A special stainless steel pipe for undersea oxygen pipelines and its preparation method

Abstract

The present invention belongs to the technical field of stainless steel pipes, and particularly relates to a special stainless steel pipe for subsea oxygen pipelines and a preparation method thereof. The special stainless steel pipe for subsea oxygen pipelines comprises the following components in weight percentage: Cr 17.2-18.3%, Ni 11.7-12.6%, Mo 2.3-3.0%, C 0.02-0.03%, Mn 1.0-2.0%, P 0.019-0.031%, S 0.010-0.017%, Al 0.8-1.7%, Nd 0.3-1.0%, Ce 1.3-2.4%, Cu 0.5-1.7%, Hf 1.2-2.4%, Nb 1.1-1.9%, and the balance is Fe and inevitable impurities. The special stainless steel pipe for subsea oxygen pipelines meets the chemical composition standard of GB / T 40317-2021 of the national standard, and at the same time has excellent mechanical properties, high corrosion resistance and high low-temperature impact performance.

Description

Technical Field

[0001] The present invention belongs to the technical field of stainless steel pipes, and particularly relates to a special stainless steel pipe for subsea oxygen pipelines and a preparation method thereof. Background Art

[0002] Stainless steel pipes are widely used in many fields, such as construction, furniture, and machinery manufacturing. Among them, oxygen pipelines are an important field of stainless steel application. Since oxygen pipelines need to transport high-purity oxygen, very high requirements are imposed on stainless steel seamless pipes, and a series of strict standards and requirements need to be met.

[0003] The special stainless steel pipe for subsea oxygen pipelines is a special pipe for transporting oxygen, mainly used in ocean engineering, such as subsea oil and gas field development, underwater laboratories, and marine research facilities. There are more stringent requirements for the design and material selection of such pipes: (1) The subsea environment is extremely complex, and the salts, microorganisms, and marine organisms in seawater will all corrode the stainless steel pipe. Especially in oxygen pipelines, due to the presence of oxygen, the corrosion rate of metals will be accelerated. Therefore, it is very important to select a suitable stainless steel material to ensure its sufficient corrosion resistance. (2) The pressure in the subsea environment is extremely high, especially in deep-sea areas. Therefore, the stainless steel pipe must have sufficient strength and toughness to withstand the external pressure without cracking or deformation. (3) Physical damage may also occur during the laying and maintenance process. Therefore, the impact resistance and wear resistance of the material are also very important. The stainless steel seamless pipe must also have high mechanical strength to ensure that the pipeline will not be deformed or damaged due to external forces. (4) The temperature change in the subsea environment is relatively large, especially at the junction of deep sea and shallow sea, and the temperature difference is particularly obvious. Therefore, the stainless steel pipe needs to have good temperature resistance and be able to maintain stable performance under different temperature conditions.

[0004] In the prior art, austenitic stainless steel and duplex stainless steel are often prepared to improve the seawater corrosion resistance of the special stainless steel pipe for subsea oxygen pipelines. However, the mechanical properties of these stainless steels on the market still need to be improved, especially the low-temperature impact resistance still has a large room for improvement.

[0005] Therefore, there is an urgent need for a special stainless steel pipe for subsea oxygen pipelines and a preparation method thereof. Summary of the Invention

[0006] The purpose of the present invention is to provide a special stainless steel pipe for subsea oxygen pipelines and a preparation method thereof.

[0007] To achieve the above purpose, the present invention provides the following technical solutions:

[0008] A special stainless steel pipe for submarine oxygen pipelines, comprising components with the following weight percentages: Cr 17.2 - 18.3%, Ni 11.7 - 12.6%, Mo 2.3 - 3.0%, C 0.02 - 0.03%, Mn 1.0 - 2.0%, P 0.019 - 0.031%, S 0.010 - 0.017%, Al 0.8 - 1.7%, Nd 0.3 - 1.0%, Ce 1.3 - 2.4%, Cu 0.5 - 1.7%, Hf 1.2 - 2.4%, Nb 1.1 - 1.9%, and the balance being Fe and inevitable impurities.

[0009] Further, the sum of the mass percentages of Cu and Hf is greater than the mass percentage of Mo.

[0010] In fact, the chemical compositions of many stainless steel seamless pipes for oxygen pipelines on the market currently do not conform to the standards of the national standard "Stainless Steel Seamless Pipes for Oxygen Pipelines". The present invention provides a stainless steel pipe with the grade 06Cr17Ni12Mo2, and the composition of the components fully conforms to the standards of GB / T 40317 - 2021 "Stainless Steel Seamless Pipes for Oxygen Pipelines". Through a large number of experiments, the present invention has found that when the sum of the mass percentages of Cu and Hf in the components is greater than the mass percentage of Mo, the tensile strength of the stainless steel pipe can be improved. After Cu and Hf are added as alloying elements to the stainless steel, they can enter the matrix lattice to form a solid solution, and this solid solution strengthening effect can increase the hardness and strength of the material. The atomic radii of Cu and Hf are relatively large, and the mismatch with the iron matrix lattice is high. Therefore, large lattice distortions are generated in the lattice, thereby enhancing the anti-deformation ability of the material. Molybdenum is one of the main elements to improve the corrosion resistance of stainless steel, especially resistance to pitting corrosion and crevice corrosion. The combined action of Cu and Hf can optimize the microstructure of the material. In addition, the addition of Cu and Hf can also improve the corrosion resistance and high-temperature performance of the material, further enhancing its comprehensive performance. Although Mo itself is also an important strengthening element, which can improve the strength and corrosion resistance of stainless steel, too high a Mo content will lead to grain coarsening and increased brittleness. When the sum of the mass percentages of Cu and Hf is greater than Mo, the negative effects brought by Mo can be effectively balanced, and the overall performance of the material can be further optimized. However, the improvement effect of the elongation after fracture is not ideal.

[0011] Further, the ratio of the mass percentages of Ce and Nd is 2 ≤ Ce / Nd ≤ 6.

[0012] In the present invention, when the ratio of the mass percentage content of Ce to Nd is 2 ≤ Ce / Nd ≤ 6, the elongation after fracture of the stainless steel pipe can be improved. The rare earth elements Nd and Ce have a microalloying effect and can form fine second-phase particles in the stainless steel. These particles can promote grain refinement, and the fine grain structure can improve the plasticity and toughness of the material, thereby improving the elongation after fracture. When the ratio of the mass percentage content of Ce to Nd is 2 ≤ Ce / Nd ≤ 6, the elongation after fracture of the stainless steel pipe can be significantly improved through the microalloying mechanism. At the same time, the low-temperature impact performance of the stainless steel pipe is improved under this condition.

[0013] Furthermore, the ratio of the mass percentage content of Hf to the sum of the mass percentage contents of Ce and Nb is (1.8 - 2) : (3 - 4).

[0014] The seabed environment contains a large amount of chloride ions. By testing the performance of the pipeline material in hydrochloric acid, the corrosion resistance of the material in a chloride ion-containing environment can be evaluated. It is found in a large number of experiments that the ratio of the mass percentage content of Hf to the sum of the mass percentage contents of Ce and Nb affects the corrosion resistance. When the ratio of the mass percentage content of Hf to the sum of the mass percentage contents of Ce and Nb is (1.8 - 2) : (3 - 4), the stainless steel pipe has high corrosion resistance at room temperature. Hafnium and cerium act together to purify the grain boundaries, reduce the aggregation of harmful impurities, and at the same time refine the grains, reducing the occurrence points of corrosion. The precipitation strengthening effect of niobium and hafnium can improve the strength of the material, and at the same time, the solid solution strengthening effect of niobium can increase the lattice distortion and improve the corrosion resistance of the material. These elements can act synergistically under this ratio, and through various mechanisms such as grain boundary purification, grain refinement, and precipitation strengthening, significantly improve the corrosion resistance of the stainless steel pipe.

[0015] The present invention also provides a method for preparing the special stainless steel pipe for the seabed oxygen pipeline, including the following steps:

[0016] (1) Place metallic iron in an electric arc furnace with a furnace temperature of 1550 - 1600 °C to obtain hot metal from the blast furnace after melting. Put the remaining components into an intermediate frequency furnace with a furnace temperature of 1600 - 1650 °C to obtain an alloy solution. Inject the hot metal from the blast furnace and the alloy solution into an argon oxygen decarburization furnace through a ladle. Blow Ar gas during smelting to obtain molten steel. Adjust the alloy composition, take samples for analysis. When each alloy composition meets the requirements, put the molten steel into a continuous casting machine for forging. Cut when discharging the billet, repeatedly upset and draw in the cross direction 3 - 4 times, then roll it into a stainless steel plate, and finally air-cool it to room temperature;

[0017] (2) Clean the welding parts of the stainless steel plate with absolute ethanol 6 - 7 times, then place the cleaned stainless steel plate on a pipe rolling and forming machine. The pipe rolling and forming machine curls the stainless steel plate to form a pipe to be welded. Then weld the pipe to be welded by plasma arc welding, and finally perform pickling treatment to obtain a pipe blank;

[0018] (3) Place the tube blank into a tube rolling mill for rolling to obtain a rough tube;

[0019] (4) Use a continuous furnace to perform solution treatment on the rough tube. After the rough tube reaches 1000 - 1200 °C, cut off the power and quench it with water to obtain a solution-treated tube blank;

[0020] (5) Process the solution-treated tube blank by means of drawing with a fixed plug to obtain a pre-stainless steel tube;

[0021] (6) Anneal the pre-stainless steel tube. After annealing, heat it to 1150 - 1200 °C, hold for 2 - 2.2 h, then perform quenching and tempering treatments. After cooling to room temperature, obtain a special stainless steel tube for subsea oxygen pipelines.

[0022] Further, the welding width of the tube to be welded is 3 - 3.5 mm, and the conditions for plasma arc welding are: current 220 A, voltage 20 V, welding speed 270 - 280 mm / min, and the flow rate of the shielding gas is 32 - 35 L / min.

[0023] Further, the heating temperature of the initial rolling bloom of the tube rolling mill is controlled according to the following requirements: the preheating section is: 720 - 770 °C, the heating section is: 950 - 1000 °C, the soaking section: 1150 - 1200 °C; its starting rolling temperature is: 1130 - 1150 °C; the final rolling and laying temperature is: 950 - 980 °C, and the finishing rolling speed is controlled at 30 - 35 m / s.

[0024] Further, the temperature of the continuous furnace is 940 - 960 °C.

[0025] Further, the wall thickness reduction of the solution-treated tube blank is 0.04 - 0.06 mm, the outer diameter reduction is 1.2 - 1.5 mm, the total cold working drawing rate is 25 - 27%, and the number of cold working drawing passes is 3 passes.

[0026] Further, the annealing is carried out in five stages. The first stage: temperature 750 - 760 °C, holding time 6 - 7 minutes; the second stage: temperature 970 - 980 °C, holding time 8 - 9 min; the third stage: temperature 1030 - 1040 °C, holding time 6 - 7 min; the fourth stage: temperature 1080 - 1090 °C, holding time 6 - 7 min; the fifth stage: temperature 1060 - 1070 °C, holding time 8 - 9 min.

[0027] The temperature change in the subsea environment is relatively large, especially at the junction of the deep sea and the shallow sea, where the temperature difference is particularly obvious. In the development of deep-sea oil and gas fields, the temperature of the wellhead and subsea pipelines can be as high as over 150 °C, and sometimes even higher. Under the conditions of the chemical composition of the present invention, using the above annealing steps can improve the corrosion resistance of the stainless steel tube at high temperatures.

[0028] Further, the quenching temperature is 950 - 1050 °C, the tempering temperature is 500 - 700 °C, and the tempering time is 1.5 - 2 h.

[0029] Compared with the prior art, the advantages and beneficial effects of the present invention are as follows:

[0030] 1. When the sum of the mass percentages of Cu and Hf in the components of the present invention is greater than the mass percentage of Mo, the tensile strength of the stainless steel pipe can be improved.

[0031] 2. When the ratio of the mass percentages of Ce and Nd in the present invention is 2 ≤ Ce / Nd ≤ 6, the elongation after fracture of the stainless steel pipe can be improved. At the same time, the low-temperature impact performance of the stainless steel pipe is improved under this condition.

[0032] 3. The ratio of the mass percentage of Hf to the sum of the mass percentages of Ce and Nb is (1.8 - 2) : (3 - 4), and the stainless steel pipe has high corrosion resistance at room temperature.

[0033] 4. Under the conditions of the chemical composition of the present invention, by using the annealing step of the present invention, the corrosion resistance of the stainless steel pipe at high temperature can be improved. Specific Embodiments

[0034] The technical solutions in the embodiments of the present invention will be clearly and completely described below. Obviously, the described embodiments are only a part of the embodiments of the present invention, rather than all of the embodiments. All other embodiments obtained by those of ordinary skill in the art without making creative efforts based on the embodiments of the present invention belong to the scope of protection of the present invention.

[0035] Example 1

[0036] This example provides a special stainless steel pipe for subsea oxygen pipelines, including the following components by weight percentage: Cr 17.8%, Ni 12.2%, Mo 2.8%, C 0.026%, Mn 1.4%, P 0.026%, S 0.013%, Al 1.3%, Nd 0.5%, Ce 2.0%, Cu 1.5%, Hf 2.0%, Nb 1.5%, and the balance is Fe and unavoidable impurities.

[0037] The preparation method of the special stainless steel pipe for subsea oxygen pipelines includes the following steps:

[0038] (1) Place metallic iron in an electric arc furnace with a temperature of 1580 °C inside. After melting, hot metal from the blast furnace is obtained. Put the remaining components into an intermediate frequency furnace with a temperature of 1620 °C inside to obtain an alloy solution. Inject the hot metal from the blast furnace and the alloy solution into an argon oxygen decarburization furnace through a ladle. During smelting, blow in Ar gas to obtain molten steel. Adjust the alloy composition and take samples for analysis. When each alloy composition meets the requirements, put the molten steel into a continuous casting machine for forging. Cut when billets come out, upset and draw out repeatedly in the cross direction 3 times, then roll into a stainless steel plate, and finally air-cool to room temperature;

[0039] (2) Clean the parts to be welded of the stainless steel plate 6 times with absolute ethanol. Then place the cleaned stainless steel plate on a pipe rolling and forming machine. The pipe rolling and forming machine curls the stainless steel plate to form a pipe to be welded. Then weld the pipe to be welded by the method of plasma arc welding. Finally, carry out pickling treatment to obtain a pipe blank; The welding width of the pipe to be welded is 3 mm. The conditions of plasma arc welding are: current 220 A, voltage 20 V, welding speed 270 mm / min, and the flow rate of the shielding gas 32 L / min.

[0040] (3) Put the pipe blank into a tube rolling mill for rolling. The heating temperature of the blooming billet of the tube rolling mill is controlled according to the following requirements: preheating section: 750 °C, heating section: 980 °C, soaking section: 1170 °C; Its starting rolling temperature is: 1140 °C; The final rolling and laying temperature is: 960 °C, and the finishing rolling speed is controlled at 35 m / s; Obtain a rough tube;

[0041] (4) Carry out solution treatment on the rough tube in a continuous furnace. The temperature of the continuous furnace is 950 °C; After the rough tube reaches 1100 °C, cut off the power supply and quench with water to obtain a solution-treated tube blank;

[0042] (5) Process the solution-treated tube blank by the method of drawing with a fixed mandrel to obtain a pre-stainless steel tube; The wall thickness reduction of the solution-treated tube blank is 0.05 mm, and the outer diameter reduction is 1.3 mm. The total cold working drawing ratio is 26%, and the number of cold working drawing passes is 3 passes;

[0043] (6) Carry out annealing treatment on the pre-stainless steel tube. After annealing, heat it to 1160 °C and hold for 2 h, then carry out quenching and tempering treatment. Among them, annealing is carried out in five stages. The first stage: temperature 755 °C, holding time 6.5 minutes; The second stage: temperature 975 °C, holding time 8.5 min; The third stage: temperature 1035 °C, holding time 6.5 min; The fourth stage: temperature 1085 °C, holding time 6.5 min; The fifth stage: temperature 1065 °C, holding time 8.5 min; The quenching temperature is 1000 °C, the tempering temperature is 600 °C, and the tempering time is 1.7 h. After cooling to room temperature, obtain a special stainless steel tube for subsea oxygen pipelines.

[0044] Example 2

[0045] This embodiment provides a special stainless steel pipe for subsea oxygen pipelines, comprising components with the following weight percentages: Cr 17.2%, Ni 12.6%, Mo 2.3%, C 0.03%, Mn 1.0%, P 0.031%, S 0.010%, Al 1.7%, Nd 0.4%, Ce 2.4%, Cu 1.7%, Hf 1.9%, Nb 1.1%, and the balance being Fe and unavoidable impurities.

[0046] The preparation method of the special stainless steel pipe for subsea oxygen pipelines comprises the following steps:

[0047] (1) Place metallic iron in an electric arc furnace with a furnace temperature of 1600 °C. After melting, hot metal from the blast furnace is obtained. Put the remaining components into an intermediate frequency furnace with a furnace temperature of 1650 °C to obtain an alloy solution. Inject the hot metal from the blast furnace and the alloy solution into an argon oxygen decarburization furnace through a ladle. During smelting, blow Ar gas to obtain molten steel. Adjust the alloy composition, take samples for analysis. When each alloy composition meets the requirements, put the molten steel into a continuous casting machine for forging. Cut when billets are taken out, repeatedly upset and draw in the cross direction 3 times, then roll into a stainless steel plate, and finally air-cool to room temperature.

[0048] (2) Clean the parts to be welded of the stainless steel plate 7 times with absolute ethanol. Then place the cleaned stainless steel plate on a pipe rolling and forming machine. The pipe rolling and forming machine curls the stainless steel plate to form a pipe to be welded. Then weld the pipe to be welded by plasma arc welding method, and finally perform pickling treatment to obtain a pipe blank. The welding width of the pipe to be welded is 3.5 mm. The conditions for plasma arc welding are: current 220 A, voltage 20 V, welding speed 280 mm / min, and the flow rate of the shielding gas 35 L / min.

[0049] (3) Put the pipe blank into a pipe rolling mill for rolling. The heating temperature of the initial rolling bloom of the pipe rolling mill is controlled according to the following requirements: preheating section: 770 °C, heating section: 950 °C, soaking section: 1200 °C; its starting rolling temperature is: 1150 °C; the final rolling and wire spitting temperature is: 950 °C, and the finishing rolling speed is controlled at 35 m / s; obtain a rough pipe.

[0050] (4) Perform solution treatment on the rough pipe using a continuous furnace with a temperature of 960 °C; when the rough pipe reaches 1000 °C, cut off the power and quench with water to obtain a solution-treated pipe blank.

[0051] (5) Process the solution-treated pipe blank by fixed mandrel drawing to obtain a pre-stainless steel pipe. The wall thickness reduction of the solution-treated pipe blank is 0.06 mm, the outer diameter reduction is 1.2 mm, the total cold working drawing rate is 27%, and the number of cold working drawing passes is 3 passes.

[0052] (6) Anneal the pre-stainless steel pipe. After annealing, heat it to 1200 °C, hold for 2 h, and then perform quenching and tempering treatments. Among them, the annealing is carried out in five stages. The first stage: temperature 760 °C, holding time 7 minutes; the second stage: temperature 970 °C, holding time 9 min; the third stage: temperature 1030 °C, holding time 7 min; the fourth stage: temperature 1080 °C, holding time 7 min; the fifth stage: temperature 1070 °C, holding time 9 min; the quenching temperature is 1050 °C, the tempering temperature is 50 °C, and the tempering time is 2 h. After cooling to room temperature, a special stainless steel pipe for submarine oxygen pipelines is obtained.

[0053] Comparative Example 1

[0054] The difference between this comparative example and Example 1 is: the component ratios are different.

[0055] Specifically: A special stainless steel pipe for submarine oxygen pipelines includes the following components by weight percentage: Cr 17.8%, Ni 12.2%, Mo 2.8%, C 0.026%, Mn 1.4%, P 0.026%, S 0.013%, Al 0.3%, Nd 1.5%, Ce 1.0%, Cu 2.5%, Hf 1.0%, Nb 2.5%, and the balance is Fe and unavoidable impurities.

[0056] Comparative Example 2

[0057] The difference between this comparative example and Example 1 is: the sum of the mass percentages of Cu and Hf is less than the mass percentage of Mo.

[0058] A special stainless steel pipe for submarine oxygen pipelines includes the following components by weight percentage: Cr 17.8%, Ni 12.2%, Mo 3.7%, C 0.026%, Mn 1.4%, P 0.026%, S 0.013%, Al 1.3%, Nd 0.5%, Ce 2.0%, Cu 1.0%, Hf 1.5%, Nb 1.5%, and the balance is Fe and unavoidable impurities.

[0059] Comparative Example 3

[0060] The difference between this comparative example and Example 1 is: the ratio of the mass percentages of Ce and Nd is different.

[0061] A special stainless steel pipe for submarine oxygen pipelines includes the following components by weight percentage: Cr 17.8%, Ni 12.2%, Mo 2.8%, C 0.026%, Mn 1.4%, P 0.026%, S 0.013%, Al 1.3%, Nd 0.1%, Ce 2.4%, Cu 1.5%, Hf 2.0%, Nb 1.5%, and the balance is Fe and unavoidable impurities.

[0062] Comparative Example 4

[0063] The difference between this comparative example and Example 1 is as follows: the ratio of the mass percentage content of Hf to the sum of the mass percentage contents of Ce and Nb is different.

[0064] A special stainless steel pipe for subsea oxygen pipelines comprises the following components in weight percentage: Cr 17.8%, Ni 12.2%, Mo 2.8%, C 0.026%, Mn 1.4%, P 0.026%, S 0.013%, Al 1.3%, Nd 0.5%, Ce 1.2%, Cu 1.5%, Hf 3.3%, Nb 1.0%, and the balance is Fe and unavoidable impurities.

[0065] Comparative Example 5

[0066] The difference between this comparative example and Example 1 is as follows: the annealing is carried out in five stages. The first stage: temperature 780 °C, holding time 9 minutes; the second stage: temperature 950 °C, holding time 6 min; the third stage: temperature 1070 °C, holding time 9 min; the fourth stage: temperature 1200 °C, holding time 4 min; the fifth stage: temperature 1030 °C, holding time 7 min.

[0067] Comparative Example 6

[0068] The difference between this comparative example and Example 1 is as follows: the annealing temperature of the annealing treatment is 900 °C, and the holding time is 37 min.

[0069] Performance test

[0070] Refer to GB / T40317-2021 "Stainless Steel Seamless Tubes for Oxygen Pipelines" to conduct performance tests on the special stainless steel pipes for subsea oxygen pipelines prepared in Examples 1-2 and Comparative Examples 1-6. Among them, the specimen size for low-temperature impact: width 10 mm, length 10 mm.

[0071] Corrosion resistance: The special stainless steel pipes for subsea oxygen pipelines are placed in a hydrochloric acid solution with a mass fraction of 30%, and are corroded at normal temperature and 100 °C for 72 h respectively, and the corrosion degree of each seamless stainless steel pipe is detected.

[0072] The results are shown in Table 1.

[0073] Table 1 Performance test results

[0074]

[0075]

[0076] As can be seen from the above performance test results, the special stainless steel pipes for subsea oxygen pipelines in Examples 1-2 have excellent mechanical properties, high corrosion resistance, and high low-temperature impact performance. In particular, the comprehensive performance of Example 1 is the most prominent, mainly due to the combined effect of the compounding of components and the preparation process.

[0077] In the comparative examples, since the necessary technical solutions were not adopted, their performance in the corresponding performance tests was significantly worse than that of the examples. In Comparative Example 1, the component ratio was changed, and it can be seen that the comprehensive effect of the stainless steel decreased. In Comparative Example 2, the sum of the mass percentages of Cu and Hf was less than the mass percentage of Mo, and the tensile strength of the stainless steel pipe decreased. In Comparative Example 3, the ratio of the mass percentages of Nd and Ce was changed, and the elongation after fracture of the stainless steel pipe decreased; at the same time, the low-temperature impact performance of the stainless steel pipe also decreased. In Comparative Example 4, the ratio of the mass percentage of Hf to the sum of the mass percentages of Ce and Nb was changed, and the corrosion resistance of the stainless steel pipe at room temperature decreased. In Comparative Examples 5 and 6, different annealing steps were used, and the corrosion resistance of the stainless steel pipe at high temperature decreased. The above experimental results further prove the importance of the technical solutions defined in the present invention for its technical effects.

[0078] The above is the preferred embodiment of the present invention. It should be noted that for those of ordinary skill in the art, without departing from the principle described in the present invention, several improvements and refinements can be made, and these improvements and refinements should also be regarded as the protection scope of the present invention.

Claims

1. A special stainless steel pipe for subsea oxygen pipelines, characterized in that, Comprising components with the following weight percentages: Cr 17.2 - 18.3%, Ni 11.7 - 12.6%, Mo 2.3 - 3.0%, C 0.02 - 0.03%, Mn 1.0 - 2.0%, P 0.019 - 0.031%, S 0.010 - 0.017%, Al 0.8 - 1.7%, Nd 0.3 - 1.0%, Ce 1.3 - 2.4%, Cu 0.5 - 1.7%, Hf 1.2 - 2.4%, Nb 1.1 - 1.9%, the balance being Fe and inevitable impurities; The sum of the mass percentages of Cu and Hf is greater than the mass percentage of Mo; The ratio of the mass percentages of Ce and Nd is 2 ≤ Ce / Nd ≤ 6; The ratio of the mass percentage of Hf to the sum of the mass percentages of Ce and Nb is (1.8 - 2) : (3 - 4).

2. A method for preparing a special stainless steel pipe for subsea oxygen pipelines according to claim 1, characterized in that, Including the following steps: (1) Place metallic iron in an electric arc furnace with the furnace temperature at 1550 - 1600 °C, and obtain hot metal after melting; Put the remaining components into an intermediate frequency furnace with the furnace temperature at 1600 - 1650 °C to obtain an alloy solution; Inject the hot metal and the alloy solution into an argon oxygen decarburization furnace, blow Ar gas during smelting to obtain molten steel, adjust the alloy composition, take samples for analysis, when each alloy composition meets the requirements, then put the molten steel into a continuous casting machine for forging, cut when billets come out, perform upsetting and drawing out repeatedly in the cross direction 3 - 4 times, then roll into a stainless steel plate, and finally air cool to room temperature; (2) Clean the parts to be welded of the stainless steel plate with absolute ethanol 6 - 7 times, then place the cleaned stainless steel plate on a pipe rolling forming machine, and roll the stainless steel plate by the pipe rolling forming machine to form a pipe to be welded, then weld the pipe to be welded by plasma arc welding method, and finally perform pickling treatment to obtain a pipe blank; (3) Put the pipe blank into a tube rolling mill for rolling to obtain a rough tube; (4) Perform solution treatment on the rough tube in a continuous furnace, cut off the power supply and quench with water when the rough tube reaches 1000 - 1200 °C to obtain a solution-treated tube blank; (5) Process the solution-treated tube blank by fixed mandrel drawing to obtain a pre-stainless steel tube; (6) Perform annealing treatment on the pre-stainless steel tube, heat it to 1150 - 1200 °C after annealing, hold for 2 - 2.2 h, then perform quenching and tempering treatment, and after cooling to room temperature, obtain a special stainless steel tube for subsea oxygen pipelines.

3. The preparation method of the special stainless steel pipe for submarine oxygen pipelines according to claim 2, characterized in that, The conditions for plasma arc welding are: current 220 A, voltage 20 V, welding speed 270 - 280 mm / min, and the flow rate of the shielding gas 32 - 35 L / min.

4. The preparation method of the special stainless steel pipe for submarine oxygen pipelines according to claim 3, characterized in that, The heating temperature of the initial rolling bloom of the said tube rolling mill is controlled according to the following requirements: preheating section: 720 - 770 °C, heating section: 950 - 1000 °C, soaking section: 1150 - 1200 °C; its starting rolling temperature is: 1130 - 1150 °C; the final rolling and coiling temperature is: 950 - 980 °C, and the finishing rolling speed is controlled at 30 - 35 m / s.

5. The preparation method of the special stainless steel pipe for submarine oxygen pipelines according to claim 4, characterized in that, The temperature of the continuous furnace is 940 - 960 °C.

6. The preparation method of the special stainless steel pipe for the submarine oxygen pipeline according to claim 5, characterized in that, The annealing is carried out in five stages. The first stage: the temperature is 750 - 760 °C and the heat preservation time is 6 - 7 minutes; the second stage: the temperature is 970 - 980 °C and the heat preservation time is 8 - 9 minutes; the third stage: the temperature is 1030 - 1040 °C and the heat preservation time is 6 - 7 minutes; the fourth stage: the temperature is 1080 - 1090 °C and the heat preservation time is 6 - 7 minutes; the fifth stage: the temperature is 1060 - 1070 °C and the heat preservation time is 8 - 9 minutes.

7. The preparation method of the special stainless steel pipe for the submarine oxygen pipeline according to claim 6, characterized in that, The quenching temperature is 950 - 1050 °C, the tempering temperature is 500 - 700 °C, and the tempering time is 1.5 - 2 hours.

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