Ultra-thick L360 pipeline steel with excellent low-temperature toughness

Abstract

The invention discloses ultra-thick L360 pipeline steel with excellent low-temperature toughness, which belongs to the technical field of materials and metallurgy and comprises the following components in percentage by weight: 0.050%-0.058% of C, 0.10%-0.20% of Si, 1.0%-1.1% of Mn, 0.040%-0.050% of Nb, 0.019%-0.021% of Ti, 0.18%-0.24% of Cr, 0.15%-0.20% of Ni, less than or equal to 0.015% of P, less than or equal to 0.003% of S, less than or equal to 0.004% of N, 0.118%-0.130% of Pcm and the balance of Fe and inevitable impurities. The low-carbon, low-manganese and low-Pcm alloy design is adopted, Nb and Ti are added to improve the strength, an acicular ferrite structure is obtained through the synergistic effect of Cr and Ni and a proper cooling system, and good obdurability matching is achieved.

Description

Technical Field

[0001] The present invention relates to the technical fields of materials and metallurgy, and more specifically, to an extra-thick specification L360 pipeline steel with excellent low-temperature toughness. Background Art

[0002] With the rapid development of the economy and society, the human demand for oil and natural gas energy is continuously increasing, and the pipeline transportation volume is also gradually increasing. In order to meet the requirements of long-distance and high-pressure transportation, the steel used for oil and gas pipelines is developing towards large diameter, large wall thickness and high strength and toughness. However, for the hot-rolled products of pipeline steel with a thickness specification of more than 20 mm, due to insufficient reduction ratio, major steel mills pay particular attention to the strength and toughness indexes of the products when undertaking contracts, especially the low-temperature toughness, which has become a common technical difficulty in the production of extra-thick specification products.

[0003] At present, for the hot-rolled coils of L360 grade pipeline steel shown in the public information, the lowest impact test temperature is -20°C. Chinese Patent with Application No. 201610803301.7 discloses a thick-specification X52 pipeline steel and its production method, in which the impact energy at -20°C is only 120 J, and the toughness is significantly insufficient. At the same time, the Cr content in this invention is relatively high, resulting in a relatively high alloy cost. The finished product thickness is 12 - 20 mm, and the coiling temperature is 350 - 520°C. There are great risks in low-temperature coiling during the production of thick-specification pipeline steel, and this invention does not describe the production method of X52 with a thickness specification above 20 mm. Chinese Patent with Application No. 201310585644.7 discloses a production method of low-cost X52 pipeline steel and the pipeline steel. The components in the steel contain, by weight percentage, C: 0.08% - 0.12%, Si ≤ 0.35%, Mn: 1.10% - 1.40%, S ≤ 0.025%, P ≤ 0.025%, Ti: 0.008% - 0.022%. The maximum thickness specification of the finished product of this invention is only 10 mm, the yield strength is 387 - 417 MPa, the impact energy at -20°C is only 153 J, and there is no Charpy V-notch requirement. After the product is formed into a spiral pipe, due to the Bauschinger effect, the strength decreases. Therefore, after pipe forming, the risk that the yield strength of the product is lower than 360 MPa is extremely high. Chinese Patent with Application No. 201010243258.6 discloses an X52 pipeline steel and its production method. The components in the steel contain, by weight percentage, C: 0.07% - 0.09%, Si: 0.15% - 0.30%, Mn: 1.10% - 1.30%, S ≤ 0.006%, P ≤ 0.020%, Nb: 0.02% - 0.04%, Ti: 0.01% - 0.02%, N ≤ 0.008%, Als: 0.020% - 0.040%. This invention controls the S content below 0.006%, and the steelmaking cost is relatively high. The soaking temperature is 1200 - 1240°C. The relatively high soaking temperature will coarsen the austenite grains, affecting the subsequent tensile properties. After rough rolling, it waits for temperature for 20 - 50 seconds and then enters the finish rolling stage, and the production efficiency is relatively low. At the same time, there is no description of the product thickness specification. In addition, its mechanical properties only meet the API 5L standard, and there is no low-temperature toughness index. Chinese Patent with Application No. 201910949817.6 discloses a hot-rolled coil plate of ultra-thick specification X52 pipeline steel and its production method. After rough rolling in this invention, there is no temperature waiting stage and it directly enters the finish rolling, which is likely to cause mixed grains and affect the toughness index. Therefore, only the impact performance at 0°C is described in this invention, and there is no description of the Charpy V-notch performance, especially the low-temperature Charpy V-notch. The prior art "Research and Development of Hot-Rolled Medium Wide Strip Steel X52 for Oil and Gas Transmission Pipelines" proposes a production method of hot-rolled coil plates of X52 grade pipeline steel with a thickness of 5 - 7 mm. This technology does not propose a method for producing thick-specification (≥20 mm) X52 steel strips from thin slab billets. Summary of the Invention

[0004] The object of the present invention is to overcome the above-mentioned defects existing in the prior art, and to provide an extra-thick L360 pipeline steel with excellent low-temperature toughness. Through the production process of "hot metal pretreatment + converter smelting + secondary refining + LF refining + continuous casting + heating + rolling, cooling and coiling" and alloy composition design, the pipeline steel prepared has a thickness of 20 mm to 24.2 mm, its -20 °C drop-weight shear area (SA) ≥ 96%, and its -40 °C impact energy ≥ 263 J, showing excellent low-temperature toughness.

[0005] To achieve the above object, the technical solution of the present invention is as follows: An extra-thick L360 pipeline steel with excellent low-temperature toughness, comprising the following components in weight percentages: C: 0.050% - 0.058%, Si: 0.10% - 0.20%, Mn: 1.0% - 1.1%, Nb: 0.040% - 0.050%, Ti: 0.019% - 0.021%, Cr: 0.18% - 0.24%, Ni: 0.15% - 0.20%, P ≤ 0.015%, S ≤ 0.003%, N ≤ 0.004%, and Pcm: 0.118% - 0.130%, the balance being Fe and inevitable impurities; the preparation method of the above-mentioned extra-thick L360 pipeline steel with excellent low-temperature toughness comprises the following steps: hot metal pretreatment, converter smelting, secondary refining, LF refining, continuous casting, heating, rolling, cooling and coiling; in the heating, rolling, cooling and coiling, after the continuous casting billet is heated to 1140 °C - 1150 °C, a two-stage controlled rolling and controlled cooling technology is adopted; then 5 passes of rough rolling are completed, the finish rolling temperature ≥ 1015 °C, the reduction ratio in the rough rolling stage increases gradually with each pass, the reduction ratio of the first pass is 20% - 20.8%, the reduction ratio of the second pass is 20.6% - 21.1%, the reduction ratio of the third pass is 20.9% - 21.8%, the reduction ratio of the fourth pass is 21.4% - 22.8%, the reduction ratio of the fifth pass is 21.8% - 24.2%, and the cumulative reduction ratio is 69.4% - 70.6%; the intermediate billet formed after rough rolling is held at 950 °C - 958 °C and then enters finish rolling, and the finish rolling temperature is 790 °C - 810 °C; after rolling, it is cooled at a speed of 20.7 °C - 22.8 °C / S to 522 °C - 543 °C for coiling.

[0006] Optionally, the thickness of the L360 pipeline steel is 20 mm - 24.2 mm.

[0007] Optionally, the yield strength of the L360 pipeline steel ≥ 407 MPa, the tensile strength ≥ 496 MPa, the yield ratio is 0.79 - 0.84, A 50 is 48% - 53%, the -20 °C drop-weight shear area SA ≥ 96%, and the -40 °C impact energy ≥ 263 J.

[0008] Optionally, the metallographic structure of the L360 pipeline steel is acicular ferrite and dispersively distributed MA islands, wherein the volume fraction of the acicular ferrite is 97% - 98%.

[0009] Optionally, the thickness of the intermediate billet is 50 mm - 52 mm.

[0010] Optionally, the number of rolling passes in finish rolling is 7. The reduction ratio in the first pass of finish rolling is 13.5% - 15.8%, the reduction ratio in the second pass is 11.8% - 15.1%, the reduction ratio in the third pass is 10.9% - 14.0%, the reduction ratio in the fourth pass is 9.7% - 12.9%, the reduction ratio in the fifth pass is 9.4% - 11.7%, the reduction ratio in the sixth pass is 8.1% - 11.2%, and the reduction ratio in the last pass is 5.1% - 8.0%. The cumulative reduction ratio in finish rolling is 53.5% - 60.0%.

[0011] Optionally, in the secondary refining and LF refining, the molten steel after tapping from the converter smelting is subjected to RH vacuum treatment, followed by light desulfurization treatment in the LF furnace and calcium treatment.

[0012] Optionally, in continuous casting, electromagnetic stirring or dynamic soft reduction is applied to the refined molten steel to obtain a continuous casting billet with a thickness of 170 mm.

[0013] Implementing the embodiments of the present invention will have the following beneficial effects: The present invention relates to an extra-thick specification L360 pipeline steel with excellent low-temperature toughness, which is particularly suitable for hot-rolled coils used in oil and gas transportation pipelines. Through the production process of "hot metal pretreatment + converter smelting + secondary refining + LF refining + continuous casting + heating + rolling, cooling and coiling" and alloy composition design, the pipeline steel produced has a thickness of 20 mm - 24.2 mm, the -20°C drop-weight shear area (SA) ≥ 96%, and the -40°C impact energy ≥ 263 J, showing excellent low-temperature toughness.

[0014] The present invention adopts a low-Pcm alloy design with low carbon and low manganese, adds Nb and Ti to improve strength, and through the synergistic effect of Cr and Ni and a suitable cooling regime, obtains an acicular ferrite structure, thus achieving a good strength-ductility match. This design not only solves the technical problem of poor strength-ductility match when producing thick-specification products with thin billets, but also ensures the stability of product performance and the excellent performance of low-temperature toughness indicators. In addition, the present invention adopts a composition design with low Mn and low Si, which provides a basic guarantee for the excellent strength and ductility of the finished product, and further optimizes the microstructure and mechanical properties through an accurate cooling regime. Detailed Embodiments

[0015] The following further illustrates the present invention with specific embodiments, but does not limit the present invention in any way.

[0016] I. Chemical Composition and Mechanical Properties The present invention discloses an extra-thick L360 pipeline steel with excellent low-temperature toughness, comprising the following components in weight percentages: C: 0.050% - 0.058%, Si: 0.10% - 0.20%, Mn: 1.0% - 1.1%, Nb: 0.040% - 0.050%, Ti: 0.019% - 0.021%, Cr: 0.18% - 0.24%, Ni: 0.15% - 0.20%, P ≤ 0.015%, S ≤ 0.003%, N ≤ 0.004%, and Pcm: 0.118% - 0.130%, with the balance being Fe and inevitable impurities.

[0017] Compared with the existing L360 pipeline steel, the composition of the extra-thick high-toughness L360 pipeline steel of the present invention adopts a low-C and low-Mn design. By utilizing the microalloying mechanism of Nb-Ti, Cr is added simultaneously to improve the tissue uniformity of thick-specification products. At the same time, in combination with a cooling process with an appropriate proportion of Ni, a acicular ferrite structure is obtained, ensuring that the product has appropriate strength and good low-temperature toughness. Specifically, the mechanism of action of each alloy component in the extra-thick L360 pipeline steel with excellent low-temperature toughness of the present invention is as follows: C: Carbon is the most important element in steel after iron and is also the most economical element to improve the strength of products. However, with the increase of C content, it directly leads to an increase in the Pcm value, and the plasticity, toughness, and welding performance of the steel will be significantly reduced. The design of low carbon content is the top priority of the alloy formula of the present invention and is the basic guarantee for the good strength-ductility matching of the products of the present invention. Therefore, the C content in the present invention is controlled to be 0.050% - 0.058%.

[0018] Si: Silicon is generally introduced as a reducing agent and deoxidizer during the steelmaking process. In addition, Si can also dissolve in ferrite and austenite to play a strengthening role and improve the strength of the steel. However, at the same time, plasticity and toughness will be lost. Therefore, a low-Si design is also adopted in the present invention. Therefore, the Si content in the present invention is controlled to be 0.10% - 0.20%.

[0019] Mn: Manganese is the second most important strengthening element in steel after C, with a solid-solution strengthening effect. It can also increase the stability of austenite, promote the formation of a certain amount of M / A, and is also beneficial to improving hardenability, effectively ensuring the strength of the steel. However, too high a manganese content will cause serious segregation, forming a segregated banded structure and strongly losing toughness. In addition, Mn and S are likely to form MnS inclusions that deform along the rolling direction, cutting the matrix and losing toughness. Therefore, in steels with strict requirements for toughness, the manganese content should not be too high. The manganese content in the present invention is controlled to be 1.0% - 1.1%.

[0020] P, S, and N: These are inevitable impurity elements in steel. Lower levels are preferred, but overly low requirements would increase production costs. In the present invention, P ≤ 0.015%, S ≤ 0.005%, and N ≤ 0.006%.

[0021] Ti: Titanium is an extremely strong nitride-forming element with a significant nitrogen-fixing effect. The stoichiometric ratio of Ti / N is 3.42. Only about 0.012% of Ti is required to completely fix the 40 ppm of N in the present invention. Under the high-temperature conditions of continuous casting, stable and fine TiN particles can be formed. On the one hand, it can effectively prevent the growth of austenite during the reheating of the slab. On the other hand, it can improve the impact toughness of the heat-affected zone of welding. For welded structural steel, the addition of Ti is particularly important. In addition, since the binding force between Ti and S is higher than that of Mn, Ti can "take away" S from MnS inclusions to form Ti4C2S2, which is not easily deformed and will not damage the continuity of the matrix, thereby improving the toughness index. Since the present invention adopts a low-Mn and low-S design, only a small amount of MnS will be formed. Therefore, for the addition of Ti in the present invention, in addition to the 0.012% of Ti used for nitrogen fixation, an additional addition of about 0.006% can modify MnS to Ti4C2S2. Therefore, the Ti content in the present invention is 0.019% - 0.021%.

[0022] Nb: Niobium is the most important element for controlled rolling in modern microalloyed pipeline steel. It can significantly increase the austenite recrystallization temperature of steel and expand the range of the non-recrystallized zone, that is, the rough rolling process window can be carried out in the high-temperature range, reducing the mill load. The strain-induced precipitation of NbC generated at this stage hinders the recovery and recrystallization of deformed austenite, continuously refining the original austenite grain structure and playing a role in grain refinement. In the subsequent finish rolling and cooling stages, Nb can also improve the properties of steel through various strengthening mechanisms such as precipitation strengthening, dispersion strengthening, and phase transformation strengthening. However, due to the low C content in the present invention, even if the addition of Nb is too high, it cannot play a role and will instead form "insoluble Nb blocks", which affects the comprehensive properties of the material. Therefore, the Nb content in the present invention is controlled to be 0.040% - 0.050%.

[0023] Cr: Chromium is the most economical element to improve hardenability. Cr can effectively improve the tissue uniformity in the thickness direction of thick-specification products. Mo can promote the formation of acicular ferrite tissue, but it is expensive, while the price of Cr is only 1 / 9 of Mo. When added together with 1.2 times the Ni content and combined with a suitable cooling process, it can effectively promote the formation of acicular ferrite tissue and martensite-austenite islands (M / A). On the one hand, it can improve the strength and toughness of the product. On the other hand, it also reduces the strength loss during the pipe-making process. Therefore, the Cr content in the present invention is 0.18% - 0.24%.

[0024] Ni: Nickel is an austenite stabilizing element second only to C, N, and Mn. It can effectively prevent grain growth at high temperatures and maintain a fine-grained structure, ensuring that the product structure has fine original austenite grains. Ni can also play a role in solid solution strengthening to increase strength. In addition, Ni can promote cross-slip of dislocations and effectively improve the toughness index of the product. In addition, in the present invention, by using an appropriate ratio of Cr and Ni and combining with the cooling process, a needle-like ferrite structure is obtained, further improving the strength and toughness. However, the cost of Ni alloy is relatively high, and excessive addition will affect the economy of the product. Therefore, the Ni content in the present invention is 0.15% - 0.20%.

[0025] In a specific embodiment, the thickness of the L360 pipeline steel is 20 mm - 24.2 mm.

[0026] In a specific embodiment, the final mechanical properties of the L360 pipeline steel coil plate are as follows: yield strength ≥ 407 MPa, tensile strength ≥ 496 MPa, yield ratio is 0.79 - 0.84, A 50 is 48% - 53%, the -20°C drop hammer shear area SA ≥ 96%, and the -40°C impact energy ≥ 263 J.

[0027] In a specific embodiment, the metallographic structure of the L360 pipeline steel is needle-like ferrite and a small amount of diffusely distributed martensite-austenite islands, where the volume fraction of needle-like ferrite is 97% - 98%, and the volume fraction of martensite-austenite islands is 2% - 3%.

[0028] II. Production process technology The present invention also discloses a preparation method of an extra-thick L360 pipeline steel with excellent low-temperature toughness as in any embodiment of the present invention, including the following steps: hot metal pretreatment, converter smelting, secondary refining, LF refining, continuous casting, heating, rolling, cooling, and coiling. Specifically, it includes the following steps: S1. In hot metal pretreatment, desulfurize the hot metal to obtain desulfurized hot metal.

[0029] S2. In converter smelting, subject the desulfurized hot metal to converter smelting and tap the steel.

[0030] Specifically, the hot metal pretreatment and converter smelting steps of the present invention can be operated according to conventional preparation methods.

[0031] S3. In secondary refining and LF refining, subject the molten steel tapped from the converter smelting to RH vacuum treatment, and then perform LF furnace mild desulfurization treatment and calcium treatment to control the inclusion morphology and improve the ductility, toughness, and cold bending performance of the steel.

[0032] S4. In continuous casting, subject the refined molten steel to electromagnetic stirring or dynamic soft reduction to obtain a continuous casting slab with a thickness of 170 mm.

[0033] In S5, during heating, rolling, cooling, and coiling, after the continuous casting billet is heated to 1140°C - 1150°C, the two-stage controlled rolling and controlled cooling technology is adopted. Since the Nb content in the present invention is not high, at a relatively low heating temperature, it can ensure that alloys such as Nb, Ti, Cr, and Ni are fully dissolved, and play a role in the subsequent controlled rolling and controlled cooling. Subsequently, 5 passes of rough rolling are quickly completed, and the finishing rolling temperature ≥ 1015°C. The reduction ratio in the rough rolling stage increases gradually for each pass. The reduction ratio of the first pass is 20% - 20.8%, the reduction ratio of the second pass is 20.6% - 21.1%, the reduction ratio of the third pass is 20.9% - 21.8%, the reduction ratio of the fourth pass is 21.4% - 22.8%, the reduction ratio of the fifth pass is 21.8% - 24.2%, and the cumulative reduction ratio is 69.4% - 70.6%. This stage ensures that recrystallization occurs in all 5 passes of rough rolling to continuously refine the structure. The intermediate billet with a thickness of 50 mm - 52 mm formed after rough rolling is heated to 950°C - 958°C and then enters the finishing rolling. The finishing rolling temperature is 790°C - 810°C, and after rolling, it is cooled at a speed of 20.7°C - 22.8°C / S to 522°C - 543°C for coiling.

[0034] In a specific embodiment, the number of rolling passes in the finishing rolling is 7. The reduction ratio of the first pass in the finishing rolling is 13.5% - 15.8%, the reduction ratio of the second pass is 11.8% - 15.1%, the reduction ratio of the third pass is 10.9% - 14.0%, the reduction ratio of the fourth pass is 9.7% - 12.9%, the reduction ratio of the fifth pass is 9.4% - 11.7%, the reduction ratio of the sixth pass is 8.1% - 11.2%, and the reduction ratio of the last pass is 5.1% - 8.0%. The cumulative reduction ratio in the finishing rolling is 53.5% - 60.0%. The reduction ratio in the finishing rolling stage is relatively low, generating substructures such as dislocations with a lower density, which can ensure excellent toughness indicators of the product.

[0035] The present invention adopts the synergistic effect of Cr - Ni. Under this cooling process, it can promote the formation of acicular ferrite structure, and at the same time generate 2% - 3% of Martensite - Austenite (M - A) islands, which are dispersedly distributed in the acicular ferrite matrix, ensuring that the hot coil has good strength and low-temperature toughness.

[0036] The following are specific embodiments The preparation method of the ultra-thick specification L360 pipeline steel with excellent low-temperature toughness in this embodiment includes the following steps: The hot metal pretreatment and converter smelting links can be operated according to the conventional preparation methods. Subsequently, the molten steel discharged from the converter smelting is subjected to RH vacuum treatment, followed by LF furnace light desulfurization treatment and calcium treatment to control the inclusion morphology and improve the ductility, toughness, and cold bending performance of the steel. The molten steel after refining treatment is subjected to electromagnetic stirring or dynamic soft reduction to obtain a continuous casting billet with a thickness of 170 mm, and finally, it is obtained through heating, rolling, cooling, and coiling.

[0037] Examples 1 - 10 The chemical compositions of Examples 1 - 10 are shown in Table 1, the heating, rolling, and cooling process parameters are shown in Tables 2 - 4, and the mechanical property test results are shown in Table 5.

[0038] Table 1 Chemical Compositions of Examples 1 - 10 (%)

[0039] Table 2 Production Process Parameters of the Steel in Examples 1 - 10

[0040] Table 3 Production Process Parameters of the Steel in Examples 1 - 10

[0041] Table 4 Production Process Parameters of the Steel in Examples 1 - 10

[0042] Table 5 Mechanical Properties and Microstructure Proportions of Examples 1 - 10

[0043] As can be seen from Tables 1 - 5, by adopting the production process scheme of the present invention, the produced hot-rolled coils of ultra-thick specification L360 pipeline steel have excellent strength and toughness, and the low-temperature toughness index is particularly excellent.

[0044] The above-described embodiments only represent several implementation manners of the present invention. Their descriptions are relatively specific and detailed, but they should not be construed as limiting the scope of the patent application. It should be noted that for those of ordinary skill in the art, without departing from the concept of the present invention, several modifications and improvements can still be made, and these all belong to the protection scope of the present invention. Therefore, the protection scope of this invention patent shall be subject to the appended claims.

Claims

1. A super-thick L360 pipeline steel with excellent low-temperature toughness, characterized in that, It comprises components with the following weight percentages: C: 0.050% - 0.058%, Si: 0.10% - 0.20%, Mn: 1.0% - 1.1%, Nb: 0.040% - 0.050%, Ti: 0.019% - 0.021%, Cr: 0.18% - 0.24%, Ni: 0.15% - 0.20%, P ≤ 0.015%, S ≤ 0.003%, N ≤ 0.004%, and Pcm: 0.118% - 0.130%, with the balance being Fe and inevitable impurities; The preparation method of the ultra - thick - specification L360 pipeline steel with excellent low - temperature toughness includes the following steps: hot metal pretreatment, converter smelting, secondary refining, LF refining, continuous casting, heating, rolling, cooling, and coiling; During the heating, rolling, cooling, and coiling, after the continuous casting billet is heated to 1140°C - 1150°C, a two - stage controlled rolling and controlled cooling technology is adopted; then 5 passes of rough rolling are completed, the finish rolling temperature ≥ 1015°C, the reduction ratio in the rough rolling stage increases pass by pass. The reduction ratio of the first pass is 20% - 20.8%, the reduction ratio of the second pass is 20.6% - 21.1%, the reduction ratio of the third pass is 20.9% - 21.8%, the reduction ratio of the fourth pass is 21.4% - 22.8%, the reduction ratio of the fifth pass is 21.8% - 24.2%, and the cumulative reduction ratio is 69.4% - 70.6%; the intermediate billet formed after rough rolling is held at 950°C - 958°C and then enters finish rolling, and the finish rolling temperature is 790°C - 810°C; after rolling, it is cooled at a speed of 20.7°C - 22.8°C / S to 522°C - 543°C for coiling.

2. The extra-thick L360 pipeline steel with excellent low-temperature toughness according to claim 1, characterized in that, The thickness of the L360 pipeline steel is 20mm - 24.2mm.

3. The ultra-thick specification L360 pipeline steel with excellent low-temperature toughness according to claim 1, wherein The yield strength of the L360 pipeline steel is ≥407 MPa, the tensile strength is ≥496 MPa, the yield ratio is 0.79 - 0.84, A 50 is 48% - 53%, the shear area SA of the drop hammer at -20°C is ≥96%, and the impact energy at -40°C is ≥263 J.

4. The extra-thick L360 pipeline steel with excellent low-temperature toughness according to claim 1, wherein The metallographic structure of the L360 pipeline steel is acicular ferrite and dispersively distributed MA islands, among which the volume fraction of the acicular ferrite is 97% - 98%.

5. The ultra-thick specification L360 pipeline steel with excellent low-temperature toughness according to claim 1, characterized in that, The thickness of the intermediate billet is 50mm - 52mm.

6. The ultra-thick specification L360 pipeline steel with excellent low-temperature toughness according to claim 1, wherein The number of rolling passes in finish rolling is 7. The reduction ratio of the first pass in finish rolling is 13.5% - 15.8%, the reduction ratio of the second pass is 11.8% - 15.1%, the reduction ratio of the third pass is 10.9% - 14.0%, the reduction ratio of the fourth pass is 9.7% - 12.9%, the reduction ratio of the fifth pass is 9.4% - 11.7%, the reduction ratio of the sixth pass is 8.1% - 11.2%, and the reduction ratio of the last pass is 5.1% - 8.0%. The cumulative reduction ratio in finish rolling is 53.5% - 60.0%.

7. The extra-thick L360 pipeline steel with excellent low-temperature toughness according to claim 1, characterized in that, During the secondary refining and LF refining, the molten steel tapped from the converter smelting is subjected to RH vacuum treatment, followed by light desulfurization treatment in the LF furnace and calcium treatment.

8. The ultra-thick L360 pipeline steel with excellent low-temperature toughness according to claim 1, characterized in that, During the continuous casting, electromagnetic stirring or dynamic soft reduction is applied to the refined molten steel to obtain a continuous casting billet with a thickness of 170mm.

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