Low yield strength ratio, high crack arrest toughness, low cost L555M pipeline steel coil and its manufacturing method

Abstract

This invention discloses a low-yield-to-tensile ratio, high crack-arresting toughness, and low-cost L555M pipeline steel coil and its manufacturing method. The chemical composition (Wt%) is: C 0.035~0.055, Nb 0.065~0.095, V 0.030~0.065, Ti 0.012~0.023, Cr 0.20~0.30, Cu 0.15~0.28, B 0.0006~0.0015, Mg 0.0012~0.0030, Ca 0.0010~0.0020, Ca / Mg 0.8~1.1. A C-Mn-Nb-B-Cu design is adopted, and the coil R... t0.5 585~690MPa, R m 645~750MPa, A 50 ≥28%, -40℃ A kv Average value ≥320J, DWTT average value ≥90% at -20℃, hardness value HV 10 ≤230, yield strength ratio ≤0.83. This invention solves the problems of high alloy cost, complex rolling process, high cost due to multiple rolling passes, and loose low-temperature crack arrest toughness index and high yield strength of the product.

Description

Technical Field

[0001] This invention belongs to the technical field of high-strength low-alloy steel, and relates to the manufacture of hot-rolled coils of high-grade spiral submerged arc welded pipe steel, particularly to a low-yield-to-tensile ratio, high crack arrest toughness, and low-cost L555M pipeline steel coil and its manufacturing method. Background Technology

[0002] X80M grade pipeline steel, as the mainstream material for long-distance pipeline projects both domestically and internationally, has become the core choice for efficient and safe transportation of oil and natural gas due to its advantages of high strength, large diameter, and high transmission pressure. As pipeline construction gradually extends to complex and harsh environments such as polar regions, deserts, and deep seas, while existing X80M pipeline steel has improved strength through the addition of precious alloying elements such as Cr, Mo, Ni, and Cu, its yield strength and tensile strength have increased simultaneously, resulting in a persistently high yield-to-tensile strength ratio. A high yield-to-tensile strength ratio significantly reduces the material's plasticity reserve, increasing the risk of local buckling or brittle fracture under external forces such as earthquakes, frozen soil displacement, or settlement. Therefore, it is urgent to improve the material's deformation adaptability and safety by reducing the yield-to-tensile strength ratio. At the same time, against the backdrop of intensified market competition in the steel industry, fluctuating alloy raw material prices, and the green and low-carbon transformation, the production cost of existing X80M pipeline steel remains high due to its reliance on a large number of precious alloying elements. How to achieve low-cost manufacturing while ensuring the high strength and toughness of X80M grade pipeline steel has become a key technical challenge that the industry urgently needs to overcome.

[0003] Although there are many patents and documents both domestically and internationally concerning hot-rolled coils of 555MPa grade pipeline steel, they are all significantly different from this invention. Patents and documents that are closer to this invention include:

[0004] 1) Chinese patent application CN102534408A, a low-cost, high-strength and high-toughness X80 pipeline steel coil and its production method. The composition is C: 0.03%~0.07%, Si: 0.10%~0.25%, Mn: 1.25%~1.95%, P: ≤0.018%, S: ≤0.005%, Nb: 0.06%~0.12%, Mo: 0.15%~0.30%, Ti: 0.008%~0.020%, Cr: 0.15%~0.35%, Als: 0.020%~0.045%, Ca: 0.0015%~0.0035%, N: ≤0.0060%, with the remainder being Fe and unavoidable impurities. The patented composition includes the precious element Mo, resulting in higher alloy costs; it does not specify the exact rolling and cooling processes; and it has relatively lenient requirements for low-temperature toughness, with an impact test temperature of -20°C and a DWTT test temperature of -15°C, which differs significantly from the present invention.

[0005] 2) Chinese patent application CN116145048A, an economical high-toughness L555M pipeline steel hot-rolled coil and its production method. The composition is C: 0.055%~0.075%, Si: 0.20%~0.30%, Mn: 1.45%~1.65%, P: ≤0.012%, S: ≤0.002%, Ti: 0.010%~0.022%, Nb: 0.050%~0.065%, Cr: 0.15%~0.25%, Als: 0.020%~0.050%, N: ≤0.0050%, with the remainder being Fe and unavoidable impurities. Although the alloy design of this patent is relatively low, it uses a billet thickness of 230mm, which is relatively thick; the final product specifications are 6-10mm, which is relatively thin, resulting in a large overall compression ratio; and the rough rolling process is relatively complex, using a 5-pass rolling process, which increases the process cost; at the same time, the yield strength ratio of the final product is also relatively high.

[0006] 3) Chinese Patent CN101413090B, a high-strength and high-toughness spiral submerged arc welded pipe X80 hot-rolled coil and its production method. The composition includes C: 0.03%~0.08%, Si: 0.10%~0.25%, Mn: 1.60%~1.95%, Nb: 0.09%~0.11%, V: 0.020%~0.030%, Ti: 0.010%~0.020%, Mo: 0.10%~0.30%, Cu: 0.10%~0.30%, Ni: 0.10%~0.30%, Cr: 0%~0.30%, P: ≤0.018%, S: ≤0.003%, N: ≤0.006%, B: 0~0.0005%, H: ≤0.0003%, with the balance being iron and unavoidable trace impurities. The patented composition contains a high content of precious metal elements such as Mo, Ni, and Cu, resulting in a high alloy cost. Secondly, the patented composition uses a 230 mm billet thickness, which differs from the present invention. Furthermore, the rolling process is not specified, and the final rolling temperature is high, requiring extremely high precision in subsequent cooling rate and control. The patented composition also does not specify the cooling process. The low-temperature toughness index is relatively lenient, with an impact test temperature of -20℃ and a DWTT test temperature of -15℃.

[0007] 4) Chinese patent application CN104561825A, a low-cost X80 pipeline steel and its manufacturing method. The composition contains C: 0.065%~0.085%, Mn: 1.0%~2.0%, Si: 0.25%~0.35%, Cu: 0.10%~0.25%, Ni: 0.10%~0.30%, Cr: 0.10%~0.50%, Nb: 0.02%~0.04%, Ti: 0.005%~0.030%, V: 0.02%~0.04%, Als: 0.02%~0.06%, Ca: ≤0.006%, P: ≤0.015%, S: ≤0.003%, N: ≤0.012%, with the balance being Fe and trace amounts of unavoidable impurities. The patented composition contains precious metal elements Ni and Cu, resulting in high alloy costs; the patented production of X80 flat plates using a wide and thick plate rolling mill is not comparable to the manufacturing process and usage conditions of hot-rolled coils; the patented use a billet thickness of 230mm, which differs from this invention; the patented low-temperature toughness requirements are relatively lenient, with an impact test temperature of -20℃ and a DWTT test temperature of -15℃.

[0008] 5) Chinese Patent CN102851614B, a hot-rolled coil of X80 pipeline steel with low yield strength ratio and its manufacturing method. The composition is C: 0.030%~0.055%, Si: 0.15%~0.30%, Mn: 1.60%~1.85%, Nb: 0.080%~0.120%, Cr: 0.20%~0.35%, Ni: 0.15%~0.25%, Cu: 0.18%~0.28%, Ti: 0.015%~0.030%, Als: 0.015%~0.050%, with the remainder being Fe and unavoidable impurities. The alloy composition of this patent contains precious metal elements such as Ni and Cu, resulting in high alloy costs; the rolling process is relatively complex, leading to high process costs; the thickness of the cast billet is not specified; as can be seen from the examples, the final product is 10-14mm, which is different from the present invention; in addition, the low-temperature toughness index requirements of this patent are relatively lenient, the impact test temperature is -20℃, and the low-temperature DWTT test is not conducted.

[0009] 6) Chinese Patent Application CN101962733A, a low-cost, high-strength and tough X80 pipeline steel resistant to large deformation and its production method. The composition contains C: 0.02%~0.08%, Si: ≤0.40%, Mn: 1.2%~2.0%, P: ≤0.015%, S: ≤0.004%, Nb: 0.03%~0.08%, Ti: 0.005~0.030%, Mo: 0.10%~0.30%, Cu: ≤0.40%, Ni: ≤0.30%, with the remainder being Fe and unavoidable impurities. The patented composition contains precious metal elements such as Mo, Ni, and Cu, resulting in high alloy costs and a wide range of compositions, which can easily lead to fluctuations in product performance. The patented composition uses a billet thickness of 210-300 mm, which differs significantly from the present invention. Furthermore, the patented composition is for producing X80 flat plates using a heavy plate rolling mill, and its manufacturing method and usage conditions are not comparable to those of hot-rolled coil manufacturing methods and usage conditions. The patented composition also does not specify requirements for low-temperature toughness.

[0010] 7) Chinese Patent CN107881421B, Pipeline steel with 550MPa grade high temperature resistance and good low temperature crack arrest toughness and its manufacturing method. The composition contains C: 0.061%~0.120%, Mn: 1.70%~2.20%, Mo: 0.15%~0.39%, Cu: 0.15%~0.30%, Ni: 0.15%~0.50%, Nb: 0.035%~0.080%, V: 0.005%~0.054%, Ti: 0.005%~0.030%, Al: 0.015%~0.040%, Ca: 0.005%~0.035%, with the remainder being Fe and unavoidable impurities. This patent adds a high content of precious metal elements such as Mo and Ni, resulting in a high alloy cost; the rolling process and cooling process of this patent are not specified.

[0011] While there are numerous publicly available patents and publications on low-cost L555M pipeline steel hot-rolled coils, most of them incorporate expensive elements such as Ni and Mo in their composition design, resulting in higher alloy costs. Furthermore, the rolling process is complex, requiring multiple rolling passes, leading to higher process costs. The final product exhibits relatively lenient low-temperature crack arrest toughness 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 crack-arresting toughness, and low-cost L555M pipeline steel coil and its manufacturing method. The invention employs a C-Mn-Nb-B-Cu design, without adding precious metals such as Mo and Ni. To reduce costs, a slow, high-reduction roughing process is used to achieve extreme austenite grain refinement, optimizing overall performance through fine-grain strengthening. The austenite is fully flattened through gradual strengthening deformation resistance control in the finishing roll. After finishing roll, an RPC+ relaxation process is used to reduce the yield-strength ratio of the coil, ultimately achieving a low-yield-strength ratio, high crack-arresting toughness, and low-cost production method for L555M 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 L555M 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.

[0014] One of the technical solutions of this invention is to propose a low yield strength ratio, high crack arrest toughness, and low cost L555M pipeline steel coil, with the following chemical composition (weight, %):

[0015] C: 0.035%~0.055%, Si: 0.23%~0.35%, Mn: 1.85%~2.10%, P: ≤0.010%, S: ≤0.0015%, Nb: 0.065%~0.095%, V: 0.03 0%~0.065%, Ti: 0.012%~0.023%, Cr: 0.20%~0.30%, Cu: 0.15%~0.28%, Als: 0.02%~0.055%, B: 0.0006%~0.0015% The composition of the hot-rolled coil is as follows: Mg: 0.0012%~0.0030%, Ca: 0.0010%~0.0020%, N: ≤0.005%, Ca / Mg: 0.8~1.1, with the remainder being Fe and unavoidable elements. The metallographic structure of the hot-rolled coil is a mixture of acicular ferrite and Mao islands, with the volume fraction of acicular ferrite being 80%~90% and the volume fraction of Mao islands being 10%~20%. The width of the acicular ferrite lath bundles is 1.5~3.0μm, and the grain size of the metallographic structure is grade 13.0~14.0.

[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 can improve hardenability and guarantee the strength and hardness of materials. Its role 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 0.035% to 0.055%.

[0018] Si (Silicon): 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, excessive silicon content will reduce the weldability of steel and significantly reduce its plasticity and toughness, with a range of 0.23% to 0.35%.

[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 the hardenability of steel, effectively ensuring its strength. Manganese can compensate for the strength decrease caused by a reduction in carbon content, making it the most important and economical strengthening element. Manganese shifts the C-curve to the right, promotes bainite transformation, and is conducive to 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 1.85%~2.10%.

[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 ≤ 0.010%, S ≤ 0.0015%, 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 martensite-austenite components, reducing the toughness of the weld heat-affected zone. Considering cost, its range is 0.065%~0.095%.

[0022] Vanadium (V): It exhibits strong precipitation strengthening and weak grain refinement strengthening. Vanadium forms stable compounds with carbon and nitrogen, primarily existing in steel as carbides. These vanadium carbonitrides precipitate uniformly in ferrite in a fine, dispersed form, significantly improving the material's strength. However, excessively high content does not significantly enhance strength but increases alloy costs; the suitable range is 0.030%~0.065%.

[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 0.012%~0.023%.

[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 0.20% to 0.30%.

[0025] Cu (Cu) is a key, economical strengthening element. Its core function is to precipitate nanoscale ε-Cu phases during relaxation and coiling, generating a strong precipitation strengthening effect and significantly improving the tensile strength of the material. Simultaneously, the Cu atoms in solid solution also provide some solid solution strengthening and slightly improve hardenability, perfectly replacing some of the functions of expensive alloying elements. Its content is precisely controlled within the range of 0.15% to 0.28%, aiming to obtain sufficient precipitation strengthening while strictly avoiding the risk of surface hot brittleness caused by excessive content.

[0026] 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. The recommended range is 0.02% to 0.055%.

[0027] Bainite (B): A highly hardenable element, it strongly inhibits the γ-α transformation, shifting the CCT curve to the right and increasing hardenability. Trace amounts of B significantly inhibit ferrite nucleation at austenite grain boundaries, exhibiting a strong synergistic effect with Mo and Nb. It can replace some expensive alloying elements (such as Ni, Cr, and Mo), reducing production costs while maintaining or improving pipeline steel performance. Furthermore, it flattens the bainite transformation curve, allowing for the acquisition of bainitic microstructure even over a wide range of cooling rates. The synergistic design with Ti and N (using TiN to fix N and protect B) ensures that B exists and functions effectively in a solid solution state, guaranteeing a high-strength and high-toughness microstructure without adding Mo. However, excessively high B content can negatively impact weldability, requiring more stringent welding processes and increasing process difficulty. Therefore, this invention controls the B content to 0.0006~0.0015%.

[0028] 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 0.0012%~0.0030%.

[0029] 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 0.0010%~0.0020%.

[0030] The second technical solution of this invention proposes a manufacturing method for low yield strength ratio, high crack arrest toughness, and low cost L555M 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 1180-1220℃ and a total furnace time of 160-250 minutes, of which the soaking section time is ≥130 minutes. The rough rolling process adopts a slow-speed, high-reduction process, with a single-pass reduction rate of 26%-35%, and a rolling speed range of 1.0-2.0 m / s. The roughing rolling temperature is 1030~1050℃, and the austenite recrystallized grain size obtained after roughing is 13~19μm; the finishing rolling temperature is 910~950℃, the single-pass reduction rate of the first two passes is 25%~28%, the reduction rate of the last pass is ≥6%, the cumulative reduction rate is 62%~70%, and the finishing rolling temperature is 810~850℃; after rolling, it is first cooled to the target temperature of 530~550℃ by ultra-fast cooling at a cooling rate of 30~60℃ / s, and then relaxed by air cooling for 5~15s in the target temperature range, and the coiling temperature is 450~500℃.

[0031] 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 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, with a net argon blowing time of ≥10min and a calming time of ≥15min.

[0032] 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.

[0033] Furthermore, the roughing rolling process consists of 3 passes, with a reduction rate of 26% to 30% in the first pass and 32% to 35% in the second and third passes.

[0034] Furthermore, the winding process employs a high-power winding machine with a drum tension coefficient of 2.2~3.0, an auxiliary winding roller pressure of 550~620MPa, a guide roller pressure of 60~90 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 15%~16%, an auxiliary winding roller WR lead rate of 18%~19%, and a pinch roller PR lead rate of 12%~13%.

[0035] Compared with the prior art, the present invention has the following advantages:

[0036] This invention employs a C-Mn-Nb-B-Cu design, eliminating the need for precious metals such as Mo and Ni, resulting in low alloy costs. Simultaneously, a calcium-magnesium composite treatment process 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 crack arrest toughness of the steel and completely eliminates the hazards of large-size MnS inclusions, fundamentally enhancing the material's low-temperature fracture toughness.

[0037] In the rough rolling stage, an innovative slow-speed, high-reduction process is adopted. By applying high strain energy in a concentrated manner, the austenite undergoes full recrystallization, successfully refining the original austenite grains to 13-19 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.

[0038] 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.

[0039] The PRC+ relaxation cooling process firstly suppresses high-temperature microstructure through ultra-fast cooling, laying the foundation for obtaining a medium-temperature transformation microstructure. During the relaxation stage, austenite recovery reduces dislocation density, directly weakening the yield strength and inducing strain-induced precipitation of nanoscale carbonitrides or ε-Cu phases of elements such as Nb, V, and Cu. After final cooling, a multiphase microstructure of "soft matrix + hard particles" is formed—the recovered acicular ferrite dominates the low yield strength, while the dispersed martensite-austenite components and nanoprecipitates ensure high tensile strength through continuous work hardening. Ultimately, a product with high strength, good low-temperature crack arrest toughness, and a low yield strength ratio (≤0.83) is obtained, making the product safer in pipeline engineering.

[0040] The hot-rolled coil of this invention possesses excellent comprehensive performance, meeting all the technical specifications of L555M coil. The hot-rolled coil has a thickness of 18-23 mm, a yield strength of 585-690 MPa, a tensile strength of 645-750 MPa, an elongation ≥28%, an average impact energy Akv ≥320 J at -40℃, an average drop weight DWTT ≥90% at -20℃, and a hardness value HV. 10 ≤230, and the yield strength ratio is not higher than 0.83. Attached Figure Description

[0041] Figure 1 Metallographic diagram of the rough-rolled austenite grain size in Example 1.

[0042] Figure 2 The image shows the metallographic structure of the hot-rolled coil in Example 1.

[0043] Figure 3 This is a diagram showing the resistance control of incremental deformation during finishing rolling implemented in Example 1. Detailed Implementation

[0044] 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.

[0045] The present invention will be described in more detail below through examples.

[0046] Compared with existing technologies, this solution adopts a C-Mn-Nb-B-Cu 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, optimizing its overall performance through grain refinement strengthening. The "progressive strengthening" finishing rolling deformation resistance control maintains stable grain flattening and elongation, fully flattening the austenite. After finishing rolling, an RPC+ relaxation process is used to reduce the plate's yield strength ratio, ultimately obtaining a product with high strength, good low-temperature crack arrest toughness, and a low yield strength ratio (≤0.83), resulting in higher product safety.

[0047] A low-yield-to-tensile ratio, high crack-arresting toughness, and low-cost L555M pipeline steel coil has the following chemical composition by weight percentage: C: 0.035%~0.055%, Si: 0.23%~0.35%, Mn: 1.85%~2.10%, P: ≤0.010%, S: ≤0.0015%, Nb: 0.065%~0.095%, V: 0.030%~0.065%, Ti: 0.012%~ 0.023%, Cr: 0.20%~0.30%, Cu: 0.15%~0.28%, Als: 0.02%~0.055%, B: 0.0006%~0.0015%, Mg: 0.0012%~0.0030%, Ca: 0.0010%~0.0020%, N: ≤0.005%, Ca / Mg: 0.8~1.1, with the remainder being Fe and unavoidable impurities. The chemical compositions of Examples 1-6 are shown in Table 1.

[0048] Table 1. Chemical composition of steel in the examples (Wt, %)

[0049] Example C Si Mn P S Nb V Ti Cr Cu Als B Ca Mg Ca / Mg N Mo Ni 1 0.045 0.29 1.95 0.008 0.0012 0.075 0.045 0.018 0.25 0.20 0.039 0.0009 0.0018 0.0022 0.82 0.004 / / 2 0.049 0.28 1.94 0.006 0.0013 0.082 0.042 0.015 0.23 0.19 0.038 0.0008 0.0015 0.0018 0.83 0.003 / / 3 0.043 0.30 1.95 0.008 0.0012 0.074 0.052 0.016 0.25 0.21 0.035 0.0010 0.0017 0.0019 0.89 0.003 / / 4 0.042 0.31 1.98 0.005 0.0012 0.075 0.048 0.015 0.28 0.18 0.040 0.0009 0.0017 0.0021 0.81 0.004 / / 5 0.039 0.28 1.92 0.009 0.0014 0.079 0.040 0.016 0.22 0.19 0.042 0.0012 0.0018 0.0021 0.86 0.002 / / 6 0.050 0.29 1.90 0.010 0.0011 0.078 0.050 0.015 0.23 0.18 0.036 0.0008 0.0016 0.0020 0.80 0.003 / / Comparative Example 1 0.040 0.20 1.85 0.009 0.002 0.10 0.025 0.015 0.01 0.24 0.035 0.0005 / / / 0.004 0.24 0.24 Comparative Example 2 0.040 0.20 1.85 0.009 0.002 0.10 0.025 0.015 0.01 0.24 0.035 0.0005 / / / 0.004 0.24 0.24

[0050] A method for manufacturing low-yield-to-tensile ratio, high crack-arresting toughness, and low-cost L555M 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:

[0051] 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 time for the inclusions that have been modified by calcium and magnesium and become low-melting-point spherical inclusions to float. Then, by bottom blowing argon stirring with a net argon blowing time ≥10min and a calming time ≥15min, 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.

[0052] 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 entire casting process is protected. At the end of solidification, dynamic light pressure is applied. 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.

[0053] Continuous casting slab heating process: The continuously cast slab is directly heated in a hot-charging furnace at 500~850℃, then heated in a walking beam furnace at 1180~1220℃, with a total furnace time of 160~250min, including a soaking time of ≥130min, 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.

[0054] 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 32%~35%. 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 1030~1050℃. 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 (13~19 μm), laying the microstructure foundation for subsequent controlled rolling. Therefore, using this rolling scheme, the austenite recrystallized grain size obtained after rough rolling is 13~19 μm (see appendix for details). Figure 1 The metallographic diagram of the rough-rolled austenite grain size shows that the ultra-fine recrystallized austenite was achieved, which significantly reduced the dependence on precious metals such as Mo and Ni, and realized "process instead of alloy", which significantly reduced the alloy cost and the process cost at the same time.

[0055] Finishing rolling process: The finishing rolling process consists of 7 passes. The reduction rate of the first two passes is 21%~26%, and the reduction rate of the last pass is ≥6%, with a cumulative reduction rate of 62%~70%. The initial rolling temperature is 910~950℃, and the final rolling temperature is 810~850℃. 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.

[0056] The heating, roughing, and finishing process parameters for Examples 1 to 6 are shown in Table 2.

[0057] Table 2. Heating, roughing, and finishing rolling process parameters for each embodiment.

[0058] 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 54 1195 165 1042 29 32 32 68.2 17.5 2 170 54 1198 168 1038 30 31 32 68.2 17.2 3 170 54 1195 165 1045 29 31 32 68.2 17.3 4 190 56 1205 171 1041 32 33 33 70.5 16.1 5 190 56 1203 170 1036 33 34 33 70.5 15.9 6 190 56 1208 175 1038 32 32 34 70.5 16.0 Comparative Example 1 230 / 1180 / 1060 / / / / / Comparative Example 2 230 / 1150 / 1020 / / / / / 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 / % / / 1 938 825 3 65.9 22 23 23 7 / / 2 935 822 3 65.9 21 22 24 6 / / 3 937 829 3 65.9 23 23 22 7 / / 4 935 830 2 61.8 23 23 20 6 / / 5 938 828 2 61.8 23 22 20 7 / / 6 940 826 3 61.8 22 22 21 6 / / Comparative Example 1 950 845 / / / / / / / / Comparative Example 2 980 830 / / / / / / / /

[0059] 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 530-550℃ at a cooling rate of 30-60℃ / 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 5-15s relaxation air cooling process is performed within the target temperature range, with a coiling temperature of 450-500℃. 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 or ε-Cu phases of elements such as Nb, V, and Cu. Ultimately, during the final cooling process, the restored "softened" austenite transforms into a matrix dominated by acicular ferrite, while the dispersed hard martensite-austenite components and nano-precipitates act as strong dislocation barriers, ensuring high tensile strength. This results in a mixed microstructure of uniformly sized acicular ferrite and martensite-austenite components (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 (≤0.83) is obtained, which makes the product safer in pipeline engineering.

[0060] Winding process: After ultra-fast cooling, a high-power winder is used for winding. The winding temperature is 450~500℃, the drum tension coefficient is 2.2~3.0, the auxiliary winding roller pressure is 550~620MPa, the guide roller pressure is 60~90kN, the drum expansion speed is 0.6 times, the number of skip turns is 5~7, the main drive motor MD lead rate is 15%~16%, the auxiliary winding roller WR lead rate is 18%~19%, the pinch roller PR lead rate is 12%~13%, and the coil shape is good with no hard bends.

[0061] 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.

[0062] Table 3 shows the cooling and winding process parameters and the volume percentage of metallographic structures for each embodiment.

[0063] Example Ultra-fast cooling rate / ℃·s-1 Relaxation temperature / ℃ Air cooling time / s Final cooling temperature / ℃ drum tension coefficient Auxiliary roller pressure / MPa Guide gauge pressure / kN Acicular ferrite / % Martensitic-austenitic components / % 1 48 535 8 485 2.5 585 82 85 15 2 52 542 10 476 2.4 591 79 86 14 3 40 538 9 478 2.5 586 80 84 16 4 45 540 11 467 2.8 596 87 88 12 5 46 539 8 483 2.6 587 84 90 10 6 45 535 8 481 2.5 594 85 87 13 Comparative Example 1 / / / 410 / / / / / Comparative Example 2 / / / 450 / / / / /

[0064] Table 4 Main mechanical properties of each embodiment

[0065] Example Finished product specifications / mm Rt0.5 / MPa Rm / MPa A50mm / % Rt0.5 / Rm 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 hammer at -20℃ / % <![CDATA[Hardness HV 10 > 1 18.4 612 746 28 0.82 355 356 378 363 100 100 100 218 2 18.4 594 733 32 0.81 366 358 381 368 100 100 100 214 3 18.4 605 747 29 0.81 359 386 344 363 100 98 99 217 4 21.4 578 713 34 0.81 349 375 364 363 100 98 99 212 5 21.4 564 714 35 0.79 368 353 355 359 100 100 100 208 6 21.4 582 718 31 0.81 369 372 377 373 100 100 100 214 Comparative Example 1 18.4 620 720 24 0.86 / / / / / / / 242 Comparative Example 2 18.4 590 705 26 0.84 / / / / / / / 239

[0066] Comparative Examples 1 and 2 are from the patent: A high-strength and high-toughness spiral submerged arc welded pipe X80 hot-rolled coil and its production method (CN101413090B).

[0067] 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 0.83. In contrast, the comparative example, perhaps due to a lack of systematic microstructure refinement and phase transformation control processes, has a higher yield strength ratio of 0.84~0.86, and its low-temperature crack arrest toughness index (351 J impact energy at -20℃ and 100% drop shear area at -15℃) is far below the level of this invention.

[0068] In summary, the hot-rolled coil of this invention possesses excellent comprehensive performance, meeting all the technical specifications of L555M coil. The hot-rolled coil has a thickness of 18-23 mm, a yield strength of 585-690 MPa, a tensile strength of 645-750 MPa, an elongation ≥28%, an average impact energy Akv ≥320 J at -40℃, an average drop weight DWTT ≥90% at -20℃, and a hardness value HV. 10 ≤230, and the yield strength ratio is not higher than 0.83.

[0069] 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-to-tensile strength ratio, high crack-arresting toughness, and low-cost L555M pipeline steel coil, characterized in that, The chemical composition of the coil, by weight percentage, is as follows: C: 0.035%~0.055%, Si: 0.23%~0.35%, Mn: 1.85%~2.10%, P: ≤0.010%, S: ≤0.0015%, Nb: 0.065%~0.095%, V: 0.030%~0.065%, Ti: 0.012%~0.023%, Cr: 0.20%~0.30%, Cu: 0.15%~0.28%, Als: 0.02%~0.055%, B: 0.0006%~0.0015%, Mg: 0.0012%~0.0030%, Ca: 0.0010%~0.0020%, N: ≤0.005%, Ca / Mg: 0.8~1.1, with the remainder being Fe and unavoidable impurities. The microstructure of the hot-rolled coil is a mixture of acicular ferrite and martensite-austenite, with acicular ferrite comprising 80%–90% by volume and martensite-austenite comprising 10%–20% by volume. The width of the acicular ferrite lath bundles is 1.5–3.0 μm, and the grain size is 13.0–14.

0. The manufacturing process includes smelting, continuous casting, billet heating, rough rolling, finish rolling, cooling, and coiling. Specifically, the continuously cast slab is directly heated in a hot-charging furnace at 500–850℃, with a heating temperature of 1180–1220℃ and a total furnace time of 160–250 min, including a soaking time of ≥130 min. The rough rolling process uses a slow speed. The large reduction process has a single-pass reduction rate of 26%~35%, a rolling speed range of 1.0~2.0m / s, a roughing rolling finishing temperature of 1030~1050℃, and a recrystallized austenite grain size of 13~19μm after roughing. The finishing rolling starting temperature is 910~950℃, the single-pass reduction rate of the first two passes is 25%~28%, the reduction rate of the last pass is ≥6%, the cumulative reduction rate is 62%~70%, and the finishing rolling temperature is 810~850℃. After rolling, the material is first cooled to the target temperature of 530~550℃ using an ultra-fast cooling method at a cooling rate of 30~60℃ / s, and then relaxed by air cooling for 5~15s within the target temperature range. The coiling temperature is 450~500℃.

2. The low yield strength ratio, high crack arrest toughness, and low-cost L555M pipeline steel coil according to claim 1, characterized in that, The thickness of the hot-rolled coil is 18~23mm, the yield strength is 585~690MPa, the tensile strength is 645~750MPa, the elongation is ≥28%, and the yield strength ratio is not higher than 0.

83.

3. The low yield strength ratio, high crack arrest toughness, and low-cost L555M pipeline steel coil according to claim 1, characterized in that, Hot-rolled coil: -40℃ average impact energy Akv ≥ 320J, -20℃ drop hammer DWTT average ≥ 90%, hardness value HV 10 ≤230.

4. A method for manufacturing low yield strength ratio, high crack arrest toughness, and low cost L555M 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 1180~1220℃ and a total furnace time of 160~250min, including a soaking time of ≥130min. The rough rolling process employs a slow-speed, high-reduction process, with a single-pass reduction rate of 26%~35%, a rolling speed range of 1.0~2.0m / s, and a finishing rolling temperature of 1030~1050℃. After rough rolling, the austenite recrystallized grain size is 1... 3~19μm; the finishing rolling temperature is 910~950℃, the single-pass reduction rate of the first two passes is 25%~28%, the reduction rate of the last pass is ≥6%, the cumulative reduction rate is 62%~70%, and the final rolling temperature is 810~850℃; after rolling, the temperature is first cooled to the target temperature of 530~550℃ by ultra-fast cooling at a rate of 30~60℃ / s, and then relaxed by air cooling for 5~15s within the target temperature range. The coiling temperature is 450~500℃.

5. The method for manufacturing low-yield-to-tensile ratio, high crack-arresting toughness, and low-cost L555M pipeline steel coils according to claim 4, characterized in that, In the smelting process, after converter smelting and ladle refining, the composition is qualified. Inclusions in steel were modified by using calcium-magnesium composite material. The calcium-magnesium feed rate was ≥4 m / s, the soft blowing time after calcium-magnesium treatment was ≥5 min, and then the steel was stirred by bottom argon blowing. The net argon blowing time was ≥10 min, and the calming time was ≥15 min.

6. The method for manufacturing low-yield-to-tensile ratio, high crack-arresting toughness, and low-cost L555M pipeline steel coils 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 low-yield-to-tensile ratio, high crack-arresting toughness, and low-cost L555M pipeline steel coils 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 32% to 35% in the second and third passes.

8. The method for manufacturing low-yield-to-tensile ratio, high crack-arresting toughness, and low-cost L555M pipeline steel coils according to claim 4, characterized in that, The winding process uses a high-strength winding machine with a drum tension coefficient of 2.2~3.0, an auxiliary winding roller pressure of 550~620 MPa, a guide roller pressure of 60~90 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 15%~16%, an auxiliary winding roller WR lead rate of 18%~19%, and a pinch roller PR lead rate of 12%~13%.

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