Drug core wire and x70 pipeline steel welded joint

Abstract

The application provides a flux-cored wire and an X70 pipeline steel welded joint, the flux-cored wire comprises a welding wire sheath and a flux core, the flux core comprises the following components in percentage by mass: TiO2 28-34%, CaF2 10-14%, carbonate 12-16%, oxide 2-4%, Al 16-20%, Ni 7-11%, Mn 4-6%, Si 1-2%, P less than or equal to 0.025%, S less than or equal to 0.025%, and the balance is Fe; wherein the carbonate is composed of CaCO3 and MgCO3, and the oxide is composed of SiO2 and Al2O3. The application can improve the hydrogen embrittlement resistance of the X70 pipeline steel welded joint.

Description

Technical Field

[0001] This invention belongs to the field of welding materials technology, specifically relating to a flux-cored welding wire and an X70 pipeline steel welding joint. Background Technology

[0002] Pipeline steel serves as a crucial carrier for the transportation of natural gas, oil, and hydrogen, exhibiting excellent service capabilities under harsh conditions such as long distances and high pressures. However, existing pipeline steel welded joints suffer from issues such as uneven microstructure, insufficient toughness, and high sensitivity to hydrogen embrittlement, resulting in a significant risk of hydrogen-induced cracking (HIC). Summary of the Invention

[0003] In view of this, the present invention provides a flux-cored welding wire and an X70 pipeline steel welding joint, which can improve the hydrogen embrittlement resistance of the pipeline steel welding joint.

[0004] To achieve the above objectives, the present invention adopts the following technical solution: In a first aspect, the present invention provides a flux-cored welding wire, comprising a wire sheath and a flux core, wherein the flux core comprises the following components in mass percentage: TiO2 28~34%, CaF2 10~14%, carbonate 12~16%, oxide 2~4%, Al 16~20%, Ni 7~11%, Mn 4~6%, Si 1~2%, P≤0.025%, S≤0.025%, with the balance being Fe; wherein the carbonate is composed of CaCO3 and MgCO3, and the oxide is composed of SiO2 and Al2O3.

[0005] Preferably, the outer sheath of the welding wire is made of low-carbon steel strip, which comprises the following components by mass percentage: C 0.03~0.08%, Si≤0.03%, Mn 0.25~0.50%, P≤0.020%, S≤0.020%, with the balance being Fe and unavoidable impurities.

[0006] Preferably, the flux-cored wire contains 22-28% flux by mass.

[0007] Secondly, the present invention also provides an X70 pipeline steel welded joint, which is obtained by welding X70 pipeline steel using the flux-cored welding wire described in the first aspect.

[0008] Preferably, the X70 pipeline steel comprises the following components by mass percentage: C 0.030~0.045%, Si 0.2~0.3%, Mn 1.0~1.2%, V 0.03~0.05%, Ni 0.07~0.15%, Mo 0.1~0.2%, Cu≤0.35%, Cr 0.2~0.3%, P≤0.01%, S≤0.003%, Nb 0.03~0.07%, Ca≤0.005%, with the balance being Fe and unavoidable impurities.

[0009] Preferably, the X70 pipeline steel comprises the following components by mass percentage: C 0.030~0.045%, Si 0.2~0.3%, Mn 1.0~1.2%, V 0.03~0.05%, Ni 0.07~0.15%, Mo 0.1~0.2%, Cu≤0.35%, Cr 0.2~0.3%, P≤0.01%, S≤0.003%, Nb 0.03~0.07%, Ca≤0.005%, Ti 0.010~0.018%, Mg 0.002%, with the balance being Fe and unavoidable impurities.

[0010] Preferably, the X70 pipeline steel comprises the following components by mass percentage: C 0.030~0.045%, Si 0.2~0.3%, Mn 1.0~1.2%, V 0.03~0.05%, Ni 0.07~0.15%, Mo 0.1~0.2%, Cu≤0.35%, Cr 0.2~0.3%, P≤0.01%, S≤0.003%, Nb 0.03~0.07%, Ca≤0.005%, Ti 0.010~0.018%, Mg 0.003%; the balance being Fe and unavoidable impurities.

[0011] Preferably, the X70 pipeline steel comprises the following components by mass percentage: C 0.030~0.045%, Si 0.2~0.3%, Mn 1.0~1.2%, V 0.03~0.05%, Ni 0.07~0.15%, Mo 0.1~0.2%, Cu≤0.35%, Cr 0.2~0.3%, P≤0.01%, S≤0.003%, Nb 0.03~0.07%, Ca≤0.005%, Ti 0.010~0.018%, Mg 0.004%, with the balance being Fe and unavoidable impurities.

[0012] Preferably, the welding is performed using a self-shielded flux-cored wire semi-automatic welding method.

[0013] Preferably, the welding voltage is 18~25 V, the welding current is 120~220 A; the joint of the X70 pipeline steel to be welded is processed into a single V-shaped bevel with a bevel angle of 60±5°; the angle between the welding torch and the tangent direction of the X70 pipeline steel is 10~15°; the welding speed is 150~200 mm / min; and the wire extension is 15~20 mm.

[0014] Compared with the prior art, the beneficial effects of the present invention are as follows: In this invention, by adding Mg and Al elements to the flux-cored welding wire, a synergistic effect of deoxidation, inclusion control, and grain refinement is achieved, taking into account the purity of the steel, harmlessness of inclusions, grain refinement, and crack resistance. This can promote the acquisition of excellent microstructure and inclusions in pipeline steel welding, thereby improving the hydrogen embrittlement resistance of pipeline steel welded joints. Attached Figure Description

[0015] Figure 1 This is an appearance diagram of the flux-cored welding wire provided by the present invention; Figure 2 Microstructure diagram of the X70 pipeline steel welded joint provided in Embodiment 1 of the present invention; Figure 3 This is a SEM image of the inclusions in the X70 pipeline steel welded joint provided in Embodiment 1 of the present invention. Figure 4 Microstructure diagram of the X70 pipeline steel welded joint provided in Embodiment 2 of the present invention; Figure 5 This is a SEM image of the inclusions in the X70 pipeline steel welded joint provided in Embodiment 2 of the present invention. Figure 6 Microstructure diagram of the X70 pipeline steel welded joint provided in Embodiment 3 of the present invention; Figure 7 This is a SEM image of the inclusions in the X70 pipeline steel welded joint provided in Embodiment 3 of the present invention. Figure 8 Microstructure diagram of the X70 pipeline steel welded joint provided in Embodiment 4 of the present invention; Figure 9 This is a SEM image of the inclusions in the X70 pipeline steel welded joint provided in Embodiment 4 of the present invention. Figure 10 Microstructure diagram of the X70 pipeline steel welded joint provided in Comparative Example 1 of the present invention; Figure 11 This is a SEM image of the inclusions in the welded joint of X70 pipeline steel provided in Comparative Example 1 of the present invention. Detailed Implementation

[0016] The present invention will now be described in further detail with reference to specific embodiments, so that those skilled in the art can more clearly understand the present invention.

[0017] In a first aspect, the present invention provides a flux-cored welding wire (see appearance drawing). Figure 1 The welding wire comprises a sheath and a flux core, wherein the flux core comprises the following components by mass percentage: TiO2 28~34%, CaF2 10~14%, carbonate 12~16%, oxide 2~4%, Al 16~20%, Ni 7~11%, Mn 4~6%, Si 1~2%, P≤0.025%, S≤0.025%, with the balance being Fe; wherein the carbonate is composed of CaCO3 and MgCO3, and the oxide is composed of SiO2 and Al2O3.

[0018] In this invention, by adding Mg and Al elements to the flux-cored welding wire, a synergistic effect of deoxidation, inclusion control, and grain refinement is achieved, taking into account the purity of the steel, harmlessness of inclusions, grain refinement, and crack resistance. This can promote the acquisition of excellent microstructure and inclusions in pipeline steel welding, thereby improving the hydrogen embrittlement resistance of pipeline steel welded joints.

[0019] In some embodiments, the mass ratio of CaCO3 to MgCO3 in the carbonate is 2:1.

[0020] In some embodiments, the mass ratio of SiO2 to Al2O3 in the oxide is 3:1.

[0021] In some embodiments, the welding wire sheath is made of low-carbon steel strip, which comprises the following components by mass percentage: C 0.03~0.08%, Si≤0.03%, Mn 0.25~0.50%, P≤0.020%, S≤0.020%, with the balance being Fe and unavoidable impurities. It is understood that the flux core is filled within the welding wire sheath, which is made of low-carbon steel strip, and the low-carbon steel strip can be a dedicated low-carbon rimmed steel strip.

[0022] In some embodiments, the flux-cored wire comprises 22-28% of the mass of the flux core.

[0023] It should be noted that carbon (C) is a key element that is strictly controlled in flux-cored welding wire. It plays a role in improving the strength of weld metal (which is formed by the solidification of filler metal and partially melted base metal). Low carbon design can avoid the formation of too much cementite, ensure weld toughness, and prevent welding cold cracks.

[0024] Nickel (Ni) is a core performance-enhancing element in flux-cored welding wire. An appropriate amount of Ni can significantly lower the ductile-brittle transition temperature of steel, allowing the weld metal to maintain good impact toughness at -30 to -40°C, meeting the low-temperature service requirements of pipeline steel. Furthermore, Ni forms a substitutional solid solution with Fe, which helps improve the strength of the weld metal, while also reducing susceptibility to hydrogen-induced cracking.

[0025] Manganese (Mn) and silicon (Si) are important basic alloying elements. Mn has solid solution strengthening effect and can improve weld fluidity. In Mg microalloyed welding wire, MnS can be modified into fine spherical MgO-MnS or MgS composite inclusions. Si, as a deoxidizer, can improve weld strength, but excessive Si will lead to increased brittleness.

[0026] Sulfur (S) and phosphorus (P) are strictly controlled harmful elements. It is necessary to prevent S from forming FeS-Fe eutectic during welding, which leads to hot cracks and harmful MnS inclusions; and to prevent P from segregating at grain boundaries, which leads to cold brittleness and cold cracks.

[0027] Magnesium (Mg) and aluminum (Al), as multifunctional auxiliary elements, can combine with oxygen to form MgO-Al2O3 spinel composite inclusions, which improve the morphology of inclusions, refine the size of inclusions, and help the inclusions disperse and distribute, thus reducing the sensitivity to hydrogen-induced cracking.

[0028] Secondly, the present invention also provides a welded joint for X70 pipeline steel, which is obtained by welding X70 pipeline steel using the flux-cored welding wire described in the first aspect. It should be noted that the microstructure of the X70 pipeline steel can be quasi-polygonal ferrite (PF) and pearlite (P).

[0029] In some embodiments, the welded metal in the X70 pipeline steel welded joint comprises the following components by mass percentage: C≤0.12%, Si≤0.80%, Mn 1.50~1.75%, Mg≤1.60%, Al≤1.80%, Ni 0.80~1.20%, Mo≤0.35%, P≤0.030%, S≤0.030%, with the balance being Fe and unavoidable impurities.

[0030] In some embodiments, the X70 pipeline steel comprises the following components by mass percentage: C 0.030~0.045%, Si 0.2~0.3%, Mn 1.0~1.2%, V 0.03~0.05%, Ni 0.07~0.15%, Mo 0.1~0.2%, Cu≤0.35%, Cr 0.2~0.3%, P≤0.01%, S≤0.003%, Nb 0.03~0.07%, Ca≤0.005%, with the balance being Fe and unavoidable impurities. Using X70 pipeline steel with the above composition helps ensure the hydrogen embrittlement resistance of X70 pipeline steel welded joints.

[0031] In some embodiments, the X70 pipeline steel comprises the following components by mass percentage: C 0.030~0.045%, Si 0.2~0.3%, Mn 1.0~1.2%, V 0.03~0.05%, Ni 0.07~0.15%, Mo 0.1~0.2%, Cu≤0.35%, Cr 0.2~0.3%, P≤0.01%, S≤0.003%, Nb 0.03~0.07%, Ca≤0.005%, Ti 0.010~0.018%, Mg 0.002%, with the balance being Fe and unavoidable impurities. The Ti-Mg composite modified X70 pipeline steel with the above composition is beneficial for improving the hydrogen embrittlement resistance of X70 pipeline steel welded joints.

[0032] In some embodiments, the X70 pipeline steel comprises the following components by mass percentage: C 0.030~0.045%, Si 0.2~0.3%, Mn 1.0~1.2%, V 0.03~0.05%, Ni 0.07~0.15%, Mo 0.1~0.2%, Cu≤0.35%, Cr 0.2~0.3%, P≤0.01%, S≤0.003%, Nb 0.03~0.07%, Ca≤0.005%, Ti 0.010~0.018%, Mg 0.003%; the balance being Fe and unavoidable impurities. The Ti-Mg composite modified X70 pipeline steel with the above composition is beneficial for improving the hydrogen embrittlement resistance of X70 pipeline steel welded joints.

[0033] In some embodiments, the X70 pipeline steel comprises the following components by mass percentage: C 0.030~0.045%, Si 0.2~0.3%, Mn 1.0~1.2%, V 0.03~0.05%, Ni 0.07~0.15%, Mo 0.1~0.2%, Cu≤0.35%, Cr 0.2~0.3%, P≤0.01%, S≤0.003%, Nb 0.03~0.07%, Ca≤0.005%, Ti 0.01~0.018%, Mg 0.004%, with the balance being Fe and unavoidable impurities. The Ti-Mg composite modified X70 pipeline steel with the above composition is beneficial for improving the hydrogen embrittlement resistance of X70 pipeline steel welded joints.

[0034] In some embodiments, the smelting method of X70 pipeline steel includes the following steps: S1. The low S and P industrial pure iron rod test raw material is surface pretreated, and then the furnace is charged for each heat of steel (46~48 kg / heat), and the dried casting mold is placed in the furnace in advance. S2. Calculate the yield based on the alloy mass and composition ratio, and then weigh and batch the materials according to the calculation results, placing them in the furnace top feeder; S3. Use a vacuum pump to evacuate the vacuum induction furnace to a vacuum level of 2×10⁻⁶. -3 After Pa, the induction furnace is started for heating and melting; after the industrial pure iron is completely melted, alloy material is added into the furnace; S4. After the alloy material has been fully melted and mixed evenly, argon gas is introduced into the vacuum induction furnace, and then casting is carried out; after casting is completed, argon gas is introduced into the vacuum induction furnace to make the furnace pressure reach atmospheric pressure, then the induction furnace is opened and the mold is removed. S5. Once the steel ingot has completely solidified in the mold and cooled to below 600°C, demold it. S6. Heat the steel ingot to 1250℃, hold for 2.5 h, and then cool to 1100℃ to obtain the steel ingot to be rolled. S7. The steel ingot to be rolled is rolled, wherein the roughing rolling temperature is 1000~1100℃, the finishing rolling temperature is 940~960℃, and the finishing rolling temperature is 880~900℃; the slab reddening temperature is 420~450℃, and it is cooled in the furnace. S8. After rolling, the steel plate is sprayed with water for rapid cooling in the two-phase region at 700~850℃. When the steel plate cools to 600℃, the water spraying is stopped, and the plate is placed on a rack for air cooling to obtain X70 pipeline steel.

[0035] In some embodiments, the welding is performed using a self-shielded flux-cored wire semi-automatic welding method.

[0036] In some embodiments, the welding voltage is 18~25 V, the welding current is 120~220 A; the joint of the X70 pipeline steel to be welded is processed into a single V-shaped bevel with a bevel angle of 60±5°; the angle between the welding torch and the tangent direction of the X70 pipeline steel is 10~15°; the welding speed is 150~200 mm / min; and the wire extension is 15~20 mm.

[0037] In some embodiments, the thickness of the X70 pipeline steel to be welded is 20 mm and the dimensions are 110 × 250 mm.

[0038] In some embodiments, a small-amplitude oscillation is used to move the wire during the welding process.

[0039] In some embodiments, the X70 pipeline steel to be welded undergoes surface treatment before welding. Further, the surface treatment steps involve mechanically polishing the X70 pipeline steel to be welded with sandpaper, followed by cleaning the X70 pipeline steel to be welded with acetone. It is understood that mechanical polishing with sandpaper is to remove the oxide layer on the surface of the X70 pipeline steel, and cleaning with acetone is to remove organic impurities such as grease from the surface of the X70 pipeline steel.

[0040] Example 1 This embodiment provides a welded joint for X70 pipeline steel, which is manufactured by welding X70 pipeline steel using flux-cored welding wire; wherein: Flux-cored welding wire consists of a wire sheath made of low-carbon steel strip and a flux core filled within the sheath. The wire sheath comprises the following components by mass percentage: C: 0.05%, Si: 0.0024%, Mn: 0.27%, P: 0.0011%, S: 0.008%, with the balance being Fe and unavoidable impurities. The flux core comprises the following components by mass percentage: TiO2: 30%, CaF2: 10%, carbonates: 13%, oxides: 2%, Al: 20%, Ni: 7%, Mn: 4%, Si: 1%, P: 0.019%, S: 0.02%, with the balance being Fe. The carbonates consist of CaCO3 and MgCO3 in a mass ratio of 2:1, and the oxides consist of SiO2 and Al2O3 in a mass ratio of 3:1. The flux core accounts for 24% of the total mass of the flux-cored welding wire. X70 pipeline steel is composed of the following components by weight percentage: C: 0.045%, Si: 0.243%, Mn: 1.150%, V: 0.039%, Ni: 0.128%, Mo: 0.121%, Cu: 0.092%, Cr: 0.249%, P: 0.004%, S: 0.002%, Nb: 0.038%; the balance being Fe and unavoidable impurities. Before welding, the X70 pipeline steel is surface treated. The thickness of the X70 pipeline steel is 20mm and the size is 110×250mm. The welding method is semi-automatic arc welding with self-shielded flux-cored wire. The welding process and parameters are as follows: the wire is moved with slight oscillation during the welding process, the welding voltage is 18~25 V, the welding current is 120~220 A, the plate to be welded is a V-groove with a groove angle of 60±5°, the angle between the welding torch and the tangent of the pipe is 10~15°, the welding speed is 150~200 mm / min, and the extension length is 15~20 mm.

[0041] Example 2 This embodiment provides a welded joint for X70 pipeline steel, which is basically the same as that in Embodiment 1, except that the X70 pipeline steel is composed of the following components by mass percentage: C: 0.040%, Si: 0.253%, Mn: 1.110%, V: 0.039%, Ni: 0.097%, Mo: 0.121%, Cu: 0.070%, Cr: 0.247%, P: 0.004%, S: 0.003%, Nb: 0.039%, Ti: 0.01%, Mg: 0.002%; the balance being Fe and unavoidable impurities.

[0042] Example 3 This embodiment provides a welded joint for X70 pipeline steel, which is basically the same as that in Embodiment 1, except that the X70 pipeline steel is composed of the following components by mass percentage: C: 0.045%, Si: 0.257%, Mn: 1.121%, V: 0.039%, Ni: 0.124%, Mo: 0.121%, Cu: 0.075%, Cr: 0.244%, P: 0.004%, S: 0.002%, Nb: 0.041%, Ti: 0.01%, Mg: 0.003%; the balance being Fe and unavoidable impurities.

[0043] Example 4 This embodiment provides a welded joint for X70 pipeline steel, which is basically the same as that in Embodiment 1, except that the X70 pipeline steel is composed of the following components by mass percentage: C: 0.040%, Si: 0.251%, Mn: 1.110%, V: 0.039%, Ni: 0.128%, Mo: 0.121%, Cu: 0.092%, Cr: 0.249%, P: 0.004%, S: 0.002%, Nb: 0.038%, Ti: 0.01%, Mg: 0.004%; the balance being Fe and unavoidable impurities.

[0044] Comparative Example 1 This comparative example provides an X70 pipeline steel welded joint, which is basically the same as Example 1, except that the welding wire is Tianjin Jinqiao ER80S-G. X70 pipeline steel is composed of the following components by weight percentage: C: 0.040%, Si: 0.250%, Mn: 1.120%, V: 0.039%, Ni: 0.097%, Mo: 0.121%, Cu: 0.070%, Cr: 0.247%, P: 0.004%, S: 0.003%, Nb: 0.039%; the balance being Fe and unavoidable impurities.

[0045] Performance Tests and Results The microstructure of the X70 pipeline steel welded joints of Examples 1-4 and Comparative Example 1 was observed using a metallographic microscope, and the inclusion morphology of the X70 pipeline steel welded joints of Examples 1-4 and Comparative Example 1 was observed using a scanning electron microscope. The results are shown in the figure. Figures 2-11 .

[0046] Depend on Figure 2 It can be seen that the weld microstructure of the X70 pipeline steel welded joint in Example 1 is granular bainite (GB) + acicular ferrite (AF) + quasi-polygonal ferrite (PF), and the heat-affected zone microstructure is quasi-polygonal ferrite (PF); Figure 3It can be seen that the inclusions in the base material are long and irregular in shape. After welding, the inclusions in the weld zone and heat-affected zone are nearly spherical and diffusely distributed small inclusions.

[0047] Depend on Figure 4 It can be seen that the weld microstructure of the X70 pipeline steel welded joint in Example 2 is granular bainite (GB) + acicular ferrite (AF), and the heat-affected zone microstructure is quasi-polygonal ferrite (PF); Figure 5 It can be seen that the inclusions in the three regions are all nearly spherical, but the welds are finer and more dispersed.

[0048] Depend on Figure 6 It can be seen that the weld microstructure of the X70 pipeline steel welded joint in Example 3 is granular bainite (GB) + acicular ferrite (AF), and the microstructure of the heat-affected zone is quasi-polygonal ferrite (PF); Figure 7 It can be seen that the inclusions in the three regions are all nearly spherical, but the inclusions in the parent material region are coarser than those in the others.

[0049] Depend on Figure 8 It can be seen that the weld microstructure of the X70 pipeline steel welded joint in Example 4 is granular bainite (GB) + lath bainite (LB), and the heat-affected zone microstructure is quasi-polygonal ferrite (PF); Figure 9 It can be seen that although the inclusions in the three regions are all nearly spherical, their number density is relatively low.

[0050] Depend on Figure 10 It can be seen that the weld microstructure of the X70 pipeline steel welded joint in Comparative Example 1 is granular bainite (GB) + quasi-polygonal ferrite (PF), and the heat-affected zone microstructure is quasi-polygonal ferrite (PF); Figure 11 It can be seen that the inclusions in the base material area are long, irregular, and relatively large, while the inclusions in the weld and heat-affected zone are nearly spherical and relatively larger, with poor dispersion effect.

[0051] Electrochemical hydrogen permeation experiments were conducted on the X70 pipeline steel welded joints of Examples 1-4 and Comparative Example 1 to calculate the effective hydrogen diffusion coefficient (D). eff ), hydrogen diffusion flux (J ∞ ), Cathode hydrogen concentration measurement (C o ), total hydrogen trap (N t The results are shown in Table 1.

[0052] Table 1. Electrochemical hydrogen permeation results of X70 pipeline steel welded joints in each embodiment and comparative example.

[0053] As shown in Table 1, compared to Comparative Example 1, the effective hydrogen diffusion coefficient, hydrogen diffusion flux, and cathode hydrogen concentration of the X70 pipeline steel welded joint in Example 1 are significantly reduced, while the total hydrogen trapping is significantly increased. The hydrogen diffusion behavior in both the weld and the heat-affected zone is significantly suppressed, indicating that the flux-cored wire provided by this invention can improve the hydrogen embrittlement resistance of the X70 pipeline steel welded joint. Compared to Example 1, the effective hydrogen diffusion coefficient, hydrogen diffusion flux, and cathode hydrogen concentration of the X70 pipeline steel welded joints in Examples 2-4 are significantly reduced, indicating that the flux-cored wire provided by this invention, in synergy with Ti-Mg composite modified X70 pipeline steel, is beneficial for further improving the hydrogen embrittlement resistance of the X70 pipeline steel welded joint.

[0054] Unless otherwise specified, all raw materials used in this invention are existing substances that can be purchased directly from the market.

[0055] The above are merely preferred embodiments of the present invention and are not intended to limit the scope of protection of the present invention. Any modifications, equivalent substitutions, improvements, etc., made within the spirit and principles of the present invention should be included within the scope of protection of the present invention.

Claims

1. A flux-cored welding wire, characterized in that, The welding wire comprises an outer sheath and a flux core, wherein the flux core comprises the following components by mass percentage: TiO2 28~34%, CaF2 10~14%, carbonate 12~16%, oxide 2~4%, Al 16~20%, Ni 7~11%, Mn 4~6%, Si 1~2%, P≤0.025%, S≤0.025%, with the balance being Fe; wherein the carbonate is composed of CaCO3 and MgCO3, and the oxide is composed of SiO2 and Al2O3.

2. The flux-cored welding wire according to claim 1, characterized in that, The outer sheath of the welding wire is made of low-carbon steel strip, which comprises the following components by mass percentage: C 0.03~0.08%, Si≤0.03%, Mn 0.25~0.50%, P≤0.020%, S≤0.020%, with the balance being Fe and unavoidable impurities.

3. The flux-cored welding wire according to claim 1, characterized in that, In the flux-cored welding wire, the mass percentage of the flux core is 22-28%.

4. A welded joint for X70 pipeline steel, characterized in that, X70 pipeline steel is obtained by welding with the flux-cored welding wire as described in any one of claims 1 to 3.

5. The X70 pipeline steel welded joint according to claim 4, characterized in that, The X70 pipeline steel comprises the following components by mass percentage: C 0.030~0.045%, Si 0.2~0.3%, Mn 1.0~1.2%, V 0.03~0.05%, Ni 0.07~0.15%, Mo 0.1~0.2%, Cu≤0.35%, Cr 0.2~0.3%, P≤0.01%, S≤0.003%, Nb 0.03~0.07%, Ca≤0.005%, with the balance being Fe and unavoidable impurities.

6. The X70 pipeline steel welded joint according to claim 4, characterized in that, The X70 pipeline steel comprises the following components by mass percentage: C 0.030~0.045%, Si 0.2~0.3%, Mn 1.0~1.2%, V 0.03~0.05%, Ni 0.07~0.15%, Mo 0.1~0.2%, Cu≤0.35%, Cr 0.2~0.3%, P≤0.01%, S≤0.003%, Nb 0.03~0.07%, Ca≤0.005%, Ti 0.010~0.018%, Mg 0.002%, with the balance being Fe and unavoidable impurities.

7. The X70 pipeline steel welded joint according to claim 4, characterized in that, The X70 pipeline steel comprises the following components by mass percentage: C 0.030~0.045%, Si 0.2~0.3%, Mn 1.0~1.2%, V 0.03~0.05%, Ni 0.07~0.15%, Mo 0.1~0.2%, Cu≤0.35%, Cr 0.2~0.3%, P≤0.01%, S≤0.003%, Nb 0.03~0.07%, Ca≤0.005%, Ti 0.010~0.018%, Mg 0.003%; the balance being Fe and unavoidable impurities.

8. The X70 pipeline steel welded joint according to claim 4, characterized in that, The X70 pipeline steel comprises the following components by mass percentage: C 0.030~0.045%, Si 0.2~0.3%, Mn 1.0~1.2%, V 0.03~0.05%, Ni 0.07~0.15%, Mo 0.1~0.2%, Cu≤0.35%, Cr 0.2~0.3%, P≤0.01%, S≤0.003%, Nb 0.03~0.07%, Ca≤0.005%, Ti 0.010~0.018%, Mg 0.004%, with the balance being Fe and unavoidable impurities.

9. The X70 pipeline steel welded joint according to claim 4, characterized in that, The welding is performed using a self-shielded flux-cored wire semi-automatic welding method.

10. The X70 pipeline steel welded joint according to claim 9, characterized in that, The welding voltage is 18~25 V, and the welding current is 120~220 A; the joint of the X70 pipeline steel to be welded is processed into a single V-shaped bevel with a bevel angle of 60±5°; the angle between the welding torch and the tangent of the X70 pipeline steel is 10~15°; the welding speed is 150~200 mm / min; and the wire extension is 15~20 mm.

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