Alkaline flux-cored wire for welding oil and gas pipelines and preparation method of alkaline flux-cored wire

Abstract

The invention relates to an alkaline flux-cored wire for welding oil and gas pipelines and a preparation method of the alkaline flux-cored wire, and belongs to the technical field of preparation of high-strength and high-toughness welding materials. The flux-cored wire is prepared from a steel belt and powder through steel belt rolling, powder filling, welding wire drawing and coil dividing. By controlling the components of the flux powder of the flux-cored wire, it is determined that the component system of the flux powder is composed of BF-Fe2O3-Al2O3-NiO-MgO-MnO, the component system is a main barium fluoride slag system supplemented with aluminum oxide, magnesium oxide strong deoxidizers and the like, and under the protection of carbon dioxide, the slag system is stable in electric arc, small in splashing, high in porosity resistance and good in fusion performance; the mechanical property of a ring welding joint welded by the obtained welding wire, the mechanical property of deposited metal and the diffusible hydrogen content meet the GB / T36233-2018 requirements.

Description

Technical Field

[0001] The invention belongs to the technical field of preparation of high-strength and high-toughness welding materials, and particularly relates to an alkaline flux-cored welding wire for oil and gas pipeline welding and a preparation method thereof. Background Art

[0002] Although the output of gas shielded welding wire has grown rapidly in recent years, high-end products still need to be imported in large quantities every year. Long-distance oil and gas pipelines all use thick-walled and large-diameter steel pipes, and the strength of the steel pipes is guaranteed by a variety of alloy elements, microstructure types and grain refinement. Solid wire gas shielded automatic welding and gas shielded flux-cored wire automatic welding are the development direction of large-diameter, thick-walled oil and gas pipeline welding. However, for this welding process, the existing welding materials are difficult to apply to thick-walled, high-grade pipeline steel pipes with different chemical compositions, and it is difficult to meet the high toughness requirements of the weld fracture toughness CTOD (Crack Tip Opening Displacement) of 0.254mm, which hinders the large-scale application of high-grade oil and gas pipeline girth weld welding materials. Summary of the invention

[0003] In view of the above problems, the present invention provides a basic flux-cored welding wire for oil and gas pipeline welding and a preparation method thereof.

[0004] The first object of the present invention is to provide a basic flux-cored welding wire for welding oil and gas pipelines, which is made of steel strip and flux powder through steel strip rolling, flux powder filling, welding wire drawing and coiling;

[0005] The powder ingredients, by mass percentage, include: BaF ≥ 15%, SiO 2 ≤2%, CaO0.12~0.35%, Mg 6~7%, MnO 5~10%, TiO 2 0.03~0.05%, Ni 10~15%, K 2 O0.03~0.05%、Na 2 O 0.2~0.4%、Al 2 O 3 ≤25%、ZrO 2 <0.010%, CeO 2 0.8~1%、P 2 O 5 <0.010%、Bi 2 O 3 <0.010%、La 2 O 3 0.081%、Nb 2 O 5 <0.010, the balance is Fe 2 O 3 .

[0006] In a specific embodiment of the present invention, the target value of the filling ratio of the medicine powder is 16-20%, and the floating range of the target value is ±0.6%.

[0007] In a specific embodiment of the present invention, the alloy composition of the steel strip, by mass percentage, includes: C 0.020-0.060%, Mn 0.15-0.35%, Si≤0.03%, S≤0.015%, P≤0.015%, Al≤0.05%, N≤0.005%, B≤0.001%, and the balance is iron.

[0008] In a specific embodiment of the present invention, the mechanical properties of the steel strip are as follows: tensile strength is 280-420 MPa, yield strength is ≥140 MPa, and elongation is ≥38.0%.

[0009] In a specific embodiment of the present invention, the chemical composition of the deposited metal of the flux-cored welding wire, by mass percentage, includes: C 0.040-0.080%, Mn 0.8-1.4%, Si≤0.02%, S≤0.010%, P≤0.012%, Cr≤0.15%, Ni≤1.8-2.2%, Mo 0.15-0.30%, N<0.018%, and the balance is iron.

[0010] In a specific embodiment of the present invention, the mechanical properties of the deposited metal of the flux-cored welding wire are as follows: tensile strength is 630-645 MPa, yield strength is 500-570 MPa, elongation is 22-23.0%, and impact energy at -40°C is ≥80J.

[0011] In a specific embodiment of the present invention, the tensile strength of the girth weld joint of the flux-cored welding wire is not less than 625MPa, the displacement of the girth weld crack tip of the flux-cored welding wire is ≥0.254mm, and the single value of the -20°C Charpy impact toughness of the girth weld joint of the flux-cored welding wire is not less than 38J, and the average value is not less than 50J.

[0012] The second object of the present invention is to provide a method for preparing a basic flux-cored welding wire for oil and gas pipeline welding, comprising:

[0013] The steel strip is rolled to obtain a steel strip with a "U"-shaped groove;

[0014] The powder is filled into a steel strip with a "U"-shaped groove, and then drawn, fine-drawn and rolled to obtain flux-cored welding wire.

[0015] In a specific embodiment of the present invention, the oil content on the surface of the steel strip is ≤0.5g / 10kg.

[0016] In a specific embodiment of the present invention, the drawing line speed in the fine drawing is ≤1500m / min, and the oil content range is: 0.2-1.0g / 10kg.

[0017] In a specific embodiment of the present invention, the forming speed of the flux-cored welding wire is 100±50 m / min.

[0018] Beneficial effects of the present invention:

[0019] The present invention provides an alkaline flux-cored welding wire for oil and gas pipeline welding and a preparation method thereof. By controlling the composition of the powder of the flux-cored welding wire, the composition system of the powder is determined to be BF-Fe 2 O 3 -Al 2 O 3 -NiO-MgO-MnO is composed of barium fluoride slag system, supplemented by strong deoxidizers such as alumina and magnesium oxide. Under the protection of carbon dioxide, the slag system has stable arc, small spatter, strong porosity resistance and good fusibility. Not only the mechanical properties of the girth weld joint welded by the obtained welding wire, the mechanical properties of the deposited metal, and the diffusible hydrogen content requirements meet the requirements of GB / T 36233-2018, but also the mechanical properties indicators of the girth weld joint welded by the obtained welding wire meet DEC-NGP-G-WD-002-2020-1, meet the high toughness requirements of weld fracture toughness CTOD 0.254mm, and meet the large-scale welding materials of high-grade oil and gas pipeline girth welds.

[0020] Other features and advantages of the present invention will be described in the following description, and partly become apparent from the description, or understood by practicing the present invention. The purpose and other advantages of the present invention can be realized and obtained by the structures pointed out in the description, claims and drawings. BRIEF DESCRIPTION OF THE DRAWINGS

[0021] In order to more clearly illustrate the embodiments of the present invention or the technical solutions in the prior art, the following briefly introduces the drawings required for use in the embodiments or the description of the prior art. Obviously, the drawings described below are some embodiments of the present invention. For ordinary technicians in this field, other drawings can be obtained based on these drawings without paying creative work.

[0022] Figure 1 A welding scene diagram of a flux-cored welding wire according to an embodiment of the present invention in a Weiyuan-Leshan gas pipeline project is shown;

[0023] Figure 2 A macroscopic photograph of a girth weld after flux-cored wire dynamic welding according to an embodiment of the present invention is shown;

[0024] Figure 3A schematic diagram of sampling for testing mechanical properties of a weld after flux-cored wire dynamic welding according to an embodiment of the present invention is shown;

[0025] Figure 4 The stress-strain curves of the T0-WM sample and the T6-WM sample in the tensile test after dynamic welding of the flux-cored wire according to the embodiment of the present invention are shown;

[0026] Figure 5 A schematic diagram of the position of the indentation in a Vickers hardness test after dynamic welding with a flux-cored wire according to an embodiment of the present invention is shown;

[0027] Figure 6 The macroscopic morphology of M0 (flat welding position weld) in the metallographic analysis after the flux-cored wire dynamic welding according to the embodiment of the present invention is shown;

[0028] Figure 7 The macroscopic morphology of M3 (vertical welding position weld) in the metallographic analysis after dynamic welding of flux-cored wire according to an embodiment of the present invention is shown;

[0029] Figure 8 The macroscopic morphology of M6 (overhead welding position weld) in the metallographic analysis after dynamic welding of flux-cored wire according to an embodiment of the present invention is shown;

[0030] Fig. 9 The test curve and morphology of the flat welding position weld (D0-WM) in the fracture toughness test after dynamic welding of the flux-cored wire according to the embodiment of the present invention are shown;

[0031] Fig.10 The test curve and morphology of the heat affected zone (D0-HAZ) at the flat welding position in the fracture toughness test after dynamic welding of the flux-cored welding wire according to the embodiment of the present invention are shown;

[0032] Fig.11 The test curve and morphology of the neutral position weld (D3-WM) of the flux-cored wire dynamic welding fracture toughness test according to the embodiment of the present invention are shown.

[0033] Fig.12 The test curve and morphology of the heat affected zone (D3-HAZ) at the neutral welding position in the fracture toughness test of the flux-cored welding wire after dynamic welding according to an embodiment of the present invention are shown;

[0034] Fig.13 The test curve and morphology of the overhead welding position weld (D6-WM) in the fracture toughness test after dynamic welding of the flux-cored wire according to the embodiment of the present invention are shown;

[0035] Fig.14 The test curve and morphology diagram of the heat affected zone (D6-HAZ) at the overhead welding position in the fracture toughness test after dynamic welding of the flux-cored wire according to the embodiment of the present invention are shown. DETAILED DESCRIPTION

[0036] In order to make the purpose, technical solution and advantages of the embodiments of the present invention clearer, the technical solution in the embodiments of the present invention will be clearly and completely described below in conjunction with the drawings in the embodiments of the present invention. Obviously, the described embodiments are part of the embodiments of the present invention, not all of the embodiments. Based on the embodiments of the present invention, all other embodiments obtained by ordinary technicians in this field without creative work are within the scope of protection of the present invention.

[0037] A method for preparing a basic flux-cored welding wire for oil and gas pipeline welding according to an embodiment of the present invention comprises:

[0038] Step 1: rolling the steel strip to obtain a steel strip with a "U"-shaped groove, wherein the "U"-shaped groove is located in the middle of the cross section of the steel strip;

[0039] Step 2: Fill the powder into the steel strip with a "U"-shaped groove, and then draw, fine-draw and roll it to obtain the flux-cored welding wire.

[0040] In step 1, the oil content on the surface of the steel strip is ≤0.5g / 10kg. In some embodiments of the present invention, the steel strip is manufactured by using SPCC cold-rolled bright steel strip, and a layer of protective oil is coated on the surface of the steel strip, and the oil content is ≤800mg / (surface×m 2 ), which exceeds the requirements for producing flux-cored welding wire. Before production, the steel strip produced by SPCC cold-rolled bright steel strip needs to be surface treated to remove surface grease. The oil stains on the surface of the steel strip should be cleaned with detergent and water. The oil content after cleaning should be controlled at ≤0.5g / 10kg.

[0041] In step 1, the alloy composition of the steel strip, by mass percentage, includes: C 0.020-0.060%, Mn 0.15-0.35%, Si≤0.03%, S≤0.015%, P≤0.015%, Al≤0.05%, N≤0.005%, B≤0.001%, and the balance is iron;

[0042] The mechanical properties of the steel strip are as follows: tensile strength of 280-420 MPa, yield strength ≥140 MPa, elongation ≥38.0%, and hardness (HRB) ≥40.

[0043] In step 2, the composition system of the powder is BF-Fe 2 O 3 -Al 2 O 3 -NiO-MgO-MnO, which is mainly composed of barium fluoride slag, supplemented by strong deoxidizers such as alumina and magnesium oxide. Under the protection of carbon dioxide, the slag has stable arc, small spatter, strong anti-porosity and good fusion. The components of the powder, by mass percentage, include: BaF ≥ 15%, SiO 2≤2%, CaO0.12~0.35%, Mg 6~7%, MnO 5~10%, TiO 2 0.03~0.05%, Ni 10~15%, K 2 O 0.03~0.05%、Na 2 O0.2~0.4%、Al 2 O 3 ≤25%、ZrO 2 <0.010%, CeO 2 0.8~1%、P 2 O 5 <0.010%、Bi 2 O 3 <0.010%、La 2 O 3 0.081%、Nb 2 O 5 <0.010, the balance is Fe 2 O 3 ;

[0044] The target value of the filling ratio of the medicine powder is 16-20%, and the floating range of the target value is ±0.6%. The calculation formula of the filling ratio is as follows:

[0045]

[0046] In step 2, during the drawing process, the deviation range of the welding wire diameter is standard wire diameter -0.03 to -0.01 mm, wherein the standard wire diameter is set according to the actual application of thick-walled, high-grade pipeline steel pipes with different chemical compositions. Generally, the core diameter of automatic welding is 1.2 mm, and the core diameter of semi-automatic welding is 1.6 mm.

[0047] In step 2, the fine drawing is achieved by drawing with a wire drawing die on the production line. Each deformation drawing of the cross-sectional diameter size passes through a wire drawing die hole of a fixed shape and size, and finally reaches the required diameter size of the finished product, so as to achieve strict control of the wire diameter, the wire drawing speed is ≤1500m / min, and the oil content range is: 0.2~1.0g / 10kg.

[0048] In step 2, the steel strip specifications are different, the production speed is different, and the forming speed of the flux-cored welding wire is 100±50m / min.

[0049] In the embodiment of the present invention, the chemical composition of the deposited metal of the flux-cored welding wire, by mass percentage, includes: C 0.040-0.080%, the C content is less than 0.040%, and the strength requirements of the deposit and process joint cannot be met at a low Mn content. If the C content is greater than 0.080%, the tensile strength of the deposited metal is likely to be higher than 760 MPa, which affects the elongation after fracture, impact absorption capacity or CTOD requirements of the deposited metal and the process joint;

[0050] Mn 0.8~1.4%, Si≤0.02%, Mo 0.15~0.30%, Ni≤1.8-2.2%, increase C content and reduce Mn content, ensure the tensile strength of deposited metal and girth welding performance, this combination can obtain good impact performance, elongation and CTOD value;

[0051] S≤0.010%, P≤0.012%. High P and S content will reduce the low-temperature impact value. Reduce the S and P content as much as possible to improve the low-temperature impact performance, reduce the ductile-brittle transition temperature, increase the CTOD value, and reduce the diffusible hydrogen content.

[0052] Cr≤0.15%, N<0.018%;

[0053] Trace amounts of V, trace amounts of Cu, trace amounts of Nb, and the balance is iron.

[0054] In the embodiment of the present invention, the mechanical properties of the deposited metal of the flux-cored welding wire are as follows: tensile strength is 630-645 MPa, yield strength is 500-570 MPa, elongation is 22-23.0%, and impact energy at -40°C is ≥80J.

[0055] In an embodiment of the present invention, the tensile strength of the girth weld joint of the flux-cored welding wire is not less than 625MPa, the displacement of the girth weld crack tip of the flux-cored welding wire is ≥0.254mm, the single value of the -20°C Charpy impact toughness of the girth weld joint of the flux-cored welding wire is not less than 38J, and the average value is not less than 50J; the hardness value of the weld metal and the heat-affected zone of the girth weld joint of the flux-cored welding wire is not greater than 300HV10 for root welding, and not greater than 325HV10 for the rest.

[0056] In a preferred embodiment of the present invention, the powder composition of the flux-cored welding wire is shown in Table 1, the alloy composition and mechanical properties of the steel strip are shown in Table 2, and the remaining process parameters of the preparation process are controlled according to the above method to obtain the flux-cored welding wire.

[0057] The obtained flux-cored welding wire (sample number 1) was subjected to a deposited metal performance test, and the chemical composition of the deposited metal of the flux-cored welding wire was measured as shown in Table 3, and the yield strength, tensile strength and -40°C impact performance tests were performed, and the results are shown in Table 4;

[0058] The welding wires that passed the deposited metal test were subjected to on-site combined automatic welding test. The welding processability is shown in Table 5. The on-site welding construction photos are shown in Figure 1 , the macroscopic photo of the girth weld after welding is shown in Figure 2 After welding, nondestructive testing is performed, and mechanical properties testing is performed after nondestructive testing. The sampling diagram of the mechanical properties test of the weld is shown in Figure 3 ;

[0059] Mechanical properties tests are:

[0060] The tensile test is based on GB / T 228.1-2021. The test results are shown in Table 6 and Figure 4 As shown;

[0061] The Charpy impact test is based on GB / T 229-2020, and the test results are shown in Table 7;

[0062] The notch hammer fracture test is based on the standard GB / T 31032-2014, and the test results are shown in Table 8;

[0063] The guide bending test is based on the standard GB / T 2653-2008, and the test results are shown in Table 9;

[0064] The Vickers hardness test is based on the standard GB / T 4340.1-2009. The test results are shown in Table 10. The indentation position is as follows: Figure 5 As shown;

[0065] The metallographic analysis was based on GB / T 13298-2015, and the test results are shown in Table 11;

[0066] Fracture toughness (CTOD) test:

[0067] The yield strength and tensile strength of the sample material at the test temperature are:

[0068] Yield strength Rp0.2 = 759 MPa;

[0069] The tensile strength is Rm=834MPa, the Poisson's ratio is taken as 0.3 during the calculation process, and the elastic modulus is E=210GPa.

[0070] Take one specimen from each of the weld and heat-affected zone at the flat welding position, vertical welding position and overhead welding position of the weld, and the specimen numbers are D0-WM, D0-HAZ, D3-WM, D3-HAZ, D6-WM and D6-HAZ. The specimen is a three-point bending specimen with a nominal size of W = 36mm and B = 18mm. An integral knife-edge specimen is used, i.e. Z = 0. The prefabricated fatigue crack data is shown in Table 12.

[0071] The test was carried out according to the standard GB / T 21143-2014, with a loading speed of 1 mm / min. The COD gauge model for monitoring the crack mouth opening displacement was 3541-005M-100M-ST. The crack length was measured using a fractographic analyzer with a measurement accuracy of 0.001 mm. The crack size measurement results are shown in Table 13.

[0072] The CTOD test results are shown in Table 14, and the validity verification of the results is shown in Table 15.

[0073] Table 1 (mass percentage %)

[0074]

[0075] Table 2

[0076]

[0077]

[0078] Table 3 (mass percentage %)

[0079]

[0080] From the data in Table 3, it can be seen that the chemical composition of the deposited metal of the obtained flux-cored welding wire meets the requirements of GB / T36233, T624T5-1C1A-GN3M1, AWS A5.29 and E91T5-GC standards.

[0081] Table 4

[0082]

[0083] From Table 4, it can be seen that the mechanical properties of the deposited metal of the flux-cored welding wire obtained in the embodiment of the present invention meet the requirements of GB / T 36233-2018 "High-strength Steel Flux-cored Welding Wire".

[0084] Table 5

[0085]

[0086] Table 6

[0087]

[0088]

[0089] Note: T0-WM and T6-WM are full weld specimens, and T1, T2, T3 and T4 are welded joint specimens.

[0090] It can be seen from the data in Table 6 that the performance of the flux-cored welding wire provided in the embodiment of the present invention after girth welding meets the requirements of DEC-OGP-G-WD-002-2020-1. Figure 4 The stress-strain curves of T0-WM and T6-WM specimens are given in Figure 4 a is the stress-strain curve of T0-WM sample, Figure 4 (b) is the stress-strain curve of the T6-WM sample.

[0091] Table 7

[0092]

[0093]

[0094] It can be seen from the data in Table 7 that the performance of the flux-cored welding wire provided by the embodiment of the present invention after girth welding meets the requirements of XQ03T02-GI001#EPL-RP-0801.

[0095] Table 8

[0096]

[0097] It can be seen from the data in Table 8 that the performance of the flux-cored welding wire provided in the embodiment of the present invention after girth welding meets the requirements of DEC-OGP-G-WD-002-2020-1.

[0098] Table 9

[0099]

[0100]

[0101] It can be seen from the data in Table 9 that the performance of the flux-cored welding wire provided in the embodiment of the present invention after girth welding meets the requirements of DEC-OGP-G-WD-002-2020-1.

[0102] Table 10

[0103]

[0104] It can be seen from the data in Table 10 that the performance of the flux-cored welding wire after girth welding provided by the embodiment of the present invention meets the requirements of DEC-OGP-G-WD-002-2020-1.

[0105] Table 11

[0106]

[0107] It can be seen from the data in Table 11 that the performance of the flux-cored welding wire after girth welding provided by the embodiment of the present invention meets the requirements of DEC-OGP-G-WD-002-2020-1. Figure 6 The macroscopic morphology of the weld at the M0 flat welding position is given. Figure 7 The macroscopic morphology of the weld at the M3 vertical welding position is given. Figure 8 The macroscopic morphology of the weld in M6 overhead welding position is given.

[0108] Table 12

[0109]

[0110] Table 13

[0111]

[0112] Table 14

[0113]

[0114]

[0115] Note: (1) Calculated according to formula (19) of GB / T 21143-2014.

[0116] (2) Calculated according to formula (7) of ISO 15653-2018.

[0117] Table 15

[0118]

[0119] It can be seen from Tables 12-15 that the requirements for the CTOD test of the flux-cored welding wire provided in the embodiments of the present invention are met.

[0120] Fig. 9 The test curve and fracture morphology of sample D0-WM are given. Fig. 9 a is the corresponding test curve graph, Fig. 9 b is the corresponding fracture morphology;

[0121] Fig.10 The test curve and fracture morphology of the sample D0-HAZ are given. Fig.10 a is the corresponding test curve graph, Fig.10 b is the corresponding fracture morphology;

[0122] Fig.11 The test curve and fracture morphology of sample D3-WM are given. Fig.11 a is the corresponding test curve graph, Fig.11 b is the corresponding fracture morphology;

[0123] Fig.12 The test curve and fracture morphology of sample D3-HAZ are given. Fig.12 a is the corresponding test curve graph, Fig.12 b is the corresponding fracture morphology;

[0124] Fig.13 The test curve and fracture morphology of sample D6-WM are given. Fig.13 a is the corresponding test curve graph, Fig.13 b is the corresponding fracture morphology;

[0125] Fig.14 The test curve and fracture morphology of sample D6-HAZ are given. Fig.14 a is the corresponding test curve graph, Fig.14 Figure b is the corresponding fracture morphology.

[0126] Although the present invention has been described in detail with reference to the aforementioned embodiments, those skilled in the art should understand that they can still modify the technical solutions described in the aforementioned embodiments, or make equivalent substitutions for some of the technical features therein; and these modifications or substitutions do not make the essence of the corresponding technical solutions deviate from the spirit and scope of the technical solutions of the embodiments of the present invention.

Claims

1. A basic flux-cored welding wire for oil and gas pipeline welding, It is characterized in that It is made of steel strip and powder through steel strip rolling, powder filling, welding wire drawing and coiling; The powder ingredients, by mass percentage, include: BaF ≥ 15%, SiO 2 ≤2%, CaO0.12~0.35%, Mg 6~7%, MnO 5~10%, TiO 2 0.03~0.05%, Ni 10~15%, K 2 O0.03~0.05%、Na 2 O 0.2~0.4%、Al 2 O 3 ≤25%、ZrO 2 <0.010%, CeO 2 0.8~1%、P 2 O 5 <0.010%、Bi 2 O 3 <0.010%、La 2 O 3 0.081%、Nb 2 O 5 <0.010, the balance is Fe 2 O 3 .

2. The basic flux-cored welding wire for oil and gas pipeline welding according to claim 1, It is characterized in that The target value of the filling ratio of the medicine powder is 16-20%, and the floating range of the target value is ±0.6%.

3. The basic flux-cored welding wire for oil and gas pipeline welding according to claim 1, It is characterized in that The alloy composition of the steel strip includes, by mass percentage, C 0.020-0.060%, Mn 0.15-0.35%, Si≤0.03%, S≤0.015%, P≤0.015%, Al≤0.05%, N≤0.005%, B≤0.001%, and the balance is iron.

4. The basic flux-cored welding wire for oil and gas pipeline welding according to claim 1, It is characterized in that The mechanical properties of the steel strip are as follows: tensile strength is 280-420 MPa, yield strength is ≥140 MPa, and elongation is ≥38.0%.

5. The basic flux-cored welding wire for oil and gas pipeline welding according to claim 1, It is characterized in that The chemical composition of the deposited metal of the flux-cored welding wire, by mass percentage, includes: C 0.040-0.080%, Mn 0.8-1.4%, Si≤0.02%, S≤0.010%, P≤0.012%, Cr≤0.15%, Ni≤1.8-2.2%, Mo 0.15-0.30%, N<0.018%, and the balance is iron.

6. The basic flux-cored welding wire for oil and gas pipeline welding according to claim 1, It is characterized in that The mechanical properties of the deposited metal of the flux-cored welding wire are as follows: tensile strength is 630-645 MPa, yield strength is 500-570 MPa, elongation is 22-23.0%, and impact energy at -40°C is ≥80J.

7. A basic flux-cored welding wire for oil and gas pipeline welding according to any one of claims 1 to 6, It is characterized in that The tensile strength of the girth weld joint of the flux-cored welding wire is not less than 625MPa, the displacement of the girth weld crack tip of the flux-cored welding wire is ≥0.254mm, and the single value of the -20°C Charpy impact toughness of the girth weld joint of the flux-cored welding wire is not less than 38J, and the average value is not less than 50J.

8. A method for preparing a basic flux-cored welding wire for oil and gas pipeline welding, It is characterized in that The basic flux-cored welding wire for oil and gas pipeline welding according to any one of claims 1 to 7 is made, comprising: The steel strip is rolled to obtain a steel strip with a "U"-shaped groove; The powder is filled into a steel strip with a "U"-shaped groove, followed by drawing, fine drawing and coiling to obtain flux-cored welding wire.

9. A method for preparing a basic flux-cored welding wire for oil and gas pipeline welding according to claim 8, It is characterized in that The oil content on the surface of the steel strip is ≤0.5g / 10kg.

10. The method for preparing a basic flux-cored welding wire for oil and gas pipeline welding according to claim 8, It is characterized in that The drawing line speed in the fine drawing is ≤1500m / min, and the oil content range is: 0.2-1.0g / 10kg.

11. A method for preparing a basic flux-cored welding wire for oil and gas pipeline welding according to any one of claims 8 to 10, It is characterized in that The forming speed of the flux-cored welding wire is 100±50m / min.

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