Low-density high-strength corrosion-resistant coiled tubing steel and manufacturing method thereof

Abstract

The invention discloses steel for a low-density high-strength corrosion-resistant continuous pipe and a manufacturing method of the steel. The steel comprises the following chemical components in percentage by weight: 6.0 to 9.0 percent of Al, 8.0 to 12.0 percent of Mn, 1.10 to 1.50 percent of Si, 1.0 to 3.0 percent of Cr, 0.12 to 0.25 percent of C, 0.20 to 0.50 percent of Cu, 0.10 to 0.50 percent of Ni, 0.20 to 0.70 percent of Mo, 0.02 to 0.15 percent of V, 0.10 to 0.35 percent of Ti, 0.02 to 0.09 percent of Nb, less than or equal to 0.01 percent of P, less than or equal to 0.006 percent of S and the balance of Fe and inevitable impurities. The steel manufacturing method comprises the steps of smelting, casting, homogenization treatment, hot rolling, cooling, coiling, stress relief annealing and sand blasting derusting. The manufacturing method of the continuous pipe comprises the following steps: preparing a rolled steel plate, longitudinally shearing the rolled plate, lengthening a steel belt, forming and welding, and performing off-line heat treatment. Alloy elements such as Al, Si, Mn and Cr are reasonably added into steel, and a manufacturing process matched with the components is provided, so that the low-density, high-strength and corrosion-resistant coiled tubing is manufactured, and the safety, economical efficiency and practicability of the coiled tubing serving in a complex working condition environment are effectively guaranteed.

Description

Technical Field

[0001] This invention relates to the field of oil and gas pipeline technology, and in particular to a low-density, high-strength, corrosion-resistant steel for continuous pipelines, its manufacturing method, and the manufacturing method and application of the continuous pipeline thereon. Background Technology

[0002] Coiled tubing (CT) is an oil and gas pipeline made of several steel strips joined together, formed and welded, and can be wound on a large-diameter drum. It can be continuously and efficiently run in and out of oil wells and is used in oilfield workover, drilling, well completion, logging, production enhancement, oil production, and gas production.

[0003] With the increasing number of deep, ultra-deep, and ultra-long horizontal wells developed in my country, higher requirements are being placed on the single-disc length of coiled tubing. However, limitations imposed by the maximum load capacity of road transport trucks and the inconvenient transportation conditions caused by the mountainous and winding roads in the Sichuan-Chongqing region restrict the length of single-disc coiled tubing, making it difficult to meet the exploration and development requirements of deep, ultra-deep, and ultra-long horizontal wells. If the density of the coiled tubing material is reduced, the single-disc length of the coiled tubing can be effectively increased under the same transportation or load-bearing conditions. Therefore, the market demand for lightweight coiled tubing is very urgent. Summary of the Invention

[0004] The density of existing carbon steel continuous tubing is approximately 7.85 g / cm³. 3 This invention is based on the continuous tube manufacturing process. By rationally adjusting lightweight alloying elements such as Al, Si, and Mn into the steel, they are dissolved in the steel to form a substitutional solid solution. This reduces the molar mass while increasing the lattice constant, thereby effectively reducing the density of the steel and achieving lightweight continuous tube material. This results in the manufacture of low-density, corrosion-resistant continuous tubes, which are convenient for transportation in the mountainous areas of Sichuan and Chongqing. Under the same transportation or load-bearing conditions, the single-disc length of the continuous tube can be effectively increased to meet the needs of ultra-deep oil and gas development. It is also suitable for the lightweight equipment requirements of continuous tube marine operations.

[0005] The first aspect of this invention provides a low-density corrosion-resistant special steel suitable for continuous tube manufacturing processes and a method for preparing the same; the second aspect provides a low-density corrosion-resistant continuous tube and a method for preparing the same. The technical solutions adopted are as follows:

[0006] The chemical composition (by mass percentage) of a low-density, high-strength, corrosion-resistant continuous pipe steel is as follows:

[0007] C: 0.12-0.25%; Al: 6.0-9.0%; Mn: 8.0-12.0%; Si: 1.10-1.50%; Cu: 0.20-0.50%; Cr: 1.0-3.0%; Ni: 0.10-0.50%; Mo: 0.20-0.70%; V: 0.02-0.15%; Ti: 0.10-0.35%; Nb: 0.02-0.09%; P: ≤0.01%; S: ≤0.006%, with the remainder being Fe and unavoidable impurities.

[0008] The preferred weight percentage of Al is 7.0%-8.5%; the preferred weight percentage of Mn is 9.5%-11.5%; the preferred weight percentage of Si is 1.20%-1.40%; and the preferred weight percentage of Cr is 1.20%-2.00%.

[0009] The main functions of each alloying element are as follows:

[0010] Carbon (C) is a strengthening element in steel, but excessive carbon content has adverse effects on the toughness, plasticity, and weldability of pipes, increasing cold brittleness and aging sensitivity. The design range of C in this invention is 0.12-0.25%.

[0011] Aluminum (Al) is the most important lightweight element in this invention, but excessive Al content will worsen the hot workability of steel, causing cracking of the steel plate during hot rolling, reducing the weldability of the steel, and increasing the brittleness of the material. The Al content in this invention is designed to be 6.0-9.0%.

[0012] Manganese (Mn) can improve the strength and plasticity of steel. However, with increasing Mn content, the tendency of steel billets to crack during rapid heating and cooling increases, while also reducing the weldability of the steel. The designed Mn content in this invention is 8.0-12.0%.

[0013] Silicon (Si) is an important lightweight element in this invention. However, when the Si content is too high, dephosphorization becomes more difficult, and the surface iron oxide scale formed by Si in the heating furnace is hard to remove, resulting in poor surface finish. The Si content in this invention is designed to be in the range of 1.1-1.50%.

[0014] Copper (Cu) enhances the strength, hardenability, and high-temperature stability of steel through solid solution strengthening. However, excessive Cu addition negatively impacts the low-temperature toughness and hot workability of steel. Furthermore, excessive Cu can form the CuAl B2 phase with Al, reducing the steel's ductility and toughness. In this invention, the Cu content is designed to be 0.20-0.50%.

[0015] Chromium (Cr) is an important element for enhancing corrosion resistance in this invention; however, high chromium content can easily lead to defects such as inclusions and gray plating during welding, as well as hardened structures that are difficult to eliminate. The design range for Cr in this invention is 1.0-3.0%.

[0016] Nickel (Ni) is an element that improves hardenability, but Ni is a precious metal and therefore expensive. In this invention, the designed Ni content is 0.10-0.50%.

[0017] Molybdenum (Mo) is the most effective H2S resistant element and can improve the passivation and corrosion resistance of steel, but it is expensive. The design range of Mo in this invention is 0.20-0.70%.

[0018] V, Nb, Ti: Vanadium (V), titanium (Ti), and niobium (Nb) can refine grains, but when their content is too high, the effect is not significant and the cost of the pipe increases. In this invention, the design range of V is 0.02-0.15%, the design range of Ti is 0.10-0.35%, and the design range of Nb is 0.02-0.09%.

[0019] P and S: Phosphorus (P) and sulfur (S) are both harmful elements that easily produce inclusions such as FeS and MnS, as well as P eutectics, reducing the toughness of steel plates and the bending fatigue resistance of continuous pipes. In this invention, the P content is controlled below 0.01%, and the S content is targeted to be controlled below 0.006%.

[0020] The preparation method of low-density, high-strength, corrosion-resistant continuous pipe steel is as follows:

[0021] Smelting and casting: Molten iron is used as the main raw material and undergoes pre-desulfurization treatment. After smelting in a top and bottom blown converter, LF heating desulfurization, VD vacuum decarburization, and Ca treatment, the composition and mass percentage of the molten steel meet the above requirements and can be tapped. The steel is then cast into continuous casting billets, and electromagnetic stirring is used to control the casting of 80-200mm thick slabs.

[0022] Homogenization treatment: The slab is subjected to high-pressure water descaling before entering the furnace; the slab is slowly heated to 1250±20℃ and held for 2 hours before being taken out of the furnace (furnace time ≥4 hours) for homogenization treatment; the homogenized slab is subjected to a second high-pressure water descaling; the descaling water pressure is 50±5MPa; the descaling temperature of the slab is ≥1180℃ to ensure the removal of low-melting-point FeO·Fe2SiO4 eutectic compounds.

[0023] Hot rolling: Rolling is controlled using a roller mill. The roughing end temperature is controlled at 1050-1160℃, and the slab thickness after roughing is controlled at 40-65mm. The finishing rolling start temperature is controlled at 960-1020℃, and the finish temperature is controlled at 860-920℃, rolling it into a hot-rolled plate with a thickness of 2.4-6.5mm, a width of 1000-1500mm, and a length greater than 300m.

[0024] Cooling and winding: The hot-rolled plate is subjected to laminar flow cooling at a rate of 30-40℃ / s and a winding temperature of 600-650℃.

[0025] Stress-relief annealing: Hold the wound material at 450-550℃ for 10-20 minutes to perform stress-relief annealing.

[0026] Sandblasting for rust removal: Due to the high content of Al and Si elements in the design composition, oxidation and decarburization are prone to occur. Therefore, the design adopts a water washing and sandblasting process to fully remove the oxide scale on the surface of the coil. The steps include: uncoiling - straightening and leveling - water washing - sandblasting - water washing - drying - electrostatic oiling - coiling, and then coiling into a roll to obtain the continuous low-density corrosion-resistant coil for pipes.

[0027] The preparation method of low-density corrosion-resistant continuous tubing is as follows:

[0028] (1) Uncoil the sheet, level it, and slit it longitudinally to obtain steel strip;

[0029] (2) The steel strip is welded together at the beginning and end to form a continuous steel strip;

[0030] (3) The continuous strip steel is continuously formed, welded, and heat-treated for weld seam, and then rolled to obtain a continuous tube blank;

[0031] (4) The entire roll of continuous tube blank is subjected to offline heat treatment, non-destructive testing, and tube rewinding to obtain a low-density corrosion-resistant continuous tube.

[0032] According to the present invention, in step (1) above, after the coil is uncoiled and leveled, it is divided into multiple steel strips with a width of 80-440mm by a slitting machine according to the corresponding requirements of the continuous tube specifications. The slitting machine uses high hardness and high strength hard alloy material shear blades to ensure the edge quality of the steel strips after slitting.

[0033] According to the present invention, in step (2) above, the ends of the two steel strips to be welded are beveled at 40°-50°. The beveled end face is reasonably processed according to the thickness of the steel strip. The bevel is I-shaped, V-shaped or U-shaped and then assembled and spliced. The steel strip welding needs to be carried out under the protection of an inert atmosphere. Two methods can be used: no filler wire welding and filler wire welding. No filler wire welding can be laser welding, friction stir welding or electron beam welding; filler wire welding can be plasma welding, TIG welding (tungsten inert gas welding), MIG welding (metal inert gas welding), etc. The strength of the welding wire used for butt welding is matched with the base material of the steel strip with weak strength or equal strength. After welding, the butt weld is rapidly heated to 880-1020°C and the weld is rolled or forged with a pressure of 6-12MPa to further refine the weld grain. The above heating temperature is maintained and the butt weld is held at the temperature for 1-3 minutes under the protection of an inert atmosphere. Then, it is rapidly cooled with water and the weld reinforcement is ground and non-destructively inspected. The "hot deformation + solution treatment" of the weld increases the diffusion rate of alloy elements, reduces the uneven distribution of weld composition, reduces residual stress in the weld joint, optimizes the weld microstructure, refines weld grains, and improves the quality of oblique weld joints. The above strip butt welding process is repeated to form a continuous strip, which is then coiled into a strip storage tray. The length of the butt welded strip depends on the required continuous pipe length.

[0034] According to the present invention, in step (3) above, based on the requirements of the outer diameter and wall thickness of the final continuous tube, before forming, the side of the continuous strip is milled into an I-shaped bevel to remove burrs from the side of the strip and precisely control the consistency of the width of the extended strip; oil stains on the side of the strip are removed with cleaning agents such as alcohol to ensure the quality of subsequent continuous forming and welding; the strip forming is controlled by a roller forming method, and the tube blank is welded using laser welding technology. Nitrogen gas with a purity ≥99.7% is used for protection during the tube blank welding process; the forming bevel gap is controlled to be ≤0.1mm, and the decoking amount is - Welding is performed continuously at a laser power of 2-16Kw and a welding speed of 1.5-10m / min. After welding, the weld is heat-treated by medium-frequency induction heating. Under inert atmosphere protection, the weld is rapidly heated to 900-960℃, held for 20-100s, and then air-cooled or spray-cooled to ≤250℃. The burrs on the weld are removed by scraping or grinding. The tube blank is continuously wound onto the production drum by a coiler to obtain a continuous tube blank with a diameter of Φ25.4-Φ139.7mm and a wall thickness of 2.4-6.5mm.

[0035] According to the present invention, in step (4) above, the entire roll of continuous tube blank is unwound and straightened on an offline heat treatment line. The tube blank is rapidly heated to 795-880°C at a rate of 20-50°C / s using a medium-frequency heating furnace. After being held in a muffle furnace for 3-5 minutes, the tube blank is quenched to room temperature using water or oil medium. The surface of the tube body is dried with cold air, rapidly heated to 300-500°C using a medium-frequency furnace, held in a muffle furnace for 3-10 minutes, air-cooled to <150°C, and then water-cooled. After non-destructive testing and surface coating with an anti-corrosion agent, it is wound onto a roll suitable for transportation to obtain a low-density corrosion-resistant continuous tube of the required length.

[0036] The aforementioned low-density corrosion-resistant continuous tubing has the following properties: yield strength 621-965 MPa (90-140 ksi), tensile strength 689-1102 MPa (100-160 ksi), elongation ≥22%, and hardness ≤42 HRC. The continuous tubing geometries include: constant wall thickness continuous tubing and variable wall thickness continuous tubing, with an outer diameter range of Φ25.4-Φ139.7 mm, a wall thickness range of 2.4-6.5 mm, and a length range of 100-12000 m. It can be wound onto drums of appropriate core diameter for transportation and use.

[0037] The aforementioned low-density corrosion-resistant continuous tubing has a density of 6.9-7.2 g / cm³. 3 Compared with carbon steel continuous pipe, the density is reduced by more than 8%; the pipe structure is composed of ferrite, martensite and retained austenite, of which the volume fraction of ferrite is 50%-65%, the volume fraction of martensite is 20%-40%; the remainder is retained austenite.

[0038] The aforementioned low-density corrosion-resistant continuous tubing contains high levels of Al and Cr, exhibiting good resistance to CO2 corrosion and Cl-. - Its corrosion resistance is more than twice that of conventional continuous tubes; it also exhibits good resistance to hydrogen-induced cracking. In the hydrogen-induced cracking (HIC) test, after immersion in a saturated H2S solution (A solution) for 96 hours, the sample showed no cracking or hydrogen blistering. In the stress corrosion cracking (SCC) test, with a stress load of 80% of the nominal yield strength, after immersion in a saturated H2S solution (A solution) for 720 hours, the sample showed no cracking.

[0039] The advantages of this invention are:

[0040] 1. This invention involves the rational addition of lightweight alloying elements such as Al, Si, and Mn to steel during the steelmaking process. These elements dissolve in the steel to form a substitutional solid solution, thereby reducing the molar mass while increasing the lattice constant. The resulting steel density is reduced to 6.9-7.2 g / cm³. 3 Compared to carbon steel continuous tubes, the density is reduced by more than 8%, thus producing low-density continuous tubes.

[0041] 2. The low-density, high-strength, corrosion-resistant continuous pipe provided by this invention contains Al and Cr, which are corrosion-resistant alloying elements, exhibiting good resistance to CO2 corrosion and Cl. - Its ability to resist corrosion and H2S stress corrosion cracking can effectively ensure the safety and economy of continuous tubing in complex working conditions.

[0042] 3. This invention provides a continuous tube steel strip butt welding and laser welding process that matches the composition of the coiled plate. After offline heat treatment, the tube structure consists of ferrite, martensite, and retained austenite. Ferrite is the soft phase, followed by retained austenite, and martensite is the hard phase. The volume fraction of ferrite and martensite is reasonably distributed, achieving a reasonable match between the strength and plasticity of the tube. This ensures high strength while giving the tube good plasticity. At the same time, offline heat treatment effectively reduces the residual stress in the weld and the entire tube, resulting in good resistance to bending fatigue during the continuous tube's lifting and lowering operation. Detailed Implementation

[0043] Example 1

[0044] This embodiment describes a φ50.8×4.44mm-CT110 grade low-density high-strength corrosion-resistant continuous pipe and its manufacturing method. The main manufacturing steps are as follows:

[0045] (I) Preparation of low-density corrosion-resistant special coils

[0046] Smelting and casting: Molten iron is used as the main raw material for pre-desulfurization treatment. After smelting in a top and bottom blown converter, LF heating desulfurization, VD vacuum decarburization, and Ca treatment, the composition and mass percentage of the molten steel meet the requirements of Table 1 and can be tapped. The steel is then cast into a continuous casting billet, and a 120±5mm thick slab is cast using electromagnetic stirring control in continuous casting technology.

[0047] Table 1 Chemical composition of the rolled steel sheets (wt, %)

[0048]

[0049] Homogenization treatment: Before the slab enters the furnace, it is subjected to high-pressure water descaling with a descaling water pressure of 50±5MPa; the slab is slowly heated to 1235℃ and held at that temperature for at least 2 hours before being taken out of the furnace and left in the furnace for ≥4 hours for homogenization treatment; the homogenized slab is subjected to a second high-pressure water descaling with a descaling water pressure of 50±5MPa; the descaling temperature of the slab is ≥1180℃ to ensure the removal of low-melting-point FeO·Fe2SiO4 eutectic compounds.

[0050] Hot rolling: Rolling is controlled using a roller mill. The roughing end temperature is controlled at 1050-1160℃, and the slab thickness after roughing is controlled at 60±3mm. The finishing rolling start temperature is controlled at 960-1020℃, and the finish temperature is controlled at 860-920℃, rolling it into a hot-rolled plate with a thickness of 4.44mm, a width of 1225mm, and a length greater than 500m.

[0051] Cooling and winding: The hot-rolled plate is subjected to laminar flow cooling at a rate of 30-40℃ / s and a winding temperature of 620℃.

[0052] Stress-relief annealing: The coiled material is held at 470℃ for 12 minutes to perform stress-relief annealing.

[0053] Sandblasting for rust removal: Due to the high Al and Si content in the design composition, oxidation and decarburization are prone to occur. A water-washing sandblasting process is used to fully remove the oxide scale from the surface of the coil. The steps include: uncoiling - straightening and leveling - water washing - sandblasting - water washing - drying - electrostatic oiling - rewinding, and winding into a coil to obtain the continuous low-density corrosion-resistant coil for pipes. The main mechanical properties of the sheet are shown in Table 2.

[0054] Table 2 Main Mechanical Properties of Coil

[0055]

[0056] (II) Preparation of low-density corrosion-resistant continuous tube blanks

[0057] Slitting and splicing of coiled steel: After uncoiling and leveling, the coiled steel is divided into 7 steel strips with a width of 159mm using a slitting machine according to the corresponding requirements of continuous pipe specification φ50.8×4.44mm. The ends of the front and rear steel strips to be welded are beveled at 45° to process I-type bevels. TIG welding is used, and the welding process parameters are shown in Table 3.

[0058] Table 3 Welding process parameters

[0059]

[0060] After welding, the butt weld was rapidly heated to 910℃ and forged with a pressure of 7MPa to refine the weld grains. The heating temperature was maintained, and the butt weld was held at this temperature for 1.5 minutes under argon protection. The weld was then water-cooled, the weld reinforcement was ground, and non-destructive testing was performed. The above strip butt welding process was repeated, and the strip was wound into a strip storage tray to form a 10050m continuous strip.

[0061] Preparation of tube blank: The side of the continuous strip steel is machined into an I-bevel by milling to remove burrs from the side of the strip steel, and the oil stains on the side of the strip steel are removed with alcohol; continuous roll forming, laser welding, nitrogen protection with a purity of ≥99.7%, forming bevel gap of 0.05mm, defocusing amount of +6mm, laser power of 7.5Kw, and welding speed of 5m / min; the weld is rapidly heated to 930℃ by medium frequency induction heating under inert atmosphere protection, held at the temperature for 40s, spray cooled to ≤250℃, and the external burrs of the weld are removed by grinding with a grinding wheel; the tube blank is continuously wound onto the production drum by a coiler to obtain a continuous tube blank with specifications of Φ50.8×4.44mm.

[0062] (III) Offline heat treatment of continuous tube blank

[0063] The entire coil of continuous pipe blank is uncoiled and straightened on an offline heat treatment line. The blank is then rapidly heated to 820°C at a rate of 40°C / s using a medium-frequency induction furnace. After holding at this temperature in a muffle furnace for 3.5 minutes, the blank is quenched to room temperature with water. The surface of the pipe is dried with cold air, then rapidly heated to 350°C in a medium-frequency induction furnace. After holding at this temperature in a muffle furnace for 4 minutes, the blank is air-cooled to <150°C and then water-cooled. After non-destructive testing and surface coating with an anti-corrosion agent, the blank is wound onto a roll suitable for transportation to obtain a 10,000m long, Φ50.8×4.44mm CT110 grade low-density high-strength corrosion-resistant continuous pipe.

[0064] The main performance indicators of the obtained CT110 (Φ50.8×4.44mm) low-density high-strength corrosion-resistant continuous tube T1 are shown in Table 4.

[0065] Table 4 Main Performance Test Results

[0066]

[0067] Example 2

[0068] This embodiment describes a φ50.8×4.44mm-CT100 grade low-density high-strength corrosion-resistant continuous pipe and its manufacturing method. The main manufacturing steps are based on Embodiment 1, but are achieved by adjusting the offline heat treatment process. The main manufacturing steps are as follows:

[0069] (I) Preparation of low-density corrosion-resistant special coils

[0070] Same as Example 1 above.

[0071] (II) Preparation of low-density corrosion-resistant continuous tube blanks

[0072] Same as Example 1 above.

[0073] (III) Offline heat treatment of continuous tube blank

[0074] The quenching process for the whole continuous tube blank is the same as in Example 1 above.

[0075] The surface of the pipe is dried with cold air, then rapidly heated to 420℃ in a medium-frequency furnace, held at that temperature in a muffle furnace for 4.5 minutes, air-cooled to <150℃, and then water-cooled. After non-destructive testing and surface coating with an anti-corrosion agent, it is wound onto a roll suitable for transportation to obtain a 10,000m long, Φ50.8×4.44mm-CT100 grade low-density high-strength corrosion-resistant continuous pipe.

[0076] The main performance indicators of the obtained CT100 (Φ50.8×4.44mm) low-density high-strength corrosion-resistant continuous tube T2 are shown in Table 5.

[0077] Table 5 Main Performance Test Results

[0078]

[0079] Example 3

[0080] This embodiment describes a φ60.3×5.2mm-CT130 grade low-density high-strength corrosion-resistant continuous pipe and its manufacturing method. The main manufacturing steps are as follows:

[0081] (I) Preparation of low-density corrosion-resistant special coils

[0082] Smelting and casting: Molten iron is used as the main raw material for pre-desulfurization treatment. After smelting in a top and bottom combined blowing converter, LF heating desulfurization, VD vacuum decarburization, and Ca treatment, the composition and mass percentage of the molten steel meet the requirements of Table 6 and can be tapped. The steel is then cast into continuous casting billets, and electromagnetic stirring is used to control the casting of 100±5mm thick slabs.

[0083] Table 6 Chemical composition of the rolled steel sheets (wt, %)

[0084]

[0085] Homogenization treatment: Before the slab enters the furnace, it is subjected to high-pressure water descaling with a descaling water pressure of 50±5MPa; the slab is slowly heated to 1260℃ and held at that temperature for at least 2 hours before being taken out of the furnace and left in the furnace for ≥4 hours for homogenization treatment; the homogenized slab is subjected to a second high-pressure water descaling with a descaling water pressure of 50±5MPa; the descaling temperature of the slab is ≥1180℃ to ensure the removal of low-melting-point FeO·Fe2SiO4 eutectic compounds.

[0086] Hot rolling: Rolling is controlled using a roller mill. The roughing end temperature is controlled at 1050-1160℃, and the slab thickness after roughing is controlled at 45±3mm. The finishing rolling start temperature is controlled at 960-1020℃, and the finish temperature is controlled at 860-920℃, rolling it into a hot-rolled plate with a thickness of 5.2mm, a width of 1200mm, and a length greater than 450m.

[0087] Cooling and winding: The hot-rolled plate is subjected to laminar flow cooling at a rate of 30-40℃ / s and a winding temperature of 640℃.

[0088] Stress-relief annealing: The coiled material is held at 520℃ for 15 minutes to perform stress-relief annealing.

[0089] Sandblasting for rust removal: Due to the high Al and Si content in the design composition, oxidation and decarburization are prone to occur. Therefore, a water-washing sandblasting process is adopted to fully remove the oxide scale from the surface of the coil. The steps include: uncoiling - straightening and leveling - water washing - sandblasting - water washing - drying - electrostatic oiling - rewinding, and winding into a coil to obtain the continuous low-density corrosion-resistant coil for pipes. The main mechanical properties of the sheet are shown in Table 7.

[0090] Table 7 Main Mechanical Properties of Coiled Steel

[0091]

[0092] (II) Preparation of low-density corrosion-resistant continuous tube blanks

[0093] Slitting and splicing of coiled steel: After uncoiling and leveling, the coiled steel is divided into 6 steel strips with a width of 189mm using a slitting machine according to the corresponding requirements of continuous pipe specification φ60.3×5.2mm. The ends of the front and rear steel strips to be welded are beveled at 45° to form an I-bevel. TIG welding is used, and the welding process parameters are shown in Table 8.

[0094] Table 8 Welding process parameters

[0095]

[0096] After welding, the above-mentioned butt weld was rapidly heated to 980℃, and the weld was forged with a pressure of 10MPa to refine the weld grains. The above heating temperature was maintained, and the butt weld was held at this temperature for 2 minutes under argon protection. The weld was then water-cooled, the weld reinforcement was ground, and non-destructive testing was performed. The above strip butt welding process was repeated, and the strip was wound into a strip storage tray to form an 8000m continuous strip.

[0097] Preparation of tube blank: The side of the continuous strip steel is machined into an I-bevel by milling to remove burrs from the side of the strip steel, and oil stains on the side of the strip steel are removed with alcohol; continuous roll forming, laser welding, nitrogen protection with a purity of ≥99.7%, forming bevel gap of 0.05mm, defocusing amount of +4mm, laser power of 9Kw, and welding speed of 3.5m / min; the weld is rapidly heated to 1000℃ by medium frequency induction heating under inert atmosphere protection, held at 60s, spray cooled to ≤250℃, and the external burrs of the weld are removed by grinding with a grinding wheel; the tube blank is continuously wound onto the production drum by a coiler to obtain a continuous tube blank with specifications of Φ60.3×5.2mm.

[0098] (III) Offline heat treatment of continuous tube blank

[0099] The entire coil of continuous pipe blank is uncoiled and straightened on an offline heat treatment line. The blank is then rapidly heated to 860°C at a rate of 35°C / s using a medium-frequency induction furnace. After holding at this temperature in a muffle furnace for 4 minutes, the blank is quenched to room temperature with water. The surface of the pipe is dried with cold air, then rapidly heated to 400°C in a medium-frequency induction furnace. After holding at this temperature in a muffle furnace for 5 minutes, the blank is air-cooled to <150°C and then water-cooled. After non-destructive testing and surface coating with an anti-corrosion agent, the blank is wound onto a roll suitable for transportation to obtain a continuous pipe with a length of 8050m and a specification of Φ60.3×5.2mm-CT130 steel grade, low density, high strength, and corrosion resistance.

[0100] The main performance indicators of the obtained CT130 (Φ60.3×5.2mm) low-density high-strength corrosion-resistant continuous tube T3 are shown in Table 9.

[0101] Table 9 Main Performance Test Results

[0102]

[0103] Example 4

[0104] This embodiment describes a φ60.3×5.2mm-CT120 grade low-density high-strength corrosion-resistant continuous pipe and its manufacturing method. The main manufacturing steps are based on Embodiment 3, but are achieved by adjusting the offline heat treatment process. The main manufacturing steps are as follows:

[0105] (I) Preparation of low-density corrosion-resistant special coils

[0106] Same as Example 3 above.

[0107] (II) Preparation of low-density corrosion-resistant continuous tube blanks

[0108] Same as Example 3 above.

[0109] (III) Offline heat treatment of continuous tube blank

[0110] The quenching process for the whole continuous tube blank is the same as in Example 3 above.

[0111] The surface of the pipe is dried with cold air, then rapidly heated to 450℃ in a medium-frequency furnace, held at that temperature in a muffle furnace for 6 minutes, air-cooled to <150℃, and then water-cooled. After non-destructive testing and surface coating with an anti-corrosion agent, it is wound onto a roll suitable for transportation to obtain a continuous pipe with a length of 8050m and a specification of Φ60.3×5.2mm-CT120 steel grade, low density, high strength, and corrosion resistance.

[0112] The main performance indicators of the obtained CT120 (Φ60.3×5.2mm) low-density high-strength corrosion-resistant continuous tube T4 are shown in Table 10.

[0113] Table 10 Main Performance Test Results

[0114]

[0115] In the embodiments, the corrosion resistance of the low-density, high-strength, corrosion-resistant continuous tubing and the comparative conventional continuous tubing are as follows:

[0116] Table 11 Results of CO2 Corrosion Resistance Test

[0117]

[0118]

[0119] Table 12 SCC Sensitivity Test Results

[0120]

[0121] It is evident that the low-density, high-strength, corrosion-resistant continuous tubing produced by this method: 1) not only produces high-strength continuous tubing that meets standard requirements, but also reduces the density by more than 8% compared to carbon steel continuous tubing; 2) the added Al and Cr are corrosion-resistant alloying elements, exhibiting good resistance to CO2 corrosion and Cl... - 3) The continuous pipe produced by this method undergoes offline heat treatment, which makes the weld structure of the pipe body and the base material structure more consistent, effectively reducing the residual stress of the weld and the entire pipe body, improving the SSC resistance of the continuous pipe, and significantly increasing the service life and operational safety of the continuous pipe during operation.

Claims

1. A low-density, high-strength, corrosion-resistant steel for continuous pipes, characterized in that, Its chemical composition, by weight percentage, includes: Al: 6.0-9.0%, Mn: 8.0-12.0%, Si: 1.10-1.50%, Cr: 1.0-3.0%, C: 0.12-0.25%, Cu: 0.20-0.50%, Ni: 0.10-0.50%, Mo: 0.20-0.70%, V: 0.02-0.15%, Ti: 0.10-0.35%, Nb: 0.02-0.09%, P: ≤0.01%, S: ≤0.006%, with the balance being Fe and unavoidable impurities.

2. The low-density, high-strength, corrosion-resistant steel for continuous tubing according to claim 1, characterized in that, The preferred weight percentage of Al is 7.0%-8.5%.

3. The method for manufacturing low-density, high-strength, corrosion-resistant continuous pipe steel as described in claim 1, characterized in that, Includes the following steps: Smelting, casting, homogenization, hot rolling, cooling, coiling, stress-relief annealing, sandblasting and rust removal; in, The casting is continuous casting; The homogenization process includes heating the slab and descaling, wherein the temperature of the heated slab is 1250±20℃, the pressure of the descaling water is 50±5Mpa, and the descaling temperature is ≥1180℃. The final rolling temperature of the hot rolling is 860-920℃; The cooling rate is 30-40℃ / s; The winding temperature is 600-650℃; The stress-relief annealing temperature is 450-550℃.

4. The method for manufacturing low-density, high-strength, corrosion-resistant continuous pipe steel according to claim 3, characterized in that, The sandblasting rust removal process includes the following steps: uncoiling, straightening and leveling, water washing, sandblasting, water washing, drying, electrostatic oiling, and rewinding.

5. A low-density, high-strength, corrosion-resistant continuous pipe, characterized in that, It is prepared from the continuous tubular steel as described in claim 1.

6. The low-density, high-strength, corrosion-resistant continuous pipe according to claim 5, characterized in that, The microstructure of the continuous tube consists of ferrite, martensite and retained austenite, wherein the volume fraction of ferrite is 50%-65%, the volume fraction of martensite is 20%-40%, and the remainder is retained austenite.

7. The method for manufacturing the low-density, high-strength, corrosion-resistant continuous tube according to claim 5 or 6, characterized in that, Includes the following steps: Preparation of steel coils; The coiled sheet is longitudinally cut, and the steel strip is spliced. Forming and welding processes; Offline heat treatment; in, The offline heat treatment is as follows: the tube blank is heated to 795-880℃ in a medium-frequency heating furnace, held at the temperature, and quenched to room temperature; the tube surface is dried, the tube blank is heated to 300-500℃ in a medium-frequency heating furnace, held at the temperature, cooled to below 150℃ and then water-cooled, and the continuous tube is wound onto a drum by a coiler to obtain a continuous tube of the required length.

8. The method for manufacturing a low-density, high-strength, corrosion-resistant continuous tube according to claim 7, characterized in that, The steel strip splicing involves beveling the ends of two steel strips to be welded at 40°-50°. The beveled ends are then processed with I-shaped, V-shaped, or U-shaped bevels according to the thickness of the steel strips and then assembled and spliced. Welding is performed using either non-filler wire welding or filler wire welding. The weld is then subjected to post-weld rolling deformation heat treatment and rapid cooling.

9. The method for manufacturing a low-density, high-strength, corrosion-resistant continuous tube according to claim 7 or 8, characterized in that, The forming and welding process involves, before forming, machining the side of the continuous strip steel into an I-shaped bevel, welding the tube blank using laser welding technology, heat-treating the weld, and then rolling the tube blank onto a production roll.

10. The method for manufacturing a low-density, high-strength, corrosion-resistant continuous tube according to claim 9, characterized in that, The welding process of the tube blank adopts inert gas protection, controls the forming bevel gap to be ≤0.1mm, the defocusing amount to be -5~10mm, the laser power to be 2-16Kw, and the welding speed to be 1.5-10m / min.

11. The method for manufacturing a low-density, high-strength, corrosion-resistant continuous tube according to claim 9, characterized in that, The heat treatment of the weld involves rapidly heating the weld to 900-960℃ under an inert atmosphere and holding it at that temperature for 20-100 seconds.

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