Austenitic heat-resistant steel continuous pipe and preparation method thereof

Abstract

The invention provides an austenitic heat-resistant steel continuous pipe and a preparation method thereof. The austenitic heat-resistant steel continuous pipe comprises the following chemical components in percentage by weight: 0.03%-0.05% of C, less than or equal to 0.4% of Si, 0.7%-0.9% of Mn, less than or equal to 0.02% of P, less than or equal to 0.007% of S, 19.00%-21.00% of Ni, 24.00%-26.00% of Cr, 2.0%-3.0% of Al, 2.50%-3.00% of Cu, 0.1%-0.3% of Nb and the balance of Fe and inevitable impurities. The problems that in the prior art, austenitic stainless steel is small in bearing capacity and prone to creep deformation, cracking and corrosion in the high-temperature environment are solved.

Description

Technical Field

[0001] This invention relates to the field of alloy materials technology, and more specifically, to an austenitic heat-resistant steel continuous pipe and its preparation method. Background Technology

[0002] Currently, my country's oil and gas resources have entered a development stage that emphasizes both conventional and unconventional resources. Unconventional oil and gas accounts for 41% of the country's cumulative proven reserves and 20% of total oil and gas production. The development of unconventional oil and gas resources is directly related to national energy security. Heavy oil, a typical unconventional resource, has proven reserves of 19.87 billion tons in my country, making it the world's fourth largest heavy oil producer after the United States, Canada, and Venezuela. Heavy oil development primarily utilizes thermal recovery technologies, including steam injection, steam drive, reservoir combustion, and steam-assisted gravity drainage (SAGD). When developing heavy oil using these technologies, the bottom-hole temperature can reach over 100℃, placing high demands on the heat resistance and oxidation resistance of oil and gas pipeline materials. Simultaneously, the produced water from the bottom of the well has high salinity and contains Cl. - SO4 2+ HCO3 - Corrosive media, such as CO2 and H2S, are present in the bottom gas phase of wells, placing stringent requirements on the corrosion resistance of oil and gas pipelines in high-temperature downhole environments. Coiled tubing (CT), a new type of oil pipe with single-piece lengths reaching several kilometers and without threaded connections, is transported and used wound on a reel. In conjunction with oilfield tools, it can be used for dozens of operations in oil and gas field workover, logging, drilling, completion, and oil and gas transportation, playing a vital role in oil and gas field exploration, development, operation, and production enhancement.

[0003] The primary material used for manufactured coiled tubing both domestically and internationally is low-alloy carbon steel. This steel exhibits poor heat resistance and is prone to corrosion at high temperatures, leading to thinning of the tubing wall and subsequent fractures, severely hindering the development of thermal recovery technology for heavy oil wells in my country. Currently, some Chinese manufacturers also produce small quantities of coiled tubing made from high-alloy materials (duplex stainless steel and austenitic stainless steel). However, duplex stainless steel is prone to chromium formation at grain boundaries at high temperatures. 23 C6 and other precipitates reduce the high-temperature corrosion resistance of the pipe. Simultaneously, under high-temperature and applied yield stress, the austenitic structure in duplex stainless steel transforms into martensite, reducing the pipe's plasticity and leading to brittle fracture. Duplex stainless steel continuous pipes can only operate for extended periods below 280℃. Ordinary austenitic stainless steel continuous pipes have low yield strength (≤200MPa) at high temperatures (100–400℃), failing to meet the requirements for load-bearing and compressive strength. Furthermore, long-term operation in the high-temperature environment at the bottom of wells makes them prone to creep, cracking, and corrosion. Summary of the Invention

[0004] The main objective of this invention is to provide an austenitic heat-resistant steel, a continuous austenitic heat-resistant steel pipe, and a method for preparing the same, in order to solve the problems of low load-bearing capacity and easy creep, cracking, and corrosion of austenitic stainless steel in the prior art under high-temperature conditions.

[0005] To achieve the above objectives, according to one aspect of the present invention, an austenitic heat-resistant steel continuous pipe is provided, comprising the following chemical composition by weight percentage: C 0.03%–0.05%, Si ≤0.4%, Mn 0.7%–0.9%, P ≤0.02%, S ≤0.007%, Ni 19.00%–21.00%, Cr 24.00%–26.00%, Al 2.0%–3.0%, Cu 2.50%–3.00%, Nb 0.1%–0.3%, with the balance being Fe and unavoidable impurities.

[0006] Furthermore, in the austenitic heat-resistant steel continuous pipe, the weight percentage content of Si is 0.2% to 0.4%.

[0007] Furthermore, in the austenitic heat-resistant steel continuous pipe, the weight percentage content of Ni to the weight percentage content of Cr is (0.75~0.82):1.

[0008] According to another aspect of the present invention, a method for preparing the above-mentioned austenitic heat-resistant steel continuous tube is provided, comprising the following steps:

[0009] Step S1: The materials are prepared according to the composition of the target austenitic heat-resistant steel continuous pipe and added to a vacuum induction furnace to be melted into molten steel.

[0010] Step S2: Cast the molten steel into a slab;

[0011] Step S3: The slab is rolled into a hot-rolled plate at 1000-1150℃, and then the hot-rolled plate is subjected to air cooling, tempering, pickling, and coiling into a coil in sequence to obtain a coil plate; the tempering temperature of the hot-rolled plate is 450-650℃.

[0012] Step S4: The coil is longitudinally sheared to obtain strip steel. The strip steel is butt-welded to form a continuous strip using cold metal transfer welding. Then, the butt weld is subjected to solution treatment and tempering treatment. The solution treatment temperature of the butt weld is 1000-1150℃, and the tempering treatment temperature is 400-600℃.

[0013] Step S5: The strip is continuously and stably formed into a blank tube shape, and during the forming process, the blank tube shape is longitudinally welded using a cold metal transition welding method to form a continuous welded tube.

[0014] Step S6: The longitudinal weld seam on the welded pipe is subjected to solution treatment and tempering treatment, and then the entire welded pipe is subjected to bright annealing heat treatment; the solution treatment temperature of the longitudinal weld seam is 1000~1150℃, and the tempering treatment temperature is 400~600℃.

[0015] Further, in step S4, under a protective gas, the strip steel is butt-welded to form a continuous strip using a single-wire cold metal transfer welding method; wherein the welding current of the welding wire is 100-200A, the welding voltage is 20-28V, the welding speed is 300-600mm / min, the gas flow rate is 12-25L / min, the welding wire diameter is 1.0-1.2mm, and the protective gas is a mixture of argon and hydrogen, with the volume fraction of argon in the protective gas being 90%-98%.

[0016] Furthermore, in step S4, the steps of solution treatment and tempering of the butt weld are as follows:

[0017] The butt weld is heated to 1000-1150℃ in a vacuum furnace at a heating rate of ≥10℃ / s and held in vacuum for 40-60s. Then, the butt weld is rolled with a rolling pressure of 4-6 tons. After rolling, the butt weld is cooled rapidly with nitrogen gas to below 150℃ at a cooling rate of ≥12℃ / s. Finally, the butt weld is tempered at 400-600℃.

[0018] Further, in step S5, the blank tube is longitudinally welded using a double-wire cold metal transfer welding method; wherein, the welding current of each welding wire is 160-280A, the welding voltage is 22-34V, the welding speed is 1000-3500mm / min, the gas flow rate is 12-28L / min, the welding wire diameter is 1.0-1.6mm, and the shielding gas is a mixture of argon and hydrogen, with the volume fraction of argon in the shielding gas being 90%-98%.

[0019] Furthermore, in step S6, the steps of solution treatment and tempering of the longitudinal weld are as follows:

[0020] The longitudinal weld is heated to 1000-1150℃ at a heating rate of ≥10℃ / s. Then, the upper surface of the longitudinal weld is piled up and rolled with an extrusion roller until it is flush with the outer surface of the pipe. The longitudinal weld is then tempered at 400-600℃. After the tempering is completed, the longitudinal weld is air-cooled to room temperature.

[0021] Further, in step S6, the bright annealing heat treatment is performed as follows: Under hydrogen protection, the entire welded pipe is subjected to bright annealing heat treatment at a temperature of 980–1080°C.

[0022] Furthermore, the melting temperature is 1550–1700℃;

[0023] Preferably, step S1 further includes refining the molten steel and carrying out homogenization treatment in a refining furnace.

[0024] By applying the technical solution of this invention and controlling the alloy composition and content, an austenitic heat-resistant steel with excellent high-temperature oxidation resistance and corrosion resistance, high high-temperature creep strength, and good thermoplasticity was obtained. Specifically, the addition of higher contents of Cr and Ni elements makes the passivation film on the surface of the austenitic heat-resistant steel denser and more stable, reducing the risk of passivation film cracking and improving the oxidation resistance and stability of the austenitic heat-resistant steel at high temperatures. The addition of Cu, Al, and Nb alloying elements further improves the high-temperature oxidation resistance and corrosion resistance of the austenitic heat-resistant steel, effectively increasing the high-temperature creep strength and high-temperature load-bearing capacity. Controlling the addition of Mn element improves the thermoplasticity of the austenitic heat-resistant steel, while also allowing Mn to form MnS with S elements in the steel, reducing the harmful effects of S on the heat-resistant steel.

[0025] The chemical composition of this invention can be used to prepare austenitic heat-resistant steel continuous pipe with excellent high-temperature oxidation resistance and corrosion resistance, high high-temperature creep strength and good thermoplasticity. Under the high-temperature working conditions of heavy oil thermal recovery wells, it has strong oxidation resistance, strong corrosion resistance, high creep strength and large load-bearing capacity. Attached Figure Description

[0026] Figure 1 This is a microstructure photograph of the austenitic heat-resistant steel continuous tube in Example 1 of the present invention;

[0027] Figure 2 The increase in the mass of the oxide film on the surface of the austenitic heat-resistant steel continuous tube in Example 1 under different oxidation times;

[0028] Figure 3 The increase in the mass of the surface oxide film of the austenitic heat-resistant steel continuous tube prepared in Example 1 after continuous oxidation for 100 hours under different high-temperature environments;

[0029] Figure 4 The image shows the intergranular corrosion performance test results of the austenitic heat-resistant steel continuous tube sample prepared in Example 1.

[0030] Figure 5 The image shows the stress corrosion performance test results of the austenitic heat-resistant steel continuous tube sample prepared in Example 1. Detailed Implementation

[0031] It should be noted that, unless otherwise specified, the embodiments and features described in this application can be combined with each other. The present invention will now be described in detail with reference to the embodiments.

[0032] As described in the background section, existing austenitic stainless steels suffer from low load-bearing capacity at high temperatures and are prone to creep, cracking, and corrosion. To address these problems, according to one aspect of the present invention, an austenitic heat-resistant steel continuous pipe is provided, comprising the following chemical composition by weight percentage: C 0.03%–0.05%, Si ≤0.4%, Mn 0.7%–0.9%, P ≤0.02%, S ≤0.007%, Ni 19.00%–21.00%, Cr 24.00%–26.00%, Al 2.0%–3.0%, Cu 2.50%–3.00%, Nb 0.1%–0.3%, with the balance being Fe and unavoidable impurities.

[0033] This invention, through control of alloy composition and content, yields austenitic heat-resistant steel with excellent high-temperature oxidation and corrosion resistance, high high-temperature creep strength, and good thermoplasticity. Specifically, the addition of higher amounts of Cr and Ni elements makes the passivation film on the surface of the austenitic heat-resistant steel denser and more stable, reducing the risk of passivation film cracking and improving the oxidation resistance and stability of the austenitic heat-resistant steel at high temperatures. The addition of Cu, Al, and Nb alloying elements further enhances the high-temperature oxidation and corrosion resistance of the austenitic heat-resistant steel, effectively improving the high-temperature creep strength and high-temperature load-bearing capacity. Controlling the addition of Mn element improves the thermoplasticity of the austenitic heat-resistant steel, while also allowing Mn to form MnS with S elements in the steel, reducing the harmful effects of S on the heat-resistant steel. The chemical composition of this invention can be used to prepare austenitic heat-resistant steel coiled tubing with excellent high-temperature oxidation resistance, corrosion resistance, high high-temperature creep strength, and good thermoplasticity. Under the high-temperature operating conditions of heavy oil thermal recovery wells, it exhibits strong oxidation resistance, strong corrosion resistance, high creep strength, and high load-bearing capacity. Moreover, the cost of the austenitic heat-resistant steel coiled tubing of this invention is much lower than that of nickel-based alloy coiled tubing and titanium alloy coiled tubing, making it more suitable for widespread application in heavy oil thermal recovery wells.

[0034] In the austenitic heat-resistant steel of the present invention, the functions of each main component are as follows:

[0035] Chromium (Cr): In this invention, Cr is an important element for improving the oxidation resistance and stability of austenitic heat-resistant steel at high temperatures. Cr can enable the formation of a dense Cr₂O₃ passivation film on the surface of the heat-resistant steel at high temperatures, preventing oxidation, promoting long-term passivation, and improving the stability of the heat-resistant steel. Furthermore, a higher Cr content can increase the solubility of carbon (C) and reduce Cr depletion, which has a positive effect on the resistance of heat-resistant steel to intergranular corrosion. However, Cr is a strong ferrite-forming element, and excessively high Cr content can destabilize the austenitic matrix. Therefore, this invention controls the Cr content in the austenitic heat-resistant steel continuous pipe material to be between 24% and 26%.

[0036] Nickel (Ni): In this invention, Ni is an important element for improving the oxidation resistance and stability of austenitic heat-resistant steel at high temperatures. A higher Ni content allows the thermal expansion coefficient of the matrix to be closer to that of the passivation film, reducing the risk of passivation film rupture due to the inconsistency in expansion coefficients between the matrix and the passivation film at high temperatures. This improves the oxidation resistance and stability of the heat-resistant steel at high temperatures. Furthermore, Ni is an austenite stabilizing element, contributing to improved stability of the heat-resistant steel. However, adding too much Ni significantly increases the cost of the alloy and reduces the plasticity of the austenitic heat-resistant steel. Therefore, in this invention, the Ni content is controlled at 19.00–21.00%.

[0037] Copper (Cu): Cu can improve the high-temperature oxidation resistance and corrosion resistance of austenitic heat-resistant steel. During long-term aging at high temperatures, Cu in austenitic heat-resistant steel can form nanoscale precipitates, which can significantly improve the high-temperature creep strength of the heat-resistant steel. Furthermore, the addition of Cu can improve the corrosion resistance of the heat-resistant steel; however, excessive Cu content leads to too many precipitates, affecting the plasticity of the austenitic heat-resistant steel. In this invention, the Cu content is controlled at 2.50–3.00%.

[0038] Aluminum (Al): Al can improve the high-temperature oxidation resistance and corrosion resistance of austenitic heat-resistant steel. The addition of Al will cause a dense Al2O3 passivation film to form on the surface of the heat-resistant steel on the basis of the Cr2O3 passivation film. The relatively stable Al2O3 at high temperatures will significantly improve the high-temperature oxidation resistance of the heat-resistant steel; however, excessive Al will make the austenitic matrix in the heat-resistant steel unstable and reduce the creep strength of the material. Therefore, the Al content is controlled at 2.0% to 3.0% in this invention.

[0039] Niobium (Nb): Nb can improve the high-temperature oxidation resistance and corrosion resistance of austenitic heat-resistant steel. Nb can form very stable niobium carbides in heat-resistant steel, improving the high-temperature creep strength of the material through carbide precipitation and dispersion strengthening. Nb also has a certain stabilizing effect on carbon, preventing chromium depletion around grain boundaries and improving material strength and corrosion resistance. However, excessive Nb content can easily lead to the precipitation of Nb-rich carbides (such as NbC), reducing the plasticity of austenitic heat-resistant steel. Therefore, this invention controls the Nb content to be 0.1%–0.3%.

[0040] Carbon (C): Carbon is a strong austenite stabilizing element. Carbon can improve the strength of austenitic heat-resistant steel through solid solution strengthening. However, when the carbon content is too high, high-Cr deposits tend to form at grain boundaries. 23 C6-type carbides lead to chromium depletion near grain boundaries, weakening intergranular corrosion resistance and reducing the steel's corrosion resistance. Therefore, the carbon content is controlled at 0.03–0.05% in this invention.

[0041] Manganese (Mn): Mn plays a role in stabilizing austenite and improving thermoplasticity in heat-resistant steel. Furthermore, Mn can combine with sulfur (S) to form MnS, reducing the harmful effects of sulfur on heat-resistant steel. However, the addition of Mn also weakens the surface oxide film, reducing the oxidation resistance and corrosion resistance of the heat-resistant steel. Therefore, this invention controls the Mn content to be 0.7%–0.9%.

[0042] Silicon (Si): Si can improve the oxidation and corrosion resistance of austenitic heat-resistant steel. In high-temperature environments or strong oxidizing media, Si can form a Si-rich SiO2 oxide layer on the surface of heat-resistant steel pipes, significantly improving the pipe's oxidation and corrosion resistance. Furthermore, adding Si can inhibit the pitting corrosion tendency of heat-resistant steel in Cl- ion media. However, excessive Si content will greatly increase the brittleness of the pipe; therefore, in this invention, the Si content is controlled at Si ≤ 0.5%.

[0043] In a preferred embodiment, the weight percentage of Si in the austenitic heat-resistant steel continuous pipe is 0.2% to 0.4% in order to obtain austenitic heat-resistant steel with high oxidation resistance, corrosion resistance and plasticity.

[0044] In a preferred embodiment, the weight percentage of Ni to the weight percentage of Cr in the austenitic heat-resistant steel continuous tube is (0.75 to 0.82):1, so that the thermal expansion coefficient of the matrix is ​​closer to that of the passivation film, further reducing the risk of passivation film rupture at high temperature and improving the oxidation resistance and stability of the heat-resistant steel at high temperature.

[0045] Preferably, the outer diameter of the austenitic heat-resistant steel coiled tubing is 25.4–60.3 mm, the wall thickness is 2.4–5.2 mm, and the length is 1000–3500 m. These parameters allow the austenitic heat-resistant steel coiled tubing to better meet the requirements of heavy oil thermal recovery wells in the market, including length requirements, temperature requirements (100–400℃), high load-bearing capacity, and service life.

[0046] According to another aspect of the present invention, a method for preparing the above-mentioned austenitic heat-resistant steel continuous tube is provided, comprising the following steps:

[0047] Step S1: The materials are prepared according to the composition of the target austenitic heat-resistant steel continuous pipe and added to a vacuum induction furnace to be melted into molten steel.

[0048] Step S2: Cast the molten steel into a slab;

[0049] Step S3: The slab is rolled into a hot-rolled plate at 1000-1150℃, and then the hot-rolled plate is subjected to air cooling, tempering, pickling, and coiling into a coil in sequence to obtain a coil plate; the tempering temperature of the hot-rolled plate is 450-650℃.

[0050] Step S4: The coil is longitudinally sheared to obtain strip steel. The strip steel is butt-welded to form a continuous strip using a single-wire cold metal transfer welding method. Then, the butt weld formed by the butt weld is subjected to solution treatment and tempering treatment. The solution treatment temperature of the butt weld is 1000~1150℃, and the tempering treatment temperature is 400~600℃.

[0051] Step S5: The strip is continuously and stably formed into a blank tube shape. During the forming process, the blank tube shape is longitudinally welded using a double-wire cold metal transition welding method to form a continuous welded tube.

[0052] Step S6: The longitudinal weld seam on the welded pipe is subjected to solution treatment and tempering treatment, and then the entire welded pipe is subjected to bright annealing heat treatment; the solution treatment temperature of the longitudinal weld seam is 1000~1150℃, and the tempering treatment temperature is 400~600℃.

[0053] The method for preparing heat-resistant austenitic stainless steel continuous tubes based on the present invention involves melting, casting, high-temperature rolling, and air cooling of a specific alloy composition. Tempering is then used to reduce or eliminate residual stress generated within the stainless steel material during high-temperature hot rolling, thereby reducing the risk of deformation and cracking during subsequent processing or use. This yields austenitic heat-resistant steel coils with high-temperature oxidation and corrosion resistance, high high-temperature creep strength, and good thermoplasticity. Subsequent butt welding and longitudinal welding are performed using a single-wire cold metal transfer welding method to control and minimize weld buildup and reduce the heat-affected zone of the weld. Further, solution treatment is used to re-dissolve precipitates in the weld area back into the cell, and tempering further reduces residual stress in the weld and surrounding areas after solution treatment, improving corrosion resistance and high-temperature creep strength in the weld area and reducing the risk of corrosion and cracking in the austenitic heat-resistant steel continuous tube. Finally, bright annealing heat treatment is applied to the entire welded tube to reduce overall stress and improve overall performance and aesthetics. The austenitic heat-resistant steel continuous pipe made using the alloy composition and method of this invention has excellent high-temperature oxidation resistance, corrosion resistance, high high-temperature creep strength, and good thermoplasticity. It has strong oxidation resistance, strong corrosion resistance, high creep strength, and large load-bearing capacity under high-temperature working conditions in heavy oil thermal recovery wells, making it suitable for application under heavy oil thermal recovery well working conditions (100-400℃).

[0054] Single-wire cold metal transfer welding is used for welding the ends of steel strips, resulting in less spatter, less heat input, and a smaller heat-affected zone in the weld. The welding process has minimal impact on the substrate, effectively preventing problems such as decreased high-temperature oxidation resistance and corrosion resistance caused by welding. In a preferred embodiment, to better control and reduce the heat-affected zone of the weld, in step S4, under a shielding gas, single-wire cold metal transfer welding is used to butt-weld the steel strip to form a continuous strip. The welding current is 100–200 A, the welding voltage is 20–28 V, the welding speed is 300–600 mm / min, the gas flow rate is 12–25 L / min, the welding wire diameter is 1.0–1.2 mm, and the shielding gas is a mixture of argon and hydrogen, with argon comprising 90%–98% of the gas volume.

[0055] In a preferred embodiment, to further improve the corrosion resistance and high-temperature creep strength of the weld zone, the steps of solution treatment and tempering of the butt weld in step S4 are as follows: the butt weld is heated to 1000-1150°C in a vacuum furnace at a heating rate ≥10°C / s, and held in vacuum for 40-60s. Then, the butt weld is rolled with a rolling pressure of 4-6 tons. After rolling, the butt weld is rapidly cooled with nitrogen gas to below 150°C at a cooling rate ≥12°C / s. Finally, the butt weld is tempered at 400-600°C.

[0056] The use of double-wire cold metal transfer welding in the longitudinal welding of continuous pipe forming results in a small heat-affected zone and effectively reduces the burr height on the inner wall of the pipe, improving weld quality, increasing the inner diameter for oil and gas flow within the continuous pipe, and avoiding phenomena such as groove corrosion. In a preferred embodiment, to better control the heat-affected zone of the weld and improve welding efficiency and weld quality, in step S5, a double-wire cold metal transfer welding method is used to longitudinally weld the blank pipe shape; wherein, the welding current of each welding wire is 160-280A, the welding voltage is 22-34V, the welding speed is 1000-3500mm / min, the gas flow rate is 12-28L / min, the welding wire diameter is 1.0-1.6mm, and the shielding gas is a mixture of argon and hydrogen, with the volume fraction of argon in the shielding gas being 90%-98%.

[0057] In a preferred embodiment, step S6 involves solution treatment and tempering of the longitudinal weld as follows: the longitudinal weld is heated to 1000-1150°C at a heating rate of ≥10°C / s, and then the upper surface of the longitudinal weld is piled up and rolled with an extrusion roller until it is flush with the outer surface of the pipe. Then, the longitudinal weld is tempered at 400-600°C. After the tempering is completed, the longitudinal weld is air-cooled to room temperature.

[0058] In a preferred embodiment, the bright annealing heat treatment in step S6 is as follows: the welded pipe is subjected to bright annealing heat treatment under hydrogen protection, and the annealing heat treatment temperature is 980-1080℃.

[0059] In a preferred embodiment, the melting temperature is 1550–1700°C; preferred step S1 further includes refining and homogenizing the molten steel in a refining furnace to further improve the purity and uniformity of the molten steel and produce higher quality alloy steel.

[0060] The present application will be further described in detail below with reference to specific embodiments, which should not be construed as limiting the scope of protection claimed in the present application.

[0061] Example 1

[0062] A continuous austenitic heat-resistant steel pipe comprises the following chemical composition by weight percentage: C 0.04%, Si 0.3%, Mn 0.8%, P 0.01%, S 0.002%, Ni 20%, Cr 25%, Al 2.5%, Cu 2.8%, Nb 0.2%, with the balance being Fe and unavoidable impurities.

[0063] The preparation method of austenitic heat-resistant steel continuous pipe includes the following steps:

[0064] Step 1: Prepare austenitic heat-resistant steel coils. The specific steps are as follows:

[0065] 1) Smelting: The raw materials are prepared according to the composition of the target austenitic heat-resistant steel continuous pipe and added to a vacuum induction furnace to be smelted into molten steel. Then, the steel is refined and homogenized in a refining furnace. The smelting temperature is 1650℃.

[0066] 2) Cast slab: Molten steel is poured into a slab, and the thickness of the slab is 25mm;

[0067] 3) Preparation of coils: The slab is rolled into a hot-rolled plate at 1000℃, and then the hot-rolled plate is subjected to air cooling, tempering at 550℃, pickling, and coiling into a coil to obtain 500-meter coils.

[0068] Step 2: The coil is longitudinally cut into multiple strips of steel ranging from 75 to 182 mm. The multiple strips are butt-welded to form a 3000 m continuous strip using a single-wire cold metal transfer welding method. The butt welds are then subjected to solution treatment and tempering treatment to regulate the microstructure.

[0069] Before welding, the end of the steel strip is processed into a 45° bevel, and a V-shaped bevel is opened on the bevel with an included angle of 45°.

[0070] The welding conditions are as follows: shielding gas is 95% argon and 5% hydrogen (volume fraction), welding current is 140A, welding voltage is 22V, welding speed is 400mm / min, gas flow rate is 20L / min, and welding wire diameter is 1.0mm.

[0071] The specific steps for solution treatment and tempering are as follows: The butt weld is heated to 1050℃ in a vacuum furnace at a heating rate of ≥10℃ / s and held in vacuum for 45s. Then, the butt weld is rolled with a rolling pressure of 4 tons. After rolling, the butt weld is cooled by rapid blowing of nitrogen gas to below 150℃ at a cooling rate of ≥12℃ / s. Finally, the butt weld is tempered at 450℃.

[0072] Step 3: The strip is continuously and stably formed into a blank tube shape, and the blank tube shape is longitudinally welded during the forming process to form a continuous welded tube. The specific steps are as follows:

[0073] 1) Mill the edges on both sides of the extended steel strip and make a V-shaped bevel with an included angle of 30-45°;

[0074] 2) The UOE roll forming method is used to control the continuous and stable forming of the strip into a blank tube shape. During the forming process, the blank tube shape is longitudinally welded using the double-wire cold metal transition welding (double-wire CMT welding) method to form a continuous welded tube.

[0075] The conditions for longitudinal welding are as follows: the shielding gas is 95% argon and 5% hydrogen (volume fraction), the welding current for each wire in the twin-wire CMT welding is 260A, the welding voltage is 24V, the welding speed is 3000mm / min, the gas flow rate is 22L / min, and the wire diameter is 1.0mm.

[0076] Step four: Perform solution treatment and tempering on the longitudinal weld seam of the welded pipe, and then perform bright annealing heat treatment on the entire welded pipe. The specific steps are as follows:

[0077] 1) Use a strip-shaped medium-frequency induction heater to heat the longitudinal weld to 1050℃ with a heating rate ≥10℃ / s. Use a rolling extrusion roller to pile up and roll the upper surface of the longitudinal weld until it is flush with the outer surface of the pipe. Then, temper the longitudinal weld at 500℃. After tempering, air cool the longitudinal weld to room temperature.

[0078] 2) Under hydrogen protection, the welded pipe is subjected to bright annealing heat treatment at a temperature of 1050℃.

[0079] Example 2

[0080] A continuous austenitic heat-resistant steel pipe comprises the following chemical composition by weight percentage: C 0.03%, Si 0.2%, Mn 0.7%, P 0.005%, S 0.001%, Ni 19.00%, Cr 24.00%, Al 2.0%, Cu 2.50%, Nb 0.1%, with the balance being Fe and unavoidable impurities.

[0081] The preparation method of austenitic heat-resistant steel continuous pipe includes the following steps:

[0082] Step 1: Prepare austenitic heat-resistant steel coils. The specific steps are as follows:

[0083] 1) Smelting: The raw materials are prepared according to the composition of the target austenitic heat-resistant steel continuous pipe and added to a vacuum induction furnace to be smelted into molten steel. Then, the steel is refined and homogenized in a refining furnace. The smelting temperature is 1550℃.

[0084] 2) Cast slab: Molten steel is poured into a slab, and the thickness of the slab is 20mm;

[0085] 3) Preparation of coils: The slab is rolled into a hot-rolled plate at 1000℃, and then the hot-rolled plate is subjected to air cooling, tempering at 480℃, pickling, and coiling into a coil to obtain 400-meter coils.

[0086] Step 2: The coil is longitudinally cut into multiple strips of steel ranging from 75 to 182 mm. The multiple strips are butt-welded to form a 1000 m continuous strip using a single-wire cold metal transfer welding method. Then, the butt welds are subjected to solution treatment and tempering treatment to regulate the microstructure.

[0087] Before welding, the end of the steel strip is processed into a 45° bevel, and a V-shaped bevel is opened on the bevel with an included angle of 45°.

[0088] The welding conditions are as follows: shielding gas is 95% argon and 5% hydrogen (volume fraction), welding current is 100A, welding voltage is 20V, welding speed is 300mm / min, gas flow rate is 12L / min, and welding wire diameter is 1.0mm.

[0089] The specific steps for solution treatment and tempering are as follows: The butt weld is heated to 1000℃ in a vacuum furnace at a heating rate of ≥10℃ / s and held in vacuum for 40s. Then, the butt weld is rolled with a rolling pressure of 4 tons. After rolling, the butt weld is cooled rapidly with nitrogen gas to below 150℃ at a cooling rate of ≥12℃ / s. Finally, the butt weld is tempered at 400℃.

[0090] Step 3: The strip is continuously and stably formed into a blank tube shape, and the blank tube shape is longitudinally welded during the forming process to form a continuous welded tube. The specific steps are as follows:

[0091] 1) Mill the edges on both sides of the extended steel strip and make a V-shaped bevel with an included angle of 30-45°;

[0092] 2) The UOE roll forming method is used to control the continuous and stable forming of strip into a blank tube shape. During the forming process, the blank tube shape is longitudinally welded using a double-wire cold metal transfer welding (double-wire CMT welding) method to form a continuous welded tube. The longitudinal welding conditions are as follows: the shielding gas is 95% argon and 5% hydrogen (volume fraction), the welding current of each welding wire in the double-wire CMT welding is 160A, the welding voltage is 22V, the welding speed is 1000mm / min, the gas flow rate is 12L / min, and the welding wire diameter is 1.0mm.

[0093] Step four: Perform solution treatment and tempering on the longitudinal weld seam of the welded pipe, and then perform bright annealing heat treatment on the entire welded pipe. The specific steps are as follows:

[0094] 1) Use a strip-shaped medium-frequency induction heater to heat the longitudinal weld to 1000℃ with a heating rate of ≥10℃ / s. Use a rolling extrusion roller to pile up and roll the upper surface of the longitudinal weld until it is flush with the outer surface of the pipe. Then, temper the longitudinal weld at 400℃. After tempering, air cool the longitudinal weld to room temperature.

[0095] 2) Under hydrogen protection, the welded pipe is subjected to bright annealing heat treatment at a temperature of 980℃.

[0096] Example 3

[0097] A continuous austenitic heat-resistant steel pipe comprises the following chemical composition by weight percentage: C 0.05%, Si 0.4%, Mn 0.9%, P 0.02%, S 0.007%, Ni 21.00%, Cr 26.00%, Al 3.0%, Cu 3.00%, Nb 0.3%, with the balance being Fe and unavoidable impurities.

[0098] The preparation method of austenitic heat-resistant steel continuous pipe includes the following steps:

[0099] Step 1: Prepare austenitic heat-resistant steel coils. The specific steps are as follows:

[0100] 1) Smelting: The raw materials are prepared according to the composition of the target austenitic heat-resistant steel continuous pipe and added to a vacuum induction furnace to be smelted into molten steel. Then, the steel is refined and homogenized in a refining furnace. The smelting temperature is 1700℃.

[0101] 2) Cast slab: Molten steel is poured into a slab, and the thickness of the slab is 40mm;

[0102] 3) Preparation of coils: The slab is rolled into a hot-rolled plate at 1150℃, and then the hot-rolled plate is subjected to air cooling, tempering at 650℃, pickling, and coiling into a coil to obtain 600-meter coils.

[0103] Step 2: The coil is longitudinally cut into multiple strips of steel ranging from 75 to 182 mm. The multiple strips are butt-welded to form a 2000 m continuous strip using a single-wire cold metal transfer welding method. Then, the butt welds are subjected to solution treatment and tempering treatment to regulate the microstructure.

[0104] Before welding, the end of the steel strip is processed into a 45° bevel, and a V-shaped bevel is opened on the bevel with an included angle of 45°.

[0105] The welding conditions were as follows: shielding gas consisted of 95% argon and 5% hydrogen (volume fraction); welding current was 200A; welding voltage was 28V; welding speed was 600mm / min; gas flow rate was 25L / min; and welding wire diameter was 1.2mm. Solution treatment and...

[0106] The specific steps of the tempering treatment are as follows: The butt weld is heated to 1150℃ in a vacuum furnace at a heating rate of ≥10℃ / s and held in vacuum for 60s. Then, the butt weld is rolled with a rolling pressure of 6 tons. After rolling, the butt weld is cooled by rapid blowing of nitrogen gas to below 150℃ at a cooling rate of ≥12℃ / s. Finally, the butt weld is tempered at 600℃.

[0107] Step 3: The strip is continuously and stably formed into a blank tube shape, and the blank tube shape is longitudinally welded during the forming process to form a continuous welded tube. The specific steps are as follows:

[0108] 1) Mill the edges on both sides of the extended steel strip and make a V-shaped bevel with an included angle of 30-45°;

[0109] 2) The UOE roll forming method is used to control the continuous and stable forming of strip into a blank tube shape. During the forming process, the blank tube shape is longitudinally welded using a double-wire cold metal transfer welding (double-wire CMT welding) method to form a continuous welded tube. The longitudinal welding conditions are as follows: the shielding gas is 95% argon and 5% hydrogen (volume fraction), the welding current of each welding wire in the double-wire CMT welding is 280A, the welding voltage is 34V, the welding speed is 3500mm / min, the gas flow rate is 28L / min, and the welding wire diameter is 1.6mm.

[0110] Step four: Perform solution treatment and tempering on the longitudinal weld seam of the welded pipe, and then perform bright annealing heat treatment on the entire welded pipe. The specific steps are as follows:

[0111] 1) Use a strip-shaped medium-frequency induction heater to heat the longitudinal weld to 1150℃ with a heating rate of ≥10℃ / s. Use a rolling extrusion roller to pile up and roll the upper surface of the longitudinal weld until it is flush with the outer surface of the pipe. Then, temper the longitudinal weld at 600℃. After tempering, air cool the longitudinal weld to room temperature.

[0112] 2) Under hydrogen protection, the welded pipe is subjected to bright annealing heat treatment at a temperature of 1080℃.

[0113] Example 4

[0114] The only difference between it and Example 1 is that in the austenitic heat-resistant steel continuous tube, the weight percentage content of Ni is 19% and the weight percentage content of Cr is 26%.

[0115] Example 5

[0116] The only difference between it and Example 1 is that in the austenitic heat-resistant steel continuous tube, the weight percentage content of Ni is 21% and the weight percentage content of Cr is 24%.

[0117] Comparative Example 1

[0118] A continuous austenitic heat-resistant steel pipe comprises the following chemical composition by weight percentage: C 0.01%, Si 0.1%, Mn 0.4%, P 0.01%, S 0.002%, Ni 15.00%, Cr 20.00%, Al 1.0%, Cu 1%, Nb 0.1%, with the balance being Fe and unavoidable impurities.

[0119] The preparation method of the austenitic heat-resistant steel continuous tube is the same as in Example 1.

[0120] Comparative Example 2

[0121] A continuous austenitic heat-resistant steel pipe comprises the following chemical composition by weight percentage: C 0.4%, Si 1%, Mn 2%, P 0.01%, S 0.002%, Ni 24.00%, Cr 30.00%, Al 5%, Cu 5%, Nb 1%, with the balance being Fe and unavoidable impurities.

[0122] The preparation method of the austenitic heat-resistant steel continuous tube is the same as in Example 1.

[0123] Comparative Example 3

[0124] The only difference between it and Example 1 is that the carbon content in the austenitic heat-resistant steel continuous tube is 0.4%.

[0125] Comparative Example 4

[0126] The only difference between it and Example 1 is that the silicon content in the austenitic heat-resistant steel continuous tube is 1%.

[0127] Comparative Example 5

[0128] The only difference between it and Example 1 is that the manganese content in the austenitic heat-resistant steel continuous tube is 2%.

[0129] Comparative Example 6

[0130] The only difference between it and Example 1 is that the nickel content in the austenitic heat-resistant steel continuous tube is 15%.

[0131] Comparative Example 7

[0132] The only difference between it and Example 1 is that the chromium content in the austenitic heat-resistant steel continuous tube is 30%.

[0133] Comparative Example 8

[0134] The only difference between it and Example 1 is that the aluminum content in the austenitic heat-resistant steel continuous tube is 5%.

[0135] Comparative Example 9

[0136] The only difference between it and Example 1 is that the copper content in the austenitic heat-resistant steel continuous tube is 5%.

[0137] Comparative Example 10

[0138] The only difference between it and Example 1 is that the Nb content in the austenitic heat-resistant steel continuous tube is 0.1%.

[0139] Performance testing

[0140] 1) The microstructure of the austenitic heat-resistant steel continuous pipe prepared in Example 1 was examined, and the results were as follows: Figure 1 The metallographic diagram shown is from Figure 1 It can be seen from the data that the microstructure of the prepared austenitic heat-resistant steel continuous pipe is austenitic.

[0141] 2) The yield strength and tensile strength of the austenitic heat-resistant steel continuous tubes prepared in Examples 1 to 5 and Comparative Examples 1 to 10, as well as the commercially available 18Cr austenitic stainless steel continuous tubes, were tested at a high temperature of 300℃. The results are shown in Table 1. The specifications of each continuous tube are Φ25.4×2.5mm.

[0142] 3) The oxidation resistance of the austenitic heat-resistant steel continuous tubes prepared in Examples 1 to 5 and Comparative Examples 1 to 10, as well as commercially available 18Cr austenitic stainless steel continuous tubes, was tested: The oxidation resistance was tested using the static oxidation discontinuous weighing method according to the national standard GB / T13303 "Determination of Oxidation Resistance of Steel". The oxidation resistance of the continuous tubes was tested at 400℃ for different oxidation times. Specifically, weight gain measurements were performed every 20 hours until the 100-hour measurement period was completed.

[0143] The increase in oxide film mass on each continuous tube after 100 hours is shown in Table 1. The test results for the austenitic heat-resistant steel continuous tube prepared in Example 1 are as follows: Figure 2 As shown, by Figure 2 It can be seen that under high temperature conditions, the quality of the oxide film on the surface of austenitic heat-resistant steel continuous pipe does not change significantly with the passage of time, indicating that it has relatively stable high-temperature oxidation resistance.

[0144] 4) At room temperature, the hardness of the austenitic heat-resistant steel continuous tubes prepared in Examples 1 to 5 and Comparative Examples 1 to 10 was measured using a Rockwell hardness tester. The results are shown in Table 1.

[0145] 5) In accordance with the national standard GB / T13303 "Determination of oxidation resistance of steel", the austenitic heat-resistant steel continuous tube prepared in Example 1 was tested for the increase in the mass of the oxide film on the surface of the austenitic heat-resistant steel continuous tube after continuous oxidation for 100 hours at different high temperature environments of 100℃, 150℃, 200℃, 250℃, 300℃, 350℃ and 400℃.

[0146] The results are as follows Figure 3 As shown, the mass increase of the oxide film on the surface of austenitic heat-resistant steel continuous pipe is not significant, indicating relatively stable high-temperature oxidation resistance.

[0147] 6) Intergranular corrosion resistance test: According to Method E in GB / T4334-2020, the austenitic heat-resistant steel continuous tube sample prepared in Example 1 was placed in CuSO4 solution and subjected to bending test after boiling for 16 hours.

[0148] The results are as follows Figure 4 As shown, no cracks appeared on the surface of the austenitic heat-resistant steel continuous pipe sample, and no cracks were observed on the surface of the sample under a 10x magnifying glass, indicating that the pipe has good resistance to intergranular corrosion.

[0149] 7) Stress corrosion resistance test: According to YB / T 5362-2006 standard, the austenitic heat-resistant steel continuous tube sample prepared in Example 1 was immersed in 42% magnesium chloride solution at 143℃ for 48 hours and the test surface was observed.

[0150] The results are as follows Figure 5As shown, the surface of the austenitic heat-resistant steel continuous pipe sample showed no stress corrosion cracking, indicating that the pipe has good resistance to stress corrosion.

[0151]

[0152] As shown in Table 1, compared with the austenitic heat-resistant steel continuous tubes prepared in Comparative Examples 1-10 and the commercially available 18Cr austenitic stainless steel continuous tubes, the austenitic heat-resistant steel continuous tubes prepared in Examples 1-5 have higher yield strength and tensile strength at 300℃, and also have higher elongation, which can effectively improve the high-temperature creep strength, high-temperature load-bearing capacity and thermoplasticity of the austenitic heat-resistant steel continuous tubes; and the austenitic heat-resistant steel continuous tubes prepared in Examples 1-5 have more stable high-temperature oxidation resistance, good resistance to intergranular corrosion and stress corrosion resistance.

[0153] The above are merely preferred embodiments of the present invention and are not intended to limit the scope of the invention. Various modifications and variations are possible with respect to this invention. Any modifications, equivalent substitutions, or improvements made within the spirit and principles of this invention should be included within the scope of protection of this invention.

Claims

1. A continuous austenitic heat-resistant steel pipe, characterized in that, It contains the following chemical composition by weight percentage: C 0.03%–0.05%, Si ≤0.4%, Mn 0.7%–0.9%, P ≤0.02%, S ≤0.007%, Ni 19.00%–21.00%, Cr 24.00%–26.00%, Al 2.0%–3.0%, Cu 2.50%–3.00%, Nb 0.1%–0.3%, with the balance being Fe and unavoidable impurities.

2. The austenitic heat-resistant steel continuous pipe according to claim 1, characterized in that, In the austenitic heat-resistant steel continuous pipe, the weight percentage content of Si is 0.2% to 0.4%.

3. The austenitic heat-resistant steel continuous pipe according to claim 1 or 2, characterized in that, In the austenitic heat-resistant steel continuous pipe, the weight percentage content of Ni to the weight percentage content of Cr is (0.75~0.82):

1.

4. The method for preparing austenitic heat-resistant steel continuous pipe according to any one of claims 1 to 3, characterized in that, Includes the following steps: Step S1: The materials are prepared according to the composition of the target austenitic heat-resistant steel continuous pipe and added to a vacuum induction furnace to be melted into molten steel. Step S2: Cast the molten steel into a slab; Step S3: The slab is rolled into a hot-rolled plate at 1000-1150°C, and then the hot-rolled plate is sequentially subjected to air cooling, tempering, pickling, and coiling into a coil to obtain a coil plate; the tempering temperature of the hot-rolled plate is 450-650°C. Step S4: The coil is longitudinally sheared to obtain strip steel, and the strip steel is butt-welded to form a continuous strip using a cold metal transfer welding method. Then, the butt weld formed by the butt weld is subjected to solution treatment and tempering treatment. The solution treatment temperature of the butt weld is 1000-1150℃, and the tempering treatment temperature is 400-600℃. Step S5: The strip is continuously and stably formed into a blank tube shape, and during the forming process, the blank tube shape is longitudinally welded using a cold metal transition welding method to form a continuous welded tube. Step S6: The longitudinal weld seam on the welded pipe is subjected to solution treatment and tempering treatment, and then the entire welded pipe is subjected to bright annealing heat treatment; the solution treatment temperature of the longitudinal weld seam is 1000~1150℃, and the tempering treatment temperature is 400~600℃.

5. The method for preparing austenitic heat-resistant steel continuous pipe according to claim 4, characterized in that, In step S4, under a protective gas, the strip steel is butt-welded to form a continuous strip using a single-wire cold metal transfer welding method; wherein the welding current of the welding wire is 100-200A, the welding voltage is 20-28V, the welding speed is 300-600mm / min, the gas flow rate is 12-25L / min, the welding wire diameter is 1.0-1.2mm, and the protective gas is a mixture of argon and hydrogen, with the volume fraction of argon in the protective gas being 90%-98%.

6. The method for preparing austenitic heat-resistant steel continuous pipe according to claim 5, characterized in that, In step S4, the solution treatment and tempering treatment of the butt weld are performed as follows: The butt weld is heated to 1000-1150℃ in a vacuum furnace at a heating rate of ≥10℃ / s and held in vacuum for 40-60s. Then, the butt weld is rolled with a rolling pressure of 4-6 tons. After rolling, the butt weld is rapidly cooled with nitrogen gas to below 150℃ at a cooling rate of ≥12℃ / s. Finally, the butt weld is tempered at 400-600℃.

7. The method for preparing austenitic heat-resistant steel continuous pipe according to any one of claims 4 to 6, characterized in that, In step S5, the longitudinal welding of the blank tube is performed using a double-wire cold metal transfer welding method; wherein, the welding current of each welding wire is 160-280A, the welding voltage is 22-34V, the welding speed is 1000-3500mm / min, the gas flow rate is 12-28L / min, the welding wire diameter is 1.0-1.6mm, and the shielding gas is a mixture of argon and hydrogen, with the volume fraction of argon in the shielding gas being 90%-98%.

8. The method for preparing austenitic heat-resistant steel continuous pipe according to claim 7, characterized in that, In step S6, the solution treatment and tempering treatment of the longitudinal weld are performed as follows: The longitudinal weld is heated to 1000-1150℃ at a heating rate of ≥10℃ / s. Then, the upper surface of the longitudinal weld is piled up and rolled with an extrusion roller until it is flush with the outer surface of the pipe. The longitudinal weld is then tempered at 400-600℃. After the tempering is completed, the longitudinal weld is air-cooled to room temperature.

9. The method for preparing austenitic heat-resistant steel continuous pipe according to any one of claims 4 to 6, characterized in that, In step S6, the bright annealing heat treatment process is as follows: Under hydrogen protection, the welded pipe is subjected to bright annealing heat treatment at a temperature of 980–1080°C.

10. The method for preparing a continuous austenitic heat-resistant steel tube according to any one of claims 4 to 6, characterized in that, The melting temperature is 1550–1700℃; Preferably, step S1 further includes refining and homogenizing the molten steel in a refining furnace.

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