Process for improving toughness of x70 pipeline steel with high deformability

Abstract

The application discloses a process method for improving toughness of X70 anti-large-deformation pipeline steel, and ideal mechanical properties and organizational morphology are obtained through control of components, continuous casting, heating, rolling, cooling and other processes, finally, the organizational morphology of 70-80% ferrite and hard phase components such as bainite and martensite is obtained, the uniform elongation and toughness of the pipeline steel are improved, the yield strength ratio is low, and the deformation resistance and anti-large-deformation capacity are improved. The X70 anti-large-deformation pipeline steel plate has a Charpy impact toughness of 350 J or more at-20 DEG C, a drop hammer shearing area of 95% or more, a yield strength of 480-550 MPa, a tensile strength of 600-680 MPa, a yield strength ratio of 0.78 or less, a UEL of 12% or more, and a R t1.5 / R t0.5 of 1.15 or more. The product is widely applied to pipeline engineering prone to large plastic deformation such as ground slip deformation in earthquake-prone areas, ground liquefaction settlement caused by permafrost and submarine pipeline laying.

Description

Technical Field

[0001] This invention belongs to the field of steel rolling technology, specifically relating to a process method for improving the toughness of X70 pipeline steel resistant to large deformation. Background Technology

[0002] Currently, most of my country's natural gas and oil resources are located in remote areas with extremely harsh environments, such as earthquake-prone zones, oceans, permafrost areas, landslide zones, and deserts. Pipeline transportation is the most reasonable and economical mode of transporting natural gas and oil. To reduce the construction and operating costs of pipelines, the strength and pressure of pipeline steel are gradually increasing. However, considering strength alone is no longer sufficient to meet the safety requirements of pipeline transportation; operational safety under special geological conditions must also be considered. Therefore, when transporting natural gas and oil in permafrost, debris flow, and earthquake-prone areas, ground displacement can easily induce large strains, requiring pipelines with higher resistance to compressive and tensile strains. Pipeline design must fully consider both strain capacity design and stress design. The pipeline steel used has a larger wall thickness and has higher requirements for low-temperature toughness, strength, weldability, and resistance to large deformations, making it a high-value-added product. Moreover, with the increase in infrastructure construction in my country, the demand for pipeline steel resistant to large deformations will continue to increase.

[0003] Generally, pipeline steel resistant to large deformation has the following characteristics: good drop hammer performance and low-temperature impact toughness; large uniform elongation and work hardening index; and a tensile curve without a yield plateau, exhibiting a continuous yield dome shape. The performance of this type of pipeline steel mainly depends on: first, the alloying elements added in the composition design and their proportions; second, the proportion of hard and soft phases in the microstructure; and third, the morphology and size of the grains after rolling.

[0004] In the prior art, Chinese patent CN102534377A discloses "X70 grade pipeline steel plate with excellent toughness and large deformation resistance and its preparation method". The alloy formula adopts C 0.03-0.08%, Nb 0.01-0.05%, Ti 0.004-0.014%, and also contains 1-4 of the following: Mo 0-0.30%, Ni 0-0.30%, Cr 0-0.30%, Cu 0-0.30%. The alloy formula adopts a low carbon ratio and adds precious alloys such as molybdenum, nickel and niobium. The alloy design is beneficial to the transformation of microstructure and the improvement of performance, but the alloy price is high and the production cost is high. Chinese patent CN101456034B discloses "A method for producing medium-thick plates of X80 grade anti-deformation pipeline steel". The alloy formula uses the following composition: C 0.02-0.05%, Ni 0.2-0.4%, Nb 0.05-0.1%, Cr 0.2-0.5%. It uses low carbon and adds a large amount of precious alloying elements. This alloy design is beneficial to microstructure transformation and performance improvement, but it is expensive and has high production costs. Chinese patent CN101962733A discloses "A low-cost, high-strength and high-toughness X80 anti-deformation pipeline steel and its production method". The alloy design uses C 0.02-0.08% and Mo 0.10-0.30%. The billet soaking temperature is 1200-1250℃, and the ferrite content in the microstructure is 20-40%. The high heating temperature and low ferrite content in the microstructure are detrimental to low-temperature toughness. The Charpy impact toughness at -20℃ is low, only about 250J. Summary of the Invention

[0005] To overcome the shortcomings of the existing technology, this invention produces a high-toughness X70 pipeline steel with large deformation through a "relaxation + controlled rolling + strong cooling" process. This overcomes the problems of high cost, low uniform elongation, and high yield strength caused by the addition of large amounts of alloys such as Nb, Mo, V, and Ni in the existing technology.

[0006] To achieve the above-mentioned objectives, this invention provides a process for improving the toughness of X70 pipeline steel resistant to large deformations, comprising the steps of top-and-bottom blowing converter smelting, ladle refining, large slab continuous casting, heating, controlled rolling (relaxation), and controlled cooling. The method includes the following technical solutions:

[0007] ① The continuous casting billet casting speed is 0.9~1.2m / min, and the continuous casting billet thickness is 230~250mm.

[0008] Using 230-250mm thick slabs for continuous casting, and optimizing the casting machine speed to 1.0-1.2m / min, can improve the quality of the core of the continuously cast slab, reduce the level of segregation in the center of the slab, improve the material's resistance to large deformation, and improve the uniformity of the slab's microstructure and composition. The purpose of the low-temperature heating regime is to effectively prevent the growth of the original austenite grains while ensuring that the alloy is fully dissolved, which is beneficial to improving the material's low-temperature impact performance.

[0009] ② The continuously cast billet adopts a low-temperature heating process, with a heating temperature of 1130~1160℃ and a heating time of 1.5~2.0h;

[0010] ③ The continuously cast billet is rolled in stages using a "relaxation + controlled rolling + forced cooling" process. The first stage is relaxation and temperature preparation. The controlled rolling starts at 1100–1120℃, using a "0+7" rolling process, with a relaxation time of 1 min 10 s to 1 min 40 s. When the temperature reaches below 960℃, it enters the finishing mill, with a finishing compression ratio ≥3.0. The second stage is controlled rolling, with a finishing temperature of 940–960℃ and an ending temperature of 830–850℃. The third stage is ultra-rapid cooling, with a controlled rolling starting temperature of 810–830℃. After exiting F7, the first five stages of ultra-rapid cooling are used at a cooling rate of 40–50℃ / s, with a final cooling temperature of 380–400℃. This ultra-rapid cooling process controls the final product's metallographic composition, grain size, and percentage content.

[0011] By employing a "relaxation + controlled rolling + strong cooling + slow cooling" process, the original austenite grains can be sufficiently refined, and the final rolling temperature can be precisely controlled above 810℃ to prepare for subsequent phase transformation. After exiting the finishing mill F7, the first five sets of ultra-fast cooling processes are used, with a cooling rate of 40-50℃ / s. This provides sufficient time for ferrite precipitation and inhibits ferrite grain growth, resulting in hard phase structures such as bainite and martensite, a dome-shaped tensile curve, and a high uniform elongation (UEL) and a low yield strength ratio.

[0012] The chemical composition of a large deformation resistant X70 pipeline steel prepared by the above process is as follows (by mass percentage): C 0.02-0.05%, Si 0.20-0.30%, Mn 1.8-2.0%, P ≤0.015%, S ≤0.003%, Alt 0.02-0.035%, N ≤0.0030%, H ≤0.0002%, O ≤0.0015%; the chemical composition also contains at least one of Cr 0.50-0.80%, Cu 0.30-0.60%, and Mo 0.40-0.60%, with the balance being iron and unavoidable impurities.

[0013] The basis for the chemical composition ratio is as follows: using C and Mn as solid solution strengthening elements can effectively increase the pearlite content, control the carbon content in the steel within the range of 0.02-0.05%, ensure the strength of the material while reducing the yield strength ratio, thus lowering the carbon equivalent and ductile-brittle transition temperature. With Mn in the range of 1.80-2.0%, the strength of the material is improved through a combination of phase transformation strengthening and solid solution strengthening, reducing the yield strength ratio and enhancing the material's resistance to large deformations.

[0014] Furthermore, the aforementioned X70 pipeline steel resistant to large deformation consists of acicular ferrite as the first phase, and bainite and MA components as the second phase; the proportion of the first phase is 70-80%.

[0015] Furthermore, the aforementioned X70 steel plate for pipelines resistant to large deformation exhibits a transverse Charpy impact energy (KV2) at -20℃ ≥ 320 J and a drop weight (DWTT) shear area ≥ 95%. The tensile properties of the steel plate are: yield strength 480–550 MPa, tensile strength 600–680 MPa, yield-to-tensile ratio ≤ 0.78, uniform elongation (UEL) ≥ 12%, and R... t1.5 / R t0.5 ≥1.15, R t2.0 / R t1.0 ≥1.10, R t5.0 / R t1.0 ≥1.15. The material has excellent resistance to large deformation and good toughness.

[0016] The application of the aforementioned X70 steel plate, which is resistant to large deformation pipeline steel, in pipeline projects prone to large plastic deformation, such as ground slip deformation in earthquake-prone areas, ground liquefaction and subsidence caused by permafrost, and submarine pipeline laying.

[0017] Compared with the prior art, the beneficial effects of the present invention are as follows:

[0018] ①This invention provides a chemical composition formula design to improve the toughness of X70 grade pipeline steel with high deformation resistance. By controlling the composition, continuous casting, heating, rolling and cooling processes, ideal mechanical properties and microstructure are obtained.

[0019] ②The steel contains a first phase composed of acicular ferrite and a second phase containing bainite and MA components; wherein the first phase accounts for 70-80%.

[0020] ③ The X70 steel plate for pipelines resistant to large deformation has a transverse Charpy impact energy (KV2) at -20℃ ≥ 350 J and a drop hammer (DWTT) shear area ≥ 95%. The tensile properties of the steel plate meet the following requirements: yield strength 480~550MPa, tensile strength 600~680MPa, yield-to-tensile ratio ≤ 0.78, uniform elongation (UEL) ≥ 12%, R... t1.5 / R t0.5 ≥1.15, Rt2.0 / R t1.0 ≥1.10, R t5.0 / R t1.0 ≥1.15. The material has excellent resistance to large deformation and good toughness. Attached Figure Description

[0021] Figure 1 Metallographic diagram of the X70 grade pipeline steel plate resistant to large deformation prepared in Example 1;

[0022] Figure 2 The image shows the microstructure of the X70 grade pipeline steel plate with high deformation resistance prepared in Example 1.

[0023] Figure 3 The tensile property curves of the X70 grade pipeline steel plate resistant to large deformation prepared in Example 1 are shown. Detailed Implementation

[0024] The present invention will be further described below with reference to specific embodiments, but this does not limit the invention in any way. To avoid redundancy, unless otherwise specified, the raw materials used in the following embodiments are all commercially available products, and the methods used are all conventional methods unless otherwise specified.

[0025] According to a process method for improving the toughness of X70 pipeline steel with resistance to large deformation according to the present invention, the steel is smelted in a 180-ton top-and-bottom blowing converter and produced by controlled rolling and controlled cooling in a 2300mm hot continuous rolling mill. The present invention is further described below through Examples 1-5. The chemical composition of the steel plates in the examples is shown in Table 1, and the process regime of the examples is described below.

[0026] Table 1. Chemical composition (wt%) of X70 anti-deformation pipeline steel plate in Example X70

[0027] C Si Mn P S Alt Cr Cu Mo N H O Example 1 0.02 0.21 1.82 0.009 0.002 0.025 0.31 0.42 0.0018 0.00005 0.0008 Example 2 0.03 0.27 1.88 0.012 0.001 0.03 0.55 0.35 0.0020 0.00009 0.0010 Example 3 0.03 0.25 1.91 0.014 0.003 0.035 0.58 0.51 0.0021 0.00011 0.0012 Example 4 0.05 0.22 1.95 0.010 0.001 0.03 0.78 0.49 0.0025 0.00015 0.0013 Example 5 0.04 0.29 1.98 0.008 0.003 0.02 0.52 0.57 0.0029 0.0002 0.0015

[0028] Example 1

[0029] A process for improving the toughness of X70 grade pipeline steel plates resistant to large deformation:

[0030] Product specifications: 21.4mm × 1550mm steel plate, continuous casting slab thickness 250mm, casting speed 1.0m / min, dynamic light reduction technology, slab heating temperature 1155℃, heating time 2h, first stage relaxation and waiting stage, controlled rolling start temperature 1118℃, initial rolling using "0+7" rolling, intermediate slab thickness 65mm, relaxation and waiting stage 1min40s, at which time the slab temperature reaches 960℃, entering the finishing mill; second stage controlled rolling stage, controlled rolling start temperature 958℃, cooling water between stands introduced, controlled rolling end temperature 850℃; third stage forced cooling stage, controlled rolling start temperature 830℃, after exiting F7, the first 5 groups of ultra-fast cooling are used, cooling rate 48℃ / s, controlling phase transformation, final cooling temperature 395℃. Through ultra-fast cooling process, the final product metallographic structure, grain size and percentage content are controlled. Example 1: Metallographic structure of the prepared steel plate as follows Figure 1 As shown, the tissue under scanning electron microscopy is as follows Figure 2 As shown, the tensile property curve of the steel plate is dome-shaped, as... Figure 3 As shown in Table 2, the comprehensive mechanical properties of the steel plate are as follows.

[0031] Example 2

[0032] A process for improving the toughness of X70 grade pipeline steel plates resistant to large deformation:

[0033] Product specifications: 18.4mm × 1500mm steel plate, continuous casting slab thickness 235mm, casting speed 1.1m / min, dynamic light reduction technology, slab heating temperature 1142℃, heating time 1h 50min, first stage relaxation and waiting stage, controlled rolling start temperature 1115℃, initial rolling using "0+7" rolling, intermediate slab thickness 60mm, relaxation and waiting stage 1min 35s, at which time the slab temperature reaches 954℃, entering the finishing mill; second stage controlled rolling stage, controlled rolling start temperature 950℃, inter-stand cooling water introduced, controlled rolling end temperature 846℃; third stage forced cooling stage, controlled rolling start temperature 832℃, after F7, the first 5 groups of ultra-fast cooling, cooling rate 45℃ / s, controlled phase transformation, final cooling temperature 390℃. Through ultra-fast cooling process, the final product metallographic structure, grain size and percentage content are controlled. The comprehensive mechanical properties of the steel plate are shown in Table 2.

[0034] Example 3

[0035] A process for improving the toughness of X70 grade pipeline steel plates resistant to large deformation:

[0036] Product specifications: 21.4mm × 1500mm steel plate, continuous casting slab thickness 250mm, casting speed 1.0m / min, dynamic light reduction technology, slab heating temperature 1145℃, heating time 1h 45min, first stage relaxation and waiting stage, controlled rolling start temperature 1112℃, initial rolling using "0+7" rolling, intermediate slab thickness 63mm, relaxation and waiting stage 1min 30s, at which time the slab temperature reaches 952℃, entering the finishing mill; second stage controlled rolling stage, controlled rolling start temperature 950℃, cooling water between stands introduced, controlled rolling end temperature 842℃; third stage forced cooling stage, controlled rolling start temperature 828℃, after exiting F7, the first 5 groups of ultra-fast cooling, cooling rate 43℃ / s, controlled phase transformation, final cooling temperature 389℃. Through ultra-fast cooling process, the final product metallographic structure, grain size and percentage content are controlled. The comprehensive mechanical properties of the steel plate are shown in Table 2.

[0037] Example 4

[0038] A process for improving the toughness of X70 grade pipeline steel plates resistant to large deformation:

[0039] Product specifications: 18.4mm × 1500mm steel plate, continuous casting slab thickness 235mm, casting speed 1.10m / min, dynamic light reduction technology, slab heating temperature 1135℃, heating time 1h 40min, first stage relaxation and waiting stage, controlled rolling start temperature 1108℃, initial rolling using "0+7" rolling, intermediate slab thickness 60mm, relaxation and waiting stage 1min 20s, at which time the slab temperature reaches 952℃, entering the finishing mill; second stage controlled rolling stage, controlled rolling start temperature 958℃, cooling water between stands introduced, controlled rolling end temperature 850℃; third stage forced cooling stage, controlled rolling start temperature 819℃, after exiting F7, the first 5 groups of ultra-fast cooling, cooling rate 42℃ / s, controlling phase transformation, final cooling temperature 385℃. Through ultra-fast cooling process, the final product metallographic structure, grain size and percentage content are controlled. The comprehensive mechanical properties of the steel plate are shown in Table 2.

[0040] Example 5

[0041] A process for improving the toughness of X70 grade pipeline steel plates resistant to large deformation:

[0042] Product specifications: 21.4mm × 1550mm steel plate, continuous casting slab thickness 250mm, casting speed 1.20m / min, dynamic light reduction technology, slab heating temperature 1130℃, heating time 1h 35min, first stage relaxation and waiting stage, controlled rolling start temperature 1105℃, initial rolling using "0+7" rolling, intermediate slab thickness 65mm, relaxation and waiting stage 1min 15s, at which time the slab temperature reaches 950℃, entering the finishing mill; second stage controlled rolling stage, controlled rolling start temperature 935℃, cooling water between stands introduced, controlled rolling end temperature 835℃; third stage ultra-rapid cooling stage, controlled rolling start temperature 815℃, after F7, the first 5 sets of ultra-rapid cooling are used, cooling rate 40℃ / s, controlling phase transformation, final cooling temperature 380℃. Through the ultra-rapid cooling process, the final product's metallographic structure, grain size and percentage content are controlled. The comprehensive mechanical properties of the steel plate are shown in Table 2.

[0043] Table 2 Mechanical properties of steel plates in Examples 1-5

[0044]

[0045] This invention discloses a process for improving the toughness of X70 grade pipeline steel plates resistant to large deformation. The X70 pipeline steel plates produced using this method have a microstructure consisting of ferrite as the first phase and bainite and martensite as the second phase, with the first phase accounting for 70-80%. By employing an optimized alloy formulation and corresponding preparation process, a refined phase transformation microstructure is obtained, ultimately achieving a microstructure containing ferrite and hard phase components such as bainite and martensite. This significantly improves the uniform elongation and toughness of the X70 pipeline steel, resulting in a low yield strength ratio and enhanced deformation resistance and resistance to large deformations.

[0046] The X70 grade pipeline steel plate prepared in the example exhibits a Charpy impact toughness of over 350 J at -20℃, a drop weight (DWTT) shear area of ​​over 95%, and tensile properties of the steel plate as follows: yield strength 480–550 MPa, tensile strength 600–680 MPa, yield-to-tensile ratio ≤0.78, uniform elongation (UEL) ≥12%, and R... t1.5 / R t0.5 ≥1.15, R t2.0 / R t1.0 ≥1.10, R t5.0 / R t1.0 ≥1.15. The product can be widely used in pipeline projects that are prone to large plastic deformation, such as ground slip deformation in earthquake-prone areas, ground liquefaction and subsidence caused by permafrost, and submarine pipeline laying.

[0047] For anyone skilled in the art, many possible variations and modifications can be made to the technical solutions of this invention, or equivalent embodiments can be modified based on the disclosed technical content, without departing from the scope of the technical solutions of this invention. Therefore, any simple modifications, equivalent changes, and modifications made to the above embodiments based on the technical essence of this invention without departing from the content of the technical solutions of this invention should still fall within the protection scope of the technical solutions of this invention.

Claims

1. A process for improving the toughness of X70 pipeline steel with resistance to large deformation, comprising top-and-bottom blowing converter smelting, ladle refining, large slab continuous casting, heating, controlled rolling, and controlled cooling processes, characterized in that, The method includes the following technical solutions: The X70 anti-deformation pipeline steel has the following chemical composition by mass percentage: C: 0.02%–0.05%, Si: 0.20%–0.30%, Mn: 1.8%–2.0%, P≤0.015%, S≤0.003%, Alt: 0.020%–0.035%, N≤0.0030%, H≤0.0002%, O≤0.0015%; the chemical composition also contains at least one of the following: Cr: 0.50%–0.80%, Cu: 0.31%–0.60%, Mo: 0.40%–0.60%, with the balance being iron and unavoidable impurities. ① The continuous casting billet casting speed is 0.9~1.2m / min, and the continuous casting billet thickness is 230~250mm; ② The continuously cast billet adopts a low-temperature heating process, with a heating temperature of 1130℃~1155℃ and a heating time of 1.5~2.0h; ③ The continuously cast billet is rolled in stages, using a "relaxation + controlled rolling + forced cooling" process technology: The first stage of the continuous casting billet is a relaxation and temperature waiting process, with a controlled rolling start temperature of 1100℃~1120℃, using "0+7" rolling, and a relaxation and temperature waiting time of 1min10s~1min40s. When the temperature reaches below 960℃, it enters the finishing mill, with a finishing rolling compression ratio ≥3.

0. The second stage of the continuous casting billet is a controlled rolling stage, with a finishing rolling start temperature of 940℃~958℃ and an end temperature of 842℃~850℃. The third stage of the continuous casting billet rolling process is an ultra-rapid cooling stage. The controlled rolling start temperature is 810℃~830℃. After exiting F7, the first 5 groups of ultra-rapid cooling are used, with a cooling rate of 42~50℃ / S. The final cooling temperature is 380℃~400℃. The X70 anti-large deformation pipeline steel consists of acicular ferrite as the first phase and bainite and MA components as the second phase; the proportion of the first phase is 70% to 80%. The pipeline steel has a transverse Charpy impact energy (KV2) at -20℃ ≥ 320 J, and a drop hammer shear area ≥ 95%; the tensile properties of the steel plate meet the following requirements: yield strength 480~550MPa, tensile strength 600~680MPa, yield-to-tensile ratio ≤ 0.78, uniform elongation (UEL) ≥ 12%, and R... t1.5 / R t0.5 ≥1.15, R t2.0 / R t1.0 ≥1.10, R t5.0 / R t1.0 ≥1.15.

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