Thin-gauge pipeline steel plate with low-temperature toughness stable control based on strain design and manufacturing method of thin-gauge pipeline steel plate

Abstract

The invention discloses a thin-gauge pipeline steel plate with low-temperature toughness stable control based on strain design and a manufacturing method of the thin-gauge pipeline steel plate, and belongs to the technical field of low-carbon microalloyed steel production. The method comprises the steps of molten steel smelting, external refining (LF refining), RH vacuum degassing, continuous casting, casting blank heating, controlled rolling, relaxation, heat compensation in a heat preservation box, cooling and air cooling to the room temperature. According to the invention, the low-temperature toughness stability control of the L450M pipeline steel plate for rolling the gas pipeline based on the strain design is realized, wherein the L450M pipeline steel plate is used for rolling the thin specification (9-13mm) and the pipe diameter of the gas pipeline is less than phi 1016mm.

Description

Technical Field

[0001] This invention belongs to the field of low-carbon microalloyed steel production technology, and specifically relates to an L450M steel plate for oil and gas pipelines with a thickness of 9-13mm and a pipe diameter of less than Φ1016mm, based on strain design and low-temperature toughness stability control, and its manufacturing method. Background Technology

[0002] In recent years, the construction of energy transmission pipelines has developed rapidly. To improve transmission efficiency and operational safety, the requirements for the strength and toughness of the selected steel raw materials are becoming increasingly stringent. Currently, major domestic and international pipeline trunk lines mainly use L485M and L555M steel grades, while branch lines and urban pipe networks mostly use thinner specifications (9-13mm) and smaller diameters (below Φ1016mm) of L450M steel grade. When pipelines are located in extremely cold regions or earthquake-prone areas, higher requirements are placed on the low-temperature toughness and plastic deformation capacity of raw materials from the perspective of operational stability and safety. For example, the low-temperature drop hammer test and ductile-brittle transition temperature require lower testing temperatures, and the Charpy impact energy requirement is higher. At the same time, stringent requirements are also placed on the longitudinal uniform elongation of the steel pipe. Therefore, these projects have put forward higher personalized requirements, and simply meeting the API SPEC 5L specification is far from sufficient. In addition, facing the severe market situation, user requirements, and manufacturing cost pressures in the steel industry, it is particularly important to reduce alloy and energy costs while ensuring product quality and minimizing resource consumption in the production line. L450M pipeline steel is typically produced using a two-stage controlled rolling process. To ensure the cumulative reduction rate during the finishing rolling stage and to facilitate static recrystallization in the intermediate billet, a relatively thick intermediate billet is required for heating. This results in a prolonged heating time for the intermediate billet, and the final rolling temperature may be controlled below 850℃ or even lower, significantly increasing the rolling difficulty. Firstly, the mill load, the longitudinal temperature difference between the head, middle, and tail of the thin-gauge steel plate, the uniformity of microstructure and properties, and the control of plate shape all face severe challenges. Secondly, improving the steel plate's performance depends primarily on the content of impurities such as phosphorus and sulfur, the content of alloying elements, and the control of key process parameters in smelting and rolling. Therefore, solving the aforementioned problems in producing thin-gauge L450M pipeline steel plates based on strain design and achieving stable low-temperature toughness control is crucial for the development of thin-gauge, strain-design-based L450M pipeline steel.

[0003] Compared with existing technologies:

[0004] To date, there are very few reports, both domestically and internationally, on the stability control of low-temperature toughness in thin-gauge L450M pipeline steel plates using strain design. Prior to this invention, application number CN201010243241.0 disclosed an X65 pipeline steel and its production method. The weight percentage composition of this patent is: C 0.055%–0.090%, Si 0.15%–0.35%, Mn 1.50%–1.65%, P≤0.020%, S≤0.005%, Nb 0.040%–0.055%, V 0.040%–0.070%, Ti 0.010%–0.025%, N≤0.008%, and Als 0.005%–0.060%. This steel grade has a high V content, and its manufacturing process uses natural air cooling, which is inefficient and cannot fully utilize the water-displacement alloy to reduce costs and improve strength and toughness. Furthermore, the specific strength, toughness, and plastic deformation indicators of the actual product are not clearly defined, and the low-temperature toughness stability control indicators for the steel plate are not specified. Application number CN201110179945.0 discloses an X65 pipeline steel with excellent low-temperature toughness and its manufacturing method. The patented composition by weight percentage 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%, Als 0.010%–0.040%, P≤0.018%, S≤0.005%, N≤0.006%. This steel grade contains V, resulting in high alloy costs. The laminar flow cooling process cannot fully utilize the water-substitution alloy to reduce costs and improve strength and toughness, and it does not meet the requirements for strain-design pipeline steel. Application number KR20020027013(A) discloses a steel product for stress relief heat treatment guaranteed API-X65 grade linepipes and method for manufacturing the same. The patented composition has the following weight percentages: C 0.07%–0.09%, Si 0.2%–0.5%, Mn 1.40%–1.60%, P≤0.025%, S≤0.005%, Nb 0.035%–0.045%, V 0.04%–0.05%, Mo 0.03%–0.07%, Cr 0.05%–0.15%, Ti 0.005%–0.015%, N 0.002%–0.007%, and Als 0.015%–0.050%. This steel contains V and Mo, resulting in high cost and insufficient low-temperature toughness in the finished product.

[0005] While the steels disclosed in the aforementioned patent documents possess high strength and toughness, they are either rolled plates or have high production costs, failing to meet the technical requirements (plastic deformation index - uniform elongation) for crossing mining areas. Furthermore, most employ two-stage low-temperature controlled rolling, making them unsuitable for producing low-cost, low-temperature toughness-stable thin-gauge L450M pipeline steel plates based on strain design. The technical solution provided by this invention effectively overcomes these shortcomings, enabling the production of high-strength L450M hot-rolled steel plates for gas pipelines with a thickness of 9-13mm and a pipe diameter of Φ1016mm or less, based on strain design, using continuously cast billets with a thickness of 135-170mm. These steel plates possess high strength, excellent and stable low-temperature toughness, and high resistance to plastic deformation. Summary of the Invention

[0006] The purpose of this invention is to overcome the aforementioned technical problems and deficiencies, and to provide a method for manufacturing thin-gauge pipeline steel plates with a thickness of 9-13 mm and a pipe diameter of Φ1016 mm or less, based on strain design and low-temperature toughness stability control. This invention solves problems such as low rolling efficiency, high rolling resistance, large longitudinal temperature difference between the head, middle, and tail of thin-gauge steel plates, poor uniformity of microstructure and properties, and difficulty in controlling plate shape. The resulting steel plate possesses low cost, excellent and stable low-temperature toughness, high resistance to plastic deformation, and can ensure the safety of gas pipelines.

[0007] To achieve the above objectives, the technical solution of the present invention is as follows: A thin-gauge pipeline steel plate based on strain design for low-temperature toughness stability control has the following chemical composition by weight percentage: C 0.05%–0.08%, Si 0.15%–0.25%, Mn 1.48%–1.58%, P≤0.020%, S≤0.015%, Nb 0.02%–0.04%, Ti 0.008%–0.020%, Al 0.015%–0.04%, Cr 0.10%–0.20%, Mo 0.04%–0.09%, Ni 0.1%–0.12%, N 0.003%–0.006%, with the balance being Fe and unavoidable impurities.

[0008] The roles of the main elements in the chemical composition of the steel plate of this invention are as follows: Carbon (C): The most economical and basic strengthening element in steel. It has a significant effect on improving the strength of steel through solid solution strengthening and precipitation strengthening. However, increasing the C content has a negative impact on the plasticity, toughness, and weldability of steel. Therefore, this invention sets the C content range to 0.05% to 0.08%.

[0009] Mn: It enhances the strength of steel through solid solution strengthening, while compensating for the strength loss caused by the reduction in carbon content. Furthermore, it lowers the γ-α phase transformation temperature, thereby refining ferrite grains and contributing to the acquisition of fine low-temperature phase transformation products, thus improving toughness. However, increasing the Mn content exacerbates center segregation in continuously cast billets, which is detrimental to improving the low-temperature toughness of the steel plate and cannot guarantee the uniformity of the cross-sectional microstructure. Therefore, the Mn content range of this invention is designed to be 1.48%–1.58%.

[0010] Si (Si) plays a role in deoxidation in steelmaking and improving the strength of the matrix. Increasing the Si content can purify ferrite and reduce the content of pearlite, which is beneficial for reducing the Bauschinger effect in the matrix material. However, excessive Si will reduce the toughness of the heat-affected zone in the weld. Therefore, the Si content is set at 0.15% to 0.25% in this invention.

[0011] Nitrogen (Nb) is a commonly used element in modern microalloyed pipeline steel, exhibiting excellent grain refinement and precipitation strengthening effects; it also delays austenite recrystallization. However, excessive Nb increases production costs and complicates continuous casting process control. This invention selects an Nb content range of 0.02% to 0.04%, and, combined with appropriate heating and rolling processes, obtains a uniform composite phase dominated by pearlite, polygonal ferrite, and acicular ferrite, resulting in good toughness.

[0012] N: In steel, nitrogen (N) has no other significant role besides forming fine TiN particles to refine austenite grains. Therefore, it needs to be kept at a low content level. The N content range selected in this invention is 0.003% to 0.006%.

[0013] Ti is a strong solid nitrogen element, existing in the form of TiN in continuously cast billets. Fine TiN particles can effectively inhibit austenite grain growth during reheating of the continuously cast billet and help improve the solid solubility of Nb in austenite, thus improving the impact toughness of the weld heat-affected zone. When the Ti content exceeds a certain value, the TiN particles coarsen, increasing the stress concentration level at the particle interface and the matrix. Therefore, this invention selects a Ti content range of 0.008% to 0.02%.

[0014] Al: Commonly used as a deoxidizer in steel, it can also refine the microstructure if it forms AlN. When the Al content exceeds 0.04%, excessive alumina inclusions will reduce the cleanliness of the steel. If the Al content is too low, deoxidation will be insufficient, and easily oxidized elements such as Ti will form oxides. Therefore, the lower limit for Al content is set at 0.015%.

[0015] Cr: It is 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 the phase transformation structure, and promotes the formation of polygonal ferrite, pearlite and acicular ferrite with a large number of dislocations in the grain in the medium and low temperature range, thereby improving the strength, plasticity and toughness of steel plates. The Cr content range selected in this invention is 0.10% to 0.20%.

[0016] Mo can significantly improve hardenability, increase strength, promote the transformation of microstructure at medium and low temperatures, and optimize the microstructure and properties of steel plates and heat-affected zones at welds. However, excessively high molybdenum content will increase production costs. Therefore, this invention controls the Mo content to be between 0.04% and 0.09%.

[0017] Ni: Nickel's phase transformation behavior in steel is similar to that of Mo. It can lower the phase transformation temperature of steel, improve the microstructure, refine the grains, and increase the strength of steel while maintaining good plasticity and toughness. However, nickel is expensive. Therefore, this invention controls the Ni content to be between 0.10% and 0.12%.

[0018] P and S are unavoidable impurity elements in steel, and their content should be as low as possible. However, due to considerations of smelting costs and processes, their content cannot be infinitely low. Therefore, this invention sets the upper limits for P and S content at 0.020% and 0.015%, respectively.

[0019] Furthermore, the target thickness of the steel plate is 9-13 mm, and it is produced on a medium-thick plate reciprocating rolling mill using a continuous casting billet with a thickness of 135-170 mm, with water as the cooling medium.

[0020] Furthermore, the steel plate is a high-strength L450M hot-rolled steel plate.

[0021] Furthermore, the steel plate is used to manufacture gas transmission pipelines with a diameter of Φ1016mm or less.

[0022] The objective of this invention is achieved through the following technical solution: This invention provides a method for manufacturing thin-gauge pipeline steel plates based on strain design and low-temperature toughness stability control, comprising: steel smelting → ladle refining (LF refining), RH vacuum degassing → continuous casting → billet heating → controlled rolling → relaxation + insulated box reheating + cooling → air cooling to room temperature; specifically including the following steps: 1) Steel smelting and continuous casting: Smelting is carried out according to the following composition, the chemical composition by weight percentage is C 0.05%~0.08%, Si 0.15%~0.25%, Mn 1.48%~1.58%, P≤0.020%, S≤0.015%, Nb 0.02%~0.04%, Ti 0.008%~0.020%, Al 0.015%~0.04%, Cr 0.10%~0.20%, Mo 0.04%~0.09%, Ni 0.1%~0.12%, N 0.003%~0.006%, the balance being Fe and unavoidable impurities, and the total amount of other impurity elements is less than 0.05%. Molten iron undergoes pretreatment, converter smelting, LF refining, RH vacuum degassing, and continuous casting. The superheat during continuous casting is 8–13℃, and the casting speed is 0.8–1.1 m / min. At the end of solidification, the billet is subjected to heavy pressure, with a reduction of 16–20 mm (the heavy pressure promotes core grain breakage, ensuring the strength and toughness of the subsequent steel plate). After casting, the billet is stacked for slow cooling at a temperature not lower than 700℃ for a duration of not less than 48 hours (to promote the diffusion of elements such as Mn, Cr, and H, and reduce their influence on microstructure and properties due to component segregation).

[0023] 2) Billet Heating: The billet is fed into a walking beam furnace for heating, passing through a preheating section, a heating section, and a soaking section before exiting the furnace. The preheating section temperature range is 750–900℃ (to promote the rapid and complete dissolution and diffusion of Nb, Cr, and Ti carbides and nitrides into the matrix), the heating section temperature range is 1190–1200℃, and the soaking section temperature range is 1160–1175℃. The total time spent in the furnace during the heating and soaking sections is controlled between 2.0 and 3.0 seconds. For hours, ensure the temperature difference between the upper and lower surfaces of the billet is within 15℃ (control the temperature of the heating section to ensure temperature uniformity in all parts of the billet, further promote the diffusion of alloying elements such as Mn and C, reduce their influence on microstructure and properties due to compositional segregation, and improve the uniformity of transverse and longitudinal metal flow on the steel plate surface; while ensuring the temperature uniformity of the billet and promoting the diffusion of alloying elements, suppress the influence of the coarsening of the original austenite grains on the low-temperature DWTT performance by reducing the heating temperature of the billet and the heating temperature of the soaking section). 3) High-pressure water descaling and controlled rolling: Before rolling, the billet after exiting the furnace is descaled with high-pressure water for 1-2 minutes, with a descaling machine pressure of 15-25 MPa; a transverse and longitudinal rolling mode is adopted. In the transverse rolling stage, the reduction rate of each pass in the first two passes is greater than 25% (using a large reduction rate to promote dynamic re-crystallization of austenite and refine the original austenite grains); in the longitudinal rolling stage, the reduction rate of each pass in the first three passes is greater than 20%, and the reduction rate of the last pass is less than 5%. The final rolling temperature of the steel plate is 780-830℃ (the first three passes in 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 and the increase of deformation resistance, the last pass uses a small reduction rate to flatten the shape of the steel plate and reduce the internal stress of the steel plate). 4) Relaxation + Heating in an Insulating Box + Cooling: After rolling, the steel plate undergoes relaxation and is kept at a temperature of 680-690℃. Pre-straightening is then initiated, with the inlet roll position at -1.0mm to -1.5mm and the outlet roll position at -2.0mm to -2.5mm. The straightening force is between 800KN and 1100KN (pre-straightening ensures the initial plate shape upon entering the water). Subsequently, heating in an insulating box is applied. The steel plate enters the insulating box at a speed of 0.3-0.5m / s, with a temperature of 650-660℃ (heating through the steel plate ensures the steel plate...). During water immersion, temperature uniformity is ensured across the width and length of the rolled steel plate to guarantee stable low-temperature toughness control. Laminar flow cooling is then employed, with an initial cooling temperature range of 640–650℃ and a final cooling temperature range of 405–425℃. The cooling rate is controlled at 10–15℃ / s (slow cooling is used to avoid Mn accumulation in the core of the billet, which could lead to martensite formation and ensure the low-temperature DWTT performance of the steel plate). After the steel plate exits controlled cooling, side spraying and air purging are activated, with side spraying pressure and water flow rate of 5–10 MPa and 50–100 m³ / s, respectively. 3 / h, the air purging pressure is 5~10MPa (by measuring the spray and air purging input, the uniformity of steel plate performance is ensured).

[0024] 5) Air cool to room temperature.

[0025] Furthermore, in step 1), the raw material is pretreated with KR molten iron to control the S content to be below 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 between 0.05% and 0.08% at the end of converter smelting. Argon gas is blown for 20 to 30 minutes when tapping the steel; then LF refining and RH vacuum degassing are carried out, and the RH vacuum is maintained for more than 30 minutes; after that, continuous casting is carried out.

[0026] Furthermore, during the converter smelting process, a double-slag method is used for phosphorus removal.

[0027] Furthermore, the thickness of the cast billet is 135–170 mm.

[0028] Furthermore, in step 4), the number of controlled cooling manifold groups opened is 3 to 6, and the water flow rate per manifold is 155 to 180 m³. 3 / h.

[0029] By adopting the above-mentioned composition and steelmaking continuous casting, controlled rolling and controlled cooling scheme, the shortcomings of the existing technology are overcome, and a low-temperature toughness stability control of L450M pipeline steel plate for gas transmission pipelines with a diameter of less than Φ1016mm based on strain design is achieved.

[0030] The beneficial effects of this invention are: 1. After deep desulfurization pretreatment of KR hot metal, the slag is thoroughly removed. The converter employs a double-slag method for P removal, resulting in lower P and S content in the billet. Controlling the argon blowing time and maintaining the RH vacuum degassing time overcomes defects such as central segregation, inclusions, and excessive H and O content in the billet caused by high Mn, Cr, and C content, thus improving the plasticity and toughness of pipeline steel. Reducing superheat and continuous casting speed can improve macroscopic segregation in the continuously cast billet, decreasing the spacing of secondary dendrite arms in the solidification structure, which helps reduce billet segregation and internal structural defects. Simultaneously, applying heavy pressure promotes core grain breakage, ensuring the strength and toughness of the subsequent steel plate. Furthermore, the billets are stacked and slowly cooled after casting, with controlled stacking temperature and cooling time, which helps promote the diffusion of Mn and Cr elements, mitigating their impact on microstructure and properties due to component segregation.

[0031] 2. Billet Heating: The temperatures and times of the billet in the preheating, heating, and soaking zones are limited to ensure the rapid and complete dissolution and diffusion of Nb, Cr, and Ti carbides and nitrides into the matrix. Controlling the heating zone temperature ensures temperature uniformity across the billet, further promoting the diffusion of alloying elements such as Mn and C, mitigating their impact on microstructure and properties due to component segregation, and improving the uniformity of transverse and longitudinal metal flow on the steel plate surface. To ensure billet temperature uniformity and promote alloying element diffusion, the heating temperatures of the billet and soaking zones are lowered to suppress the impact of initial austenite grain coarsening on low-temperature DWTT performance. 3. A transverse and longitudinal rolling mode is adopted. In the first few passes of the transverse and longitudinal rolling stage, the mill capacity is maximized by using a large reduction rate to promote dynamic re-crystallization of austenite, refine the original austenite grains, and improve the strength and toughness of the steel plate. In the later stages of rolling, due to the temperature drop of the steel plate and the increase in deformation resistance, a small reduction rate is used in the last pass to flatten the shape of the steel plate and reduce the internal stress. Pre-straightening is used to ensure the initial shape of the plate upon entering the water. The steel plate is heated in an insulated box to ensure the temperature uniformity of all positions in the rolling width and length direction when the steel plate enters the water, thereby ensuring the stable control of the low-temperature toughness (DWTT) of the steel plate. A slow cooling rate is adopted to avoid the accumulation of Mn elements in the core of the billet, which would lead to the formation of martensite phase and ensure the low-temperature DWTT performance of the steel plate. Side spraying and air purging are used to facilitate the control of the steel plate shape and improve the uniformity of the steel plate performance.

[0032] 4. This invention, through rational composition design and by controlling the steelmaking, continuous casting, heating, controlled rolling, relaxation, heat treatment in the insulation box, and controlled cooling processes, achieves a highly efficient rolling process for L450M pipeline steel plates with thickness specifications (9-13mm) and pipe diameters below Φ1016mm, based on strain design and with stable low-temperature toughness control. The steel plate exhibits excellent low-temperature toughness. Specific performance indicators are as follows: Transverse: tensile yield strength between 450-520MPa, tensile strength between 560-620MPa, yield strength ratio less than 0.85, elongation ≥25%. Transverse Charpy impact energy at -40℃ ≥150J, DWTT transverse shear area at the head-middle-tail positions of the steel plate at -20℃ not less than 90%, longitudinal: uniform elongation ≥10%. Detailed Implementation

[0033] The following examples are used to illustrate the content of the present invention. These examples are only general descriptions of the content of the present invention and do not limit the content of the present invention.

[0034] A thin-gauge pipeline steel plate with low-temperature toughness stability control based on strain design has the following chemical composition by weight percentage: C 0.05%–0.08%, Si 0.15%–0.25%, Mn 1.48%–1.58%, P≤0.020%, S≤0.015%, Nb 0.02%–0.04%, Ti 0.008%–0.020%, Al 0.015%–0.04%, Cr 0.10%–0.20%, Mo 0.04%–0.09%, Ni 0.1%–0.12%, N 0.003%–0.006%, with the balance being Fe and unavoidable impurities. The steel plate is manufactured with a thickness of 9–13 mm, using continuously cast billets with a thickness of 135–170 mm on a medium-thickness reciprocating rolling mill, with water as the cooling medium. The steel plate is used to manufacture gas transmission pipelines with a diameter of Φ1016mm or less.

[0035] The aforementioned manufacturing method for thin-gauge pipeline steel plates based on strain design for low-temperature toughness stability control includes steel smelting → ladle refining (LF refining), RH vacuum degassing → continuous casting → billet heating → controlled rolling → relaxation + insulated box reheating + cooling → air cooling to room temperature; specifically, it includes the following steps: 1) Steelmaking and continuous casting: Smelting is carried out according to the above composition. The raw materials are pretreated with KR hot metal to control the S content to be less than 0.015%. After slag removal, they are fed into the converter. In the converter smelting, the double slag method is used to remove P, and the P content is controlled to be ≤0.02%. At the end of the converter smelting, the C content is controlled to be 0.05-0.08%. Argon gas is blown for 20-30 minutes when tapping the steel. Then, LF refining and RH vacuum degassing are carried out. The RH vacuum is maintained for more than 30 minutes. After that, slab continuous casting is carried out. The superheat of continuous casting is 8-13℃, and the continuous casting speed is 0.8-1.1m / min. At the end of solidification, the continuous casting billet is subjected to heavy pressure, and the reduction of the continuous casting billet is 16-20mm. After the billet is removed from the line, it is stacked and slowly cooled. The stacking temperature is not lower than 700℃, and the slow cooling time is not less than 48 hours.

[0036] 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 750-900℃, the temperature range of the heating section is 1190-1200℃, and the temperature range of the soaking section is 1160-1175℃. The total time in the furnace in the heating section and soaking section is controlled at 2.0-3.0 hours to ensure that the temperature difference between the upper and lower surfaces of the billet is within 15℃. 3) High-pressure water descaling and controlled rolling: Before rolling, the billet after exiting the furnace is descaled with high-pressure water for 1-2 minutes, with the descaling machine pressure being 15-25 MPa; a transverse and longitudinal rolling mode is adopted. In the transverse rolling stage, the reduction rate of each pass in the first two rolling passes is greater than 25%; in the longitudinal rolling stage, the reduction rate of each pass in the first three rolling passes is greater than 20%, and the reduction rate of the last pass is less than 5%. The final rolling temperature of the steel plate is 780-830℃. 4) Relaxation + Insulation Box Heating + Cooling: After rolling, the steel plate relaxes and waits at a temperature of 680-690℃. Pre-straightening is then initiated, with the inlet roll position at -1.0mm to -1.5mm and the outlet roll position at -2.0mm to -2.5mm. The straightening force is between 800KN and 1100KN. Subsequently, insulation box heating is applied, with the steel plate entering the box at a speed of 0.3-0.5m / s and a temperature of 650-660℃. Laminar flow cooling is then employed, with an initial cooling temperature range of 640-650℃ and a final cooling temperature range of 405-425℃. The number of controlled cooling manifold groups is 3-6, and the water flow rate per manifold is 155-180m³. 3 The cooling rate is controlled at 10~15℃ / s. After the steel plate exits controlled cooling, the side spray and air purging are activated. The side spray pressure and water flow rate are 5~10MPa and 50~100m³ / h, respectively.3 / h, the air purging pressure is 5~10MPa.

[0037] 5) Air cool to room temperature.

[0038] Examples 1-6 Table 1 shows the chemical composition of the steel in the examples; Table 2 shows the smelting and stacking process of the steel in the examples; Table 3 shows the heating process of the billet and the high-pressure water descaling process before rolling of the continuously cast billet of the steel in the examples; Table 4 shows the rolling process of the steel in the examples; Table 5 shows the controlled cooling process of the steel in the examples; and Table 6 shows the properties of the steel plates in the examples.

[0039] Table 1 Chemical composition (wt, %) of the examples

[0040] Note: Impurity elements in steel: P≤0.02%; S≤0.015%; O≤0.0050%.

[0041] Table 2. Smelting and stacking process regulations for steel in the examples.

[0042] Table 3 Heating regime of steel billets and high-pressure water descaling process before rolling of continuously cast billets in the examples

[0043] Table 4 Rolling and pre-straightening processes of the steel in the examples

[0044] Table 5. Process parameters for heat replenishment and cooling control of the steel insulation box in the embodiment.

[0045] Table 6. Steel Plate Properties in Examples

[0046] Therefore, compared with existing technologies, this invention reduces alloy costs through simple composition design and provides a method for controlling the low-temperature toughness stability of L450M steel plates for oil and gas pipelines with thicknesses of 9-13mm and pipe diameters of Φ1016mm or less, based on strain design, by regulating the processes of steelmaking, continuous casting, heating, controlled rolling, relaxation, heat treatment in the insulation box, and controlled cooling. This solves problems such as low steel plate rolling efficiency, high rolling resistance, large longitudinal temperature differences (head-middle-tail) in thin-gauge steel plates, poor uniformity of microstructure and properties, and difficulty in controlling plate shape. The resulting steel plate possesses low cost, excellent and stable low-temperature toughness, high resistance to plastic deformation, and can ensure the safety of gas pipelines.

[0047] The above embodiments are only for illustrating the technical concept and features of the present invention, and are intended to enable those skilled in the art to understand the content of the present invention and implement it accordingly. They should not be construed as limiting the scope of protection of the present invention. All equivalent changes or modifications made in accordance with the spirit and essence of the present invention should be covered within the scope of protection of the present invention.

Claims

1. A thin-gauge pipeline steel plate with low-temperature toughness stability control based on strain design, characterized in that, Its chemical composition by weight percentage is as follows: C 0.05%~0.08%, Si 0.15%~0.25%, Mn 1.48%~1.58%, P≤0.020%, S≤0.015%, Nb 0.02%~0.04%, Ti 0.008%~0.020%, Al 0.015%~0.04%, Cr 0.10%~0.20%, Mo 0.04%~0.09%, Ni 0.1%~0.12%, N 0.003%~0.006%, with the balance being Fe and unavoidable impurities.

2. The thin-gauge pipeline steel plate based on strain design for low-temperature toughness stability control according to claim 1, characterized in that, The thickness of the steel plate is 9–13 mm.

3. The thin-gauge pipeline steel plate based on strain design for low-temperature toughness stability control according to claim 1, characterized in that, The steel plate is used to manufacture gas transmission pipelines with a diameter of Φ1016mm or less.

4. The thin-gauge pipeline steel plate based on strain design for low-temperature toughness stability control according to claim 1, characterized in that, The steel plate has a transverse tensile yield strength of 450-520 MPa, a tensile strength of 560-620 MPa, a yield strength ratio of less than 0.85, an elongation of ≥25%, a transverse Charpy impact energy of ≥150 J at -40℃, a transverse shear area of ​​not less than 90% at each position from the head to the middle to the tail at -20℃ DWTT, and a longitudinal uniform elongation of ≥10%.

5. The method for manufacturing thin-gauge pipeline steel plates based on strain design for low-temperature toughness stability control as described in any one of claims 1-4, characterized in that, Includes the following steps: 1) Steelmaking and continuous casting: Molten iron undergoes pretreatment, converter smelting, LF refining, RH vacuum degassing, and continuous casting; the superheat of continuous casting is 8-13℃, and the continuous casting billet pulling speed is 0.8-1.1m / min; at the end of solidification, it is put into heavy pressure, and the reduction of the continuous casting billet is 16-20mm. After the billet is removed from the line, it is stacked and slowly cooled, with a stacking temperature of not less than 700℃ and a slow cooling time of not less than 48h. 2) Billet heating: The billet is sent into the heating furnace for heating. The billet passes through the preheating section, heating section and soaking section in sequence before exiting the furnace. The temperature of the preheating section is 750-900℃, the temperature of the heating section is 1190-1200℃, and the temperature of the soaking section is 1160-1175℃. The total time in the furnace in the heating section and soaking section is controlled at 2.0-3.0 hours to ensure that the temperature difference between the upper and lower surfaces of the billet is within 15℃. 3) High-pressure water descaling and controlled rolling: Before rolling, the billet after exiting the furnace is descaled with high-pressure water for 1-2 minutes, with the descaling machine pressure being 15-25 MPa; a transverse and longitudinal rolling mode is adopted. In the transverse rolling stage, the reduction rate of each pass in the first two rolling passes is greater than 25%; in the longitudinal rolling stage, the reduction rate of each pass in the first three rolling passes is greater than 20%, and the reduction rate of the last pass is less than 5%. The final rolling temperature of the steel plate is 780-830℃. 4) Relaxation + Insulation Box Heating + Cooling: After rolling, the steel plate relaxes and waits at a temperature of 680-690℃. Pre-straightening is then initiated, with the inlet roll position at -1.0mm to -1.5mm and the outlet roll position at -2.0mm to -2.5mm. The straightening force is between 800KN and 1100KN. The steel plate then enters an insulation box for reheating at a speed of 0.3-0.5m / s and a temperature of 650-660℃. Laminar flow cooling is then employed, with an initial cooling temperature of 640-650℃ and a final cooling temperature of 405-425℃. The cooling rate is controlled at 10-15℃ / s. After the steel plate exits controlled cooling, side spraying and air purging are activated, with side spray pressure and water flow rate of 5-10MPa and 50-100m³ / s, respectively. 3 / h, the air purging pressure is 5~10MPa; 5) Air cool to room temperature.

6. The manufacturing method 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 be below 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 between 0.05% and 0.08% at the end of converter smelting. Argon gas is blown for 20 to 30 minutes when tapping the steel. Then, LF refining and RH vacuum degassing are carried out, and the RH vacuum is maintained for more than 30 minutes. After that, continuous casting is carried out.

7. The manufacturing method according to claim 6, characterized in that, During the converter smelting process, the double slag method is used for phosphorus removal.

8. The manufacturing method according to claim 5, characterized in that, The thickness of the cast billet is 135–170 mm.

9. The manufacturing method according to claim 5, characterized in that, In step 4), the number of controlled cooling manifold groups opened is 3 to 6, and the water flow rate of a single manifold is 155 to 180 m³. 3 / h.

10. The manufacturing method according to claim 5, characterized in that, The steel plate is produced using cast billets on a medium-thickness reciprocating rolling mill, with water as the cooling medium.

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