A welded pipe for marine riser with high fracture toughness and preparation method thereof

Abstract

The present invention relates to a high fracture toughness marine riser welded pipe and a preparation method thereof, which belongs to the field of marine pipeline steel manufacturing, and solves the problem that the existing marine riser welded pipe cannot ensure high strength of the weld and high fracture toughness at the same time. A high fracture toughness marine riser welded pipe comprises a pipe body and a weld, and the chemical composition of the weld is calculated by mass percentage: C: 0.03-0.06%, Si: 0.20-0.50%, Mn: 1.00-1.50%, Ni: 1.00-1.50%, Mo: 0.80-1.50%, Cr: 0.10-0.20%, Cu: 0.10-0.20%, S≤0.005%, P≤0.005%, and the balance is Fe and unavoidable impurities. The weld of the marine riser welded pipe of the present invention has both high strength and high fracture toughness.

Description

Technical Field

[0001] The invention relates to the field of marine pipeline steel manufacturing, and in particular to a high fracture toughness welded pipe for marine riser and a preparation method thereof. Background Art

[0002] Fracture toughness is a measure of a material's ability to resist brittle fracture in the presence of cracks. It is an inherent property of the material and is related to the material itself, heat treatment and processing technology.

[0003] Marine risers are in a complex marine environment for a long time and are affected by various factors such as waves, tides, and corrosion. Therefore, their fracture toughness is particularly important. Good fracture toughness can ensure that the riser is not prone to sudden fracture when subjected to various external loads and internal stresses, thereby ensuring the safety of marine oil and gas transportation. For long-distance transportation of deep-sea oil and gas, welding is the most common and economical way to connect deep-sea pipelines. Welding is required every 12m in a marine riser, so welded joints have become an inevitable structure in marine oil and gas engineering. The weld undergoes a process of rapid heating and cooling during the welding process, forming a casting structure, which is looser than the structure of the parent material after pressure processing, and may have defects such as dendrite structure, pores, shrinkage, internal stress, etc. (these defects are within the allowable range of non-destructive testing). These defects and loose organizational structure make the fracture toughness of the weld usually lower than that of the parent material, and it is the weakest part of the toughness in the entire riser structure.

[0004] The weld performance of existing marine riser welded pipes cannot ensure high weld strength and high fracture toughness at the same time. Summary of the invention

[0005] In view of the above analysis, the present invention aims to provide a high fracture toughness marine riser welded pipe and a preparation method thereof, so as to solve the problem that the weld performance of existing marine riser welded pipes cannot ensure high strength of the weld and high fracture toughness at the same time.

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

[0007] The present invention provides a welded pipe for a high fracture toughness marine riser, comprising a pipe body and a weld, wherein the chemical composition of the weld is as follows by mass percentage: C: 0.03-0.06%, Si: 0.20-0.50%, Mn: 1.00-1.50%, Ni: 1.00-1.50%, Mo: 0.80-1.50%, Cr: 0.10-0.20%, Cu: 0.10-0.20%, S≤0.005%, P≤0.005%, and the balance is Fe and unavoidable impurities.

[0008] Furthermore, the welded pipe for marine riser comprises a pipe body and a weld, and the chemical composition of the weld is as follows by mass percentage: C: 0.04-0.058%, Si: 0.24-0.46%, Mn: 1.10-1.43%, Ni: 1.11-1.46%, Mo: 0.89-1.41%, Cr: 0.12-0.18%, Cu: 0.10-0.15%, S≤0.003%, P≤0.004%, and the balance is Fe and unavoidable impurities.

[0009] Furthermore, the microstructure of the weld of the marine riser welded pipe is acicular ferrite, polygonal ferrite and side plate strip ferrite.

[0010] Furthermore, the volume content of the acicular ferrite is ≥80%, the volume content of the polygonal ferrite is ≤15%, and the volume content of the side plate ferrite is ≤5%.

[0011] Furthermore, the effective grain size of the acicular ferrite is ≤3 μm.

[0012] The present invention also provides a method for preparing a welded pipe for a marine riser, which is used to prepare the welded pipe for a marine riser, and comprises the following steps:

[0013] Preparation of steel plates for welded pipes for offshore risers - ultrasonic inspection of steel plates - edge milling - pre-bending - JCO forming - pre-welding - internal welding - external welding - weld X-ray inspection - steel pipe expansion - hydrostatic pressure test - weld ultrasonic inspection - pipe end X-ray inspection - chamfering - pipe end magnetic particle inspection - appearance quality inspection;

[0014] Pre-welding, internal welding and external welding all adopt mixed gas shielded welding, ER70S welding wire is selected, and the mixed gas is 50%~70% CO2 and 30%~50% Ar.

[0015] Furthermore, the flow rate of the mixed gas is 65-80 L / min.

[0016] Furthermore, the pre-welding speed is 25-30 cm / min.

[0017] Furthermore, the welding speeds of the inner welding and the outer welding are both 18-23 cm / min, and the interlayer temperature is both 130-170°C.

[0018] Furthermore, the expansion rate of the steel pipe is 0.70% to 0.90%.

[0019] Compared with the prior art, the present invention can achieve at least one of the following beneficial effects:

[0020] 1. The alloy composition of the weld in the marine riser welded pipe of the present invention adopts a high Mo and high Ni weld alloy design, accurately controls the type and proportion of the structure in the weld, and cooperates with appropriate pre-welding speed, internal and external welding speeds and interlayer temperature to effectively regulate the microstructure state of the weld, including the phase ratio and effective grain size, so that the microstructure of the weld is acicular ferrite, polygonal ferrite and side plate strip ferrite; ensuring that the weld of the marine riser welded pipe has both high strength and high fracture toughness.

[0021] 2. The present invention ensures a reasonable composition design of the weld through the synergistic effect of the design of the tube body base material composition and strict control of the welding process (such as mixed gas shielded welding, selection of welding wire type and quantity, pre-welding, internal welding, external welding, etc.), so that the microstructure of the weld is acicular ferrite, polygonal ferrite and side strip ferrite. In the weld dominated by acicular ferrite, the side strip ferrite grows into the grain along the original austenite grain boundary, destroying the continuity of the matrix. During the crack propagation process, it usually directly extends along the parallel strips of the side strip ferrite, greatly reducing the toughness of the weld. At the same time, polygonal ferrite, as a soft and tough phase in the organization, is prone to form microcracks at the two-phase interface with the hard phase acicular ferrite, thereby destroying the toughness. Refining the effective grain size of acicular ferrite can improve both strength and toughness. Therefore, strictly controlling the proportion of side plate ferrite and grain boundary polygonal ferrite in the organization and refining the acicular ferrite can ensure that the weld obtains excellent fracture toughness, so that the weld of the welded pipe for offshore riser has both high strength and high fracture toughness.

[0022] 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

[0023] 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;

[0024] Figure 1 is a metallographic photograph of the weld in Example 1 of the present invention;

[0025] Figure 2 This is a SEM photograph of the acicular ferrite structure in the weld in Example 1 of the present invention. DETAILED DESCRIPTION

[0026] The preferred embodiments of the present invention are described in detail below in conjunction with the accompanying drawings, wherein the accompanying drawings constitute a part of this application and are used together with the embodiments of the present invention to illustrate the principles of the present invention, but are not used to limit the scope of the present invention.

[0027] The present invention provides a high fracture toughness welded pipe for marine riser, comprising a pipe body and a weld, wherein the chemical composition of the weld is calculated by mass percentage as follows: C: 0.03-0.06%, Si: 0.20-0.50%, Mn: 1.00-1.50%, Ni: 1.00-1.50%, Mo: 0.80-1.50%, Cr: 0.10-0.20%, Cu: 0.10-0.20%, S≤0.005%, P≤0.005%, and the balance is Fe and unavoidable impurities.

[0028] The reasons for limiting the chemical composition of the weld of the welded pipe for offshore riser and the method for producing the same in the present invention are explained below. The mass percentage in the composition is expressed only in %.

[0029] C: Carbon forms carbides with vanadium and titanium in the base metal of the tube body, which can prevent the growth of austenite grains in the coarse-grained zone of the heat-affected zone and ensure the fracture toughness of the hot zone; however, too high a content will significantly damage the welding performance; therefore, the C content is controlled at 0.03~0.06%.

[0030] Si: Silicon is a deoxidizer. When Si exists in austenite in the form of a solid solution, it can improve the strength of steel. However, too high a content will damage the toughness of the steel. Therefore, the Si content is controlled at 0.2~0.5%.

[0031] Mn: Manganese is the main solid solution strengthening element used to ensure the strength of steel, but too much Mn will increase the carbon equivalent and affect weldability. Therefore, the Mn content is controlled at 1.00-1.50%.

[0032] Ni: Nickel can improve the solid solution strengthening increment and hardenability of steel, ensure that the weld has good plasticity and low-temperature toughness, and at the same time ensure that the molten iron has good fluidity when the weld solidifies, thereby improving the weldability. Therefore, the Ni content is controlled at 1.00~1.50%.

[0033] Mo: Molybdenum can reduce the phase transition temperature of supercooled austenite, thereby inhibiting the formation of polygonal ferrite and side plate ferrite in the weld and promoting the transformation of acicular ferrite. At the same time, molybdenum can improve the hardenability and toughness of the weld. Therefore, the Mo content is controlled at 0.80~1.50%.

[0034] Cr, Cu: Both chromium and copper can improve the hardenability, strength and hardness of the weld, but excessive content is not good for the toughness of the weld. Therefore, the Cr content is controlled at 0.10~0.20%, and the Cu content is controlled at 0.10~0.20%.

[0035] P: Phosphorus is an impurity element in steel, which will damage the toughness of offshore riser steel. Therefore, the P content is controlled below 0.005%.

[0036] S: Sulfur is an impurity element in steel. It will form sulfide inclusions and become a crack source. Too much S will also deteriorate weldability. Therefore, the S content is controlled below 0.005%.

[0037] Preferably, the present invention provides a welded pipe for a high fracture toughness marine riser, comprising a pipe body and a weld, wherein the chemical composition of the weld is, by mass percentage, C: 0.04-0.058%, Si: 0.24-0.46%, Mn: 1.10-1.43%, Ni: 1.11-1.46%, Mo: 0.89-1.41%, Cr: 0.12-0.18%, Cu: 0.10-0.15%, S≤0.003%, P≤0.004%, and the balance is Fe and unavoidable impurities.

[0038] The present invention also provides a method for preparing a welded pipe for a marine riser, which is used to prepare the welded pipe for a marine riser, and comprises the following steps:

[0039] Preparation of steel plates for welded pipes for offshore risers - ultrasonic inspection of steel plates - edge milling - pre-bending - JCO forming (JCO-Expanding forming) - pre-welding - internal welding - external welding - weld X-ray inspection - steel pipe expansion - hydrostatic pressure test - weld ultrasonic inspection - pipe end X-ray inspection - chamfering - pipe end magnetic particle inspection - appearance quality inspection;

[0040] Specifically, the preparation of steel plates for welded pipes for marine risers is completed through the steps of molten iron pretreatment - converter smelting - refining - continuous casting - controlled rolling and controlled cooling.

[0041] Specifically, pre-welding, internal welding and external welding all use mixed gas shielded welding, ER70S welding wire is selected, the mixed gas is 50%~70% CO2 and 30%~50% Ar, and the gas flow rate is 65~80L / min.

[0042] Specifically, pre-welding is the preliminary connection or fixation of the welded joint before formal welding. Check the preparation of the welded joint, including the size, shape, cleanliness, etc. of the joint, reduce the deformation and residual stress that may be generated during the welding process, so as to ensure that high-quality welds can be formed during formal welding. This is especially important for offshore riser structures that need to withstand high pressure and high stress environments; the pre-welding speed in the present invention is 25-30 cm / min;

[0043] Internal welding can usually provide more uniform heat distribution, which helps to obtain higher quality welds. External welding can significantly enhance the strength of the pipeline, enabling it to better withstand various loads and stresses in the marine environment; by controlling the appropriate welding speed and interlayer temperature, an ideal weld structure can be obtained; in the present invention, both internal and external welding adopts mixed gas shielded welding, with a welding speed of 18-23cm / min and an interlayer temperature of 130-170℃;

[0044] It should be noted that in the welding of offshore risers, the control of welding speed is crucial, which directly affects the quality of the weld, welding efficiency and the overall performance of the pipeline; too slow welding speed may cause the weld to be too high or too wide, with an uneven shape, and may even burn through the thin plate; while too fast welding speed may cause the weld to be narrow and cause defects such as incomplete penetration; the welding speed also affects the oxidation degree of the weld and the formation of non-metallic inclusions. Inclusions can provide nucleation sites for acicular ferrite, which is beneficial to the phase transformation of acicular ferrite; when the welding speed is slow, the edge of the heated tube blank is exposed to the air for a long time, which is easy to produce deeper non-metallic oxides. These oxides are difficult to be completely squeezed out of the weld during the subsequent crystallization process, thereby reducing the strength of the weld; on the contrary, when the welding speed is fast, the oxidation time is short, the non-metallic oxides produced are less and limited to the surface layer, which are easily squeezed out of the weld, which is not conducive to the formation of acicular ferrite in the weld; in addition, the faster the welding speed, the easier it is to refine the grains of the weld structure. Therefore, in the present invention, the pre-welding speed is 25-30 cm / min; the welding speeds of the internal welding and the external welding are 18-23 cm / min.

[0045] Specifically, the steel pipe expansion can make the pipe force uniform, improve the residual stress distribution at the welded joint, help reduce stress concentration, and improve the overall strength of the pipe. After mechanical expansion, the pipe quality of the welded pipe is improved, which helps to ensure the stable operation of the welded pipe for marine risers in complex marine environments; the expansion rate in the present invention is 0.70% to 0.90%.

[0046] It should be noted that the present invention ensures a reasonable composition design of the weld through the synergistic effect of the design of the pipe body base material composition and the strict control of the welding process (such as mixed gas shielded welding, the selection of welding wire types and quantities, pre-welding, internal welding, external welding, etc.), the alloy composition of the weld has a high Mo and high Ni design, and the organization type and proportion in the weld are accurately controlled. The Mo element can reduce the phase transition temperature of the supercooled austenite, thereby inhibiting the formation of polygonal ferrite and side plate ferrite in the weld, and promoting the transformation of acicular ferrite; at the same time, Mo can improve the hardenability and toughness of the weld. The Ni element can ensure that the weld has good plasticity and low-temperature toughness, and at the same time, it can ensure that the molten iron has good fluidity when the weld solidifies, and improve the weldability of the weld. With appropriate pre-welding speed, internal and external welding speed and interlayer temperature, the microstructure state of the weld can be effectively regulated, including phase ratio and effective grain size, so that the microstructure of the weld is acicular ferrite, polygonal ferrite and side plate ferrite.

[0047] In welds dominated by acicular ferrite, the side lath ferrite grows into the grain along the original austenite grain boundary, destroying the continuity of the matrix. During the crack propagation process, it usually directly extends along the parallel laths of the side lath ferrite, greatly reducing the toughness of the weld. At the same time, polygonal ferrite, as a soft and tough phase in the organization, is prone to form microcracks at the two-phase interface with the hard phase acicular ferrite, thereby destroying the toughness. Refining the effective grain size of acicular ferrite can improve both strength and toughness. Therefore, strictly controlling the ratio of side lath ferrite and grain boundary polygonal ferrite in the organization, while refining the acicular ferrite, can ensure that the weld obtains excellent fracture toughness.

[0048] The welded pipe for marine riser of the present invention has a microstructure of the pipe body of fine-grained ferrite and ultra-low carbon bainite, and a microstructure of the weld of acicular ferrite, polygonal ferrite and side plate ferrite, wherein the volume content of acicular ferrite is ≥80% (for example, 82% to 88%), the volume content of polygonal ferrite is ≤15% (for example, 10% to 15%), and the volume content of side plate ferrite is ≤5% (for example, 2% to 3%); the effective grain size of the acicular ferrite is ≤3μm (for example, 2.2μm to 2.8μm).

[0049] The high fracture toughness welded pipe for marine riser prepared by the present invention has a yield strength of ≥490 MPa (e.g. 491-552 MPa); a tensile strength of ≥580 MPa (e.g. 586-676 MPa); a -20°C weld transverse impact energy of ≥180 J (e.g. 188-236 J); and a CTOD (-20°C) of ≥0.90 mm (e.g. 0.93-1.16 mm). It has excellent fracture toughness while ensuring strength.

[0050] 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.

[0051] Example 1

[0052] The present embodiment provides a welded pipe for a marine riser with high fracture toughness, comprising a pipe body and a weld, wherein the chemical composition of the weld is, by mass percentage, C: 0.04%, Si: 0.24%, Mn: 1.10%, Ni: 1.11%, Mo: 0.89%, Cr: 0.12%, Cu: 0.12%, S: 0.003%, P: 0.004%, and the balance is Fe and unavoidable impurities.

[0053] This embodiment also provides a method for preparing a welded pipe for a marine riser, which is used to prepare the welded pipe for a marine riser, comprising the following steps:

[0054] Preparation of steel plates for welded pipes for offshore risers - ultrasonic inspection of steel plates - edge milling - pre-bending - JCO forming (JCO-Expanding forming) - pre-welding - internal welding - external welding - weld X-ray inspection - steel pipe expansion - hydrostatic pressure test - weld ultrasonic inspection - pipe end X-ray inspection - chamfering - pipe end magnetic particle inspection - appearance quality inspection;

[0055] Among them, the preparation of steel plates for welded pipes for marine risers is completed through the steps of molten iron pretreatment - converter smelting - refining - continuous casting - controlled rolling and controlled cooling;

[0056] Among them, the welding adopts mixed gas shielded welding, the welding wire is ER70S welding wire, the mixed gas is 50% CO2 and 50% Ar, and the gas flow rate is 65L / min. The pre-welding speed is 26cm / min; the internal and external welding adopts four-wire submerged arc automatic welding, the welding speed is 18.5cm / min, the interlayer temperature is 135℃; the expansion rate is 0.72%.

[0057] The welded pipe for marine riser prepared in this embodiment has a yield strength of 491 MPa, a tensile strength of 586 MPa, a weld transverse impact energy (-20°C) of 188 J, a weld transverse CTOD (-20°C) of 0.93 mm, and a weld microstructure of acicular ferrite (82% by volume) + polygonal ferrite (15% by volume) + side plate ferrite (3% by volume), and the effective grain size of the acicular ferrite is 2.8 μm.

[0058] Example 2

[0059] The present embodiment provides a welded pipe for a marine riser with high fracture toughness, comprising a pipe body and a weld, wherein the chemical composition of the weld is, by mass percentage, C: 0.058%, Si: 0.46%, Mn: 1.43%, Ni: 1.46%, Mo: 1.41%, Cr: 0.18%, Cu: 0.15%, S: 0.002%, P: 0.002%, and the remainder is Fe and unavoidable impurities.

[0060] This embodiment also provides a method for preparing a welded pipe for a marine riser, which is used to prepare the welded pipe for a marine riser, comprising the following steps:

[0061] Preparation of steel plates for welded pipes for offshore risers - ultrasonic inspection of steel plates - edge milling - pre-bending - JCO forming (JCO-Expanding forming) - pre-welding - internal welding - external welding - weld X-ray inspection - steel pipe expansion - hydrostatic pressure test - weld ultrasonic inspection - pipe end X-ray inspection - chamfering - pipe end magnetic particle inspection - appearance quality inspection;

[0062] Among them, the preparation of steel plates for welded pipes for marine risers is completed through the steps of molten iron pretreatment - converter smelting - refining - continuous casting - controlled rolling and controlled cooling;

[0063] Among them, the welding adopts mixed gas shielded welding, the welding wire is ER70S welding wire, the mixed gas is 70% CO2 and 30%% Ar, and the gas flow rate is 80L / min. The pre-welding speed is 29cm / min; the internal and external welding adopts four-wire submerged arc automatic welding, the welding speed is 22cm / min, the interlayer temperature is 165℃; the expansion rate is 0.86%.

[0064] The welded pipe for marine riser prepared in this embodiment has a yield strength of 552 MPa, a tensile strength of 676 MPa, a weld transverse impact energy (-20°C) of 236 J, a weld transverse CTOD (-20°C) of 1.16 mm, and a weld microstructure of acicular ferrite (88% by volume) + polygonal ferrite (10% by volume) + side plate ferrite (2% by volume), and the effective grain size of the acicular ferrite is 2.2 μm.

[0065] Comparative Example 1

[0066] This comparative example provides a welded pipe for an offshore riser, comprising a pipe body and a weld, wherein the chemical composition of the weld is, by mass percentage, C: 0.044%, Si: 0.25%, Mn: 1.15%, Ni: 0.90%, Mo: 0.70%, Cr: 0.09%, Cu: 0.11%, S: 0.003%, P: 0.004%, and the balance is Fe and unavoidable impurities.

[0067] This comparative example also provides a method for preparing a welded pipe for a marine riser, which is used to prepare the welded pipe for a marine riser, comprising the following steps:

[0068] Preparation of steel plates for welded pipes for offshore risers - ultrasonic inspection of steel plates - edge milling - pre-bending - JCO forming (JCO-Expanding forming) - pre-welding - internal welding - external welding - weld X-ray inspection - steel pipe expansion - hydrostatic pressure test - weld ultrasonic inspection - pipe end X-ray inspection - chamfering - pipe end magnetic particle inspection - appearance quality inspection;

[0069] Among them, the preparation of steel plates for welded pipes for marine risers is completed through the steps of molten iron pretreatment - converter smelting - refining - continuous casting - controlled rolling and controlled cooling;

[0070] Among them, the welding adopts mixed gas shielded welding, the welding wire is ER70S welding wire, the mixed gas is 50% CO2 and 50% Ar, and the gas flow rate is 65L / min. The pre-welding speed is 26cm / min; the internal and external welding adopts four-wire submerged arc automatic welding, the welding speed is 19cm / min, the interlayer temperature is 132℃; the expansion rate is 0.75%.

[0071] The welded pipe for marine riser prepared in this comparative example has a yield strength of 482 MPa, a tensile strength of 580 MPa, a weld transverse impact energy (-20°C) of 98 J, a weld transverse CTOD (-20°C) of 0.54 mm, and a weld structure of acicular ferrite (71% by volume) + polygonal ferrite (23% by volume) + side plate ferrite (6% by volume), and the effective grain size of the acicular ferrite is 2.9 μm.

[0072] Comparative Example 2

[0073] This comparative example provides a welded pipe for an offshore riser, comprising a pipe body and a weld, wherein the chemical composition of the weld is, by mass percentage, C: 0.04%, Si: 0.30%, Mn: 1.22%, Ni: 1.29%, Mo: 1.16%, Cr: 0.13%, Cu: 0.11%, S: 0.003%, P: 0.004%, and the balance is Fe and unavoidable impurities.

[0074] This comparative example also provides a method for preparing a welded pipe for a marine riser, which is used to prepare the welded pipe for a marine riser, comprising the following steps:

[0075] Preparation of steel plates for welded pipes for offshore risers - ultrasonic inspection of steel plates - edge milling - pre-bending - JCO forming (JCO-Expanding forming) - pre-welding - internal welding - external welding - weld X-ray inspection - steel pipe expansion - hydrostatic pressure test - weld ultrasonic inspection - pipe end X-ray inspection - chamfering - pipe end magnetic particle inspection - appearance quality inspection;

[0076] Among them, the preparation of steel plates for welded pipes for marine risers is completed through the steps of molten iron pretreatment - converter smelting - refining - continuous casting - controlled rolling and controlled cooling;

[0077] Among them, the welding adopts mixed gas shielded welding, the welding wire is ER70S welding wire, the mixed gas is 70% CO2 and 30% Ar, and the gas flow rate is 80L / min. The pre-welding speed is 24cm / min; the internal and external welding adopts four-wire submerged arc automatic welding, the welding speed is 17cm / min, the interlayer temperature is 125℃; the expansion rate is 0.66%.

[0078] The welded pipe for marine riser prepared in this comparative example has a yield strength of 533 MPa, a tensile strength of 628 MPa, a weld transverse impact energy (-20°C) of 111 J, a weld transverse CTOD (-20°C) of 0.66 mm, and a weld structure of acicular ferrite (75% by volume) + polygonal ferrite (20% by volume) + side plate ferrite (5% by volume), and the effective grain size of the acicular ferrite is 4.3 μm.

[0079] Comparative Example 3

[0080] This comparative example provides a welded pipe for an offshore riser, comprising a pipe body and a weld, wherein the chemical composition of the weld is, by mass percentage, C: 0.077%, Si: 0.50%, Mn: 1.50%, Ni: 0.81%, Mo: 0.62%, Cr: 0.22%, Cu: 0.21%, S: 0.005%, P: 0.005%, and the balance is Fe and unavoidable impurities.

[0081] This comparative example also provides a method for preparing a welded pipe for a marine riser, which is used to prepare the welded pipe for a marine riser, comprising the following steps:

[0082] Preparation of steel plates for welded pipes for offshore risers - ultrasonic inspection of steel plates - edge milling - pre-bending - JCO forming (JCO-Expanding forming) - pre-welding - internal welding - external welding - weld X-ray inspection - steel pipe expansion - hydrostatic pressure test - weld ultrasonic inspection - pipe end X-ray inspection - chamfering - pipe end magnetic particle inspection - appearance quality inspection;

[0083] Among them, the preparation of steel plates for welded pipes for marine risers is completed through the steps of molten iron pretreatment - converter smelting - refining - continuous casting - controlled rolling and controlled cooling;

[0084] Among them, SJ102 flux is used for welding. The pre-welding speed is 25cm / min; submerged arc welding is used for both internal and external welding, with a welding speed of 24cm / min, an interlayer temperature of 150℃, and a diameter expansion rate of 0.70%.

[0085] The welded pipe for marine riser prepared in this comparative example has a yield strength of 502 MPa, a tensile strength of 613 MPa, a weld transverse impact energy (-20°C) of 86 J, a weld transverse CTOD (-20°C) of 0.38 mm, and a weld structure of acicular ferrite (69% by volume) + polygonal ferrite (19% by volume) + side plate ferrite (12% by volume), and the effective grain size of the acicular ferrite is 4.7 μm.

[0086] 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.

[0087]

[0088]

[0089]

[0090] It can be seen from the table that in Comparative Example 1, the welded pipe preparation process meets the requirements of the present invention, but the contents of Mo, Ni and Cr in the chemical composition do not meet the requirements of the present invention, resulting in a decrease in the proportion of acicular ferrite in the weld structure and a higher proportion of polygonal ferrite, thereby reducing the fracture toughness of the weld.

[0091] In Comparative Example 2, the chemical composition meets the requirements of the present invention, but the parameters such as the welding speed, interlayer temperature and expansion rate of the pre-welding, internal welding and external welding in the preparation process of the welded pipe do not meet the range required by the present invention, resulting in a slightly low proportion of acicular ferrite in the structure of the weld, and the effective grain size of the acicular ferrite is not effectively refined.

[0092] In Comparative Example 3, the composition and welding process of the weld in the conventional welded pipe were used, so that the proportion of acicular ferrite in the weld was lower, the effective grain size was larger, and the proportion of side plate ferrite was higher, resulting in a significant decrease in the fracture toughness of the weld.

[0093] 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 high fracture toughness marine riser welded pipe, comprising a pipe body and a weld, characterized in that: The chemical composition of the weld is calculated by mass percentage as follows: C: 0.03-0.06%, Si: 0.46-0.50%, Mn: 1.00-1.50%, Ni: 1.00-1.50%, Mo: 0.80-1.50%, Cr: 0.10-0.18%, Cu: 0.10-0.15%, S≤0.005%, P≤0.005%, the balance is Fe and unavoidable impurities; The microstructure of the weld is acicular ferrite, polygonal ferrite and side plate strip ferrite; The volume content of the needle-shaped ferrite is ≥80%, the volume content of the polygonal ferrite is ≤15%, and the volume content of the side plate ferrite is ≤5%; The effective grain size of the acicular ferrite is ≤3 μm; The marine riser welded pipe is prepared by the following process: preparing steel plate for marine riser welded pipe - ultrasonic inspection of steel plate - edge milling - pre-bending - JCO forming - pre-welding - internal welding - external welding - weld seam X-ray inspection - steel pipe diameter expansion - hydrostatic pressure test - weld seam ultrasonic inspection - pipe end X-ray inspection - chamfering - pipe end magnetic particle inspection - appearance quality inspection; Pre-welding, internal welding and external welding all adopt mixed gas shielded welding, ER70S welding wire is used, and the mixed gas is 50%~70% CO2 and 30%~50% Ar; The flow rate of the mixed gas is 65-80 L / min; The pre-welding speed is 25-30 cm / min; The welding speed of the internal welding and the external welding is 18-23 cm / min, and the interlayer temperature is 130-170°C; The marine riser welded pipe has a yield strength of ≥490 MPa, a tensile strength of ≥580 MPa, a -20°C weld transverse impact energy of ≥180 J, and a -20°C weld transverse CTOD of ≥0.90 mm.

2. The welded pipe for offshore riser according to claim 1, characterized in that: The invention comprises a pipe body and a weld, wherein the chemical composition of the weld is as follows by mass percentage: C: 0.04-0.058%, Si: 0.46-0.50%, Mn: 1.10-1.43%, Ni: 1.11-1.46%, Mo: 0.89-1.41%, Cr: 0.12-0.18%, Cu: 0.10-0.15%, S≤0.003%, P≤0.004%, and the balance is Fe and unavoidable impurities.

3. A method for preparing a welded pipe for a marine riser, for preparing the welded pipe for a marine riser according to claim 1 or 2, comprising the following steps: Preparation of steel plates for welded pipes for offshore risers - ultrasonic inspection of steel plates - edge milling - pre-bending - JCO forming - pre-welding - internal welding - external welding - weld X-ray inspection - steel pipe expansion - hydrostatic pressure test - weld ultrasonic inspection - pipe end X-ray inspection - chamfering - pipe end magnetic particle inspection - appearance quality inspection; Mixed gas shielded welding is used for pre-welding, internal welding and external welding. ER70S welding wire is used, and the mixed gas is 50%~70% CO2 and 30%~50% Ar.

4. The preparation method according to claim 3, characterized in that: The pre-welding speed is 26-30 cm / min.

5. The preparation method according to claim 4, characterized in that: The welding speeds of the inner welding and the outer welding are both 18.5-23 cm / min, and the interlayer temperature is both 135-170°C.

6. The preparation method according to claim 5, characterized in that: The expansion rate of the steel pipe is 0.70% to 0.90%.

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