A seamless pipe for fatigue-resistant marine riser and a preparation method thereof

Abstract

The invention relates to a seamless pipe for fatigue-resistant marine riser and a preparation method thereof, belongs to the field of marine pipeline steel manufacturing, and solves the problem that the existing seamless pipe cannot have both high strength and toughness and fatigue resistance. Its chemical composition by mass percentage (%) is: C: 0.08-0.12, Si: 0.2-0.4, Mn: 0.035-0.07, Nb: 0.01-0.03, V: 0.03-0.08, Ti: 0.01-0.02, Cr: 0.20-0.50, Ni: 0.20-0.45, Mo: 0.20-0.40, P: <0.003, S: <0.003, H: <0.00015, O: <0.0008, N: <0.004, As≤0.008, Sb≤0.004, Sn≤0.008, Pb≤0.004, Bi≤0.004, and the rest are Fe and unavoidable impurities.

Description

Technical Field

[0001] The invention relates to the field of marine pipeline steel manufacturing, and in particular to a seamless pipe for fatigue-resistant marine riser and a preparation method thereof. Background Art

[0002] As onshore oil and gas resources gradually shrink, humans are gradually shifting their oil exploitation to the ocean. From shallow sea to deep sea, humans are building oil exploration and production systems in deeper waters. This is the development trend of the world's marine oil and gas industry. As the water depth continues to increase, the technical equipment for deepwater development is constantly facing new challenges, and strict requirements are also put forward for the risers connecting the submarine pipeline and the platform.

[0003] Marine risers are one of the key facilities in deepwater oil and gas development projects. They connect offshore floating devices with submarine pipelines (or underwater wellheads) and are an important part of offshore oil and gas field development systems. Marine risers are subjected to external multi-flow field loads such as wind, waves, and ocean currents for a long time, which may cause vortex-induced vibration (VIV), leading to pipeline fatigue damage and endangering production safety. As the exploitation waters continue to deepen, the aspect ratio of the riser continues to increase, and VIV becomes more complex, which increases the risk of riser fatigue damage. This will not only bring huge economic losses to the project, but also cause serious secondary disasters to the natural environment.

[0004] Improving the fatigue resistance of marine risers can reduce the requirements for pipe span length and support conditions during pipeline management, and reduce the operating costs of submarine pipeline management. The existing seamless pipes for marine risers have too high volume fractions of banded structure and M / A structure (i.e., martensite-austenite) in the thickness section, which results in low fatigue resistance although the existing seamless pipes for marine risers can meet the requirements of strength and toughness. In response to the urgent needs of submarine oil and gas production, it is urgent to develop a new type of seamless pipe for marine risers, which can significantly improve the fatigue performance of the pipe on the basis of meeting the strength and toughness. In order to ensure strength and toughness while significantly improving the fatigue resistance of seamless pipes for marine risers, reasonable organizational design and process control are essential. Summary of the invention

[0005] In view of the above analysis, the present invention aims to provide a fatigue-resistant seamless pipe for marine risers and a preparation method thereof, so as to solve at least one of the following problems: 1. The existing seamless pipes for marine risers have too high a banded structure in thickness section and too high a volume fraction of M / A components, resulting in poor fatigue resistance; 2. The existing seamless pipes for marine risers cannot have both good strength and toughness and high fatigue resistance.

[0006] The purpose of the present invention is mainly achieved through the following technical solutions:

[0007] The invention provides a seamless pipe for fatigue-resistant marine riser, the chemical composition of which is as follows by mass percentage: C: 0.08-0.12%, Si: 0.2-0.4%, Mn: 0.035-0.07%, Nb: 0.01-0.03%, V: 0.03-0.08%, Ti: 0.01-0.02%, Cr: 0.20-0.50%, Ni: 0.20-0.45%, Mo: 0.20-0.40%, P: <0.003%, S: <0.003%, H: <0.00015%, O: <0.0008%, N: <0.004%, As≤0.008%, Sb≤0.004%, Sn≤0.008%, Pb≤0.004%, Bi≤0.004%, and the rest are Fe and unavoidable impurities.

[0008] Further, the chemical composition by mass percentage is: C: 0.083-0.10%, Si: 0.3-0.35%, Mn: 0.042-0.067%, Nb: 0.012-0.014%, V: 0.038-0.073%, Ti: 0.014-0.015%, Cr: 0.29-0.47%, Ni: 0.23-0.43%, Mo: 0.26-0.35%, P: <0.002%, S: <0.0018%, H: <0.00012%, O: <0.0006%, N: <0.0032%, As≤0.006%, Sb≤0.0035%, Sn≤0.007%, Pb≤0.0034%, Bi≤0.0031%, and the rest are Fe and unavoidable impurities.

[0009] Furthermore, the microstructure of the fatigue-resistant seamless pipe for marine riser is fine-grained tempered troostite, there is no banded structure in the full thickness direction of the structure, and the average effective grain size is ≤4μm.

[0010] The present invention also provides a method for preparing a fatigue-resistant seamless pipe for marine riser, which is used to prepare the above-mentioned fatigue-resistant seamless pipe for marine riser, through the process of ironmaking → steelmaking → hot rolling pipe → cold drawing → steel pipe tempering heat treatment → straightening → outer surface polishing → inner surface shot blasting → pipe end processing.

[0011] Furthermore, in the hot rolling tube making step, the rolling ratio of the hot rolling tube making process is ≥9.

[0012] Furthermore, in the cold drawing step, a suitable cold drawing die shape and size are selected to control the inner diameter tolerance of the tube end to be ≤±0.35mm and the wall thickness tolerance to be ≤±1.0mm.

[0013] Furthermore, in the steel pipe tempering heat treatment step, the quenching temperature of the steel pipe tempering heat treatment is 910-950°C.

[0014] Furthermore, the cooling rate of the steel pipe during the tempering heat treatment process is ≥35°C / s.

[0015] Furthermore, in the steel pipe tempering heat treatment step, the tempering temperature of the steel pipe tempering heat treatment is 670-710°C.

[0016] Furthermore, in the tube end processing step, the inner diameter tolerance of the tube end is ≤±0.22 mm.

[0017] Compared with the prior art, the present invention can achieve at least one of the following beneficial effects:

[0018] 1. Segregation-prone elements such as C and Mn are prone to produce banded structure problems, which have little effect on conventional use scenarios, but will affect the fatigue performance of marine risers. The present invention reduces the Mn content and increases the content of alloying elements such as Cr, Ni and Mo that also have solid solution strengthening effects. In combination with increasing the rolling ratio of the hot rolling pipe making process, controlling the quenching temperature of the tempering heat treatment and increasing the cooling rate during the preparation process, the problem of banded structure of the seamless pipe for marine risers is effectively solved. The prepared seamless pipe for marine risers has both good strength and toughness and high fatigue resistance.

[0019] 2. The present invention provides a heat treatment process for marine risers with a new composition system by reasonable steel composition design and strict control of process parameters during the preparation process, thereby obtaining fine and uniform fine-grained tempered troostite with no banded structure in the full thickness direction and an average effective grain size of ≤4μm, thereby improving the fatigue performance of the marine riser seamless pipe. The prepared marine riser seamless pipe has both good strength and toughness and high fatigue resistance.

[0020] In the present invention, the above-mentioned technical solutions can also be combined with each other to achieve more preferred combination solutions. Other features and advantages of the present invention will be described in the subsequent description, and some advantages can become obvious from the description, or can be understood by practicing the present invention. The purpose and other advantages of the present invention can be realized and obtained through the contents particularly pointed out in the description and the drawings. BRIEF DESCRIPTION OF THE DRAWINGS

[0021] The accompanying drawings are only used for the purpose of illustrating specific embodiments and are not to be considered as limiting the present invention. In the entire drawings, the same reference symbols represent the same components;

[0022] Figure 1 The metallographic structure diagram of the seamless pipe for marine riser provided in Example 1 of the present invention;

[0023] Figure 2 This is the EBSD structure diagram of the seamless pipe for marine riser provided in Example 1 of the present invention. DETAILED DESCRIPTION

[0024] The present invention provides a fatigue-resistant seamless pipe for marine riser. The chemical composition of the fatigue-resistant seamless pipe for marine riser is as follows by mass percentage: C: 0.08-0.12%, Si: 0.2-0.4%, Mn: 0.035-0.07%, Nb: 0.01-0.03%, V: 0.03-0.08%, Ti: 0.01-0.02%, Cr: 0.20-0.50%, Ni: 0.20-0.45%, Mo: 0.20-0.40%, P: <0.003%, S: <0.003%, H: <0.00015%, O: <0.0008%, N: <0.004%, As≤0.008%, Sb≤0.004%, Sn≤0.008%, Pb≤0.004%, Bi≤0.004%, and the rest are Fe and unavoidable impurities.

[0025] At present, the volume fraction of the banded structure and M / A structure (i.e., martensite-austenite) in the thickness section of the existing seamless pipe for marine risers is too high, which cannot meet the requirements of having good strength and toughness while also having high fatigue resistance. The present invention ensures that the microstructure of the seamless pipe for marine risers is fine-grained tempered bainite by controlling the element types, element contents and preparation process of the seamless pipe for risers, and controls the structure to have no banded structure in the full thickness direction, thereby solving the problem that the existing seamless pipe for marine risers cannot have both good strength and toughness and high fatigue resistance.

[0026] The reasons for limiting the composition of the cast slab of the seamless pipe for fatigue-resistant marine riser and the method for producing the same in the present invention will be described below. The mass percentage in the composition will be expressed only in %.

[0027] C: Carbon is an element that ensures the strength of steel. It forms carbides with vanadium and titanium to delay the recrystallization of austenite and thus refine the grains. The carbon content has a great influence on the impact toughness of steel. Too high a content will significantly damage the welding performance. Therefore, the C content is controlled at 0.08~0.12%.

[0028] Si: A deoxidizer in the steel smelting process, it has a strong solid solution strengthening effect, but too high a content will damage the toughness of the steel; therefore, the Si content is controlled at 0.2~0.4%.

[0029] Mn: Too high manganese content will cause component segregation in the thickness section of the pipe body, thereby forming a banded structure, resulting in uneven tissue distribution and seriously damaging the fatigue resistance of the pipe; but a reduction in manganese content will lead to a decrease in its solid solution strengthening increment, and it is necessary to increase the strength of the steel by other means; therefore, the Mn content is controlled at 0.035-0.07%.

[0030] Nb: An element that promotes the formation of M / A structure. If its content is too high, the volume fraction of M / A components in the structure will increase. As a hard and brittle phase in the structure, M / A components are prone to induce fatigue cracks, resulting in reduced fatigue resistance of steel and also damaging toughness. Therefore, the Nb content is controlled at 0.01-0.03%.

[0031] V: A nanoscale second phase is precipitated during the tempering process, which improves the strength of the marine riser through precipitation strengthening; the V content is controlled at 0.03-0.08%.

[0032] Ti: Stable TiN is formed at high temperature, which inhibits the growth of austenite grains during reheating. Too high a content will form large-sized precipitates, affecting the toughness of the steel. The Ti content is controlled at 0.01~0.02%, which means micro-titanium treatment.

[0033] Mo: Molybdenum can reduce the phase transition temperature of supercooled austenite, inhibit the formation of polygonal ferrite, and promote the transformation of acicular ferrite. In Nb-containing pipeline steel, Mo can increase the solid solubility of Nb (C, N) in austenite, reduce the precipitation temperature of Nb (C, N), and make more Nb (C, N) precipitate in low-temperature α, thereby improving the precipitation strengthening effect of Nb (C, N). At the same time, Mo is also a carbide-forming element. During the welding thermal cycle, it can be integrated into the lattice of microalloy carbonitrides to reduce the mismatch energy, thereby promoting the precipitation of microalloy carbonitrides. Therefore, the Mo content is controlled at 0.20~0.40%.

[0034] Cr and Ni: As supplementary elements of molybdenum, their effects on the phase transformation process are similar to those of molybdenum; they can increase the solid solution strengthening increment and hardenability of steel, so the Ni content is controlled at 0.20~0.45% and the Cr content is controlled at 0.20~0.50%.

[0035] P: Phosphorus is an impurity element in steel and will damage the toughness of offshore riser steel; therefore, the P content is controlled below 0.003%.

[0036] S: Sulfur is an impurity element in steel, which will form sulfide inclusions and become a crack source; therefore, the S content is controlled below 0.003%.

[0037] H: Hydrogen can cause hydrogen embrittlement of steel, resulting in a sharp decrease in the ductility and toughness of steel. Controlling the hydrogen content in offshore risers can reduce the risk of embrittlement or even cracking of deep-sea risers under the combined action of hydrogen and stress, and improve the safety and service life of the risers; therefore, the H content is controlled below 0.00015.

[0038] O: Oxygen mainly exists in steel in the form of inclusions, reducing the strength, plasticity and toughness of steel. Controlling the oxygen content can reduce the number of inclusions and improve the purity of steel. In addition, controlling the oxygen content in the offshore riser can also significantly reduce the risk of material performance degradation and structural damage caused by oxygen corrosion in deep-sea risers, and extend the service life of the riser; therefore, the O content is controlled below 0.0008%.

[0039] N: Nitrogen will significantly reduce the mechanical properties, welding performance and safety reliability of the riser, and increase the risk of accidents such as breakage and leakage of deep-sea risers during use; therefore, the N content is controlled below 0.004%.

[0040] As: Arsenic will increase the cold brittleness of steel and reduce welding performance. In order to ensure the low temperature toughness and welding performance of steel, the As content is controlled below 0.008%.

[0041] Sn: Tin will significantly reduce the high temperature mechanical properties of steel and alloys and increase temper brittleness. In order to ensure the strength and toughness of steel after high temperature tempering during quenching and tempering heat treatment, the Sn content is controlled below 0.008%.

[0042] Sb: Antimony, like tin, will also reduce the toughness and plasticity of steel. In order to ensure the mechanical properties and processing properties of the offshore riser, the Sb content is controlled below 0.004%.

[0043] Pb: The presence of lead in steel will reduce the mechanical properties of the steel and also pollute the environment; in order to improve the purity of the steel, the Pb content is controlled below 0.004%.

[0044] Bi: Bismuth in steel will also reduce the mechanical properties and processing properties of steel. In order to ensure the quality stability and reliability of steel, the Bi content is controlled below 0.004%.

[0045] Optimally, a fatigue-resistant seamless pipe for marine riser, the chemical composition of the fatigue-resistant seamless pipe for marine riser is as follows by mass percentage: C: 0.083-0.10%, Si: 0.3-0.35%, Mn: 0.042-0.067%, Nb: 0.012-0.014%, V: 0.038-0.073%, Ti: 0.014-0.015%, Cr: 0.29-0.47%, Ni: 0.23~0.43%, Mo: 0.26~0.35%, P: <0.002%, S: <0.0018%, H: <0.00012%, O: <0.0006%, N: <0.0032%, As≤0.006%, Sb≤0.0035%, Sn≤0.007%, Pb≤0.0034%, Bi≤0.0031%, the rest are Fe and unavoidable impurities.

[0046] The present invention also provides a method for preparing a fatigue-resistant seamless pipe for marine riser, wherein the seamless pipe for marine riser is prepared through the process of ironmaking → steelmaking → hot rolling pipe → cold drawing → steel pipe tempering heat treatment → straightening → outer surface polishing → inner surface shot blasting → pipe end processing.

[0047] Specifically, in the hot rolling tube making step, the rolling ratio of the hot rolling tube making process is ≥9. The high rolling ratio can produce a large number of crystal defects during the deformation process, such as broken dendrites, curved grain boundaries, deformation bands, and strain-induced second phase precipitation particles. These crystal defects provide nucleation sites for phase transformation during the subsequent steel tube tempering heat treatment process, thereby refining the grain size of the reheated austenite. At the same time, the second phase particles pin the grain boundaries, which is beneficial to hinder the grain growth.

[0048] Specifically, in the cold drawing step, a suitable cold drawing die shape and size are selected to control the inner diameter tolerance of the pipe end to ≤±0.35mm and the wall thickness tolerance to ≤±1.0mm. This is because dimensional accuracy has a great influence on the fatigue performance of the marine riser. The cold drawing process can significantly improve the dimensional accuracy of the seamless pipe, including the accuracy of the outer diameter and inner diameter, as well as the dimensional accuracy of the wall thickness.

[0049] Due to the relatively high content of alloying elements such as Cr, Ni and Mo, in order to ensure that the pipe is fully austenitized without grain growth, the quenching temperature of the steel pipe tempering heat treatment process is controlled at 910-950℃, the holding time is determined according to the thickness of the pipe, and the cooling rate is ≥35℃ / s, which is conducive to preventing grain growth; the tempering temperature is controlled at 670℃~710℃, so as to obtain a good comprehensive strength and toughness match. Finally, the inner diameter is processed by turning to further control the inner diameter tolerance of the pipe end to ≤±0.22mm.

[0050] Elements that are prone to segregation, such as C and Mn, are prone to produce banded structure problems, which have little effect on conventional usage scenarios, but will affect the fatigue performance of marine risers; the present invention reduces the Mn content while increasing the content of alloying elements that also have a solid solution strengthening effect, such as Cr, Ni and Mo, combined with operations such as increasing the rolling ratio of the hot rolling pipe making process, controlling the quenching temperature of the tempering heat treatment and increasing the cooling rate during the preparation process, thereby effectively solving the problem of banded structure of seamless pipes for marine risers; the microstructure of the seamless pipe for marine risers of the present invention is fine-grained tempered troostite, there is no banded structure in the full thickness direction of the structure, and the average effective grain size is ≤4μm.

[0051] The seamless pipe for marine riser prepared by the present invention has a yield strength of 485-605 MPa, a tensile strength of 560-700 MPa, a CVN (-20°C) ≥ 400 J, a CTOD (-20°C) ≥ 2.0 mm, and a full-size fatigue cycle number of ≥ 3 × 10 at a stress level of 25 Ksi. 6; On the basis of meeting good strength and toughness properties, it has good fatigue resistance and resistance to heat-affected zone softening.

[0052] The advantages of the precise control of the chemical composition, content and preparation process parameters of the present invention will be demonstrated below with specific examples and comparative examples.

[0053] Example 1

[0054] The present embodiment provides a fatigue-resistant seamless pipe for marine riser, and the chemical composition of the fatigue-resistant seamless pipe for marine riser is as follows by mass percentage: C: 0.095%, Si: 0.30wt.%, Mn: 0.056%, Nb: 0.014%, V: 0.052%, Ti: 0.015%, Cr: 0.35%, Ni: 0.31%, Mo: 0.30%, P: 0.002%, S: 0.0018%, H: 0.00011%, O: 0.0006%, N: 0.0031%, As: 0.006%, Sb: 0.003%, Sn: 0.007%, Pb: 0.003%, Bi: 0.0027%, and the rest are Fe and unavoidable impurities.

[0055] Preparation through the process of iron making → steel making → hot rolling → cold drawing → steel pipe quenching and tempering heat treatment → straightening → outer surface polishing → inner surface shot blasting → pipe end processing:

[0056] Among them, the rolling ratio of the hot rolling pipe making process is 9.2, the inner diameter tolerance of the pipe end after cold drawing is ±0.33mm, and the wall thickness tolerance is ±0.09mm. The quenching temperature of the steel pipe tempering heat treatment process is 925℃, the cooling rate is 38℃ / s, the tempering temperature is 688℃, and the inner diameter tolerance of the pipe end after processing is ±0.21mm.

[0057] The yield strength of the seamless pipe for offshore riser prepared in this embodiment is 523 MPa, the tensile strength is 611 MPa, the CVN (-20°C) is 468 J, the CTOD (-20°C) is 2.46 mm, and the full-size fatigue cycle number at a stress level of 25 Ksi is 3.77×10 6 The microstructure is fine-grained tempered troostite, with no banded structure in the full thickness direction, and the average effective grain size is 3.46μm.

[0058] Example 2

[0059] The present embodiment provides a fatigue-resistant seamless pipe for marine riser, and the chemical composition of the fatigue-resistant seamless pipe for marine riser is as follows by mass percentage: C: 0.083%, Si: 0.35wt.%, Mn: 0.042%, Nb: 0.012%, V: 0.073%, Ti: 0.014%, Cr: 0.47%, Ni: 0.43%, Mo: 0.35%, P: 0.0018%, S: 0.0016%, H: 0.00012%, O: 0.0005%, N: 0.0032%, As: 0.0053%, Sb: 0.0035%, Sn: 0.0051%, Pb: 0.003%, Bi: 0.0022%, and the rest are Fe and unavoidable impurities.

[0060] Preparation through the process of iron making → steel making → hot rolling → cold drawing → steel pipe quenching and tempering heat treatment → straightening → outer surface polishing → inner surface shot blasting → pipe end processing:

[0061] Among them, the rolling ratio of the hot-rolled pipe making process is 9.5, the inner diameter tolerance of the pipe end after cold drawing is ±0.31mm, and the wall thickness tolerance is ±0.09mm. The quenching temperature of the steel pipe tempering heat treatment process is 946℃, the cooling rate is 39℃ / s, the tempering temperature is 705℃, and the inner diameter tolerance of the pipe end after processing is ±0.22mm.

[0062] The yield strength of the seamless pipe for offshore riser prepared in this embodiment is 556 MPa, the tensile strength is 652 MPa, the CVN (-20°C) is 435 J, the CTOD (-20°C) is 2.31 mm, and the number of full-size fatigue cycles at a stress level of 25 Ksi is 3.33×10 6 The microstructure is fine-grained tempered troostite, with no banded structure in the full thickness direction and an average effective grain size of 3.52μm.

[0063] Example 3

[0064] The present embodiment provides a fatigue-resistant seamless pipe for marine riser, and the chemical composition of the fatigue-resistant seamless pipe for marine riser is as follows by mass percentage: C: 0.10%, Si: 0.34%, Mn: 0.067%, Nb: 0.013%, V: 0.038%, Ti: 0.015%, Cr: 0.29%, Ni: 0.23%, Mo: 0.26%, P: 0.0011%, S: 0.0012%, H: 0.00011%, O: 0.0006%, N: 0.003%, As: 0.0055%, Sb: 0.0035%, Sn: 0.006%, Pb: 0.0034%, Bi: 0.0031%, and the rest are Fe and unavoidable impurities.

[0065] Preparation through the process of iron making → steel making → hot rolling → cold drawing → steel pipe quenching and tempering heat treatment → straightening → outer surface polishing → inner surface shot blasting → pipe end processing:

[0066] Among them, the rolling ratio of the hot-rolled pipe making process is 9.8, the inner diameter tolerance of the pipe end after cold drawing is ±0.30mm, and the wall thickness tolerance is ±0.08mm. The quenching temperature of the steel pipe tempering heat treatment process is 911℃, the cooling rate is 35℃ / s, the tempering temperature is 676℃, and the inner diameter tolerance of the pipe end after processing is ±0.21mm.

[0067] The yield strength of the seamless pipe for offshore riser prepared in this embodiment is 540 MPa, the tensile strength is 633 MPa, the CVN (-20°C) is 411 J, the CTOD (-20°C) is 2.17 mm, and the number of full-size fatigue cycles at a stress level of 25 Ksi is 3.19×10 6 The microstructure is fine-grained tempered troostite, with no banded structure in the full thickness direction, and the average effective grain size is 3.87μm.

[0068] Comparative Example 1

[0069] This comparative example provides a seamless pipe for marine riser, whose chemical composition, by mass percentage, is: C: 0.15%, Si: 0.28%, Mn: 0.088%, Nb: 0.02%, V: 0.025%, Ti: 0.015%, Cr: 0.16%, Ni: 0.15%, Mo: 0.18%, P: 0.005%, S: 0.005%, H: 0.0002%, O: 0.0009%, N: 0.005%, As: 0.009%, Sb: 0.005%, Sn: 0.009%, Pb: 0.005%, Bi: 0.005%, and the rest is Fe and unavoidable impurities.

[0070] Preparation through the process of iron making → steel making → hot rolling → cold drawing → steel pipe quenching and tempering heat treatment → straightening → outer surface polishing → inner surface shot blasting → pipe end processing:

[0071] Among them, the rolling ratio of the hot rolling pipe making process is 9.1, the inner diameter tolerance of the pipe end after cold drawing is ±0.35mm, and the wall thickness tolerance is ±0.09mm. The quenching temperature of the steel pipe tempering heat treatment process is 924℃, the cooling rate is 36℃ / s, the tempering temperature is 681℃, and the inner diameter tolerance of the pipe end after processing is ±0.22mm.

[0072] The yield strength of the seamless pipe prepared in this comparative example is 537MPa, the tensile strength is 632MPa, the CVN (-20℃) is 347J, the CTOD (-20℃) is 1.23mm, and the full-size fatigue cycle number at the 25Ksi stress level is 2.34×10 6The microstructure is fine-grained tempered troostite, with banded structure on the thickness section and an average effective grain size of 3.91 μm.

[0073] Comparative Example 2

[0074] This comparative example provides a seamless pipe for marine riser, whose chemical composition, by mass percentage, is: C: 0.083%, Si: 0.35wt.%, Mn: 0.042%, Nb: 0.012%, V: 0.073%, Ti: 0.014%, Cr: 0.47%, Ni: 0.43%, Mo: 0.35%, P: 0.0018%, S: 0.0016%, H: 0.00012%, O: 0.0005%, N: 0.0032%, As: 0.0053%, Sb: 0.0035%, Sn: 0.0051%, Pb: 0.003%, Bi: 0.0022%, and the rest is Fe and unavoidable impurities.

[0075] Preparation through the process of iron making → steel making → hot rolling → cold drawing → steel pipe quenching and tempering heat treatment → straightening → outer surface polishing → inner surface shot blasting → pipe end processing:

[0076] Among them, the rolling ratio of the hot rolling pipe making process is 8, the inner diameter tolerance of the pipe end after cold drawing is ±0.5mm, and the wall thickness tolerance is ±0.15mm. The quenching temperature of the steel pipe tempering heat treatment process is 896℃, the cooling rate is 30℃ / s, the tempering temperature is 660℃, and the inner diameter tolerance of the pipe end after processing is ±0.40mm.

[0077] The yield strength of the seamless pipe prepared in this comparative example is 516MPa, the tensile strength is 603MPa, the CVN (-20℃) is 377J, the CTOD (-20℃) is 1.66mm, and the number of full-size fatigue cycles at a stress level of 25Ksi is 2.56×10 6 The microstructure is fine-grained tempered troostite, with banded structure on the thickness section and an average effective grain size of 5.02μm.

[0078] Comparative Example 3

[0079] This comparative example provides a seamless pipe for marine risers, whose chemical composition, by mass percentage, is: C: 0.092%, Si: 0.30%, Mn: 1.50%, Nb: 0.04%, V: 0.03%, Ti: 0.015%, Cr: 0.12%, Ni: 0.20%, Mo: 0.15%, P: 0.015%, S: 0.003%, As: 0.01%, Sb: 0.005%, Sn: 0.01%, Pb: 0.005%, Bi: 0.005%, and the rest is Fe and unavoidable impurities.

[0080] Preparation through the process of iron making → steel making → hot rolling → cold drawing → steel pipe quenching and tempering heat treatment → straightening → outer surface polishing → inner surface shot blasting → pipe end processing:

[0081] Among them, the rolling ratio of the hot rolling pipe making process is 8, the inner diameter tolerance of the pipe end after cold drawing is ±0.50mm, and the wall thickness tolerance is ±0.10mm. The quenching temperature of the steel pipe tempering heat treatment process is 900℃, the cooling rate is 30℃ / s, the tempering temperature is 670℃, and the inner diameter tolerance of the pipe end after processing is ±0.50mm.

[0082] The yield strength of the seamless pipe prepared in this comparative example is 505MPa, the tensile strength is 596MPa, the CVN (-20℃) is 365J, the CTOD (-20℃) is 1.09mm, and the number of full-size fatigue cycles at a stress level of 25Ksi is 2.14×10 6 The microstructure is fine-grained tempered troostite, with banded structure on the thickness section and an average effective grain size of 5.55 μm.

[0083] Table 1 shows the chemical composition and content of the steel of the embodiment and the comparative example; Table 2 shows the process parameters of the preparation process of the steel of the embodiment and the comparative example; Table 3 shows the mechanical properties of the steel of the embodiment and the comparative example.

[0084]

[0085]

[0086]

[0087] It can be seen from the table that in Comparative Example 1, the pipe preparation process meets the requirements of the present invention, but various factors in the chemical composition do not meet the requirements of the present invention (C, Mn, V, Cr, Ni, Mo, P, S, H, O, N, As, Sb, Sn, Pb, Bi), resulting in the appearance of banded structure in the final steel structure, thereby significantly reducing the fatigue performance.

[0088] In Comparative Example 2, the chemical composition is the same as that of Example 2, but the rolling ratio, cold drawing dimensional accuracy control and overall quenching and tempering process parameters in the pipe preparation process do not meet the range required by the present invention, resulting in that the effective grain size of the tempered bainite structure in the final steel structure is not effectively refined, and the toughness and fatigue properties of the seamless pipe finally prepared are reduced.

[0089] In Comparative Example 3, the composition and pipe making process of a conventional seamless pipe were used, resulting in a larger effective grain size of the tempered bainite structure and the presence of a banded structure in the thickness section, resulting in reduced toughness and fatigue properties.

[0090] The above description is only a preferred specific implementation manner of the present invention, but the protection scope of the present invention is not limited thereto. Any changes or substitutions that can be easily conceived by any technician familiar with the technical field within the technical scope disclosed by the present invention should be covered within the protection scope of the present invention.

Claims

1. A seamless pipe for fatigue-resistant marine riser, characterized in that: The chemical composition is calculated by mass percentage: C: 0.08-0.12%, Si: 0.2-0.4%, Mn: 0.035-0.07%, Nb: 0.01-0.03%, V: 0.03-0.08%, Ti: 0.01-0.02%, Cr: 0.20-0.50%, Ni: 0.20-0.45%, Mo: 0.20-0.40%, P: <0.003%, S: <0.003%, H: <0.00015%, O: <0.0008%, N: <0.004%, As≤0.008%, Sb≤0.004%, Sn≤0.008%, Pb≤0.004%, Bi≤0.004%, the rest are Fe and unavoidable impurities; The microstructure of the seamless pipe for fatigue-resistant marine riser is fine-grained tempered troostite, there is no banded structure in the full thickness direction of the structure, and the average effective grain size is ≤4μm.

2. The fatigue-resistant seamless pipe for marine riser according to claim 1, characterized in that: The chemical composition by mass percentage is: C: 0.083~0.10%, Si: 0.3~0.35%, Mn: 0.042~0.067%, Nb: 0.012~0.014%, V: 0.038~0.073%, Ti: 0.014~0.015%, Cr: 0.29~0.47%, Ni: 0.23~0.43%, Mo: 0.26~0.35%, P: <0.002%, S: <0.0018%, H: <0.00012%, O: <0.0006%, N: <0.0032%, As≤0.006%, Sb≤0.0035%, Sn≤0.007%, Pb≤0.0034%, Bi≤0.0031%, and the rest are Fe and unavoidable impurities.

3. A method for preparing a fatigue-resistant seamless pipe for marine riser, which is used to prepare the fatigue-resistant seamless pipe for marine riser according to any one of claims 1 or 2, through the process of ironmaking → steelmaking → hot rolling pipe → cold drawing → steel pipe quenching and tempering heat treatment → straightening → outer surface polishing → inner surface shot blasting → pipe end processing; In the hot rolling tube making step, the rolling ratio of the hot rolling tube making process is ≥9.

4. The preparation method according to claim 3, characterized in that: In the cold drawing step, a suitable cold drawing die shape and size are selected to control the inner diameter tolerance of the tube end to be ≤±0.35mm and the wall thickness tolerance to be ≤±1.0mm.

5. The preparation method according to claim 4, characterized in that: In the steel pipe tempering heat treatment step, the quenching temperature of the steel pipe tempering heat treatment is 910-950°C.

6. The preparation method according to claim 5, characterized in that: The cooling rate of the steel pipe during the tempering heat treatment process is ≥35°C / s.

7. The preparation method according to claim 6, characterized in that: In the steel pipe tempering heat treatment step, the tempering temperature of the steel pipe tempering heat treatment is 670-710°C.

8. The preparation method according to claim 7, characterized in that: In the tube end processing step, the inner diameter tolerance of the tube end is ≤±0.22 mm.

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