the 5th to 8th sections is 950-1100L / min, and at the same time, light reduction is applied at the end of solidification, and the continuous casting billet reduction is 8-15mm; the billets after coming off the line are stacked and slowly cooled, the stacking temperature is not lower than 650°C, and the slow cooling time is not less than 48h.

[0033] 2) Billet heating: The billet is fed into a walking beam furnace for heating. The billet passes through the preheating section, heating section, and soaking section in sequence before exiting the furnace. The temperature range of the preheating section is 950-1100°C, the temperature range of the heating section is 1210-1230°C, and the temperature range of the soaking section is 1100-1125°C. The total time in the heating and soaking sections is controlled within 2.5-3.5 hours to ensure that the temperature difference between the upper and lower surfaces of the billet is within 15°C. 3) High-pressure water descaling and controlled rolling: Before rolling, high-pressure water is used to descale the cast billet after it leaves the furnace for 1 to 2 minutes, and the descaling machine pressure is 15 to 25 MPa; the rolling mode of transverse and longitudinal rolling is adopted. In the transverse rolling stage, the reduction rate of each of the first two passes of rolling is greater than 20%, and the first pass of rolling is sprayed with water for descaling, the time is 1 to 1.5 minutes, and the pressure is 20 to 25 MPa; in the longitudinal rolling stage, the reduction rate of each of the first three passes of rolling is greater than 25%, and the reduction rate of the last pass is less than 5%. The final rolling temperature of the steel plate is 780 to 810 ° C; 4) Relaxation + cooling: After rolling, the steel plate is relaxed and kept warm, and pre-straightened (to ensure the initial plate shape before entering the water, and to control the water temperature of the steel plate to ensure that the ferrite ratio of the steel plate before entering the water is not less than 65%), followed by laminar cooling. The cooling temperature range is 610-630℃, the final cooling temperature range is 370-400℃, the number of cooling manifolds opened is 3-6, and the water volume of a single manifold is 120-150m 3 / h, the cooling rate is controlled at 10~15℃ / s to ensure that the bainite ratio is below 30%. After the steel plate is cooled, the side spraying and wind purge are turned on, and the side spraying pressure and water volume are 5~10MPa and 35~50m 3 / h, the wind blowing pressure is 5~10MPa, and then hot straightening (three-pass straightening) is carried out, the position of the lead-in roller is -1.5mm~-2.6mm, the position of the lead-out roller is -3.3mm~-4.1mm, and the straightening force is between 2500KN and 3000KN; 5) Air cool to room temperature.

[0034] Examples 1-6 Table 1 shows the chemical composition of the example steel, Table 2 shows the smelting to continuous casting and stacking process system of the example steel, Table 3 shows the heating system of the cast slab and the high-pressure water descaling process before rolling of the continuous casting slab of the example steel; Table 4 shows the rolling and partial cooling process of the example steel; Table 5 shows the main process parameters of the cooling control of the example steel; Table 6 shows the properties of the example steel plate.

[0035] Table 1 Chemical composition of example steel (wt, %)

[0036] Note: Impurity elements P in steel ≤ 0.02%; S ≤ 0.015%; the total amount of other impurity elements is less than 0.05%.

[0037] Table 2 Process system from smelting to continuous casting and stacking of the example steel

[0038] Table 3 Heating system of the cast steel and high-pressure water descaling process before continuous casting

[0039] Table 4 Rolling and partial cooling process of example steel

[0040] Table 5 Main process parameters of controlled cooling of example steel

[0041] Table 6 Properties of the steel plates of the examples

[0042] It can be seen that compared with the existing technology, the present invention reduces alloy costs through simple composition design, and achieves the production and application of L450M pipeline steel plates for gas pipelines with efficient rolling thickness specifications (9-15mm) and pipe diameters below Φ914mm based on strain design by regulating steelmaking, continuous casting, heating, controlled rolling and cooling, and hot straightening processes. The steel plate has a complex phase structure mainly composed of bainite and polygonal ferrite, of which polygonal ferrite accounts for more than 65%, and the steel plate has good low-temperature toughness and plasticity.

[0043] The above embodiments are intended only to illustrate the technical concepts and features of the present invention. Their purpose is to enable those skilled in the art to understand the contents of the present invention and implement them accordingly. They are not intended to limit the scope of protection of the present invention. Any equivalent changes or modifications made in accordance with the spirit of the present invention are intended to be covered by the scope of protection of the present invention.

Claims

1. A thin gauge L450M pipeline steel plate based on strain design, characterized in that: Its chemical composition by weight percentage includes: C 0.05%~0.08%, Si 0.15%~0.25%, Mn 1.55%~1.70%, P≤0.020%, S≤0.015%, Nb0.02%~0.04%, Ti 0.008%~0.020%, Al 0.015%~0.04%, Cr 0.15%~0.25%, Mo 0.03%~0.06%, N 0.003%~0.006%, the balance is Fe and unavoidable impurities, and the total amount of other impurity elements is less than 0.05%.

2. The thin gauge L450M pipeline steel plate based on strain design according to claim 1, characterized in that: The thickness of the steel plate is 9-15 mm.

3. The thin gauge L450M pipeline steel plate based on strain design according to claim 1, characterized in that: The diameter of the steel plate is less than 914 mm.

4. The thin gauge L450M pipeline steel plate based on strain design according to claim 1, characterized in that: The steel plate has a transverse tensile yield strength of 480-520 MPa, a tensile strength of 570-630 MPa, a yield strength ratio of less than 0.85, and an elongation of ≥25%; a transverse Charpy impact energy of ≥150 J at -40°C, a transverse shear area of ​​not less than 90% at -20°C DWTT, and a longitudinal uniform elongation of ≥10%.

5. The method for manufacturing thin-gauge L450M pipeline steel plate based on strain design according to any one of claims 1 to 4, characterized in that: The steps include: 1) From molten steel smelting to continuous casting and slow cooling of billet stacking: The molten iron undergoes pretreatment, converter smelting, LF refining, RH vacuum degassing, and continuous casting. The continuous casting superheat is 9-15°C, and the continuous casting billet drawing rate is 0.7-1.0 m / min. In the fan-shaped section, strong cooling is adopted, with the total cooling water volume in the first half being 1000-1200 L / min, and the total cooling water volume in the second half being 950-1100 L / min. Simultaneously, soft reduction is applied at the end of solidification, with a continuous casting billet reduction of 8-15 mm. After leaving the line, the billets are stacked for slow cooling, with the stacking temperature not lower than 650°C and the slow cooling time not less than 48 hours. 2) Billet heating: The billet is fed into a walking beam furnace for heating. The billet passes through the preheating section, heating section, and soaking section in sequence before being discharged from the furnace. The temperature in the preheating section is 950-1100°C, the temperature in the heating section is 1210-1230°C, and the temperature in the soaking section is 1100-1125°C. The total time in the heating and soaking sections is controlled within 2.5-3.5 hours to ensure that the temperature difference between the upper and lower surfaces of the billet is within 15°C. 3) Controlled rolling: adopt the rolling mode of transverse and longitudinal rolling. In the transverse rolling stage, the reduction rate of each of the first two passes is greater than 20%, and the first pass of rolling is sprayed to remove scale water, the time is 1-1.5 minutes, and the pressure is 20-25MPa; in the longitudinal rolling stage, the reduction rate of each of the first three passes is greater than 25%, and the reduction rate of the last pass is less than 5%. The final rolling temperature of the steel plate is 780-810℃; 4) Relaxation + cooling: After rolling, the steel plate is relaxed and kept warm, pre-straightened, and then cooled by laminar flow. The starting cooling temperature is 610-630℃, the final cooling temperature is 370-400℃, and the cooling rate is controlled at 10-15℃ / s; then hot straightening is performed; 5) Air cool to room temperature.

6. The method for manufacturing thin-gauge L450M pipeline steel plate based on strain design according to claim 5, characterized in that: The thickness of the cast slab is 135-200 mm; the steel plate is obtained by rolling the cast slab on a medium and thick plate reciprocating rolling mill.

7. The method for manufacturing thin-gauge L450M pipeline steel plate based on strain design according to claim 5, characterized in that: In step 1), the raw material is pretreated with KR molten iron to control the S content to less than 0.015%, and then enters the converter after slag removal. During converter smelting, the P content is controlled to be ≤0.02%, and the C content is controlled to be 0.05-0.08% at the end of converter smelting. Argon is blown for 15-20 minutes during tapping. LF refining and RH vacuum degassing are then carried out, and the RH vacuum is maintained for more than 30 minutes. Continuous casting is then carried out.

8. The method for manufacturing thin-gauge L450M pipeline steel plate based on strain design according to claim 5, characterized in that: During converter smelting, double slag method is used to remove phosphorus; During the continuous casting process, the first half is sections 1-4, and the second half is sections 5-8.

9. The method for manufacturing thin-gauge L450M pipeline steel plate based on strain design according to claim 5, characterized in that: In step 3), high-pressure water is used to descale the cast slab after it leaves the furnace for 1 to 2 minutes before rolling, and the descaling machine pressure is 15 to 25 MPa; after the steel plate leaves the controlled cooling, the side spraying and wind blowing are turned on, and the side spraying pressure and water volume are 5 to 10 MPa and 35 to 50 m3 respectively. 3 / h, the wind blowing pressure is 5~10MPa, and then hot straightening.

10. The method for manufacturing thin-gauge L450M pipeline steel plate based on strain design according to claim 5, characterized in that: In step 4), the cooling medium for laminar cooling is water; the number of cooling manifolds opened is 3 to 6, and the water volume of a single manifold is 120 to 150 m3. 3 / h; hot straightening is a three-pass straightening process, with the lead-in roller position at -1.5mm to -2.6mm, the lead-out roller position at -3.3mm to -4.1mm, and the straightening force between 2500KN and 3000KN.

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