Stainless steel seamless steel tube with extra-large wall thickness for ocean platform and manufacturing method of stainless steel seamless steel tube

Abstract

The invention relates to the technical field of steel pipe manufacturing, and discloses an extra-large-wall-thickness stainless steel seamless steel pipe for an ocean platform, and the steel pipe is manufactured by adopting the following manufacturing method: step A, smelting; step B, die casting; c, forging, wherein the steel ingot is forged into a round billet; d, perforating and rolling: heating the round steel, and perforating to prepare a hollow billet; e, heat treatment, wherein fine grain treatment is conducted on the pierced billet; f, cold rolling and passivation are conducted, specifically, the steel pipe subjected to heat treatment is fed into a cold-rolling mill for finished product rolling; g, inspection is conducted, specifically, the steel pipe is subjected to final inspection; the steel pipe comprises, but is not limited to, about 0.25% of Co, about 0.15% of V, about 0.10% of Cu and about 0.045% of P. By adopting a refining process of'three-removal of molten iron and intermediate frequency furnace pre-solution, AOD, RH and die casting 'and combining an RH vacuum treatment technology, the dehydrogenation rate is more than 80%, and the content of harmful elements is effectively controlled, so that the corrosion resistance of the steel pipe is remarkably improved.

Description

Technical Field

[0001] The invention relates to the technical field of steel pipe manufacturing, in particular to an extra-thick stainless steel seamless pipe for an offshore platform and a manufacturing method thereof. Background Art

[0002] The material selection of offshore platform wellhead pipelines is a complex process, which requires comprehensive consideration of the pressure, temperature, corrosiveness of the wellhead logistics, as well as the type of material, wall thickness, procurement cycle, weight and other factors. Since the wellhead is in a complex environment with high temperature, high humidity, high salt sea breeze, salt spray, seawater splash, etc., corrosion resistance becomes the main consideration for material selection. Among them, the corrosion damage of the pipeline surface by the chloride ions in the high humidity sea breeze and salt spray, as well as the internal corrosion caused by the fluid in the pipeline, especially CO 2 The corrosion caused by corrosion is the main source of corrosion.

[0003] After searching, the Chinese patent number CN204163637U discloses a reinforced offshore platform oil production pipeline, including a seamless steel pipe and a plurality of support frames, which are welded into one. Its characteristics also include integral vulcanized rubber and steel wire rope, the steel wire rope is wound around the seamless steel pipe and the support frame along the spiral direction, the outer surface of the vulcanized rubber is cylindrical, and the steel wire rope, the seamless steel pipe and the support frame are all wrapped inside, and there is no gap, no void, and no connection interface between the two adjacent parts inside the vulcanized rubber, and the whole is a continuous phase, and the vulcanized rubber is tightly bonded to the surface of the seamless steel pipe, the surface of the support frame and the surface of the steel wire rope.

[0004] After searching, the Chinese patent number CN221683867U discloses a welded steel pipe for an offshore platform under harsh working conditions, which belongs to the technical field of welded steel pipes. The welded steel pipe comprises a first pipeline; a second pipeline, which is arranged on the upper side of the first pipeline; and a docking mechanism, which is arranged between the first pipeline and the second pipeline to dock the first pipeline and the second pipeline. On an offshore platform under harsh working conditions, during the threaded docking process of the internal threaded hole and the internal threaded pipe, a plurality of plug-in blocks are plugged into a plurality of fixed blocks to ensure accurate docking of the first pipeline and the second pipeline, reduce the difficulty of docking the first pipeline and the second pipeline, and effectively improve the docking construction efficiency between welded steel pipes.

[0005] The above-mentioned offshore platforms are based on the good corrosion resistance of stainless steel, especially the prevention of CO 2In order to improve the corrosion resistance, stainless steel is generally selected as the main material. However, the outer surface of stainless steel is easily affected by 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 of some offshore platform wellheads, in order to meet the requirements of design pressure, pipeline outer diameter and mechanical margin, it is necessary to increase the wall thickness, but this will also increase the platform load too much. Based on this, the present invention designs a stainless steel seamless pipe with extra-large wall thickness 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 stainless steel seamless pipe with extra-large wall thickness for an offshore platform and a manufacturing method thereof, which solves the problems of local corrosion and increased platform load due to 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 thick stainless steel seamless pipe for an offshore platform, the steel pipe being manufactured by the following manufacturing method:

[0009] Step A: Smelting, using the refining process of "three-dehydration water + medium frequency furnace pre-solution → AOD → RH → mold 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 miniaturization and dispersion refining technology; Step B: Mold casting, using large square steel ingots for mold casting, the ingot size is 40", the weight is 10.98 tons, and protective casting measures are taken to prevent secondary oxidation of molten steel, Step C: Forging, forging the steel ingot into a round billet, using a multi-fire precision forging process, including one fire flat anvil drawing to 650mm² square, two fire 800℃ V anvil Φ550mm rough round, three fire room temperature tabletop heating furnace heated to 1160℃ forging, the total compression ratio is 4.58, and defect-free round steel is obtained; Step D: Perforation and rolling, the round steel is heated and perforated to prepare a rough pipe, and a large diameter tapered roller perforating unit is used for perforation A rough pipe of Φ420×95mm is obtained, and then secondary extension rolling deformation is performed to obtain a rough pipe of Φ405×85mm, and 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, and the rough pipe is heated to 1020°C in a desktop heat treatment furnace and rapidly heated for 40 minutes, and then taken out of the furnace and quenched with water to obtain a steel pipe with an average grain size of 5.5 and mixed crystal elimination; Step F: cold rolling and passivation, the heat-treated steel pipe is put on a cold rolling mill for finished product rolling, and then pickled and passivated in a pickling and passivation cylinder. The acid 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%, the pure weight ratio of hydrofluoric acid is 5%, and the pickling time is 40 minutes to achieve a completely clean passivation degree; Step G: inspection, the steel pipe is subjected to a final inspection to ensure that it meets the technical requirements of super thick stainless seamless steel pipes for offshore platforms;

[0010] The chemical composition of the steel pipe includes but is not limited to Co≈0.25%, V≈0.15%, Cu≈0.10%, and 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 can reach 93%.

[0011] Preferably, the refining process of step A further comprises the following steps: preparing high content MgO+Al 2 O 3 The slag system and strong aluminum deoxidation are used to further reduce the total oxygen content in the steel to below 15ppm, and the size of inclusions in the molten steel is controlled below 15µm and the number is significantly reduced, eliminating large-sized inclusions.

[0012] Preferably, the protective casting measures in step B include protecting the surface of the molten steel rising in the mold from secondary oxidation, and protecting the steel flow from the ladle to the central casting pipe from secondary oxidation. The bottom casting method is adopted to inject molten steel into the central casting pipe, and inject it into the ingot mold from the bottom through the soup channel built on the ingot plate. Multiple ingots are cast at the same time to improve the surface quality of the ingots.

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

[0014] Preferably, the perforation preparation of the rough tube in step D adopts a tapered roller perforation unit, wherein the rolling angle β of the tapered roller is designed to be 15°, and the advancing angle γ is 11°, so as to control the circumferential shear deformation and surface twisting deformation, and ensure the quality of the rough tube; the secondary extension rolling deformation in step D is carried out in an oblique tube rolling mill, and a short mandrel is used to roll a mandrel with a length of up to 60 mm to obtain a rough tube with precise wall thickness and a smooth and corrugated inner surface. The grain size of the rough tube is at an average level of 2.5, and fine grain treatment is required.

[0015] Preferably, the fine grain treatment in step E also includes heating the rough pipe to 1020°C in the austenite stable region in a desktop heat treatment furnace, rapidly heating for 40 minutes, and then quenching with water after taking it out of the furnace. The water temperature is 20° and the cooling time is 2 minutes, so as to increase the grain size to an average of 5.5 and eliminate the mixed crystal phenomenon.

[0016] Preferably, 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 a high-efficiency 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 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%, and after 40 minutes of pickling, the steel pipe is taken out and rinsed with clean water and then soaked in hot water for 10 minutes to achieve a completely clean and passivated degree.

[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, the steel pipe is also required to undergo environmental adaptability tests, including but not limited to salt spray resistance tests and seawater corrosion resistance tests, to ensure the long-term stability of the steel pipe in the marine environment; after manufacturing, the steel pipe is also required to be packaged and stored, using moisture-proof and corrosion-resistant packaging materials, 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 comprehensive testing 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 the step F, after cold rolling, straightening treatment is required to ensure the straightness and roundness of the steel pipe. A continuous straightening machine is used in the straightening process to achieve precise straightening of the steel pipe by adjusting the roll gap and pressure. After the passivation treatment in the step F, a final surface inspection is 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:

[0021] 1. In the present invention, by adopting the refining process of "three-dehydration water + medium frequency furnace pre-solution → AOD → RH → mold casting", combined with RH vacuum treatment technology, the dehydrogenation rate of more than 80% is achieved, the content of harmful elements is effectively controlled, and the corrosion resistance of the steel pipe is significantly improved, especially in preventing CO 2 Corrosion; in addition, through ultra-low oxygen and inclusion miniaturization and dispersion refining technology, the total oxygen content and inclusion size 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 high pitting corrosion resistance equivalent value (PRE value is 24.2), which meets the minimum standard for stainless steel selection in the offshore platform environment, effectively reduces the risk of pitting corrosion and stress corrosion, and thus extends the service life of the steel pipe; by precisely controlling the wall thickness of the steel pipe, when the outer diameter is 335.6mm, the wall thickness needs to reach 82.55mm, ensuring that the steel pipe meets the design pressure and mechanical margin requirements while avoiding excessive increase in the platform's load, thereby achieving structural optimization.

[0023] 3. In the present invention, through multi-fire precision forging process and fine grain processing technology, the present invention makes the internal grains of the steel pipe more refined, the organizational performance is optimized, and the overall strength and toughness of the steel pipe are improved; strict size control and surface treatment technology are adopted in the manufacturing process, including ultrasonic cleaning and pickling passivation treatment, to ensure the dimensional accuracy and surface quality of the steel pipe, and meet the strict requirements of the offshore platform for steel pipes. BRIEF DESCRIPTION OF THE DRAWINGS

[0024] Figure 1 The characteristics of the steel pipe of the present invention and the flow chart of the method for manufacturing the steel pipe;

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

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

[0027] Figure 4 is a composition diagram of non-metallic inclusions in the steel pipe of the present invention;

[0028] Figure 5 This is a test diagram of the mechanical properties of the steel pipe of the present invention. DETAILED DESCRIPTION

[0029] The following will be combined with the drawings in the embodiments of the present invention to clearly and completely describe the technical solutions in the embodiments of the present invention. Obviously, the described embodiments are only part of the embodiments of the present invention, not all of the embodiments. Based on the embodiments of the present invention, all other embodiments obtained by ordinary technicians in this field without creative work are within the scope of protection of the present invention.

[0030] Embodiment 1;

[0031] See also Figure 1-Figure 2 In an embodiment of the present invention, a thick stainless steel seamless pipe for an offshore platform is manufactured by the following manufacturing method:

[0032] Step A: Smelting, using the refining process of "three-dehydration water + medium frequency furnace pre-solution → AOD → RH → mold 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 miniaturization and dispersion refining technology;

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

[0034] Step C: Forging, forging the steel ingot into a round billet, using a multi-fire precision forging process, including one fire flat anvil drawing to 650mm² square, two fires 800℃ V anvil Φ550mm rough round, three fires room temperature desktop heating furnace heated to 1160℃ before forging, the total compression ratio is 4.58, and defect-free round steel is obtained;

[0035] Step D: perforation and rolling, the round steel is heated and perforated to prepare a rough pipe, a large-diameter tapered roller perforating unit is used to perforate to obtain a rough pipe of Φ420×95mm, and then a secondary extension rolling deformation is performed to obtain a rough pipe of Φ405×85mm, and the wall thickness of the rough pipe is between 84.3-85.5;

[0036] Step E: heat treatment, the rough pipe is subjected to grain refinement treatment, and the rough pipe is heated to 1020°C in a desktop heat treatment furnace and rapidly heated for 40 minutes, and then taken out of the furnace and quenched with water to obtain a steel pipe with an average grain size of 5.5 and mixed crystal elimination;

[0037] Step F: cold rolling and passivation, the heat-treated steel pipe is put on a cold rolling mill for finished product rolling, and then pickled and passivated in a pickling and passivation tank. The acid 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 pickling time is 40 minutes to achieve a completely clean passivation degree;

[0038] Step G: Inspection: Conduct final inspection on the steel pipe to ensure that it meets the technical requirements of extra-thickness seamless stainless steel pipes for offshore platforms;

[0039] The chemical composition of the steel pipe includes but is not limited to Co≈0.25%, V≈0.15%, Cu≈0.10%, and P≈0.045%. Through the above process, the content of these harmful metal elements can be 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: LF prepares high content MgO+Al 2 O 3 The slag system and strong aluminum deoxidation are used to further reduce the total oxygen content in the steel to below 15ppm, and the size of inclusions in the molten steel is controlled below 15µm and the number is significantly reduced, eliminating large-sized inclusions.

[0041] The protective casting measures in step B include protecting the surface of the rising molten steel in the mold from secondary oxidation, and protecting the steel flow from the ladle to the central casting pipe from secondary oxidation. The bottom casting method is adopted to inject molten steel into the central casting pipe, and then inject it into the ingot mold from the bottom through the soup channel built on the ingot plate. Multiple ingots are cast at the same time to improve the surface quality of the ingots.

[0042] The multi-fire precision forging process in step C also includes completely cooling the round steel before each heating forging to eliminate internal stress, refine the grains and optimize the organizational properties. The round steel after three-fire forging is reduced by 20% to ensure that there are no defects.

[0043] The perforation preparation of the rough tube in step D adopts a tapered roller perforation unit, wherein the rolling angle β of the tapered roller is designed to be 15° and the advancing angle γ is 11°, so as to control the circumferential shear deformation and surface twisting deformation and ensure the quality of the rough tube; the secondary extension rolling deformation in step D is carried out in an oblique tube rolling mill, using a short mandrel rolling strip with a mandrel as long as 60 mm to obtain a rough tube with precise wall thickness and a smooth inner surface without corrugation. The grain size of the rough tube is an average of 2.5, and fine grain treatment is required.

[0044] The working principle of the embodiment of the present invention is that the material selection of the wellhead pipeline of the offshore platform needs to comprehensively consider the pressure, temperature, corrosiveness and other factors of the wellhead logistics. In such a complex environment, the pipeline is exposed to erosion by high humidity sea breeze, salt spray, seawater splash, etc., especially the damage of chloride ions to the pipeline surface. In addition, the corrosion caused by the fluid inside the pipeline is also a complex and changeable process, in which CO 2 The corrosion effect of CO is particularly prominent. 2 The concentration, partial pressure, water content and temperature of CO are closely related. 2 The content range is from 5.7% to 0.03% by mole percentage, the maximum partial pressure can reach 1.44MPa, the design temperature range is -14℃ to 85℃, and the working pressure is 32.3MPa. According to the Dewarrd & Milliman formula, the maximum corrosion rate can reach 0.5mm / a. Stainless steel has excellent resistance to CO 2 In contrast, carbon steel pipes are more susceptible to corrosion in high CO 2 The maximum corrosion rate under the content can reach 5.57mm / a. If no anti-corrosion measures are taken, its working life of more than 10 years in the high-pressure process system will face great risks. Even if the corrosion allowance is increased by about 27mm, the load of the platform will increase. Therefore, stainless steel is selected to resist CO 2 Corrosion is particularly important.

[0045] However, the outer surface of stainless steel pipes is also susceptible to pitting corrosion and stress corrosion in marine environments. Pitting corrosion usually occurs in environments 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 offshore platform environments, 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, so TP316L becomes the minimum standard for stainless steel selection in marine environments. At some wellheads in the East China Sea oil and gas fields in my country, the maximum temperature is close to 68°C, and the strength of stainless steel determines the choice of wall thickness. According to the design pressure, pipeline outer diameter, allowable stress of TP316L, and mechanical margin, when the outer diameter is 335.6mm, the wall thickness needs to reach 82.55mm, the total length is more than 18 meters, and the diameter-to-wall ratio (D / S) is only 4.06, which belongs to the category of ultra-thick wall seamless pipe.

[0046] In the smelting of step A, a refining process of "three-dehydrogenation water + medium frequency furnace pre-solution → AOD → RH → mold casting" was developed. The RH vacuum treatment used for stainless steel dehydrogenation can achieve a dehydrogenation rate of more than 80%. This process route mainly reflects the control of harmful elements. Ordinary austenitic stainless steel is smelted with scrap steel and other raw materials. Co≈0.25%, V≈0.15%, Cu≈0.10%, and P≈0.045% in the steel, 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 P, V, Cu, and Co in stainless steel by more than 60%, and the Co element reduction rate can reach 93%. In order to meet the high requirements of offshore platform pipeline TP316L on oxygen content and inclusions in steel, an ultra-low oxygen and inclusion miniaturization and dispersion refining technology for stainless steel was developed. The main features are the preparation of a high-content MgO+Al2O3 slag system in one LF and the use of strong aluminum deoxidation. The total oxygen content in the steel is reduced from more than 30ppm to less than 15ppm, the size of inclusions in the molten steel is controlled below 15µm and the number is significantly reduced, eliminating large-sized inclusions.

[0047] In the die casting of step B, stainless steel is a multi-element high-alloy steel, especially TP316L high-performance stainless steel, which contains a variety of easily oxidizable metal elements. If the exposed molten steel comes into contact with air, it will react with the oxygen in the air. 2 、N 2 The reaction forms conjunctiva, crust, skin, and heavy skin during casting, which leads to an increase in inclusions, seriously affecting the surface quality and internal quality of the steel ingot. Therefore, protective casting measures are taken in stainless steel casting. The protection of mold casting is mainly carried out from two aspects: one is to protect the surface of the rising molten steel in the mold from secondary oxidation; the other is to protect the steel flow from the ladle to the middle casting pipe from secondary oxidation. The bottom casting method injects molten steel into the central casting pipe, and injects it into the ingot mold from the bottom through the soup channel built on the ingot plate, and multiple ingots can be cast at the same time. The advantage is that the molten steel rises steadily in the ingot, does not cause the steel flow to splash, and is convenient to take protective casting measures, and the surface quality of the cast ingot is good. The large square steel ingot with a weight of 40" and 10.98 tons is selected to increase the compression ratio when forging the round billet, so that the internal grains of the round billet are finer, the organizational properties are more optimized, and the tendency of central cracks that may occur during perforation is reduced.

[0048] In step C, forging round steel is a key link in the manufacturing process, which directly affects the inner and outer surface quality of the perforated rough pipe and the secondary elongated rolled rough pipe. The heating temperature and reduction amount in the forging process are two core process parameters.

[0049] One-fire forging: The flat anvil is drawn to 650mm², the initial forging temperature is controlled at 1180℃, the final forging temperature is maintained at 850℃, and the reduction is set to 32%.

[0050] Second fire forging: load the furnace at 800℃, use V anvil Φ550mm rough circle, keep the forging temperature the same as the first fire, and adjust the reduction to 15%.

[0051] Three-fire forging: Before 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 forgings to be twisted and crushed, and transformed into equiaxed recrystallized structures with finer grains and uniform size. Complete cooling helps solid phase transformation, creates conditions for regeneration and crystallization refinement of hot forging again; curbs embrittlement caused by δ phase precipitation and surface cracks of re-hot forging; alleviates the increase in internal stress caused by two consecutive hot forgings, and reduces the risk of defects in the core of the re-forged parts. The temperature of three-fire forging is set at 1160℃, the furnace is loaded at room temperature, the heating time of the desktop heating furnace is 440 minutes, the initial forging temperature is 1190℃, the final forging temperature is 900℃, the anvil is Φ450mm round, air cooling, and the reduction is 20%. The entire forging process uses a 1600-ton rapid forging unit, with a die forging method with drawing as the main deformation, and the total compression ratio reaches 4.58.

[0052] In step D, since the product is an extra-large thick-walled tube, its diameter-to-wall ratio (D / S) is only 4.38, which belongs to the category of super-extra-large thick-walled tube. Considering that cold rolling overload may lead to failure to meet technical index requirements, this process policy adopts "large thermoplastic deformation and precision cold-controlled deformation".

[0053] Hot piercing: Stainless steel round steel is subjected to complex forces during oblique rolling, which can easily lead to defects on the inner and outer surfaces. To prevent defects, high-quality rough tubes are provided for secondary hot plastic elongation rolling, and large-diameter tapered roller (mushroom-type) piercing units are used to pierce rough tubes of Φ420×95mm. The main difference between tapered rollers and ordinary drum rollers is their "large in and small out" tapered design, as well as the rolling angle in addition to the advancing angle (bite angle). This design increases the axial pulling force before the secondary bite of the round steel, effectively alleviating the rotary forging effect, surface distortion and circumferential shear deformation caused by shear stress, and greatly improving the quality of the inner wall of the rough tube. The β angle is designed to be 15°, the γ angle is 11°, and the rough tube quality is best when γ+β≥25°.

[0054] Heating parameters: round steel is heated in an inclined bottom furnace, with a tapping temperature of 1160°C. After staying at 720°C for 80 minutes, it starts to move forward, with a travel time of 300 minutes to the tapping port, and then is heated and kept warm for 60 minutes before being taken out of the furnace for perforation.

[0055] Secondary elongation rolling: rough pipes close to the finished product size require fine technical requirements. Secondary elongation deformation rolling aims to achieve rough pipe wall thickness reaching or close to the wall thickness of the final product, while ensuring good surface quality and internal structure. After the rough pipe is formed, it needs to be water quenched, and the cooling rate is greater than 500℃ / minute to solid solution and enhance plasticity. The rough pipe needs to be further inspected, and the surface defects are removed before secondary heating for elongation rolling. The secondary elongation rolling is carried out in the inclined tube mill, using a short mandrel to roll a mandrel as long as 60mm to obtain a rough pipe with precise wall thickness and a smooth inner surface without corrugation. The grain size of the rough pipe is an average of 2.5, and fine grain treatment is required.

[0056] Embodiment 2;

[0057] See also Figure 1-Figure 2 In the embodiment of the present invention, the fine grain treatment in step E also includes heating the rough pipe to 1020°C in the austenite stable region in a desktop heat treatment furnace, rapidly heating for 40 minutes, and then quenching with water after taking it out of the furnace. The water temperature is 20° and the cooling time is 2 minutes to increase the grain size to an average of 5.5 and eliminate the mixed crystal phenomenon.

[0058] 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 a high-efficiency 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 passivation treatment in step F also includes pickling in a pickling passivation tank with 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%, and after the pickling time is 40 minutes, it is taken out and rinsed with clean water and then soaked in hot water for 10 minutes to achieve a completely clean passivation degree.

[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, the steel pipe is also subject to environmental adaptability tests, including but not limited to salt spray resistance tests and seawater corrosion resistance tests, to ensure the long-term stability of the steel pipe in the marine environment; after manufacturing, the steel pipe is also required to be packaged and stored, using moisture-proof and corrosion-resistant packaging materials, and stored 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 testing 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 offshore platforms.

[0061] In step F, after cold rolling, straightening treatment is required to ensure the straightness and roundness of the steel pipe. A continuous straightening machine is used in the straightening process to adjust the roll gap and pressure to achieve precise straightening of the steel pipe. After the passivation treatment in step F, a final surface inspection is 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: the rough pipe is inspected, repaired and ground to eliminate surface defects before being put on the cold rolling mill, and the finished product is rolled on the LD350 three-roller rolling mill. The adjustment parameters include feed value (per half stroke) 3mm, stroke number 22 times / min, elongation coefficient 1.21, and surface reduction rate 18%. The rollers of the three-roller cold rolling mill are evenly distributed in the vertical plane, forming a ring-shaped hole at 120° from each other. The rolling force is transmitted to the fixed thick-walled frame sleeve through the slideway to uniformly apply 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 cleaned before solid solution heat treatment can be carried out to avoid the "chromium depletion" phenomenon caused by local high temperature. The ultrasonic cleaning method is adopted, the cleaning medium is a high-efficiency weak acid environmentally friendly degreasing agent, and the ultrasonic cleaning machine used is a piezoelectric 200KHz high-frequency transducer. The cleaned steel pipe is soaked in a hot water tank for 30 minutes, fished out and rinsed with a clean water high-pressure gun to meet the standard of "water dripping without beads".

[0063] In order to make the solution heat treatment effective and to refine the grains by quenching again, and to obtain the organizational properties of TP316L stainless steel pipe, the final heat treatment of the finished product is a very critical link. In the continuous heat treatment furnace, the high temperature section temperature is set to 1050℃, and the insulation time is long, so that the intermetallic compounds are completely precipitated, and then the solid solution is strengthened by water quenching. The temperature and insulation time of each section are as follows: preheating section, temperature 800-960℃, time 40 minutes; heating section, temperature 960-1060℃, time 30 minutes; soaking section, temperature 1050℃, time 20 minutes; spray room, water temperature 20℃, water flow rate of each spray pipe is 1.2 m / s.

[0064] The passivation of stainless steel is an indispensable and important process. 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 an alloy element in the form of a solid solution. The higher chromium and nickel can comprehensively improve the passivation performance of stainless steel in the alloy element, and can form a more stable oxide film, thereby improving its corrosion resistance. During pickling and passivation, attention should be paid to the ratio of the passivation cylinder and the passivation time. The pickling of the finished stainless steel pipe is to remove the surface iron oxide scale and dirt, and to passivate the steel base to generate a protective film of chromium and low-valent oxides. Since TP316L is an austenitic stainless steel, the chromium trioxide contained in its iron oxide scale is insoluble in general acid, and pickling is relatively difficult. This application uses a mixed acid of nitric acid with a concentration of 68% of a strong oxidizing acid and hydrofluoric acid with a concentration of 40%. Nitric acid can convert low-valent oxides of chromium and iron into high-valent oxides. Only when low-valent oxides are oxidized to high-valent oxides can they be dissolved by acid. In addition, nitric acid can passivate the steel base to form a protective film of chromium and low-valent oxides. Hydrofluoric acid is used to dissolve high-valent oxides of iron, chromium, and nickel. However, there must be a certain ratio between nitric acid and hydrofluoric acid. If the ratio of nitric acid is too low, ferrous oxide is not oxidized to iron, and hydrofluoric acid cannot dissolve ferric oxide, but can only dissolve chromium oxide and nickel, which will form acid corrosion spots on the surface of the steel pipe, becoming an over-pickling defect. The molar concentration of nitric acid with a concentration of 68% is 15.2mol / L. The proportion should be calculated according to the concentration content converted into a pure 100% concentration, which is between 12-15%; the molar concentration of hydrofluoric acid with a concentration of 40% is 22.4mol / L, which is calculated as 3-5%. In practical applications, there are different adjustments according to the material and size of the pipe. For the pickling and passivation of TP316L extra-large thick-walled pipe, the pure weight ratio of nitric acid used in this application is 17% and the pure weight ratio of hydrofluoric acid is 5%. The pickling time is 40 minutes, and then the pipe is taken out and rinsed with clean water and then soaked in hot water for 10 minutes to achieve a completely clean passivation degree.

[0065] Embodiment 3;

[0066] See also Figure 1-Figure 5 In the embodiment of the present invention, the steel pipe manufactured by the manufacturing method is inspected:

[0067] 1. Surface and dimensional inspection

[0068] (1) Surface quality and roughness

[0069] The TP316L extra-large thick-walled stainless steel pipe in this application has been inspected and has no obvious defects on the inner and outer surfaces. The surface roughness reaches Ra3.6, which meets the standard requirements for pickling passivation pipes.

[0070] (2) Dimension tolerance

[0071] According to the platform standard Φ355.6×82.55×18000mm, the outer diameter, wall thickness and length were inspected for dimensional tolerance. The outer diameter tolerance is +2.4mm to -0.8mm, and the actual measured value is 356.3mm; the wall thickness tolerance is +0.00 to -8.2mm, and the actual measured value is 80.4mm; the length tolerance is +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 was inspected using integrated ultrasonic and eddy current flaw detection technology, and no alarm signals were found, indicating that there were no defects inside the steel pipe.

[0074] 2. Physical, chemical and performance testing

[0075] (1) Chemical composition

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

[0077] (2) Non-metallic inclusions

[0078] The non-metallic inclusions were analyzed and the test results were in line with the platform standards. The fine and coarse inclusions of Class A to Class D were within the standard range. The specific data are as follows: Figure 4 shown.

[0079] (3) Metallographic structure

[0080] The grain size test results show that the platform requirement is greater than level 5 or finer, the actual measured value is level 6, and the level difference is 2. The α phase content is measured to be 8%, and the level is 1.5.

[0081] (4) Intergranular corrosion test

[0082] The intergranular corrosion sensitivity test was carried out using ASTM A262 standard procedure E method. No corrosion cracks were found on the sample and it was evaluated as qualified.

[0083] (5) Mechanical properties test

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

[0085] The working principle of the embodiment of the present invention is: after strict inspection and testing and platform actual application verification, 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 meet or exceed the standard requirements.

[0086] Working principle: This application proposes a method for manufacturing extra-thick stainless steel seamless pipes for offshore platforms, which ensures the high performance and reliability of the steel pipe through a series of fine process steps. First, the refining process of "three de-iron water + medium frequency furnace pre-solution → AOD → RH → die casting" is adopted, combined with RH vacuum treatment technology, to achieve a dehydrogenation rate of more than 80%, effectively control the content of harmful elements, and reduce the total oxygen content and inclusion size in the steel. During the die casting process, large square steel ingots and protective casting measures are adopted to prevent the secondary oxidation of molten steel and ensure the surface quality of the ingot. In the forging link, the application of multi-fire precision forging process makes the internal structure of round steel uniform and improves the plasticity and mechanical properties. In the perforation and rolling steps, the application of large-diameter tapered roller perforation unit optimizes the rough pipe quality, and the secondary extension rolling deformation ensures the precise control of the rough pipe wall thickness. In the heat treatment step, the grain refinement and quenching technology improves the grain size of the rough pipe to level 5.5 and eliminates the mixed crystal 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 meet or exceed the standard requirements.

[0087] Although the embodiments of the present invention have been shown and described, it will be appreciated by those skilled in the art that various changes, modifications, substitutions and alterations may be made to the embodiments without departing from the principles and spirit thereof, and the scope of the present invention is defined by the appended claims and their equivalents.

Claims

1. A thick stainless steel seamless pipe for offshore platforms, characterized in that: The steel pipe is made by the following manufacturing method: Step A: Smelting, using the refining process of "three-dehydration water + medium frequency furnace pre-solution → AOD → RH → mold 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 miniaturization and dispersion refining technology; Step B: die casting, using a large square steel ingot for die casting, the ingot size is 40", the weight is 10.98 tons, and protective casting measures are taken to prevent secondary oxidation of the molten steel to obtain a steel ingot with good surface quality; Step C: Forging, forging the steel ingot into a round billet, using a multi-fire precision forging process, including one fire flat anvil drawing to 650mm² square, two fires 800℃ V anvil Φ550mm rough round, three fires room temperature desktop heating furnace heated to 1160℃ before forging, the total compression ratio is 4.58, and defect-free round steel is obtained; Step D: perforation and rolling, the round steel is heated and perforated to prepare a rough pipe, a large-diameter tapered roller perforating unit is used to perforate to obtain a rough pipe of Φ420×95mm, and then a secondary extension rolling deformation is performed to obtain a rough pipe of Φ405×85mm, and the wall thickness of the rough pipe is between 84.3-85.5; Step E: heat treatment, the rough pipe is subjected to grain refinement treatment, and the rough pipe is heated to 1020°C in a desktop heat treatment furnace and rapidly heated for 40 minutes, and then taken out of the furnace and quenched with water to obtain a steel pipe with an average grain size of 5.5 and mixed crystal elimination; Step F: cold rolling and passivation, the heat-treated steel pipe is put on a cold rolling mill for finished product rolling, and then pickled and passivated in a pickling and passivation tank. The acid 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 pickling time is 40 minutes to achieve a completely clean passivation degree; Step G: Inspection: Conduct final inspection on the steel pipe to ensure that it meets the technical requirements of extra-thickness seamless stainless steel pipes for offshore platforms; The chemical composition of the steel pipe includes but is not limited to Co≈0.25%, V≈0.15%, Cu≈0.10%, and 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 can reach 93%.

2. The method for manufacturing a thick stainless steel seamless pipe for an offshore platform according to claim 1, characterized in that: The refining process of step A also includes the following steps: LF prepares a high-content MgO+Al2O3 slag system and uses strong aluminum deoxidation to further reduce the total oxygen content in the steel to below 15ppm, and controls the size of inclusions in the molten steel to below 15µm and significantly reduces the number, thereby eliminating large-sized inclusions.

3. The method for manufacturing a thick stainless steel seamless pipe for an offshore platform according to claim 1, characterized in that: The protective casting measures in step B include protecting the surface of the rising molten steel in the mold from secondary oxidation, and protecting the steel flow from the ladle to the central casting pipe from secondary oxidation. The bottom casting method is adopted to inject molten steel into the central casting pipe, and then inject it into the ingot mold from the bottom through the soup channel built on the ingot plate. Multiple ingots are cast at the same time to improve the surface quality of the ingots.

4. The method for manufacturing a thick stainless steel seamless pipe for an offshore platform according to claim 1, characterized in that: The multi-fire precision forging process in step C also includes completely cooling the round steel before each heating forging to eliminate internal stress, refine the grains and optimize the organizational properties. The round steel after three-fire forging is reduced by 20% to ensure that there are no defects.

5. The method for manufacturing a thick stainless steel seamless pipe for an offshore platform according to claim 1, characterized in that: The perforation preparation of the rough tube in step D adopts a tapered roller perforation unit, wherein the rolling angle β of the tapered roller is designed to be 15° and the advancing angle γ is 11°, so as to control the circumferential shear deformation and surface twisting deformation and ensure the quality of the rough tube; the secondary extension rolling deformation in step D is carried out in an oblique tube rolling mill, using a short mandrel to roll a mandrel with a length of 60 mm to obtain a rough tube with precise wall thickness and a smooth inner surface without corrugations. The grain size of the rough tube is at an average level of 2.5 and needs to be refined.

6. The method for manufacturing a thick stainless steel seamless pipe for an offshore platform according to claim 1, characterized in that: The fine grain treatment in step E also includes heating the rough pipe to 1020°C in the austenite stable region in a desktop heat treatment furnace, rapidly heating for 40 minutes, and then quenching with water at a temperature of 20° and a cooling time of 2 minutes, so as to increase the grain size to an average of 5.5 and eliminate the mixed crystal phenomenon.

7. The method for manufacturing a thick stainless steel seamless pipe for an offshore platform according to claim 1, characterized in that: 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 a high-efficiency 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 passivation treatment in step F also includes pickling in a pickling passivation tank with 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%, and after the pickling time is 40 minutes, the steel pipe is taken out and rinsed with clean water and then soaked in hot water for 10 minutes to achieve a completely clean passivation degree.

8. The method for manufacturing a thick stainless steel seamless pipe for an offshore platform according to claim 1, characterized in that: 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, the steel pipe is also required to undergo environmental adaptability tests, including but not limited to salt spray resistance tests and seawater corrosion resistance tests, to ensure the long-term stability of the steel pipe in the marine environment; after manufacturing, the steel pipe is also required to be packaged and stored, using moisture-proof and corrosion-resistant packaging materials, and stored in a dry and ventilated environment to protect the steel pipe from the influence of the external environment.

9. The method for manufacturing a thick stainless steel seamless pipe for an offshore platform according to claim 1, characterized in that: The inspection in step G includes comprehensive testing 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.

10. The method for manufacturing a thick stainless steel seamless pipe for an offshore platform according to claim 1, characterized in that: In the step F, after cold rolling, straightening treatment is required to ensure the straightness and roundness of the steel pipe. A continuous straightening machine is used in the straightening process to achieve precise straightening of the steel pipe by adjusting the roll gap and pressure. After the passivation treatment in the step F, a final surface inspection is 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.

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