Carbon dioxide pipeline steel, steel billet casting method and steel plate rolling method

Abstract

The present invention discloses a carbon dioxide pipeline steel, a casting method for a steel billet, and a rolling method for a steel plate. The chemical composition includes: C 0.026-0.071%, Si 0.11-0.17%, Mn 0.89-1.02%, P 0.0060-0.0099%, Cr 0.31-0.53%, Ni 0.09-0.23%, Mo 0.08-0.22%, Cu 0.15-0.33%, Nb 0.058-0.073%, Ti 0.010-0.022%, Al 0.022-0.049%, Mg 0.0005-0.0012%, Ca 0.0012-0.0042%, and the remainder is iron and impurities.

Description

Technical Field

[0001] The present invention belongs to the technical field of steel material preparation, and relates to a carbon dioxide pipeline steel, a casting method of a steel billet, and a rolling method of a steel plate. Background Art

[0002] Carbon dioxide exists in solid, gaseous, liquid, dense phase and supercritical states. Based on existing transportation experience and transportation cost comparison, supercritical and dense phase carbon dioxide pipeline transportation is the most economical for long-distance and large-scale carbon dioxide transportation.

[0003] Dry, pure carbon dioxide is non-corrosive to pipelines. However, carbon dioxide transported through pipelines inevitably contains a certain amount of impurities such as H2O, O2, and SO2. Once a free water phase forms in the pipeline, carbon dioxide will dissolve in water to form H2CO3, causing corrosion of the pipeline. The presence of gaseous impurities will especially exacerbate pipeline corrosion, requiring the steel plates used for pipelines to have excellent corrosion resistance.

[0004] Some existing carbon dioxide transmission pipeline steels generally use high-chromium alloy composition systems in their chemical composition. For example, patented technologies such as CN107502823A, CN105132822A, CN104862607A and CN103334055 have Cr contents above 1.5%, even reaching 7.0%. The high alloy content leads to increased production costs.

[0005] How to achieve excellent corrosion resistance at low cost is one of the important issues in pipeline steel for carbon dioxide transportation. Summary of the Invention

[0006] The object of the present invention is to provide a carbon dioxide pipeline steel, a casting method of a steel billet and a rolling method of a steel plate.

[0007] To achieve the above-mentioned object, one embodiment of the present invention provides a carbon dioxide pipeline steel. The chemical composition of the pipeline steel, in percentage by mass, includes: C 0.026-0.071%, Si 0.11-0.17%, Mn 0.89-1.02%, P 0.0060-0.0099%, Cr 0.31-0.53%, Ni 0.09-0.23%, Mo 0.08-0.22%, Cu 0.15-0.33%, Nb 0.058-0.073%, Ti 0.010-0.022%, Al 0.022-0.049%, Mg 0.0005-0.0012%, Ca 0.0012-0.0042%, with the remainder being iron and unavoidable impurities.

[0008] 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.

[0009] Preferably, the number of inclusions with a size of ≥10 μm on the cross section of the pipeline steel is ≤10 / cm 2 , the maximum size of inclusions does not exceed 15μm, and the average size of inclusions is 0.1~2μm.

[0010] Preferably, part or all of the non-metallic inclusions in the pipeline steel are of a core-shell structure, wherein the core is MgO·Al2O3 and the shell is MnS, CaS, TiN and is coated on the outer surface of the inner shell.

[0011] Preferably, the yield strength R of the pipeline steel t0.5 ≥420MPa, tensile strength R m ≥560MPa, elongation A 50mm ≥30%, yield strength ratio R t0.5 / R m ≤0.82.

[0012] 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, and an impact energy KV2 of ≥250J at -80°C.

[0013] Preferably, the ductile-brittle transition temperature of the pipeline steel is t50%US ≤-100℃.

[0014] Preferably, the pipeline steel has a -10°C DWTT drop weight shear area fraction of 100%, a -20°C DWTT drop weight shear area fraction ≥90%, a -30°C DWTT drop weight shear area fraction ≥80%, and a -20°C CTOD crack tip opening displacement ≥1.5 mm.

[0015] Preferably, the hardness of the pipeline steel is ≤210HV 10 .

[0016] Preferably, the microstructure of the pipeline steel is a duplex microstructure of quasi-polygonal ferrite + acicular ferrite + bainite;

[0017] The total volume proportion of the quasi-polygonal ferrite structure and the acicular ferrite structure is more than 70%.

[0018] Preferably, the volume proportion of the quasi-polygonal ferrite structure is 15-65%, the volume proportion of the acicular ferrite structure is 15-70%, and the volume proportion of the bainite structure is 5-30%.

[0019] Preferably, the average grain size of the quasi-polygonal ferrite and the acicular ferrite is 3-10 μm.

[0020] Preferably, the chemical composition of the pipeline steel, in terms of mass percentage, further satisfies:

[0021] 13≤Mn / C≤36; and / or,

[0022] CEV = [C] + [Mn] / 6 + ( [Cr] + [Mo]) / 5 + ( [Cu] + [Ni]) / 15 = 0.294 to 0.403; and / or,

[0023] Pcm=[C]+[Si] / 30+([Mn]+[Cu]+[Cr]) / 20+[Ni] / 60+[Mo] / 15=0.118~0.176.

[0024] To achieve the above-mentioned purpose, an embodiment of the present invention provides a casting method for the pipeline steel billet. The casting method includes KR desulfurization, converter smelting, LF refining, RH vacuum refining and continuous casting to prepare a continuous casting billet;

[0025] In KR desulfurization, magnesium powder and lime powder are mixed and sprayed into the molten iron to control the sulfur content in the molten iron to within 0.0012% by mass. The injection rate of magnesium powder is 0.35-0.45 kg per ton of molten iron.

[0026] During converter smelting: tapping temperature is controlled at 1645~1665℃, P≤0.006%;

[0027] LF refining: alloying and controlling the O content below 0.005%;

[0028] RH vacuum refining: No oxygen blowing during the whole process, under the condition of vacuum degree ≤2.5mbar, degassing for 18~23min, net circulation for 7~9min; then, at 4~5Nm 3 / h flow rate of argon blowing for 1-2 minutes, then vacuum static, and then break the vacuum to feed calcium wire to adjust the Ca / S mass ratio of the molten steel to 2-4 and the Ca / Al mass ratio to above 0.06; then static stirring for 2-3 minutes, add magnesium alloy to adjust the Mg mass ratio of the molten steel to 0.0005-0.0012%, and finally blow argon from the bottom;

[0029] Continuous casting: superheat is controlled at 26~32℃.

[0030] Preferably, KR desulfurization: the outlet molten iron temperature is ≥1300℃, S ≤0.0020%.

[0031] Preferably, in converter smelting: the slag basicity is controlled at 1.9-2.1, and the total iron in the slag is 13-16%; the final slag basicity is controlled at 3.6-4.2, and the final slag total iron is controlled at 14-17%.

[0032] Preferably, LF refining: first deoxidize with a slag surface deoxidizer, 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°C; wherein, during strong stirring, the argon flow rate 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 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 15 minutes.

[0033] Preferably, RH vacuum refining: the tapping ladle is left standing on the turntable for 12 to 16 minutes, and the tapping temperature is regulated so that the temperature of the start-casting furnace is 1587±5°C and the temperature of the continuous casting furnace is 1577±5°C.

[0034] Preferably, continuous casting: adopts secondary cooling electromagnetic stirring and dynamic soft reduction, 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℃.

[0035] To achieve the above-mentioned purpose, one embodiment of the present invention provides a method for rolling the pipeline steel plate. The rolling method comprises heating, first hot rolling, water cooling, second hot rolling, ultra-rapid cooling, air cooling and auto-tempering a continuous casting billet with a thickness of 300-320 mm, to produce a steel plate with a thickness of t≥38 mm.

[0036] During the heating process: the soaking temperature is T NbC +(40~100)℃, soaking time is 30~50min;

[0037] In the first hot rolling process: the thickness of the rolled plate is (3.2~4.2)t, and the starting rolling temperature is T nr +(5~25)℃, final rolling temperature T nr +(0~20)℃;

[0038] In the water cooling process: the final cooling temperature is Ar3-20℃~Ar3+15℃;

[0039] In the second hot rolling process: the starting and finishing temperatures are both Ar3-20℃~Ar3+15℃;

[0040] In the ultra-fast cooling process: water inlet temperature ≥A r3 -30℃, final cooling temperature is B s-(460~300)℃, cooling rate is 9~22℃ / s;

[0041] In the air-cooling autotempering process: the steel plate is air-cooled to room temperature on the cooling bed.

[0042] Preferably, the rolling method further comprises a stacking process before the heating process:

[0043] The continuous casting billets are stacked warm, with a stacking temperature ≥ 500℃ and a destacking temperature of 230~280℃.

[0044] Preferably, in the first hot rolling process:

[0045] The reduction of the initial rolling pass of the non-widening pass is ≥42mm, and the minimum reduction of each pass is ≥31mm.

[0046] Preferably, in the second hot rolling process:

[0047] The reduction of the last pass is ≤8mm, and the reduction of the remaining passes is ≥22mm.

[0048] Preferably, the starting and finishing temperatures of the second hot rolling process are both Ar3-20°C~Ar3, and the finishing temperature of the ultra-fast cooling process is B s -(360~320)℃; or

[0049] The starting and finishing temperatures of the second hot rolling process are both Ar3-15℃~Ar3+5℃, and the finishing temperature of the ultra-fast cooling process is B s -(360~320)℃; or

[0050] The starting and finishing temperatures of the second hot rolling process are both Ar3-5℃~Ar3+15℃, and the finishing temperature of the ultra-fast cooling process is B s -(340~300)℃.

[0051] Compared with the prior art, the present invention has the following beneficial effects:

[0052] (1) Reduce the C content and Cr content in the steel, and add appropriate amounts of carbide-forming elements Nb and Ti to reduce the carbonization 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 steel plate 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 makes steelmaking easy and has low production costs;

[0053] (2) On the basis of modifying the inclusions by using trace amounts of magnesium and calcium, further ensuring the formation of sufficient nucleation points to promote the formation of soft sulfides and avoid the generation and agglomeration of long strips of sulfides, thereby increasing and optimizing the non-metallic inclusions with a core-shell structure and improving corrosion resistance and low-temperature toughness;

[0054] (3) Combining the special controlled rolling and controlled cooling process of "continuous casting billet heating → recrystallization zone + water cooling + two-phase zone special controlled rolling → ultra-fast cooling + air cooling and self-tempering special controlled cooling", the steel plate is improved in terms of strength, yield strength ratio, hardness, low temperature toughness and corrosion resistance; and the alloy content is low, and the alloy cost is low; furthermore, no additional heat treatment process is required, the production process is short, the cost is low, and the efficiency is high. BRIEF DESCRIPTION OF THE DRAWINGS

[0055] Figure 1 is a metallographic structure diagram of the finished steel plate of Example 1 of the present invention;

[0056] Figure 2 is a metallographic structure diagram of the finished steel plate of Example 3 of the present invention;

[0057] Figure 3 is a metallographic structure diagram of the finished steel plate of Example 5 of the present invention;

[0058] Figure 4 This is a metallographic structure diagram of the finished steel plate of Example 5 of the present invention, concentrated on non-metallic inclusions. DETAILED DESCRIPTION

[0059] 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.

[0060] The present invention provides a pipeline steel material with improved chemical composition, thereby achieving excellent corrosion resistance at a low cost. The steel material is particularly excellent in corrosion resistance for carbon dioxide transmission pipelines.

[0061] Specifically, the chemical composition of the steel includes, by mass percentage, C 0.026-0.071%, Si 0.11-0.17%, Mn 0.89-1.02%, P 0.0060-0.0099%, Cr 0.31-0.53%, Ni 0.09-0.23%, Mo 0.08-0.22%, Cu 0.15-0.33%, Nb 0.058-0.073%, Ti 0.010-0.022%, Al 0.022-0.049%, Mg 0.0005-0.0012%, Ca 0.0012-0.0042%, and the remainder is iron and unavoidable impurities.

[0062] The following is a detailed analysis of the main functions of each element and the selection of its dosage.

[0063] 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 steel plates, but excessive carbon content can lead to poor low-temperature toughness and weldability, reducing the steel plate's low-temperature drop hammer performance. 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 the present invention, the carbon content is controlled to 0.026-0.071%.

[0064] 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%.

[0065] Mn: Manganese plays a role in solid solution strengthening in steel, improving strength and hardness. A reasonable manganese content can ensure the strength of the steel plate at a low cost. As the manganese content increases, the strength of the steel plate increases significantly, while the ductile-brittle transition temperature hardly changes. Excessive 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 be 0.89~1.02%.

[0066] 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%.

[0067] 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 increases the microhardness of the steel plate and reduces low-temperature toughness. In one embodiment, the chromium content is controlled to 0.31-0.53%.

[0068] 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 self-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 between 0.09% and 0.23%.

[0069] Mo: Molybdenum significantly improves the hardenability of steel, increasing its strength and toughness. It also refines grains and enhances corrosion resistance. Molybdenum is a medium-strong carbide-forming element that combines with carbon, allowing more chromium to exist in the steel as a solid solution. This maximizes chromium's corrosion resistance and improves the utilization efficiency of the matrix alloying elements. Molybdenum delays ferrite transformation, resulting in an acicular ferrite structure, which contributes to the strength and toughness of the steel plate. However, excessive molybdenum content significantly increases alloy cost. Therefore, considering corrosion resistance, strength, low-temperature toughness, and alloy cost, the molybdenum content is controlled between 0.08% and 0.22%.

[0070] Cu: Copper promotes niobium precipitation and compensates for the strength loss caused by the decrease in carbon content. Adding a certain amount of nickel along 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 0.15-0.33%.

[0071] Nb: Niobium is a key grain-refining element in steel. During hot rolling, niobium strongly inhibits austenite recrystallization and its precipitation within the austenite, pinning austenite grain boundaries and refining the recrystallized grains. During cooling, dissolved niobium can continue to precipitate as niobium carbonitrides, significantly refining the resulting microstructure after phase transformation and 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 reduce the low-temperature toughness of the heat-affected zone (HAZ) of welded joints. Therefore, considering all factors, the niobium content is controlled between 0.058% and 0.073%.

[0072] Titanium: Titanium is a nitrogen-fixing element in steel. It forms dispersed titanium nitride particles, which inhibit austenite grain coarsening during billet heating and rolling. However, excessive addition can easily lead to the formation of coarse carbon / nitride precipitation in the core of the billet, affecting the low-temperature toughness of the steel plate. Therefore, the titanium content is controlled between 0.010% and 0.022%.

[0073] 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%.

[0074] Ca: can modify the morphology of sulfides and oxides and improve the toughness of steel; if added in excess, the beneficial effects cannot be equivalently improved; taking all factors into consideration, in one embodiment, the calcium content is controlled within a range of 0.0012-0.0042%.

[0075] Mg: Trace amounts of magnesium can modify Al2O3 into fine magnesium-aluminum spinel (MgO·Al2O3), improving the toughness of steel. However, excessive magnesium addition can produce excessive Al-Mg-O-Mn-S inclusions. These inclusions are difficult to disperse in molten steel and tend to aggregate and merge into larger inclusions, even forming chain-like inclusions, which is detrimental to improving low-temperature toughness. Research has shown that in one embodiment, the magnesium content is controlled between 0.0005% and 0.0012%. This present invention offers at least the following advantages over existing technologies:

[0076] (1) Reduce the C content in the steel and add appropriate amounts of carbide-forming elements Nb and Ti to reduce the carbonization of Cr, so that Cr exists in the steel in the form of a solid solution. In this way, the corrosion resistance effect can be fully exerted with less Cr addition;

[0077] (2) Cr is added in combination with Ni, Mo, and Cu, and P is used as a corrosion-resistant element, which makes the steel plate have excellent corrosion resistance in the use of carbon dioxide transportation.

[0078] (3) It does not contain expensive alloys such as V, and the 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. The integration of these factors makes the steel easy to produce, with low production costs and low alloy costs;

[0079] (4) On the basis of modifying the inclusions by using trace amounts of magnesium and calcium, it is further ensured that sufficient nucleation points are formed to promote the formation of soft sulfides and avoid the generation and agglomeration of long strips of sulfides, thereby increasing and optimizing the non-metallic inclusions with a core-shell structure and improving corrosion resistance and low-temperature toughness.

[0080] Based on the aforementioned chemical composition, the steel of the present invention exhibits excellent corrosion resistance. Specifically, at 20°C, a CO2 pressure of 1.0 MPa, and a liquid phase velocity of 1 m / s, the pipeline steel exhibits a uniform corrosion rate of 0.10 mm / year or less. This is unprecedented in the field of CO2 transport pipeline steel.

[0081] Even 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 ≤0.09 mm / year.

[0082] 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%.

[0083] Furthermore, the chemical composition of the steel, measured in mass percentage, satisfies the following conditions: 13 ≤ Mn / C ≤ 36. This allows for a low carbon content while maintaining a manganese-carbon ratio within the range of 13-36, significantly reducing segregation and improving the low-temperature toughness of the pipeline steel.

[0084] Furthermore, the chemical composition of the steel material, in terms of mass percentage, also satisfies:

[0085] CEV = [C] + [Mn] / 6 + ( [Cr] + [Mo]) / 5 + ( [Cu] + [Ni]) / 15 = 0.294 to 0.403; and / or,

[0086] Pcm=[C]+[Si] / 30+([Mn]+[Cu]+[Cr]) / 20+[Ni] / 60+[Mo] / 15=0.118~0.176.

[0087] 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.

[0088] Preferably, the contents of some impurity elements in the chemical composition of the steel are described as follows.

[0089] 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, S is controlled to below 0.0015%.

[0090] Considering that too low sulfur content will significantly increase steelmaking costs, the sulfur content is preferably controlled at 0.0009~0.0015%.

[0091] O: Oxygen is an impurity element in steel that easily forms various oxide inclusions. In one embodiment, the O content is controlled to be below 0.0025%.

[0092] Too low oxygen will increase steelmaking costs, so the oxygen content is preferably controlled at 0.0011~0.0025%.

[0093] Nitrogen: Nitrogen is an impurity element in steel. It easily forms brittle inclusions with edges and corners with Ti, Al, etc., reducing the plasticity and toughness of the steel plate. The nitrogen content should be controlled below 0.0055%.

[0094] Too low nitrogen content will increase steelmaking costs, so the nitrogen content is preferably controlled at 0.0025~0.0051%.

[0095] H: Hydrogen is an impurity element in steel that can easily cause hydrogen embrittlement in steel, leading to cracking in continuous casting slabs and steel plates. In one embodiment, H is controlled to be below 0.00020%.

[0096] Too low a hydrogen content will increase the cost of steelmaking, therefore, the hydrogen content is preferably controlled between 0.00005 and 0.00018%.

[0097] Furthermore, the number of inclusions with a size of ≥10 μm on the cross section of the steel is ≤10 / cm 2 .

[0098] 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.

[0099] More preferably, the maximum size of inclusions in the cross section of the steel material does not exceed 15 μm, and the average size of inclusions is 0.1-2 μm. This further reduces the adverse effects of inclusions on corrosion resistance and low-temperature toughness, improves corrosion resistance and low-temperature resistance, and ensures the steel's performance in carbon dioxide transportation projects.

[0100] Furthermore, part or all of the non-metallic inclusions in the steel material are of a core-shell structure, wherein the core is MgO·Al2O3 and the shell is MnS, CaS, TiN and is coated on the outer surface of the inner shell.

[0101] In one embodiment, more than 95% (or even more than 98%) of the non-metallic inclusions are of the core-shell structure.

[0102] The core MgO·Al2O3 is an insulating oxide with a large potential difference from the steel matrix (Fe), making it prone to forming microbatteries. Shell materials, such as TiN, have an electrode potential closer to that of the steel matrix, reducing the potential difference between inclusions and the matrix and weakening the driving force of galvanic corrosion. Furthermore, the low conductivity of the MnS / CaS shell can also reduce electron transfer at the interface, inhibiting localized corrosion.

[0103] In addition, the central segregation of the steel material is ≤ level 0.5.

[0104] 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.

[0105] Furthermore, the central porosity of the steel is ≤ level 0.5.

[0106] 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.

[0107] 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.

[0108] Furthermore, the A, B, C, and D inclusion ratings of the steel are all ≤ Level 1, and the sum of the A, B, C, and D inclusion ratings A+B+C+D ≤ Level 3.0.

[0109] 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.

[0110] Furthermore, the structure of the steel is a complex phase structure of quasi-polygonal ferrite+acicular ferrite+bainite.

[0111] The sum of the volume proportions of the quasi-polygonal ferrite structure and the acicular ferrite structure is greater than 70%, or preferably greater than 80%, and even more preferably greater than 90%.

[0112] Specifically, the volume proportion of the quasi-polygonal ferrite structure is 15-65%, the volume proportion of the acicular ferrite structure is 15-70%, and the volume proportion of the bainite structure is 5-30%.

[0113] This complex phase structure can ensure good matching of various mechanical properties of the steel, such as mechanical strength, low-temperature toughness, yield strength ratio, hardness, and drop hammer performance.

[0114] Furthermore, the average grain size of the quasi-polygonal ferrite and the acicular ferrite is 3-10 μm, thereby ensuring excellent low-temperature toughness of the steel.

[0115] Furthermore, the steel has excellent mechanical properties. For example, the yield strength R t0.5 ≥420MPa, tensile strength R m ≥560MPa, elongation A 50mm ≥30%.

[0116] For another example, the yield strength ratio R of the steel t0.5 / R m ≤0.82.

[0117] More specifically, the yield strength R t0.5 It is 420~570MPa.

[0118] Tensile strength R m It is 560~680MPa.

[0119] Elongation A 50mm It is 30~60%.

[0120] Yield strength ratio R t0.5 / R m It is 0.65~0.82.

[0121] Here, the steel can be sampled and mechanical properties tested in accordance with GB / T 2975-2018 "Sampling location and specimen preparation for mechanical properties test of steel and steel products" and GB / T 228.1-2021 "Tensile test of metallic materials Part 1: Room temperature test method".

[0122] The steel has excellent low temperature performance. Thus, the steel can be suitable for transporting supercritical or dense phase carbon dioxide and exhibit excellent toughness in low temperature environments.

[0123] For example, the steel has an impact energy KV2 of ≥400J at -20°C.

[0124] The -40°C impact energy KV2 of the steel is ≥380J.

[0125] The -60°C impact energy KV2 of the steel is ≥320J.

[0126] The -80°C impact energy KV2 of the steel is ≥250J.

[0127] The ductile-brittle transition temperature T t50%US ≤-100℃.

[0128] The -10°C DWTT drop weight shear area fraction of the steel is 100%.

[0129] The -20°C DWTT drop weight shear area fraction of the steel is ≥90%.

[0130] The -30°C DWTT drop weight shear area fraction of the steel is ≥80%.

[0131] The -20°C CTOD crack tip opening displacement of the steel is ≥1.5 mm.

[0132] Here, the steel 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".

[0133] In addition, the hardness of the steel is ≤210HV 10 .

[0134] For example, the hardness is 170~210HV 10 .

[0135] One embodiment of the present invention also provides a steel billet casting method for the steel material. Based on this casting method, a continuous casting billet with excellent structure can be obtained, including excellent inclusion characteristics, center segregation, center porosity, etc. of the continuous casting billet.

[0136] The casting method comprises KR desulfurization, converter smelting, LF refining, RH vacuum refining and continuous casting to prepare a continuous casting billet.

[0137] In KR desulfurization, magnesium powder and lime powder are mixed and sprayed into the molten iron to control the sulfur content in the molten iron to within 0.0012% by mass. The injection rate of magnesium powder is 0.35-0.45 kg per ton of molten iron.

[0138] During converter smelting: tapping temperature is controlled at 1645~1665℃, P≤0.006%;

[0139] LF refining: alloying and controlling the O content below 0.005%;

[0140] RH vacuum refining: No oxygen blowing during the whole process, under the condition of vacuum degree ≤2.5mbar, degassing for 18~23min, net circulation for 7~9min; then, at 4~5Nm 3 / h flow rate of argon blowing for 1-2 minutes, then vacuum static, and then break the vacuum to feed calcium wire to adjust the Ca / S mass ratio of the molten steel to 2-4 and the Ca / Al mass ratio to above 0.06; then static stirring for 2-3 minutes, add magnesium alloy to adjust the Mg mass ratio of the molten steel to 0.0005-0.0012%, and finally blow argon from the bottom;

[0141] Continuous casting: superheat is controlled at 26~32℃.

[0142] In this way, on the basis of sulfur control by KR, phosphorus control by converter, and oxygen control by LF, calcium treatment is first performed in RH vacuum refining to completely transform Al2O3 in the molten steel into liquid or semi-liquid calcium aluminate, thereby promoting the spheroidization of inclusions. Then, combined with the addition of trace magnesium, the liquid or semi-liquid calcium aluminate formed during static stirring is modified to form fine, spherical magnesium-aluminate spinel (MgO·Al2O3). In addition, this casting process can also form uniform and fine inclusions.

[0143] In addition, low superheat casting is helpful to reduce the segregation in the core of the ingot.

[0144] In summary, the casting method of the present application can not only improve the corrosion resistance and ensure low alloy cost based on chemical composition, but also improve the corrosion resistance and low-temperature toughness of steel from the aspects of inclusion characteristics and center improvement through inclusion modification and precise control of inclusion size and quantity, as well as center quality control.

[0145] Further preferably, in the KR desulfurization process, the outlet molten iron temperature is ≥1300°C and S is ≤0.0020%.

[0146] 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%.

[0147] Preferably, in the converter smelting process: the slag basicity is controlled at 1.9-2.1, and the total iron in the slag is 13-16%; the final slag basicity is controlled at 3.6-4.2, and the final slag total iron is controlled at 14-17%.

[0148] 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.

[0149] 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.

[0150] Further preferably, in the LF refining process: first deoxidation is performed by using a slag surface deoxidizer, argon is blown from the bottom of the ladle for strong stirring, alloying is then performed, and finally power is applied to control the tapping temperature to 1624±5°C.

[0151] 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.

[0152] 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.

[0153] 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.

[0154] 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.

[0155] Further preferably, in the RH vacuum refining process: the tapping ladle is left standing on the turntable for 12 to 16 minutes, and the tapping temperature is regulated so that the temperature of the start-casting furnace is 1587±5°C and the temperature of the continuous casting furnace is 1577±5°C.

[0156] In this way, letting the steel stand for 12 to 16 minutes before pouring can further promote the floating of inclusions, improve the purity of the molten steel, and greatly reduce the size and number of inclusions.

[0157] Further preferably, in the continuous casting process: secondary cooling electromagnetic stirring and dynamic soft reduction are adopted, the reduction amount 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℃.

[0158] In this way, the secondary cooling electromagnetic stirring and dynamic soft reduction technology are further adopted, especially the control of the inlet water temperature and the outlet water temperature difference (that is, the difference between the outlet water temperature and the inlet water temperature), to increase the equiaxed grain rate of the ingot, reduce the segregation and looseness in the core of the ingot, and improve the size and number of inclusions, ultimately laying the foundation for achieving excellent corrosion resistance and low-temperature toughness.

[0159] In addition, full-protection casting can be used in the continuous casting process.

[0160] 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.

[0161] 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.

[0162] 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.

[0163] 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.

[0164] Furthermore, the frequency of the secondary cooling electromagnetic stirring is 4.5~5.5Hz, and the current is 520~620A.

[0165] The taper of the continuous casting machine can be controlled to 1.15±0.1%.

[0166] The water volume of the crystallizer is 420~550L / min.

[0167] Next, the present invention also provides a steel plate rolling method of the steel material.

[0168] Based on the chemical composition, the rolling method prepares a steel plate with a thickness of t≥38 mm from a continuous casting billet with a thickness of 300-320 mm through heating, first hot rolling, water cooling, second hot rolling, ultra-fast cooling, and air cooling and self-tempering.

[0169] Among them, in the heating process: the soaking temperature is T NbC +(40~100)℃, soaking time is 30~50min.

[0170] 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.

[0171] 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.

[0172] In this way, by controlling the soaking temperature and duration, 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 in the subsequent first hot rolling process.

[0173] In the first hot rolling process: including multiple passes, the thickness of the rolled plate is (3.2~4.2)t, and the starting rolling temperature is T nr +(5~25)℃, final rolling temperature T nr +(0~20)℃.

[0174] 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.

[0175] 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.

[0176] 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.

[0177] In the water cooling process: after the steel plate leaves the hot rolling mill of the first hot rolling process, it enters the water cooling device for water cooling.

[0178] The final cooling temperature is Ar3-20℃~Ar3+15℃. That is, the water is discharged when the temperature of the steel plate is cooled to Ar3-20℃~Ar3+15℃.

[0179] After the first hot rolling, fine recrystallized grains are obtained. After a water cooling process, they are quickly cooled to below the recrystallization temperature through a water cooling device. On the one hand, this prevents the recrystallized grains from growing rapidly during the waiting process, which leads to the deterioration of the low-temperature toughness of the final steel plate; on the other hand, it reduces the waiting time and improves the rolling efficiency.

[0180] In the second hot rolling process, which includes multiple passes, the starting rolling temperature and the final rolling temperature are both Ar3-20℃~Ar3+15℃.

[0181] 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.

[0182] 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 ​​of Ar3 known to those skilled in the art based on the concept of Ar3 can be applied to the present invention.

[0183] 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;

[0184] On the other hand, the steel plate is rolled at the two-phase critical point, that is, the rolling deformation is carried out within 30°C around the starting point of the two-phase transformation. This can ensure that the deformation-induced ferrite content is not too high, which leads to a low yield strength, and not too low, which leads to an excessively high yield strength ratio. The deformation-induced ferrite content is kept within an optimal range.

[0185] 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. 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. The final rolling temperature control of the present application ensures low yield strength, high tensile strength, and high toughness.

[0186] In the ultra-fast cooling process: after the steel plate leaves the hot rolling mill of the second hot rolling process, it directly enters the ultra-fast cooling system for ultra-fast cooling without being coiled.

[0187] Water inlet temperature ≥A r3 -30℃, final cooling temperature is B s -(460~300)℃, cooling rate is 9~22℃ / s.

[0188] 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.

[0189] 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.

[0190] In this way, by controlling the cooling rate and the final cooling temperature, higher tensile strength and low yield ratio can be obtained, as well as excellent plate shape; if the cooling rate is too fast or too slow, martensite or pearlite phase will be formed, which is not conducive to improvement; if the final cooling temperature is too high, the bainite phase transformation will be incomplete; if the final cooling temperature is too low, the large temperature drop will be detrimental to plate shape control.

[0191] In the air-cooling autotempering process: the steel plate is air-cooled to room temperature on a cooling bed.

[0192] The temperature of the upper cooling bed may not be lower than 150°C.

[0193] In this way, after ultra-fast cooling, there is no need for coiling or other heat treatment, and internal stress can be eliminated or weakened by air cooling and self-tempering on the cooling bed. On the other hand, during the phase change process, pipeline steel plates produce MA hard phase, which is not conducive to the toughness of the steel plates and easily leads to high local hardness of the steel plates. Air cooling and self-tempering cooling can decompose MA, improve toughness and local hard points.

[0194] In summary, the rolling method of the present invention can achieve improvements in strength, yield strength ratio, hardness, low-temperature toughness and corrosion resistance of steel plates through the process of "recrystallization zone + water cooling + special controlled rolling in two-phase zone → ultra-fast cooling + air cooling and self-tempering special controlled cooling"; and has low alloy content and low alloy cost; furthermore, no additional heat treatment or other processes are required, the production process is short, the cost is low and the efficiency is high.

[0195] Preferably, the heating process includes preheating, first heating, second heating and soaking in sequence.

[0196] The preheating temperature is ≤950°C, the first heating temperature is 1050±15°C, and the second heating temperature is 1150±15°C. 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.

[0197] Preferably, the first hot rolling process includes several widening passes and several non-widening passes.

[0198] Among them, the reduction of the initial rolling pass without widening pass is ≥42mm.

[0199] The minimum reduction for each pass is ≥31mm.

[0200] In this way, when the total reduction 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.

[0201] Preferably, in the multiple passes of the second hot rolling process, the reduction in the last pass is ≤8 mm, and the reduction in the remaining passes is ≥22 mm.

[0202] 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.

[0203] Preferably, when ultra-fast cooling is performed on the ultra-fast cooling system, the water pressure can be controlled to be 0.20~0.50MPa, the upper and lower water ratio can be controlled to be 0.91~0.95, and the roller speed can be controlled to be 0.6~1.4m / s.

[0204] Furthermore, the rolling method also includes a stacking process.

[0205] That is, the continuous casting strands are directly stacked while still warm after leaving the continuous casting machine.

[0206] The stacking temperature is ≥500℃, that is, stacking begins after the continuous casting billet leaves the continuous casting machine and before the temperature cools down to 500℃.

[0207] The destacking temperature is 230~280℃.

[0208] 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.

[0209] Next, a first preferred embodiment of the present invention will be described.

[0210] In a first embodiment, the chemical composition of the steel comprises, 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%, Mg 0.0005-0.0012%, Ca 0.0012-0.0042%, and the remainder is iron and unavoidable impurities.

[0211] CEV = 0.294-0.380. Preferably, CEV = 0.294-0.337. More preferably, CEV = 0.333-0.337.

[0212] Pcm = 0.125-0.176. Preferably, Pcm = 0.142-0.176. More preferably, Pcm = 0.142-0.153.

[0213] The starting and finishing temperatures of the second hot rolling process are both Ar3-20℃~Ar3.

[0214] The final cooling temperature of the ultra-fast cooling process is B s -(460~400)℃.

[0215] The roller speed of the ultra-fast cooling process is 0.6~1.0m / s.

[0216] Furthermore, in the first embodiment, the structure of the steel plate is a duplex structure of quasi-polygonal ferrite+acicular ferrite+bainite.

[0217] 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%.

[0218] The average grain size of quasi-polygonal ferrite and acicular ferrite is 5~10μm.

[0219] Furthermore, the yield strength R t0.5 ≥420MPa, tensile strength R m ≥560MPa, elongation A 50mm ≥32%.

[0220] For example, the yield strength ratio R of the steel plate t0.5 / R m ≤0.72.

[0221] More specifically, the yield strength R t0.5 It is 420~500MPa.

[0222] Tensile strength R m It is 560~650MPa.

[0223] Elongation A 50mm It is 32~60%.

[0224] Yield strength ratio R t0.5 / R m It is 0.65~0.72.

[0225] The hardness of the finished steel plate is ≤200HV 10 .

[0226] For example, the hardness is 170~200HV 10 .

[0227] Next, a second preferred embodiment of the present invention will be described.

[0228] In a second embodiment, the chemical composition of the steel comprises, 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%, Mg 0.0005-0.0012%, Ca 0.0012-0.0042%, and the remainder is iron and unavoidable impurities.

[0229] CEV = 0.303-0.389. Preferably, CEV = 0.343-0.350.

[0230] Pcm = 0.121-0.172. Preferably, Pcm = 0.142-0.172. More preferably, Pcm = 0.142-0.149.

[0231] The starting and finishing temperatures of the second hot rolling process are both Ar3-15°C~Ar3+5°C.

[0232] The final cooling temperature of the ultra-fast cooling process is B s -(360~320)℃.

[0233] The roller speed of the ultra-fast cooling process is 1.0~1.4m / s.

[0234] In the second embodiment, the structure of the steel plate is a duplex structure of quasi-polygonal ferrite+acicular ferrite+bainite.

[0235] Specifically, 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%.

[0236] The average grain size of quasi-polygonal ferrite and acicular ferrite is 4~9μm.

[0237] Furthermore, the yield strength R of the finished steel plate t0.5 ≥460MPa, tensile strength R m ≥580MPa, elongation A 50mm ≥31%, yield strength ratio R t0.5 / R m ≤0.80.

[0238] Furthermore, the yield strength R t0.5 It is 460~540MPa.

[0239] Tensile strength R m It is 580~660MPa.

[0240] Elongation A 50mm It is 31~58%.

[0241] Yield strength ratio R t0.5 / R m It is 0.70~0.80.

[0242] The hardness of the finished steel plate is ≤205HV 10 .

[0243] For example, the hardness is 175~205HV 10 .

[0244] Next, a third preferred embodiment of the present invention will be described.

[0245] In a third embodiment, the chemical composition of the steel comprises, by mass percentage, C 0.026-0.056%, Si 0.11-0.17%, Mn 0.94-1.02%, P 0.0060-0.0099%, Cr 0.45-0.53%, Ni 0.09-0.17%, Mo 0.14-0.22%, Cu 0.15-0.23%, Nb 0.065-0.073%, Ti 0.014-0.022%, Al 0.022-0.049%, Mg 0.0005-0.0012%, Ca 0.0012-0.0042%, and the remainder is iron and unavoidable impurities.

[0246] CEV = 0.317-0.403. Preferably, CEV = 0.347-0.403. More preferably, CEV = 0.347-0.361.

[0247] Pcm = 0.118-0.168. Preferably, Pcm = 0.137-0.150. More preferably, Pcm = 0.137-0.146.

[0248] The starting and finishing temperatures of the second hot rolling process are both Ar3-5°C~Ar3+15°C.

[0249] The final cooling temperature of the ultra-fast cooling process is B s -(340~300)℃.

[0250] The roller speed of the ultra-fast cooling process is 0.8~1.2m / s.

[0251] In a third embodiment, the structure of the steel plate is a duplex structure of quasi-polygonal ferrite+acicular ferrite+bainite.

[0252] Specifically, the volume proportion of the quasi-polygonal ferrite structure is 15-25%, the volume proportion of the acicular ferrite structure is 60-70%, and the volume proportion of the bainite structure is 5-15%.

[0253] The average grain size of quasi-polygonal ferrite and acicular ferrite is 3~6μm.

[0254] Yield strength R of finished steel plate t0.5 ≥490MPa, tensile strength R m ≥600MPa, elongation A 50mm ≥30%, yield strength ratio R t0.5 / R m ≤0.82.

[0255] Furthermore, the yield strength R t0.5 It is 490~570MPa.

[0256] Tensile strength R m It is 600~680MPa.

[0257] Elongation A 50mm It is 30~56%.

[0258] Yield strength ratio R t0.5 / R m It is 0.75~0.82.

[0259] The hardness of the finished steel plate is ≤210HV 10 .

[0260] For example, the hardness is 180~210HV 10 .

[0261] The above describes the technical purpose of the present invention and the basic conditions of each embodiment. The following describes several experimental examples of the present invention. Of course, these experimental examples are only a part of the many variations of the present invention, not all of them.

[0262] The implementation process of these test cases is as follows:

[0263] (1) A continuous casting billet is prepared by KR desulfurization, converter smelting, LF refining, RH vacuum refining and continuous casting; the specific process operation is as described above;

[0264] The chemical composition of the continuous casting slab is shown in Table 1;

[0265] The results of the continuous casting slab thickness, center mass and inclusion detection are shown in Table 2;

[0266] [Table 1]

[0267]

[0268] [Table 2]

[0269]

[0270] (2) without coiling and additional heat treatment, the finished steel plate is prepared by stacking, heating, first hot rolling, water cooling, second hot rolling, ultra-fast cooling and air cooling and self-tempering;

[0271] Some important process parameters are shown in Table 3.

[0272] [Table 3]

[0273]

[0274] [Table 3 continued]

[0275]

[0276] The steel plates of each test case were tested for structure and performance, and the test results are as follows:

[0277] (1) All of them are duplex structures of quasi-polygonal ferrite + acicular ferrite + bainite; the metallographic structures of test cases 1, 3, and 5 are respectively Figure 1 、 2 , 3; Among them, the average grain size of quasi-polygonal ferrite and acicular ferrite, and the volume proportion of each structure in the complex phase structure are shown in Table 4 respectively;

[0278] (2) In the steel plates of each test example, some or all of the non-metallic inclusions are of a core-shell structure, where the core is MgO·Al2O3 and the shell is MnS, CaS, TiN and is coated on the surface of the inner shell; for example, Figure 4 The metallographic structure of the steel plate of Test Example 5 at a core-shell structure inclusion is shown;

[0279] (3) For the steel plates of each test case, the impact energy KV2 at -20℃ is ≥400J, the impact energy KV2 at -40℃ is ≥380J, the impact energy KV2 at -60℃ is ≥320J, the impact energy KV2 at -80℃ is ≥250J, and the 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;

[0280] [Table 4]

[0281]

[0282] (4) 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 steel plate is also shown in Table 4;

[0283] (5) The strength, elongation, hardness, etc. of the steel plates of each test example are shown in Table 4.

Claims

1. A carbon dioxide pipeline steel, characterized in that: The chemical composition of the pipeline steel includes, by mass percentage, C 0.026-0.071%, Si 0.11-0.17%, Mn 0.89-1.02%, P 0.0060-0.0099%, Cr 0.31-0.47%, Ni 0.09-0.23%, Mo 0.08-0.22%, Cu 0.15-0.33%, Nb 0.058-0.073%, Ti 0.010-0.022%, Al 0.022-0.049%, Mg 0.0005-0.0012%, Ca 0.0012-0.0042%, and the remainder is iron and unavoidable impurities; The microstructure of the pipeline steel is a duplex microstructure of quasi-polygonal ferrite + acicular ferrite + bainite; The total volume proportion of the quasi-polygonal ferrite structure and the acicular ferrite structure is more than 70%, the volume proportion of the quasi-polygonal ferrite structure is 15-65%, the volume proportion of the acicular ferrite structure is 15-70%, and the volume proportion of the bainite structure is 5-30%; Part or all of the non-metallic inclusions in the pipeline steel form a core-shell structure, wherein the core is MgO·Al2O3 and the shell is MnS, CaS, TiN and is coated on the surface of the inner shell; 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.

2. The carbon dioxide pipeline steel 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 , the maximum size of inclusions does not exceed 15μm, and the average size of inclusions is 0.1~2μm.

3. The carbon dioxide pipeline steel 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 ≥30%, yield strength ratio R t0.5 / R m ≤0.

82.

4. The carbon dioxide pipeline steel according to claim 1, characterized in that The pipeline steel has an impact energy KV2 of 400J or more at -20°C, 380J or more at -40°C, 320J or more at -60°C, and 250J or more at -80°C.

5. The carbon dioxide pipeline steel according to claim 1, characterized in that: The ductile-brittle transition temperature T t50%US ≤-100℃.

6. The carbon dioxide pipeline steel according to claim 1, characterized in that The pipeline steel has a -10°C DWTT drop weight shear area fraction of 100%, a -20°C DWTT drop weight shear area fraction ≥90%, a -30°C DWTT drop weight shear area fraction ≥80%, and a -20°C CTOD crack tip opening displacement ≥1.5 mm.

7. The carbon dioxide pipeline steel according to claim 1, characterized in that The hardness of the pipeline steel is ≤210HV 10 .

8. The carbon dioxide pipeline steel according to claim 1, characterized in that: The average grain size of quasi-polygonal ferrite and acicular ferrite is 3~10μm.

9. The carbon dioxide pipeline steel according to claim 1, 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 = 0.294 to 0.403; and / or, Pcm=[C]+[Si] / 30+([Mn]+[Cu]+[Cr]) / 20+[Ni] / 60+[Mo] / 15=0.118~0.

176.

10. A method for casting a carbon dioxide pipeline steel billet according to any one of claims 1 to 9, characterized in that: The casting method includes KR desulfurization, converter smelting, LF refining, RH vacuum refining and continuous casting to prepare a continuous casting billet; In KR desulfurization, magnesium powder and lime powder are mixed and sprayed into the molten iron to control the sulfur content in the molten iron to within 0.0012% by mass. The injection rate of magnesium powder is 0.35-0.45 kg per ton of molten iron. During converter smelting: tapping temperature is controlled at 1645~1665℃, P≤0.006%; LF refining: alloying and controlling the O content below 0.005%; RH vacuum refining: No oxygen blowing during the whole process, under the condition of vacuum degree ≤2.5mbar, degassing for 18~23min, net circulation for 7~9min; then, at 4~5Nm 3 / h flow rate of argon blowing for 1-2 minutes, then vacuum static, and then break the vacuum to feed calcium wire to adjust the Ca / S mass ratio of the molten steel to 2-4 and the Ca / Al mass ratio to above 0.06; then static stirring for 2-3 minutes, add magnesium alloy to adjust the Mg mass ratio of the molten steel to 0.0005-0.0012%, and finally blow argon from the bottom; Continuous casting: superheat is controlled at 26~32℃.

11. The casting method of carbon dioxide pipeline steel billet according to claim 10, characterized in that: KR desulfurization: molten iron temperature outgoing from the station ≥1300℃, S ≤0.0020%.

12. The casting method of carbon dioxide pipeline steel billet according to claim 10, characterized in that: Converter smelting: slag basicity is controlled at 1.9~2.1, and the total iron content in the slag is 13~16%; final slag basicity is controlled at 3.6~4.2, and the total iron content in the final slag is controlled at 14~17%.

13. The casting method of carbon dioxide pipeline steel billet according to claim 10, characterized in that: 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.

14. The method for casting a carbon dioxide pipeline steel billet according to claim 10, wherein: RH vacuum refining: The steel ladle is left on the turntable for 12 to 16 minutes, and the tapping temperature is adjusted so that the temperature of the open-cast furnace is 1587±5℃ and the temperature of the continuous-casting furnace is 1577±5℃.

15. The method for casting a carbon dioxide pipeline steel billet according to claim 10, wherein: Continuous casting: secondary cooling electromagnetic stirring and dynamic soft reduction are used, with the reduction amount being 4~7% of the continuous casting billet thickness. The water volume of the continuous casting machine's crystallizer is 420~550L / min, the water inlet temperature is 25~40℃, and the water outlet temperature difference is 4~10℃.

16. A method for rolling steel plates for carbon dioxide pipeline steel, characterized in that: The chemical composition of the pipeline steel includes, by mass percentage, C 0.026-0.071%, Si 0.11-0.17%, Mn 0.89-1.02%, P 0.0060-0.0099%, Cr 0.31-0.53%, Ni 0.09-0.23%, Mo 0.08-0.22%, Cu 0.15-0.33%, Nb 0.058-0.073%, Ti 0.010-0.022%, Al 0.022-0.049%, Mg 0.0005-0.0012%, Ca 0.0012-0.0042%, and the remainder is iron and unavoidable impurities; The rolling method comprises heating, first hot rolling, water cooling, second hot rolling, ultra-fast cooling, air cooling and self-tempering of a continuous casting billet with a thickness of 300-320 mm to prepare a steel plate with a thickness of t≥38 mm; During the heating process: the soaking temperature is T NbC +(40~100)℃, soaking time is 30~50min; T NbC is the starting precipitation temperature of NbC; In the first hot rolling process: the thickness of the rolled plate is (3.2~4.2)t, and the starting rolling temperature is T nr +(5~25)℃, final rolling temperature T nr +(0~20)℃;T nr Indicates the minimum temperature of austenite recrystallization; In the water cooling process: the final cooling temperature is Ar3-20℃~Ar3+15℃; In the second hot rolling process: the starting and finishing temperatures are both Ar3-20℃~Ar3+15℃; In the ultra-fast cooling process: water inlet temperature ≥A r3 -30℃, final cooling temperature is B s -(460~300)℃, cooling rate is 9~22℃ / s; B s is the temperature at which the bainite transformation begins; In the air-cooling autotempering process: the steel plate is air-cooled to room temperature on the cooling bed.

17. The method for rolling steel plates for carbon dioxide pipeline steel according to claim 16, wherein: The rolling method further comprises a stacking process before the heating process: The continuous casting billets are stacked warm, with a stacking temperature ≥ 500℃ and a destacking temperature of 230~280℃.

18. The method for rolling steel plates for carbon dioxide pipeline steel according to claim 16, wherein: In the first hot rolling process: The reduction of the initial rolling pass of the non-widening pass is ≥42mm, and the minimum reduction of each pass is ≥31mm.

19. The method for rolling a steel plate for a carbon dioxide pipeline according to claim 16, wherein: In the second hot rolling process: The reduction of the last pass is ≤8mm, and the reduction of the remaining passes is ≥22mm.

20. The method for rolling steel plates for carbon dioxide pipeline steel according to claim 16, wherein: The starting and finishing temperatures of the second hot rolling process are both Ar3-20℃~Ar3, and the finishing temperature of the ultra-fast cooling process is B s -(360~320)℃; or, The starting and finishing temperatures of the second hot rolling process are both Ar3-15℃~Ar3+5℃, and the finishing temperature of the ultra-fast cooling process is B s -(360~320)℃; or, The starting and finishing temperatures of the second hot rolling process are both Ar3-5℃~Ar3+15℃, and the finishing temperature of the ultra-fast cooling process is B s -(340~300)℃.

21. The method for rolling a steel plate for a carbon dioxide pipeline according to claim 16, wherein: 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 = 0.294 to 0.403; and / or, Pcm=[C]+[Si] / 30+([Mn]+[Cu]+[Cr]) / 20+[Ni] / 60+[Mo] / 15=0.118~0.

176.

22. The method for rolling a steel plate for a carbon dioxide pipeline according to claim 16, wherein: The number of inclusions with a size of ≥10 μm on the cross section of the pipeline steel is ≤10 / cm 2 , the maximum size of inclusions does not exceed 15μm, and the average size of inclusions is 0.1~2μm.

23. The method for rolling steel plates for carbon dioxide pipeline steel according to claim 16, wherein: Part or all of the non-metallic inclusions in the pipeline steel form a core-shell structure, wherein the core is MgO·Al2O3 and the shell is MnS, CaS, TiN and is coated on the outer surface of the inner shell.

24. The method for rolling a steel plate for a carbon dioxide pipeline according to claim 16, wherein: The microstructure of the pipeline steel is a complex phase structure of quasi-polygonal ferrite + acicular ferrite + bainite; the total volume proportion of the quasi-polygonal ferrite structure and the acicular ferrite structure is more than 70%.

25. The method for rolling steel plates for carbon dioxide pipeline steel according to claim 24, characterized in that: The volume proportion of quasi-polygonal ferrite structure is 15~65%, the volume proportion of acicular ferrite structure is 15~70%, and the volume proportion of bainite structure is 5~30%.

26. The method for rolling steel plates for carbon dioxide pipeline steel according to claim 24, wherein: The average grain size of quasi-polygonal ferrite and acicular ferrite is 3~10μm.

27. The method for rolling a steel plate for a carbon dioxide pipeline according to claim 16, wherein: 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.

28. The method for rolling a steel plate for a carbon dioxide pipeline according to claim 16, wherein: The yield strength R of the pipeline steel t0.5 ≥420MPa, tensile strength R m ≥560MPa, elongation A 50mm ≥30%, yield strength ratio R t0.5 / R m ≤0.82, -40℃ impact energy KV2≥380J.

29. The method for rolling a steel plate for a carbon dioxide pipeline according to claim 16, wherein: The pipeline steel has an impact energy of KV2≥400J at -20℃, 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℃.

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