Economical thin gauge l450m pipeline steel and method of controlling banded structure

Abstract

The present application relates to the technical field of low-carbon microalloyed steel production, and particularly relates to an economic thin-gauge L450M pipeline steel and a method for controlling banded structure, wherein the chemical composition of the steel is as follows in terms of percentage by weight: C: 0.07% to 0.09%, Si: 0.10% to 0.25%, Mn: 1.70% to 1.80%, P: less than or equal to 0.020%, S: less than or equal to 0.015%, Nb: 0.01% to 0.02%, Ti: 0.008% to 0.015%, Al: 0.015% to 0.04%, Cr: 0.05% to 0.15%, N: 0.003% to 0.006%, and the balance of Fe and inevitable impurities, and the total amount of impurity elements is less than 0.05%. The steel plate produced by the present application has low cost, high strength and toughness, low yield ratio and high flatness, and can ensure the service safety of gas pipelines.

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 and a method for controlling banded structure. 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 (9-15mm) and smaller diameter (Φ914mm and below) L450M steel grades. When pipelines are located in extremely cold regions, from the perspective of operational stability and safety, higher requirements are placed on the low-temperature toughness and other indicators of raw materials. 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 is 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 of steel plates, higher requirements are placed on banded microstructure. The presence of banded microstructure can severely reduce the low-temperature toughness and crack arrest performance of steel plates (ferrite and pearlite are distributed in bands along the rolling direction at different parts of the plate thickness section, and the banding level is greater than 3.5 or even up to 4. During the external deformation process, stress concentration is easily generated here, inducing cracks on the lower surface with the greatest tensile stress). At present, in order to ensure the low-temperature toughness of economical pipeline steel, the intermediate billet is usually kept thicker for heating, and a two-stage or even three-stage controlled rolling process is adopted to control the banded microstructure of pipeline steel. This results in the final rolling temperature of the steel plate being controlled below 820℃, or even lower, which greatly increases the rolling difficulty. First, the mill load, steel plate temperature uniformity, and plate shape control all face severe challenges. Second, the improvement of steel plate performance mainly depends 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 in the smelting and rolling processes. Therefore, the key to the development of thin-gauge, economical L450M pipeline steel lies in solving the problems mentioned above in the production of economical thin-gauge L450M pipeline steel, controlling the banded microstructure grade of economical thin-gauge L450M pipeline steel, reducing the alloy cost of pipeline steel, and ensuring its excellent strength and toughness.

[0003] Compared with existing technologies:

[0004] 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] Application number 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%. 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] Application No. 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 product.

[0007] JP2005194607 discloses a high-strength steel plate with excellent crack arrest 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, and the method for controlling banded structure is not clearly defined.

[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. Furthermore, they do not provide a clear method for controlling the banded structure of thin-gauge pipeline steel. Therefore, they are not suitable for producing low-cost, thin-gauge, high-straightness L450M pipeline steel plates with excellent low-temperature toughness. Summary of the Invention

[0009] This invention provides an economical thin-gauge L450M pipeline steel and a method for controlling the banded structure. The method is used to control the banded structure of economical thin-gauge L450M pipeline steel with a thickness of 9-15mm and a pipe diameter of Φ914mm and below for gas transmission pipelines. The steel plate has low cost, high strength and toughness, low yield strength ratio, and high straightness, which can ensure the service safety of gas transmission pipelines.

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

[0011] An economical, thin-gauge L450M pipeline steel has the following chemical composition by weight percentage: C: 0.07%–0.09%, Si: 0.10%–0.25%, Mn: 1.70%–1.80%, P≤0.020%, S≤0.015%, Nb: 0.01%–0.02%, Ti: 0.008%–0.015%, Al: 0.015%–0.04%, Cr: 0.05%–0.15%, N: 0.003%–0.006%, with the balance being Fe and unavoidable impurities, and the total amount of impurity elements being less than 0.05%.

[0012] A method for controlling the banded microstructure of economical, thin-gauge L450M pipeline steel, the method comprising: steelmaking → ladle refining and degassing → continuous casting → billet heating → controlled rolling → controlled cooling → air cooling to room temperature, as detailed below:

[0013] 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 0.07-0.09%. Argon gas is blown for 15-20 minutes when tapping the steel.

[0014] 2) Ladle refining and degassing: Perform LF refining and RH vacuum degassing, maintaining RH vacuum for more than 30 minutes;

[0015] 3) Continuous casting: Slabs are continuously cast, with a superheat of 8-13℃ and a casting speed of 0.6-0.8 m / min.

[0016] Electromagnetic stirring is used in the secondary cooling zone of the continuous casting stage. The stirring method is alternating forward and reverse stirring, with forward stirring time of 10-20s and reverse stirring time of 5-10s. The current is 1000-1500A and the frequency is 10-20Hz. This allows the molten steel to be continuously cast into a billet, and the isometric crystal ratio of the billet is not less than 80%. At the same time, strong cooling is used in the fan-shaped section. The total cooling water volume of the first to fourth sections is 1200-1500L / min, and the total cooling water volume of the fifth to eighth sections is 1100-1300L / min. At the end of solidification, light reduction is applied, and the reduction of the billet is 8-12mm. After the billet is removed from the line, it is stacked for slow cooling. The stacking temperature is not lower than 850℃ and the slow cooling time is not less than 36h.

[0017] 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 1240-1260℃, and the temperature range of the soaking section is 1210-1225℃. The time spent in the heating and soaking sections in the furnace is not less than 4.5-5.5 hours.

[0018] 5) Controlled rolling: 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 carried out 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℃. The rough rolling should not exceed seven passes, and the reduction ratio of the first four passes should be guaranteed to be above 30% in two of them. The thickness of the intermediate billet is 1.5-1.8 times that of the finished product.

[0019] The second stage is non-recrystallization rolling, i.e., finishing rolling. The initial rolling temperature range is 970-1010℃, and the final rolling temperature range is 850-880℃. There are no more than four finishing rolling passes. The reduction system for each pass is to ensure a reduction rate of more than 20% in the first two passes. The rolling speed is increased, and the steel is quickly ejected after rolling at a speed of 4-6.5m / s. Pre-straightening is not used.

[0020] 6) Controlled Cooling: A combination of ultra-rapid cooling and laminar flow cooling is employed. The initial cooling temperature range is 770–790℃, with a cooling rate of 30–45℃ / s during the ultra-rapid cooling stage. After cooling to below 650℃, laminar flow cooling is used, with a cooling rate of 5–15℃ / s during the laminar flow cooling stage. The final cooling temperature range is 550–570℃. After the steel plate exits controlled cooling, side spraying and air purging are activated. The side spraying pressure and water flow rate are 2–5MPa and 50–100m³ / s, respectively. 3 / h, the air purging pressure is 10~20MPa, 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.

[0021] Furthermore, the finished thickness of the L450M pipeline steel is 9-15mm, and it is used to manufacture gas transmission pipelines with a diameter of Φ914mm and below.

[0022] Furthermore, the mechanical properties of the finished steel plate are as follows: the yield strength in the transverse tensile test is between 480 and 520 MPa, the tensile strength is between 570 and 630 MPa, the yield-to-tensile ratio is less than 0.88, the elongation is ≥25%, the transverse Charpy impact energy at -35℃ is ≥270 J, the transverse shear area (SA) at -20℃ (DWTT) is not less than 95%, and the banded structure is grade 2 or below.

[0023] Furthermore, the thickness of the continuously cast billet is 150-200 mm, and it is rolled on a medium-thick plate reciprocating rolling mill.

[0024] Compared with the prior art, the beneficial effects of the present invention are:

[0025] 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. 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 and Cr content are overcome. This improves the plasticity and toughness of pipeline steel, reduces superheat, lowers continuous casting speed, and improves macroscopic segregation in the continuously cast billet. It also reduces the spacing between secondary dendrite arms in the solidification structure of the billet, helping to reduce billet segregation, decrease internal structural defects, and refine austenite grains. Rolling can reduce the width of ferrite strips in the banded structure; by optimizing the electromagnetic stirring process in the continuous casting stage, the equiaxed crystal ratio of the continuous casting billet can be greatly improved; strong cooling is used to ensure the temperature gradient in the thickness direction of the billet; and light pressure can help reduce billet segregation, reduce internal structural defects, and promote core grain breakage, thus ensuring the strength and toughness of the subsequent steel plate; at the same time, the billet is stacked and slowly cooled after it comes 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.

[0026] 2) Heating the billet: The temperature and time of the billet in the preheating section, heating section and soaking section are limited to ensure that the carbides and nitrides of Nb and Cr are quickly and fully dissolved in the matrix and fully diffused. At the same time, the diffusion of alloying elements Mn, Cr and H is promoted, and the influence of their compositional segregation on the microstructure and properties is reduced.

[0027] 3) The composition of this invention is reasonable and the amount of alloy added is low. By replacing Mo with Mn and 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 of each pass and ensuring the comprehensive performance of the super steel plate.

[0028] 4) A two-stage controlled rolling process is adopted to control the reduction in roughing and finishing passes, optimize the intermediate billet thickness, shorten the intermediate billet warming time, ensure core rolling penetration, suppress the formation of wide banded structures in the core, and eliminate the need for post-rolling relaxation. Rapid post-rolling steel removal and pre-straightening without intervention prevent the precipitation of proferrite due to pre-straightening temperature drop, which could cause widening of the banded structure and affect the steel plate's toughness. The cooling rates at different stages of water immersion are also controlled to maintain the final cooling temperature of the steel plate at 550–500°C. At 70℃, the proportion of acicular ferrite is maintained at 30%–40%, and the proportion of polygonal ferrite and pearlite is maintained at 60%–70%. This avoids the formation of granular bainite in the microstructure and inhibits the diffusion and segregation of carbon elements, preventing the formation of banded structures. This ensures the low-temperature DWTT performance and yield strength ratio of the steel plate. By using side spraying, air blowing, and hot straightening, it is beneficial to control the shape of the steel plate, improve the uniformity of the steel plate performance, reduce the probability of problems with the shape of the top and bottom of the plate, and save on the investment cost of subsequent cold straightening equipment.

[0029] 5) This invention reduces alloy costs through simple composition design and obtains a method for controlling the banded structure of economical thin-gauge L450M pipeline steel for gas transmission pipelines with thicknesses of 9-15mm and diameters 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 acicular ferrite, pearlite, and polygonal ferrite, with acicular ferrite accounting for 30%-40%, and polygonal ferrite and pearlite accounting for 60%-70%. It has good low-temperature toughness and high straightness. Specifically, the yield strength in the transverse tensile test is between 480-520MPa, the tensile strength is between 570-630MPa, the strength range is narrow, the yield strength ratio is less than 0.88, the elongation is ≥25%, the transverse Charpy impact energy at -35℃ is ≥270J, the transverse shear area (SA) at -20℃ is not less than 95%, and the banded structure is grade 2 and below. Detailed Implementation

[0030] The specific embodiments of the present invention will be further described below:

[0031] This invention discloses an economical, thin-gauge L450M pipeline steel and a method for controlling banded microstructure. The finished product thickness is 9–15 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 gas pipelines with a diameter of Φ914 mm and below. The chemical composition by weight percentage is: C: 0.07%–0.09%, Si: 0.10%–0.25%, Mn: 1.70%–1.80%, P≤0.020%, S≤0.015%, Nb: 0.01%–0.02%, Ti: 0.008%–0.015%, Al: 0.015%–0.04%, Cr: 0.05%–0.15%, N: 0.003%–0.006%, with the balance being Fe and unavoidable impurities, and the total amount of impurity elements being less than 0.05%.

[0032] The roles of the main elements in the chemical composition of the steel plate of this invention are as follows:

[0033] 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 0.07% to 0.09%.

[0034] 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, which helps to obtain fine low-temperature phase transformation products and improve 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 1.70% to 1.80%.

[0035] Si has the function of 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.

[0036] 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 0.01% to 0.02% 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.

[0037] 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%.

[0038] 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%.

[0039] 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%.

[0040] 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 plates. The Cr content range selected in this invention is 0.05% to 0.15%.

[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 controlling the banded microstructure of economical, thin-gauge L450M pipeline steel. The production process includes steelmaking → ladle refining and degassing → continuous casting → billet heating → controlled rolling → controlled cooling → air cooling to room temperature, as detailed below:

[0043] 1) Steel smelting to continuous casting: The steel is smelted according to the following composition by weight percentage: C: 0.07%–0.09%, Si: 0.10%–0.25%, Mn: 1.70%–1.80%, P≤0.020%, S≤0.015%, Nb: 0.01%–0.02%, Ti: 0.008%–0.015%, Al: 0.015%–0.04%, Cr: 0.05%–0.15%, N: 0.003%–0.006%, with the balance being Fe and unavoidable impurities, and the total amount of impurity elements is low. The raw material is pretreated with KR hot metal to control the S content to be below 0.015%, and then enters the converter after slag removal. During converter smelting, a double-slag method is used to remove P, controlling the P content to ≤0.02%. At the end of converter smelting, the C content is controlled to be between 0.07% and 0.09%. Argon gas is blown for 15–20 minutes during tapping. This is followed by LF refining and RH vacuum degassing, with the RH vacuum maintained for at least 30 minutes. Afterwards, slab continuous casting is performed, with a superheat of 8–13°C and a casting speed of 0.6–0.8 m / min. Electromagnetic stirring is used in the secondary cooling zone during the continuous casting stage. The stirring method involves alternating forward and reverse stirring, with forward stirring lasting 10–20 seconds and reverse stirring lasting 5–10 seconds. The current is 1000–1500 A, and the frequency is 10–20 Hz. This process ensures that the molten steel is continuously cast into a billet with an isometric crystal ratio of no less than 80%. In the fan-shaped section, strong cooling is employed, with a total cooling water flow of 1200–1500 L / min for sections 1–4 and 1100–1300 L / min for sections 5–8. Simultaneously, a light reduction is applied at the end of solidification, with a billet reduction of 8–12 mm. After casting, the billet proceeds... The billet is stacked and cooled slowly, with a stacking temperature not lower than 850℃ and a slow cooling time not lower than 36 hours. By optimizing the electromagnetic stirring process in the continuous casting stage, the equiaxed crystal ratio of the continuously cast billet is significantly improved. Strong cooling is used to ensure the temperature gradient in the thickness direction of the billet. At the same time, light pressure is applied, which helps to reduce billet segregation, reduce internal structural defects, and promote core grain breakage, thus ensuring the strength and toughness of the subsequent steel plate. Meanwhile, the billet is stacked and cooled slowly after it comes off the line, and the stacking temperature is not lower than the slow cooling time limit, which helps to promote the diffusion of Mn, Cr, and H elements and reduce their impact 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, Cr, and Ti carbides and nitrides into the matrix and allows for sufficient diffusion. The temperature range of the heating section is 1240-1260℃, and the temperature range of the soaking section is 1210-1225℃. 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 and reduces their influence on microstructure and properties due to compositional segregation.

[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 rolling temperature range of 990–1050℃. The rough rolling process consists of no more than seven passes, with two of the first four passes ensuring a reduction rate of over 30%. The resulting intermediate billet thickness is 1.5–1.8 times the finished product thickness, reducing the intermediate billet's waiting-to-heat thickness, accelerating its temperature drop, improving rolling efficiency, and ensuring the final rolling temperature. The second stage is non-recrystallization rolling (finish rolling), with an initial rolling temperature range of 970–1010℃ and a final rolling temperature range of 850–880℃. The rolling process involves no more than four passes. The first two passes ensure a reduction rate of over 20% to guarantee core penetration and promote dynamic recrystallization of austenite grains. As the reduction gradually increases, the rolled piece undergoes flattening and elongation deformation. Simultaneously, due to the combined effects of dynamic and static recrystallization, new grains nucleate and grow at grain boundaries, forming new, fine recrystallized grains. This suppresses banding width. The rolling speed is increased, and the two-stage rolling process refines the grain structure at different stages and introduces a certain degree of pre-deformation to reduce the grain size after phase transformation. Controlling the final rolling temperature ensures that large-angle grain boundaries occupy a significant portion of the surface area. Compared to other methods, this improves the strength and toughness of the steel plate. After rolling, rapid steel blasting is performed at a speed of 4–6.5 m / s, without pre-straightening to prevent the precipitation of proferrite due to temperature drop during pre-straightening, which would widen the banded structure and affect the toughness of the steel plate. Subsequently, a combination of ultra-fast cooling and laminar flow cooling is used. The initial cooling temperature range is 770–790℃, with a cooling rate of 30–45℃ / s during the ultra-fast cooling stage. After cooling to below 650℃, laminar flow cooling is employed at a rate of 5–15℃ / s, with a final cooling temperature range of 550–570℃. A rapid cooling rate is used in the initial stage of cooling to maintain... To ensure the proportion of acicular ferrite is 30%–40%, while avoiding the precipitation of granular bainite and inhibiting carbon diffusion and segregation to prevent banded structures, a slow cooling rate is adopted after 650℃ to promote surface temperature recovery and prevent inconsistent microstructure along the thickness direction of the steel plate. A low cooling rate is also used to avoid the formation of granular bainite. The proportion of acicular ferrite is controlled at 30%–40%, and the proportion of polygonal ferrite and pearlite at 60%–70% to ensure the yield strength ratio 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 2–5 MPa and 50–100 μm. 3 / h, the air purging pressure is 10~20MPa, 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 existing technologies are overcome, and a method for controlling the banded structure of economical thin-gauge L450M pipeline steel for gas pipelines with thickness specifications of 9-15mm and pipe diameters of Φ914mm and below is realized. This steel plate has low cost, high strength and toughness, low yield strength ratio, and high straightness, which can ensure the service safety of gas pipelines. The microstructure of the steel plate is a multiphase structure mainly composed of acicular ferrite, pearlite and polygonal ferrite, of which acicular ferrite accounts for 30%-40%, polygonal ferrite and pearlite account for 60%-70%, and the banded structure is level 2 and below. This steel plate has low cost, high strength and toughness, 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 microstructure and 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 the examples.

[0055]

[0056] Table 3. Heating regime of steel billets and high-pressure water descaling process before rolling of continuously cast billets in the examples.

[0057]

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

[0059]

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

[0061]

[0062] Table 6. Steel plate microstructure and performance indicators of embodiments of the present invention

[0063]

[0064] Therefore, compared with the existing technology, the composition design and steelmaking continuous casting, heating and controlled rolling and cooling scheme of this invention overcome the shortcomings of the existing technology, and realize a method for controlling the banded structure of economical thin-gauge L450M pipeline steel for gas pipelines with a thickness of 9-15mm and a pipe diameter of Φ914mm and below. This steel plate has low cost, high strength and toughness, low yield strength ratio and high straightness, which can ensure the service safety of gas pipelines. The microstructure of the steel plate is a multiphase structure mainly composed of acicular ferrite, pearlite and polygonal ferrite, of which acicular ferrite accounts for 30%-40%, polygonal ferrite and pearlite account for 60%-70%, and the banded structure is level 2 and below. This steel plate has low cost, high strength and toughness, 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. An economic thin gauge L450M pipeline steel characterized in that, The chemical composition of the steel is as follows in percentage by weight: C: 0.07%~0.09%, Si: 0.10%~0.25%, Mn: 1.70%~1.80%, P≤0.020%, S≤0.015%, Nb: 0.01%~0.02%, Ti: 0.008%~0.015%, Al: 0.015%~0.04%, Cr: 0.05%~0.15%, N: 0.003%~0.006%, and the balance of Fe and inevitable impurities, and the total amount of impurities is less than 0.05%; The method for controlling banded structure of the economic thin-gauge L450M pipeline steel, wherein: Controlled rolling: the cast blank after tapping is descaled for 1-2 minutes by using high-pressure water before rough rolling, and the pressure of the descaling machine is 20-25 MPa; two-stage rolling: the first stage is recrystallization rolling, i.e. rough rolling, the open rolling temperature interval is 1150-1180℃, the rough rolling final rolling temperature interval is 990-1050℃, and the rough rolling is not more than seven passes, and the pass reduction system is that the first four passes have a pass reduction rate of more than 30%; the thickness of the obtained intermediate blank is 1.5-1.8 times of the thickness of the finished product; The second stage is non-recrystallization rolling, i.e. finish rolling, the open rolling temperature interval is 970-1010℃, the final rolling temperature interval is 850-880℃, and the finish rolling is not more than four passes, and the pass reduction system is that the first two passes have a pass reduction rate of more than 20%; speed increasing rolling, and the cast blank is thrown at a speed of 4-6.5 m / s after rolling, and the pre-straightening is not put into; Controlled cooling: adopt the cooling method of combining ultrafast cooling and laminar cooling, the open cooling temperature range is 770-790℃, the cooling speed of ultrafast cooling stage is 30-45℃ / s, laminar cooling is adopted after cooling to below 650℃, the cooling speed of laminar cooling stage is 5-15℃ / s, the final cooling temperature interval is 550-570℃, the side spray and air blowing are opened after the steel plate is out of the controlled cooling, the side spray pressure and water volume are 2-5MPa, 50-100m 3 / h, the air blowing pressure is 10-20MPa, hot straightening is performed again for three straightening, the lead-in roller position is-1.5mm--2.5mm, the lead-out roller position is-2.8mm--3.6mm, and then air cooling to room temperature.

2. A method of controlling banded structure of an economic thin gauge L450M pipeline steel as claimed in claim 1, characterized in that, The method comprises the following steps of: molten steel smelting, external refining and degassing, continuous casting, cast blank heating, controlled rolling, controlled cooling, and air cooling to room temperature, and the specific steps are as follows: 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 lower than 0.015%, and the molten iron is poured into a converter after slagging; a double-slag method is used to remove P in the converter smelting, the content of P is controlled to be lower than or equal to 0.02%, the content of C is controlled to be 0.07-0.09% at the end of the converter smelting, and argon is blown for 15-20 minutes when the molten steel is tapped; 2) External refining and degassing: LF refining and RH vacuum degassing are performed, and the RH vacuum is maintained for more than 30 minutes; 3) Continuous casting: slab continuous casting is performed, the continuous casting superheat is 8-13℃, and the continuous casting withdrawal rate is 0.6-0.8 m / min; Electromagnetic stirring is used in the secondary cooling zone in the continuous casting stage, the stirring mode is positive and reverse alternating stirring, the positive stirring time is 10-20 s, the reverse stirring time is 5-10 s, the current is 1000-1500 A, and the frequency is 10-20 Hz, so that the molten steel obtains a continuous casting blank, the proportion of the intermediate crystal in the continuous casting blank is not less than 80%, meanwhile, strong cooling is used in the fan-shaped section, the total cooling water quantity of the first to fourth sections is 1200-1500 L / min, the total cooling water quantity of the fifth to eighth sections is 1100-1300 L / min, the end of solidification is simultaneously subjected to light reduction, the reduction amount of the continuous casting blank is 8-12 mm, the cast blank is subjected to stacking and slow cooling after being withdrawn, the stacking temperature is not lower than 850℃, and the slow cooling time is not less than 36 h; 4) Billet heating: the billet is sent into the walking beam heating furnace for heating, and the billet is sequentially sent through the preheating section, the heating section and the soaking section and then discharged from the furnace; the temperature range of the preheating section is 900-1150℃, the temperature range of the heating section is 1240-1260℃, the temperature range of the soaking section is 1210-1225℃, and the heating and soaking section time in the furnace is not less than 4.5-5.5h; 5) Controlled rolling: the billet discharged from the furnace is descaled by high-pressure water for 1-2min before opening rolling, and the descaling machine pressure is 20-25MPa; the rolling is divided into two stages: the first stage is recrystallization rolling, i.e. rough rolling, the opening rolling temperature range is 1150-1180℃, the rough rolling finishing temperature range is 990-1050℃, and 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 30%, and the obtained intermediate billet thickness is 1.5-1.8 times the finished product thickness; the second stage is non-recrystallization rolling, i.e. finish rolling, the opening rolling temperature range is 970-1010℃, the finishing temperature range is 850-880℃, and 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 increasing rolling, rapid steel throwing after rolling, the throwing speed is 4-6.5m / s, and the pre-straightening is not put into; 6) Control cooling: adopt the cooling method of combining ultrafast cooling and laminar cooling, the open cooling temperature range is 770-790℃, the cooling speed of the ultrafast cooling stage is 30-45℃ / s, laminar cooling is adopted after cooling to below 650℃, the cooling speed of the laminar cooling stage is 5-15℃ / s, the final cooling temperature interval is 550-570℃, 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 is 2-5MPa, 50-100m 3 / h, the air blowing pressure is 10-20MPa, hot straightening is performed again for three straightening, the lead-in roller position is -1.5mm--2.5mm, the lead-out roller position is -2.8mm--3.6mm, and then air cooling to room temperature.

3. A method of controlling banded structure of an economic thin gauge L450M pipeline steel according to claim 2, characterized in that, the finished product thickness of the L450M pipeline steel is 9-15mm, and is used for manufacturing gas pipeline with a pipe diameter of Φ914mm and below.

4. A method of controlling banded structure of economical thin gauge L450M pipeline steel as claimed in claim 2, wherein, The mechanical properties of the finished steel plate are: the yield strength of transverse tensile property is between 480-520MPa, the tensile strength is between 570-630MPa, the yield strength ratio is less than 0.88, the elongation is ≥25%, the -35℃ transverse Charpy impact energy is ≥270J, the -20℃ DWTT transverse shear area SA is not less than 95%, and the banded structure is at level 2 and below.

5. The method of controlling banding of economical thin gauge L450M pipeline steel of claim 2, wherein, The continuous casting billet thickness is 150-200mm, and is rolled on a medium plate reciprocating mill.

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