X80 pipeline steel and preparation method thereof

Abstract

The invention discloses X80 pipeline steel and a preparation method thereof, relates to the technical field of metallurgical production processes, and aims to solve the problem of low safety of circumferential welds. The X80 pipeline steel comprises the following components in percentage by weight: 0.045% to 0.060% of C, 0.18% to 0.22% of Si, 1.50% to 1.60% of Mn, 0 to 0.012% of P, less than or equal to 0.0030% of S, 0.15% to 0.20% of Cr, 0.08% to 0.12% of Ni, 0.12% to 0.16% of Mo, 0.035% to 0.055% of V, 0.035% to 0.055% of Nb, 0.012% to 0.016% of Ti and the balance of iron and inevitable trace impurities. And the value range of the cold crack sensitivity coefficient Pcm is 0.15%-0.17%.

Description

Technical Field

[0001] The present application relates to the technical field of metallurgical production technology, and in particular to X80 pipeline steel and a preparation method thereof. Background Art

[0002] As the demand for oil and gas resources continues to rise, the mileage of oil and gas pipelines continues to increase, and oil and gas pipelines are developing towards high strength levels, high transmission pressures and large diameters. However, with the increase in pipeline mileage and service life, the number of pipeline accidents is also increasing, causing huge economic losses, casualties and adverse social impacts.

[0003] In pipeline failure cases, the poor strength matching of girth welds due to the large dispersion of pipe strength is particularly prominent. The service safety of girth welds has become an important engineering issue currently facing pipeline safety service, and has put forward higher requirements on the strength and toughness of gas pipelines, especially the stability of mechanical properties. Summary of the invention

[0004] The purpose of this application is to provide an X80 pipeline steel and a preparation method thereof, aiming to solve the problem of low service safety of girth welds, improve the toughness and mechanical stability of X80 pipeline steel, and improve the service safety of girth welds.

[0005] On the one hand, in order to achieve the above-mentioned purpose, the present application adopts the following technical scheme: the weight percentage of the components of X80 pipeline steel is: C: 0.045% ~ 0.060%, Si: 0.18% ~ 0.22%, Mn: 1.50% ~ 1.60%, P: 0 ~ 0.012%, S: ≤ 0.0030%, Cr: 0.15% ~ 0.20%, Ni: 0.08% ~ 0.12%, Mo: 0.12% ~ 0.16%, V: 0.035% ~ 0.055%, Nb: 0.035% ~ 0.055%, Ti: 0.012% ~ 0.016%, and the remainder is iron and inevitable trace impurities; the value range of the cold crack sensitivity coefficient is 0.15% ~ 0.17%.

[0006] The embodiment of the present application can ensure that the steel pipe produced by the product has good comprehensive mechanical properties such as strength and low-temperature toughness through the composite strengthening effect of alloy elements such as Ni, Cr, Mo, Nb and Ti. At the same time, through the precise control of the C element and the cold crack sensitivity coefficient, the fine grain strengthening effect of the Nb element and the optimization of the low-temperature toughness of the Ni element, the microstructure of the heat-affected zone of the steel pipe after girth welding can still ensure good low-temperature toughness, so that the heat-affected zone of the steel pipe after girth welding has a technical effect of high toughness, and the strength of the steel pipe has a smaller fluctuation range, achieving the purpose of controlling the stability of the mechanical properties of the steel pipe.

[0007] On the other hand, a preparation method of X80 pipeline steel is provided, the method comprising: providing the X80 pipeline steel as described in any of the above embodiments; subjecting the components of the X80 pipeline steel to continuous casting to obtain a cast billet; placing the cast billet into a heating furnace, with a heating temperature ranging from 1170°C to 1190°C and a heating time ranging from 180min to 240min; rough rolling, with the number of rough rolling being less than or equal to 8 passes; finish rolling, with the number of finish rolling being less than or equal to 9 passes; cooling, with a cooling rate ranging from 18°C / s to 24°C / s; and a final cooling temperature ranging from 410°C to 460°C.

[0008] In some embodiments, during the continuous casting, protected casting is used throughout the entire process, and the continuous casting process adopts low superheat and dynamic light reduction processes.

[0009] In some embodiments, in the rough rolling, the final rolling reduction ratio is ≥25%, the final rolling temperature is ≤960° C., and the intermediate billet thickness is 3.5 t.

[0010] In some embodiments, during the finishing rolling, the finishing rolling inlet temperature is ≤940° C., and the finishing rolling cumulative reduction ratio is ≥55%.

[0011] In some embodiments, during the cooling, the cooling rate ranges from 18° C. / s to 22° C. / s.

[0012] In some embodiments, during the cooling, the final cooling temperature ranges from 420°C to 440°C.

[0013] The embodiments of the present application improve the manufacturing method from the perspective of detailed control of continuous casting, rough rolling and finish rolling, thereby achieving the effect of refining the final organizational state of the product, thereby utilizing the genetic effect. Although the organizational structure at the heat affected zone of the girth weld has undergone a thermal cycle and changed its initial organizational state, the organizational structure at the heat affected zone of the girth weld still inherits part of the fine grain effect of the parent material, thereby achieving the goal of improving the low-temperature toughness of the heat affected zone of the girth weld. In addition, the hot rolling production process provided by the present application appropriately weakens the excessively high requirements for equipment by adjusting the pass reduction rate and key point temperature parameters, which can have a favorable effect on equipment maintenance during the mass production of high-strength thick-gauge pipeline steel.

[0014] The embodiments of the present application provide X80 pipeline steel with strong toughness and stable mechanical properties through the X80 pipeline steel and the preparation method thereof as described in the above embodiments, which can improve the service safety of the girth weld. BRIEF DESCRIPTION OF THE DRAWINGS

[0015] In order to more clearly illustrate the technical solutions of the embodiments of the present application, the drawings required for use in the description of the embodiments will be briefly introduced below. Obviously, the drawings described below are only some embodiments of the present application. For ordinary technicians in this field, other drawings can be obtained based on these drawings without paying any creative work.

[0016] Figure 1 A flow chart of a method for preparing X80 pipeline steel provided according to an embodiment of the present application;

[0017] Figure 2 A metallographic photograph of the microstructure of the X80 pipeline steel provided according to an embodiment of the present application;

[0018] Figure 3 The microstructure photograph of the heat-affected zone of the girth weld joint of X80 pipeline steel after pipe making and girth welding provided in an embodiment of the present application. DETAILED DESCRIPTION

[0019] The following will be combined with the drawings in the embodiments of the present application to clearly and completely describe the technical solutions in the embodiments of the present application. Obviously, the described embodiments are only part of the embodiments of the present application, not all of the embodiments. Based on the embodiments in the present application, all other embodiments obtained by ordinary technicians in this field without creative work are within the scope of protection of this application.

[0020] In the present application, the terms "comprises", "comprising" or any other variants thereof are intended to cover non-exclusive inclusion, so that a process, article or device including a series of elements includes not only those elements, but also includes other elements not explicitly listed, or also includes elements inherent to such process, article or device. In the absence of further restrictions, an element defined by the sentence "comprising a ..." does not exclude the presence of other identical elements in the process, article or device including the element.

[0021] In the embodiments of the present application, words such as "exemplary" or "for example" are used to indicate examples, illustrations or descriptions. Any embodiment or design described as "exemplary" or "for example" in the embodiments of the present application should not be interpreted as being more preferred or more advantageous than other embodiments or designs. Specifically, the use of words such as "exemplary" or "for example" is intended to present related concepts in a specific way.

[0022] In the description of this specification, specific features, structures, materials or characteristics may be combined in an appropriate manner in any one or more embodiments or examples.

[0023] In some embodiments, the present application provides an X80 pipeline steel.

[0024] X80 is the US classification model for high-strength pipeline steel, equivalent to China's L555.

[0025] In some examples, the weight percentage of the composition of X80 pipeline steel is: C: 0.045% ~ 0.060%, Si: 0.18% ~ 0.22%, Mn: 1.50% ~ 1.60%, P: 0 ~ 0.012%, S: ≤ 0.0030%, Cr: 0.15% ~ 0.20%, Ni: 0.08% ~ 0.12%, Mo: 0.12% ~ 0.16%, V: 0.035% ~ 0.055%, Nb: 0.035% ~ 0.055%, Ti: 0.012% ~ 0.016%, and the remainder is iron and unavoidable trace impurities; the value range of the cold crack sensitivity coefficient Pcm is 0.15% ~ 0.17%.

[0026] For example, in the composition of X80 pipeline steel, the weight percentage of C is 0.045%, 0.048%, 0.050%, 0.052% or 0.060%, etc., which is not limited here.

[0027] C is the most basic and economical strengthening element in steel, and is also the main element that affects the low-temperature toughness, fracture resistance, ductility and formability of pipeline steel. The inventors found that high or low C content will have an adverse effect on the low-temperature toughness of the girth weld heat affected zone. In the composition of X80 pipeline steel, the weight percentage of C is in the range of 0.045% to 0.060%, which is the optimal range.

[0028] For example, in the composition of X80 pipeline steel, the weight percentage of Si is 0.18%, 0.19%, 0.20%, 0.21% or 0.22%, etc., which is not limited here.

[0029] Si is also a basic strengthening element in steel.

[0030] For example, in the composition of X80 pipeline steel, the weight percentage of Mn is 1.5%, 1.55% or 1.6%, etc., which is not limited here.

[0031] Mn improves the strength of steel through solid solution strengthening and is the most important and economical strengthening element in pipeline steel to compensate for the strength loss caused by the decrease in C content.

[0032] For example, in the composition of X80 pipeline steel, the weight percentage of P is 0, 0.003%, 0.005%, 0.007%, 0.01%, 0.011% or 0.012%, etc., which is not limited here. In the composition of X80 pipeline steel, the weight percentage of S is 0, 0.001%, 0.002% or 0.003%, etc., which is not limited here.

[0033] In the examples of the present application, the contents of P and S are both low.

[0034] For example, in the composition of X80 pipeline steel, the weight percentage of Cr is 0.15%, 0.16%, 0.17%, 0.18%, 0.19% or 0.20%, etc., which is not limited here.

[0035] Gr can improve the weather resistance and corrosion resistance of steel. High-strength thick-gauge X80 pipeline steel does not have high requirements for weather resistance and corrosion resistance, and adding 0.15% to 0.20% Cr can meet the requirements for weather resistance and corrosion resistance.

[0036] For example, in the composition of X80 pipeline steel, the weight percentage of Ni is 0.08%, 0.09%, 0.10%, 0.11% or 0.12%, etc., which is not limited here.

[0037] The increase of Ni content is beneficial to improving the low-temperature toughness of pipeline steel, but the price of Ni alloy is relatively high. Under the premise of ensuring the performance requirements of the pipe, the Ni content is reduced to 0.08% to 0.12% for the purpose of reducing metallurgical costs.

[0038] For example, in the composition of X80 pipeline steel, the weight percentage of Mo is 0.12%, 0.13%, 0.14%, 0.15% or 0.16%, etc., which is not limited here.

[0039] Mo can reduce the phase transformation temperature of supercooled austenite, inhibit the formation of polygonal ferrite, and promote the transformation of acicular ferrite.

[0040] Among them, austenite is a lamellar microstructure of steel. Austenite has good plasticity, low strength, certain toughness, and no ferromagnetism. Because austenite is face-centered cubic, the tetrahedral gap is larger and can accommodate more carbon.

[0041] Polygonal ferrite and acicular ferrite are both common microstructure forms in steel materials. Their formation conditions, morphological characteristics and effects on steel properties are unique.

[0042] Polygonal ferrite is usually formed in the non-recrystallized zone of austenite through continuous cooling or phase transformation. It has a lower dislocation density and a more relaxed substructure, so its hardness and strength are relatively low, but its plasticity and toughness are good. This ferrite morphology is very beneficial for improving the ductility and formability of steel.

[0043] Acicular ferrite is formed at a lower temperature during the transformation of austenite to ferrite. It usually presents a fine needle or lath-like morphology. The formation mechanism of acicular ferrite involves the preferential orientation growth of ferrite, which can achieve phase transformation without forming Fe3C (cementite). The presence of acicular ferrite allows the steel to have good toughness and fatigue properties while maintaining high strength.

[0044] For example, in the composition of X80 pipeline steel, the weight percentage of V is 0.035%, 0.040%, 0.045%, 0.050% or 0.055%, etc., which is not limited here.

[0045] For example, in the composition of X80 pipeline steel, the weight percentage of Nb is 0.035%, 0.040%, 0.045%, 0.050% or 0.055%, etc., which is not limited here.

[0046] For example, in the composition of X80 pipeline steel, the weight percentage of Ti is 0.012%, 0.013%, 0.014%, 0.015% or 0.016%, etc., which is not limited here.

[0047] Among them, Nb has significant grain refining effect and moderate precipitation strengthening effect, which increases strength while reducing the ductile-brittle transition temperature. However, Nb is a precious metal element and the strengthening and toughening effect is no longer obvious after adding a certain amount; Ti has significant precipitation strengthening effect and moderate grain refining effect, and can also improve the distribution morphology of sulfides to improve the lateral properties of steel, but it will deteriorate low-temperature toughness and increase the ductile-brittle transition temperature; V has higher precipitation strengthening and weaker grain refining effect, but it will deteriorate low-temperature toughness and increase the ductile-brittle transition temperature. It is generally not used alone in the alloy design of pipeline steel.

[0048] The inventors found that when the Nb content exceeds 0.06%, it is easy to cause a large number of M / A components that are not conducive to toughness to appear in the heat-affected zone of the girth weld joint. Therefore, it is necessary to appropriately reduce the addition of the Nb element. In order to compensate for the strength loss caused by the reduction of Nb, an appropriate amount of V element is added. Among them, the MA component is generally formed at a medium cooling rate and is the result of an increase in the carbon content in austenite. The presence of the MA component will cause the product to become embrittled, and the more MA components there are, the more severe the embrittlement. The MA component essentially becomes a potential crack source and has the effect of stress concentration.

[0049] Exemplarily, the value of the cold crack sensitivity coefficient Pcm is 0.15%, 0.16% or 0.17%, etc., which is not limited here.

[0050] Among them, Pcm is the chemical composition crack sensitivity coefficient of the weld. The cold crack sensitivity coefficient Pcm ranges from 0.17% to 0.19%. The alloy elements and carbon content in the steel are reasonably matched, the hardening tendency of the steel is low, and it is not easy to form a brittle and hard structure in the heat-affected zone. Under welding stress, the strength and toughness of the steel are well matched, which can relieve stress concentration. And when the weld is cooled, it is easy to form a uniform, fine and tough structure to ensure that the steel has good welding performance.

[0051] This application can ensure that the steel pipe produced by the product has good comprehensive mechanical properties such as strength and low-temperature toughness through the composite strengthening effect of alloy elements such as Ni, Cr, Mo, Nb and Ti. At the same time, through the precise control of C element and cold crack sensitivity coefficient Pcm (also known as carbon equivalent), the fine grain strengthening effect of Nb element and the optimization of low-temperature toughness by Ni element, the microstructure of the heat-affected zone of the steel pipe after girth welding can still ensure good low-temperature toughness, achieving the technical effect of high toughness of the heat-affected zone of the steel pipe after girth welding, and the product strength has a smaller fluctuation range, achieving the purpose of controlling the stability of the mechanical properties of the steel pipe.

[0052] The embodiment of the present application also provides a method for preparing X80 pipeline steel, such as Figure 1 As shown, the method includes steps: R1 to R6.

[0053] R1. Provide X80 pipeline steel as described in the above embodiment.

[0054] Exemplarily, the molten steel is subjected to converter smelting and LF / RH refining to obtain X80 pipeline steel with the composition as in the above embodiment, which is pure molten steel with low P, low S and other impurity elements, and is used as a metallurgical component to prepare X80 pipeline steel.

[0055] Among them, the LF furnace (ladle refining furnace) is mainly used for desulfurization and heating of molten steel. It is a special form of electric arc furnace. It refines by stirring white slag and argon in the barrel, and can strictly adjust the composition and temperature of molten steel. The LF furnace can cooperate with electric furnace or converter to produce high-quality alloy steel and control the composition and temperature on the continuous casting production line.

[0056] RH furnace (vacuum cycle degassing) is mainly used for dehydrogenation of molten steel, and is usually equipped with an oxygen lance for decarburization and heating. The working principle of RH furnace is to remove hydrogen and other gases in molten steel through vacuum cycle degassing, as well as to adjust the composition.

[0057] R2. The X80 pipeline steel with the above composition is continuously cast to obtain a cast ingot.

[0058] In this step, protected casting is adopted throughout the whole process, and the continuous casting process adopts low superheat and dynamic light reduction technology.

[0059] Full-process protective casting refers to an important technical measure to protect the molten steel exposed to the air during the continuous casting process to avoid secondary oxidation of the molten steel by the air.

[0060] The whole process low superheat process refers to the dynamic light reduction process with a low superheat of 15℃~20℃ above the liquidus line and a total reduction of 4mm~6mm during the continuous casting process.

[0061] The full-process dynamic light reduction process refers to an important technical measure to improve the center porosity and center segregation of the continuous casting billet by applying pressure to the billet by changing the roll gap near the solidification end of the continuous casting billet to produce a certain amount of reduction to compensate for the solidification shrinkage of the billet.

[0062] R3. Place the ingot into a heating furnace with a heating temperature range of 1170°C to 1190°C and a heating time range of 180min to 240min.

[0063] Exemplarily, the cold ingot is put into a heating furnace for reheating.

[0064] Illustratively, the heating temperature of the ingot in the heating furnace is 1170° C., 1175° C., 1180° C., 1185° C. or 1190° C., etc., which is not limited here.

[0065] Exemplarily, the heating time of the ingot in the heating furnace is 180 min, 190 min, 200 min, 210 min, 220 min, 230 min or 240 min, etc., which is not limited here.

[0066] Heating the ingot at 1170℃~1190℃ for 180min~240min can make the product structure uniform and the alloy elements fully dissolved.

[0067] R4, rough rolling, the number of rough rolling is less than or equal to 8 times.

[0068] Exemplarily, hot rolling is performed by a hot rolling mill, and the hot rolling includes rough rolling.

[0069] In rough rolling, the reduction rate of the last pass is ≥25%, and the reduction rate of the last pass is greater than or equal to 25%, for example, the reduction rate of the last pass is 25%, 26%, 27%, 28%, 29% or 30%, etc., and there is no limit here; the rolling temperature of the last pass is ≤960℃, and the rolling temperature of the last pass is less than or equal to 960℃, for example, the rolling temperature of the last pass is 960℃, 950℃, 940℃, 930℃, 920℃ or 910℃, etc., and there is no limit here.

[0070] The reduction ratio refers to the ratio of the deformation amount in the rolling pass to the thickness before deformation.

[0071] For example, in the rough rolling, the intermediate billet thickness is 3.5t. Wherein, t is the abbreviation of thickness, which refers to the final thickness of the steel plate. The thickness of the X80 steel plate will vary according to the engineering requirements. Therefore, t represents the range of the final thickness of the steel plate.

[0072] R5, finishing rolling, the number of finishing rolling is less than or equal to 9 times.

[0073] Exemplarily, hot rolling is performed by a hot rolling mill, and the hot rolling includes finish rolling.

[0074] In finishing rolling, the finishing inlet temperature is ≤940°C, that is, the finishing inlet temperature is less than or equal to 940°C, for example, the finishing inlet temperature is 940°C, 930°C, 920°C, 910°C or 900°C, etc., and there is no limit here; the finishing cumulative reduction rate is ≥55%, that is, the finishing cumulative reduction rate is greater than or equal to 55%, and the finishing cumulative reduction rate is 55%, 56%, 57%, 58%, 59% or 60%, etc., and there is no limit here.

[0075] Rough rolling is less than or equal to 8 passes and finishing rolling is less than or equal to 9 passes, which can refine the product grains and control the plate size.

[0076] Since the more rough rolling times and finishing rolling times are required, the higher the capacity requirement of the rolling mill equipment is. The embodiment of the present application has more flexible requirements on the rough rolling times and finishing rolling times, and can be applied to more rolling mill equipment.

[0077] R6, cooling, the cooling rate range is 18℃ / s~24℃ / s; the final cooling temperature range is 410℃~460℃.

[0078] Exemplarily, the cooling rate is 18°C / s, 19°C / s, 20°C / s, 21°C / s, 22°C / s, 23°C / s or 24°C / s, etc., but is not limited thereto. Preferably, during cooling, the cooling rate ranges from 18°C / s to 22°C / s.

[0079] Exemplarily, the final cooling temperature is 410°C, 420°C, 430°C, 440°C, 450°C or 460°C, etc., which is not limited here. Preferably, during cooling, the final cooling temperature ranges from 420°C to 440°C.

[0080] The cooling rate range is 18℃ / s~24℃ / s and the final cooling temperature range is 410℃~460℃, which enables the product to form a fine acicular ferrite structure with a good balance of strength and toughness.

[0081] The embodiments of the present application improve the manufacturing method from the perspective of detailed control of continuous casting, rough rolling and finish rolling, thereby achieving the effect of refining the final organizational state of the product, thereby utilizing the genetic effect so that although the organizational structure at the heat-affected zone of the girth weld has undergone a thermal cycle and changed its initial organizational state, it still inherits part of the fine-grained effect of the parent material, thereby achieving the goal of improving the low-temperature toughness of the heat-affected zone of the girth weld; in addition, the hot rolling production process provided by the present application appropriately weakens the excessively high requirements for equipment by adjusting the pass reduction rate and key point temperature parameters, which can have a favorable effect on equipment maintenance during the mass production of high-strength thick-gauge pipeline steel.

[0082] The embodiments of the present application also provide an X80 pipeline steel prepared by the method for preparing the X80 pipeline steel as described in any of the above embodiments.

[0083] Figure 2 The following is a metallographic photograph of the microstructure of the X80 pipeline steel provided according to the embodiments of the present application.

[0084] from Figure 2 It can be seen that it is characterized by a typical acicular ferrite mixed phase structure, including granular bainite, bainitic ferrite and martensite / austenite components.

[0085] Figure 3 The microstructure photograph of the heat-affected zone of the girth weld joint of X80 pipeline steel after pipe making and girth welding provided in an embodiment of the present application. Figure 3 (a) is the coarse grain structure. Figure 3 (b) is the fine grain structure. Figure 3 (c) in the figure is the critical region organization. Figure 3 (d) in the figure is the structure of the tempering zone.

[0086] The following is a test of the performance of the X80 pipeline steel provided in the examples of the present application.

[0087] The embodiments of the present application carry out converter smelting - refining outside the furnace - continuous casting - slab heating - rough rolling - finishing rolling - controlled cooling - coiling according to the metallurgical composition and hot rolling process as described in any of the above embodiments. The specific composition and parameters are shown below.

[0088] The present disclosure provides Example 1, Example 2, Example 3 and Example 4 according to the above technical solution. The metallurgical compositions of Example 1, Example 2, Example 3 and Example 4 are shown in Table 1.

[0089] Table 1 Metallurgical composition of Example 1, Example 2, Example 3 and Example 4 (wt.%)

[0090] serial number C Si Mn P S Cr Ni Mo Nb V Ti Pc Example 1 0.060 0.20 1.50 0.0086 0.0030 0.20 0.08 0.15 0.040 0.035 0.015 0.166 Example 2 0.055 0.18 1.55 0.0095 0.0018 0.15 0.10 0.16 0.035 0.05 0.014 0.163 Example 3 0.045 0.19 1.60 0.0120 0.0015 0.18 0.12 0.14 0.055 0.055 0.015 0.157 Example 4 0.050 0.22 1.57 0.0095 0.0016 0.16 0.09 0.12 0.045 0.045 0.016 0.158

[0091] Table 1 shows the weight percentage (wt.%) of each component.

[0092] The present application also provides Comparative Example 1 and Comparative Example 2. The metallurgical compositions of Comparative Example 1 and Comparative Example 2 are shown in Table 2.

[0093] Table 2 Metallurgical composition of Comparative Example 1 and Comparative Example 2 (wt.%)

[0094] serial number C Si Mn P S Cu Cr Ni Mo Nb Ti Pc Comparative Example 1 0.060 0.26 1.75 ≤0.015 0.0020 0.15 0.26 0.15 0.24 0.065 0.015 0.195 Comparative Example 2 0.058 0.22 1.72 ≤0.015 0.0025 0.20 0.28 0.22 0.28 0.068 0.016 0.198

[0095] The process parameters of Example 1, Example 2, Example 3, Example 4, Comparative Example 1 and Comparative Example 2 of the present disclosure are shown in Table 3.

[0096] Table 3 Process parameters

[0097] serial number Slab heating temperature Slab heating time Intermediate billet thickness Rough rolling and final rolling temperature Finish rolling temperature Coiling temperature Example 1 1190℃ 240min 58mm 930℃ 825℃ 445℃ Example 2 1180℃ 195min 56mm 925℃ 830℃ 420℃ Example 3 1170℃ 180min 57mm 950℃ 847℃ 460℃ Example 4 1185℃ 225min 60mm 960℃ 835℃ 455℃ Comparative Example 1 1220℃ 215min 60mm 905℃ 802℃ 340℃ Comparative Example 2 1215℃ 230min 58mm 895℃ 810℃ 360℃

[0098] The mechanical properties of the steel coils of Example 1, Example 2, Example 3, Example 4, Comparative Example 1 and Comparative Example 2 of the present disclosure are shown in Table 4. Steel coil refers to a product in the state of a steel plate, which is used to make a steel pipe.

[0099] Table 4 Mechanical properties of steel coil

[0100] serial number Rt0.5 / MPa Rm / MPa Rt0.5 / Rm A / ％ -20℃CVN / J -15℃DWTT / % Example 1 595 720 0.83 29 340 100 Example 2 605 726 0.83 27 325 98 Example 3 585 716 0.82 32 365 100 Example 4 589 724 0.81 30 345 98 Comparative Example 1 584 681 0.86 26.5 312 96 Comparative Example 2 614 702 0.87 28.0 298 100

[0101] Rt0.5 represents the yield strength, which refers to the maximum stress that a material can withstand before it begins to deform plastically. For steel, yield strength is a very important performance indicator because it determines the deformation capacity of steel when subjected to stress. The unit of yield strength is MPa (megapascal).

[0102] Rm stands for tensile strength, which refers to the maximum stress that a material can withstand during stretching. When the material reaches the tensile strength, it will break. Tensile strength is also an important indicator for measuring the strength of steel. The unit of tensile strength is MPa (megapascal).

[0103] Rt0.5 / Rm represents the yield strength ratio.

[0104] A represents: A represents the elongation after fracture, which refers to the percentage of the elongation of the gauge length after the metal material breaks under the tensile load to the original gauge length. The calculation formula is: A = (L u -L 0 ) / L 0 , where A is the elongation after fracture and L u The length of the gauge after breaking is L 0 Original gauge length. Elongation after fracture mainly reflects the degree of plastic deformation that steel can withstand before breaking. Steel with a larger elongation after fracture can produce a larger deformation without breaking during the stretching process.

[0105] -20℃CNV indicates the Charpy impact energy of the material at -20℃. Charpy impact energy refers to the energy absorbed by a specimen with a V-notch when it is broken by a pendulum impact in a Charpy impact test, and the unit is joule (J). The magnitude of the CNV Charpy impact energy reflects the toughness of the material. The larger the absorbed energy value, the better the toughness of the material, and it is insensitive to notches or other stress concentrations in the structure. It is an important indicator for evaluating the ability of metal materials to resist fracture under dynamic loading and the presence of notches, and is of great significance for material selection, quality control, and structural safety assessment. The Charpy impact energy test results should meet the design requirements.

[0106] -15℃ DWTT (Drop Weight Tear Test) is a drop weight tear test conducted at a specific temperature of -15℃. The drop weight tear test is a test item used to detect the toughness of metal materials. It is mainly used for the study of low-temperature brittleness of metal materials. By observing the metallurgical defects, fracture properties, morphology and other characteristics on the fracture of the sample, measuring the ratio of the ductile fracture area to the net cross-sectional area used to evaluate the fracture (i.e., shear area percentage, recorded as SA%) and other indicators, the metallurgical quality and anti-fracture ability of the material are comprehensively evaluated.

[0107] The mechanical properties of the steel pipes of Example 1, Example 2, Example 3, Example 4, Comparative Example 1 and Comparative Example 2 of the present application are shown in Table 5.

[0108] Table 5 Mechanical properties of steel pipe

[0109]

[0110]

[0111] From the comparison of the mechanical properties of the steel coils and the mechanical properties of the steel pipes of Examples 1 to 4 and Comparative Examples 1 and 2, it can be seen that there is little difference between the examples and the comparative examples in terms of yield strength Rt0.5 and tensile strength Rm indicators, and the strength of the example products of the present application is slightly higher than that of the comparative examples.

[0112] The mechanical properties of the heat-affected zone of the steel pipe girth weld joints of Example 1, Example 2, Example 3, Example 4, Comparative Example 1 and Comparative Example 2 of the present application are shown in Table 6.

[0113] Table 6 Mechanical properties of heat affected zone of steel pipe girth weld joint

[0114]

[0115] The CTOD (Crack Tip Opening Displacement) test is an effective method to evaluate the fracture resistance of materials and welded joints. It is used to evaluate the low-temperature toughness of the heat-affected zone of the steel pipe girth weld.

[0116] In terms of the Charpy impact energy CVN and drop weight tear area DWTT indicators of the low-temperature toughness of the base material, the embodiment is slightly better than the comparative example, especially when the composition range of the embodiment is in the preferred range provided in this application, the low-temperature toughness value is greatly improved compared with the comparative example. It can be seen from Table 6 that after the steel pipe is subjected to girth welding, the girth weld mechanical properties are compared. The low-temperature toughness value (Charpy impact energy and CTOD value) of the hot line affected zone of the girth weld joint of the embodiment product is much higher than that of the comparative example, and the Charpy impact energy value of the comparative example shows a certain volatility, while the Charpy impact energy value of the embodiment is relatively stable.

[0117] From the comparison of the above performance parameters, it can be seen that through the design of the metallurgical composition and production process of the present application, the product has excellent low-temperature toughness in the welding heat affected zone.

[0118] The above are only specific implementations of the present application, but the protection scope of the present application is not limited thereto. Any technician familiar with the technical field can easily think of changes or substitutions within the technical scope disclosed in the present application, which should be included in the protection scope of the present application. Therefore, the protection scope of the present application should be based on the protection scope of the claims.

Claims

1. An X80 pipeline steel, characterized in that: The X80 pipeline steel has the following components in percentage by weight: C: 0.045% to 0.060%, Si: 0.18% to 0.22%, Mn: 1.50% to 1.60%, P: 0 to 0.012%, S: ≤0.0030%, Cr: 0.15% to 0.20%, Ni: 0.08% to 0.12%, Mo: 0.12% to 0.16%, V: 0.035% to 0.055%, Nb: 0.035% to 0.055%, Ti: 0.012% to 0.016%, and the remainder is iron and unavoidable trace impurities; the cold crack sensitivity coefficient has a value range of 0.15% to 0.17%.

2. A method for preparing X80 pipeline steel, characterized in that: include: Providing the X80 pipeline steel as claimed in claim 1; The components of the X80 pipeline steel are continuously cast to obtain a casting ingot; The ingot is placed in a heating furnace, the heating temperature ranges from 1170° C. to 1190° C., and the heating time ranges from 180 min to 240 min; Rough rolling: the number of rough rolling passes is less than or equal to 8; Finish rolling, the number of finishing rolling is less than or equal to 9 times; Cooling, the cooling rate range is 18℃ / s~24℃ / s; the final cooling temperature range is 410℃~460℃.

3. The method for preparing X80 pipeline steel according to claim 2, characterized in that: In the continuous casting, protected casting is adopted throughout the whole process, and the continuous casting process adopts low superheat and dynamic light reduction technology.

4. The method for preparing X80 pipeline steel according to claim 2, characterized in that: In the rough rolling, the final rolling reduction ratio is ≥25%, the final rolling temperature is ≤960°C, and the intermediate billet thickness is 3.5t.

5. The method for preparing X80 pipeline steel according to claim 2, characterized in that: In the finishing rolling, the finishing rolling inlet temperature is ≤940°C, and the finishing rolling cumulative reduction ratio is ≥55%.

6. The method for preparing X80 pipeline steel according to claim 2, characterized in that: In the cooling, the cooling rate ranges from 18°C / s to 22°C / s.

7. The method for preparing X80 pipeline steel according to claim 2, characterized in that: In the cooling, the final cooling temperature ranges from 420°C to 440°C.

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