Low-cost high-performance l450m pipeline steel hot-rolled coil and manufacturing method thereof

Abstract

This invention discloses a low-cost, high-performance L450M pipeline steel hot-rolled coil and its manufacturing method. The chemical composition (Wt%) of the hot-rolled coil is: C 0.05~0.08, Si 0.15~0.30, Mn 1.35~1.85, P≤0.013, S≤0.005, Nb 0.035~0.045, Ti 0.009~0.018, Cr 0.10~0.18, Als 0.015~0.055, Mg 0.0008~0.0032, Ca 0.0005~0.0020, N≤0.005, Ca / Mg 0.3~1. This invention adopts a C-Mn-Nb-Mg-Ca design, combined with rolling and cooling processes, to produce a coil with a R... t0.5 490~590MPa, R m 550~700MPa, A 50 ≥30%, Average impact energy A at -40℃ kv ≥300J, -20℃ drop weight DWTT average ≥90%, hardness value HV 10 ≤210, yield strength ratio not higher than 0.78. This invention solves the problems of existing rolling processes being relatively complex, having multiple rolling passes and high costs, and having relatively lenient low-temperature toughness indicators for products.

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-cost, high-performance L450M pipeline steel hot-rolled coil and its manufacturing method. Background Technology

[0002] L450M pipeline steel, as the mainstream material for medium- and high-pressure oil and gas pipeline projects both domestically and internationally, is widely used in onshore long-distance pipelines, urban gas networks, and shallow-sea oil and gas gathering and transportation systems due to its moderate strength, good weldability, and economy. With the continuous expansion of global energy infrastructure, pipeline construction is gradually developing towards more complex geological conditions and more diversified transport media. L450M pipeline steel, with its advantages in both safety and cost, has become a core material in the medium-pressure transmission field. However, the yield strength ratio of traditional L450M pipeline steel is generally maintained in the range of 0.80 to 0.85, which is insufficient to meet the demand for high plasticity reserves in complex terrain areas (such as active fault zones, permafrost regions, and high-strain environments). A high yield strength ratio can lead to insufficient strain coordination capacity of the material under conditions such as seismic activity, foundation settlement, or low-temperature shrinkage, increasing the risk of local buckling, crack initiation, and brittle fracture. At the same time, the steel industry faces the dual challenges of rising resource costs and low-carbon transformation. Although traditional L450M steel has a lower alloy content than high-grade pipeline steel (such as X80), some companies still rely on precious elements such as Ni and Mo to improve its performance, resulting in high production costs. Driven by shrinking industry profits and green manufacturing requirements, there is an urgent need to reduce the consumption of precious alloys through low-cost alloy design and process innovation (such as rolling processes and cooling technologies) to achieve a synergistic improvement in material strength, toughness, and yield strength ratio. This has become a key technological path for enhancing the adaptability of L450M steel to complex environments.

[0003] Although there are many patents and documents both domestically and internationally concerning hot-rolled coils of 450MPa 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 CN101082106B, a method for low-cost production of X65 pipeline steel. The composition is C: 0.046%~0.060%, Si: 0.15%~0.30%, Mn: 1.40%~1.60%, P: 0.006%~0.015%, S: 0.001%~0.008%, Nb: 0.045%~0.050%, Ti: 0.015%~0.025%, N: 0.005%~0.007%, with the balance being Fe and unavoidable impurities. Although this patent does not add precious alloys such as Mo, Ni, and Cu, it is only applicable to the production of ultra-thin (less than 10mm) X65 pipeline steel on CSP production lines, and cannot be used in various complex and extreme service environments; it uses a billet thickness of 60mm, which is significantly different from this invention; the patent does not specify the specific rolling and cooling processes; in addition, the low-temperature toughness requirement is -10℃ impact, and the low-temperature toughness index is relatively lenient.

[0005] 2) Chinese Patent CN102851599B, ​​a thick-walled, low-cost X65 hot-rolled coil for spiral welded pipes and its manufacturing method. The composition is: C: 0.05%~0.10%, Si: 0.10%~0.30%, Mn: 1.50%~1.65%, P: ≤0.020%, S: ≤0.008%, Ti: 0.008%~0.020%, Als: 0.02%~0.06%, Nb: 0.04%~0.07%, Cr: 0.10%~0.20%, V: 0.03%~0.06%, N: <0.008%, Ceq≤0.40%, Pcm≤0.21%, with the remainder being Fe and unavoidable elements. The patented composition contains V but does not contain Mg, and the alloy design system is different. The patented cooling process uses laminar flow cooling, which cannot fully utilize the water-substitute alloy to reduce costs and improve strength and toughness. In addition, the patented low-temperature toughness requirements are also relatively lenient, with an impact test temperature of 0°C and no low-temperature DWTT test.

[0006] 3) Chinese Patent CN109385576A, a low-cost X65 pipeline steel based on magnesium treatment and its manufacturing method. The composition is C: 0.07%~0.09%, Si: 0.1%~0.30%, Mn: 1.35%~1.45%, P: ≤0.015%, S: ≤0.006%, Nb: 0.035%~0.045%, Ti: 0.010%~0.025%, Mg: 0.0010%~0.0030%, Als: 0.020%~0.030%, N: ≤0.0060%, with the remainder being Fe and unavoidable impurities. Although the alloy cost of this patent is relatively low, the rolling process is more complex. The rough rolling uses 6 passes, which increases the process cost. The thickness of the continuously cast billet is 210~230mm, which is different from the present invention. The cooling process uses laminar flow cooling, which cannot fully utilize the water-substitute alloy to reduce costs and improve strength and toughness. In addition, the low-temperature toughness index is not specified.

[0007] 4) Chinese Patent CN118441224A, Thin-gauge, Low-yield-strength-ratio, Low-temperature-service L450M pipeline steel and its production method. The composition is: C: 0.062%~0.078%, Si: 0.15%~0.25%, Mn: 1.50%~1.60%, Nb: 0.032%~0.043%, Ti: 0.026%~0.029%, Mo: 0.06%~0.12%, Cr: 0.06%~0.12%, Als: 0.015%~0.045%, P≤0.015%, S≤0.004%, N≤0.004%, with the remainder being iron and unavoidable impurities. The patented composition includes the precious metal element Mo but does not include Mg, resulting in a different alloy system and higher alloy cost. In addition, the patented composition uses a relatively thick 200-230mm billet, resulting in a thinner final product (7-9mm), requiring more rolling passes, making the process more complex and increasing process costs. The cooling process uses laminar flow cooling, which cannot fully utilize the water-substitute alloy to reduce costs and improve strength and toughness.

[0008] 5) Chinese Patent CN104046896A, an economical X65 oil and gas pipeline steel and its production method. The composition is C: 0.116%~0.14%, Si: 0.15%~0.40%, Mn: 1.10%~1.40%, P: ≤0.015%, S≤0.0020%, Nb: 0.025%~0.045%, V: 0.057%~0.060%, Ti: 0.005~0.020%, Al: 0.020%~0.050%, N: ≤0.008%, with the balance being Fe and unavoidable inclusions. This patent is for the production of X65 flat plates by a heavy plate rolling mill. Its manufacturing method and usage conditions are not comparable to those of hot-rolled coil manufacturing methods and usage conditions. The thickness of the billet and the thickness of the final product are not specified. The rolling process and cooling process are not specified. In addition, the low-temperature toughness requirements are -20℃ impact and -15℃ drop hammer, which are relatively lenient low-temperature toughness indicators.

[0009] 6) Chinese Patent CN112410647B, a method for producing low-cost X65 acid-resistant pipeline steel plates for use under ultra-low temperature conditions using a coiling furnace. The composition is as follows: C: 0.02%~0.04%, Si: 0.15%~0.30%, Mn: 1.35%~1.50%, P: ≤0.008%, S: ≤0.0008%, Al: 0.02%~0.04%, Nb: 0.020%~0.040%, V: ≤0.10%, Cr+Cu: 0.25%~0.35%, Mo+Ni: 0.20%~0.30%; Ti: 0.010%~0.020%, with the remainder being Fe and unavoidable impurities. The patented alloy design contains precious metal elements such as Mo, Ni, and Cu, resulting in higher alloy costs. This patent is for producing X65 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.

[0010] While there are many publicly available patents and documents on low-cost L450M pipeline steel hot-rolled coils, most of them involve complex rolling processes with multiple rolling passes, resulting in high process costs and thinner final product specifications with relatively lenient low-temperature toughness indicators. Summary of the Invention

[0011] The purpose of this invention is to provide a low-cost, high-performance L450M pipeline steel hot-rolled 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 achieves extreme austenite grain refinement through a "slow-speed, high-reduction" rough rolling process, optimizing its overall performance through fine-grain strengthening. The austenite is fully flattened through "progressive strengthening" deformation resistance control during finish rolling. After finish rolling, a relaxation + RPC process is used to reduce the yield strength ratio of the coil, ultimately achieving the production of a low-yield-strength-ratio, high-performance, and low-cost L450M pipeline steel hot-rolled coil.

[0012] To achieve the above objectives, the technical solution adopted by the present invention is as follows:

[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 L450M 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-cost, high-performance L450M pipeline steel hot-rolled coil, with the following chemical composition (weight, %):

[0015] C: 0.05%~0.08%, Si: 0.15%~0.30%, Mn: 1.35%~1.85%, P: ≤0.013%, S: ≤0.005%, Nb: 0.035%~0.045%, Ti: 0.009%~0.018%, Cr: 0.10%~0.18%, Als: 0.015%~0.055%, Mg: 0.0008%~0.0032%, Ca: 0.0005%~0.0020%, N: ≤0.005%, with the remainder being Fe and unavoidable impurities. The microstructure of the hot-rolled coil is mainly composed of acicular ferrite (volume fraction ≥90%), interspersed with fine MA islands (volume fraction ≤5%) and a small amount of bainite. The microstructure is uniform and fine, with an ASTM grain size level ≥12, and the average width of the obtained acicular ferrite lath bundles is controlled within the range of 3.0~5.0μm.

[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 material strength and hardness. 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, and it is also necessary to ensure the formation of a certain amount of strengthening phases (such as M / A islands). 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 0.05% to 0.08%.

[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 the steel plate in the transverse and longitudinal directions. 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 0.15% to 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 1.35%~1.85%.

[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 ≤ 0.013%, S ≤ 0.005%, 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 relaxation and cooling, 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 0.035%~0.045%.

[0022] 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 range is 0.009%~0.018%.

[0023] 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, inhibiting the formation of polygonal ferrite, and promoting the formation of acicular ferrite, thereby improving the strength and hardness of 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 optimal chromium content is 0.10% to 0.18%.

[0024] 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 acceptable range for this range is 0.015% to 0.055%.

[0025] Mg can reduce the oxygen and sulfur content and the number of inclusions in steel, purifying the molten steel and greatly reducing the harm of inclusions to steel properties. It can also significantly modify inclusions in steel, generating fine and dispersed inclusions that provide nucleation sites for sulfide and carbonitride precipitation, which helps optimize the distribution of inclusions in steel. At the same time, it can induce the nucleation of acicular ferrite (AF), thereby refining the microstructure of steel. It can also increase yield strength and tensile strength by more than 5%, while maintaining plasticity basically unchanged, thus reducing the yield strength ratio. It can also improve the low-temperature toughness of steel. In addition, by utilizing the microalloying effect of Mg and designing a reasonable amount of Mg added, the amount of expensive microalloying metals such as Nb, V, and Ti can be reduced, thereby reducing the alloying production cost. The range is 0.0008%~0.0032%.

[0026] 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 0.0005%~0.0020%.

[0027] The second technical solution of this invention proposes a low-cost, high-performance hot-rolled coil manufacturing method for L450M pipeline steel, 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 550~900℃, with a heating temperature of 1160~1190℃ and a total furnace time of 140~280 min, of which the soaking section furnace time is ≥120 min. The rough rolling process adopts a slow-speed, high-reduction process, with a single-pass reduction rate of 29%~35%, and a rolling speed range of 1.0~ The rolling speed is 2.0 m / s, the roughing rolling temperature is 1010~1050℃, and the austenite recrystallized grain size after roughing is 22~28μm; the finishing rolling temperature is 920~950℃, the single-pass reduction rate of the first 3 passes is 22%~27%, the reduction rate of the last pass is ≥8%, the cumulative reduction rate is 71%~85%, and the finishing rolling temperature is 870~900℃; after rolling, the rolling is first relaxed by air cooling for 10~40s, and then cooled by ultra-fast cooling at a cooling rate of 28~50℃ / s; the coiling temperature is 380~440℃.

[0028] 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 (using a small argon flow rate) time after calcium-magnesium treatment is ≥5min, and then the bottom blowing argon agitation is carried out, with a net argon blowing time of ≥8min and a calming time of ≥12min.

[0029] Furthermore, the superheat during casting is controlled at 20~30℃, 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.

[0030] Furthermore, the roughing rolling process consists of 3 passes, with a reduction rate of 29% to 32% in the first pass and 31% to 35% in the second and third single passes.

[0031] Furthermore, the winding machine drum tension coefficient is 1.8~2.4, the auxiliary winding roller pressure is 370~420 MPa, the guide roller pressure is 38~55kN, the drum expansion speed is 0.6 times, the number of skipped turns is 5~7, the main drive motor MD lead rate is 12%~14%, the auxiliary winding roller WR lead rate is 16%~17%, and the pinch roller PR lead rate is 12%~13%.

[0032] Compared with the prior art, the present invention has the following advantages:

[0033] 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.

[0034] 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 20-28 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.

[0035] 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.

[0036] The relaxation + PRC cooling process is employed. During the relaxation stage, the distorted austenite recovers, reducing dislocation density and thus actively lowering the yield strength. Subsequently, the PRC process guides the supercooled austenite to transform into ultrafine acicular ferrite, accompanied by nanoscale NbC precipitation, jointly ensuring high tensile strength. The final product exhibits high strength, good low-temperature toughness, and a low yield strength ratio (≤0.78), enhancing its safety in pipeline engineering.

[0037] The hot-rolled coil of this invention has excellent comprehensive performance and can meet all the technical specifications of L450M coil. The hot-rolled coil has a thickness of 10~16mm, yield strength of 490~590MPa, tensile strength of 550~700MPa, elongation ≥30%, average impact energy Akv ≥300J at -40℃, average drop weight DWTT ≥90% at -20℃, and hardness HV. 10 ≤210, and the yield strength ratio is not higher than 0.78. Attached Figure Description

[0038] Figure 1 Metallographic diagram of the rough-rolled austenite grain size in Example 1.

[0039] Figure 2 The image shows the metallographic structure of the hot-rolled coil in Example 1.

[0040] Figure 3 This is a diagram showing the resistance control of incremental deformation during finishing rolling implemented in Example 1. Detailed Implementation

[0041] 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.

[0042] The present invention will be described in more detail below through examples.

[0043] Compared with existing technologies, this invention adopts a C-Mn-Nb-Mg-Ca design, without adding precious metal elements such as Mo, Ni, and Cu, 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. Through "progressive strengthening" fine rolling deformation resistance control, it maintains a stable grain flattening and elongation state, fully flattening the austenite. After fine rolling, a relaxation + RPC process is used to reduce the plate's yield strength ratio, ultimately obtaining a product with high strength, good low-temperature toughness, and a low yield strength ratio (≤0.78), resulting in higher product safety.

[0044] A low-cost, high-performance L450M pipeline steel hot-rolled coil has the following chemical composition by weight percentage: C: 0.05%~0.08%, Si: 0.15%~0.30%, Mn: 1.35%~1.85%, P: ≤0.013%, S: ≤0.005%, Nb: 0.035%~0.045%, Ti: 0.009%~0.018%, Cr: 0.10%~0.18%, Als: 0.015%~0.055%, Mg: 0.0008%~0.0032%, Ca: 0.0005%~0.0020%, N: ≤0.005%, Ca / Mg: 0.3~1, with the remainder being Fe and unavoidable impurities; the chemical compositions of Examples 1-6 are shown in Table 1.

[0045] Table 1. Chemical composition of steel in the examples (Wt, %)

[0046] Example C Si Mn P S Nb Ti Cr Als Ca Mg Ca / Mg N 1 0.065 0.25 1.53 0.008 0.0011 0.042 0.010 0.15 0.029 0.0012 0.0028 0.43 0.004 2 0.062 0.26 1.50 0.010 0.0016 0.040 0.009 0.12 0.031 0.0010 0.0025 0.40 0.003 3 0.071 0.25 1.51 0.010 0.0015 0.044 0.013 0.14 0.031 0.0014 0.0025 0.56 0.003 4 0.069 0.29 1.55 0.009 0.0018 0.039 0.011 0.13 0.028 0.0009 0.0022 0.41 0.004 5 0.066 0.27 1.52 0.011 0.0020 0.041 0.010 0.15 0.030 0.0015 0.0026 0.58 0.004 6 0.065 0.25 1.48 0.010 0.0019 0.040 0.008 0.15 0.035 0.0012 0.0026 0.46 0.004 Comparative Example 1 0.070 0.22 1.54 0.014 0.0020 0.050 0.012 0.17 0.020 / / / 0.003 Comparative Example 2 0.072 0.30 1.41 0.012 0.0034 0.041 0.016 0.15 0.022 / 0.0020 / 0.0032

[0047] A method for manufacturing low-cost, high-performance L450M pipeline steel hot-rolled 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. Wherein:

[0048] 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.

[0049] Continuous casting process: The superheat of the billet is controlled at 20~30℃ 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.

[0050] Continuous casting slab heating process: The continuously cast slab is directly hot-charged at 550~900℃ and heated in a walking beam furnace at 1160~1190℃, with a total furnace time of 140~280min, including a soaking time of ≥120min, to ensure thorough heating and complete austenitization. Within this heating temperature range, elements such as Cr and Nb are fully dissolved, and 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.

[0051] 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 29%~32%, and the reduction rates in the second and third single passes are 31%~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 1010~1050℃. Temperatures above this range ensure complete recrystallization during rough rolling while preventing austenite grain growth, refining the grains, and improving strength. Therefore, using this rolling scheme, the austenite recrystallized grain size obtained after rough rolling is 22~28 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.

[0052] Finishing rolling process: The finishing rolling process consists of 7 passes. The reduction rate per pass in the first 3 passes is 22%~27%, and the reduction rate in the last pass is ≥8%, with a cumulative reduction rate of 71%~85%. The initial rolling temperature is 920~950℃, and the final rolling temperature is 870~900℃. The temperature range of the final rolling is conducive to Nb precipitation, making full use of the "dragging effect" of Nb to ensure that the austenite is flattened in the non-recrystallization zone. However, the higher temperature enhances the dislocation recovery ability. While obtaining flattened austenite (ensuring high tensile strength), the deformation dislocation density is effectively reduced (thus reducing yield strength). At the same time, the deformation resistance during the finishing rolling process is controlled to keep it in a continuously increasing state (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.

[0053] The heating, roughing, and finishing process parameters for Examples 1 to 6 are shown in Table 2.

[0054] Table 2. Heating, roughing, and finishing rolling process parameters for each embodiment.

[0055] 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 52 1178 125 1032 30% 31% 31% 69.4% 24.5 2 170 52 1182 125 1035 29% 31% 32% 69.4% 23.6 3 170 52 1175 125 1028 29% 32% 31% 69.4% 23.8 4 190 58 1169 135 1030 31% 34% 33% 69.5% 22.5 5 190 58 1181 135 1032 30% 33% 33% 69.5% 22.9 6 190 58 1185 135 1029 32% 33% 32% 69.5% 22.7 Comparative Example 1 / / 1220 / 1048 / / / / / Comparative Example 2 210~230 / 1227 219 1018 / / / / / Example Finishing rolling start temperature / ℃ Finishing rolling temperature / ℃ Passes with a reduction rate ≥ 25% Total reduction rate in finishing rolling / % First pass reduction rate / % Second pass reduction rate / % Third pass reduction rate / % 4th pass reduction rate / % 7th pass reduction rate / % 1 938 885 4 80.8% 26% 27% 28% 25% 10% 2 935 883 4 80.8% 25% 26% 28% 25% 9% 3 939 879 4 80.8% 26% 26% 28% 27% 9% 4 942 881 3 75.9% 26% 25% 25% 17% 10% 5 945 891 3 75.9% 25% 25% 27% 16% 11% 6 937 886 3 75.9% 25% 26% 26% 16% 10% Comparative Example 1 931 819 / / / / / / / Comparative Example 2 / 848 / / / / / / /

[0056] Cooling Process: After rolling, a relaxation process combined with ultra-fast cooling is employed. Air cooling for 10-40 seconds is first performed, and this brief high-temperature hold promotes the recrystallization of austenite grains, resulting in more uniform grain size and eliminating localized stress concentrations. Simultaneously, carbonitrides of Nb and Ti microalloying elements (such as Nb(C,N) and TiN) may partially precipitate. The uniform distribution of these precipitates reduces excessive yield strength growth, while tensile strength remains high due to grain refinement and transformation strengthening, thus lowering the yield-to-tensile ratio. Furthermore, relaxation before rapid cooling provides more uniform initial conditions for the rapid transformation of austenite to ferrite / bainite, resulting in a finer and more uniform microstructure. After relaxation, ultra-fast cooling at a rate of 28-50°C / s is used, ultimately yielding a mixed microstructure of uniformly sized acicular ferrite and a small amount of bainite (see appendix for details). Figure 2 The metallographic diagram of the hot-rolled sheet coil shows that the final product not only has high strength and good low-temperature toughness, but also a low yield strength ratio of ≤0.78.

[0057] Winding process: After ultra-fast cooling, a high-power winding machine is used for winding. The winding temperature is 380~440℃, the drum tension coefficient is 1.8~2.4, the auxiliary winding roller pressure is 370~420 MPa, the guide roller pressure is 38~55kN, the drum expansion speed is 0.6 times, the number of skipped turns is 5~7, the main drive motor MD lead rate is 12%~14%, the auxiliary winding roller WR lead rate is 16%~17%, and the pinch roller PR lead rate is 12%~13%. The coil shape is good, with no hard bends.

[0058] 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.

[0059] Table 3 shows the cooling and winding process parameters and the volume percentage of metallographic structures for each embodiment.

[0060] Example Air cooling time / s Final cooling temperature / ℃ Ultra-fast cooling rate / ℃·s-1 drum tension coefficient Auxiliary roller pressure / MPa Guide gauge pressure / kN Acicular ferrite / % Granular bainite / % 1 35 425 38 1.8 386 42 93% 7% 2 32 426 34 1.7 381 39 94% 6% 3 29 408 37 2.0 376 40 94% 6% 4 38 417 41 1.9 381 37 96% 4% 5 35 433 39 1.8 375 44 95% 5% 6 33 395 40 1.8 396 40 96% 4% Comparative Example 1 / 560 7.0 / / / / / Comparative Example 2 / 557 Laminar flow cooling / / / / /

[0061] Table 4 Main mechanical properties of each embodiment

[0062] Example Finished product specifications / mm Rt0.5 / MPa Rm / MPa A50mm / % Rt0.5 / Rm Charpy impact energy at -40℃ (single value / J) Charpy impact energy at -40℃ (single value / J) Charpy impact energy at -40℃ (single value / J) Average Charpy impact energy at -40℃ / J -20℃ drop hammer shear area (single value / %) -20℃ drop hammer shear area (single value / %) -20℃ Drop Weight Shear Area (Single Value Mean) / % Hardness HV10 1 10.0 505 673 38 0.75 335 332 325 331 100 98 99 207 2 10.0 498 673 40 0.74 346 338 335 340 100 100 100 202 3 10.0 503 680 37 0.74 329 328 336 331 100 100 100 208 4 14.0 495 660 36 0.75 328 326 341 332 100 100 100 203 5 14.0 502 678 34 0.74 343 319 327 330 98 100 99 206 6 14.0 497 681 36 0.73 322 337 329 330 100 98 99 205 Comparative Example 1 16.0 505 610 33 0.83 250 250 255 252 / / / / Comparative Example 1 10.5 552 654 26 0.84 / / / / / / / /

[0063] The comparative analysis of composition, process, and performance in Tables 1-4 shows that although the alloy composition design of this invention is only slightly adjusted compared to the comparative example, it achieves a qualitative leap in the comprehensive performance of the product through a series of innovative process designs in roughing, finishing, and cooling stages. Specifically, the large-strain roughing process adopted in this invention drives the full static recrystallization of austenite through high energy input, laying the foundation for ultra-fine microstructure; the incremental deformation resistance control in the finishing stage stably achieves the high flattening of deformed austenite, ensuring microstructure uniformity and avoiding mixed crystals; the final "relaxation-RPC" 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 0.78. In contrast, Comparative Examples 1 and 2, perhaps due to the lack of systematic microstructure refinement and phase transformation control processes, have yield strength ratios as high as 0.83~0.84, and their low-temperature toughness indicators (the 0℃ impact energy of Comparative Example 1 is 252J, with no drop hammer data; Comparative Example 2 has no toughness indicators) are far from reaching the level of this invention.

[0064] In summary, the hot-rolled coil produced by this invention possesses excellent comprehensive performance, meeting all the technical specifications of L450M coils, including a yield strength of 490~590MPa, tensile strength of 550~700MPa, elongation ≥30%, average impact energy Akv ≥300J at -40℃, average drop weight DWTT ≥90% at -20℃, and hardness HV. 10 ≤210, and the yield strength ratio is not higher than 0.78.

[0065] 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-cost, high-performance L450M pipeline steel hot-rolled coil, characterized in that, The chemical composition of the hot-rolled coil (by weight percentage) is as follows: C: 0.05%~0.08%, Si: 0.15%~0.30%, Mn: 1.35%~1.85%, P: ≤0.013%, S: ≤0.005%, Nb: 0.035%~0.045%, Ti: 0.009%~0.018%, Cr: 0.10%~0.18%, Als: 0.015%~0.055%, Mg: 0.0008%~0.0032%, Ca: 0.0005%~0.0020%, N: ≤0.005%, Ca / Mg: 0.3~1, with the remainder being Fe and unavoidable impurities. The microstructure of the hot-rolled coil is mainly composed of acicular ferrite, with a volume fraction ≥90%, interspersed with fine MA islands, a volume fraction ≤5%, and a small amount of bainite; the microstructure is uniform and fine, with an ASTM grain size grade ≥12, and the average width of the obtained acicular ferrite lath bundles is controlled within the range of 3.0~5.0μm; the manufacturing method includes smelting, continuous casting, billet heating, rough rolling, finish rolling, cooling, and coiling. The continuously cast slab is directly heated in a hot furnace at 550~900℃, with a heating temperature of 1160~1190℃ and a total furnace time of 140~280min, of which the soaking time is ≥120min; rough rolling... The process employs a slow-speed, high-reduction process, with a single-pass reduction rate of 29%~35%, a rolling speed range of 1.0~2.0 m / s, and a roughing rolling finishing temperature of 1010~1050℃. After roughing, the austenite recrystallized grain size obtained is 22~28 μm. The finishing rolling starting temperature is 920~950℃, with a single-pass reduction rate of 22%~27% for the first three passes, a reduction rate of ≥8% for the last pass, a cumulative reduction rate of 71%~85%, and a finishing rolling temperature of 870~900℃. After rolling, the material undergoes 10~40s of relaxation and air cooling, followed by ultra-fast cooling at a rate of 28~50℃ / s. The coiling temperature is 380~440℃.

2. The low-cost, high-performance L450M pipeline steel hot-rolled coil according to claim 1, characterized in that, The thickness of the hot-rolled coil is 10~16mm, the yield strength is 490~590MPa, the tensile strength is 550~700MPa, the elongation is ≥30%, and the yield strength ratio is not higher than 0.

78.

3. The low-cost, high-performance L450M pipeline steel hot-rolled coil according to claim 1, characterized in that, Hot-rolled coil: Average impact energy Akv ≥ 300J at -40℃, average drop weight DWTT ≥ 90% at -20℃, hardness value HV 10 ≤210.

4. A method for manufacturing a low-cost, high-performance L450M pipeline steel hot-rolled 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 550~900℃, with a heating temperature of 1160~1190℃ and a total furnace time of 140~280min, including a soaking time of ≥120min. The rough rolling process employs a slow-speed, high-reduction technique, with a single-pass reduction rate of 29%~35%, a rolling speed range of 1.0~2.0m / s, and a finishing temperature of 1010~1050℃. Austenitic slab is obtained after rough rolling. The recrystallized grain size is 22~28μm; the finishing rolling temperature is 920~950℃, the single-pass reduction rate of the first 3 passes is 22%~27%, the reduction rate of the last pass is ≥8%, the cumulative reduction rate is 71%~85%, and the final rolling temperature is 870~900℃; after rolling, the rolling process is first relaxed by air cooling for 10~40s, and then cooled by ultra-fast cooling at a rate of 28~50℃ / s; the coiling temperature is 380~440℃.

5. The method for manufacturing a low-cost, high-performance L450M pipeline steel hot-rolled 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 method for manufacturing a low-cost, high-performance L450M pipeline steel hot-rolled coil according to claim 4, characterized in that, The superheat of the billet is controlled at 20~30℃ 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-cost, high-performance L450M pipeline steel hot-rolled coil according to claim 4, characterized in that, The roughing mill has three passes, with a reduction rate of 29% to 32% in the first pass and 31% to 35% in the second and third passes.

8. The method for manufacturing a low-cost, high-performance L450M pipeline steel hot-rolled coil according to claim 4, characterized in that, The winding machine has a drum tension coefficient of 1.8~2.4, an auxiliary winding roller pressure of 370~420 MPa, a guide roller pressure of 38~55 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 12%~14%, an auxiliary winding roller WR lead rate of 15%~17%, and a pinch roller PR lead rate of 12%~13%.

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