A low yield strength ratio, high toughness, and low cost L485M pipeline steel coil and its manufacturing method

Abstract

This invention discloses a low yield strength ratio, high toughness, and low-cost L485M pipeline steel coil and its manufacturing method. The chemical composition (Wt%) is: C 0.045~0.070, Nb 0.045~0.075, V 0.028~0.045, Ti 0.008~0.018, Cr 0.15~0.28, Als 0.012~0.045, Mg 0.0010~0.0025, Ca 0.0008~0.0018, Ca / Mg 0.6~1.2. Using a C-Mn-Nb-Mg-Ca design, the coil has a yield strength of 530~600MPa, a tensile strength of 630~730MPa, an elongation ≥28%, and an A value at -40℃. kv Average value ≥280J, DWTT average value ≥90% at -20℃, hardness value HV 10 ≤220, yield strength ratio ≤0.82. This invention solves the problems of complex existing rolling processes, high cost due to multiple rolling passes, and loose low-temperature toughness index and high yield strength ratio of the product.

Description

Technical Field

[0001] This invention belongs to the field of low alloy steel technology, and relates to the manufacture of hot-rolled coils of pipeline steel for spiral submerged arc welded pipes, and particularly to a low yield strength ratio, high toughness, and low cost L485M pipeline steel coil and its manufacturing method. Background Technology

[0002] Pipeline transportation is the most economical, safe, and efficient mode of transporting oil and natural gas. To improve transportation efficiency, high strength, large diameter, and high transmission pressure have become the main development trends for long-distance pipelines. Currently, X70 / X80 pipeline steel remains the mainstream material in both domestic and international pipeline engineering applications. Although the alloy composition design and production processes for hot-rolled X70 pipeline steel coils vary among steel companies, they all add large amounts of precious alloying elements such as Mo and Ni. While adding these precious metals can improve the strength performance of the coils, the yield strength and tensile strength increase simultaneously, making it difficult to control the yield-to-tensile strength ratio at a lower level. However, with the continuous exploitation of energy resources, oil and natural gas extraction is gradually extending to polar regions, deserts, and areas with even harsher geographical environments. Therefore, pipeline steel needs higher resistance to compressive and tensile strains, i.e., a lower yield-to-tensile strength ratio. Simultaneously, increasingly fierce competition in the steel industry and a significant rise in alloy prices have led to high production costs and decreased product market competitiveness, creating a challenge of matching low-cost manufacturing of pipeline steel with excellent high strength and toughness.

[0003] Although there are many patents and documents both domestically and internationally concerning hot-rolled coils of 485MPa grade pipeline steel, they are all significantly different from this invention. The patents and documents that are closest to this invention include:

[0004] 1) Chinese patent application CN117467891A, a low-cost X70 grade high-strength pipeline steel plate coil and its production method. The composition is C: 0.05%~0.09%, Si: 0.2%~0.3%, Mn: 1.06%~1.38%, Nb: 0.03%~0.06%, Cr: ≤0.19%, Ti: 0.015%~0.025%, Ca: 0.0002%~0.005%, Mg: 0.0002%~0.0025%, (Ca+Mg) / S≥1.5, Mn / C: 13~30, with the remainder being Fe and unavoidable impurities. The invention has several problems: although the alloy design cost is low, the rolling process is complex, with 5-7 passes of rolling deformation in the roughing stage and a small reduction rate per pass, which fails to achieve the effect of fine grain strengthening; at the same time, the billet thickness is not clearly defined; in addition, the product performance does not have a low yield ratio and the low temperature toughness index is also relatively lenient.

[0005] 2) Chinese patent application CN102560260A discloses a low-cost, high-strength, and high-toughness X70 pipeline steel coil and its production method. The composition is as follows: C: 0.03%~0.07%, Si: 0.1%~0.25%, Mn: 1.10%~1.80%, P: ≤0.018%, S: ≤0.005%, Nb: 0.05%~0.10%, Mo: 0.05%~0.12%, Ti: 0.008%~0.020%, Cr: 0.15%~0.35%, Als: 0.020%~0.045%, Ca: 0.0015%~0.0030%, N: ≤0.0060%, with the remainder being Fe and unavoidable impurities. This patent includes the precious element Mo and has a high Cr content, resulting in a high alloy composition. The specific rolling process is not clearly defined; furthermore, the low-temperature impact and drop hammer test temperatures are relatively high, indicating lenient requirements.

[0006] 3) Chinese patent application CN116970870A, a hot-rolled coil of X70 grade pipeline steel with low yield strength ratio and its manufacturing method. The composition is C: 0.05%~0.08%, Si: 0.1%~0.30%, Mn: 1.40%~1.60%, P: ≤0.015%, S: ≤0.005%, Nb: 0.045%~0.065%, Ti: 0.010%~0.020%, Cr: 0.15%~0.25%, Mo: 0.08%~0.10%, Ni: 0.10%~0.20%, Als: 0.015%~0.045%, with the remainder being Fe and unavoidable impurities. The patented composition includes precious metal elements such as Mo and Ni, resulting in a high alloy content; the billet thickness is 200-230mm, which is relatively thick; the final product specifications are 9-13mm, which is relatively thin; the low-temperature toughness requirements are -20℃ impact and -15℃ drop hammer, which are relatively lenient requirements.

[0007] 4) Chinese patent application CN103045945A, Economical High-Toughness X70 Pipeline Steel Hot-Rolled Coil and its Preparation Method. The composition is C: 0.060%~0.075%, Si: 0.15%~0.30%, Mn: 1.20%~1.65%, Nb: 0.070%~0.080%, Ti: 0.010%~0.020%, Cr: ≤0.25%, with the remainder being Fe and unavoidable impurities. Although this patent does not add precious metal elements such as Mo and Ni to the alloy composition, the rolling process is complex, involving 8-10 passes of rolling deformation in the roughing stage, with a small reduction rate per pass, failing to achieve the effect of fine grain strengthening; it also does not specify the billet thickness; the cooling process does not employ ultra-fast cooling, failing to utilize the water-based alloying method; and the final product thickness is below 15mm, significantly different from this design.

[0008] 5) Chinese patent application CN101348881A, a method for producing low-cost, high-performance X70 pipeline steel. The composition is: C: 0.02%~0.06%, Si: 0.10%~0.30%, Mn: 1.4%~1.9%, V: 0.04%~0.07%, Nb: 0.040%~0.070%, Als: 0.020%~0.040%, B: 0.0005%~0.0035%, N: ≤0.0060%, P: ≤0.012%, S: ≤0.006%, with the remainder being Fe and unavoidable impurities. Although this patent has a low alloy cost, it adds boron (B) to the alloy design. Because boron easily causes product quality fluctuations, its addition is currently prohibited in pipeline engineering both domestically and internationally. This production method is not suitable for actual production and engineering applications. Furthermore, the slab thickness used is 70~90mm, resulting in insufficient rolling compression ratio, failing to achieve the effect of fine-grain strengthening.

[0009] 6) Chinese Patent CN103805865B, an economical X70 oil and gas pipeline steel and its production method. The composition is: C: 0.10%~0.14%, Si: 0.15%~0.19%, Mn: 1.4%~1.5%, P: ≤0.015%, S: ≤0.0020%, Cr: 0.16%~0.24%, Nb: 0.025%~0.045%, V: 0.041%~0.050%, Ti: 0.008%~0.020%, Als: 0.038%~0.050%, N: ≤0.0080%, with the remainder being Fe and unavoidable impurities. This patent uses a high C content in its alloy design, which easily leads to fluctuations in low-temperature toughness and weldability; the thickness of the cast billet and the final product are not clearly defined; furthermore, its finishing rolling process uses a two-stage control method, which is significantly different from the finishing rolling process in this design.

[0010] 7) Chinese Patent CN112680659B, a low-compression-ratio economical X70 pipeline steel and its production method. The composition is: C: 0.05%~0.08%, Si: 0.10%~0.40%, Mn: 1.60%~1.80%, Nb: 0.050%~0.065%, Ti: 0.010%~0.030%, P: ≤0.025%, S: ≤0.0015%, Als: 0.015%~0.060%, with the remainder being Fe and unavoidable impurities. This patent uses a 150mm thick continuously cast billet, a thickness significantly different from this design; the roughing rolling process uses 5 passes, and the finishing rolling process uses 6 passes, also significantly different from this design; furthermore, the cooling process is not specified.

[0011] While there are numerous publicly available patents and publications on low-cost L485M pipeline steel hot-rolled coils, most involve high alloy costs and complex rolling processes, resulting in high operating costs due to the large number of rolling passes. The final product exhibits relatively lenient low-temperature toughness specifications and a high yield strength ratio. Summary of the Invention

[0012] The purpose of this invention is to provide a low yield strength ratio, high toughness, and low cost L485M pipeline steel coil and its manufacturing method. It employs a C-Mn-Nb-Mg-Ca design, without adding precious metal elements such as Mo and Ni. The invention utilizes a "slow-speed, high-reduction" rough rolling process to achieve extreme austenite grain refinement, optimizing its overall performance through fine-grain strengthening. "Progressive strengthening" in the finishing rolling deformation resistance control fully flattens the austenite. After finishing rolling, an RPC+ relaxation process is used to reduce the yield strength ratio of the coil, ultimately achieving a low yield strength ratio, high strength and toughness, and low cost production method for L485M pipeline steel hot-rolled coil.

[0013] Due to increasingly fierce competition in the steel product market and a sharp rise in alloy prices, and in response to the current problems of high production costs and declining market competitiveness of L485M pipeline steel hot-rolled coils, this invention proposes a technical solution to reduce production costs, fully utilize the potential of tooling and equipment, and achieve good strength and toughness properties.

[0014] One of the technical solutions of this invention is to propose a low yield strength ratio, high toughness, and low cost L485M pipeline steel coil, with the following chemical composition (weight, %):

[0015] C: 0.045%~0.070%, Si: 0.20%~0.30%, Mn: 1.75%~2.0%, P: ≤0.012%, S: ≤0.0020%, Nb: 0.045%~0.075%, V: 0.028%-0.045%, Ti: 0.008%~0.018%, Cr: 0.15%~0.28%, Als: 0.012%~0.045%, Mg: 0.0010%~0.0025%, Ca: 0.0008%~0.0018%, N: ≤0.005%, Ca / Mg: 0.6~1.2, with the remainder being Fe and unavoidable impurities. The metallographic structure of the hot-rolled coil is a mixture of acicular ferrite and MA, wherein the volume fraction of acicular ferrite is 85%~92% and the volume fraction of MA component is 8%~15%. The width of the acicular ferrite lath bundles is 2.5~4.0μm, with an equivalent ASTM grain size grade of 12.5~13.0.

[0016] The reason for selecting the above alloying elements and their contents in this invention is because:

[0017] Carbon (C) is a carbide-forming element and the most effective element for ensuring strength. It improves hardenability and guarantees the strength and hardness of materials. Its effect is second only to phosphorus, and stronger than elements such as manganese, nickel, chromium, tungsten, molybdenum, and vanadium. It is the most economical and effective strengthening element in steel. Carbon significantly improves strength through solid solution strengthening and phase transformation strengthening. Only with sufficient carbon can enough acicular ferrite be formed. If the carbon content is too low, the strength and hardness of the material cannot be guaranteed; however, if the content is too high, it is easy to cause center segregation in steel plates, reducing the toughness and plasticity of steel and affecting the weldability of products. As the strength level of steel increases, the C content in pipeline steel shows a gradual decreasing trend, tending towards low-carbon or ultra-low-carbon, with a range of 0.045%~0.070%.

[0018] Si (Si): It can dissolve in ferrite and austenite, playing a certain role in solid solution strengthening. It can significantly improve the hardness and strength of steel, while promoting ferrite grain coarsening and reducing the anisotropy of steel plates in the transverse and longitudinal properties. Si also helps to improve the corrosion resistance and high-temperature oxidation resistance of steel. However, excessively high silicon content will reduce the weldability of steel and significantly reduce its plasticity and toughness; the range is 0.20%~0.30%.

[0019] Mn (manganese): Manganese has a solid solution strengthening effect. The solid solution formed by manganese and iron increases the hardness and strength of ferrite and austenite in steel. It is also a carbide-forming element, entering cementite to replace some iron atoms. In steel, manganese lowers the critical transformation temperature, increases austenite stability, and strongly increases hardenability, effectively ensuring steel strength. Manganese can compensate for the strength decrease caused by reduced carbon content, making it the most important and economical strengthening element. Manganese shifts the C-curve to the right, promotes bainite transformation, and is beneficial for the formation of acicular ferrite, significantly improving strength with minimal decrease in toughness. However, excessive manganese content can increase the tendency for center segregation in continuously cast billets, leading to an increase in banded structures in steel plates, increasing brittleness and decreasing plasticity. The range for this is 1.75%~2.0%.

[0020] P, S, and N are unavoidable impurity elements in steel, and it is desirable to keep them as low as possible. However, excessively low requirements will increase production costs. In this invention, P ≤ 0.012%, S ≤ 0.0020%, and N ≤ 0.005%.

[0021] Nitrogen (Nb) is one of the main microalloying elements in pipeline steel. It can significantly increase the recrystallization temperature of austenite, expand the non-recrystallized region, facilitate high-temperature controlled rolling, and reduce mill load. Simultaneously, it can inhibit austenite grain growth, exhibiting a very significant grain-refining strengthening effect. Nb can combine with carbon (C) and nitrogen (N). During the finishing rolling stage, strain-induced precipitation of Nb(C,N) strongly pins the austenite grain boundaries, preventing recrystallization and forming flattened, elongated "pancake-shaped" austenite, greatly increasing the effective grain boundary area. During cooling and relaxation, the precipitation of fine Nb(C,N) two-phase particles produces precipitation strengthening, thereby increasing the steel's strength. Excessive Nb content promotes the formation of austenite islands (MA islands), reducing the toughness of the weld heat-affected zone. Considering cost, its range is 0.045%~0.075%.

[0022] Vanadium (V): It has a strong precipitation strengthening effect and a weak grain refinement strengthening effect. Vanadium forms stable compounds with carbon and nitrogen, mainly existing in steel in the form of carbides. Vanadium carbonitrides precipitate uniformly in ferrite in a fine and dispersed form, which can significantly improve the strength of the material. However, excessively high content does not significantly improve the strength but increases the cost of the alloy. Its range is 0.028%-0.045%.

[0023] Titanium (Ti): Titanium is a strong nitrogen-fixing element that can form high-temperature stable and fine TiN precipitates during slab continuous casting. These fine TiN precipitates effectively prevent the growth of austenite grains during heating of the continuously cast slab and also significantly improve the toughness of the heat-affected zone during steel welding. Similar to niobium in steel, titanium has strong grain-refining and precipitation-strengthening effects. Furthermore, Ti can prolong the precipitation incubation period of NbC, resulting in a later start time for carbide precipitation in Nb-Ti composite steel compared to Nb steel, leading to finer and more dispersed precipitates. However, excessively high content diminishes the effect and easily forms large inclusions; the optimal content is 0.008%~0.018%.

[0024] Cr: Chromium can improve strength through solid solution strengthening. Like Mn, Cr can dissolve into solid solutions, improving the hardenability of steel and thus increasing strength. It is also inexpensive, effectively replacing expensive alloying elements like Mo and Ni, reducing alloy costs. When Cr dissolves into austenite, it increases the stability of supercooled austenite, shifting the C-curve to the right and promoting the formation of low-carbon bainite in supercooled structures, thereby improving the strength and hardness of the steel. Furthermore, chromium has excellent weather resistance and corrosion resistance. However, excessive chromium content significantly increases the brittle transition temperature of steel, reduces elongation, and easily forms coarse carbides, leading to a deterioration in toughness. The recommended chromium content is 0.15% to 0.28%.

[0025] Aluminum (AlN) is a commonly used deoxidizer in steel. The addition of aluminum significantly improves the strength of steel. Through reaction with elements such as nitrogen in the steel, the generated AlN helps refine austenite grains, thereby achieving a strengthening effect and improving strength and low-temperature impact toughness. Aluminum improves toughness by reducing the steel's susceptibility to cracking, especially at low temperatures, where it can significantly lower the ductile transformation temperature. Aluminum's solid solution strengthening effect is significant, strongly limiting the expansion of the austenite phase region, which is beneficial for improving strength. However, excessively high aluminum content in steel can easily lead to a significant increase in aluminum oxide inclusions, reducing the steel's purity and negatively impacting its low-temperature toughness. Its optimal range is 0.012%~0.045%.

[0026] Mg can reduce the oxygen and sulfur content and the number of inclusions in steel, purifying the molten steel and significantly reducing the harmful effects of inclusions on steel properties. It also has a significant modifying effect on inclusions in steel, generating fine, dispersed inclusions that provide nucleation sites for sulfides and carbonitrides, thus contributing to the optimized distribution of inclusions in the steel. Simultaneously, it can induce the nucleation of acicular ferrite (AF), thereby refining the microstructure of the steel. Furthermore, it can increase yield strength and tensile strength by more than 5%, while maintaining plasticity essentially unchanged, effectively reducing the yield-to-tensile ratio. It can also improve the low-temperature toughness of steel. In addition, by utilizing the microalloying effect of Mg and designing a reasonable Mg addition amount, the amount of expensive microalloying metals such as Nb, V, and Ti can be reduced, thereby lowering the alloying production cost. The range is 0.0010%~0.0025%.

[0027] Ca: It has a strong affinity for sulfur in steel and plays a role in desulfurization. At the same time, calcium treatment can change the morphology of sulfides, improve the anisotropy of steel, and modify inclusions to achieve the purpose of spheroidizing inclusions, thus ensuring the toughness of steel. If the content is too high, large particles of inclusions will be formed. The range is 0.0008%~0.0018%.

[0028] The second technical solution of this invention proposes a manufacturing method for low yield strength ratio, high toughness, and low cost L485M pipeline steel coils, including smelting, continuous casting, billet heating, rough rolling, finish rolling, cooling, and coiling. The continuously cast slab is directly heated in a hot-charging furnace at 500-850℃, with a heating temperature of 1170-1200℃ and a total furnace time of 150-250 minutes, of which the soaking section time is ≥120 minutes. The rough rolling process adopts a slow-speed, high-reduction process, with a single-pass reduction rate of 26%-34% and a rolling speed range of 1.0-2.0 m / s. The final rolling temperature is 1020~1040℃, and the austenite recrystallized grain size after rough rolling is 18~24μm; the initial rolling temperature of finish rolling is 900~960℃, the single-pass reduction rate of the first two passes is 22%~27%, the reduction rate of the last pass is ≥6%, the cumulative reduction rate is 62%~69%, and the final rolling temperature is 780~840℃; after rolling, it is first cooled to the target temperature of 490~510℃ by ultra-fast cooling at a cooling rate of 30~50℃ / s, and then relaxed by air cooling for 10~25s in the target temperature range. The coiling temperature is 400~480℃.

[0029] Furthermore, in the smelting process, after converter smelting and ladle refining, and after the composition is qualified, calcium-magnesium composite is used to modify the inclusions in the steel. The calcium-magnesium feed linear velocity is ≥4m / s, the soft blowing time after calcium-magnesium treatment is ≥5min, and then the steel is stirred by bottom blowing argon, with a net argon blowing time of ≥8min and a calming time of ≥12min.

[0030] Furthermore, the superheat during casting is controlled at 15~30℃, and the slab is continuously cast into a continuous casting slab. The casting process is protected throughout, and dynamic light pressure is applied at the end of solidification. The light pressure of the casting machine is ≥4.5mm, and the slab adopts a medium thickness of 170~190mm.

[0031] Furthermore, the roughing rolling process consists of 3 passes, with a reduction rate of 26% to 30% in the first pass and 31% to 34% in the second and third single passes.

[0032] Furthermore, the winding process employs a high-strength winding machine with a drum tension coefficient of 2.0~2.8, an auxiliary winding roller pressure of 450~560MPa, a guide roller pressure of 55~88 kN, a drum expansion speed of 0.6 times, a skip number of turns of 5~7, a main drive motor MD lead rate of 13%~15%, an auxiliary winding roller WR lead rate of 16%~17%, and a pinch roller PR lead rate of 12%~13%.

[0033] Compared with the prior art, the present invention has the following advantages:

[0034] This invention employs an economical C-Mn-Nb-Mg-Ca alloy design, eliminating the need for precious metals such as Mo and Ni, resulting in low alloy costs. A calcium-magnesium composite treatment is used to achieve deep modification of inclusions: Ca promotes the low-melting-point spheroidization of non-deformable inclusions such as Al2O3, effectively preventing nozzle clogging and ensuring smooth casting; Mg generates nano-sized MgO·Al2O3 spinel, acting as a nucleation core for needle-like ferrite, inducing intragranular ferrite nucleation and achieving intragranular refinement. This significantly improves the low-temperature toughness of the steel and completely eliminates the hazards of large-size MnS inclusions, fundamentally enhancing the material's low-temperature fracture toughness.

[0035] In the roughing stage, an innovative slow-speed, high-reduction process was adopted. By applying high strain energy in a concentrated manner, the austenite underwent full recrystallization, successfully refining the original austenite grains to 18-24 μm. This process achieves "process-for-alloy substitution," compensating for the strength loss caused by the reduction in alloy design with its extreme grain refinement (fine-grain strengthening). While significantly reducing reliance on precious metals and alloy costs, it also simplifies the rolling process, achieving dual economic benefits in both materials and manufacturing.

[0036] An incremental deformation resistance control strategy is implemented during the finishing rolling stage to ensure that austenite is continuously and uniformly flattened and elongated in the non-recrystallization region. This technology effectively stabilizes the flattened state of deformed austenite, avoids mixed crystal phenomena caused by uneven deformation, and provides high-density, uniformly distributed nucleation sites for subsequent phase transformation, fundamentally guaranteeing the product's excellent low-temperature toughness and microstructure uniformity.

[0037] Employing the PRC+ relaxation cooling process, the ultra-rapid cooling process first rapidly supercools the distorted austenite to the phase transformation critical region, effectively suppressing the formation of proeutectoid ferrite and laying the foundation for obtaining a non-equilibrium intermediate-temperature transformation microstructure. Subsequently, the relaxation stage causes significant recovery of the distorted austenite, leading to stress release and a decrease in dislocation density, thereby actively reducing the yield strength. Simultaneously, this stage induces strain-induced precipitation of nanoscale carbonitrides of elements such as Nb and V. Finally, during the final cooling process, the recovered "softened" austenite transforms into a matrix dominated by acicular ferrite, while the dispersed hard MA islands and nanoprecipitates act as strong dislocation barriers, ensuring high tensile strength. This results in a product with high strength, good low-temperature toughness, and a low yield strength ratio (≤0.82), enhancing its safety in pipeline engineering.

[0038] The hot-rolled coil of this invention has excellent comprehensive performance and can meet all the technical specifications of L485M coil. The hot-rolled coil has a thickness of 17~22mm, yield strength of 530~600MPa, tensile strength of 630~730MPa, elongation ≥28%, average impact energy Akv ≥280J at -40℃, average drop weight DWTT ≥90% at -20℃, and hardness HV.10 ≤220, and the yield strength ratio is not higher than 0.82. Attached Figure Description

[0039] Figure 1 Metallographic diagram of the rough-rolled austenite grain size in Example 1.

[0040] Figure 2 The image shows the metallographic structure of the hot-rolled coil in Example 1.

[0041] Figure 3 This is a diagram showing the resistance control of incremental deformation during finishing rolling implemented in Example 1. Detailed Implementation

[0042] It should be noted that, unless otherwise specified, the embodiments and features described in this invention can be combined with each other. The described embodiments are merely some, not all, of the embodiments of this invention. The following description of at least one exemplary embodiment is merely illustrative and is in no way intended to limit the invention or its application or use. All other embodiments obtained by those skilled in the art based on the embodiments of this invention without inventive effort are within the scope of protection of this invention.

[0043] The present invention will be described in more detail below through examples.

[0044] Compared with existing technologies, this invention adopts a C-Mn-Nb-Mg-Ca design, without adding precious metal elements such as Mo and Ni, thus reducing alloy costs; it achieves extreme austenite grain refinement through a "slow-speed, high-reduction" rough rolling process, and optimizes its comprehensive performance through fine grain strengthening; it maintains stable grain flattening and elongation state through "progressive strengthening" fine rolling deformation resistance control, fully flattening the austenite; after fine rolling, it adopts RPC+ relaxation process to reduce the plate yield strength ratio, ultimately obtaining a product with high strength, good low-temperature toughness, and a low yield strength ratio (≤0.82), making the product safer.

[0045] A low-yield-to-tensile ratio, high-toughness, and low-cost L485M pipeline steel coil has the following chemical composition by weight percentage: C: 0.045%~0.070%, Si: 0.20%~0.30%, Mn: 1.75%~2.0%, P: ≤0.012%, S: ≤0.0020%, Nb: 0.045%~0.075%, V: 0.028%-0.045%, Ti: 0.008%~0.018%, Cr: 0.15%~0.28%, Als: 0.012%~0.045%, Mg: 0.0010%~0.0025%, Ca: 0.0008%~0.0018%, N: ≤0.005%, Ca / Mg: 0.6~1.2, with the remainder being Fe and unavoidable impurities. The chemical compositions of Examples 1-6 are shown in Table 1.

[0046] Table 1. Chemical composition of steel in the examples (Wt, %)

[0047] Example C Si Mn P S Nb V Ti Cr Als Ca Mg Ca / Mg N Mo 1 0.052 0.24 1.83 0.010 0.0017 0.061 0.032 0.013 0.18 0.033 0.0015 0.0022 0.68 0.003 / 2 0.052 0.25 1.88 0.009 0.0016 0.062 0.035 0.012 0.15 0.028 0.0012 0.0018 0.67 0.004 / 3 0.055 0.22 1.81 0.011 0.0015 0.058 0.035 0.015 0.17 0.035 0.0014 0.0020 0.70 0.004 / 4 0.054 0.24 1.82 0.010 0.0015 0.065 0.031 0.013 0.17 0.039 0.0015 0.0023 0.65 0.003 / 5 0.058 0.24 1.82 0.011 0.0014 0.067 0.030 0.015 0.16 0.032 0.0012 0.0019 0.63 0.004 / 6 0.056 0.26 1.80 0.008 0.0017 0.060 0.034 0.014 0.18 0.036 0.0012 0.0020 0.60 0.004 / Comparative Example 1 0.062 0.24 1.61 0.009 0.0020 0.092 / 0.009 0.28 0.031 0.0026 / / 0.0038 0.096 Comparative Example 2 0.112 0.19 1.48 0.009 0.0015 0.027 0.044 0.014 0.25 0.049 / / / 0.0046 /

[0048] A method for manufacturing low-yield-strength-ratio, high-toughness, and low-cost L485M pipeline steel coils includes hot metal pretreatment, converter smelting, ladle refining, RH furnace control of hydrogen and oxygen content, LF furnace light desulfurization treatment, calcium and magnesium treatment to control inclusion morphology and improve the steel's ductility, toughness, and cold bending performance, slab continuous casting using electromagnetic stirring or dynamic light reduction, and continuous casting slab heating, rolling, cooling, and coiling. Among these:

[0049] Smelting Process: After hot metal pretreatment, converter smelting employs top-blowing or top-bottom combined blowing; ladle refining utilizes RH vacuum treatment to control hydrogen and oxygen content, followed by LF furnace light desulfurization treatment. After the composition meets the requirements, a calcium-magnesium composite treatment is then used to modify inclusions in the steel. Magnesium acts as a nucleation site, responsible for generating numerous fine core inclusions to serve as nuclei for subsequent heterogeneous precipitation phases. This is one of the keys to achieving "oxide metallurgy," refining grains, and improving toughness. Calcium plays a role in deep modification and protection, responsible for ultimately "processing" these core inclusions into harmless spherical shapes to ensure sulfur... The spheroidization effect of inclusions of sulfides and oxides is achieved by feeding calcium and magnesium linear velocity ≥4m / s and soft blowing time ≥5min after treatment. The stable molten steel flow rate generated by small airflow stirring provides sufficient floating time for the calcium and magnesium modified inclusions that have become low-melting-point spherical inclusions. Then, by bottom blowing argon stirring with a net argon blowing time ≥8min and a calming time ≥12min, large inclusions float to the surface and are adsorbed by the top slag. Small inclusions remaining in the molten steel are modified into spherical inclusions. These spherical inclusions become nucleation sites for sulfur during solidification, further reducing the precipitation of large sulfides.

[0050] Continuous casting process: The superheat of the billet is controlled at 15~30℃ during casting. The slab is continuously cast into a continuous casting slab. The casting process is protected throughout. Dynamic light pressure is applied at the end of solidification. The light pressure of the casting machine is ≥4.5mm. The billet adopts a medium thickness of 170~190mm, which can ensure the compression ratio and make the billet heat through quickly and evenly, reduce energy consumption and save costs.

[0051] Continuous casting slab heating process: The continuously cast slab is directly hot-charged at 500~850℃ and heated in a walking beam furnace at 1170~1200℃, with a total furnace time of 150~250min, including a soaking time of ≥120min, to ensure thorough heating and complete austenitization. This process also allows the carbonitrides of Nb, V, and Ti to fully dissolve in the austenite, providing sufficient solute atoms for precipitation strengthening during subsequent rolling and cooling. Furthermore, Ti and N form extremely stable TiN particles at high temperatures. These particles effectively prevent austenite grain coarsening during slab heating, providing a fine and uniform initial microstructure for subsequent rolling. Simultaneously, the fixed N avoids the detrimental effects of free N on toughness.

[0052] Rough rolling process: The rough rolling process consists of 3 passes, using slow rolling at a speed range of 1.0~2.0 m / s. The reduction rate in the first pass is 26%~30%, and the reduction rates in the second and third passes are 31%~34%. This large reduction in rough rolling allows for complete recrystallization of austenite, refining the original austenite grains and providing a foundation for obtaining fine and uniform ferrite grains. The final rough rolling temperature is 1020~1040℃. This temperature range ensures complete recrystallization during rough rolling while preventing austenite grain growth. Large deformation is completed above the recrystallization temperature. Through repeated "deformation-recrystallization" cycles, the as-cast structure is fully broken down, resulting in uniform and fine original austenite grains (18~24 μm), laying the microstructure foundation for subsequent controlled rolling. Therefore, using this rolling scheme, the austenite recrystallized grain size obtained after rough rolling is 18~24 μm (see appendix for details). Figure 1 The metallographic diagram of the rough-rolled austenite grain size shows that the recrystallized austenite has been made ultra-fine, which significantly reduces the dependence on precious metals such as Mo, Ni and Cu, and realizes "process instead of alloy", which significantly reduces the cost of alloys and process costs at the same time.

[0053] Finishing rolling process: The finishing rolling process consists of 7 passes. The reduction rate of the first two passes is 22%~27%, and the reduction rate of the last pass is ≥6%, with a cumulative reduction rate of 62%~69%. The initial rolling temperature is 900℃~960℃, and the final rolling temperature is 780~840℃. The lower final rolling temperature ensures that the austenite achieves sufficient flattening and distortion energy. At the same time, it controls the deformation resistance during the finishing rolling process, ensuring that it continues to increase (see appendix for details). Figure 3(The control diagram of increasing deformation resistance in fine rolling) keeps the grain flattening and elongation state stable, so that the austenite grains are fully refined and elongated, avoiding mixed grains and ensuring low-temperature impact and drop hammer performance.

[0054] The heating, roughing, and finishing process parameters for Examples 1 to 6 are shown in Table 2.

[0055] Table 2. Heating, roughing, and finishing rolling process parameters for each embodiment.

[0056] Example Slab thickness / mm Intermediate billet thickness / mm Heating temperature / ℃ Insulation time / min Roughing and finishing rolling temperatures / ℃ First pass reduction rate / % Second pass reduction rate / % Third pass reduction rate / % Total reduction rate / % Austenite recrystallization grain size / μm 1 170 56 1185 155 1025 28 31 31 67 21.5 2 170 56 1187 158 1028 27 32 32 67 22.1 3 170 56 1182 161 1023 28 31 31 67 19.8 4 190 58 1190 165 1034 33 33 32 69.5 18.5 5 190 58 1185 162 1025 32 33 33 69.5 18.6 6 190 58 1184 165 1030 32 32 34 69.5 19.2 Comparative Example 1 / 52 1150 / 950 / / / / / Comparative Example 2 / / 1130 / 1012 / / / / / Example Finishing rolling start temperature / ℃ Finishing rolling temperature / ℃ Passes with a reduction rate ≥ 22% Total reduction rate in finishing rolling / % First pass reduction rate / % Second pass reduction rate / % Third pass reduction rate / % 7th pass reduction rate / % / / 1 940 796 3 68.75 23 23 24 8 / / 2 942 792 3 68.75 22 23 25 7 / / 3 938 788 3 68.75 23 24 22 7 / / 4 942 801 2 63.79 23 22 21 6 / / 5 943 795 2 63.79 23 23 21 7 / / 6 939 790 3 63.79 22 23 22 6 / / Comparative Example 1 / 750 / 66.35 / / / / / / Comparative Example 2 835-922 745-790 / 75.0 / / / / / /

[0057] Cooling Process: After rolling, an ultra-rapid cooling method combined with a relaxation process is employed. First, ultra-rapid cooling is used to reach the target relaxation temperature of 490-510℃ at a cooling rate of 30-50℃ / s. This process rapidly supercools the distorted austenite, allowing it to quickly "skip" the ferrite / pearlite phase transformation zone, preventing insufficient austenite stability due to the absence of Mo and premature formation of soft phase structures. Simultaneously, it effectively suppresses the formation of proeutectoid ferrite, laying the foundation for obtaining a non-equilibrium, medium-temperature transformation microstructure. Subsequently, a 10-25s relaxation air cooling process is performed within the target temperature range, with a coiling temperature of 400-480℃. The relaxation stage significantly restores the distorted austenite, leading to stress release and a decrease in dislocation density, thereby actively reducing the yield strength. Simultaneously, this stage induces strain-induced precipitation of nanoscale carbonitrides of elements such as Nb and V. Ultimately, during the final cooling process, the restored "softened" austenite transforms into a matrix dominated by acicular ferrite, while the dispersed hard MA islands and nano-precipitates act as strong dislocation barriers, ensuring high tensile strength. This results in a mixed microstructure of uniformly sized acicular ferrite and MA (see appendix for details). Figure 2 The metallographic diagram of the hot-rolled sheet coil is shown. Finally, a product with high strength, good low-temperature toughness, and low yield strength ratio (≤0.82) is obtained, which makes the product safer in pipeline engineering.

[0058] Winding process: The winding adopts a high-strength winding machine with a winding temperature of 400~480℃, a drum tension coefficient of 2.0~2.8, an auxiliary winding roller pressure of 450~560 MPa, a guide roller pressure of 55~88 kN, a drum expansion speed of 0.6 times, a skip number of turns of 5~7, a main drive motor MD lead rate of 13%~15%, an auxiliary winding roller WR lead rate of 16%~17%, a pinch roller PR lead rate of 12%~13%, and the coiled plate has a good roll shape without hard bends.

[0059] The cooling and winding process parameters for Examples 1 to 6 are shown in Table 3, and the mechanical property test results are shown in Table 4.

[0060] Table 3 shows the cooling and winding process parameters and the volume percentage of metallographic structures for each embodiment.

[0061] Example <![CDATA[Ultra-fast cooling rate / °C·s -1 > Relaxation temperature / ℃ Air cooling time / s Final cooling temperature / ℃ drum tension coefficient Auxiliary roller pressure / MPa Guide gauge pressure / kN Acicular ferrite / % MA components / % 1 40 505 15 435 2.3 485 75 90 10 2 42 502 12 436 2.2 495 69 92 8 3 38 508 19 428 2.2 488 72 90 10 4 45 498 16 427 2.3 492 67 88 12 5 46 506 15 423 2.3 497 74 89 11 6 45 505 13 431 2.5 505 70 90 10 Comparative Example 1 15 / / 300 / / / / / Comparative Example 2 12.6 / / 604 / / / / /

[0062] Table 4 Main mechanical properties of each embodiment

[0063] Example Finished product specifications / mm <![CDATA[R t0.5 / MPa]]> <![CDATA[R m / MPa]]> <![CDATA[A 50 mm / %]]> <![CDATA[R t0.5 / R m ]]> Charpy impact work at -40℃ (single J) Charpy impact work at -40℃ (single J) Charpy impact work at -40℃ (single J) Average Charpy impact energy at -40℃ / J -20℃ Drop Weight Shear Area (single value / %) -20℃ Drop Weight Shear Area (single value / %) Average shear area of ​​drop weight at -20℃ / % <![CDATA[Hardness HV 10 > 1 17.5 565 706 34 0.80 317 316 325 319 100 100 100 212 2 17.5 562 694 32 0.81 316 318 315 316 98 100 99 215 3 17.5 558 698 32 0.80 309 328 312 316 100 100 100 208 4 21 548 685 35 0.80 308 305 308 307 100 100 100 214 5 21 552 690 36 0.80 313 313 306 311 100 98 99 212 6 21 555 685 34 0.81 312 307 309 309 100 100 100 215 Comparative Example 1 17.5 520 600 24 0.84 / / / / / / / 230 Comparative Example 2 / 522 605 28 0.86 / / / / / / / /

[0064] Comparative Example 1 is from a Chinese patent application: A low-cost, high-strength and high-toughness X70 pipeline steel coil and its production method (CN102560260A); Comparative Example 2 is from a Chinese patent: An economical X70 oil and gas pipeline steel and its production method (CN103805865B).

[0065] The comparative analysis of composition, process, and performance in Tables 1-4 shows that the large-strain roughing process used in this invention drives the full static recrystallization of austenite through high energy input, laying the foundation for ultra-refined microstructure. The incremental deformation resistance control during the finishing rolling stage stably achieves highly flattened deformed austenite, ensuring microstructure uniformity and avoiding mixed crystals. The final "ultra-fast cooling + relaxation" synergistic cooling path, through precise control of the austenite state before phase transformation and optimization of phase transformation kinetics, achieves high strength and high toughness while significantly reducing the yield strength ratio to below 0.82. In contrast, Comparative Examples 1 and 2, perhaps due to a lack of systematic microstructure refinement and phase transformation control processes, have yield strength ratios that are 0.84~0.86 higher, and their low-temperature toughness indicators (Comparative Example 1: -20℃ impact energy 335J, -15℃ drop shear area 100%; Comparative Example 2: -20℃ impact energy 287J, -15℃ drop shear area 98%) are far below the level of this invention.

[0066] In summary, the hot-rolled coil produced by this invention possesses excellent comprehensive performance, meeting all the technical specifications of L485M coils, including a yield strength of 530~600MPa, tensile strength of 630~730MPa, elongation ≥28%, average impact energy Akv ≥280J at -40℃, average drop weight DWTT ≥90% at -20℃, and hardness HV. 10 ≤220, and the yield strength ratio is not higher than 0.82.

[0067] It is hereby noted that the above embodiments are only for illustrating the technical concept and features of the present invention, and are not intended to limit the present invention. Any equivalent substitutions or modifications made without departing from the essence of the present invention fall within the protection scope of the present invention.

Claims

1. A low yield strength ratio, high toughness, and low cost L485M pipeline steel coil, characterized in that, The chemical composition of the coiled plate by weight percentage is as follows: C: 0.045%~0.070%, Si: 0.20%~0.30%, Mn: 1.75%~2.0%, P: ≤0.012%, S: ≤0.0020%, Nb: 0.045%~0.075%, V: 0.028%~0.045%, Ti: 0.008%~0.018%, Cr: 0.15%~0.28%, Als: 0.012%~0.045%, Mg: 0.0010%~0.0025%, Ca: 0.0008%~0.0018%, N: ≤0.005%, Ca / Mg: 0.6~1.2, with the remainder being Fe and unavoidable impurities; The metallographic structure of the hot-rolled coil is a mixture of acicular ferrite and MA, wherein the volume fraction of acicular ferrite is 85%~92% and the volume fraction of MA component is 8%~15%. The width of the acicular ferrite lath bundles is 2.5~4.0μm, with an equivalent ASTM grain size grade of 12.5~13.

0.

2. The low yield strength ratio, high toughness, and low cost L485M pipeline steel coil according to claim 1, characterized in that, The thickness of the hot-rolled coil is 17~22mm, the yield strength is 530~600MPa, the tensile strength is 630~730MPa, the elongation is ≥28%, and the yield strength ratio is not higher than 0.

82.

3. The low yield strength ratio, high toughness, and low cost L485M pipeline steel coil according to claim 1, characterized in that, Hot-rolled coil: -40℃ average impact energy Akv ≥ 280J, -20℃ drop hammer DWTT average ≥ 90%, hardness value HV 10 ≤220.

4. A method for manufacturing the low yield strength ratio, high toughness, and low cost L485M pipeline steel coil as described in any one of claims 1 to 3, comprising smelting, continuous casting, billet heating, rough rolling, finish rolling, cooling, and coiling, characterized in that: The continuously cast slab is directly heated in a hot-charging furnace at 500~850℃, with a heating temperature of 1170~1200℃ and a total furnace time of 150~250min, including a soaking time of ≥120min. The rough rolling process employs a slow-speed, high-reduction technique, with a single-pass reduction rate of 26%~34%, a rolling speed range of 1.0~2.0m / s, and a finishing rolling temperature of 1020~1040℃. After rough rolling, the austenite recrystallized grain size is 18. ~24μm; the finishing rolling temperature is 900~960℃, the single-pass reduction rate of the first two passes is 22%~27%, the reduction rate of the last pass is ≥6%, the cumulative reduction rate is 62%~69%, and the final rolling temperature is 780~840℃; after rolling, the temperature is first cooled to the target temperature of 490~510℃ by ultra-fast cooling at a rate of 30~50℃ / s, and then relaxed by air cooling for 10~25s within the target temperature range. The coiling temperature is 400~480℃.

5. The method for manufacturing a low yield strength ratio, high toughness, and low-cost L485M pipeline steel coil according to claim 4, characterized in that, In the smelting process, after converter smelting and ladle refining, and after the composition is qualified, calcium-magnesium composite is used to modify the inclusions in the steel. The calcium-magnesium feed line speed is ≥4m / s, the soft blowing time after calcium-magnesium treatment is ≥5min, and then the steel is stirred by bottom blowing argon. The net argon blowing time is ≥8min, and the calming time is ≥12min.

6. The manufacturing method of a low yield strength ratio, high toughness, and low cost L485M pipeline steel coil according to claim 4, characterized in that, The superheat of the billet is controlled at 15~30℃ during casting. The slab is continuously cast into a continuous casting slab. The casting process is protected. Dynamic light pressure is applied at the end of solidification. The light pressure of the casting machine is ≥4.5mm. The billet adopts a medium thickness of 170~190mm.

7. The method for manufacturing a low yield strength ratio, high toughness, and low-cost L485M pipeline steel coil according to claim 4, characterized in that, The roughing mill has three passes, with a reduction rate of 26% to 30% in the first pass and 31% to 34% in the second and third passes.

8. The method for manufacturing a low yield strength ratio, high toughness, and low-cost L485M pipeline steel coil according to claim 4, characterized in that, The winding process uses a high-strength winding machine with a drum tension coefficient of 2.0~2.8, an auxiliary winding roller pressure of 450~560MPa, a guide roller pressure of 55~88kN, a drum speed of 0.6 times, a skip number of turns of 5~7, a main drive motor MD lead rate of 13%~15%, an auxiliary winding roller WR lead rate of 16%~17%, and a pinch roller PR lead rate of 12%~13%.

Images