X65-grade pipeline steel and preparation method thereof

Abstract

The invention discloses X65-grade pipeline steel and a preparation method thereof. The steel comprises the following partial elements in percentage by weight: 0.0060 to 0.0099 percent of P, 0.37 to 0.45 percent of Cr, 0.12 to 0.20 percent of Ni, 0.11 to 0.19 percent of Mo, 0.20 to 0.28 percent of Cu, 0.061 to 0.069 percent of Nb and 0.012 to 0.020 percent of Ti. The preparation method comprises the following steps: rolling in a recrystallization region at the initial rolling temperature of Tnr + (5-25) DEG C and the final rolling temperature of Tnr + (0-20) DEG C; rolling in a two-phase region, wherein the initial rolling temperature and the final rolling temperature are Ar3-15 DEG C to Ar3 + 5 DEG C; water cooling is conducted, the water inlet temperature is larger than or equal to Ar < 3-30 > DEG C, the final cooling temperature is Bs-(360-320) DEG C, and the cooling speed 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 an X65 grade pipeline steel 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 an X65 grade pipeline steel and a preparation method thereof, which can not only meet the high corrosion resistance requirements of pipeline steel for carbon dioxide transportation, but also have low cost, excellent mechanical properties and low-temperature toughness.

[0007] To achieve the above-mentioned objectives, one embodiment of the present invention provides an X65-grade pipeline steel suitable for transporting supercritical or dense-phase carbon dioxide. The chemical composition of the pipeline steel, in percentage by mass, includes the following: C 0.034-0.064%, Si 0.11-0.17%, Mn 0.91-0.99%, P 0.0060-0.0099%, Cr 0.37-0.45%, Ni 0.12-0.20%, Mo 0.11-0.19%, Cu 0.20-0.28%, Nb 0.061-0.069%, Ti 0.012-0.020%, Al 0.022-0.049%, with the remainder being iron and unavoidable impurities.

[0008] The following is a detailed analysis of the main functions of each element and the selection of its dosage.

[0009] 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 0.034-0.064%. Preferably, it is controlled to 0.050-0.064%, and more preferably, it is controlled to 0.056-0.064%.

[0010] 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%.

[0011] 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 localized corrosion. In one embodiment, taking into account corrosion performance, low-temperature toughness, and strength, the manganese content is controlled to 0.91~0.99%. It is preferably controlled at 0.91~0.95%, and more preferably at 0.91~0.94%.

[0012] 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%.

[0013] Cr: Chromium acts as a solid solution strengthener in steel and is the most effective element in steel against CO2 corrosion. As a ferrite-forming element, chromium produces a more acicular ferrite structure in high-niobium steel. Increasing chromium content significantly enhances CO2 corrosion resistance. However, excessive chromium content can increase the microhardness of pipeline steel and reduce low-temperature toughness. In one embodiment, the chromium content is controlled to 0.37-0.45%.

[0014] 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 improves 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 0.12-0.20%. A range of 0.12-0.18% is preferred, with a more preferred range of 0.15-0.18%.

[0015] 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 0.11~0.19%. It is preferably controlled at 0.11~0.16%, and more preferably at 0.13~0.16%.

[0016] 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 0.20-0.28%. Preferably, it is controlled to 0.23-0.28%.

[0017] 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 between 0.061% and 0.069%. A range of 0.061% to 0.067% is preferred, with a more preferred range of 0.061% to 0.066%.

[0018] 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 0.012-0.020%, preferably 0.012-0.017%, and more preferably 0.012-0.014%.

[0019] 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%.

[0020] 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.

[0021] 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.

[0022] Preferably, among the impurities in the pipeline steel, S is controlled below 0.0015%.

[0023] Considering that too low sulfur content will significantly increase steelmaking costs, the sulfur content is preferably controlled at 0.0009~0.0015%.

[0024] Preferably, the impurities in the pipeline steel include: O controlled below 0.0025%.

[0025] Too low oxygen will increase steelmaking costs, so the oxygen content is preferably controlled at 0.0011~0.0025%.

[0026] Preferably, the impurities in the pipeline steel include nitrogen content controlled below 0.0055%.

[0027] Too low nitrogen content will increase steelmaking costs, so the nitrogen content is preferably controlled at 0.0025~0.0051%.

[0028] Preferably, the impurities in the pipeline steel include: H content controlled below 0.00020%.

[0029] Too low a hydrogen content will increase the cost of steelmaking, therefore, the hydrogen content is preferably controlled between 0.00005 and 0.00018%.

[0030] Preferably, the chemical composition of the pipeline steel further satisfies the following in percentage by mass: 13≤Mn / C≤36.

[0031] 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.

[0032] Furthermore, in one embodiment, the chemical composition of the pipeline steel, in terms of mass percentage, further satisfies the following: CEV = [C] + [Mn] / 6 + ( [Cr] + [Mo]) / 5 + ( [Cu] + [Ni]) / 15 = 0.303 to 0.389.

[0033] Preferably, CEV=0.303~0.330.

[0034] Furthermore, in one embodiment, the chemical composition of the pipeline steel, in terms of mass percentage, further satisfies: Pcm=[C]+[Si] / 30+([Mn]+[Cu]+[Cr]) / 20+[Ni] / 60+[Mo] / 15=0.121~0.172.

[0035] Preferably, Pcm=0.148~0.172; more preferably, Pcm=0.168~0.172.

[0036] 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.

[0037] Preferably, the microstructure of the pipeline steel is a duplex microstructure of quasi-polygonal ferrite+acicular ferrite+bainite.

[0038] Among them, the total volume share of quasi-polygonal ferrite structure and acicular ferrite structure is more than 70%.

[0039] 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.

[0040] Preferably, the average grain size of the quasi-polygonal ferrite and the acicular ferrite is 4-9 μm, which can ensure excellent low-temperature toughness of the pipeline steel.

[0041] Preferably, the volume proportion of the quasi-polygonal ferrite structure is 45-55%, the volume proportion of the acicular ferrite structure is 15-35%, and the volume proportion of the bainite structure is 20-30%.

[0042] Preferably, the yield strength R of the pipeline steel t0.5 ≥460MPa, tensile strength R m ≥580MPa, elongation A 50mm ≥31%, yield strength ratio R t0.5 / R m ≤0.80. Thus, the pipeline steel has excellent mechanical properties.

[0043] Furthermore, the yield strength R t0.5 It is 460~540MPa.

[0044] Tensile strength R m It is 580~660MPa.

[0045] Elongation A 50mm It is 31~58%.

[0046] Yield strength ratio R t0.5 / R m It is 0.70~0.80.

[0047] 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".

[0048] Preferably, the pipeline steel has an impact energy KV2 of ≥400J at -20°C; -40℃ impact energy KV2≥380J; -60℃ impact energy KV2≥320J; -80℃ impact energy KV2≥250J; Ductile-brittle transition temperature T t50%US ≤-100℃; -10℃ DWTT drop weight shear area fraction 100%; -20℃ DWTT drop weight shear area fraction ≥90%; -30℃ DWTT drop weight shear area fraction ≥80%; -20℃ CTOD crack tip opening displacement ≥1.5mm.

[0049] It can be seen that the steel plate has excellent low-temperature performance.

[0050] 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".

[0051] Preferably, the hardness of the pipeline steel is ≤205HV 10 .

[0052] Furthermore, the hardness is 175~205HV 10 .

[0053] 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.

[0054] 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%.

[0055] It can be seen that the pipeline steel has excellent corrosion resistance, especially excellent corrosion resistance in a carbon dioxide environment.

[0056] Furthermore, the central segregation of the pipeline steel is ≤ level 0.5.

[0057] 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.

[0058] Furthermore, the central porosity of the pipeline steel is ≤ level 0.5.

[0059] 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.

[0060] 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.

[0061] 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.

[0062] 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.

[0063] Furthermore, the number of inclusions with a size of ≥10 μm on the cross section of the pipeline steel is ≤10 / cm 2 .

[0064] 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.

[0065] Furthermore, the pipeline steel may be specifically configured as a steel plate, for example, a steel plate with a thickness of t≥38 mm.

[0066] Specifically, the thickness t may be 38~50mm.

[0067] To achieve the above-mentioned purpose, an embodiment of the present invention provides a method for preparing the pipeline steel. The method includes a process of smelting, continuous casting, heating, hot rolling, and cooling. Specifically, the method includes: The 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 prepared continuous casting billet is ≤10 / cm 2 ;The thickness of the continuous casting billet can be 300~320mm; The continuous casting billet is sent into the heating furnace for heating; wherein the temperature of the soaking section is T NbC +(40~100)℃, with a soaking period of 30~50min. In this way, by controlling the temperature and duration of the soaking period, the alloying 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 rolling process. The continuous casting slab is rolled into steel plate through recrystallization zone rolling and two-phase zone rolling; Among them, the starting rolling temperature T of the recrystallization zone is nr +(5~25)℃, final rolling temperature T nr +(0~20)℃, the thickness of the rolled plate is (3.2~4.2)t; thus, 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, cooperate with the precipitation of Nb compounds during the rolling process to prevent the growth of recrystallized grains and refine the recrystallized grains; The starting and finishing temperatures of the two-phase rolling are both Ar3-15℃~Ar3+5℃, and the thickness of the resulting steel plate is t≥38mm. Thus, 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℃ near the starting point of the two-phase phase transformation, which can ensure that the content of deformation-induced ferrite is not too high. Too much deformation-induced ferrite will result in low yield strength, while the deformation-induced ferrite content will not be too little, resulting in an excessively high yield strength ratio, so that the deformation-induced ferrite content is within an optimal range; on the other hand, if the finishing temperature is too high, the subsequent cooling temperature drop will be large, resulting in high yield and tensile strength and low toughness; while if the finishing temperature is too low, the subsequent cooling temperature drop will be small, and the deformation-induced ferrite content will be high, resulting in low yield and tensile strength. The finishing temperature control of the present application ensures low yield strength, high tensile strength, and high toughness; 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-(360~320)℃, cooling rate is 9~22℃ / s; 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 low 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, its bainite phase transformation is incomplete; if the final cooling temperature is too low, the large temperature drop is not conducive to plate shape control; here, one embodiment can obtain higher tensile strength and low yield ratio, as well as excellent plate shape by controlling the cooling rate and final cooling temperature; After exiting the water, the steel plate is air-cooled on a cooling bed until it cools to room temperature. This air-cooling and self-tempering process eliminates or reduces internal stress. Meanwhile, during the phase transformation process, pipeline steel plates produce MA hard phases, which are detrimental to the toughness of the steel plate and can easily lead to high local hardness. Air-cooling and self-tempering can decompose MA, improving toughness and local hardness.

[0068] 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.

[0069] Among them, 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.

[0070] 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.

[0071] 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.

[0072] 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.

[0073] 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.

[0074] 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.

[0075] 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.

[0076] 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.

[0077] Preferably, during rolling in the two-phase region, the starting rolling temperature and the finishing rolling temperature can be further controlled to be Ar3-10°C~Ar3+5°C, more preferably Ar3-10°C~Ar3.

[0078] Preferably, 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.

[0079] Among them, the temperature of the preheating section is ≤950℃, the temperature of the first heating section is 1050±15℃, and the temperature of the second heating section is 1150±15℃.

[0080] 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.

[0081] 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.

[0082] Preferably, when rolling in the recrystallization zone, the reduction of the initial rolling pass (non-widening pass) is ≥42 mm, and the minimum reduction of each pass is ≥31 mm. In this way, when the total reduction of rolling in the recrystallization zone is large, a large reduction is also used in each pass. In the low temperature section of the recrystallization zone, the large reduction rolling is used to fully break up the continuous casting structure, obtain refined recrystallized grains, and have a significant effect on refining the austenite grains. Moreover, due to the low temperature, the grains will not grow excessively. During rolling in the two-phase region, the reduction of the last pass is ≤8mm, and the reduction of the remaining passes is ≥22mm; in this way, except for the last pass, each pass adopts a large reduction deformation 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 adopts a small reduction, which can improve the shape of the thick plate.

[0083] Preferably, the rolling temperature of each pass of the two-phase rolling is controlled within the range of Ar3-15°C to Ar3+5°C.

[0084] The rolling temperature of each pass can be further controlled to be Ar3-10℃~Ar3+5℃, more preferably Ar3-10℃~Ar3.

[0085] 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.

[0086] 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 Ar3-15°C~Ar3+5°C.

[0087] Water cooling can further control the temperature of the intermediate billet to Ar3-10℃~Ar3+5℃, and more preferably to Ar3-10℃~Ar3.

[0088] 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.

[0089] Preferably, 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 1.0-1.4 m / s. However, this application is not limited thereto.

[0090] Furthermore, after the steel plate leaves the ultra-fast cooling system, that is, after leaving the water, and before being placed on the cooling bed, it can be hot straightened, which can further improve the plate shape.

[0091] 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.

[0092] 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.

[0093] Preferably, the step of "preparing a continuous casting billet by smelting and continuous casting" includes the processes of KR desulfurization, converter smelting, LF refining, RH vacuum refining and continuous casting.

[0094] Furthermore, KR desulfurization: the outgoing molten iron temperature is ≥1300℃, S≤0.0020%.

[0095] Optionally, 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%.

[0096] Further, converter smelting: The slag basicity is controlled at 1.9~2.1, and the total iron content in the slag is 13~16%; the final slag basicity is controlled at 3.6~4.2, the final slag total iron content is controlled at 14~17%, the temperature is controlled at 1645~1665℃, and P≤0.006%; in this way, the molten pool temperature, slag basicity and total iron content are controlled, and the low temperature environment of the molten pool is used to enhance 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.

[0097] Optionally, argon is blown throughout the entire 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.

[0098] Further, LF refines: First, deoxidize the slag surface with a deoxidizer, then blow argon gas from the bottom of the ladle for strong stirring, then alloying, and finally power on to control the tapping temperature at 1624±5℃; During strong stirring, the flow rate of argon is 650~850NL / min and the duration does not exceed 6 minutes; During alloying, the bottom blowing argon flow rate is 620~820NL / min and the duration does not exceed 4 minutes; At other times, the flow rate of bottom blowing argon does not exceed 450NL / min; During the power-on period, calcium carbide is added for diffusion deoxidation, and white slag is produced, and the white slag is maintained for more than 15 minutes.

[0099] 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.

[0100] Further, RH vacuum refining: No oxygen is blown during the whole process. When the vacuum degree is ≤2.5mbar, the degassing is 18~23min, the net circulation is 7~9min, the steel ladle is left on the turntable for 12~16min, and the steel 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℃.

[0101] 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.

[0102] Furthermore, 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 inlet water temperature is 25~40℃, and the outlet water temperature difference is 4~10℃.

[0103] 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.

[0104] Optionally, in the continuous casting process, fully protected casting can be used.

[0105] 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.

[0106] 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.

[0107] 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.

[0108] 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.

[0109] Furthermore, the frequency of the secondary cooling electromagnetic stirring is 4.5~5.5Hz, and the current is 520~620A.

[0110] The taper of the continuous casting machine can be controlled to 1.15±0.1%.

[0111] The water volume of the crystallizer is 420~550L / min.

[0112] Compared with the prior art, the present invention has the following beneficial effects: (1) In terms of chemical composition, a low-alloy composition system with low carbon, low manganese, low sulfur, high niobium and a composite addition of corrosion-resistant elements Cr + Ni + Mo + Cu + P is adopted to reduce the C and Cr contents in the steel, and appropriate amounts of carbide-forming elements Nb and Ti are added 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, the composite addition of Cr with Ni, Mo, Cu, and P further improves the corrosion resistance, thus making the pipeline steel have excellent corrosion resistance in the use of carbon dioxide transportation; (2) In terms of chemical composition, it does not contain expensive alloys such as V, and the content of alloy additions such as Cr, Ni, and Mo is low. P exists as a corrosion-resistant element without the need for ultra-low content control, making it easy to produce steel and having low production costs. (3) On the basis of the low alloy composition system, combined with 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", the special controlled rolling in the two-phase critical zone is used to adjust and control the type and proportion of the finished product structure 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, meeting the performance requirements of steel in supercritical or dense phase carbon dioxide transportation. BRIEF DESCRIPTION OF THE DRAWINGS

[0113] Figure 1 This is a metallographic structure diagram of the finished steel plate of Example 1 of the present invention. DETAILED DESCRIPTION

[0114] 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 specific embodiments of the present invention. Obviously, the embodiments described are only some of the embodiments of the present invention, not all of them. 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.

[0115] Example 1 This embodiment provides a pipeline steel, whose chemical composition, in percentage by mass, includes: C 0.038%, Si 0.16%, Mn 0.97%, P 0.0076%, Cr 0.43%, Ni 0.18%, Mo 0.16%, Cu 0.26%, Nb 0.067%, Ti 0.015%, Al 0.033%, S 0.0011%, O 0.0020%, N 0.0028%, H 0.00017%, and the remainder is iron and unavoidable impurities.

[0116] The pipeline steel is specifically a steel plate with a thickness t of 40 mm.

[0117] The method for preparing the pipeline steel includes the following steps.

[0118] (1) Prepare continuous casting billets through smelting and continuous casting.

[0119] The number of inclusions with a size of ≥10 μm on the cross section of the prepared continuous casting billet is 7 / cm 2 .

[0120] The thickness of the continuous casting slab can be 320 mm.

[0121] The center segregation of continuous casting billet is ≤0.5 level, and the center porosity is ≤0.5 level.

[0122] The ratings of A, B, C and D inclusions in the continuous casting billet are all ≤ Level 1, and the sum of the ratings of the four types of inclusions A, B, C and D (A+B+C+D) is ≤ Level 3.0.

[0123] The specific smelting and continuous casting methods can be implemented according to the technology provided in this application, but are not limited thereto.

[0124] (2) After the continuous casting billets leave the continuous casting machine, they are directly stacked with a stacking temperature of 650°C and a destacking temperature of 250°C.

[0125] (3) The continuous casting billet is sent into a heating furnace for heating.

[0126] The temperature of the preheating section is ≤950℃, the temperature of the first heating section is 1050±15℃, the temperature of the second heating section is 1150±15℃, the temperature of the soaking section is 1180℃, and the soaking section time is 37 minutes.

[0127] (4) The continuous casting slab is rolled into a steel plate with a thickness of 40 mm through recrystallization zone rolling and two-phase zone rolling.

[0128] Among them, the starting rolling temperature of the recrystallization zone rolling is 1155℃, the final rolling temperature is 1145℃, and the plate thickness after rolling is 140mm.

[0129] 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.

[0130] After rolling in the recrystallization zone and before rolling in the two-phase zone, the intermediate bar is water-cooled to cool the temperature of the intermediate bar to 794°C.

[0131] Next, the starting rolling temperature of the two-phase zone rolling was 792°C and the finishing rolling temperature was 787°C.

[0132] When rolling in the two-phase region, the reduction in the last pass is ≤8mm, and the reduction in the remaining passes is ≥22mm.

[0133] (5) After leaving the rolling mill, the steel plate enters the ultra-fast cooling system for water cooling.

[0134] Among them, the water inlet temperature is 768℃, the final cooling temperature is 345℃, and the roller speed of the ultra-fast cooling system is 1.1m / s.

[0135] (6) After the steel plate is removed from the water, it is air-cooled on a cooling bed until it cools to room temperature.

[0136] It can be understood that the above describes some important parameters in the preparation method, but other important parameters not mentioned are implemented according to the embodiment of the present invention described above.

[0137] The microstructure and performance of the pipeline steel obtained in step 6 were tested as follows.

[0138] Ginseng Figure 1 The microstructure of the pipeline steel is a complex phase structure of quasi-polygonal ferrite + acicular ferrite + bainite.

[0139] The volume proportion of quasi-polygonal ferrite structure is 47%, the volume proportion of acicular ferrite structure is 28%, and the volume proportion of bainite structure is 25%.

[0140] The average grain size of quasi-polygonal ferrite and acicular ferrite is 7 μm.

[0141] The yield strength R of the pipeline steel t0.5 The tensile strength is 486MPa, m is 648MPa, elongation A 50mm It is 52%.

[0142] The pipeline steel has an impact energy KV2 of 459J at -20°C, 402J at -40°C, 378J at -60°C, and 302J at -80°C.

[0143] The ductile-brittle transition temperature T t50%USIt is -110℃.

[0144] 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 89%, and a -20°C CTOD crack tip opening displacement of 1.8 mm.

[0145] The hardness of the pipeline steel is 202HV 10 .

[0146] 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.

[0147] Example 2 This embodiment provides a pipeline steel, whose chemical composition, measured in percentage by mass, includes: C 0.056%, Si 0.13%, Mn 0.94%, P 0.0072%, Cr 0.39%, Ni 0.15%, Mo 0.15%, Cu 0.22%, Nb 0.062%, Ti 0.013%, Al 0.035%, S 0.0013%, O 0.0019%, N 0.0030%, H 0.00015%, and the remainder is iron and unavoidable impurities.

[0148] The pipeline steel is specifically a steel plate with a thickness t of 40 mm.

[0149] The method for preparing the pipeline steel includes the following steps.

[0150] (1) Prepare continuous casting billets through smelting and continuous casting.

[0151] The number of inclusions with a size of ≥10 μm on the cross section of the prepared continuous casting billet is 9 / cm 2 .

[0152] The thickness of the continuous casting slab can be 320 mm.

[0153] The center segregation of continuous casting billet is ≤0.5 level, and the center porosity is ≤0.5 level.

[0154] The ratings of A, B, C and D inclusions in the continuous casting billet are all ≤ Level 1, and the sum of the ratings of the four types of inclusions A, B, C and D (A+B+C+D) is ≤ Level 3.0.

[0155] The specific smelting and continuous casting methods can be implemented according to the technology provided in this application, but are not limited thereto.

[0156] (2) After the continuous casting billets leave the continuous casting machine, they are directly stacked. The stacking temperature is 690℃ and the destacking temperature is 260℃.

[0157] (3) The continuous casting billet is sent into a heating furnace for heating.

[0158] The temperature of the preheating section is ≤950℃, the temperature of the first heating section is 1050±15℃, the temperature of the second heating section is 1150±15℃, the temperature of the soaking section is 1205℃, and the soaking section time is 43 minutes.

[0159] (4) The continuous casting slab is rolled into a steel plate with a thickness of 40 mm through recrystallization zone rolling and two-phase zone rolling.

[0160] Among them, the starting rolling temperature of the recrystallization zone rolling is 1145℃, the final rolling temperature is 1130℃, and the plate thickness after rolling is 140mm.

[0161] 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.

[0162] After rolling in the recrystallization zone and before rolling in the two-phase zone, the intermediate bar is water-cooled to cool the temperature of the intermediate bar to 795°C.

[0163] Next, the starting rolling temperature of the two-phase zone rolling was 793°C and the finishing rolling temperature was 788°C.

[0164] When rolling in the two-phase region, the reduction in the last pass is ≤8mm, and the reduction in the remaining passes is ≥22mm.

[0165] (5) After leaving the rolling mill, the steel plate enters the ultra-fast cooling system for water cooling.

[0166] Among them, the water inlet temperature is 769℃ and the final cooling temperature is 347℃.

[0167] The roller speed of the ultra-fast cooling system is 1.1m / s.

[0168] (6) After the steel plate is removed from the water, it is air-cooled on a cooling bed until it cools to room temperature.

[0169] It can be understood that the above describes some important parameters in the preparation method, but other important parameters not mentioned are implemented according to the embodiment of the present invention described above.

[0170] The microstructure and performance of the pipeline steel obtained in step 6 were tested as follows.

[0171] The microstructure of the pipeline steel is a complex phase structure of quasi-polygonal ferrite+acicular ferrite+bainite.

[0172] The volume proportion of quasi-polygonal ferrite structure is 52%, the volume proportion of acicular ferrite structure is 26%, and the volume proportion of bainite structure is 22%.

[0173] The average grain size of quasi-polygonal ferrite and acicular ferrite is 6 μm.

[0174] The yield strength R of the pipeline steel t0.5 The tensile strength R is 473MPa. m is 625MPa, elongation A 50mm is 50%.

[0175] The pipeline steel has an impact energy KV2 of 465 J at -20°C, 423 J at -40°C, 398 J at -60°C, and 313 J at -80°C.

[0176] The ductile-brittle transition temperature T t50%US It is -105℃.

[0177] 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 87%, and a -20°C CTOD crack tip opening displacement of 1.7 mm.

[0178] The hardness of the pipeline steel is 194HV 10 .

[0179] 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.

[0180] Example 3 This embodiment provides a pipeline steel, whose chemical composition, in percentage by mass, includes: C 0.049%, Si 0.14%, Mn 0.95%, P 0.0086%, Cr 0.41%, Ni 0.16%, Mo 0.15%, Cu 0.24%, Nb 0.065%, Ti 0.016%, Al 0.035%, S 0.0012%, O 0.0019%, N 0.0031%, H 0.00016%, and the remainder is iron and unavoidable impurities.

[0181] The pipeline steel is specifically a steel plate with a thickness t of 39 mm.

[0182] The method for preparing the pipeline steel includes the following steps.

[0183] (1) Prepare continuous casting billets through smelting and continuous casting.

[0184] The number of inclusions with a size of ≥10 μm on the cross section of the prepared continuous casting billet is 9 / cm 2 .

[0185] The thickness of the continuous casting slab can be 320 mm.

[0186] The center segregation of continuous casting billet is ≤0.5 level, and the center porosity is ≤0.5 level.

[0187] The ratings of A, B, C and D inclusions in the continuous casting billet are all ≤ Level 1, and the sum of the ratings of the four types of inclusions A, B, C and D (A+B+C+D) is ≤ Level 3.0.

[0188] The specific smelting and continuous casting methods can be implemented according to the technology provided in this application, but are not limited thereto.

[0189] (2) After the continuous casting billets leave the continuous casting machine, they are directly stacked with a stacking temperature of 700°C and a destacking temperature of 270°C.

[0190] (3) The continuous casting billet is sent into a heating furnace for heating.

[0191] The temperature of the preheating section is ≤950℃, the temperature of the first heating section is 1050±15℃, the temperature of the second heating section is 1150±15℃, the temperature of the soaking section is 1205℃, and the soaking section time is 40 minutes.

[0192] (4) The continuous casting slab is rolled into a steel plate with a thickness of 39 mm through recrystallization zone rolling and two-phase zone rolling.

[0193] Among them, the starting rolling temperature of the recrystallization zone rolling is 1150℃, the final rolling temperature is 1142℃, and the plate thickness after rolling is 136mm.

[0194] 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.

[0195] 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 802°C.

[0196] Next, the starting rolling temperature of the two-phase zone rolling is 800°C and the finishing rolling temperature is 795°C.

[0197] When rolling in the two-phase region, the reduction in the last pass is ≤8mm, and the reduction in the remaining passes is ≥22mm.

[0198] (5) After leaving the rolling mill, the steel plate enters the ultra-fast cooling system for water cooling.

[0199] Among them, the water inlet temperature is 776℃ and the final cooling temperature is 344℃.

[0200] The roller speed of the ultra-fast cooling system is 1.1m / s.

[0201] (6) After the steel plate is removed from the water, it is air-cooled on a cooling bed until it cools to room temperature.

[0202] It can be understood that the above describes some important parameters in the preparation method, but other important parameters not mentioned are implemented according to the embodiment of the present invention described above.

[0203] The microstructure and performance of the pipeline steel obtained in step 6 were tested as follows.

[0204] The microstructure of the pipeline steel is a duplex microstructure of quasi-polygonal ferrite+acicular ferrite+bainite.

[0205] The volume proportion of quasi-polygonal ferrite structure is 50%, the volume proportion of acicular ferrite structure is 27%, and the volume proportion of bainite structure is 23%.

[0206] The average grain size of quasi-polygonal ferrite and acicular ferrite is 6 μm.

[0207] The yield strength R of the pipeline steel t0.5 The tensile strength R is 480MPa. m is 639MPa, elongation A 50mm It is 51%.

[0208] The pipeline steel has an impact energy KV2 of 447 J at -20°C, 398 J at -40°C, 370 J at -60°C, and 301 J at -80°C.

[0209] The ductile-brittle transition temperature T t50%US It is -105℃.

[0210] 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 95%, a -30°C DWTT drop weight shear area fraction of 86%, and a -20°C CTOD crack tip opening displacement of 1.7 mm.

[0211] The hardness of the pipeline steel is 198HV 10 .

[0212] 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.

Claims

1. A method for preparing X65 grade pipeline steel, characterized in that: The chemical composition of the pipeline steel includes, by mass percentage, C 0.034-0.064%, Si 0.11-0.17%, Mn 0.91-0.99%, P 0.0060-0.0099%, Cr 0.37-0.45%, Ni 0.12-0.20%, Mo 0.11-0.19%, Cu 0.20-0.28%, Nb 0.061-0.069%, Ti 0.012-0.020%, Al 0.022-0.049%, and the remainder is iron and unavoidable impurities; The preparation method comprises: Prepare continuous casting billets through smelting and continuous casting; 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 Ar3-15℃~Ar3+5℃, and the thickness of the resulting 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 -(360~320)℃, 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.

2. The method for preparing X65 grade pipeline steel according to claim 1, 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.

3. The method for preparing X65 grade pipeline steel according to claim 1, 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.

4. The method for preparing X65 grade pipeline steel according to claim 1, 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.

5. The method for preparing X65 grade pipeline steel according to claim 1, 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%.

6. The method for preparing X65 grade pipeline steel according to claim 1, characterized in that: The step of "preparing continuous casting billets by smelting and continuous casting" includes: LF refining: first deoxidize with a slag surface deoxidizer, then blow argon from the bottom of the ladle for strong stirring, then alloying, and finally power on to control the tapping temperature at 1624±5℃; during strong stirring, the argon flow rate is 650~850NL / min and the duration does not exceed 6min; during alloying, the bottom blowing argon flow rate is 620~820NL / min and the duration does not exceed 4min; at other times, the bottom blowing argon flow rate does not exceed 450NL / 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 15min.

7. The method for preparing X65 grade pipeline steel according to claim 1, characterized in that: The step of "preparing continuous casting billets by smelting and continuous casting" includes: RH vacuum refining: No oxygen blowing is performed during the entire process. 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℃.

8. The method for preparing X65 grade pipeline steel according to claim 1, characterized in that: The step of "preparing continuous casting billets by smelting and continuous casting" includes: 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℃.

9. An X65 grade pipeline steel, characterized in that: The chemical composition of the pipeline steel includes, by mass percentage, C 0.034-0.064%, Si 0.11-0.17%, Mn 0.91-0.99%, P 0.0060-0.0099%, Cr 0.37-0.45%, Ni 0.12-0.20%, Mo 0.11-0.19%, Cu 0.20-0.28%, Nb 0.061-0.069%, Ti 0.012-0.020%, Al 0.022-0.049%, and the remainder is iron and unavoidable impurities.

10. The X65 grade pipeline steel according to claim 9, characterized in that: The chemical composition of the pipeline steel, expressed in mass percentage, also satisfies: 13≤Mn / C≤36; and / or, CEV = [C] + [Mn] / 6 + ( [Cr] + [Mo]) / 5 + ( [Cu] + [Ni]) / 15 = 0.303 to 0.389; and / or, Pcm=[C]+[Si] / 30+([Mn]+[Cu]+[Cr]) / 20+[Ni] / 60+[Mo] / 15=0.121~0.

172.

11. The X65 grade pipeline steel according to claim 9, 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%.

12. The X65 grade pipeline steel according to claim 11, characterized in that: The average grain size of quasi-polygonal ferrite and acicular ferrite is 4–9 μm; The volume proportion of quasi-polygonal ferrite structure is 45~55%, the volume proportion of acicular ferrite structure is 15~35%, and the volume proportion of bainite structure is 20~30%.

13. The X65 grade pipeline steel according to claim 9, characterized in that: The yield strength R of the pipeline steel t0.5 ≥460MPa, tensile strength R m ≥580MPa, elongation A 50mm ≥31%, yield strength ratio R t0.5 / R m ≤0.

80.

14. The X65 grade pipeline steel according to claim 9, 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 ≤205HV 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.

15. The X65 grade pipeline steel according to claim 9, 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.

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