Thin gauge low yield ratio l245-l450 pipeline steel and method of manufacturing and using same

Abstract

This invention relates to the field of high-toughness energy steel manufacturing technology, specifically to thin-gauge, low-yield-strength-ratio L245-L450 pipeline steel, its preparation method, and applications. The thin-gauge, low-yield-strength-ratio L245-L450 pipeline steel, by mass fraction, contains 0.04-0.08% C, 0.10-0.25% Si, 0.70-1.40% Mn, P≤0.015%, S≤0.010%, 0.010-0.035% Nb, 0.010-0.020% Ti, 0-0.20% Cr, 0.010-0.035% Als, N≤0.0060%, with the remainder being Fe and unavoidable impurities. This invention aims to provide microstructure control targets and suitable composition-processes for a series of pipeline steel products with low yield strength ratios (L245-L450). By optimizing the alloy element ratios and laminar flow cooling process parameters (e.g., controlling the average cooling rate of the water-cooled section to 10-20℃ with 3-8 sets of centrally opened front-end manifolds), a feasible standardized common technology is formed, which stably controls the yield strength ratio of the product within the range of 0.82-0.86. This meets the PSL2 grade performance requirements specified in the national standard "Hot-rolled wide steel strip for oil and gas transmission pipelines" while also possessing low carbon equivalent, excellent low-temperature toughness, and a wide process window, ensuring the comprehensive mechanical properties of the material.

Description

Technical Field

[0001] This invention relates to the field of high-toughness energy steel manufacturing technology, specifically to thin-gauge, low-yield-strength-ratio L245-L450 pipeline steel, its preparation method, and applications. Background Technology

[0002] Pipeline steel is the most critical structural material in long-distance pipeline transportation projects for clean energy such as oil and natural gas. Its service performance directly determines the operational safety, stability, and service life of the oil and gas pipeline network. With the continuous advancement of oil and gas pipeline network construction in my country and the increasing complexity and sophistication of pipeline network service conditions, increasingly stringent requirements are being placed on the comprehensive performance of pipeline steel, especially its resistance to fracture failure and safe service margin.

[0003] The yield strength ratio is a key indicator characterizing the plastic deformation capacity and resistance to fracture failure risk of pipeline steel. To ensure that pipeline steel can cope with extreme conditions such as pressure fluctuations, geological deformation, and external load impacts during service, it is required that pipeline steel has sufficient plastic deformation margin before fracture failure and can withstand greater pressure increases. For natural gas pipelines, the yield strength ratio needs to be controlled at 0.88 or below.

[0004] In actual production, there is a strong correlation between the yield strength and thickness of pipeline steel: the thinner the steel plate, the more significant the increase in its yield strength, while the tensile strength changes very little with the thickness specification, which ultimately leads to the yield strength ratio of thin-gauge pipeline steel easily exceeding the standard upper limit.

[0005] Chinese patent CN110284066A discloses a method for manufacturing thin-gauge pipeline steel plates with a thickness of 12-15 mm and a low yield strength-to-tensile ratio. This invention focuses on the production of medium-thick plates with a bainitic microstructure, a yield strength ≥485 MPa, and a tensile strength ≥580 MPa, involving steel grades L485 and above. The microstructure type and microstructure control method provided by this invention cannot be used for the production of thin-gauge strip steel of low steel grades.

[0006] Chinese patent CN114836683A discloses a method for manufacturing H2S-resistant steel plates with low yield strength ratios. The invention describes a heat-treated, heat-controlled microstructure consisting of polygonal ferrite and acicular ferrite. The hot rolling cooling process employs a two-stage cooling method to obtain this dual-phase microstructure, followed by offline heat treatment to obtain a fine and uniform normalized microstructure. This method cannot achieve a uniform and stable dual-phase microstructure under continuous hot rolling conditions, and its approach to reducing the yield strength ratio is completely different from the continuous hot rolling TMCP process.

[0007] Chinese patent CN119020662A discloses a method for manufacturing low yield strength ratio pipeline steel for straight seam welding. This invention employs a ferrite + pearlite microstructure design, increasing the carbon content to 0.13-0.15% to ensure a high pearlite content. Simultaneously, to avoid deteriorating low-temperature toughness, the coiling temperature is designed to be above 600℃. This method offers some economic advantages, but its narrow production process window and relatively high carbon equivalent are detrimental to low-temperature toughness and weldability.

[0008] In summary, existing technical solutions for pipeline steel with low yield strength ratio cannot, under the conditions of hot continuous rolling TMCP process, ensure that thin-gauge pipeline steel of L245-L450 grade with a thickness ≤10 mm meets all the performance requirements of the national standard PSL2 grade for hot-rolled wide steel strip for oil and gas transmission pipelines while maintaining a stable yield strength ratio of 0.88 or below.

[0009] Furthermore, on the one hand, there are differences in understanding among those skilled in the art; on the other hand, the inventors studied a large number of documents and patents when making this invention, but due to space limitations, not all details and contents were listed in detail. However, this does not mean that the present invention does not possess the features of these prior art. On the contrary, the present invention already possesses all the features of the prior art, and the applicant reserves the right to add relevant prior art to the background art. Summary of the Invention

[0010] This invention relates to the field of high-toughness energy steel manufacturing technology, specifically to thin-gauge, low-yield-strength-ratio L245-L450 pipeline steel, its preparation method, and applications.

[0011] To address the aforementioned technical problems, one objective of this invention is to provide thin-gauge, low-yield-strength-ratio L245-L450 pipeline steel, which, by mass fraction, contains 0.04-0.08% C, 0.10-0.25% Si, 0.70-1.40% Mn, P≤0.015%, S≤0.010%, 0.010-0.035% Nb, 0.010-0.020% Ti, 0-0.20% Cr, 0.010-0.035% Als, N≤0.0060%, with the remainder being Fe and unavoidable impurities. Preferably, it contains 0.005-0.015% P, 0.002-0.005% S, and 0.0029-0.0049% N.

[0012] According to a preferred embodiment, the thin-gauge, low-yield-strength-ratio L245-L450 pipeline steel contains, by mass fraction, 0.05% C, 0.18% Si, 0.79% Mn, 0.005% P, 0.003% S, 0.014% Nb, 0.018% Ti, 0.025% Als, 0.0035% N, with the remainder being Fe and unavoidable impurities.

[0013] According to a preferred embodiment, the thin-gauge, low-yield-strength-ratio L245-L450 pipeline steel contains, by mass fraction, 0.05% C, 0.18% Si, 0.95% Mn, 0.015% P, 0.002% S, 0.024% Nb, 0.014% Ti, 0.15% Cr, 0.019% Als, 0.0029% N, with the remainder being Fe and unavoidable impurities.

[0014] According to a preferred embodiment, the thin-gauge, low-yield-strength-ratio L245-L450 pipeline steel contains, by mass fraction, 0.08% C, 0.17% Si, 0.92% Mn, 0.010% P, 0.005% S, 0.021% Nb, 0.015% Ti, 0.17% Cr, 0.030% Als, 0.0043% N, with the remainder being Fe and unavoidable impurities.

[0015] According to a preferred embodiment, the thin-gauge, low-yield-strength-ratio L245-L450 pipeline steel, by mass fraction, contains 0.08% C, 0.15% Si, 1.45% Mn, 0.012% P, 0.002% S, 0.035% Nb, 0.018% Ti, 0.20% Cr, 0.020% Als, 0.0049% N, with the remainder being Fe and unavoidable impurities.

[0016] According to a preferred embodiment, the toughness of thin-gauge, low-yield-strength-ratio L245-L450 pipeline steel meets the PSL2 grade requirements when the thickness is ≤10 mm. The yield-strength-ratio range of the thin-gauge, low-yield-strength-ratio L245-L450 pipeline steel is 0.82-0.86 when the thickness is ≤10 mm.

[0017] One of the objectives of this invention is to provide a method for preparing thin-gauge, low-yield-strength-ratio L245-L450 pipeline steel, which includes the following steps: The above-mentioned thin-gauge L245-L450 pipeline steel slabs are obtained by converter smelting-continuous casting. The above-mentioned slabs are loaded into a heating furnace for heating; After the slab is taken out of the furnace, it is descaled by high-pressure water to remove the oxide scale formed on the surface of the slab during the heating process. After descaling, the slab is rolled by a roughing mill to obtain an intermediate slab, wherein the single-pass reduction in roughing is not less than 18%, and the exit temperature of the roughing mill is ≥930℃; Cooling water is used to accelerate the cooling of the intermediate billet between the roughing and finishing rolling processes. The intermediate billet enters the finishing rolling process when the temperature is 900-930℃. The finishing rolling exit temperature is 780-840℃. The cumulative deformation during the finishing rolling stage is in the range of 75%-85%. The steel strip obtained after finishing rolling is cooled by a laminar flow cooling process. After cooling, the steel strip is wound up by a coiler to obtain the finished steel coil.

[0018] According to a preferred embodiment, the steps of loading the slab into the furnace and heating it include: sending the slab to the heating furnace to raise and homogenize the temperature, with the furnace exit temperature at 1170-1190℃; heating at 920℃ or above for no less than 100 minutes, and the total furnace time being no less than 180 minutes.

[0019] According to a preferred embodiment, the descaling water pressure is 20-23 MPa. This step is used to remove iron oxide scale.

[0020] According to a preferred embodiment, during rough rolling, the slab undergoes 5-8 passes of reciprocating rolling to obtain an intermediate slab.

[0021] According to a preferred embodiment, during finishing rolling, the intermediate billet is hot-rolled to the finished size of the hot-rolled steel strip by hot continuous rolling on a 7-stand machine.

[0022] According to a preferred embodiment, laminar flow cooling is performed using a laminar cooling system. The operation steps include: opening the first 3-8 sets of manifolds and controlling the average cooling rate of the water-cooled section between 10-20°C. The laminar cooling system is, for example, a laminar flow cooling system where the first three sets are for accelerated cooling, the middle eight sets are for conventional cooling, and the last four sets are for fine-tuning.

[0023] According to a preferred embodiment, the steel strip cooled to 380-580°C is wound up and becomes steel ready for shipment for the preparation of welded pipes.

[0024] According to a preferred embodiment, the slab is a slab with a diameter of 200-230 nm.

[0025] One of the objectives of this invention is to provide the above-mentioned thin-gauge low yield strength ratio L245-L450 pipeline steel or the thin-gauge low yield strength ratio L245-L450 pipeline steel prepared based on the above-mentioned preparation method for use in the preparation of transport steel pipes for transporting oil, natural gas or other fluid media.

[0026] The beneficial effects of this technical solution are: This technical solution provides a method for preparing thin-gauge, low-yield-strength-ratio L245-L450 pipeline steel. Through the coordinated matching of composition design and process control, precise control of the microstructure type of L245-L450 grade pipeline steel with a thickness ≤10 mm is achieved. Its beneficial effects are mainly reflected in: This technical solution optimizes the alloy element ratio and laminar flow cooling process parameters (e.g., centrally opening 3-8 sets of manifolds in the front section to control the average cooling rate of the water cooling section at 10-20℃), so that the yield strength ratio of the product is stably controlled in the range of 0.82-0.86. This meets the performance requirements of PSL2 grade while also possessing low carbon equivalent, excellent low-temperature toughness, and a wide process window. It also ensures the comprehensive mechanical properties of the materials (L245 pipeline steel is an equiaxed ferrite + a small amount of pearlite material; L360-L415 pipeline steel is an equiaxed ferrite + a small amount of bainite material; L450 pipeline steel is a quasi-polygonal ferrite + a small amount of bainite material). Meanwhile, this common technology covers all-steel thin-gauge products from L245 to L450, breaking through the limitations of traditional methods in terms of thickness and steel grade, providing standardized solutions for different application scenarios, and enhancing the market adaptability of products; Secondly, by precisely controlling the rolling and cooling processes, microstructure inhomogeneity is effectively suppressed, scrap rate caused by performance fluctuations is reduced, product performance qualification rate is significantly improved, and customers' stringent requirements for the upper limit of yield strength ratio are met. Attached Figure Description

[0027] Figure 1 This is a material structure diagram of an embodiment of the present invention, showing an equiaxed ferrite structure with a small amount of pearlite. Figure 2 This is a material structure diagram of the microstructure of equiaxed ferrite with a small amount of bainite, as shown in Embodiment 2 of the present invention. Figure 3 This is a material structure diagram of the microstructure type of quasi-polygonal ferrite with a small amount of bainite, as shown in Embodiment 4 of the present invention. Detailed Implementation

[0028] In the description of this invention, terminology is used for descriptive purposes only and should not be construed as indicating or implying relative importance or implicitly specifying the number of indicated technical features. In the description of this invention, "a plurality of" means two or more, unless otherwise explicitly defined.

[0029] Unless otherwise specified, the experimental methods used in the following examples are all conventional methods; the materials, reagents or instruments used, unless otherwise specified by the manufacturer, are all commercially available reagents and materials; the conditions not specified in the examples are all carried out according to conventional conditions or conditions recommended by the manufacturer. At the same time, the present invention does not limit the source of the raw materials used. Unless otherwise specified, the raw materials used in the present invention are all commercially available products in this technical field.

[0030] Based on the strength and toughness requirements of different steel grades, different compositions and TMCP process parameters are used to obtain, for example... Figure 1-3The different tissues shown all meet the yield strength ratio range of 0.80-0.88.

[0031] The rationale for the alloy composition design involved in this technical solution is as follows: C: Provides solid solution strengthening effect. Too low a content will not meet the strength requirements and will increase the cost of the converter steelmaking process, while too high a content will deteriorate the weldability and cause fluctuations in the yield strength ratio. Therefore, the content range is 0.04-0.08%.

[0032] Si: It plays a deoxidizing role in steelmaking and also provides a certain solid solution strengthening effect. Too low a content will affect the smelting effect, while too high a content is not conducive to welding and is prone to forming inclusions in steel. Therefore, the content range is 0.10-0.25%.

[0033] Mn: It has a solid solution strengthening effect and improves low-temperature toughness. However, excessive content will lead to an increase in the yield strength ratio. Therefore, the content range is 0.70-1.40%.

[0034] P, S, N: The main inclusion-forming elements and harmful elements in steel, which deteriorate low-temperature toughness and weldability, but have little impact on performance within the design range.

[0035] Nb: During the controlled rolling and cooling process of steel strip, it refines the grains and improves strength and toughness. Too low a content will result in poor grain refinement, while too high a content will result in a significant increase in cost. Therefore, the content range is 0.010-0.035%.

[0036] Ti: Improves the strength and toughness matching of welded joints. Higher content can easily lead to significant fluctuations in the yield strength ratio and deteriorate toughness. Therefore, the content range is 0.010-0.020%.

[0037] Cr: Inhibits pearlite phase transformation and promotes ferrite / bainite phase transformation. Excessive content can lead to changes in microstructure and fluctuations in yield strength ratio. Therefore, the content range is 0-0.20%.

[0038] Als: The main deoxidizer in the smelting process of killed steel. If the content is too high, it will increase the inclusion content in the steel. Therefore, the content range is 0.010-0.035%.

[0039] Example 1: (1) The chemical composition, by mass percentage, is 0.05%C, 0.18%Si, 0.79%Mn, 0.005%P, 0.003%S, 0.014%Nb, 0.018%Ti, 0.025%Als, 0.0035%N, with the remainder being Fe and unavoidable impurities.

[0040] (2) Steel is smelted in an oxygen-blowing converter at a temperature of 1600℃ and then passed through a slab continuous casting machine to obtain a 230 mm slab.

[0041] (3) The slab loading temperature is 309℃, the furnace time is 255 min, of which the heating time at 920℃ is 145 min, and the furnace exit temperature is 1177℃.

[0042] (4) The slab is descaled by high-pressure water at a pressure of 21 MPa.

[0043] (5) A 56 mm thick intermediate billet is obtained by five passes of rough rolling, with a single pass deformation of 18.6%-21.1% and a rolling temperature of 930-980℃.

[0044] (6) The intermediate billet is cooled to 910°C by the cooling water of the roughing mill stand and then enters the finishing mill.

[0045] (7) A steel strip with a thickness of 10 mm is obtained by continuous rolling in 7 stands. The cumulative deformation of the finishing rolling is 82%, and the final rolling temperature is 780-810℃.

[0046] (8) Before the layer cooling is turned on, there are 4 sets of manifolds, with 33 rows on the top and 35 rows on the bottom. The cooling water temperature is 31℃, the average cooling rate of the water cooling section is 11-13℃ / s, and the winding temperature is 540-580℃.

[0047] Performance test results: Yield strength 359 MPa; tensile strength 438 MPa; yield strength ratio 0.82; elongation after fracture (A50) 49%; average full-size impact energy at -20℃ 329 J; shear area ratio 97%; drop hammer test (SA) at -5℃ 94%; (If...) Figure 1 As shown, its microstructure is equiaxed ferrite with a small amount of pearlite.

[0048] Example 2: (1) The chemical composition, by mass percentage, is 0.05% C, 0.18% Si, 0.95% Mn, 0.015% P, 0.002% S, 0.024% Nb, 0.014% Ti, 0.15% Cr, 0.019% Als, 0.0029% N, with the remainder being Fe and unavoidable impurities.

[0049] (2) Steel is smelted in an oxygen-blowing converter at 1800℃ and then passed through a slab continuous casting machine to obtain a 200 mm slab.

[0050] (3) The slab loading temperature is 256℃, the furnace time is 197 min, of which the heating time at 950℃ is 115 min, and the furnace exit temperature is 1183℃.

[0051] (4) Descaling of the slab is performed by high-pressure water at a pressure of 20 MPa.

[0052] (5) A 46 mm thick intermediate billet is obtained by five passes of rough rolling, with a single pass deformation of 25%-27% and a rolling temperature of 950-990℃.

[0053] (6) The intermediate billet is cooled to 920°C by the cooling water of the roughing mill stand and then enters the finishing mill.

[0054] (7) A steel strip with a thickness of 7.1 mm is obtained by continuous rolling in 7 stands. The cumulative deformation of the finishing rolling is 85%, and the final rolling temperature is 810-840℃.

[0055] (8) Before the layer cooling is turned on, there are 6 sets of manifolds, 48 ​​rows on the top and 41 rows on the bottom. The cooling water temperature is 29℃, the average cooling rate of the water cooling section is 14-17℃ / s, and the winding temperature is 510-550℃.

[0056] Performance test results: Yield strength: 441 MPa; Tensile strength: 524 MPa; Yield-to-tensile ratio: 0.84; Elongation after fracture (A50): 46%; Average full-size impact energy at -20℃: 249 J; Shear area ratio: 97%; Drop hammer test (SA) at -5℃: 99%; Figure 2 As shown, its microstructure is equiaxed ferrite with a small amount of bainite.

[0057] Example 3: (1) The chemical composition, by mass percentage, is 0.08% C, 0.17% Si, 0.92% Mn, 0.010% P, 0.005% S, 0.021% Nb, 0.015% Ti, 0.17% Cr, 0.030% Als, 0.0043% N, with the remainder being Fe and unavoidable impurities.

[0058] (2) Steel is smelted in an oxygen-blowing converter at a temperature of 1900℃ and then passed through a slab continuous casting machine to obtain a 200 mm slab.

[0059] (3) The slab loading temperature is 224℃, the furnace time is 201 min, of which the heating time at 990℃ is 145 min, and the furnace exit temperature is 1184℃.

[0060] (4) Descaling of the slab is performed by high-pressure water at a pressure of 23 MPa.

[0061] (5) A 46 mm thick intermediate billet is obtained by five passes of rough rolling, with a single pass deformation of 25%-27% and a rolling temperature of 930-950℃.

[0062] (6) The intermediate billet is cooled to 915°C by the cooling water of the roughing mill stand and then enters the finishing mill.

[0063] (7) A steel strip with a thickness of 8.0 mm is obtained by continuous rolling in 7 stands. The cumulative deformation of the finishing rolling is 83%, and the final rolling temperature is 810-850℃.

[0064] (8) Before the start of the cooling, there are 7 sets of manifolds, with 49 rows on the top and 42 rows on the bottom. The cooling water temperature is 28℃, the average cooling rate of the water cooling section is 15-17℃ / s, and the winding temperature is 510-550℃.

[0065] Performance test results: Yield strength is 435 MPa; tensile strength is 512 MPa; yield strength ratio is 0.85; elongation after fracture (A50) is 45%; average full-size impact energy at -20℃ is 235 J, shear surface ratio is 94%; drop hammer test (SA) at -5℃ is 99%; its microstructure is equiaxed ferrite with a small amount of bainite.

[0066] Example 4: (1) The chemical composition, by mass percentage, is 0.08% C, 0.15% Si, 1.45% Mn, 0.012% P, 0.002% S, 0.035% Nb, 0.018% Ti, 0.20% Cr, 0.020% Als, 0.0049% N, with the remainder being Fe and unavoidable impurities.

[0067] (2) A 200 mm slab is obtained through the process of “hot metal pretreatment - converter smelting - LF refining - calcium treatment - continuous casting”.

[0068] (3) The slab loading temperature is 197℃, the furnace time is 240 min, of which the heating time at 1100℃ is 180 min, and the furnace exit temperature is 1188℃.

[0069] (4) Descaling of the slab is performed by high-pressure water at a pressure of 22 MPa.

[0070] (5) A 46 mm thick intermediate billet is obtained by five passes of rough rolling, with a single pass deformation of 25%-27% and a rolling temperature of 930-950℃.

[0071] (6) The intermediate billet is cooled to 915°C by the cooling water of the roughing mill stand and then enters the finishing mill.

[0072] (7) A steel strip with a thickness of 9.5 mm is obtained by continuous rolling in 7 stands, with a cumulative deformation of 79% in the finishing rolling and a final rolling temperature of 810-850℃.

[0073] (8) Before the layer cooling is turned on, there are 4 sets of manifolds, with 33 rows on the top and 33 rows on the bottom. The cooling water temperature is 24℃, the average cooling rate of the water cooling section is 16-19℃ / s, and the winding temperature is 480-520℃.

[0074] Performance test results: Yield strength 503 MPa; tensile strength 585 MPa; yield strength ratio 0.86; elongation after fracture (A50) 42%; average full-size impact energy at -20℃ 221 J; shear area ratio 93%; drop weight test (SA) at -5℃ 99%; Figure 3 As shown, its microstructure is quasi-polygonal ferrite with a small amount of bainite.

[0075] It should be noted that the specific embodiments described above are exemplary, and those skilled in the art can devise various solutions inspired by the disclosure of this invention. These solutions all fall within the scope of this invention and its protection. Those skilled in the art should understand that this specification and its accompanying drawings are illustrative and not intended to limit the scope of the claims. The scope of protection of this invention is defined by the claims and their equivalents.

Claims

1. Thin-gauge, low-yield-strength-ratio L245-L450 pipeline steel, characterized in that, By mass fraction, it contains 0.04-0.08% C, 0.10-0.25% Si, 0.70-1.40% Mn, P≤0.015%, S≤0.010%, 0.010-0.035% Nb, 0.010-0.020% Ti, 0-0.20% Cr, 0.010-0.035% Als, N≤0.0060%, with the remainder being Fe and unavoidable impurities.

2. The thin-gauge, low-yield-strength-ratio L245-L450 pipeline steel according to claim 1, characterized in that, When the thickness is ≤10mm, the strength and toughness of the pipeline steel meet the requirements of PSL2 grade.

3. The thin-gauge, low-yield-strength-ratio L245-L450 pipeline steel according to claim 1, characterized in that, When the thickness is ≤10mm, the yield strength ratio of pipeline steel ranges from 0.82 to 0.

86.

4. A method for preparing thin-gauge, low-yield-strength-ratio L245-L450 pipeline steel, characterized in that, Includes the following steps: Slabs with the composition of thin-gauge L245-L450 pipeline steel as described in any one of claims 1-3 are obtained by converter smelting-continuous casting. The above-mentioned slabs are loaded into a heating furnace for heating; After the slab is taken out of the furnace, it is descaled by high-pressure water to remove the oxide scale formed on the surface of the slab during the heating process. After descaling, the slab is rolled by a roughing mill to obtain an intermediate slab, wherein the single-pass reduction in roughing is not less than 18%, and the exit temperature of the roughing mill is ≥930℃; Cooling water is used to accelerate the cooling of the intermediate billet between the roughing and finishing rolling processes. The intermediate billet enters the finishing rolling process when the temperature is 900-930℃. The finishing rolling exit temperature is 780-840℃. The cumulative deformation during the finishing rolling stage is in the range of 75%-85%. The steel strip obtained after finishing rolling is cooled by a laminar flow cooling process. After cooling, the steel strip is wound up by a coiler to obtain the finished steel coil.

5. The method for preparing thin-gauge, low-yield-strength-ratio L245-L450 pipeline steel according to claim 4, characterized in that, The thickness of the slab obtained after continuous casting is 200-230 mm.

6. The method for preparing thin-gauge, low-yield-strength-ratio L245-L450 pipeline steel according to claim 4, characterized in that, The slab exit temperature is 1170-1190℃; the slab temperature is ≥920℃ for no less than 100 min, and the total furnace time is no less than 180 min.

7. The method for preparing thin-gauge, low-yield-strength-ratio L245-L450 pipeline steel according to claim 4, characterized in that, Descaling water pressure: 20-23 MPa.

8. The method for preparing thin-gauge, low-yield-strength-ratio L245-L450 pipeline steel according to claim 4, characterized in that, The laminar flow cooling system starts with 3-8 sets of manifolds in the front section, controlling the average cooling rate of the water-cooled section to be between 10-20℃.

9. The method for preparing thin-gauge, low-yield-strength-ratio L245-L450 pipeline steel according to claim 4, characterized in that, The winding temperature is 380-580℃.

10. The use of thin-gauge low yield strength ratio L245-L450 pipeline steel as described in any one of claims 1-3, or thin-gauge low yield strength ratio L245-L450 pipeline steel prepared according to the preparation method described in any one of claims 4-9, in the manufacture of pipeline steel pipes for transporting oil, natural gas or other fluid media.

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