Low yield ratio economic thin gauge l450m pipeline steel plate and method of manufacturing the same

Abstract

The present application relates to the technical field of low-carbon microalloyed steel production, and particularly relates to an economic low-yield-ratio thin-gauge L450M pipeline steel plate and a manufacturing method thereof, wherein the chemical composition of the steel plate is as follows in terms of percentage by weight: C: 0.05% to 0.07%, Si: 0.10% to 0.25%, Mn: 1.50% to 1.65%, P: less than or equal to 0.020%, S: less than or equal to 0.015%, Nb: 0.02% to 0.04%, Ti: 0.008% to 0.020%, Al: 0.015% to 0.04%, Cr: 0.25% to 0.30%, N: 0.003% to 0.006%, CE II W = C + Mn / 6 + (Cr + V + Mo) / 5 + (Cu + Ni) / 15: 0.36% to 0.40%, the balance being Fe and inevitable impurities, and the total amount of the impurity elements is less than 0.05%; the production process comprises the following steps: smelting of molten steel, off-furnace refining and degassing, continuous casting, heating of the cast blank, controlled rolling, relaxation, controlled cooling, and air cooling to room temperature; the present application is used for manufacturing the L450M hot-rolled steel plate with a thickness of 9 to 17.5 mm and a pipe diameter of less than or equal to 914 mm, and the problems of low rolling efficiency, large rolling resistance and difficult control of the plate shape are solved, and the steel plate has low cost, high strength and toughness, low yield ratio and high flatness.

Description

Technical Field

[0001] This invention relates to the field of low-carbon microalloyed steel production technology, and in particular to an economical thin-gauge L450M pipeline steel plate with low yield strength ratio 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, toughness, and low yield strength ratio 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 gauge (9-17.5mm) and smaller diameter (Φ914mm and below) L450M steel grades. When pipelines are located in extremely cold or earthquake-prone areas, higher requirements are placed on the low-temperature toughness, yield strength ratio, and other indicators of raw materials from the perspective of operational stability and safety. For example, the required testing temperatures for low-temperature drop hammer tests and ductile-brittle transition temperatures are lower, and the required Charpy impact energy values ​​are higher. Therefore, these projects have put forward higher personalized requirements, and simply meeting the API-SPEC-5L specification is far from sufficient. Currently, to ensure the low-temperature toughness and yield strength ratio of steel plates, a relatively thick intermediate billet is typically maintained for heating, and a two-stage or even three-stage controlled rolling process is used. This results in the final rolling temperature of the steel plate potentially being controlled below 820℃, or even lower, significantly increasing the rolling difficulty. Firstly, mill load, steel plate temperature uniformity, and plate shape control all face severe challenges. Secondly, improving the performance of the steel plate mainly depends 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 economical, thin-gauge L450M pipeline steel, while reducing the alloy cost of pipeline steel and ensuring its excellent strength, toughness, and low yield strength ratio, is key to the development of thin-gauge, low-yield-strength-ratio, economical L450M pipeline steel.

[0003] Compared with existing technologies:

[0004] To date, there have been very few reports, both domestically and internationally, on the production of economical, thin-gauge L450M pipeline steel. Prior to this invention, patent application CN201010243241.0 disclosed an X65 pipeline steel and its production method. The weight percentage of the components in 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%, Als: 0.005%~0.060%. However, this steel has a high V content, and the process uses natural air cooling, which is inefficient and cannot fully utilize the water-displacement alloy to reduce costs and improve strength and toughness. In addition, the specific strength and toughness of the actual product are not clearly defined for this steel.

[0005] Patent application CN201110179945.0 discloses an X65 pipeline steel with excellent low-temperature toughness and its manufacturing method. The weight percentage of the components in this patent 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%. However, this steel contains V, resulting in high alloy cost. The laminar flow cooling process cannot fully utilize the water-substitution alloy to reduce costs and improve strength and toughness.

[0006] Patent application number KR20020027013(A) discloses an API-X65 pipeline pipe with good aging performance and its manufacturing method. The weight percentage of the components in this patent is 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%, Als: 0.015%~0.050%. However, this steel contains V and Mo, resulting in high cost and insufficient low-temperature toughness of the actual product.

[0007] Patent JP2005194607 discloses a high-strength steel plate with excellent crack-arresting toughness and its manufacturing method. The weight percentage of the components in this patent is C: 0.02%~0.08%, Si: 0.01%~0.5%, Mn: 0.5%~1.8%, P≤0.01%, S≤0.002%, Nb: 0.005%~0.05%, Mo: 0.005%~0.5%, Ti: 0.005%~0.04%, N: 0.002%~0.007%, Al≤0.07%, Mg: 0.0005%~0.005%. However, this steel contains Mo and Mg, resulting in high cost.

[0008] Although the steels disclosed in the above patent documents have achieved high strength and toughness, they are either coils or have high production costs and low rolling efficiency, and are therefore not suitable for producing low-cost, thin-gauge, high-straightness L450M pipeline steel plates, etc. Summary of the Invention

[0009] This invention provides a low yield strength ratio, economical thin-gauge L450M pipeline steel plate and its manufacturing method. It is used to manufacture high-strength L450M hot-rolled steel plates with a thickness of 9-17.5mm and a pipe diameter of Φ914mm and below for oil pipelines with a low yield strength ratio. It solves the problems of low rolling efficiency, high rolling resistance and difficulty in controlling plate shape. The steel plate has low cost, high strength and toughness, low yield strength ratio and high straightness, which can ensure the service safety of oil pipelines.

[0010] To achieve the above objectives, the present invention employs the following technical solution:

[0011] A low yield strength ratio, economical, thin-gauge L450M pipeline steel plate, with the following chemical composition by weight percentage: C: 0.05%–0.07%, Si: 0.10%–0.25%, Mn: 1.50%–1.65%, P≤0.020%, S≤0.015%, Nb: 0.02%–0.04%, Ti: 0.008%–0.020%, Al: 0.015%–0.04%, Cr: 0.25%–0.30%, N: 0.003%–0.006%, CE II W = C + Mn / 6 + (Cr + V + Mo) / 5 + (Cu + Ni) / 15: 0.36% to 0.40%, with the balance being Fe and unavoidable impurities, and the total amount of impurity elements being less than 0.05%.

[0012] Furthermore, the finished thickness of the L450M pipeline steel plate is 9 to 17.5 mm, and it is used to manufacture oil pipelines with a diameter of Φ914 mm and below.

[0013] Furthermore, the mechanical properties of the finished steel plate are as follows: the yield strength in the transverse tensile test is between 470 and 510 MPa, the tensile strength is between 580 and 635 MPa, the strength range is narrow, the yield strength ratio is less than 0.8, the elongation is ≥25%, the transverse Charpy impact energy at -35℃ is ≥260 J, and the transverse shear area SA at -20℃ DWTT is not less than 90%.

[0014] A method for manufacturing a low-yield-strength-ratio, economical, thin-gauge L450M pipeline steel plate, comprising the following production processes: steel smelting → ladle refining and degassing → continuous casting → billet heating → controlled rolling → relaxation → controlled cooling → air cooling to room temperature.

[0015] 1) Steel smelting: Smelting is carried out according to composition. The raw materials are pretreated with KR hot metal to control the S content to be less than 0.015%. After slag removal, the raw materials are fed into the converter. In the converter smelting, the double slag method is used to remove P to control the P content to ≤0.02%. At the end of the converter smelting, the C content is controlled to be 0.05-0.07%. Argon gas is blown for 10-15 minutes when tapping the steel.

[0016] 2) Ladle refining and degassing: Perform LF refining and RH vacuum degassing, maintaining RH vacuum for more than 20 minutes;

[0017] 3) Continuous casting: Slabs are continuously cast with a superheat of 8-14℃ and a casting speed of 0.7-1.0m / min. In the horizontal sector section, i.e. the end of solidification, heavy pressure is applied to reduce the continuous casting billet by 10-20mm. After the billets are removed from the line, they are stacked and cooled slowly with a stacking temperature of not less than 700℃ and a cooling time of not less than 48h.

[0018] 4) 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 900-1150℃, the temperature range of the heating section is 1220-1240℃, and the temperature range of the soaking section is 1200-1215℃. The time spent in the heating and soaking sections in the furnace is not less than 4.5-5.5 hours.

[0019] 5) Control rolling and relaxation: Before rolling, use high-pressure water to descale the billet after it exits the furnace for 1-2 minutes, with a descaling machine pressure of 20-25 MPa; rolling is done in two stages: the first stage is recrystallization rolling, i.e. rough rolling, with an initial rolling temperature range of 1150-1180℃ and a final rough rolling temperature range of 990-1050℃. There are no more than seven rough rolling passes, and the reduction ratio is guaranteed to be above 20% in two of the first four passes. The thickness of the intermediate billet is 1.5-2.0 times that of the finished product.

[0020] The second stage is non-recrystallization rolling, i.e., finishing rolling. The initial rolling temperature range is 960–990℃, and the final rolling temperature range is 860–900℃. There are no more than four finishing rolling passes, and the reduction ratio of the first two passes is guaranteed to be above 20%. The rolling speed is increased, and the steel is quickly ejected after rolling at a speed of 4–6.5 m / s. Pre-straightening is also introduced. After rolling, the steel plate is allowed to relax and wait for the temperature to rise. It is oscillated on the roller table to ensure that the proportion of polygonal ferrite in the steel plate is not less than 40% before the steel plate enters the water.

[0021] 6) Controlled cooling: Laminar flow cooling is adopted, with an initial cooling temperature range of 710–730℃ and a final cooling temperature range of 470–510℃. The cooling rate is controlled at 30–40℃ / 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 2–5MPa and 50–100m³ / s, respectively. 3 / h, the air purging pressure is 5~15MPa, the hot straightening is followed by three straightening processes, the inlet roller position is -1.5mm~-2.5mm, the outlet roller position is -2.8mm~-3.6mm, and then it is air cooled to room temperature.

[0022] Furthermore, the thickness of the continuously cast billet is 150-200 mm, and it is rolled on a medium-thick plate reciprocating rolling mill.

[0023] Compared with the prior art, the beneficial effects of the present invention are:

[0024] 1) After deep desulfurization pretreatment of KR hot metal, the slag is thoroughly removed. The converter uses a double-slag method for P removal, resulting in lower P and S content in the billet. By controlling the argon blowing time and maintaining the RH vacuum degassing time, defects such as central segregation, inclusions, and excessive H and O content in the billet caused by high Mn, Cr, and C content are overcome, which is beneficial to improving the plasticity and toughness of pipeline steel. Reducing superheat, lowering the continuous casting speed, and placing it under heavy pressure can improve the macroscopic segregation of the continuous casting billet, reduce the spacing of secondary dendrite arms in the solidification structure of the continuous casting billet, help reduce billet segregation, reduce internal structural defects, refine austenite grains, and reduce the width of ferrite strips in the banded structure after rolling. At the same time, the billets are stacked and slowly cooled after being taken off the line, and the stacking temperature and slow cooling time are limited, which helps to promote the diffusion of Mn, Cr, and H elements and reduce their impact on the microstructure and properties due to compositional segregation.

[0025] 2) The billet is heated by limiting the temperature and time of the billet in the preheating, heating and soaking sections. This ensures that the carbides and nitrides of Nb, Cr and Ti are rapidly and fully dissolved in the matrix and diffused. At the same time, it promotes the diffusion of alloying elements Mn and Cr, reducing their influence on the microstructure and properties due to compositional segregation. The heating and soaking time in the furnace is also limited to promote the appropriate growth of the original austenite grains, reduce the contribution of fine grain strengthening to the strength of the steel plate, increase the contribution of solid solution strengthening, and ensure the yield strength ratio of the steel plate.

[0026] 3) The composition of this invention is reasonable and the amount of alloy added is low. By replacing Mo with Cr and adding a small amount of micro-alloying element Nb, the cost of alloy and the resistance to high-temperature deformation in the roughing and finishing stages are greatly reduced, which is conducive to increasing the reduction in each pass and ensuring the comprehensive performance of the super steel plate.

[0027] 4) A two-stage controlled rolling process is adopted to control the reduction in roughing and finishing passes, optimize the thickness of intermediate slabs, increase the starting temperature of finishing rolling, reduce the thickness of intermediate slabs waiting to be heated, accelerate the temperature drop of intermediate slabs, and improve rolling efficiency. At the same time, the dislocation density in the second-stage rolling process is reduced, the final rolling temperature is controlled, and the proportion of large-angle grain boundaries is ensured to be above 35%, which reduces dislocation density, increases the degree of matrix recovery, and improves the low-temperature toughness of the steel plate. Furthermore, relaxation waiting is adopted after rolling to ensure that the proportion of polygonal ferrite in the steel plate is not less than 40% before the steel plate is immersed in water, thereby reducing the yield strength ratio. A high cooling rate is adopted to control the final cooling temperature of the steel plate at 470-510℃, ensuring that there is a certain amount of granular bainite (20%-30%) in the microstructure to ensure its strength. Ultimately, the yield strength ratio and other properties of the steel plate meet the technical requirements. The use of side spraying, air blowing and hot straightening is conducive to the control of the steel plate shape, improves the uniformity of steel plate performance, reduces the probability of head and tail shape problems, and saves the investment cost of subsequent cold straightening equipment.

[0028] 5) This invention reduces alloy costs through simple composition design and obtains a low yield strength ratio economical L450M pipeline steel plate for oil pipelines with a thickness specification (9-17.5mm) and a pipe diameter of Φ914mm and below by controlling the steelmaking, continuous casting, and controlled rolling and cooling processes. The microstructure is a multiphase structure mainly composed of granular bainite and polygonal ferrite, with the proportion of polygonal ferrite not less than 40% and granular bainite 20%-30%. It has good low-temperature toughness and high straightness. The specific properties are: the yield strength in the transverse tensile test is between 470-510MPa, the tensile strength is between 580-635MPa, the strength range is narrow, the yield strength ratio is less than 0.8, the elongation is ≥25%, the transverse Charpy impact energy at -35℃ is ≥260J, and the transverse shear area SA at -20℃ is not less than 90%. Detailed Implementation

[0029] The specific embodiments of the present invention will be further described below:

[0030] This invention discloses an economical, thin-gauge L450M pipeline steel plate with a low yield strength ratio and its manufacturing method. The finished product thickness is 9–17.5 mm. It is produced using a 150–200 mm thick continuously cast billet on a medium-thick plate reciprocating rolling mill, with water as the cooling medium. It is used to manufacture high-strength L450M hot-rolled steel plates for oil pipelines with a diameter of Φ914 mm and below. The chemical composition by weight percentage is: C: 0.05%–0.07%, Si: 0.10%–0.25%, Mn: 1.50%–1.65%, P≤0.020%, S≤0.015%, Nb: 0.02%–0.04%, Ti: 0.008%–0.020%, Al: 0.015%–0.04%, Cr: 0.25%–0.30%, N: 0.003%–0.006%, CE… II W = C + Mn / 6 + (Cr + V + Mo) / 5 + (Cu + Ni) / 15: 0.36% to 0.40%, with the balance being Fe and unavoidable impurities, and the total amount of impurity elements being less than 0.05%.

[0031] The roles of the main elements in the chemical composition of the steel plate of this invention are as follows:

[0032] 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, the present invention sets the C content range to 0.05% to 0.07%.

[0033] Mn: It can improve the strength of steel through solid solution strengthening, while compensating for the strength loss of steel plate caused by the reduction of C content. In addition, it can lower the γ-α phase transformation temperature, thereby refining the ferrite grains and helping to obtain fine low-temperature phase transformation products, thus improving its toughness. However, increasing the Mn content will aggravate the center segregation of the continuously cast billet, which is not conducive to improving the low-temperature toughness of the steel plate, and cannot guarantee the uniformity of the cross-sectional structure of the steel plate. Therefore, the Mn content range of this invention is designed to be 1.50% to 1.65%.

[0034] Si plays a role in deoxidation in steelmaking and improving the strength of the matrix. However, excessive Si will reduce the toughness of the heat-affected zone of the base material. Increasing the Si content can purify ferrite and reduce the content of pearlite, which is beneficial to reducing the Bauschinger effect of the matrix material. Therefore, the Si content is set to 0.10% to 0.25% in this invention.

[0035] Nitrogen (Nb) is one of the commonly used elements in modern microalloyed pipeline steel. It has good grain refinement and precipitation strengthening effects and can also delay austenite recrystallization. However, excessive Nb will increase production costs and make continuous casting process control more difficult. This invention selects an Nb content range of 0.02% to 0.04% and, with a reasonable TMCP process, can obtain a uniform composite phase mainly composed of pearlite, polygonal ferrite and acicular ferrite, which gives it good toughness.

[0036] 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%.

[0037] Ti is a strong solid nitrogen element, existing in the form of TiN in continuously cast billets. Fine TiN particles can effectively inhibit the growth of austenite grains during reheating of continuously cast billets and help improve the solid solubility of Nb in austenite, thereby improving the impact toughness of the weld heat-affected zone. When the amount of Ti added exceeds a certain value, the TiN particles will coarsen, increasing the stress concentration level between the particle interface and the matrix. Therefore, the present invention selects a Ti content range of 0.008% to 0.02%.

[0038] Al: It is usually used as a deoxidizer in steel. If AlN is formed, it can also refine the microstructure. When the Al content exceeds 0.04%, the excessive alumina inclusions will reduce the cleanliness of the steel. If the Al content is too low, the deoxidation will be insufficient, and easily oxidized elements such as Ti will form oxides. Therefore, the lower limit of Al content is set at 0.015%.

[0039] Cr: It is a major element that can effectively improve hardenability, inhibit ferrite formation and promote bainite formation. It plays an important role in controlling phase transformation structure, 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, and improves the strength, plasticity and toughness of steel plate. The Cr content range selected in this invention is 0.25% to 0.30%.

[0040] CE II W: To ensure the weldability of the product, the present invention CE II The W content is 0.36% to 0.40%.

[0041] P and S are unavoidable impurity elements in steel, and the lower the better. However, due to considerations of smelting costs and processes, they cannot be kept indefinitely low. Therefore, this invention sets the upper limit of P and S content to 0.020% and 0.015%, respectively.

[0042] This invention discloses a method for manufacturing a low yield strength ratio, economical, thin-gauge L450M pipeline steel plate. The production process includes steel smelting → ladle refining and degassing → continuous casting → billet heating → controlled rolling → relaxation → controlled cooling → air cooling to room temperature.

[0043] 1) Steel smelting to continuous casting: Smelting according to the following composition, with the following chemical composition by weight percentage: C: 0.05%~0.07%, Si: 0.10%~0.25%, Mn: 1.50%~1.65%, P≤0.020%, S≤0.015%, Nb: 0.02%~0.04%, Ti: 0.008%~0.020%, Al: 0.015%~0.04%, Cr: 0.25%~0.30%, N: 0.003%~0.006%, CE II W = C + Mn / 6 + (Cr + V + Mo) / 5 + (Cu + Ni) / 15: 0.36%~0.40%, with the balance being Fe and unavoidable impurities, and the total amount of impurity elements being less than 0.05%; the raw material is pretreated with KR hot metal to control the S content to be less than 0.015%, and then enters the converter after slag removal; during converter smelting, the double slag method is used to remove P, controlling the P content to be ≤0.02%, and the C content is controlled at 0.05~0.07% at the end of converter smelting, and argon gas is blown for 10~15 minutes during tapping; Next, LF refining and RH vacuum degassing are carried out, with RH vacuum maintained for more than 20 minutes. Then, slab continuous casting is performed with a superheat of 8-14℃ and a casting speed of 0.7-1.0 m / min. In the horizontal sector section, i.e. the end of solidification, heavy pressure is applied to the continuously cast slab, with a reduction of 10-20 mm. After the slab 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 48 hours. This promotes the diffusion of Mn, Cr, C, and H elements and reduces their influence on microstructure and properties due to component segregation.

[0044] 2) Billet Heating: A billet with a thickness of 150-200mm is fed into a walking beam furnace for heating. The billet passes through a preheating section, a heating section, and a soaking section before exiting the furnace. The temperature range of the preheating section is 900-1150℃, which promotes the rapid and complete solidification of Nb and Cr carbides and nitrides into the matrix and allows for sufficient diffusion. The temperature range of the heating section is 1220-1240℃, and the temperature range of the soaking section is 1200-1215℃. The time spent in the heating and soaking sections is not less than 4.5-5.5 hours, which further promotes the diffusion of alloying elements Mn, Cr, and H, reduces their influence on the microstructure and properties due to compositional segregation, and promotes the appropriate growth of the original austenite grains, reduces the contribution of fine grain strengthening to the strength of the steel plate, increases the contribution of solid solution strengthening, and ensures the yield strength ratio of the steel plate.

[0045] 3) Control rolling to air cooling to room temperature: Before rolling, use high-pressure water to descale the billet after it comes out of the furnace for 1 to 2 minutes, with a descaling machine pressure of 20 to 25 MPa;

[0046] The rolling process is divided into two stages: the first stage is recrystallization rolling (rough rolling), with an initial rolling temperature range of 1150–1180℃ and a final rough rolling temperature range of 990–1050℃. The rough rolling process consists of no more than seven passes, with the first four passes ensuring a reduction rate of over 20%. The resulting intermediate billet thickness is 1.5–2.0 times the finished product thickness. This reduces the intermediate billet's waiting-to-heat thickness, accelerates its temperature drop, improves rolling efficiency, and simultaneously reduces the dislocation density during the second-stage rolling process. The first stage contributes to the strength of the steel plate and ensures the yield strength ratio. The second stage is non-recrystallization rolling (finish rolling). The initial rolling temperature range is 960-990℃, and the final rolling temperature range is 860-900℃. There are no more than four finish rolling passes. The reduction ratio of the first two passes is guaranteed to be above 20% to ensure the core rolling penetration and promote the dynamic recrystallization of austenite grains. The speed-increasing rolling, through two-stage rolling, refines the grain structure at different stages and has a certain degree of pre-deformation to reduce phase density. The modified grain size and controlled final rolling temperature ensured that the proportion of large-angle grain boundaries was above 35%, reducing dislocation density, increasing matrix recovery, improving the low-temperature toughness of the steel plate, and lowering the yield strength ratio. Rapid steel blasting was performed after rolling at a speed of 4–6.5 m / s. The rolled steel plate underwent relaxation and temperature control, oscillating on the roller conveyor (ensuring that the proportion of polygonal ferrite before immersion in water was not less than 40%). Simultaneously, the immersion temperature was controlled to suppress the precipitation of proferrite. Laminar flow cooling was then employed. The cooling temperature range is 710–730℃, and the final cooling temperature range is 470–510℃. The cooling rate is controlled at 30–40℃ / s. A high cooling rate is adopted, and the final cooling temperature of the steel plate is controlled at 470–510℃ to ensure that a certain amount of granular bainite (20%–30%) exists in the microstructure to guarantee its strength. Ultimately, the yield strength ratio and other properties of the steel plate meet the technical requirements. After the steel plate exits controlled cooling, the side spraying and air purging are started. The side spraying pressure and water volume are 2–5MPa and 50–100m³ / h, respectively. 3 / h, the air purging pressure is 5~15MPa, the hot straightening is followed by three straightening processes, the inlet roller position is -1.5mm~-2.5mm, the outlet roller position is -2.8mm~-3.6mm, and then it is air cooled to room temperature.

[0047] By adopting the above-mentioned composition and steelmaking continuous casting, controlled rolling and controlled cooling scheme, the shortcomings of the existing technology have been overcome, and a high-strength L450M hot-rolled steel plate for manufacturing low yield strength ratio economical oil pipelines with a thickness of 9-17.5mm and a pipe diameter of Φ914mm and below has been realized. The microstructure of the steel plate is a multiphase structure mainly composed of granular bainite and polygonal ferrite, of which the proportion of polygonal ferrite is not less than 40% and granular bainite is 20%-30%. This steel plate has low cost, high strength and toughness, low yield strength ratio and high straightness, which can ensure the service safety of oil pipelines.

[0048] The following embodiments are implemented based on the technical solution of the present invention, providing detailed implementation methods and specific operation processes. However, the scope of protection of the present invention is not limited to the following embodiments. Unless otherwise specified, the methods used in the following embodiments are conventional methods.

[0049] Example:

[0050] 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 billet heating process and high-pressure water descaling process before rolling of the continuously cast billet in the examples; Table 4 shows the rolling parameters of the steel in the examples; Table 5 shows the main process parameters of controlled cooling of the steel in the examples; Table 6 shows the performance indicators of the steel plates in the examples.

[0051] Table 1: Chemical composition (wt, %) of embodiments of the present invention

[0052]

[0053] Note: Impurity elements in steel: P≤0.02%; S≤0.015%; O≤0.0050%; total amount of other impurity elements is less than 0.05%.

[0054] Table 2: Smelting and Stacking Process Regulations for Steel in Examples

[0055]

[0056] Table 3: Heating regime of billet and high-pressure water descaling process before rolling of continuous casting billet for the example steel

[0057]

[0058] Table 4: Rolling parameters of the steel in the examples

[0059]

[0060] Table 5: Main process parameters for controlled cooling of steel in embodiments of the present invention

[0061]

[0062] Table 6: Performance Indicators of Steel Plates in Embodiments of the Invention

[0063]

[0064] Therefore, compared with the existing technology, the composition design and steelmaking continuous casting, heating and controlled rolling and cooling schemes of this invention overcome the shortcomings of the existing technology, and realize the production and application of a high-strength L450M hot-rolled steel plate for manufacturing low yield strength ratio economic oil pipelines with a thickness of 9-17.5mm and a pipe diameter of Φ914mm and below. The microstructure of the steel plate is a multiphase structure mainly composed of granular bainite and polygonal ferrite, wherein the proportion of polygonal ferrite is not less than 40%, granular bainite is 20%-30%, and large-angle grain boundaries account for more than 35%. This steel plate has low cost, high strength and toughness, low yield strength ratio and high straightness, which can ensure the service safety of oil pipelines.

[0065] 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 low yield ratio economical thin gauge L450M pipeline steel plate characterized in that, The chemical composition in the steel is as follows in percentage by weight: C: 0.05%~0.07%, Si: 0.10%~0.25%, Mn: 1.50%~1.65%, P≤0.020%, S≤0.015%, Nb: 0.02%~0.04%, Ti: 0.008%~0.020%, Al: 0.015%~0.04%, Cr: 0.25%~0.30%, N: 0.003%~0.006%, CE II W=C+Mn / 6+(Cr+V+Mo) / 5+(Cu+Ni) / 15: 0.36%~0.40%, the balance being Fe and unavoidable impurities, and the total amount of impurity elements being less than 0.05%; The finished thickness of the L450M pipeline steel plate is 9-17.5 mm, and is used for manufacturing oil pipelines with a pipe diameter of Φ914 mm and below; The mechanical properties of the finished steel plate are as follows: the yield strength of transverse tension is between 470-510 MPa, the tensile strength is between 580-635 MPa, the strength range is narrow, the yield strength ratio is less than 0.8, the elongation is greater than or equal to 25%, the Charpy impact energy at-35 ℃ is greater than or equal to 260 J, and the DWTT transverse shear area at-20 ℃ is not less than 90%. The production process of the steel plate includes molten steel smelting, external furnace refining and degassing, continuous casting, casting blank heating, controlled rolling, relaxation, controlled cooling and air cooling to room temperature, wherein, 1) Molten steel smelting: smelting according to the composition, the raw material is subjected to KR molten iron pretreatment, the content of S is controlled to be less than 0.015%, and after slagging, the molten steel is fed into a converter; a double-slag method is used for P removal in converter smelting, the content of P is controlled to be less than or equal to 0.02%, the content of C at the end of converter smelting is controlled to be 0.05-0.07%, and argon is blown for 10-15 min when the molten steel is tapped; 2) External furnace refining and degassing: LF refining and RH vacuum degassing are performed, and the RH vacuum is maintained for more than 20 min; 3) Continuous casting: slab continuous casting is performed, the continuous casting superheat is 8-14 ℃, and the continuous casting withdrawal rate is 0.7-1.0 m / min; heavy pressing is performed at the horizontal fan-shaped section, i.e. the solidification end, the continuous casting blank pressing amount is 10-20 mm, and the casting blank is subjected to stack slow cooling after being withdrawn, the stack temperature is not less than 700 ℃, and the slow cooling time is not less than 48 h; 4) Casting blank heating: the casting blank is sent into a walking beam heating furnace for heating, and the casting blank is sequentially discharged after passing through a preheating section, a heating section and a soaking section; the preheating section temperature range is 900-1150 ℃, the heating section temperature range is 1220-1240 ℃, the soaking section temperature range is 1200-1215 ℃, and the heating and soaking section time in the furnace is 4.5-5.5 h; 5) Controlled rolling and relaxation: the casting blank is descaled after being discharged from the furnace by using high-pressure water for 1-2 min, and the descaling machine pressure is 20-25 MPa; rolling is performed in two stages: the first stage is recrystallization rolling, i.e. rough rolling, the rough rolling temperature range is 1150-1180 ℃, the rough rolling final rolling temperature range is 990-1050 ℃, the rough rolling is not more than seven passes, and the pass reduction system is that the first four passes ensure that the reduction rate is more than 20%, and the thickness of the obtained intermediate blank is 1.5-2.0 times the finished thickness; the second stage is non-recrystallization rolling, i.e. finish rolling, the finish rolling temperature range is 960-990 ℃, the finish rolling final rolling temperature range is 860-900 ℃, the finish rolling is not more than four passes, and the pass reduction system is that the first two passes ensure that the reduction rate is more than 20%; speed rolling is performed, the steel is quickly thrown after rolling, the throwing speed is 4-6.5 m / s; the steel plate is relaxed after rolling, and the steel plate is ensured to have a polygonal ferrite proportion of not less than 40% before entering water by roller oscillation; 6) Control cooling: laminar cooling is adopted, the open cooling temperature range is 710-730℃, the final cooling temperature range is 470-510℃, the control cooling speed is 30-40℃ / s, the side spray is opened and the air blowing is opened after the steel plate is out of the control cooling, the side spray pressure and water volume are 2-5MPa, 50-100m 3 / h, the air blowing pressure is 5-15MPa, three straightening is carried out again, the leading-in roller position is -1.5mm--2.5mm, the leading-out roller position is -2.8mm--3.6mm, and then air cooling is carried out to room temperature; the structure is mainly a complex phase structure of granular bainite and polygonal ferrite, wherein the polygonal ferrite proportion is not less than 40%, and the granular bainite is 20%-30%.

2. The low yield ratio economic thin-gauge L450M pipe line steel plate according to claim 1, characterized by, the continuous casting blank thickness is 150-200 mm, and the rolling is performed on a medium plate reciprocating mill.

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