Thick gauge pipeline steel with excellent low temperature and mechanical properties

Abstract

This application discloses a thick-gauge pipeline steel with excellent low-temperature performance and mechanical properties. The chemical composition of the pipeline steel is: C 0.035~0.07%, Si 0.12~0.28%, Mn 0.9~1.68%, Cr 0.1~0.49%, Ni 0.3~0.48%, Mo 0.06~0.26%, Nb 0.027~0.059%, V≤0.044%, Ti 0.012~0.017%, Alt 0.021~0.049%, N 0.002~0.005%; the pipeline steel thickness is 36~64mm, yield strength ≥420MPa, R... m With a strength of ≥550MPa, a yield strength ratio of ≤0.88, and a shear area percentage of ≥85% in the drop hammer tear test at -57℃, this design achieves a balance between thickness, low-temperature toughness, and strength, fully meeting the high requirements for pipeline steel in harsh service environments such as polar regions and deep seas.

Description

Technical Field

[0001] This application belongs to the field of steel material preparation technology and relates to a thick-gauge pipeline steel with both excellent low-temperature performance and mechanical properties. Background Technology

[0002] With the continuous development of oil and gas resources in polar regions, deep seas, and the Gobi Desert, pipeline steel used to transport oil and gas faces severe challenges due to service conditions, including ultra-low temperature environments, high pressures, and large displacements. To improve transportation efficiency, high pressure, large capacity, and long distances have become the development trend for long-distance oil and gas pipelines. Therefore, oil and gas pipeline steel is required to have greater thickness, higher strength, better low-temperature toughness, and better ductility.

[0003] However, as the thickness of pipeline steel increases, its drop-weight performance weakens significantly compared to other properties. Therefore, existing pipeline steel typically only achieves drop-weight performance comparable to thin plates; thick plates with excellent drop-weight performance are not yet available, and even industry standards do not explicitly require superior drop-weight performance for thick plates. For example, there are currently no pipeline steel plates with excellent drop-weight performance at a test temperature of -57°C, and even fewer technologies exist for pipeline steel plates thicker than 36mm that also possess excellent drop-weight performance.

[0004] Furthermore, pipeline steel plates will experience a certain amount of strain (e.g., around 1.5%) when used for pipe manufacturing and diameter expansion. This will deteriorate the low-temperature toughness of the steel itself. Existing pipeline steel has poor low-temperature performance after strain aging, and there is even a lack of research on it.

[0005] In addition, existing pipeline steel lacks corresponding technology for crack resistance at low temperatures. Summary of the Invention

[0006] The purpose of this application is to provide a thick-gauge pipeline steel that combines excellent low-temperature performance and mechanical properties.

[0007] To achieve the aforementioned objectives, one embodiment of this application provides a thick-gauge pipeline steel possessing both excellent low-temperature performance and mechanical properties. The chemical composition of the pipeline steel, by mass percentage, includes: C 0.035~0.070%, Si 0.12~0.28%, Mn 0.90~1.68%, Cr 0.10~0.49%, Ni 0.30~0.48%, Mo 0.06~0.26%, Nb 0.027~0.059%, V 0~0.044%, Ti 0.012~0.017%, Alt 0.021~0.049%, N 0.002~0.005%, CE Pcm The content of Mn is 0.12~0.20%, and the content of Mn is 1.3~3.0%, with the remainder being iron and unavoidable impurities;

[0008] The pipeline steel is a steel plate with a thickness of 36~64mm and a yield strength R. t0.5 ≥420MPa, tensile strength R m ≥550MPa, yield strength ratio R t0.5 / R m ≤0.88; The pipeline steel exhibits a shear area percentage of ≥85% in a drop hammer tear test at a test temperature of -57℃.

[0009] As a further improvement, the pipeline steel exhibits a shear area percentage of ≥95% in a drop hammer tear test at a test temperature of -57°C.

[0010] As a further improvement, the pipeline steel has a Charpy V-notch impact energy ≥350J at a test temperature of -60℃; And / or, the strain-aged impact energy of the pipeline steel after being subjected to 3% pre-strain and aging treatment at -60℃ for 280℃×0.5h is ≥320J; And / or, the characteristic value of the crack tip opening displacement δc of the pipeline steel at a test temperature of -60℃ is ≥1.3mm.

[0011] As a further improvement, the microstructure of the pipeline steel, by volume fraction, includes: ferrite, accounting for ≥70%, with an average grain size ≤18μm; martensite-austenite, accounting for ≤2%, with a maximum grain size ≤2μm; and the remainder being tempered bainite or tempered bainite + tempered sorbite.

[0012] As a further improvement, the microstructure of the pipeline steel includes: the ferrite comprising ultrafine ferrite with a maximum grain size ≤ 5 μm, and the ultrafine ferrite accounting for 2~20%.

[0013] As a further improvement, in the microstructure of the pipeline steel: the maximum grain size of the ferrite is ≤20μm.

[0014] As a further improvement, the microstructure of the pipeline steel includes: The ferrite is ultrafine-grained ferrite with a maximum grain size ≤ 5 μm + fine-grained ferrite with a grain size > 5 μm; The proportion of ultrafine ferrite is 2-7%, 10-12%, or 17-19%; Fine-grained ferrite is polygonal ferrite or polygonal ferrite + acicular ferrite, with an average grain size of 6~18μm and a proportion of ≥70%.

[0015] As a further improvement, the microstructure of the pipeline steel is as follows: A composite microstructure consisting of polygonal ferrite, MA components, and tempered bainite; Alternatively, a composite microstructure consisting of ultrafine-grained ferrite with a maximum grain size ≤ 5 μm, fine-grained polygonal ferrite with a grain size > 5 μm, MA component, and tempered bainite, wherein the proportion of ultrafine-grained ferrite is 2~7%, and the average grain size of fine-grained polygonal ferrite is 6~18 μm, accounting for ≥80%; Alternatively, a composite microstructure consisting of ultrafine-grained ferrite with a maximum grain size ≤ 5 μm, fine-grained polygonal ferrite and fine-grained acicular ferrite with a grain size > 5 μm, MA component, tempered bainite, and tempered sorbite, wherein the proportion of ultrafine-grained ferrite is 10~19%, and the average grain size of fine-grained polygonal ferrite and fine-grained acicular ferrite is 6~15 μm, accounting for ≥70%.

[0016] As a further improvement, the banded structure of the pipeline steel is ≤0.5 grade.

[0017] As a further improvement, the chemical composition of the pipeline steel, by mass percentage, includes: C 0.055~0.070%, Si 0.12~0.18%, Mn 0.90~1.10%, Cr 0.10~0.18%, Ni 0.30~0.36%, Mo 0.06~0.13%, Nb 0.027~0.034%; and CE Pcm The yield strength R of the pipeline steel is 0.12~0.15%, and ∑Mn is 1.3~1.6%. t0.5 The tensile strength is 420~520MPa, and the tensile strength R is... m The pressure is 550~650MPa.

[0018] As a further improvement, the chemical composition of the pipeline steel, by mass percentage, includes: C 0.045~0.060%, Si 0.15~0.21%, Mn 1.10~1.30%, Cr 0.15~0.23%, Ni 0.30~0.36%, Mo 0.10~0.17%, Nb 0.032~0.039%, V 0.012~0.019%; and CE Pcm The yield strength R of the pipeline steel is 0.13~0.16%, and ∑Mn is 1.8~2.1%; t0.5 The tensile strength is 450~550MPa, and the tensile strength R is... m The pressure is 560~660MPa.

[0019] As a further improvement, the chemical composition of the pipeline steel, by mass percentage, includes: C 0.035~0.050%, Si 0.19~0.25%, Mn 1.38~1.58%, Cr 0.32~0.39%, Ni 0.42~0.48%, Mo 0.17~0.24%, Nb 0.041~0.048%, V 0.027~0.035%; and CE Pcm The yield strength R of the pipeline steel is 0.15~0.18%, and ∑Mn is 2.4~2.7%. t0.5 The tensile strength is 485~585MPa, and the tensile strength R is... m It is 600~700MPa.

[0020] As a further improvement, the chemical composition of the pipeline steel, by mass percentage, includes: C 0.035~0.050%, Si 0.22~0.28%, Mn 1.48~1.68%, Cr 0.42~0.49%, Ni 0.42~0.48%, Mo 0.19~0.26%, Nb 0.052~0.059%, V 0.037~0.044%; and CE Pcm The yield strength R of the pipeline steel is 0.17~0.20%, and ∑Mn is 2.7~3.0%. t0.5 The tensile strength is 555~655MPa, and the tensile strength R is... m The pressure is 680~780MPa.

[0021] Compared with existing technologies, the beneficial effects of this application include: adopting a low-carbon design, combined with controlled content of Si, Mn, Cr, Ni, Mo, Nb, V, and Ti, promoting precise microstructure control in steel, achieving refined grain size, and improving low-temperature toughness in large thicknesses of 36~64mm. For example, at a test temperature of -57℃, the shear area percentage in the drop hammer tear test is ≥85%. It also ensures the strength of the steel, for example, the yield strength R... t0.5 ≥420MPa, tensile strength R m ≥550MPa, yield strength ratio R t0.5 / R m With a thickness of ≤0.88, it achieves a balance between large thickness, low-temperature toughness, and strength, fully meeting the high requirements of pipeline steel for harsh service environments such as polar regions and deep seas. Attached Figure Description

[0022] Figure 1 This is a metallographic diagram of the pipeline steel plate of Embodiment 1 of the present invention; Figure 2 This is a metallographic diagram of the pipeline steel plate of Embodiment 5 of the present invention; Figure 3This is a metallographic diagram of the pipeline steel plate of Embodiment 9 of the present invention; Figure 4 This is a metallographic diagram of the pipeline steel plate of Embodiment 11 of the present invention. Detailed Implementation

[0023] To make the objectives, technical solutions, and advantages of this application clearer, the technical solutions of this application will be clearly and completely described below in conjunction with specific embodiments. Obviously, the described embodiments are only a part of the embodiments of this application, and not all of them. Based on the embodiments in this application, all other embodiments obtained by those skilled in the art without creative effort are within the scope of protection of this application.

[0024] First Implementation Method One embodiment of this application provides a steel plate, specifically a steel plate for pipeline steel, which can be a quenched and tempered steel plate (i.e., obtained by hot rolling and heat treatment). This steel plate is designed for pipeline steel applications under low-temperature service conditions, such as oil and gas transportation projects.

[0025] The chemical composition of the pipeline steel, by mass percentage, includes: C 0.035~0.070%, Si 0.12~0.28%, Mn 0.90~1.68%, Cr 0.10~0.49%, Ni 0.30~0.48%, Mo 0.06~0.26%, Nb 0.027~0.059%, V 0~0.044%, Ti 0.012~0.017%, Alt 0.021~0.049%, N 0.002~0.005%, with the remainder being iron and unavoidable impurities.

[0026] The following section provides a detailed explanation of the role and mechanism of each chemical element in the chemical composition.

[0027] Carbon (C): C is the most economical strengthening element in steel, playing a role in solid solution strengthening. It can also form carbides with elements such as Ti, Nb, V, Mo, and Cr, playing a role in precipitation strengthening. Increasing the C content significantly improves the strength and hardness of pipeline steel. However, higher C content significantly reduces the low-temperature toughness, weldability, and formability of pipeline steel. Existing technologies typically set the C content above 0.1%, while this application uses a low carbon content of 0.035~0.070%, achieving excellent low-temperature toughness while simultaneously ensuring excellent strength; that is, the strength is not excessively reduced due to the low carbon content.

[0028] Silicon (Si): Si mainly plays a role in solid solution strengthening, but adding excessive Si will significantly deteriorate the ductility and toughness of pipeline steel. In this application, the Si content is set to 0.12~0.28%, for example, it can be any value or range of 0.12%, 0.13%, 0.14%, 0.15%, 0.16%, 0.17%, 0.18%, 0.19%, 0.20%, 0.21%, 0.22%, 0.23%, 0.24%, 0.25%, 0.26%, 0.27%, and 0.28%.

[0029] Manganese (Mn): Mn mainly plays a role in desulfurization, deoxidation, and solid solution strengthening in steel. Mn can expand the austenite phase region, delay bainite transformation, and promote the formation of fine acicular ferrite. Excessive Mn content leads to an increase in banded segregation, reducing microstructure uniformity and toughness. In this application, the Mn content is set to 0.90~1.68%, for example, any value or range of 0.90%, 0.92%, 0.95%, 0.98%, 1.00%, 1.03%, 1.06%, 1.10%, 1.13%, 1.16%, 1.20%, 1.23%, 1.26%, 1.30%, 1.33%, 1.36%, 1.38%, 1.41%, 1.44%, 1.48%, 1.51%, 1.54%, 1.58%, 1.61%, 1.64%, and 1.68%. The preferred Mn content is set at 0.90~1.30% or 1.38~1.68%.

[0030] Chromium (Cr): Cr can improve the hardenability, wear resistance, and corrosion resistance of pipeline steel. However, adding a large amount of Cr will reduce the low-temperature toughness and weldability of pipeline steel. In this application, the Cr content is set to 0.10~0.49%, for example, it can be any value or range of 0.10%, 0.12%, 0.15%, 0.17%, 0.18%, 0.20%, 0.23%, 0.25%, 0.28%, 0.30%, 0.32%, 0.34%, 0.36%, 0.39%, 0.41%, 0.42%, 0.44%, 0.46%, 0.49%. Preferably, the Cr content is set to 0.10~0.23% or 0.32~0.49%.

[0031] Nickel (Ni): Ni can improve the strength of pipeline steel, reduce notch sensitivity and ductile-brittle transition temperature at low temperatures, and improve the service safety of pipeline steel. In this application, the Ni content is set at 0.30~0.48%, for example, any value or range of 0.30%, 0.31%, 0.33%, 0.36%, 0.37%, 0.39%, 0.42%, 0.43%, 0.45%, and 0.48%. Preferably, the Ni content is set at 0.30~0.36% or 0.42~0.48%.

[0032] Molybdenum (Mo): Mo can lower the phase transformation temperature, inhibit the formation of bulk ferrite, promote the transformation of acicular ferrite, and enhance the precipitation strengthening effect of Nb(C,N). Under high-temperature conditions, Mo can increase the resistance to temper softening and prevent temper brittleness. Mo can promote the formation of martensite-austenite (MA) components, which is detrimental to low-temperature toughness. Furthermore, Mo is a precious metal, and excessive addition will increase the cost of pipeline steel. In this application, the Mo content is set to 0.06~0.26%, for example, any value or range of 0.06%, 0.08%, 0.10%, 0.12%, 0.13%, 0.15%, 0.17%, 0.19%, 0.21%, 0.22%, 0.24%, 0.25%, and 0.26%.

[0033] Niobium (Nb): When pipeline steel is held at temperatures above the austenite recrystallization temperature, the fine, dispersed carbonitrides formed by Nb, V, Ti, C, and N pin the austenite grain boundaries, preventing austenite grain coarsening and facilitating the formation of a fine-grained microstructure. Furthermore, Nb can synergistically interact with Mo to promote bainite formation. In this application, the Nb content is set to 0.027~0.059%, for example, any value or range from 0.027%, 0.029%, 0.032%, 0.034%, 0.036%, 0.039%, 0.041%, 0.043%, 0.046%, 0.048%, 0.050%, 0.052%, 0.055%, 0.057%, and 0.059%.

[0034] Vanadium (V): V has a good precipitation strengthening effect, which can precipitate VC in ferrite and improve the strength of steel. The V content is set to 0~0.044%, for example, V can be not added, and the corresponding V content is 0 or close to 0 (existing as an impurity introduced into the raw material, for example, content <0.005%), or it can be any value in the range of 0.012~0.044%, such as any value or range of 0.012%, 0.014%, 0.016%, 0.019%, 0.027%, 0.029%, 0.031%, 0.033%, 0.035%, 0.037%, 0.039%, 0.041%, 0.044%.

[0035] Titanium (Ti): Ti has a strong bonding ability with N, making it a good solid N element in steel. TiN exhibits strong stability at high temperatures, which helps to prevent the coarsening of austenite grains during reheating of the steel plate in heat treatment. In this application, the Ti content is set to 0.012~0.017%, for example, any value or range of 0.012%, 0.013%, 0.015%, 0.016%, and 0.017%.

[0036] Aluminum (Al): Al is a commonly used deoxidizer in pipeline steel. Adding a small amount of Al to steel can refine the grains and improve low-temperature toughness. However, excessive Al content can affect the weldability and machinability of pipeline steel. In this application, the Al content is set between 0.021% and 0.049%, for example, any value or range of 0.021%, 0.023%, 0.025%, 0.027%, 0.030%, 0.032%, 0.034%, 0.036%, 0.039%, 0.041%, 0.043%, 0.045%, 0.047%, and 0.049%.

[0037] Nitrogen (N): N is a key microalloying element in pipeline steel. Appropriate amounts of dissolved N can preferentially combine with Ti to form high-temperature stable TiN nanoscale precipitates. During hot rolling and subsequent heat treatment in the recrystallization zone, TiN particles can effectively pin austenite grain boundaries, inhibit grain boundary migration, and continuously hinder abnormal austenite grain growth and coarsening, ensuring the stability of the fine-grained matrix. Simultaneously, N can combine with Nb and V to precipitate Nb(C,N) and V(C,N) composite carbonitrides. Compared to single carbides, carbonitrides have a wider precipitation temperature range, smaller precipitate size, and stronger dispersion, further enhancing grain boundary pinning and improving the uniformity of the microstructure during recrystallization. In this application, the N content is controlled within the range of 0.002% to 0.005%, for example, any value or range within 0.002%, 0.003%, 0.004%, and 0.005%.

[0038] Regarding impurity elements, for example, P ≤ 0.010% and S ≤ 0.0015%.

[0039] Specifically, phosphorus (P) and sulfur (S) are harmful elements in pipeline steel. P deteriorates the mechanical properties of pipeline steel, while S deteriorates its low-temperature ductility, toughness, and weldability. In this application, P ≤ 0.010% and S ≤ 0.0015%. More preferably, P ≤ 0.008% and S ≤ 0.0012%.

[0040] The chemical composition of the pipeline steel, by mass percentage, also meets: CE Pcm The value is 0.12~0.20%, and ∑Mn is 1.3~3.0%.

[0041] Among them, CE Pcm The full name is cold crack sensitivity coefficient, which is a professional standard term in this field. Its specific calculation method can be found in CE. Pcm =C+Si / 30+(Mn+Cr) / 20+Ni / 60+Mo / 15+V / 10.

[0042] ∑Mn refers to manganese equivalent, which is a professional standard term in this field. Its specific value can be calculated according to the formula ∑Mn=Mn+3.28Mo+0.64Cr+0.37Ni+0.03Si+0.1(Al+V+Ti+Nb).

[0043] In this application, CE Pcm ∑Mn and A in the following text r3 Mf, T nr A c1 A c3 v FB T Nb(C,N) In the relevant formulas, the element symbols represent the mass percentage (%) of the corresponding element in the steel. For example, if the C content in the steel is 0.040%, then C is substituted into 0.040 when calculating.

[0044] In this application, the pipeline steel is a steel plate with a thickness of 36-64 mm. This thickness can also be referred to as the nominal thickness, or the target thickness during production and processing. That is, the pipeline steel belongs to the ultra-thick plate with a thickness specification of 36-64 mm, but considering the production tolerances and measurement errors that are common in the material processing field, a fluctuation of 0 mm to 1 mm between its actual thickness and the nominal thickness / target thickness is allowed.

[0045] Specifically, the thickness of the pipeline steel is any value or any range of 36mm, 38mm, 39mm, 40mm, 41mm, 43mm, 46mm, 48mm, 50mm, 51mm, 53mm, 56mm, 58mm, 61mm, and 64mm.

[0046] The pipeline steel also exhibits excellent mechanical properties, with a yield strength R t0.5 ≥420MPa, tensile strength R m ≥550MPa, yield strength ratio R t0.5 / R m ≤0.88.

[0047] Specifically, the yield strength R of the pipeline steel t0.5 The tensile strength is 420~655MPa, and the tensile strength R is... m The strength is 550~780MPa, and the elongation is A. 50 ≥40%, yield strength ratio Rt0.5 / R m ≤0.88.

[0048] Furthermore, the steel plate exhibits excellent low-temperature toughness.

[0049] On the one hand, at a test temperature of -57℃, the shear area percentage of the steel plate in the drop hammer tear test is ≥85%, even reaching 90%, 92%, or as high as 95%. As mentioned in the background art, with the increase of thickness, the drop hammer performance will be drastically weakened compared with other properties. In the prior art, there are no pipeline steel plates with a thickness of 36mm or more that have excellent drop hammer performance at a test temperature of -57℃, especially pipeline steel plates with a thickness of 39.7mm or more, and the relevant drop hammer performance is not disclosed in industry standards. However, the pipeline steel of this application has ultra-thick specifications and excellent low-temperature drop hammer performance; Thus, in terms of chemical composition, this application adopts a low-carbon design and combines it with controlled content of Si, Mn, Cr, Ni, Mo, Nb, N, V, and Ti to promote precise microstructure control in the steel, achieving refined grain size. This improves low-temperature toughness even with a large thickness of 36~64mm; for example, at a test temperature of -57℃, the shear area percentage in the drop hammer tear test is ≥85%. It also ensures the strength of the steel, for example, the yield strength R... t0.5 ≥420MPa, tensile strength R m ≥550MPa, yield strength ratio R t0.5 / R m With a thickness of ≤0.88, it achieves a balance between large thickness, low-temperature toughness, and strength, fully meeting the high requirements of pipeline steel for harsh service environments such as polar regions and deep seas.

[0050] Furthermore, at a test temperature of -60℃, the Charpy V-notch impact energy of the steel plate is ≥350J.

[0051] On the other hand, the pipeline steel, after being subjected to a test temperature of -60℃ and aging treatment with 3% pre-strain and 280℃×0.5h, exhibits a strain-aged impact energy ≥320J. As mentioned in the background art, the low-temperature performance of existing pipeline steel after strain aging is poor, and there is even a lack of research on it. However, the pipeline steel of this application not only achieves a strain-aging performance improvement of about 1.5%, but also exhibits excellent strain-aging performance under 3% strain conditions, greatly improving the low-temperature toughness and safety of pipeline steel plates in subsequent applications in pipeline engineering.

[0052] Furthermore, at a test temperature of -60℃, the characteristic value of crack tip opening displacement δc in the steel plate is ≥1.3mm. Thus, the pipeline steel of this application also exhibits excellent crack resistance at low temperatures.

[0053] Furthermore, the microstructure of the pipeline steel, by volume fraction, comprises: ferrite, accounting for ≥70%, with an average grain size ≤18μm; martensite-austenite, accounting for ≤2%, with a maximum grain size ≤2μm; and the remainder being tempered bainite or tempered bainite + tempered sorbite. Thus, this composite microstructure, in addition to its overall fine grain size (e.g., the ferrite grains, which constitute the largest proportion, are fine), also incorporates trace amounts of ultrafine-grained martensite (MA) components. Their fine and dispersed distribution not only strengthens the matrix structure and ensures strength, but also increases crack propagation resistance, thereby significantly improving low-temperature toughness.

[0054] In some embodiments, the ferrite in the pipeline steel is polygonal ferrite with an average grain size of 6~18μm. That is, the microstructure of the pipeline steel is a composite microstructure of polygonal ferrite + MA component + tempered bainite, or a composite microstructure of polygonal ferrite + MA component + tempered bainite + tempered sorbite.

[0055] Furthermore, the ferrite in the pipeline steel also includes ultrafine-grained ferrite with a maximum grain size ≤5μm, accounting for 2~20%. Thus, while the ferrite, which accounts for the largest proportion, has fine grains, it also contains a certain proportion of ultrafine-grained ferrite. When cracks encounter the large-angle grain boundaries of these ultrafine-grained ferrites, they will turn, increasing the energy required for crack propagation, thereby further improving the low-temperature drop hammer performance, as well as the mechanical strength and low-temperature toughness.

[0056] In some embodiments, the proportion of ultrafine ferrite in the pipeline steel can be any value or any range of 2%, 3%, 4%, 5%, 6%, 7%, 8%, 9%, 10%, 11%, 12%, 13%, 14%, 15%, 16%, 17%, 18%, 19%, and 20%.

[0057] In addition to the ultrafine ferrite, the ferrite in the pipeline steel also includes fine ferrite with a grain size > 5 μm. These fine ferrites can specifically be polygonal ferrite, or acicular ferrite + polygonal ferrite.

[0058] The proportion of these fine-grained ferrites can be ≥70%, or even ≥75%, or even ≥80%.

[0059] The average grain size of the fine-grained ferrite described above is 6 to 18 μm, preferably 6 to 15 μm, more preferably below 12 μm, for example 10 to 12 μm.

[0060] In addition, in the pipeline steel, the maximum grain size of the ferrite does not exceed 20 μm, or even 16 μm.

[0061] Furthermore, in this application, the banded structure of the pipeline steel is ≤0.5 grade.

[0062] In this application, the microstructure and properties of pipeline steel can be sampled and tested using the following standards: (1) The steel plate was sampled and tested according to GB / T 13298-2015 "Metallic Microstructure Test Method" to obtain the microstructure test results of pipeline steel; the specific microstructure test process can be as follows: a sample is cut at 1 / 4 of the width of the steel plate, and the microstructure at 1 / 4 of the thickness direction is observed using a Zeiss Axio optical microscope (OM), and the size and proportion of each phase are statistically analyzed using Image J image processing software; (2) The steel plates were sampled and tested using API RP 5L3 "Drop-Weight Tear Tests on Line Pipe", GB / T 229-2020 "Charpy Impact Test Method for Metallic Materials", GB / T 4160-2004 "Steel Strain Aging Sensitivity Test Method (Charpy Impact Method)" and ISO 12135-2021 "Metallic materials - Unified method of test for the determination of quasistatic fracture toughness" to obtain the test results of the drop weight performance (e.g., DWTT), Charpy V-notch impact energy, low temperature performance after strain aging, and crack resistance (e.g., CTOD) of the pipeline steel. The specific test process for DWTT can be as follows: cut the sample at 1 / 4 of the width of the steel plate, reduce the thickness to 19 mm on one side, press a V-notch, and conduct a -57 ℃ DWTT test on an Instron 8150 testing machine. (3) The steel plate was sampled and tested according to GB / T 228.1-2021 "Metallic materials, tensile testing - Part 1: Room temperature test method" to obtain the mechanical property test results of pipeline steel.

[0063] In summary, the pipeline steel of this application breaks through the technical constraints of existing technologies in terms of thickness, strength, and low-temperature toughness, achieving effective refinement of the core grains and precise control of the microstructure of the steel plate, ensuring excellent low-temperature toughness, and covering the strength levels from X60Q to X80Q for thick-gauge quenched and tempered pipeline steel, fully meeting the high requirements for low-temperature toughness, strength, and thickness in harsh service environments such as polar regions and deep seas.

[0064] The quenched and tempered pipeline steel of the first embodiment will be further described below through a series of examples and comparative examples.

[0065] The chemical composition, microstructure, and properties of the pipeline steel in each embodiment and comparative example are shown in Table 1, Table 2, and Table 3, respectively.

[0066] [Table 1]

[0067] [Table 2]

[0068] [Table 3]

[0069] The preparation processes for these examples and comparative examples are roughly as follows: (1) Stacking of continuously cast billets obtained through steelmaking and continuous casting; wherein the thickness t0 of the continuously cast billets is 320mm~400mm, and the stacking start temperature is ≥A r3 Stacking time ≥ 72 hours, destacking temperature ≤ Mf; (2) The continuously cast billet is fed into a heating furnace for heating; wherein, the temperature of the continuously cast billet entering the furnace is ≤ Mf, and the heating temperature is T. Nb(C,N) +30℃~T Nb(C,N) +60℃, total furnace time 1.0t0~1.2t0min / mm, holding time 0.1t0~0.2t0min / mm; (3) The continuously cast billet is rolled into an intermediate billet through several passes of widening rolling and several passes of extension rolling; wherein the number of passes of widening rolling is 1 to 3, the widening ratio is 1.3; the initial rolling temperature is T. nr +30℃~T nr +60℃, final rolling temperature T nr ~T nr +20℃; the reduction rate of each pass in the widening rolling is 10~16%, the reduction rate of the first pass in the extended rolling is ≥8%, the reduction rate of each subsequent pass in the extended rolling increases sequentially, and the reduction rate of the last pass in the extended rolling is ≥17%. In addition, the time interval between the last two passes in the extended rolling is ≤16s; (4) The intermediate billet is cooled to a suitable temperature, specifically by water cooling first, followed by air cooling after water removal, until the starting rolling temperature for finishing rolling is reached; wherein, the thickness t1 of the intermediate billet is ±10mm, water temperature T nr -40℃~T nr -30℃; (5) The intermediate billet is rolled into a finished plate by multiple passes of finishing rolling; wherein the initial rolling temperature is A. r3 +25℃~A r3 +55℃, final rolling temperature A r3 +10℃~A r3+40℃; the reduction rate of the first finishing rolling pass is ≥18%, and the reduction rate of each subsequent finishing rolling pass decreases sequentially, with a total reduction rate ≥60%; (6) The finished rolled plate is cooled by water cooling; wherein, the hot rolled plate can relax for 40~60s or not relax before entering the water, and the water temperature is A. r3 -25℃~A r3 +5℃, final cooling temperature Mf-120℃~Mf-80℃, cooling rate v FB -6~v FB +7℃ / s; (7) The cooled steel plate is heat-treated by offline quenching and tempering, or by tempering (or online quenching and tempering) to obtain a finished steel plate with a nominal thickness of t2; wherein, the thickness of the continuously cast billet t0 is 5 to 9 times t2; when tempering, the tempering temperature is A. c1 -170℃~A c1 -80℃, holding time is 3t2+30min~3t2+50min; when using offline quenching + tempering, the quenching heating temperature is A. c3 -30℃~A c3 The furnace is quenched at -10℃ for a total time of 1.5t² + 20 min to 1.5t² + 30 min, then cooled to room temperature before tempering at temperature A. c1 -100~A c1 -80℃, heat preservation time is 3t2+30min~3t2+50min.

[0070] Where Mf is the martensitic transformation end temperature, its unit is ℃, and its value is taken from the formula Mf=561-474C-33Mn-17Cr-17Ni-21Mo-215.

[0071] T Nb(C,N) It represents the temperature at which niobium carbonitrides completely dissolve, measured in °C, and can be obtained from the formula T. Nb(C,N) =6770 / (2.16 lg((C+12N / 14)×Nb))-273.15.

[0072] T nr It represents the non-recrystallization temperature of austenite, that is, the lowest temperature at which static recrystallization of austenite is completely suppressed after hot deformation, and is expressed in °C. It can be taken from the formula T. nr =887+464C+6445Nb-644 +732V-230 +890Ti+363Al-357Si.

[0073] A r3It represents the critical temperature at which austenite begins to precipitate ferrite during the cooling process, and its unit is °C. A specific value can be obtained from formula A. r3 =910-310C-80Mn-15Cr-55Ni-80Mo.

[0074] v FB The critical cooling rate for obtaining acicular ferrite structure during cooling is characterized by its unit of °C / s, and its specific value can be obtained from the formula v. FB =e 11.82-0.43C-0.49Mn-0.78Ni+0.26Cr+0.38Mo / 3600.

[0075] A c1 It represents the critical temperature at which pearlite begins to transform into austenite during the heating process, also known as the austenitizing initiation temperature, and is measured in °C. A specific value can be obtained from formula A. c1 =727-9Mn+24Si+24Cr-14Ni+63V+63Ti+41Al.

[0076] A c3 It represents the end temperature of austenite transformation during heating, also known as the austenitizing termination temperature, and is measured in °C. Specific values ​​can be obtained from formula A. c3 =912-250C-16Mn+48Si-2Cr-16Ni+95V+96Ti+210Al.

[0077] Second Implementation Method One embodiment of this application provides a steel plate, specifically an X60Q grade steel plate for pipeline steel, which may be a quenched and tempered steel plate (e.g., obtained by hot rolling, offline quenching and tempering).

[0078] The chemical composition of the pipeline steel, by mass percentage, includes: C 0.055~0.070%, Si 0.12~0.18%, Mn 0.90~1.10%, Cr 0.10~0.18%, Ni 0.30~0.36%, Mo 0.06~0.13%, Nb 0.027~0.034%, Ti 0.012~0.017%, Alt 0.021~0.049%, N 0.002~0.005%, with the remainder being iron and unavoidable impurities.

[0079] Regarding impurity elements, for example, P ≤ 0.010% and S ≤ 0.0015%.

[0080] The chemical composition of the pipeline steel, by mass percentage, also meets: CE Pcm The value is 0.12~0.15%, and ∑Mn is 1.3~1.6%.

[0081] In this application, the pipeline steel is a steel plate with a nominal thickness of 36~64mm.

[0082] The yield strength R of the pipeline steel t0.5 The tensile strength is 420~520MPa, and the tensile strength R is... m The strength is 550~650MPa, and the elongation is A. 50 ≥40%, yield strength ratio R t0.5 / R m ≤0.88, meeting the strength requirements of pipeline steel of X60Q grade.

[0083] Furthermore, the steel plate exhibits excellent low-temperature toughness.

[0084] On the one hand, at a test temperature of -57℃, the percentage of shear area in the drop hammer tear test of the steel plate is ≥85%, even reaching 90%, 92%, or as high as 95%.

[0085] Furthermore, at a test temperature of -60℃, the Charpy V-notch impact energy of the steel plate is ≥350J.

[0086] On the other hand, the strain-aged impact energy of the pipeline steel after being subjected to 3% pre-strain and aging treatment at 280℃×0.5h at a test temperature of -60℃ is ≥320J.

[0087] In addition, at a test temperature of -60℃, the characteristic value of the crack tip opening displacement δc of the steel plate is ≥1.3mm.

[0088] Furthermore, the microstructure of the pipeline steel is a composite microstructure of polygonal ferrite + MA component + tempered bainite + tempered sorbite. Specifically, the polygonal ferrite accounts for ≥70% of the volume, with a maximum grain size ≤20μm and an average grain size of 6~18μm; the MA component accounts for ≤1% of the volume, with a maximum grain size ≤2μm.

[0089] Furthermore, the banded structure of the pipeline steel is ≤0.5 grade.

[0090] The X60Q quenched and tempered pipeline steel of the second embodiment will be further described below through a series of examples.

[0091] The chemical composition, microstructure, and properties of the pipeline steel in each embodiment are shown in Table 4, Table 5, and Table 6, respectively.

[0092] [Table 4]

[0093] [Table 5]

[0094] [Table 6]

[0095] The preparation process for these embodiments is roughly as follows: (1) Stacking of continuously cast billets obtained through steelmaking and continuous casting; wherein the thickness t0 of the continuously cast billets is 320mm~400mm, and the stacking start temperature is ≥A r3 Stacking time ≥ 72 hours, destacking temperature ≤ Mf; (2) The continuously cast billet is fed into a heating furnace for heating; wherein, the temperature of the continuously cast billet entering the furnace is ≤ Mf, and the heating temperature is T. Nb(C,N) +30℃~T Nb(C,N) +60℃, total furnace time 1.0t0~1.2t0min / mm, holding time 0.1t0~0.2t0min / mm; (3) The continuously cast billet is rolled into an intermediate billet through several passes of widening rolling and several passes of extension rolling; wherein the number of passes of widening rolling is 1 to 3, the widening ratio is 1.3; the initial rolling temperature is T. nr +30℃~T nr +60℃, final rolling temperature T nr ~T nr +20℃; the reduction rate of each pass in the widening rolling is 10~16%, the reduction rate of the first pass in the extended rolling is ≥8%, the reduction rate of each subsequent pass in the extended rolling increases sequentially, and the reduction rate of the last pass in the extended rolling is ≥17%. In addition, the time interval between the last two passes in the extended rolling is ≤16s; (4) The intermediate billet is cooled to a suitable temperature, specifically by water cooling first, followed by air cooling after water removal, until the starting rolling temperature for finishing rolling is reached; wherein, the thickness t1 of the intermediate billet is ±10mm, water temperature T nr -40℃~T nr -30℃; (5) The intermediate billet is rolled into a finished plate by multiple passes of finishing rolling; wherein the initial rolling temperature is A. r3 +25℃~A r3 +45℃, final rolling temperature A r3 +10℃~A r3 +30℃; the reduction rate of the first finishing rolling pass is ≥18%, and the reduction rate of each subsequent finishing rolling pass decreases sequentially, with a total reduction rate ≥60%; (6) The precision rolled plate is cooled by water cooling; wherein, the water immersion temperature A of the hot rolled plate is... r3 -15℃~A r3 +5℃, final cooling temperature Mf-100℃~Mf-80℃, cooling rate v FB -6~v FB -3℃ / s; (7) The cooled steel plate is heat-treated by offline quenching and tempering to obtain a finished steel plate with a nominal thickness of t2; wherein, the thickness of the continuously cast billet t0 is 5 to 9 times t2; during offline quenching and tempering, the quenching heating temperature is A. c3 -30℃~A c3 The furnace is quenched at -10℃ for a total time of 1.5t² + 20 min to 1.5t² + 30 min, then cooled to room temperature before tempering at temperature A. c1 -100~A c1 -80℃, heat preservation time is 3t2+30min~3t2+50min.

[0096] Third Implementation Method One embodiment of this application provides a steel plate, specifically an X65Q grade steel plate for pipeline steel, which may be a quenched and tempered steel plate (e.g., obtained by hot rolling, offline quenching and tempering).

[0097] The chemical composition of the pipeline steel, by mass percentage, includes: C 0.045~0.060%, Si 0.15~0.21%, Mn 1.10~1.30%, Cr 0.15~0.23%, Ni 0.30~0.36%, Mo 0.10~0.17%, Nb 0.032~0.039%, V 0.012~0.019%; Ti 0.012~0.017%, Alt 0.021~0.049%, N 0.002~0.005%, with the remainder being iron and unavoidable impurities.

[0098] Regarding impurity elements, for example, P ≤ 0.010% and S ≤ 0.0015%.

[0099] The chemical composition of the pipeline steel, by mass percentage, also meets: CE Pcm The value is 0.13~0.16%, and ∑Mn is 1.8~2.1%.

[0100] In this application, the pipeline steel is a steel plate with a nominal thickness of 36~64mm.

[0101] The yield strength R of the pipeline steel t0.5 The tensile strength is 450~550MPa, and the tensile strength R is... m The strength is 560~660MPa, and the elongation is A. 50 ≥40%, yield strength ratio R t0.5 / R m ≤0.88, meeting the strength requirements of pipeline steel of X65Q grade.

[0102] Furthermore, the steel plate exhibits excellent low-temperature toughness.

[0103] On the one hand, at a test temperature of -57℃, the percentage of shear area in the drop hammer tear test of the steel plate is ≥85%, even reaching 90%, 92%, or as high as 95%.

[0104] Furthermore, at a test temperature of -60℃, the Charpy V-notch impact energy of the steel plate is ≥350J.

[0105] On the other hand, the strain-aged impact energy of the pipeline steel after being subjected to 3% pre-strain and aging treatment at 280℃×0.5h at a test temperature of -60℃ is ≥320J.

[0106] In addition, at a test temperature of -60℃, the characteristic value of the crack tip opening displacement δc of the steel plate is ≥1.3mm.

[0107] Furthermore, the microstructure of the pipeline steel is a composite microstructure of polygonal ferrite + MA component + tempered bainite + tempered sorbite. Specifically, the polygonal ferrite accounts for ≥70% of the volume, with a maximum grain size ≤20μm and an average grain size of 6~18μm; the MA component accounts for ≤1% of the volume, with a maximum grain size ≤2μm.

[0108] Furthermore, the banded structure of the pipeline steel is ≤0.5 grade.

[0109] The X65Q quenched and tempered pipeline steel of the third embodiment will be further described below through a series of examples.

[0110] The chemical composition, microstructure, and properties of the pipeline steel in each embodiment are shown in Table 7, Table 8, and Table 9, respectively.

[0111] [Table 7]

[0112] [Table 8]

[0113] [Table 9]

[0114] The preparation process of these embodiments, steps (1) to (4) and step (7) are the same as those of embodiments A1 and 2A of the second embodiment above, except that: (5) The intermediate billet is rolled into a finished plate by multiple passes of finishing rolling; wherein the initial rolling temperature is A. r3 +30℃~A r3 +50℃, final rolling temperature A r3 +15℃~A r3 +35℃; the reduction rate of the first finishing rolling pass is ≥18%, and the reduction rate of each subsequent finishing rolling pass decreases sequentially, with a total reduction rate ≥60%; (6) The precision rolled plate is cooled by water cooling; wherein, the water immersion temperature A of the hot rolled plate is... r3 -15℃~A r3 +5℃, final cooling temperature Mf-100℃~Mf-80℃, cooling rate v FB -3~v FB ℃ / s.

[0115] Fourth Implementation Method One embodiment of this application provides a steel plate, specifically an X60Q grade steel plate for pipeline steel, which may be a quenched and tempered steel plate (e.g., obtained by hot rolling and tempering).

[0116] The pipeline steel plate of this embodiment is the same as that of the second embodiment above in terms of chemical composition, thickness, mechanical properties, low-temperature toughness, and banded structure, with the only difference being the structure.

[0117] Specifically, the microstructure of the pipeline steel is a composite microstructure consisting of ultrafine-grained ferrite (maximum grain size ≤ 5 μm), fine-grained polygonal ferrite (grain size > 5 μm), MA component, and tempered bainite. More specifically, the fine-grained polygonal ferrite accounts for ≥ 80%, with an average grain size of 6~18 μm and a maximum grain size ≤ 20 μm; the ultrafine-grained ferrite accounts for 2~5%; the MA component accounts for ≤ 1% of the volume, and the maximum grain size of the MA component is ≤ 2 μm.

[0118] The X60Q quenched and tempered pipeline steel of the fourth embodiment will be further described below through a series of examples.

[0119] The chemical composition, microstructure, and properties of the pipeline steel in each embodiment are shown in Table 10, Table 11, and Table 12, respectively.

[0120] [Table 10]

[0121] [Table 11]

[0122] [Table 12]

[0123] The preparation process of these embodiments, steps (1) to (6) are the same as those of embodiments 1A and 2A of the second embodiment above, except that: (7) The cooled steel plate is heat-treated by tempering (i.e., tempering is performed online directly after step 6) to obtain a finished steel plate with a nominal thickness of t2; wherein, the thickness of the continuously cast billet t0 is 5 to 9 times that of t2, and the tempering temperature is A. c1 -100℃~A c1-80℃, heat preservation time is 3t2+30min~3t2+50min.

[0124] Fifth Implementation Method One embodiment of this application provides a steel plate, specifically an X65Q grade steel plate for pipeline steel, which may be a quenched and tempered steel plate (e.g., obtained by hot rolling and tempering).

[0125] The pipeline steel plate of this embodiment is the same as that of the third embodiment above in terms of chemical composition, thickness, mechanical properties, low-temperature toughness, and banded structure, with the only difference being the structure.

[0126] Specifically, the microstructure of the pipeline steel is a composite microstructure consisting of ultrafine-grained ferrite (maximum grain size ≤ 5 μm), fine-grained polygonal ferrite (grain size > 5 μm), MA component, and tempered bainite. More specifically, the fine-grained polygonal ferrite accounts for ≥ 80%, with an average grain size of 6~18 μm and a maximum grain size ≤ 20 μm; the ultrafine-grained ferrite accounts for 5~7%; the volume fraction of the MA component is ≤ 1%, and the maximum grain size of the MA component is ≤ 2 μm.

[0127] The X65Q quenched and tempered pipeline steel of the fifth embodiment will be further described below through a series of examples.

[0128] The chemical composition, microstructure, and properties of the pipeline steel in each embodiment are shown in Table 13, Table 14, and Table 15, respectively.

[0129] [Table 13]

[0130] [Table 14]

[0131] [Table 15]

[0132] The preparation process of these embodiments, steps (1) to (6) are the same as those of embodiments 1C and 2C of the third embodiment above, except that: (7) The cooled steel plate is heat-treated by tempering (i.e., tempering is performed online directly after step 6) to obtain a finished steel plate with a nominal thickness of t2; wherein, the thickness of the continuously cast billet t0 is 5 to 9 times that of t2, and the tempering temperature is A. c1 -100℃~A c1 -80℃, heat preservation time is 3t2+30min~3t2+50min.

[0133] Sixth Implementation Method One embodiment of this application provides a steel plate, specifically an X70Q grade steel plate for pipeline steel, which may be a quenched and tempered steel plate (e.g., obtained by hot rolling and tempering).

[0134] The chemical composition of the pipeline steel, by mass percentage, includes: C 0.035~0.050%, Si 0.19~0.25%, Mn 1.38~1.58%, Cr 0.32~0.39%, Ni 0.42~0.48%, Mo 0.17~0.24%, Nb 0.041~0.048%, V 0.027~0.035%, Ti 0.012~0.017%, Alt 0.021~0.049%, N 0.002~0.005%, with the remainder being iron and unavoidable impurities.

[0135] Regarding impurity elements, for example, P ≤ 0.010% and S ≤ 0.0015%.

[0136] The chemical composition of the pipeline steel, by mass percentage, also meets: CE Pcm The value is 0.15~0.18%, and ∑Mn is 2.4~2.7%.

[0137] In this application, the pipeline steel is a steel plate with a nominal thickness of 36~64mm.

[0138] The yield strength R of the pipeline steel t0.5 The tensile strength is 485~585MPa, and the tensile strength R is... m The strength is 600~700MPa, and the elongation is A. 50 ≥40%, yield strength ratio R t0.5 / R m ≤0.88, meeting the strength requirements of pipeline steel of X70Q grade.

[0139] Furthermore, the steel plate exhibits excellent low-temperature toughness.

[0140] On the one hand, at a test temperature of -57℃, the percentage of shear area in the drop hammer tear test of the steel plate is ≥85%, even reaching 90%, 92%, or as high as 95%.

[0141] Furthermore, at a test temperature of -60℃, the Charpy V-notch impact energy of the steel plate is ≥350J.

[0142] On the other hand, the strain-aged impact energy of the pipeline steel after being subjected to 3% pre-strain and aging treatment at 280℃×0.5h at a test temperature of -60℃ is ≥320J.

[0143] In addition, at a test temperature of -60℃, the characteristic value of the crack tip opening displacement δc of the steel plate is ≥1.3mm.

[0144] Furthermore, the microstructure of the pipeline steel is a composite microstructure consisting of ultrafine-grained ferrite (maximum grain size ≤ 5 μm) + fine-grained acicular ferrite + fine-grained polygonal ferrite + MA component + tempered bainite. Specifically, the sum of the proportions of fine-grained acicular ferrite and fine-grained polygonal ferrite is ≥ 75%, the average grain size is 6~15 μm, the maximum grain size is ≤ 20 μm, and the proportion of ultrafine-grained ferrite is 10~12%; the volume fraction of the MA component is ≤ 2%, and the maximum grain size of the MA component is ≤ 2 μm.

[0145] Furthermore, the banded structure of the pipeline steel is ≤0.5 grade.

[0146] The X70Q quenched and tempered pipeline steel of the sixth embodiment will be further described below through a series of examples.

[0147] The chemical composition, microstructure, and properties of the pipeline steel in each embodiment are shown in Table 16, Table 17, and Table 18, respectively.

[0148] [Table 16]

[0149] [Table 17]

[0150] [Table 18]

[0151] The preparation process for these embodiments is roughly as follows: (1) Stacking of continuously cast billets obtained through steelmaking and continuous casting; wherein the thickness t0 of the continuously cast billets is 320mm~400mm, and the stacking start temperature is ≥A r3 Stacking time ≥ 72 hours, destacking temperature ≤ Mf; (2) The continuously cast billet is fed into a heating furnace for heating; wherein, the temperature of the continuously cast billet entering the furnace is ≤ Mf, and the heating temperature is T. Nb(C,N) +30℃~T Nb(C,N) +60℃, total furnace time 1.0t0~1.2t0min / mm, holding time 0.1t0~0.2t0min / mm; (3) The continuously cast billet is rolled into an intermediate billet through several passes of widening rolling and several passes of extension rolling; wherein the number of passes of widening rolling is 1 to 3, the widening ratio is 1.3; the initial rolling temperature is T. nr +30℃~T nr +60℃, final rolling temperature T nr ~T nr+20℃; the reduction rate of each pass in the widening rolling is 10~16%, the reduction rate of the first pass in the extended rolling is ≥8%, the reduction rate of each subsequent pass in the extended rolling increases sequentially, and the reduction rate of the last pass in the extended rolling is ≥17%. In addition, the time interval between the last two passes in the extended rolling is ≤16s; (4) The intermediate billet is cooled to a suitable temperature, specifically by water cooling first, followed by air cooling after water removal, until the starting rolling temperature for finishing rolling is reached; wherein, the thickness t1 of the intermediate billet is ±10mm, water temperature T nr -40℃~T nr -30℃; (5) The intermediate billet is rolled into a finished plate by multiple passes of finishing rolling; wherein the initial rolling temperature is A. r3 +35℃~A r3 +55℃, final rolling temperature A r3 +20℃~A r3 +40℃; the reduction rate of the first finishing rolling pass is ≥18%, and the reduction rate of each subsequent finishing rolling pass decreases sequentially, with a total reduction rate ≥60%; (6) The precision-rolled plate is cooled by water cooling; wherein, the hot-rolled plate can relax for 50~60s before entering the water, and the water temperature is A. r3 -25℃~A r3 -5℃, final cooling temperature Mf-120℃~Mf-100℃, cooling rate v FB +1~v FB +4℃ / s; (7) The cooled steel plate is heat-treated by tempering (i.e., tempering is performed online directly after step 6) to obtain a finished steel plate with a nominal thickness of t2; wherein, the thickness of the continuously cast billet t0 is 5 to 9 times that of t2, and the tempering temperature is A. c1 -170℃~A c1 -150℃, heat preservation time is 3t2+30min~3t2+50min.

[0152] Seventh Implementation Method One embodiment of this application provides a steel plate, specifically an X80Q grade steel plate for pipeline steel, which may be a quenched and tempered steel plate (e.g., obtained by hot rolling and tempering).

[0153] The chemical composition of the pipeline steel, by mass percentage, includes: C 0.035~0.050%, Si 0.22~0.28%, Mn 1.48~1.68%, Cr 0.42~0.49%, Ni 0.42~0.48%, Mo 0.19~0.26%, Nb 0.052~0.059%, V 0.037~0.044%, Ti 0.012~0.017%, Alt 0.021~0.049%, N 0.002~0.005%, with the remainder being iron and unavoidable impurities.

[0154] Regarding impurity elements, for example, P ≤ 0.010% and S ≤ 0.0015%.

[0155] The chemical composition of the pipeline steel, by mass percentage, also meets: CE Pcm The value is 0.17~0.20%, and ∑Mn is 2.7~3.0%.

[0156] In this application, the pipeline steel is a steel plate with a nominal thickness of 36~64mm.

[0157] The yield strength R of the pipeline steel t0.5 The tensile strength is 555~655MPa, and the tensile strength R is... m The strength is 680~780MPa, and the elongation is A. 50 ≥40%, yield strength ratio R t0.5 / R m ≤0.88, meeting the strength requirements of pipeline steel of X70Q grade.

[0158] Furthermore, the steel plate exhibits excellent low-temperature toughness.

[0159] On the one hand, at a test temperature of -57℃, the percentage of shear area in the drop hammer tear test of the steel plate is ≥85%, even reaching 90%, 92%, or as high as 95%.

[0160] Furthermore, at a test temperature of -60℃, the Charpy V-notch impact energy of the steel plate is ≥350J.

[0161] On the other hand, the strain-aged impact energy of the pipeline steel after being subjected to 3% pre-strain and aging treatment at 280℃×0.5h at a test temperature of -60℃ is ≥320J.

[0162] In addition, at a test temperature of -60℃, the characteristic value of the crack tip opening displacement δc of the steel plate is ≥1.3mm.

[0163] Furthermore, the microstructure of the pipeline steel is a composite microstructure consisting of ultrafine-grained ferrite (maximum grain size ≤ 5 μm) + fine-grained acicular ferrite + fine-grained polygonal ferrite + MA component + tempered bainite + tempered sorbite. Specifically, the sum of the proportions of fine-grained acicular ferrite and fine-grained polygonal ferrite is ≥ 70%, the average grain size is 6~12 μm, the maximum grain size is ≤ 20 μm, and the proportion of ultrafine-grained ferrite is 17~19%; the volume fraction of the MA component is ≤ 2%, and the maximum grain size of the MA component is ≤ 2 μm.

[0164] Furthermore, the banded structure of the pipeline steel is ≤0.5 grade.

[0165] The X80Q quenched and tempered pipeline steel of the seventh embodiment will be further described below through a series of examples.

[0166] The chemical composition, microstructure, and properties of the pipeline steel in each embodiment are shown in Table 19, Table 20, and Table 21, respectively.

[0167] [Table 19]

[0168] [Table 20]

[0169] [Table 21]

[0170] The preparation process of these embodiments, steps (1) to (5) and step (7) are the same as those of embodiments 1E and 2E of the sixth embodiment above, except that: (6) The precision-rolled plate is cooled by water cooling; wherein, the hot-rolled plate can relax for 40~50s before entering the water, and the water temperature is A. r3 -20℃~A r3 ℃, final cooling temperature Mf-120℃~Mf-100℃, cooling rate v FB +4~v FB +7℃ / s.

[0171] As can be seen from the above embodiments, the technology of this application breaks through the technical constraints of the prior art in terms of thickness, strength, and low-temperature toughness, and achieves effective refinement of the core grains and fine control of the microstructure of the steel plate, ensuring excellent low-temperature toughness. Moreover, the strength level covers thick-gauge quenched and tempered pipeline steel from X60Q to X80Q, fully meeting the high requirements of low-temperature toughness, strength, and thickness for harsh service environments such as polar regions and deep seas.

Claims

1. A thick-gauge pipeline steel possessing both excellent low-temperature performance and mechanical properties, characterized in that, The chemical composition of the pipeline steel, by mass percentage, includes: C 0.035~0.070%, Si 0.12~0.28%, Mn 0.90~1.68%, Cr 0.10~0.49%, Ni 0.30~0.48%, Mo 0.06~0.26%, Nb 0.027~0.059%, V 0~0.044%, Ti 0.012~0.017%, Alt 0.021~0.049%, N 0.002~0.005%, CE Pcm The content of Mn is 0.12~0.20%, and the content of Mn is 1.3~3.0%, with the remainder being iron and unavoidable impurities; The pipeline steel is a steel plate with a thickness of 36~64mm and a yield strength R. t0.5 ≥420MPa, tensile strength R m ≥550MPa, yield strength ratio R t0.5 / R m ≤0.88; The pipeline steel exhibits a shear area percentage of ≥85% in a drop hammer tear test at a test temperature of -57℃.

2. The thick-gauge pipeline steel with excellent low-temperature performance and mechanical properties according to claim 1, characterized in that, The pipeline steel exhibits a shear area percentage of ≥95% in a drop hammer tear test at a test temperature of -57℃.

3. The thick-gauge pipeline steel with excellent low-temperature performance and mechanical properties according to claim 2, characterized in that, The pipeline steel has a Charpy V-notch impact energy ≥350J at a test temperature of -60℃. And / or, the strain-aged impact energy of the pipeline steel after being subjected to 3% pre-strain and aging treatment at -60℃ for 280℃×0.5h is ≥320J; And / or, the characteristic value of the crack tip opening displacement δc of the pipeline steel at a test temperature of -60℃ is ≥1.3mm.

4. The thick-gauge pipeline steel with excellent low-temperature performance and mechanical properties according to claim 2, characterized in that, The microstructure of the pipeline steel, by volume fraction, includes: ferrite, accounting for ≥70%, with an average grain size ≤18μm; martensite-austenite, accounting for ≤2%, with a maximum grain size ≤2μm; and the remainder being tempered bainite or tempered bainite + tempered sorbite.

5. The thick-gauge pipeline steel with excellent low-temperature performance and mechanical properties according to claim 4, characterized in that, The microstructure of the pipeline steel includes ultrafine ferrite with a maximum grain size of ≤5μm, and the proportion of ultrafine ferrite is 2~20%.

6. The thick-gauge pipeline steel with excellent low-temperature performance and mechanical properties according to claim 4, characterized in that, In the microstructure of the pipeline steel: the maximum grain size of the ferrite is ≤20μm.

7. The thick-gauge pipeline steel with excellent low-temperature performance and mechanical properties according to claim 6, characterized in that, The microstructure of the pipeline steel is as follows: The ferrite is ultrafine-grained ferrite with a maximum grain size ≤ 5 μm + fine-grained ferrite with a grain size > 5 μm; The proportion of ultrafine ferrite is 2-7%, 10-12%, or 17-19%; Fine-grained ferrite is polygonal ferrite or polygonal ferrite + acicular ferrite, with an average grain size of 6~18μm and a proportion of ≥70%.

8. The thick-gauge pipeline steel with excellent low-temperature performance and mechanical properties according to claim 6, characterized in that, The microstructure of the pipeline steel is as follows: A composite microstructure consisting of polygonal ferrite, MA components, and tempered bainite; Alternatively, a composite microstructure consisting of ultrafine-grained ferrite with a maximum grain size ≤ 5 μm, fine-grained polygonal ferrite with a grain size > 5 μm, MA component, and tempered bainite, wherein the proportion of ultrafine-grained ferrite is 2~7%, and the average grain size of fine-grained polygonal ferrite is 6~18 μm, accounting for ≥80%; Alternatively, a composite microstructure consisting of ultrafine-grained ferrite with a maximum grain size ≤ 5 μm, fine-grained polygonal ferrite and fine-grained acicular ferrite with a grain size > 5 μm, MA component, tempered bainite, and tempered sorbite, wherein the proportion of ultrafine-grained ferrite is 10~19%, and the average grain size of fine-grained polygonal ferrite and fine-grained acicular ferrite is 6~15 μm, accounting for ≥70%.

9. The thick-gauge pipeline steel with excellent low-temperature performance and mechanical properties according to claim 2, characterized in that, The banded structure of the pipeline steel is ≤0.5 grade.

10. The thick-gauge pipeline steel with excellent low-temperature performance and mechanical properties according to claim 2, characterized in that, The chemical composition of the pipeline steel, by mass percentage, includes: C 0.055~0.070%, Si 0.12~0.18%, Mn 0.90~1.10%, Cr 0.10~0.18%, Ni 0.30~0.36%, Mo 0.06~0.13%, Nb 0.027~0.034%; and CE Pcm The yield strength R of the pipeline steel is 0.12~0.15%, and ∑Mn is 1.3~1.6%. t0.5 The tensile strength is 420~520MPa, and the tensile strength R is... m The pressure is 550~650MPa; Alternatively, the chemical composition of the pipeline steel, by mass percentage, includes: C 0.045~0.060%, Si 0.15~0.21%, Mn 1.10~1.30%, Cr 0.15~0.23%, Ni 0.30~0.36%, Mo 0.10~0.17%, Nb 0.032~0.039%, V 0.012~0.019%; and CE Pcm The yield strength R of the pipeline steel is 0.13~0.16%, and ∑Mn is 1.8~2.1%; t0.5 The tensile strength is 450~550MPa, and the tensile strength R is... m The pressure is 560~660MPa; Alternatively, the chemical composition of the pipeline steel, by mass percentage, includes: C 0.035~0.050%, Si 0.19~0.25%, Mn 1.38~1.58%, Cr 0.32~0.39%, Ni 0.42~0.48%, Mo 0.17~0.24%, Nb 0.041~0.048%, V 0.027~0.035%; and CE Pcm The yield strength R of the pipeline steel is 0.15~0.18%, and ∑Mn is 2.4~2.7%. t0.5 The tensile strength is 485~585MPa, and the tensile strength R is... m The pressure is 600~700MPa; Alternatively, the chemical composition of the pipeline steel, by mass percentage, includes: C 0.035~0.050%, Si 0.22~0.28%, Mn 1.48~1.68%, Cr 0.42~0.49%, Ni 0.42~0.48%, Mo 0.19~0.26%, Nb 0.052~0.059%, V 0.037~0.044%; and CE Pcm The yield strength R of the pipeline steel is 0.17~0.20%, and ∑Mn is 2.7~3.0%. t0.5 The tensile strength is 555~655MPa, and the tensile strength R is... m The pressure is 680~780MPa.

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