Extra-thick-wall stainless steel seamless steel pipe for offshore platform and manufacturing method thereof

Abstract

The present invention relates to the technical field of steel pipe manufacturing, and discloses an extra-large wall thickness stainless steel seamless steel pipe for an offshore platform. The steel pipe is manufactured by the following manufacturing method: Step A: Smelting; Step B: Die casting; Step C: Forging, forging the ingot into a round billet; Step D: Piercing and rolling, heating the round steel and then performing piercing to prepare a rough tube; Step E: Heat treatment, performing fine grain treatment on the rough tube; Step F: Cold rolling and passivation, performing finished product rolling on the heat-treated steel pipe on a cold rolling mill; Step G: Inspection, performing final inspection on the steel pipe; The chemical composition of the steel pipe includes but is not limited to Co≈0.25%, V≈0.15%, Cu≈0.10%, P≈0.045%. By adopting the refining process of "three-deironing molten iron + intermediate frequency furnace pre-solution → AOD → RH → die casting" and combining with the RH vacuum treatment technology, a dehydrogenation rate of more than 80% is achieved, effectively controlling the content of harmful elements, thereby significantly improving the corrosion resistance of the steel pipe.

Description

Technical Field

[0001] The present invention relates to the technical field of steel pipe manufacturing, and specifically to an extra-large wall-thickness stainless steel seamless steel pipe for offshore platforms and a manufacturing method thereof. Background Art

[0002] The material selection of wellhead pipelines for offshore platforms is a complex process, which requires comprehensive consideration of various factors such as the pressure, temperature, corrosiveness of wellhead logistics, as well as the type, wall thickness, procurement cycle, weight, etc. of the material. Since the wellhead is in a complex environment such as high temperature, high humidity, high-salt sea breeze, salt fog, and seawater splash, anti-corrosion becomes the main consideration factor in material selection. Among them, the erosion damage of the chloride ions in the high-humidity sea breeze and salt fog to the pipeline surface and the internal corrosion generated by the fluid inside the pipeline, especially the corrosion brought by CO2, are the main corrosion sources.

[0003] After retrieval, the patent with the Chinese patent number CN204163637U discloses a reinforced oil production pipeline for offshore platforms, which includes a seamless steel pipe and a plurality of support frames welded together. Its characteristics also include integral vulcanized rubber and steel wire ropes. The steel wire ropes are wound around the seamless steel pipe and the support frames along the helical direction. The outer surface of the vulcanized rubber is cylindrical, and the steel wire ropes, the seamless steel pipe, and the support frames are all wrapped inside. There are no gaps, voids, or connection interfaces between adjacent parts inside the vulcanized rubber, and it is a continuous phase as a whole. The vulcanized rubber is tightly bonded to the surfaces of the seamless steel pipe, the support frames, and the wire strands of the steel wire ropes that are in contact with it.

[0004] After retrieval, the patent with the Chinese patent number CN221683867U discloses a welded steel pipe for offshore platforms under harsh working conditions, belonging to the technical field of welded steel pipes. It includes a first pipeline; a second pipeline arranged on the upper side of the first pipeline; and a docking mechanism arranged between the first pipeline and the second pipeline for docking the first pipeline and the second pipeline. On an offshore platform under harsh working conditions, during the threaded docking process of the internal thread hole and the internal threaded pipe, a plurality of plug-in blocks are inserted into a plurality of fixed blocks to ensure the accurate docking of the first pipeline and the second pipeline, reduce the docking difficulty between the first pipeline and the second pipeline, and effectively improve the docking construction efficiency between the welded steel pipes.

[0005] For the above-mentioned offshore platform, due to the excellent corrosion resistance of stainless steel, especially its ability to prevent CO2 corrosion, stainless steel is generally selected as the main material. However, the outer surface of stainless steel is prone to local corrosion such as pitting corrosion and stress corrosion, which will affect the properties of the stainless steel pipe itself. At the same time, due to the high-temperature environment at the wellhead of some offshore platforms, in order to meet the requirements of design pressure, pipe outer diameter, and mechanical allowance, etc., the wall thickness needs to be increased, but this will also increase the excessive load of the platform. Based on this, the present invention designs a super-large wall thickness stainless steel seamless pipe for offshore platforms and a manufacturing method to solve the above problems. Summary of the Invention

[0006] The purpose of the present invention is to provide a super-large wall thickness stainless steel seamless pipe for offshore platforms and a manufacturing method, which solve the problems of local corrosion and increased platform load due to increased wall thickness in the background technology.

[0007] In order to solve the above technical problems, the present invention provides the following technical solutions:

[0008] A super-large wall thickness stainless steel seamless pipe for offshore platforms, the pipe is made by the following manufacturing method:

[0009] Step A: Smelting, adopting the refining process of "triple-decarburized hot metal + medium-frequency furnace pre-solution → AOD → RH → ingot casting". Through RH vacuum treatment, the dehydrogenation rate reaches over 80%, the content of harmful elements is controlled, and the total oxygen content and inclusion size are reduced by the ultra-low oxygen and inclusion refinement technologies of fining and dispersion. Step B: Ingot casting, using large square ingots for ingot casting. The ingot size is 40", and the weight is 10.98 tons. Through protective casting measures, secondary oxidation of the molten steel is prevented, and an ingot with good surface quality is obtained. Step C: Forging, forging the ingot into a round billet, adopting a multi-pass precision forging process, including drawing to a 650mm² square with a flat anvil in the first pass, rough turning to a Φ550mm blank round with a V anvil at 800°C in the second pass, and forging to the finished shape after heating to 1160°C in a bench-type heating furnace at room temperature in the third pass. The total reduction ratio is 4.58, and defect-free round steel is obtained. Step D: Piercing and rolling, heating the round steel and then piercing to prepare a mandrel tube. Using a large-diameter conical roll piercing mill to pierce to obtain a Φ420×95mm mandrel tube, and then performing secondary elongation rolling deformation to obtain a Φ405×85mm rough tube. The wall thickness of the rough tube is between 84.3 - 85.5. Step E: Heat treatment, performing fine grain treatment on the rough tube, adopting the method of grain refinement and quenching. Heating the rough tube to 1020°C in a bench-type heat treatment furnace for 40 minutes and then quenching with water to obtain a steel tube with an average grain size of 5.5 grades and elimination of mixed grains. Step F: Cold rolling and passivation, feeding the heat-treated steel tube onto a cold rolling mill for finish rolling, and then performing pickling and passivation treatment in a pickling and passivation tank. The acid solution used for pickling and passivation is a mixed acid of 68% nitric acid and 40% hydrofluoric acid. The pure weight ratio of nitric acid is 17%, and the pure weight ratio of hydrofluoric acid is 5%. The acid immersion time is 40 minutes to achieve complete and clean passivation. Step G: Inspection, performing final inspection on the steel tube to ensure compliance with the technical requirements of extra-large wall-thickness stainless steel seamless tubes for offshore platforms.

[0010] The chemical composition of the steel tube includes, but is not limited to, Co≈0.25%, V≈0.15%, Cu≈0.10%, P≈0.045%. Through the above process, the content of these harmful metal elements is reduced by over 60%, and the reduction rate of Co element can reach 93%.

[0011] Preferably, the refining process in Step A further includes the following steps: preparing a slag system with high content of MgO + Al2O3 in LF and using strong aluminum deoxidation to further reduce the total oxygen content in the steel to below 15ppm, controlling the inclusion size in the molten steel to below 15µm and significantly reducing the quantity, and eliminating large-size inclusions.

[0012] Preferably, the protective casting measures in step B include preventing secondary oxidation on the surface of the molten steel rising in the protective mold and preventing secondary oxidation of the molten steel flow from the ladle to the center casting tube. The down-casting method is adopted to inject the molten steel into the center casting tube, and it is injected into the ingot mold from the bottom through the runner built on the ingot plate, while casting multiple ingots at the same time to improve the surface quality of the ingots.

[0013] Preferably, the multi-pass precision forging process in step C also includes completely cooling the round steel before each heating forging to eliminate internal stress, refine grains and optimize the tissue performance. The reduction of the round steel after three-pass forging is 20% to ensure no defects.

[0014] Preferably, the piercing to prepare the hollow billet in step D uses a conical roll piercing mill. The rolling angle β of the conical roll is designed to be 15°, and the forward angle γ is 11° to control the circumferential shear deformation and surface distortion deformation to ensure the quality of the hollow billet. The secondary elongation rolling deformation in step D is carried out in a skew rolling mill, using a short mandrel to roll a mandrel with a length of up to 60 mm to obtain a rough tube with precise wall thickness dimensions and a smooth inner surface without corrugations. The average grain size of the rough tube is 2.5 grades, and fine grain treatment is required.

[0015] Preferably, the fine grain treatment in step E also includes heating the rough tube in a bench-type heat treatment furnace to the austenite stable region of 1020 °C, quickly heating for 40 minutes and then taking it out of the furnace and quenching it with water. The water temperature is 20 °C, and the cooling time is 2 minutes to improve the grain size to an average of 5.5 grades and eliminate the mixed grain phenomenon.

[0016] Preferably, the cold rolling and passivation in step F also include ultrasonic cleaning of the steel pipe before cold rolling to remove the oil stains and dirt on the surface. The cleaning medium is a highly efficient weak acid environmentally friendly degreasing agent, and the ultrasonic cleaning machine is a piezoelectric 200KHz high-frequency transducer. After cleaning, the steel pipe is subjected to solution heat treatment. The pickling and passivation treatment in step F also includes pickling in a pickling and passivation tank with a mixed acid of nitric acid with a concentration of 68% and hydrofluoric acid with a concentration of 40%. The pure weight ratio of nitric acid is 17% and the pure weight ratio of hydrofluoric acid is 5%. After pickling for 40 minutes, take it out and rinse it with clear water and then soak it in hot water for 10 minutes to achieve a completely clean and passivated state.

[0017] Preferably, the chemical composition of the steel pipe also includes C≤0.03%, Si≤1.0%, Mn≤2.0%, Cr: 16.0%-18.0%, Ni: 10.0%-14.0%. During the manufacturing process of the steel pipe, environmental adaptability tests are also required, including but not limited to salt spray resistance test and seawater corrosion resistance test to ensure the long-term stable performance of the steel pipe in the marine environment. After the steel pipe is manufactured, it also needs to be packaged and stored. Moisture-proof and corrosion-proof packaging materials are used and stored in a dry and ventilated environment to protect the steel pipe from the influence of the external environment.

[0018] Preferably, the inspection in step G includes a comprehensive inspection of the chemical composition, dimensional accuracy, surface quality, internal structure and mechanical properties of the steel pipe to ensure that the steel pipe meets the special requirements of the offshore platform.

[0019] Preferably, in step F, after cold rolling forming, straightening treatment is also required to ensure the straightness and roundness of the steel pipe. During the straightening process, a continuous straightening machine is used, and precise straightening of the steel pipe is achieved by adjusting the roll gap and pressure; after the passivation treatment in step F, a final surface inspection is also required, including but not limited to visual inspection and non-destructive testing, to ensure that the surface of the steel pipe is free of defects and meets the requirements of corrosion resistance.

[0020] Compared with the prior art, the beneficial effects achieved by the present invention are as follows:

[0021] 1. In the present invention, by adopting the refining process of "three-deoxidized hot metal + intermediate frequency furnace pre-solution → AOD → RH → ingot casting", combined with the RH vacuum treatment technology, a dehydrogenation rate of more than 80% is achieved, effectively controlling the content of harmful elements, thereby significantly improving the corrosion resistance of the steel pipe, especially in preventing CO2 corrosion; in addition, through the ultra-low oxygen and fine-grained and dispersed refining technology of inclusions, the total oxygen content and the size of inclusions are further reduced, enhancing the corrosion resistance of the steel pipe in the marine environment.

[0022] 2. In the present invention, the selected TP316L stainless steel material has a relatively high pitting corrosion equivalent value (PRE value is 24.2), meeting the minimum standard for stainless steel material selection in the offshore platform environment, effectively reducing the risk of pitting corrosion and stress corrosion, thereby extending the service life of the steel pipe; by precisely controlling the wall thickness of the steel pipe, when the outer diameter is 335.6 mm, the wall thickness needs to reach 82.55 mm, ensuring that the steel pipe meets the requirements of design pressure and mechanical allowance while avoiding excessive increase in the load of the platform, achieving structural optimization.

[0023] 3. In the present invention, through the multi-pass precision forging process and the fine grain treatment technology, the internal grains of the steel pipe are made finer, the tissue performance is optimized, and the overall strength and toughness of the steel pipe are improved; during the manufacturing process, strict dimensional control and surface treatment technologies are adopted, including ultrasonic cleaning and pickling and passivation treatment, ensuring the dimensional accuracy and surface quality of the steel pipe and meeting the strict requirements of the offshore platform for the steel pipe. BRIEF DESCRIPTION OF THE DRAWINGS

[0024] Figure 1 It is a flow chart of the characteristics of the steel pipe of the present invention and the manufacturing method of the steel pipe;

[0025] Figure 2 It is a detailed flow chart of the manufacturing method of the steel pipe of the present invention;

[0026] Figure 3 TP316L chemical composition diagram (mass fraction %) of the steel pipe of the present invention;

[0027] Figure 4 Non-metallic inclusion composition diagram of the steel pipe of the present invention;

[0028] Figure 5 Mechanical property test diagram of the steel pipe of the present invention. Detailed implementation manners

[0029] Next, the technical solutions in the embodiments of the present invention will be clearly and completely described in conjunction with the accompanying drawings in the embodiments of the present invention. Obviously, the described embodiments are only a part of the embodiments of the present invention, rather than all of the embodiments. Based on the embodiments of the present invention, all other embodiments obtained by those of ordinary skill in the art without creative efforts shall fall within the protection scope of the present invention.

[0030] Embodiment 1;

[0031] Please refer to Figure 1 - Figure 2 , in the embodiment of the present invention, a super-large wall-thickness stainless steel seamless steel pipe for an offshore platform is made by the following manufacturing method:

[0032] Step A: Smelting, adopting the refining process of "three-decarburized hot metal + intermediate frequency furnace pre-solution → AOD → RH → die casting", achieving a dehydrogenation rate of more than 80% through RH vacuum treatment, controlling the content of harmful elements, and reducing the total oxygen content and inclusion size through ultra-low oxygen and inclusion refinement technologies such as fining and dispersion;

[0033] Step B: Die casting, using a large square ingot for die casting, with the ingot size of 40" and the weight of 10.98 tons, preventing secondary oxidation of the molten steel through protective casting measures to obtain an ingot with good surface quality;

[0034] Step C: Forging, forging the ingot into a round billet, adopting a multi-pass precision forging process, including drawing to a 650 mm² square with a flat anvil in the first pass, rough turning to a Φ550 mm blank circle with a V anvil at 800 °C in the second pass, and forging to shape at 1160 °C in a bench-type heating furnace with room temperature loading in the third pass, with a total compression ratio of 4.58 to obtain defect-free round steel;

[0035] Step D: Piercing and rolling, heating the round steel and then piercing to prepare a shell, using a large-diameter conical roll piercing mill to pierce to obtain a shell of Φ420×95 mm, and then performing secondary elongation rolling deformation to obtain a rough pipe of Φ405×85 mm, with the wall thickness of the rough pipe between 84.3 - 85.5;

[0036] Step E: Heat treatment. The rough tube is subjected to fine grain treatment by adopting the method of grain refinement quenching. The rough tube is heated to 1020 °C in a bench-type heat treatment furnace and rapidly heated for 40 minutes, then taken out of the furnace and quenched with water to obtain a steel tube with an average grain size of 5.5 grades and mixed grains eliminated.

[0037] Step F: Cold rolling and passivation. The heat-treated steel tube is rolled into a finished product on a cold rolling mill, and then pickled and passivated in a pickling and passivation tank. The acid solution used for pickling and passivation is a mixed acid of nitric acid with a concentration of 68% and hydrofluoric acid with a concentration of 40%. The pure weight ratio of nitric acid is 17% and the pure weight ratio of hydrofluoric acid is 5%. The acid immersion time is 40 minutes to achieve a completely clean passivation degree.

[0038] Step G: Inspection. The steel tube is subjected to final inspection to ensure compliance with the technical requirements of extra-large wall thickness stainless steel seamless steel tubes for offshore platforms.

[0039] The chemical composition of the steel tube includes but is not limited to Co≈0.25%, V≈0.15%, Cu≈0.10%, P≈0.045%. Through the above process, the content of these harmful metal elements is reduced by more than 60%, and the reduction rate of Co element can reach 93%.

[0040] The refining process of Step A also includes the following steps: preparing a slag system with high content of MgO+Al2O3 in LF and using strong aluminum deoxidation to further reduce the total oxygen content in the steel to below 15 ppm, controlling the size of inclusions in the molten steel to below 15 µm and significantly reducing the quantity, and eliminating large-size inclusions.

[0041] The protective casting measures in Step B include protecting the surface of the molten steel rising in the protective mold from secondary oxidation and protecting the molten steel flow from the ladle to the central casting tube from secondary oxidation. The bottom casting method is adopted to inject the molten steel into the central casting tube, and it is injected into the ingot mold from the bottom through the runner built on the ingot plate, and multiple ingots are cast simultaneously to improve the surface quality of the ingot.

[0042] The multi-pass precision forging process in Step C also includes completely cooling the round steel before each heating and forging to eliminate internal stress, refine grains and optimize the tissue performance. The reduction of the round steel after three-pass forging is 20% to ensure no defects.

[0043] The piercing to prepare the rough tube in Step D adopts a conical roll piercing mill. The rolling angle β of the conical roll is designed to be 15° and the forward angle γ is 11° to control the circumferential shear deformation and surface distortion deformation and ensure the quality of the rough tube. The secondary elongation rolling deformation in Step D is carried out in a skew rolling mill, and a mandrel with a length of 60 mm is used to roll the strip with a short mandrel to obtain a rough tube with precise wall thickness dimensions and a smooth inner surface without corrugations. The average grain size of the rough tube is 2.5 grades and fine grain treatment is required.

[0044] The working principle of the embodiments of the present invention is as follows: The material selection of the wellhead pipeline of an offshore platform needs to comprehensively consider factors such as the pressure, temperature, and corrosiveness of the wellhead logistics. In such a complex environment, the pipeline is exposed to the erosion of high-humidity sea breeze, salt spray, seawater splashing, etc., especially the damage to the pipeline surface by chloride ions. In addition, the corrosion caused by the internal fluid of the pipeline is also a complex and variable process, among which the corrosion effect of CO2 is particularly prominent, and it is closely related to the concentration, partial pressure, water content, and temperature of CO2. The CO2 content at the wellhead ranges from 5.7% to 0.03% in mole percentage, the maximum partial pressure can reach 1.44 MPa, the design temperature range is from -14°C to 85°C, and the working pressure is 32.3 MPa. According to the Dewarrd&Milliam formula, the maximum corrosion rate can reach 0.5 mm / a. Stainless steel is preferentially considered due to its excellent CO2 corrosion resistance. In contrast, the maximum corrosion rate of carbon steel pipelines can reach 5.57 mm / a under high CO2 content. Without anti-corrosion measures, their working life of more than 10 years in a high-pressure process system will face great risks. Even if the corrosion allowance is increased by about 27 mm, it will also increase the load of the platform. Therefore, it is particularly important to select stainless steel materials to resist CO2 corrosion.

[0045] However, the outer surface of stainless steel pipelines is also prone to pitting corrosion and stress corrosion in the marine environment. Pitting corrosion usually occurs in an environment containing CI¯ (including Br¯, I¯) ions. The higher the pitting corrosion resistance equivalent value (PRE) of stainless steel, the stronger its pitting corrosion resistance. In the offshore platform environment, the PRE value of stainless steel should be at least 24. The PRE value of TP304L is 19.0, while the PRE value of TP316L is 24.2. Therefore, TP316L becomes the minimum standard for stainless steel material selection in the marine environment. At some wellheads in the East China Sea oil and gas fields in China, the highest temperature is close to 68°C, and the strength of stainless steel determines the selection of wall thickness. According to factors such as the design pressure, pipeline outer diameter, allowable stress of TP316L, and mechanical allowance, when the outer diameter is 335.6 mm, the wall thickness needs to reach 82.55 mm, the total length is more than 18 meters, and the diameter-wall ratio (D / S) is only 4.06, belonging to the category of ultra-thick-walled seamless pipes.

[0046] In the smelting process of Step A, a refining process of "three-decarburized hot metal + intermediate frequency furnace pre-solution → AOD → RH → ingot casting" was developed. The RH vacuum treatment for stainless steel dehydrogenation can achieve a dehydrogenation rate of over 80%. This process route mainly reflects the control of harmful elements. Ordinary austenitic stainless steel is smelted using scrap steel and other raw materials. In the steel, Co≈0.25%, V≈0.15%, Cu≈0.10%, P≈0.045%, which are much higher than the specified values of TP316L for offshore platform pipelines. After use, a breakthrough in the control of stainless steel purity was achieved, reducing the harmful metal elements of P, V, Cu, and Co in stainless steel by over 60%, and the reduction rate of Co element can reach 93%. To meet the high requirements of TP316L for offshore platform pipelines regarding the oxygen content and inclusions in steel, a refining technology for ultra-low oxygen and fine and dispersed inclusions in stainless steel was developed. The main feature is to prepare a high-content MgO + Al2O3 slag system in the first LF and use strong aluminum deoxidation. The total oxygen content in the steel was reduced from above 30 ppm to below 15 ppm. The size of inclusions in the molten steel was controlled below 15 µm and the quantity was significantly reduced, eliminating large-size inclusions.

[0047] In the ingot casting process of Step B, stainless steel is a multi-element high-alloy steel, especially stainless steel of the high-performance TP316L grade. The steel contains various easily oxidized metal elements. If the exposed molten steel comes into contact with air, it will react with O2 and N2 in the air, forming a film, crust, turnover skin, double skin during casting and resulting in an increase in inclusions, seriously affecting the surface quality and internal quality of the ingot. Therefore, protective casting measures are taken for stainless steel casting. The protection of ingot casting is mainly carried out from two aspects: one is to protect the molten steel surface rising in the mold from secondary oxidation; the other is to protect the steel flow from the ladle to the central casting tube from secondary oxidation. The bottom casting method injects the molten steel into the central casting tube and then into the ingot mold from the bottom through the runner built on the ingot plate, and multiple ingots can be cast simultaneously. The advantage is that the molten steel rises smoothly in the ingot, without causing splashing of the steel flow, and at the same time, it is convenient to take protective casting measures, and the surface quality of the cast ingot is good. Selecting a large square ingot with a size of 40" and a weight of 10.98 tons is to increase the compression ratio during forging into a round billet, making the internal grains of the round billet finer, the tissue performance more optimized, and reducing the tendency of central cracks that may occur during piercing.

[0048] In Step C, forging round steel is a key link in the manufacturing process, directly affecting the internal and external surface quality of the pierced seamless tube blank and the second-pass elongation rolled rough tube. The heating temperature and reduction amount during forging are two core process parameters.

[0049] One-fire forging: Adopt the process of drawing out to a square with a cross-sectional area of 650 mm² using flat anvils. The initial forging temperature is controlled at 1180 °C, the final forging temperature is maintained at 850 °C, and the reduction amount is set at 32%.

[0050] Two-fire forging: Charge the furnace at 800°C, use a V-anvil Φ550mm rough round, and keep the forging temperature the same as that of the single-fire forging. Adjust the reduction ratio to 15%.

[0051] Three-fire forging: Before performing three-fire forging, it is necessary to ensure that the round steel is completely cooled before reheating. This step is crucial because it causes the coarse dendrites and columnar grains after two hot forging processes to twist and crush, transforming into equiaxed recrystallized grains with finer and more uniform sizes. Complete cooling helps with solid-phase transformation, creating conditions for the refinement of recrystallization during the next hot forging; it curbs embrittlement caused by the precipitation of δ-phase and surface cracks during reheating forging; it alleviates the increase in internal stress caused by two consecutive hot forging processes, reducing the risk of defects in the core of the re-forged parts. The temperature for three-fire forging is set at 1160°C, charge the furnace at room temperature, the heating time in the bench-type heating furnace is 440 minutes, the initial forging temperature is 1190°C, the final forging temperature is 900°C, use a die-anvil Φ450mm round, air-cool, and the reduction ratio is 20%. The entire forging process uses a 1600-ton rapid forging unit, with die forging mainly by drawing out, and the total compression ratio reaches 4.58.

[0052] In step D, considering that the product is an extra-large thick-walled pipe with a diameter-to-wall ratio (D / S) of only 4.38, belonging to the category of ultra-extra-large thick-walled pipes. Considering that cold rolling overload may lead to failure to meet the technical index requirements, this process guideline adopts "large thermoplastic deformation and precision cold-controlled deformation".

[0053] Hot piercing: The stress on the stainless steel round steel during skew rolling is complex, and internal and external surface defects are likely to form. To prevent defects and provide high-quality billets for secondary thermoplastic elongation rolling, a large-diameter conical roll (mushroom type) piercing unit is used to pierce a billet of Φ420×95mm. The main difference between the conical roll and the ordinary drum-type roll lies in its "larger at the entrance and smaller at the exit" conical design, as well as the rolling angle in addition to the forward angle (bite angle). This design increases the axial pulling force before the round steel is bitten again for the second time, effectively alleviating the rotary forging effect and surface distortion and circumferential shear deformation caused by shear stress, and greatly improving the inner wall quality of the rough pipe. The β angle is designed to be 15°, and the γ angle is 11°. When γ+β≥25°, the quality of the billet is the best.

[0054] Heating parameters: The round steel is heated in an inclined-bottom furnace, the tapping temperature is 1160°C, it stays at 720°C for 80 minutes and then starts to move forward, the travel time is 300 minutes to the tapping port, and then it is soaked and heat-insulated for 60 minutes before being taken out of the furnace for piercing.

[0055] Secondary Stretch Rolling: The semifinished tube close to the finished product size requires delicate technical requirements. The secondary stretch deformation rolling aims to achieve that the wall thickness of the semifinished tube reaches or approaches that of the final finished product, while ensuring good surface quality and internal structure. After the blank tube is formed, it needs to be water quenched with a cooling rate greater than 500 °C / minute to solidify and enhance plasticity. The blank tube needs to be further inspected, and after removing surface defects, it is reheated for stretch rolling. The secondary stretch rolling is carried out on a skew rolling mill, using a short mandrel to roll a mandrel with a length of up to 60 mm to obtain a semifinished tube with precise wall thickness dimensions and a smooth inner surface without corrugations. The average grain size of the semifinished tube is at level 2.5, and fine grain treatment is required.

[0056] Example 2;

[0057] Please refer to Figure 1 - Figure 2 , in the embodiment of the present invention, the fine grain treatment in step E further includes heating the semifinished tube to the austenite stable region of 1020 °C in a bench-type heat treatment furnace, quickly heating for 40 minutes and then taking it out of the furnace for water quenching, the water temperature is 20 °, and the cooling time is 2 minutes to improve the grain size to an average of level 5.5 and eliminate the mixed grain phenomenon.

[0058] The cold rolling and passivation in step F also include ultrasonic cleaning of the steel pipe before cold rolling to remove surface oil stains and dirt, the cleaning medium is a highly efficient weak acid environmentally friendly degreasing agent, the ultrasonic cleaning machine is a piezoelectric 200KHz high-frequency transducer, and the cleaned steel pipe is then subjected to solution heat treatment; the pickling and passivation treatment in step F also includes pickling in a pickling and passivation tank with a mixed acid of nitric acid with a concentration of 68% and hydrofluoric acid with a concentration of 40%, the pure weight ratio of nitric acid is 17%, the pure weight ratio of hydrofluoric acid is 5%, after pickling for 40 minutes, it is fished out, rinsed with clear water and then soaked in hot water for 10 minutes to achieve complete clean passivation.

[0059] The chemical composition of the steel pipe also includes C≤0.03%, Si≤1.0%, Mn≤2.0%, Cr: 16.0%-18.0%, Ni: 10.0%-14.0%; during the manufacturing process of the steel pipe, environmental adaptability tests are also required, including but not limited to salt spray resistance test and seawater corrosion resistance test to ensure the long-term stable performance of the steel pipe in the marine environment; after the steel pipe is manufactured, packaging and storage are also required, using moisture-proof and corrosion-proof packaging materials and storing in a dry and ventilated environment to protect the steel pipe from the influence of the external environment.

[0060] The inspection in step G includes comprehensive detection of the chemical composition, dimensional accuracy, surface quality, internal structure and mechanical properties of the steel pipe to ensure that the steel pipe meets the special requirements of the offshore platform.

[0061] In step F, after cold rolling forming, straightening treatment is also required to ensure the straightness and roundness of the steel pipe. During the straightening process, a continuous straightening machine is used, and precise straightening of the steel pipe is achieved by adjusting the roll gap and pressure. After the passivation treatment in step F, a final surface inspection is also required, including but not limited to visual inspection and non-destructive testing, to ensure that the surface of the steel pipe is free of defects and meets the requirements of corrosion resistance.

[0062] The working principle of the embodiment of the present invention is as follows: After the rough pipe is inspected and surface defects are removed by grinding, it is fed onto the cold rolling mill. The finished product rolling is carried out on an LD350 three-high rolling mill. The adjusted parameters include the feed value (per half stroke) of 3 mm, the number of strokes of 22 times / min, the elongation coefficient of 1.21, and the reduction ratio of 18%. The rolls of the three-high cold rolling mill are evenly distributed in the vertical plane and form an annular pass at an angle of 120° with each other. The rolling force is transmitted through the slideway to the fixed thick-wall frame sleeve to generate a uniform positive pressure stress, so that the impact force of the rotary feed is small and the surface of the steel pipe is flat and smooth. After rolling, the oil stains on the surface of the steel pipe must be removed completely before solution heat treatment to avoid the "chromium depletion" phenomenon caused by local high temperature. The ultrasonic cleaning method is adopted, the cleaning medium is a highly efficient weak acid environmental protection degreasing agent, and the ultrasonic cleaning machine used is a piezoelectric 200KHz high-frequency transducer. The cleaned steel pipe is then soaked in a hot water tank for 30 minutes, fished out and rinsed clean with a high-pressure water gun until it meets the standard of "no water droplets forming beads".

[0063] In order to make the solution timeliness of the solution heat treatment and be able to quench again to refine the grains and obtain the tissue properties of the TP316L stainless steel pipe, the final heat treatment of the finished product is a very crucial step. In the continuous heat treatment furnace, the temperature of the high-temperature section is set at 1050°C, and the holding time should be long to completely precipitate the intermetallic compounds, and then solution strengthening is achieved by quenching in water. The temperature and holding time of each section are as follows: preheating section, temperature 800 - 960°C, time 40 minutes; heating section, temperature 960 - 1060°C, time 30 minutes; soaking section, temperature 1050°C, time 20 minutes; spray chamber, water temperature 20°C, and the water flow rate of each spray pipe is 1.2 m / s.

[0064] The passivation of stainless steel is an essential and important process, and the good corrosion resistance of stainless steel is based on its passivability. TP316L is a multi-element alloy with a certain structure. Its microstructure is alloying elements existing in the form of solid solution, with relatively high chromium and nickel. Among the alloying elements, they can comprehensively improve the passivation performance of stainless steel and form a more stable oxide film, thus enhancing its corrosion resistance. During pickling and passivation, attention should be paid to the ratio of the passivation tank and the passivation time. The pickling of finished stainless steel pipes serves two purposes: one is to remove the surface scale and dirt, and the other is to passivate the steel base to generate a protective film of chromium and low-valent oxides. Since TP316L is austenitic stainless steel, the chromium sesquioxide contained in its scale is insoluble in ordinary acids, making pickling rather difficult. The mixed acid used in this application is nitric acid with a strong oxidation acid concentration of 68% and hydrofluoric acid with a concentration of 40%. Nitric acid can convert chromium and iron low-valent oxides into high-valent oxides. Only when the low-valent oxides are oxidized into high-valent oxides can they be dissolved by the acid. In addition, nitric acid can passivate the steel base to generate a protective film of chromium and low-valent oxides. Hydrofluoric acid is used to dissolve the high-valent oxides of iron, chromium, and nickel. However, there should be a certain ratio between nitric acid and hydrofluoric acid. If the proportion of nitric acid is too low, ferrous oxide is not oxidized into iron, and hydrofluoric acid cannot dissolve iron oxide but can only dissolve chromium oxide and nickel, which will form acid etching points on the surface of the steel pipe, resulting in over-pickling defects. The molar concentration of 68% nitric acid is 15.2mol / L, and the mixing ratio should be calculated based on converting the concentration content into a pure 100% concentration, resulting in a range between 12 - 15%; the molar concentration of 40% hydrofluoric acid is 22.4mol / L, calculated to be 3 - 5%. In actual applications, there are different adjustments according to the material and size of the pipe. For TP316L extra-large thick-walled pipes, during pickling and passivation, the pure weight ratio of nitric acid used in this application is 17%, and the pure weight ratio of hydrofluoric acid is 5%. The acid immersion time is 40 minutes, then fished out and rinsed with clear water and soaked in hot water for 10 minutes to achieve a completely clean passivation degree.

[0065] Example 3;

[0066] Please refer to Figure 1 - Figure 5 , in the embodiment of the present invention, the steel pipes manufactured by the manufacturing method are inspected:

[0067] 1. Surface and dimension inspection

[0068] (1) Surface quality and roughness

[0069] For the TP316L extra-large thick-walled stainless steel pipes in this application, after inspection, there are no obvious defects on both the inner and outer surfaces, and the surface roughness reaches Ra3.6, meeting the standard requirements of pickled and passivated pipes.

[0070] (2) Dimension tolerance

[0071] According to the platform standard Φ355.6×82.55×18000mm, dimensional tolerance inspections are carried out on the outer diameter, wall thickness and length. The outer diameter tolerance is from +2.4mm to -0.8mm, and the actual measured value is 356.3mm; the wall thickness tolerance is from +0.00 to -8.2mm, and the actual measured value is 80.4mm; the length tolerance is from +6mm to -0.00, and the actual measured value is 18002mm. All dimensions meet the standard requirements and are judged to be qualified.

[0072] (3)Nondestructive testing

[0073] The steel pipe is detected by the integrated ultrasonic and eddy current flaw detection technology, and no alarm signal is found, indicating that there are no defects inside the steel pipe.

[0074] 2. Physical and chemical and performance testing

[0075] (1)Chemical composition

[0076] According to ASTM A959 standard, the chemical composition of TP316L stainless steel is analyzed, and the results are as Figure 3 shown.

[0077] (2)Non-metallic inclusions

[0078] The non-metallic inclusions are analyzed, and the test results meet the platform standard. The fine series and coarse series of inclusions from class A to class D are within the standard range. The specific data are as Figure 4 shown.

[0079] (3)Metallographic structure

[0080] The test results of grain size show that the platform requires greater than grade 5 or finer. The measured value is grade 6, and the grade difference is 2 grades. The measured α-phase content is 8%, and the grade is 1.5 grades.

[0081] (4)Intergranular corrosion test

[0082] The intergranular corrosion sensitivity test is carried out according to the standard procedure E of ASTM A262. No corrosion cracks are found in the specimen, and it is judged to be qualified.

[0083] (5)Mechanical property test

[0084] According to ASTM A312 standard, the yield strength, tensile strength and elongation are tested, and the results are as Figure 5 shown.

[0085] The working principle of the embodiment of the present invention is: Through strict inspection and verification of actual application on the platform, the above-mentioned steel pipe fully meets the technical requirements in terms of surface quality, dimensional accuracy, chemical composition, non-metallic inclusions, metallographic structure, intergranular corrosion sensitivity and mechanical properties, and all indicators reach or exceed the standard provisions.

[0086] Working principle: This application proposes a manufacturing method for extra-large wall-thickness stainless steel seamless steel pipes used in offshore platforms. Through a series of fine technological steps, the high performance and reliability of the steel pipes are ensured. First, the refining process of "three-deironing molten iron + intermediate frequency furnace pre-solution → AOD → RH → ingot casting" is adopted, combined with RH vacuum treatment technology, achieving a dehydrogenation rate of over 80%, effectively controlling the content of harmful elements, and reducing the total oxygen content and inclusion size in the steel. During the ingot casting process, large square ingots and protective casting measures are taken to prevent secondary oxidation of the molten steel and ensure the surface quality of the ingots. In the forging link, the application of multi-pass precision forging technology homogenizes the internal structure of the round steel, improving its plasticity and mechanical properties. In the piercing and rolling steps, the application of a large-diameter conical roll piercing mill optimizes the quality of the hollow billet, and the secondary elongation rolling deformation ensures precise control of the wall thickness of the rough pipe. In the heat treatment step, the grain refinement quenching technology improves the grain size of the rough pipe to grade 5.5, eliminating the mixed grain phenomenon. During the cold rolling and passivation process, the application of ultrasonic cleaning and pickling passivation technology ensures the cleanliness and corrosion resistance of the steel pipe surface. The final inspection and testing results show that the steel pipe fully meets the technical requirements in terms of surface quality, dimensional accuracy, chemical composition, non-metallic inclusions, metallographic structure, intergranular corrosion sensitivity, and mechanical properties, and all indicators reach or exceed the standard provisions.

[0087] Although the embodiments of the present invention have been shown and described, for those of ordinary skill in the art, it can be understood that various changes, modifications, substitutions, and variations can be made to these embodiments without departing from the principles and spirit of the present invention. The scope of the present invention is defined by the appended claims and their equivalents.

Claims

1. A manufacturing method of extra-thick-wall stainless steel seamless steel pipes for offshore platforms, characterized in that The steps of the manufacturing method are as follows: Step A: Smelting. The "triple-deoxidized molten iron + medium-frequency furnace pre-melted liquid → AOD → RH" refining process is adopted. Through RH vacuum treatment, the dehydrogenation rate is over 80%, the content of harmful elements is controlled, and the total oxygen content and the inclusion size are reduced by the ultra-low oxygen and inclusion refinement technologies of fine and dispersed inclusions; Step B: Ingot casting. Big square ingots are used for ingot casting. The size of the ingot is 40", and the weight is 10.98 tons. Molten steel secondary oxidation is prevented through protective casting measures to obtain ingots with good surface quality; Step C: Forging. The ingot is forged into a round billet. The multi-pass precision forging process is adopted, including one-pass upsetting to a 650mm² square on a flat anvil, the second pass heating to 800°C and forging a Φ550mm rough round on a V anvil, and the third pass heating to 1160°C in a bench-type heating furnace at room temperature and then forging into shape. The total reduction ratio is 4.58 to obtain defect-free round steel; Step D: Piercing and rolling. The round steel is heated and then pierced to prepare a pilger pipe. A large-diameter conical roll piercing mill is used for piercing to obtain a Φ420×95mm pilger pipe, and then secondary elongation rolling deformation is carried out to obtain a Φ405×85mm rough pipe. The wall thickness of the rough pipe is between 84.3 - 85.5; Step E: Heat treatment. The rough pipe is subjected to fine grain treatment. The grain refinement quenching method is adopted. The rough pipe is heated to 1020°C in a bench-type heat treatment furnace and rapidly heated for 40 minutes and then taken out of the furnace and water quenched to obtain a steel pipe with an average grain size of 5.5 grades and mixed grains eliminated; Step F: Cold rolling and passivation. The heat-treated steel pipe is put on a cold rolling mill for finish rolling, and then pickling and passivation treatment is carried out in a pickling and passivation tank. The acid solution used for pickling and passivation is a mixed acid of 68% nitric acid and 40% hydrofluoric acid. The pure weight ratio of nitric acid is 17% and the pure weight ratio of hydrofluoric acid is 5%. The acid dipping time is 40 minutes to achieve complete clean passivation; Step G: Inspection. The steel pipe is subjected to final inspection to ensure compliance with the technical requirements of extra-large wall thickness stainless steel seamless pipes for offshore platforms; The refining process in Step A further includes the following steps: LF is used to prepare a slag system with high MgO + Al2O3 content and strong aluminum deoxidation is used to further reduce the total oxygen content in the steel to below 15ppm, control the inclusion size in the molten steel to below 15µm and significantly reduce the number, and eliminate large-size inclusions; The protective casting measures in Step B include protecting the molten steel surface rising in the mold from secondary oxidation and protecting the molten steel flow from the ladle to the center casting pipe from secondary oxidation. The down-casting method is adopted to inject the molten steel into the center casting pipe, and it is injected into the ingot mold from the bottom through the runner built on the ingot plate, and multiple ingots are cast simultaneously to improve the surface quality of the ingots; The multi-pass precision forging process in Step C further includes completely cooling the round steel before each heating and forging to eliminate internal stress, refine grains and optimize the tissue performance. The reduction of the round steel after three-pass forging is 20% to ensure no defects; The piercing-prepared capillary tube in step D uses a conical roll piercing mill. The rolling angle β of the conical roll is designed to be 15°, and the forward angle γ is 11° to control the circumferential shear deformation and surface distortion deformation and ensure the quality of the capillary tube. The secondary elongation rolling deformation in step D is carried out in a skew rolling mill, using a short mandrel to roll a mandrel with a length of up to 60 mm to obtain a rough tube with precise wall thickness dimensions and a smooth inner surface without corrugations. The average grain size of the rough tube is 2.5 grades on average, and fine grain treatment is required. The fine grain treatment in step E also includes heating the rough tube in a bench-type heat treatment furnace to the austenite stable region of 1020 °C, quickly heating for 40 minutes and then taking it out of the furnace and quenching it with water. The water temperature is 20 °C, and the cooling time is 2 minutes to increase the grain size to an average of 5.5 grades and eliminate the mixed grain phenomenon. The cold rolling and passivation in step F also include ultrasonic cleaning of the steel pipe before cold rolling to remove oil stains and dirt on the surface. The cleaning medium is an efficient weak acid environmentally friendly degreasing agent, and the ultrasonic cleaning machine is a piezoelectric 200 KHz high-frequency transducer. The cleaned steel pipe is then subjected to solution heat treatment. The pickling and passivation treatment in step F also includes pickling in a pickling and passivation tank with a mixed acid of nitric acid with a concentration of 68% and hydrofluoric acid with a concentration of 40%. The pure weight ratio of nitric acid is 17%, and the pure weight ratio of hydrofluoric acid is 5%. After pickling for 40 minutes, it is fished out, rinsed with clean water and then soaked in hot water for 10 minutes to achieve a completely clean and passivated state. The chemical composition of the steel pipe includes Co = 0.25%, V = 0.15%, Cu = 0.10%, P = 0.045%. And through the above process, the content of these harmful metal elements is reduced by more than 60%, and the reduction rate of Co element reaches 93%. The chemical composition of the steel pipe also includes C ≤ 0.03%, Si ≤ 1.0%, Mn ≤ 2.0%, Cr: 16.0% - 18.0%, Ni: 10.0% - 14.0%, and the balance is iron. During the manufacturing process of the steel pipe, environmental adaptability tests are also required, including but not limited to salt spray resistance test and seawater corrosion resistance test to ensure the long-term stable performance of the steel pipe in the marine environment. After the steel pipe is manufactured, it also needs to be packaged and stored. Moisture-proof and corrosion-proof packaging materials are used and stored in a dry and ventilated environment to protect the steel pipe from the influence of the external environment.

2. The manufacturing method of a super-large wall thickness stainless steel seamless steel pipe for an offshore platform according to claim 1, characterized in that: The inspection in step G includes comprehensive detection of the chemical composition, dimensional accuracy, surface quality, internal structure and mechanical properties of the steel pipe to ensure that the steel pipe meets the special requirements of the offshore platform.

3. The manufacturing method of an extra-large wall-thickness stainless steel seamless steel pipe for an offshore platform according to claim 1, characterized in that, In step F, after cold rolling and forming, straightening treatment is also required to ensure the straightness and roundness of the steel pipe. During the straightening process, a continuous straightening machine is used to achieve precise straightening of the steel pipe by adjusting the roll gap and pressure. After the passivation treatment in step F, a final surface inspection is also required, including but not limited to visual inspection and non-destructive testing, to ensure that the surface of the steel pipe has no defects and meets the requirements of corrosion resistance performance.

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