Low-cost pipeline steel for carbon dioxide conveying and preparation method thereof

Abstract

The invention discloses low-cost pipeline steel for carbon dioxide conveying and a preparation method of the low-cost pipeline steel. The steel comprises the following chemical components: 0.041% to 0.071% of C, 0.11% to 0.17% of Si, 0.89% to 0.97% of Mn, 0.0060% to 0.0099% of P, 0.31% to 0.39% of Cr, 0.15% to 0.23% of Ni, 0.08% to 0.16% of Mo, 0.25% to 0.33% of Cu, 0.058% to 0.066% of Nb, 0.010% to 0.018% of Ti, 0.022% to 0.049% of Al and the balance of iron and impurities. The preparation method comprises the steps of recrystallization region rolling and two-phase region rolling. Then water cooling is carried out; wherein the water inlet temperature is larger than or equal to Ar < 3-30 > DEG C, the final cooling temperature is Bs-(460-400) DEG C, and the cooling rate is 9-22 DEG C / s; and performing air cooling on a cooling bed after water is discharged.

Description

Technical Field

[0001] The invention belongs to the technical field of steel material preparation, and relates to low-cost pipeline steel for carbon dioxide transportation and a preparation method thereof. Background Art

[0002] Carbon Capture, Utilization and Storage (CCUS) is an important strategic technology for achieving carbon dioxide emission reduction.

[0003] Carbon dioxide exists in solid, gaseous, liquid, dense-phase, and supercritical states. Based on existing transportation experience and cost comparisons, supercritical and dense-phase pipeline transportation is the most economical and optimal option for long-distance, large-scale carbon dioxide transportation. Carbon dioxide transportation is a key step in the application of CCUS technology.

[0004] Dry, pure carbon dioxide is non-corrosive to pipelines. However, due to limitations in gas sources, capture methods, and costs, pipeline-transported carbon dioxide inevitably contains a certain amount of impurities such as H2O, O2, and SO2. Once a free water phase forms in the pipeline, carbon dioxide dissolves in water to form H2CO3, causing corrosion in the pipeline. The presence of gaseous impurities, in particular, exacerbates pipeline corrosion, requiring pipeline steel plates to possess excellent corrosion resistance.

[0005] However, some existing carbon dioxide transmission pipeline steels generally use a high-chromium alloy component system in their chemical composition. For example, patented technologies such as CN107502823A, CN105132822A, CN104862607A and CN103334055 have Cr contents above 1.5%, and even up to 7.0%. The high alloy content increases production costs. Summary of the Invention

[0006] The object of the present invention is to provide a low-cost pipeline steel for transporting carbon dioxide and a preparation method thereof.

[0007] To achieve the above-mentioned objectives, one embodiment of the present invention provides a low-cost pipeline steel for transporting carbon dioxide. The chemical composition of the pipeline steel, in percentage by mass, includes: C 0.041-0.071%, Si 0.11-0.17%, Mn 0.89-0.97%, P 0.0060-0.0099%, Cr 0.31-0.39%, Ni 0.15-0.23%, Mo 0.08-0.16%, Cu 0.25-0.33%, Nb 0.058-0.066%, Ti 0.010-0.018%, Al 0.022-0.049%, with the remainder being iron and unavoidable impurities.

[0008] Preferably, the impurities in the pipeline steel include: S 0.0009-0.0015%, O 0.0011-0.0025%, N 0.0025-0.0051%, and H 0.00005-0.00018%.

[0009] Preferably, the chemical composition of the pipeline steel further satisfies any one or more of the following in mass percentage: 13≤Mn / C≤36, CEV=0.294-0.380, and Pcm=0.125-0.176; Where, CEV=[C]+[Mn] / 6+([Cr]+[Mo]) / 5+([Cu]+[Ni]) / 15; Pcm=[C]+[Si] / 30+([Mn]+[Cu]+[Cr]) / 20+[Ni] / 60+[Mo] / 15.

[0010] Preferably, the pipeline steel has a duplex structure of quasi-polygonal ferrite + acicular ferrite + bainite, and the sum of the volume proportions of the quasi-polygonal ferrite structure and the acicular ferrite structure is greater than 70%.

[0011] Preferably, the average grain size of the quasi-polygonal ferrite and the acicular ferrite is 5 to 10 μm; The volume proportion of quasi-polygonal ferrite structure is 55~65%, the volume proportion of acicular ferrite structure is 15~30%, and the volume proportion of bainite structure is 15~20%.

[0012] Preferably, the yield strength R of the pipeline steel t0.5 ≥420MPa, tensile strength R m ≥560MPa, elongation A 50mm ≥32%, yield strength ratio R t0.5 / R m ≤0.72.

[0013] Preferably, the pipeline steel has an impact energy KV2 of ≥400J at -20°C, an impact energy KV2 of ≥380J at -40°C, an impact energy KV2 of ≥320J at -60°C, an impact energy KV2 of ≥250J at -80°C, and a ductile-brittle transition temperature T t50%US ≤-100℃, hardness ≤200HV 10 , -10℃ DWTT drop hammer shear area fraction 100%, -20℃ DWTT drop hammer shear area fraction ≥ 90%, -30℃ DWTT drop hammer shear area fraction ≥ 80%, -20℃ CTOD crack tip opening displacement ≥ 1.5mm.

[0014] Preferably, under the conditions of 20° C., carbon dioxide pressure of 1.0 MPa, and liquid phase flow rate of 1 m / s, the uniform corrosion rate of the pipeline steel is ≤0.10 mm / year.

[0015] Preferably, the number of inclusions with a size of ≥10 μm on the cross section of the pipeline steel is ≤10 / cm 2 .

[0016] To achieve the above-mentioned purpose, one embodiment of the present invention provides a method for preparing the pipeline steel. The preparation method comprises: A continuous casting billet is prepared by smelting and continuous casting; the number of inclusions with a size of ≥10 μm on the cross section of the continuous casting billet is ≤10 / cm 2 ; The continuous casting billet is sent into the heating furnace for heating; wherein the temperature of the soaking section is T NbC +(40~100)℃, soaking period is 30~50min; The continuous casting slab is rolled into steel plate through recrystallization zone rolling and two-phase zone rolling; wherein the starting rolling temperature T nr +(5~25)℃, final rolling temperature T nr +(0~20)℃, the thickness of the rolled plate is (3.2~4.2)t; the starting and finishing temperatures of the two-phase rolling are both Ar3-20℃~Ar3, and the thickness of the obtained steel plate is t≥38mm; After the steel plate leaves the rolling mill, it enters the ultra-fast cooling system for water cooling; the water inlet temperature ≥A r3 -30℃, final cooling temperature is B s -(460~400)℃, cooling rate is 9~22℃ / s; After the steel plate comes out of the water, it is air-cooled on a cooling bed until it cools to room temperature.

[0017] Preferably, the step of "sending the continuous casting billet into a heating furnace for heating" includes a preheating section, a first heating section, a second heating section and a soaking section, the temperature of the preheating section is ≤950°C, the temperature of the first heating section is 1050±15°C, and the temperature of the second heating section is 1150±15°C.

[0018] Preferably, when rolling in the recrystallization zone, the reduction of the initial rolling pass of the non-widening pass is ≥42 mm, and the minimum reduction of each pass is ≥31 mm; When rolling in the two-phase region, the reduction in the last pass is ≤8mm, and the reduction in the remaining passes is ≥22mm.

[0019] Preferably, after the step of "preparing continuous casting billets by smelting and continuous casting", the continuous casting billets are directly stacked after leaving the continuous casting machine, with the stacking temperature being ≥500°C and the destacking temperature being 230-280°C.

[0020] Preferably, the step of "preparing continuous casting billets by smelting and continuous casting" includes, KR desulfurization: outlet molten iron temperature ≥ 1300℃, S ≤ 0.0020%; Converter smelting: slag basicity is controlled at 1.9~2.1, total iron in slag is 13~16%; final slag basicity is controlled at 3.6~4.2, total iron in final slag is controlled at 14~17%, temperature is controlled at 1645~1665℃, P≤0.006%; LF refining: Deoxidation is first carried out using a slag surface deoxidizer, followed by strong stirring with argon blowing from the ladle bottom, followed by alloying, and finally power-on to control the tapping temperature at 1624±5°C. During strong stirring, the argon flow rate is 650-850 NL / min and the duration does not exceed 6 minutes. During alloying, the bottom blowing argon flow rate is 620-820 NL / min and the duration does not exceed 4 minutes. At other times, the bottom blowing argon flow rate does not exceed 450 NL / min. During the power-on period, calcium carbide is added for diffusion deoxidation, and white slag is produced, and the white slag retention time is greater than 15 minutes. RH vacuum refining: No oxygen blowing is done during the whole process. Under the condition of vacuum degree ≤2.5mbar, degassing takes 18~23min, net circulation takes 7~9min, the steel ladle is left on the turntable for 12~16min, and the tapping temperature is regulated to make the temperature of the open-casting furnace 1587±5℃ and the temperature of the continuous casting furnace 1577±5℃; Continuous casting: The superheat is controlled at 26~32℃, and secondary cooling electromagnetic stirring and dynamic soft reduction are used. The reduction is 4~7% of the thickness of the continuous casting billet. The water volume of the crystallizer of the continuous casting machine is 420~550L / min, the water inlet temperature is 25~40℃, and the water outlet temperature difference is 4~10℃.

[0021] Compared with the prior art, the present invention has the following beneficial effects: (1) In terms of chemical composition, a low-alloy component system with low carbon, low manganese, low sulfur, high niobium and Cr + Ni + Mo + Cu + P corrosion-resistant elements is designed. On the one hand, the C content and Cr content in the steel are reduced, and carbide-forming elements Nb and Ti are added in appropriate amounts to reduce the carburization of Cr, so that Cr exists in the steel in the form of a solid solution, giving full play to the corrosion resistance of Cr; on the other hand, Cr is added in combination with Ni, Mo, Cu, and P to further improve the corrosion resistance, so that the pipeline steel has excellent corrosion resistance in the use of carbon dioxide transportation; on the other hand, it does not contain expensive alloys such as V, and the addition content of alloys such as Cr, Ni, and Mo is low. P exists as a corrosion-resistant element without the need for ultra-low content control, which is easy to steel and has low production costs. Therefore, compared with the existing technology, in terms of chemical composition, it can not only ensure the corrosion resistance of pipeline steel, but also greatly reduce production costs; (2) On the basis of the low alloy composition system, the special controlled rolling and cooling process of "continuous casting billet heating → recrystallization zone + two-phase zone special controlled rolling → water cooling + air cooling self-tempering special controlled cooling" is combined. Through the special controlled rolling in the two-phase critical zone, the type and proportion of the finished product structure are adjusted and controlled to obtain a complex phase structure, thereby achieving improvements in the strength, yield strength ratio, hardness, low temperature toughness and corrosion resistance of the steel plate. BRIEF DESCRIPTION OF THE DRAWINGS

[0022] Figure 1 This is a metallographic structure diagram of the finished steel plate of Example 1 of the present invention. DETAILED DESCRIPTION

[0023] To make the objectives, technical solutions, and advantages of the present invention more clear, the technical solutions of the present invention will be clearly and completely described below in conjunction with the specific embodiments of the present invention. Obviously, the embodiments described are only part of the embodiments of the present invention, not all of the embodiments. Based on the embodiments of the present invention, all other embodiments obtained by ordinary technicians in this field without making creative efforts are within the scope of protection of the present invention.

[0024] One embodiment of the present invention provides a steel, which may be a pipeline steel suitable for transporting carbon dioxide, and more specifically suitable for transporting supercritical or dense phase carbon dioxide.

[0025] The chemical composition of the pipeline steel includes, by mass percentage, C 0.041-0.071%, Si 0.11-0.17%, Mn 0.89-0.97%, P 0.0060-0.0099%, Cr 0.31-0.39%, Ni 0.15-0.23%, Mo 0.08-0.16%, Cu 0.25-0.33%, Nb 0.058-0.066%, Ti 0.010-0.018%, Al 0.022-0.049%, and the remainder is iron and unavoidable impurities.

[0026] The following is a detailed analysis of the main functions of each element and the selection of its dosage.

[0027] C: Carbon is the most economical strengthening element in steel, providing solid solution strengthening and forming carbides with niobium, titanium, chromium, molybdenum, and other elements, which contribute to precipitation strengthening. Increasing carbon content significantly improves the strength and hardness of pipeline steel, but excessive carbon content can lead to poor low-temperature toughness and weldability, reducing the low-temperature drop hammer performance of pipeline steel. Reducing carbon content promotes homogenization of composition and structure, increases the transformation temperature of acicular ferrite, expands the cooling rate range for acicular ferrite transformation, facilitates the formation of acicular ferrite, and minimizes the difference in electrode potential between microstructures, enhancing corrosion resistance. In one embodiment, the carbon content is controlled to 0.041-0.071%. Preferably, it is controlled to 0.060-0.071%, and more preferably, it is controlled to 0.064-0.071%.

[0028] Si: Silicon acts as a solid solution strengthener in steel. When used in combination with copper, it improves corrosion resistance. However, high silicon content can increase grain boundary segregation of elements like phosphorus and sulfur, reducing low-temperature toughness and weldability. Excessive silicon can also easily form Fe2SiO4 on the surface of the continuous casting ingot, compromising surface quality control of the steel plate. In one embodiment, the silicon content is controlled to 0.11-0.17%.

[0029] Mn: Manganese plays a role in solid solution strengthening in steel, improving strength and hardness. A reasonable manganese content can ensure the strength of pipeline steel at a low cost. As the manganese content increases, the strength of pipeline steel increases significantly, while the ductile-brittle transition temperature hardly changes. Too much manganese will cause segregation in the center of the ingot, which is detrimental to toughness. At the same time, it will increase the level of banded structure. The higher the level of banded structure, the more uneven the tissue distribution, and the more prone to local corrosion. In one embodiment, considering corrosion performance, low-temperature toughness, and strength, the manganese content is controlled to 0.89~0.97%. It is preferably controlled at 0.89~0.95%, and more preferably at 0.89~0.91%.

[0030] Phosphorus (P) is used in this application to improve corrosion resistance. Excessive phosphorus levels can lead to segregation and significantly reduce the steel's low-temperature toughness. Considering both corrosion resistance and low-temperature toughness, the phosphorus content should be controlled between 0.0060% and 0.0099%.

[0031] Cr: Chromium acts as a solid solution strengthener in steel and is also the most effective element in steel for resisting carbon dioxide corrosion. As a ferrite-forming element, chromium produces a more acicular ferrite structure in high-niobium steel. As the chromium content increases, carbon dioxide corrosion resistance also significantly improves. However, excessive chromium content increases the microhardness of pipeline steel and reduces low-temperature toughness. In one embodiment, the chromium content is controlled to 0.31-0.39%. Preferably, it is controlled to 0.31-0.37%.

[0032] Nickel: Nickel acts as a solid solution strengthener in steel, increasing its strength without significantly increasing its hardness. It also improves the steel's low-temperature toughness and weldability, shifts the steel's natural corrosion potential toward the positive side, promotes the formation of the α-FeOOH phase in the rust layer, and enhances its stability. However, excessive nickel content increases alloy cost. Therefore, considering corrosion resistance, low-temperature toughness, weldability, and alloy cost, the nickel content is controlled to 0.15-0.23%. A range of 0.18-0.23% is preferred, with a more preferred range of 0.20-0.23%.

[0033] Mo: Molybdenum can significantly improve the hardenability of steel, and increase its strength and toughness. It can also refine grains and improve corrosion resistance. Molybdenum is a medium-strong carbide-forming element that can combine with carbon to allow more chromium to exist in the steel in the form of a solid solution, maximizing the corrosion resistance of chromium and improving the utilization efficiency of matrix alloy elements. Molybdenum can delay ferrite transformation and obtain acicular ferrite structure, which is beneficial to improving the strength and toughness of pipeline steel. However, when the molybdenum content is too high, the alloy cost increases significantly. Therefore, considering the corrosion performance, strength, low-temperature toughness, and alloy cost, the molybdenum content is controlled to 0.08~0.16%. It is preferably controlled to 0.08~0.13%, and more preferably to 0.08~0.11%.

[0034] Cu: Copper promotes niobium precipitation and compensates for the strength loss caused by reduced carbon content. Adding a certain amount of nickel simultaneously with copper can effectively suppress surface cracking. Copper complements the corrosion resistance of chromium, hindering the crystallization of the rust layer, promoting the formation of α-FeOOH and amorphous Fe₃O₄, and improving pitting resistance. However, high copper content can negatively impact weldability. In one embodiment, the copper content is controlled to 0.25-0.33%. Preferably, it is controlled to 0.28-0.33%.

[0035] Nb: Niobium is a key grain-refining element in steel. During hot rolling, niobium strongly inhibits austenite recrystallization and precipitation within the austenite, pinning austenite grain boundaries and refining recrystallized grains. During cooling, dissolved niobium can continue to precipitate as niobium carbonitrides, significantly refining the resulting microstructure after phase transformation, further improving the steel's strength and toughness. Niobium is a strong carbide-forming element, preventing carbon from combining with chromium, which can affect corrosion resistance. However, high niobium levels increase alloy cost and degrade the low-temperature toughness of the heat-affected zone (HAZ) of welded joints. Therefore, based on comprehensive considerations, the niobium content is controlled within a range of 0.058-0.066%. A range of 0.058-0.062% is preferred, with a more preferred range of 0.058-0.061%.

[0036] Ti: Titanium is a nitrogen-binding element in steel. It forms dispersed titanium nitride particles, which inhibit austenite grain coarsening during billet heating and rolling. However, excessive additions can easily lead to the formation of coarse carbon / nitride precipitation in the core of the ingot, affecting the low-temperature toughness of the steel plate. Therefore, the titanium content is controlled to 0.010-0.018%, preferably 0.010-0.014%, and more preferably 0.010-0.012%.

[0037] Al: Aluminum is a deoxidizing element in steel. Excessive aluminum can easily increase Al2O3 inclusions in the steel, affecting the steel's low-temperature toughness. While ensuring the deoxidation effect, the aluminum content should be minimized. Therefore, the aluminum content is controlled at 0.022-0.049%.

[0038] Thus, in terms of chemical composition, the present invention adopts a low-alloy component system design with low carbon, low manganese, low sulfur, high niobium and a composite addition of corrosion-resistant elements Cr + Ni + Mo + Cu + P. On the one hand, the C content and Cr content in the steel are reduced, and carbide-forming elements Nb and Ti are added in appropriate amounts to reduce the carburization of Cr, so that Cr exists in the steel in the form of a solid solution, and the corrosion-resistant effect of Cr is fully exerted; on the other hand, Cr is added in combination with Ni, Mo, Cu, and P to further improve the corrosion resistance, thereby making the pipeline steel have excellent corrosion resistance in carbon dioxide transportation applications; on the other hand, it does not contain expensive alloys such as V, and the content of alloy additions such as Cr, Ni, and Mo is relatively low. P exists as a corrosion-resistant element without the need for ultra-low content control, which facilitates steelmaking production and has low production costs.

[0039] Therefore, compared with the existing technology, in terms of chemical composition, it can not only ensure the corrosion resistance of pipeline steel, but also greatly reduce production costs.

[0040] Preferably, the contents of some impurity elements in the chemical composition of the pipeline steel are described as follows.

[0041] Sulfur (S): Sulfur is an impurity element in steel that not only increases hot brittleness but also easily forms MnS inclusions with manganese, reducing the steel's low-temperature toughness. In one embodiment, the S content is controlled below 0.0015%. Considering that excessively low S content significantly increases steelmaking costs, the S content is preferably controlled between 0.0009% and 0.0015%.

[0042] Oxygen: Oxygen is an impurity element in steel, easily forming various oxide inclusions. In one embodiment, the O content is controlled below 0.0025%. Excessively low oxygen levels increase steelmaking costs, so the oxygen content is preferably controlled between 0.0011% and 0.0025%.

[0043] Nitrogen: Nitrogen is an impurity element in steel. It easily forms brittle, angular inclusions with Ti, Al, and other elements, reducing the ductility and toughness of the steel plate. In one embodiment, the nitrogen content is controlled below 0.0055%. Excessively low nitrogen content increases steelmaking costs. Therefore, the nitrogen content is preferably controlled between 0.0025% and 0.0051%.

[0044] Hydrogen: Hydrogen is an impurity element in steel that can cause hydrogen embrittlement, leading to cracking in continuous-cast slabs and steel plates. In one embodiment, hydrogen is controlled below 0.00020%. Excessively low hydrogen levels increase steelmaking costs, so the hydrogen content is preferably controlled between 0.00005% and 0.00018%.

[0045] Furthermore, in one embodiment, the chemical composition of the pipeline steel, in terms of mass percentage, further satisfies: 13≤Mn / C≤36.

[0046] That is, while keeping the carbon content low, the manganese-carbon ratio is also controlled within the range of 13 to 36, which can greatly reduce segregation and improve the low-temperature toughness of pipeline steel.

[0047] Furthermore, in one embodiment, the chemical composition of the pipeline steel, in terms of mass percentage, further satisfies: CEV=0.294~0.380.

[0048] Preferably, CEV=0.294~0.330; more preferably, CEV=0.294~0.303.

[0049] Furthermore, in one embodiment, the chemical composition of the pipeline steel, in terms of mass percentage, further satisfies: Pcm=0.125~0.176.

[0050] Preferably, Pcm=0.148~0.176; more preferably, Pcm=0.172~0.176.

[0051] CEV=[C]+[Mn] / 6+([Cr]+[Mo]) / 5+([Cu]+[Ni]) / 15.

[0052] Pcm=[C]+[Si] / 30+([Mn]+[Cu]+[Cr]) / 20+[Ni] / 60+[Mo] / 15.

[0053] It should be noted that in the various formulas in this article, [C], [Si], [Mn], [Cr], [Mo], [Cu], [Ni], [Nb], and [Ti] refer to the mass percentages of C, Si, Mn, Cr, Mo, Cu, Ni, Nb, and Ti in the steel, respectively. For example, if the mass percentage of C in a continuous casting billet is 0.050%, then the mass percentage of C [C] is 0.050.

[0054] Furthermore, the microstructure of the pipeline steel is a duplex microstructure of quasi-polygonal ferrite+acicular ferrite+bainite.

[0055] Among them, the total volume share of quasi-polygonal ferrite structure and acicular ferrite structure is more than 70%.

[0056] This complex phase structure can ensure good matching of various mechanical properties of the pipeline steel, such as mechanical strength, low-temperature toughness, yield strength ratio, hardness, and drop weight performance.

[0057] Specifically, the volume proportion of the quasi-polygonal ferrite structure is 55-65%, the volume proportion of the acicular ferrite structure is 15-30%, and the volume proportion of the bainite structure is 15-20%.

[0058] Furthermore, the average grain size of the quasi-polygonal ferrite and the acicular ferrite is 5-10 μm, thereby ensuring excellent low-temperature toughness of the pipeline steel. Furthermore, the pipeline steel has excellent mechanical properties. For example, the yield strength R t0.5 ≥420MPa, tensile strength R m ≥560MPa, elongation A 50mm ≥32%.

[0059] For another example, the yield strength ratio R of the pipeline steel t0.5 / R m ≤0.72.

[0060] More specifically, the yield strength R t0.5 It is 420~500MPa.

[0061] Tensile strength R m It is 560~650MPa.

[0062] Elongation A 50mm It is 32~60%.

[0063] Yield strength ratio R t0.5 / R m It is 0.65~0.72.

[0064] Here, the steel plate can be sampled and mechanical properties tested in accordance with GB / T 2975-2018 "Steel and Steel Products - Sampling Location and Specimen Preparation for Mechanical Properties Tests" and GB / T 228.1-2021 "Metallic Materials - Tensile Tests - Part 1: Room Temperature Test Methods".

[0065] The steel plate has excellent low-temperature properties.

[0066] For example, the pipeline steel has an impact energy KV2 of ≥400J at -20°C.

[0067] The pipeline steel has an impact energy KV2 of ≥380J at -40°C.

[0068] The pipeline steel has an impact energy KV2 of ≥320J at -60°C.

[0069] The pipeline steel has an impact energy KV2 of ≥250J at -80°C.

[0070] The ductile-brittle transition temperature T t50%US ≤-100℃.

[0071] The -10°C DWTT drop weight shear area fraction of the pipeline steel is 100%.

[0072] The pipeline steel has a -20°C DWTT drop weight shear area fraction of ≥90%.

[0073] The pipeline steel has a -30°C DWTT drop weight shear area fraction of ≥80%.

[0074] The -20°C CTOD crack tip opening displacement of the pipeline steel is ≥1.5 mm.

[0075] Here, the steel plate can be sampled and subjected to low-temperature performance testing in accordance with GB / T 2975-2018 "Steel and Steel Products - Sampling Location and Specimen Preparation for Mechanical Properties Tests" and GB / T 229-2020 "Charpy Pendulum Impact Test Method for Metallic Materials".

[0076] In addition, the hardness of the pipeline steel is ≤200HV 10 .

[0077] For example, the hardness is 170~200HV 10 Furthermore, the central segregation of the pipeline steel is ≤ level 0.5.

[0078] Here, the central segregation can be specifically determined in accordance with GB / T 34474-2017 "Methods for evaluating the microstructure of steel", by using the sulfur print method or electron probe to detect the degree of element segregation in the central area of the ingot.

[0079] Furthermore, the central porosity of the pipeline steel is ≤ level 0.5.

[0080] The central porosity level can be determined by a macro-acid immersion test in accordance with GB / T 226-2015 "Macrostructure and Defect Acid Etching Test of Steel", or by ultrasonic testing.

[0081] In this way, by controlling the center segregation and center porosity, the organizational structure at the center is made continuous, the impact toughness, fracture toughness, drop hammer performance, etc. of the steel plate are improved, and cracks are avoided.

[0082] Furthermore, the inclusion ratings of categories A, B, C, and D of the pipeline steel are all ≤ level 1, and the sum of the ratings of categories A, B, C, and D inclusions A+B+C+D ≤ level 3.0.

[0083] Here, the rating of A, B, C, and D inclusions can be specifically based on GB / T 10561-2005 "Microscopic inspection method for the determination of the content of non-metallic inclusions in steel - standard rating chart", and the rating can be compared with the standard chart under a microscope.

[0084] Furthermore, the number of inclusions with a size of ≥10 μm on the cross section of the pipeline steel is ≤10 / cm 2 .

[0085] Research has shown that inclusions are the most sensitive locations for pitting and crack initiation, and large inclusions with an equivalent circular diameter exceeding 20 μm have a significant impact on corrosion resistance and low-temperature toughness. The pipeline steel of this application has fewer large inclusions, which can improve corrosion resistance and low-temperature toughness.

[0086] Furthermore, the pipeline steel has excellent corrosion resistance, especially excellent corrosion resistance in a carbon dioxide environment.

[0087] For example, under the conditions of 20° C., carbon dioxide pressure of 1.0 MPa, and liquid phase flow rate of 1 m / s, the uniform corrosion rate of the pipeline steel is ≤0.10 mm / year.

[0088] The characteristics of the carbon dioxide used in the corrosion resistance test can be based on the standard GB / T 6052-2011 "Industrial Liquid Carbon Dioxide." For example, the carbon dioxide used in the test is in gaseous form, with a purity (volume fraction) of ≥99.5% and other impurities (water, oxygen, sulfur dioxide, nitrogen, carbon monoxide, etc.) ≤0.5%.

[0089] Furthermore, the pipeline steel may be specifically configured as a steel plate, for example, a steel plate with a thickness of t≥38 mm.

[0090] Specifically, the thickness t may be 38~50mm.

[0091] Next, one embodiment of the present invention further provides a method for preparing the pipeline steel.

[0092] The preparation method includes a process of smelting, continuous casting, heating, hot rolling and cooling. Specifically: Through smelting and continuous casting, continuous casting billets are prepared; the thickness of the continuous casting billets can be 300~320mm; The continuous casting billet is fed into a heating furnace for heating; The continuous casting slab is rolled into a steel plate with a thickness of t≥38mm through recrystallization zone rolling and two-phase zone rolling; After leaving the rolling mill, the steel plate enters the ultra-fast cooling system for water cooling; After the steel plate comes out of the water, it is air-cooled on a cooling bed until it cools to room temperature.

[0093] The number of inclusions with a size of ≥10 μm on the cross section of the prepared continuous casting billet is ≤10 / cm 2 .

[0094] When the continuous casting billet is heated, the soaking section temperature is T NbC +(40~100)℃, soaking period is 30~50min.

[0095] In this way, by controlling the temperature and duration of the soaking section, the alloy elements are effectively dissolved, ensuring that the Nb precipitates in the steel are completely dissolved and the austenite grains do not grow excessively, preparing for precipitation during the subsequent recrystallization zone rolling process.

[0096] T NbC is the precipitation start temperature of NbC, which can be analyzed by thermodynamic calculation software (such as Thermo-Calc) or laboratory measurement (such as thermal simulation test) combined with the chemical composition of the specific continuous casting billet.

[0097] For example, T NbC The formula lg([Nb]×[C] 0.875 )=3.11-7520 / (T NbC +273.15) is calculated, but T NbC The value of is not limited to this.

[0098] When rolling in the recrystallization zone, the starting rolling temperature T nr +(5~25)℃, final rolling temperature T nr +(0~20)℃, plate thickness after rolling (3.2~4.2)t.

[0099] In this way, rolling in a higher temperature range can reduce the rolling deformation resistance, which is conducive to increasing the rolling reduction, and is conducive to deformation penetration into the core of the billet, improving defects such as core segregation and looseness, while reducing banded structure; furthermore, the billet is rolled in the recrystallization zone to avoid mixed crystals, and at the same time, the precipitation of Nb compounds during the rolling process can prevent the growth of recrystallized grains and refine the recrystallized grains.

[0100] T nr It indicates the lowest temperature of austenite recrystallization, which can be obtained through various methods such as theoretical calculation, experimental measurement and empirical formula.

[0101] For example, in one embodiment, T can be calculated according to the following formula: nr :T nr=887+464[C]+(6445[Nb]-644 )+890[Ti]+363[Al]–357[Si]. But T nr The value of is not limited to this. nr Other value-taking methods that can be known based on the concept can all be applied to the present invention.

[0102] When rolling in the two-phase region, the starting rolling temperature and the finishing rolling temperature are both Ar3-20℃~Ar3, and the thickness of the obtained steel plate is t≥38mm.

[0103] In this way, on the one hand, a large amount of deformation is carried out in the non-recrystallization zone to obtain a deformed structure, so that a large number of deformation bands are accumulated in the structure, and a fine structure is obtained in the subsequent cooling process, thereby improving the low-temperature toughness of the steel plate; on the other hand, the steel plate is always rolled at the two-phase critical point, that is, it is always rolled and deformed within a range of 30°C near the starting point of the two-phase phase transformation, which can ensure that the deformation-induced ferrite content is not too much, resulting in a low yield strength, and can also ensure that the deformation-induced ferrite content is not too little, resulting in an ultra-high yield strength ratio, so that the deformation-induced ferrite content is within an optimal range; on the other hand, if the final rolling temperature is too high, the subsequent cooling temperature drop is large, resulting in high yield and tensile strength and low toughness, while if the final rolling temperature is too low, the subsequent cooling temperature drop is small, and the deformation-induced ferrite content is high, resulting in low yield and tensile strength, and the final rolling temperature control of the present application ensures low yield strength, high tensile strength, and high toughness.

[0104] Ar3 is the temperature at which austenite (γ-Fe) begins to precipitate ferrite (α-Fe) during cooling. It can be obtained by testing and measuring using a differential scanning calorimeter (DSC) or thermogravimetric analysis (TG), or by an empirical formula.

[0105] For example, in one embodiment, Ar3 can be calculated according to the following formula: Ar3 = 910-203[C] + 44.7[Si] + 30[Mn] + 70[Cr]. However, the value of Ar3 is not limited to this. Other values known to those skilled in the art based on the concept of Ar3 can be applied to the present invention.

[0106] When the ultra-fast cooling system is water-cooled, the water inlet temperature ≥A r3 -30℃, final cooling temperature is B s -(460~400)℃, cooling rate is 9~22℃ / s.

[0107] Thus, during the rolling process, deformation-induced ferrite phase transformation has occurred in the steel plate, and a large number of fine deformation-induced ferrites have been produced. The production of ferrite phase ensures that the steel plate has a lower yield strength. In order to obtain higher tensile strength and lower yield ratio, the supercooled austenite phase in the steel plate needs to be rapidly cooled to transform it into bainite. If the cooling rate is too fast or too slow, it will form martensite phase or pearlite phase. If the final cooling temperature is too high, the bainite phase transformation is incomplete. If the final cooling temperature is too low, the large temperature drop is not conducive to plate shape control.

[0108] However, the present application can obtain higher tensile strength and lower yield ratio, as well as excellent plate shape by controlling the cooling rate and final cooling temperature.

[0109] B s It is the temperature at which the bainite phase transformation begins, which can be measured from the continuous cooling transformation curve of supercooled austenite, namely the CCT curve, or obtained through an empirical formula.

[0110] For example, B s =830-270[C]-90[Mn]-37[Ni]-70[Cr]-83[Mo]. But B s The value of is not limited to this. s Other value-taking methods that can be known based on the concept can all be applied to the present invention.

[0111] After water cooling, it is air-cooled on the cooling bed. In this way, the steel plate will undergo air-cooling and self-tempering on the cooling bed. During the process, the internal stress will be eliminated or weakened. On the other hand, during the phase change process, the pipeline steel plate will produce MA hard phase, which is not conducive to the toughness of the steel plate and easily leads to high local hardness of the steel plate. Air-cooling and self-tempering cooling can decompose MA and improve toughness and local hard points.

[0112] In summary, the preparation method of one embodiment of the present invention, based on the low alloy composition system, combines the special controlled rolling and cooling process of "continuous casting billet heating → recrystallization zone + two-phase zone special controlled rolling → water cooling + air cooling and self-tempering special controlled cooling". Through the special controlled rolling of the two-phase critical zone, the type and proportion of the finished product structure are adjusted and controlled to obtain a complex phase structure, thereby achieving improvements in the strength, yield strength ratio, hardness, low-temperature toughness and corrosion resistance of the steel plate.

[0113] Preferably, during rolling in the two-phase region, the starting rolling temperature and the finishing rolling temperature can be further controlled to be Ar3-20°C to Ar3-10°C, more preferably Ar3-20°C to Ar3-15°C.

[0114] Furthermore, in one embodiment, when the continuous casting billet is heated in a heating furnace, it includes a preheating section, a first heating section, a second heating section and a soaking section, the temperature of the preheating section is ≤950°C, the temperature of the first heating section is 1050±15°C, and the temperature of the second heating section is 1150±15°C.

[0115] In this way, on the one hand, the solid solution effect of the alloy elements can be improved, and on the other hand, the billet can be heated evenly in the heating furnace.

[0116] Preferably, the total furnace time of the continuous casting billet in the heating furnace is 1.15-1.35 min / mm, that is, according to the thickness of the continuous casting billet, the total furnace time is 1.15-1.35 min per millimeter of thickness.

[0117] Furthermore, during rolling in the recrystallization zone, the reduction in the initial rolling pass of the non-widening pass is ≥42 mm, and the minimum reduction in each pass is ≥31 mm.

[0118] In this way, when the total reduction in the recrystallization zone is large, a large reduction is also used in each pass. In the low-temperature section of the recrystallization zone, large reduction rolling is used to fully break up the continuous casting structure and obtain refined recrystallized grains. The refinement effect on the austenite grains is obvious, and because the temperature is low, the grains will not grow excessively.

[0119] When rolling in the two-phase region, the reduction in the last pass is ≤8mm, and the reduction in the remaining passes is ≥22mm.

[0120] In this way, except for the last pass, each pass uses a large reduction to obtain a deformed structure, so that a large number of deformation bands accumulate in the structure, and a fine structure is obtained in the subsequent cooling process, thereby improving the low-temperature toughness of the steel plate; and the last pass uses a small reduction, which can improve the shape of the thick plate.

[0121] Preferably, the rolling temperature of each pass of the two-phase region rolling is controlled at Ar3-20°C~Ar3.

[0122] The rolling temperature of each pass can be further controlled to be Ar3-20℃~Ar3-10℃, more preferably Ar3-20℃~Ar3-15℃.

[0123] In one embodiment, the steel plate is water-cooled between two adjacent passes of two-phase rolling, which can achieve more precise control of temperature and thus ensure control of microstructure and performance.

[0124] More preferably, after rolling in the recrystallization zone and before rolling in the two-phase zone, the intermediate billet is water-cooled to cool the temperature of the intermediate billet to Ar3-20°C~Ar3.

[0125] Water cooling can further control the temperature of the intermediate billet to Ar3-20℃~Ar3-10℃, and more preferably to Ar3-20℃~Ar3-15℃.

[0126] In this way, after rolling in the recrystallization zone, fine recrystallized grains are obtained, which are quickly cooled to below the recrystallization temperature through a water-cooling cooling device. On the one hand, this prevents the recrystallized grains from growing rapidly during the waiting process, thereby causing the final low-temperature toughness of the steel plate to deteriorate; on the other hand, it reduces the waiting time and improves rolling efficiency.

[0127] When water cooling is performed on the ultra-fast cooling system, the water pressure can be controlled to be 0.20-0.50 MPa, the upper and lower water ratio can be controlled to be 0.91-0.95, and the roller speed can be controlled to be 0.6-1.0 m / s. However, this application is not limited thereto.

[0128] In one embodiment, after the steel plate exits the ultra-fast cooling system, that is, after exiting the water, and before being placed on the cooling bed, it can be hot straightened to further improve the plate shape.

[0129] Preferably, the continuous casting billets prepared by smelting and continuous casting are directly stacked after leaving the continuous casting machine, with the stacking temperature being ≥500°C and the destacking temperature being 230-280°C.

[0130] In this way, on the one hand, the cooling rate of the billet is reduced, the stress difference between the core and the surface is reduced, and the generation of microcracks is prevented; on the other hand, stack cooling within this temperature range can effectively promote the diffusion and escape of hydrogen.

[0131] In one embodiment, the step of "preparing a continuous casting billet by smelting and continuous casting" may specifically include the processes of KR desulfurization, converter smelting, LF refining, RH vacuum refining and continuous casting.

[0132] Among them, in the KR desulfurization process, the outlet molten iron temperature is ≥1300℃ and S≤0.0020%.

[0133] In one embodiment, the temperature of the molten iron arriving at the KR station is ≥1390° C., P ≤ 0.12%, S ≤ 0.040%, 0.3% ≤ Si ≤ 0.6%, and Mn ≤ 0.40%.

[0134] In the converter smelting process: the slag basicity is controlled at 1.9~2.1, the total iron in the slag is 13~16%; the final slag basicity is controlled at 3.6~4.2, the final slag total iron is controlled at 14~17%, the temperature is controlled at 1645~1665℃, and P≤0.006%.

[0135] In this way, the molten pool temperature, slag basicity and total iron content are controlled, the low temperature environment of the molten pool is used to strengthen dephosphorization in the early stage, and the slag basicity is increased in the later stage for deep dephosphorization to ensure the phosphorus content at the end of the converter.

[0136] In addition, optionally, argon is blown throughout the entire steel-tapping process of converter smelting. Before tapping, the bottom of the ladle is opened for argon blowing, and the argon blowing pressure is controlled at 0.5~0.6MPa. The molten steel surface is turbulent, and the bright ring diameter is 250~400mm; after 3 / 4 of the steel is tapped, the pressure is adjusted to 0.4~0.5MPa, and the bright ring diameter is 200~350mm.

[0137] In the LF refining process: first deoxidize with a deoxidizer on the slag surface, then blow argon from the bottom of the ladle for strong stirring, then alloying is carried out, and finally power is turned on to control the tapping temperature at 1624±5℃.

[0138] During the strong stirring of the LF refining process, the flow rate of argon gas is 650~850NL / min and the duration does not exceed 6 minutes.

[0139] During alloying in the LF refining process, the bottom blowing argon flow rate is 620~820NL / min and the duration does not exceed 4 minutes; at other times, the bottom blowing argon flow rate does not exceed 450NL / min.

[0140] During the power-on period of the LF refining process, calcium carbide is added for diffusion deoxidation, and white slag is produced, and the white slag retention time is greater than 15 minutes.

[0141] In this way, bottom blowing and stirring in the LF refining process can promote slag melting, accelerate temperature uniformity, and facilitate the floating of inclusions. In the early stages, a high bottom blowing flow rate is set to enhance molten pool stirring, promote rapid slag melting and uniform steel composition and temperature. In the later stages, a low flow rate is set for soft blowing to promote the floating and removal of inclusions.

[0142] In the RH vacuum refining process: no oxygen is blown throughout the process, and when the vacuum degree is ≤2.5mbar, degassing takes 18~23min, net circulation takes 7~9min, the steel ladle is left standing on the turntable for 12~16min, and the tapping temperature is regulated so that the temperature of the start-casting furnace is 1587±5℃ and the temperature of the continuous casting furnace is 1577±5℃.

[0143] This allows for rapid denitrification and dehydrogenation under low vacuum conditions, ensuring low nitrogen and hydrogen levels. Furthermore, strict control of the net circulation time promotes the floating and removal of inclusions, with a 12-16 minute standstill period before pouring further promoting the floating of inclusions and improving the purity of the molten steel.

[0144] In the continuous casting process: the superheat is controlled at 26~32℃, secondary cooling electromagnetic stirring and dynamic soft reduction are used, the reduction is 4~7% of the continuous casting billet thickness, the crystallizer water volume of the continuous casting machine is 420~550L / min, the water inlet temperature is 25~40℃, and the water outlet temperature difference is 4~10℃.

[0145] In this way, low superheat casting is beneficial to reducing the segregation in the core of the billet. Further use of secondary cooling electromagnetic stirring and dynamic soft reduction technology can increase the equiaxed grain rate of the billet, reduce the segregation and looseness in the core of the billet, and improve the defects in the core of the billet.

[0146] In addition, full-protection casting can be used in the continuous casting process.

[0147] For example, the argon flow rate of the long shroud of the ladle is 150~200L / min, and the depth of the shroud inserted into the molten steel is 200~300mm.

[0148] The argon flow rate of the upper water inlet is 3~6L / min, the argon flow rate of the mechanism is 3~10L / min, the argon flow rate of the stopper rod and submerged water inlet is 3~6L / min, and the depth of the water inlet inserted into the molten steel is 120~180mm.

[0149] The argon seal back pressure is ≥0.05Bar, ensuring that the continuous casting nitrogen increase is ≤0.0002% and the crystallizer liquid level fluctuation is ≤2.5mm.

[0150] The tundish covering agent uses a double-layer insulation structure of pre-melted covering agent plus carbonized rice husk, and the crystallizer protection slag uses 3 / 4SG-2 special protection slag, with a liquid slag layer thickness of 9~12mm.

[0151] Furthermore, the frequency of the secondary cooling electromagnetic stirring is 4.5~5.5Hz, and the current is 520~620A.

[0152] The taper of the continuous casting machine can be controlled to 1.15±0.1%.

[0153] The water volume of the crystallizer is 420~550L / min.

[0154] The above describes the basic situation of one embodiment of the present invention. The following describes the specific embodiments of the present invention through two embodiments. Of course, these two embodiments are only part of the many variations of the present invention, not all of them.

[0155] Example 1 This embodiment provides a pipeline steel, whose chemical composition, measured in percentage by mass, includes: C 0.046%, Si 0.15%, Mn 0.95%, P 0.0092%, Cr 0.37%, Ni 0.21%, Mo 0.13%, Cu 0.27%, Nb 0.064%, Ti 0.016%, Al 0.032%, S 0.0011%, O 0.0021%, N 0.0036%, H 0.00015%, and the remainder is iron and unavoidable impurities.

[0156] The pipeline steel is specifically a steel plate with a thickness t of 42 mm.

[0157] Ginseng Figure 1 The microstructure of the pipeline steel is a complex phase structure of quasi-polygonal ferrite + acicular ferrite + bainite.

[0158] The volume proportion of quasi-polygonal ferrite structure is 59%, the volume proportion of acicular ferrite structure is 24%, and the volume proportion of bainite structure is 17%.

[0159] The average grain size of quasi-polygonal ferrite and acicular ferrite is 8 μm.

[0160] The yield strength R of the pipeline steel t0.5 The tensile strength is 436MPa, and the m is 621MPa, elongation A 50mm It is 54%.

[0161] The pipeline steel has an impact energy KV2 of 453J at -20°C, an impact energy KV2 of 415J at -40°C, an impact energy KV2 of 354J at -60°C, and an impact energy KV2 of 298J at -80°C.

[0162] The ductile-brittle transition temperature T t50%US It is -105℃.

[0163] The pipeline steel has a -10°C DWTT drop weight shear area fraction of 100%, a -20°C DWTT drop weight shear area fraction of 96%, a -30°C DWTT drop weight shear area fraction of 82%, and a -20°C CTOD crack tip opening displacement of 1.9 mm.

[0164] The hardness of the pipeline steel is 193HV 10 .

[0165] Furthermore, the central segregation of the pipeline steel is ≤0.5 level, and the central porosity is ≤0.5 level.

[0166] Furthermore, the inclusion ratings of categories A, B, C, and D of the pipeline steel are all ≤ level 1, and the sum of the ratings of categories A, B, C, and D inclusions A+B+C+D ≤ level 3.0.

[0167] Furthermore, the number of inclusions with a size of ≥10 μm on the cross section of the pipeline steel is 8 / cm 2 .

[0168] Under the conditions of 20° C., a carbon dioxide pressure of 1.0 MPa, and a liquid phase flow rate of 1 m / s, the uniform corrosion rate of the pipeline steel is ≤0.10 mm / year.

[0169] The pipeline steel production process includes smelting, continuous casting, heating, hot rolling, and cooling. Some of the key parameters are shown in Table 1. It will be appreciated that, in addition to the parameters shown in Table 1, other key parameters in the production process are implemented as described above in this application.

[0170] [Table 1]

[0171] Example 2 This embodiment provides a pipeline steel, whose chemical composition, in percentage by mass, includes: C 0.060%, Si 0.14%, Mn 0.92%, P 0.0088%, Cr 0.33%, Ni 0.17%, Mo 0.11%, Cu 0.31%, Nb 0.060%, Ti 0.013%, Al 0.036%, S 0.0010%, O 0.0022%, N 0.0031%, H 0.00015%, and the remainder is iron and unavoidable impurities.

[0172] The pipeline steel is specifically a steel plate with a thickness t of 42 mm.

[0173] The microstructure of the pipeline steel is a complex phase structure of quasi-polygonal ferrite+acicular ferrite+bainite.

[0174] The volume proportion of quasi-polygonal ferrite structure is 63%, the volume proportion of acicular ferrite structure is 18%, and the volume proportion of bainite structure is 19%.

[0175] The average grain size of quasi-polygonal ferrite and acicular ferrite is 7 μm.

[0176] The yield strength R of the pipeline steel t0.5 The tensile strength is 427MPa, and the m is 605MPa, elongation A 50mm It is 57%.

[0177] The pipeline steel has an impact energy KV2 of 442 J at -20°C, an impact energy KV2 of 412 J at -40°C, an impact energy KV2 of 376 J at -60°C, and an impact energy KV2 of 321 J at -80°C.

[0178] The ductile-brittle transition temperature T t50%US It is -110℃.

[0179] The pipeline steel has a -10°C DWTT drop weight shear area fraction of 100%, a -20°C DWTT drop weight shear area fraction of 97%, a -30°C DWTT drop weight shear area fraction of 83%, and a -20°C CTOD crack tip opening displacement of 2.0 mm.

[0180] The hardness of the pipeline steel is 186HV 10 .

[0181] Furthermore, the central segregation of the pipeline steel is ≤0.5 level, and the central porosity is ≤0.5 level.

[0182] Furthermore, the inclusion ratings of categories A, B, C, and D of the pipeline steel are all ≤ level 1, and the sum of the ratings of categories A, B, C, and D inclusions A+B+C+D ≤ level 3.0.

[0183] Furthermore, the number of inclusions with a size of ≥10 μm on the cross section of the pipeline steel is 8 / cm 2 .

[0184] Under the conditions of 20° C., a carbon dioxide pressure of 1.0 MPa, and a liquid phase flow rate of 1 m / s, the uniform corrosion rate of the pipeline steel is ≤0.10 mm / year.

[0185] The pipeline steel production process includes smelting, continuous casting, heating, hot rolling, and cooling. Some key parameters are shown in Table 2. It will be appreciated that, in addition to the parameters shown in Table 2, other key parameters in the production process are implemented as described above in this application.

[0186] [Table 2]

Claims

1. A low-cost pipeline steel for carbon dioxide transportation, characterized in that: The chemical composition of the pipeline steel includes, by mass percentage, C 0.041-0.071%, Si 0.11-0.17%, Mn 0.89-0.97%, P 0.0060-0.0099%, Cr 0.31-0.39%, Ni 0.15-0.23%, Mo 0.08-0.16%, Cu 0.25-0.33%, Nb 0.058-0.066%, Ti 0.010-0.018%, Al 0.022-0.049%, and the remainder is iron and unavoidable impurities.

2. The low-cost pipeline steel for carbon dioxide transportation according to claim 1, characterized in that: The impurities in the pipeline steel include: S 0.0009-0.0015%, O 0.0011-0.0025%, N 0.0025-0.0051%, and H 0.00005-0.00018%.

3. The low-cost pipeline steel for carbon dioxide transportation according to claim 1, characterized in that: The chemical composition of the pipeline steel further satisfies, in percentage by mass, any one or more of: 13≤Mn / C≤36, CEV=0.294-0.380, and Pcm=0.125-0.176; Where, CEV=[C]+[Mn] / 6+([Cr]+[Mo]) / 5+([Cu]+[Ni]) / 15; Pcm=[C]+[Si] / 30+([Mn]+[Cu]+[Cr]) / 20+[Ni] / 60+[Mo] / 15.

4. The low-cost pipeline steel for carbon dioxide transportation according to claim 1, characterized in that: The pipeline steel has a complex phase structure of quasi-polygonal ferrite + acicular ferrite + bainite, and the total volume proportion of the quasi-polygonal ferrite structure and the acicular ferrite structure is more than 70%.

5. The low-cost pipeline steel for transporting carbon dioxide according to claim 4, characterized in that: The average grain size of quasi-polygonal ferrite and acicular ferrite is 5–10 μm; The volume proportion of quasi-polygonal ferrite structure is 55~65%, the volume proportion of acicular ferrite structure is 15~30%, and the volume proportion of bainite structure is 15~20%.

6. The low-cost pipeline steel for carbon dioxide transportation according to claim 1, characterized in that: The yield strength R of the pipeline steel t0.5 ≥420MPa, tensile strength R m ≥560MPa, elongation A 50mm ≥32%, yield strength ratio R t0.5 / R m ≤0.

72.

7. The low-cost pipeline steel for carbon dioxide transportation according to claim 1, characterized in that: The pipeline steel has an impact energy of KV2≥400J at -20℃, an impact energy of KV2≥380J at -40℃, an impact energy of KV2≥320J at -60℃, an impact energy of KV2≥250J at -80℃, and a ductile-brittle transition temperature T t50%US ≤-100℃, hardness ≤200HV 10 , -10℃ DWTT drop hammer shear area fraction 100%, -20℃ DWTT drop hammer shear area fraction ≥ 90%, -30℃ DWTT drop hammer shear area fraction ≥ 80%, -20℃ CTOD crack tip opening displacement ≥ 1.5mm.

8. The low-cost pipeline steel for carbon dioxide transportation according to claim 1, characterized in that: Under the conditions of 20° C., a carbon dioxide pressure of 1.0 MPa, and a liquid phase flow rate of 1 m / s, the uniform corrosion rate of the pipeline steel is ≤0.10 mm / year.

9. The low-cost pipeline steel for carbon dioxide transportation according to claim 1, characterized in that: The number of inclusions with a size of ≥10 μm on the cross section of the pipeline steel is ≤10 / cm 2 .

10. A method for preparing low-cost pipeline steel for transporting carbon dioxide according to any one of claims 1 to 9, characterized in that: The preparation method comprises: A continuous casting billet is prepared by smelting and continuous casting; the number of inclusions with a size of ≥10 μm on the cross section of the continuous casting billet is ≤10 / cm 2 ; The continuous casting billet is sent into the heating furnace for heating; wherein the temperature of the soaking section is T NbC +(40~100)℃, soaking period is 30~50min; The continuous casting slab is rolled into steel plate through recrystallization zone rolling and two-phase zone rolling; wherein the starting rolling temperature T nr +(5~25)℃, final rolling temperature T nr +(0~20)℃, the thickness of the rolled plate is (3.2~4.2)t; the starting and finishing temperatures of the two-phase rolling are both Ar3-20℃~Ar3, and the thickness of the obtained steel plate is t≥38mm; After the steel plate leaves the rolling mill, it enters the ultra-fast cooling system for water cooling; the water inlet temperature ≥A r3 -30℃, final cooling temperature is B s -(460~400)℃, cooling rate is 9~22℃ / s; After the steel plate comes out of the water, it is air-cooled on a cooling bed until it cools to room temperature.

11. The method for preparing low-cost pipeline steel for transporting carbon dioxide according to claim 10, characterized in that: The step of "feeding the continuous casting billet into a heating furnace for heating" includes a preheating section, a first heating section, a second heating section and a soaking section. The temperature of the preheating section is ≤950°C, the temperature of the first heating section is 1050±15°C, and the temperature of the second heating section is 1150±15°C.

12. The method for preparing low-cost pipeline steel for transporting carbon dioxide according to claim 10, characterized in that: When rolling in the recrystallization zone, the initial rolling reduction of the non-widening pass is ≥42mm, and the minimum rolling reduction of each pass is ≥31mm; When rolling in the two-phase region, the reduction in the last pass is ≤8mm, and the reduction in the remaining passes is ≥22mm.

13. The method for preparing low-cost pipeline steel for transporting carbon dioxide according to claim 10, characterized in that: After the step of "preparing continuous casting billets through smelting and continuous casting", the continuous casting billets are directly stacked after leaving the continuous casting machine, with the stacking temperature being ≥500°C and the destacking temperature being 230~280°C.

14. The method for preparing low-cost pipeline steel for transporting carbon dioxide according to claim 10, characterized in that: The step of "preparing continuous casting billets by smelting and continuous casting" includes: KR desulfurization: outlet molten iron temperature ≥ 1300℃, S ≤ 0.0020%; Converter smelting: slag basicity is controlled at 1.9~2.1, total iron in slag is 13~16%; final slag basicity is controlled at 3.6~4.2, total iron in final slag is controlled at 14~17%, temperature is controlled at 1645~1665℃, P≤0.006%; LF refining: Deoxidation is first carried out using a slag surface deoxidizer, followed by strong stirring with argon blowing from the ladle bottom, followed by alloying, and finally power-on to control the tapping temperature at 1624±5°C. During strong stirring, the argon flow rate is 650-850 NL / min and the duration does not exceed 6 minutes. During alloying, the bottom blowing argon flow rate is 620-820 NL / min and the duration does not exceed 4 minutes. At other times, the bottom blowing argon flow rate does not exceed 450 NL / min. During the power-on period, calcium carbide is added for diffusion deoxidation, and white slag is produced, and the white slag retention time is greater than 15 minutes. RH vacuum refining: No oxygen blowing is done during the whole process. Under the condition of vacuum degree ≤2.5mbar, degassing takes 18~23min, net circulation takes 7~9min, the steel ladle is left on the turntable for 12~16min, and the tapping temperature is regulated to make the temperature of the open-casting furnace 1587±5℃ and the temperature of the continuous casting furnace 1577±5℃; Continuous casting: The superheat is controlled at 26~32℃, and secondary cooling electromagnetic stirring and dynamic soft reduction are used. The reduction is 4~7% of the thickness of the continuous casting billet. The water volume of the crystallizer of the continuous casting machine is 420~550L / min, the water inlet temperature is 25~40℃, and the water outlet temperature difference is 4~10℃.

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