JP7789486B2 - Flux-cored wire for self-shielded arc welding - Google Patents

Description

The present invention relates to a flux-cored wire for self-shielded arc welding of 590 MPa-class high-tensile steel, which produces weld metal with high strength and excellent toughness, does not produce welding defects such as pits or blowholes, and provides good welding workability.

Flux-cored wire for self-shielded arc welding is a welding method in which a gas generating agent such as a metal fluoride or metal carbonate is added to the welding wire. During welding, the metal fluoride and metal carbonate are decomposed by the heat of the arc to generate a shielding gas that covers the molten pool. This allows welding to be performed without using a shielding gas and while the atmosphere is shut off. This welding method does not require equipment such as cylinders or piping to deliver the shielding gas, so the welding equipment has a simple structure and is easy to carry, and is widely used in civil engineering and construction fields. For example, Patent Document 1 discloses a flux-cored wire for self-shielded arc welding that is capable of all-position welding.

However, compared to conventional gas-shielded arc welding, self-shielded arc welding has problems such as an unstable arc, poor welding workability, a tendency to develop porosity defects such as pits and blowholes, and poor mechanical properties of the weld metal.

As a means of solving this problem, Patent Document 2 discloses a flux-cored wire for self-shielded arc welding that improves shielding resistance and prevents porosity defects such as pits and blowholes by adding appropriate amounts of Al, which fixes N in the molten metal and improves porosity resistance, Mg, which shields the weld, metal fluorides, and metal carbonates to the welding wire, while also providing good welding workability, such as stabilizing the arc.

Patent Document 3 also discloses a flux-cored wire for self-shield arc welding that prevents the occurrence of porosity defects such as pits and blowholes by adding Al, while simultaneously adding Al, Mg, and Ba to facilitate arc stabilization, resulting in good welding workability, and adding appropriate amounts of Mn and Ni to improve the toughness of the weld metal.

The flux-cored wires for self-shielded arc welding disclosed in Patent Documents 2 and 3 can suppress porosity defects such as pits and blowholes, stabilize the arc, and provide good welding workability. However, because they contain a large amount of Al, the microstructure of the weld metal tends to coarsen, making it difficult to obtain weld metal with excellent mechanical properties, and in particular, it is difficult to obtain sufficient weld metal toughness.

In recent years, the strength of welded structures has also increased in the civil engineering and construction fields, and high-strength steels such as 590 MPa-class steels are widely used. When welding these high-strength steels, the weld metal also needs to be quite strong. Therefore, elements that improve the strength of the weld metal, such as Mo, are sometimes added to the welding wire used in self-shielded arc welding. However, when Mo or other elements are present in the weld metal, it becomes even more difficult to obtain the toughness of the weld metal.

As a means for solving these problems, Patent Document 4 discloses a flux-cored wire for self-shielded arc welding that improves welding workability by adding appropriate amounts of Al, _BaF2 , and Sr composite oxide to stabilize the arc and thereby suppressing porosity defects such as pits and blowholes, and that improves the strength and toughness of the weld metal by adding appropriate amounts of C, Mn, Ni, and Mo. However, although the flux-cored wire for self-shielded arc welding disclosed in Patent Document 4 achieves good welding workability by stabilizing the arc and suppressing porosity defects such as pits and blowholes, it has a problem in that the strength of the weld metal becomes excessively high due to the increased amounts of C, Mn, Ni, and Mo added.

Japanese Unexamined Patent Publication No. 62-238097 Japanese Unexamined Patent Publication No. 3-118993 Japanese Patent Application Laid-Open No. 2000-301382 JP 2009-119497 A

The present invention was devised in light of the above-mentioned problems, and aims to provide a flux-cored wire for self-shielded arc welding of 590 MPa-class high-tensile steel, which produces weld metal with high strength and excellent toughness, suppresses porosity defects such as pits and blowholes, and provides good welding workability.

The gist of the present invention is a flux-cored wire for self-shield arc welding, which is made by filling a steel sheath with flux, and which contains, in mass % relative to the total mass of the wire, the steel sheath and flux together containing 0.05 to 0.25% C, 0.5 to 1.5% Mn, 1.0 to 3.0% Al, and 0.8% or less Si; and further, in mass % relative to the total mass of the wire, the flux contains 1.0 to 3.0% Mg, 0.1 to 0.5% total Si oxides in terms of SiO ₂ , 1.3 to 5% total metal fluorides in terms of F (excluding 1.3), 0.5 to 1.5% total of one or more metal carbonates selected from calcium carbonate, barium carbonate, and lithium carbonate , 0.5 to 1.5% total Na oxides and K oxides in terms of Na ₂ O and K ₂ The steel sheet is characterized by containing one or two types of iron in total, calculated as O: 0.02 to 0.11%, with the remainder consisting of the Fe content of the steel outer shell, iron powder, the Fe content of the iron alloy powder, and impurities.

Furthermore, the total mass of the steel sheath and flux is 0.5% or less Mo, expressed as a percentage by mass of the total mass of the wire.

Furthermore, the flux-cored wire for self-shielded arc welding is characterized in that, in mass % relative to the total mass of the wire, the total Ni content of the steel sheath and flux is 1.0% or less.

By applying the flux-cored wire for self-shielded arc welding of the present invention, it is possible to obtain weld metal with a high strength of 590 MPa or more and excellent toughness, and it is possible to provide a flux-cored wire for self-shielded arc welding that is free of porosity defects such as pits and blowholes and has good welding workability.

FIG. 1 is a cross-sectional view of a flux-cored ear for self-shielded arc welding fabricated in an embodiment of the present invention.

In order to solve the above-mentioned problems, the inventors conducted detailed research into the component composition of a flux-cored wire for self-shielded arc welding that would produce a weld metal with a high strength of 590 MPa or more and excellent toughness, would not produce porosity defects such as pits or blowholes, would produce a stable arc, and would provide excellent welding workability.

As a result, it was found that adding an appropriate amount of Al to flux-cored wire for self-shielded arc welding can fix N in the molten metal as AlN, thereby suppressing the occurrence of porosity defects such as pits and blowholes. However, Al coarsens the microstructure of the weld metal, reducing its toughness. Therefore, after examining various methods for improving the strength of the weld metal while suppressing the coarsening of the microstructure, it was found that adding large amounts of gas-generating metal fluorides, metal carbonates, and Mg to flux-cored wire for self-shielded arc welding can improve shielding properties, reduce the amount of Al added, and suppress the coarsening of the microstructure. Furthermore, it was found that adding appropriate amounts of C and Mn can improve the strength and toughness of the weld metal. Furthermore, it was found that increasing the amount of Mn added to achieve the required toughness of the weld metal results in the weld metal becoming too strong.

Further investigations were conducted to find a way to suppress the occurrence of porosity defects such as pits and blowholes, achieve a strength of 590 MPa, and obtain weld metal with good toughness. As a result, it was discovered that by reducing the amount of metal carbonate added to suppress the amount of slag production, keeping the amount of Al added to a minimum while increasing the amount of Mg and metal fluoride added, high shielding properties are ensured and the occurrence of porosity defects such as pits and blowholes is suppressed, and by keeping the amount of Mn added low and adjusting the amount of C added, a weld metal with a strength of 590 MPa and good toughness can be obtained. Furthermore, it was discovered that by adding appropriate amounts of Mo and Ni, the strength of the weld metal can be improved without reducing its toughness.

In addition, with regard to other welding workability, we found that adding appropriate amounts of sodium oxide and potassium oxide is effective in stabilizing the arc.

Furthermore, we discovered that by adding an appropriate amount of silicon oxide, the viscosity of the molten slag can be adjusted, improving the slag encapsulation of the weld bead and the bead shape.

The flux-cored wire for self-shielded arc welding of the present invention is made possible by the synergistic effects of each component, both alone and in combination. The reasons for adding and limiting each component are described below. In the following, the chemical components of the flux-cored wire for self-shielded welding will be expressed in mass %, which is the ratio of the total mass of the wire, and descriptions relating to mass % will be simply written as %.

[C: 0.05 to 0.25% in total for steel sheath and flux]
C has the effect of improving the strength of the weld metal through solid solution strengthening. Furthermore, CO and _CO2 generated during welding block the atmosphere from the molten pool, improving shielding and suppressing the occurrence of porosity defects such as pits and blowholes. If the C content is less than 0.05%, this effect is not fully achieved, and the weld metal strength is not obtained. Furthermore, if the C content is less than 0.05%, sufficient shielding is not ensured, and porosity defects such as pits and blowholes are more likely to occur. On the other hand, if the C content exceeds 0.25%, the strength of the weld metal becomes excessively high and its toughness decreases. Therefore, the total C content of the steel sheath and flux is set to 0.05 to 0.25%. In addition to being a component contained in the steel sheath, C can also be added from the flux as metal powder, alloy powder, etc.

[Mn: 0.5 to 1.5% in total for steel sheath and flux]
Mn acts as a deoxidizer for the weld metal and has the effect of improving the strength and toughness of the weld metal. If the Mn content is less than 0.5%, this effect is not fully achieved, and the strength and toughness of the weld metal decrease. On the other hand, if the Mn content exceeds 1.5%, the strength of the weld metal becomes excessively high and the toughness decreases. Therefore, the total Mn content of the steel sheath and flux is set to 0.5 to 1.5%. In addition to being a component contained in the steel sheath, Mn can also be added from the flux as alloy powder such as metallic Mn, Fe-Mn, or Fe-Si-Mn.

[Al: 1.0 to 3.0% in total for steel sheath and flux]
Al acts as a deoxidizer to improve the toughness of the weld metal and also fixes N that penetrates into the molten metal as AlN, thereby suppressing the occurrence of porosity defects such as pits and blowholes. If the Al content is less than 1.0%, this effect is not fully achieved, and porosity defects such as pits and blowholes are more likely to occur. On the other hand, if the Al content exceeds 3.0%, excessive Al is retained in the weld metal, causing the microstructure to coarsen and reducing the toughness of the weld metal. Therefore, the total Al content of the steel sheath and flux is set to 1.0 to 3.0%. In addition to being a component contained in the steel sheath, Al can be added from the flux as alloy powder such as metallic Al, Fe-Al, or Al-Mg.

[Si: 0.8% or less in total for steel sheath and flux]
Since excessive Si remaining in the weld metal reduces the toughness of the weld metal, the total content of the steel sheath and flux must be 0.8% or less. In addition to being a component contained in the steel sheath, Si can also be added from the flux in the form of alloy powder such as metallic Si, Fe-Si, or Fe-Si-Mn.

[Mg in flux: 1.0 to 3.0%]
Mg acts as a deoxidizer, promoting deoxidation of the weld metal and suppressing the occurrence of porosity defects such as pits and blowholes, while also improving the toughness of the weld metal. If the Mg content is less than 1.0%, this effect is not fully achieved, and porosity defects such as pits and blowholes are more likely to occur, while the toughness of the weld metal decreases. On the other hand, if the Mg content exceeds 3.0%, the arc becomes excessively strong and unstable. Therefore, the total Mg content in the steel sheath and flux is set to 1.0 to 3.0%. Mg can be added to the flux as metallic Mg or alloy powder such as Al-Mg.

[Total _SiO2 equivalent value of Si oxides in flux: 0.1 to 0.5%]
Silicon oxides have the effect of adjusting the viscosity and melting point of molten slag and improving slag encapsulation. If the total amount of silicon oxides in terms of _SiO2 is less than 0.1%, this effect is not fully achieved, and slag encapsulation deteriorates. On the other hand, if the total amount of silicon oxides in terms of _SiO2 exceeds 0.5%, the basicity of the molten slag decreases, the oxygen content of the weld metal increases, and the toughness of the weld metal decreases. Therefore, the total amount of silicon oxides in terms of _SiO2 in the flux should be 0.1 to 0.5%. Silicon oxides can be added to the flux as silica sand, potash glass, fluorite, etc.

[Metal fluorides contained in flux: 1 to 5% in total in terms of F]
Metal fluorides act as gas generators and have the effect of improving the shielding of the molten pool from the atmosphere. If the total F-equivalent value of the metal fluorides is less than 1%, this effect is not fully achieved, resulting in insufficient shielding, making porosity defects such as pits and blowholes more likely to occur, and the arc becomes unstable. On the other hand, if the total F-equivalent value of the metal fluorides exceeds 5%, the arc becomes excessively strong and unstable. Therefore, the total F-equivalent value of the metal fluorides contained in the flux should be 1 to 5%. Note that metal fluorides can be added to the flux as fluorite, sodium fluoride, lithium fluoride, magnesium fluoride, potassium silicofluoride, cryolite, aluminum fluoride, etc., and the F-equivalent value is the sum of the F amounts contained in these substances.

[Total of one or more metal carbonates contained in the flux: 0.5 to 1.5%]
Metal carbonates act as gas generators, decomposing with arc heat during welding to generate CO and _CO2 gases, which isolate the molten pool from the atmosphere, improving shielding and suppressing the occurrence of porosity defects such as pits and blowholes. If the total content of one or more metal carbonates is less than 0.5%, this effect is insufficient, and porosity defects such as pits and blowholes are more likely to occur. Furthermore, if the total content of one or more metal carbonates is less than 0.5%, the toughness of the weld metal decreases. On the other hand, if the total content of one or more metal carbonates exceeds 1.5%, the arc becomes excessively strong and unstable, resulting in slag entrapment in the weld. Therefore, the total content of one or more metal carbonates in the flux is limited to 0.5-1.5%. Metal carbonates can be added to the flux as calcium carbonate, barium carbonate, lithium carbonate, etc.

[Total of one or both of the Na ₂ O equivalent values and K ₂ O equivalent values of Na oxides and K oxides contained in the flux: 0.02 to 0.11% ]
Na oxide and K oxide have the effect of stabilizing the arc state. If the total of one or both of the _Na2O equivalent and _K2O equivalent values of Na oxide and K oxide is less than 0.02%, the arc becomes unstable. On the other hand, if the total of one or both of the _Na2O equivalent and _K2O equivalent values of Na oxide and K oxide exceeds 0.11% , the bead toe does not conform well, resulting in a poor bead shape. Therefore, the total of one or both of the _Na2O equivalent and _K2O equivalent values of Na oxide and K oxide contained in the flux should be 0.02 to 0.11% . Na oxide and K oxide can be added to the flux as powders such as potash glass and soda glass.

[Mo: 0.5% or less in total for steel sheath and flux]
Mo has the effect of increasing the strength of the weld metal. However, if Mo exceeds 0.5%, the strength of the weld metal becomes excessively high and the toughness decreases. Therefore, the total Mo content of the steel sheath and flux is set to 0.5% or less. In order to obtain the effect of increasing the strength of the weld metal, Mo is preferably 0.1% or more. In addition to being a component contained in the steel sheath, Mo can be added to the flux as metallic Mo powder or alloy powder such as Fe-Mo.

[Ni content of steel sheath and flux combined: 1.0% or less]
Ni has the effect of improving the strength and toughness of the weld metal. However, if Ni exceeds 1.0%, the strength of the weld metal becomes excessively high and the toughness decreases. Therefore, Ni content is set to 1.0% or less. In order to obtain the effect of improving the strength and toughness of the weld metal, Ni content is preferably 0.1% or more. In addition to being a component contained in the steel sheath, Ni can be added from the flux as metallic Ni, Fe-Ni alloy powder, etc.

The remainder of the flux-cored wire for self-shielded arc welding of the present invention consists of the Fe content of the steel sheath, iron powder, the Fe content of iron alloy powder such as Fe-Si, Fe-Mn, Fe-Si-Mn, and Fe-Al alloys, and impurities. The iron powder is contained in the flux to adjust the flux filling rate. There are no particular restrictions on the impurities, but from the perspective of preventing hot cracking, it is preferable that the P content be 0.010% or less.

The flux-cored wire for self-shielded arc welding of the present invention is a so-called E-section wire, in which a steel sheath is formed into a U-shape, the interior is filled with flux, and the sheath is then woven inside and formed into a wire.

There are no particular restrictions on the flux filling rate, but from a productivity perspective, it is preferable to set it at 10-30% of the total wire mass.

The effects of the present invention will be specifically explained below using examples.

A steel sheath (C: 0.001-0.10%, Si: 0.05% or less, Mn: 0.1-0.6%, P: 0.05% or less, S: 0.05% or less) was used, which was formed into a U-shape and filled with various fluxes shown in Table 1 at a filling rate of 15-25%. The sheath was then woven inside and shaped to produce a flux-cored wire with the cross section shown in Figure 1. The wire diameter was 3.2 mm. The flux used was dried before filling.

Self-shielded arc welding was performed using the prototype flux-cored wire for self-shielded arc welding, and weld metal tests and welding workability evaluations were conducted.

Weld metal testing was conducted in accordance with JIS Z 3111 and JIS Z 3313 under the welding conditions shown in Table 2. The steel plate used was JIS G 3106 SM570 (plate thickness: 20 mm). A0 tensile test pieces and V-notch impact test pieces were taken from the center of the plate thickness of the weld metal of the resulting test specimens and subjected to tensile testing and impact testing, respectively.

The criteria for evaluating the tensile properties of the weld metal were: a tensile strength in the range of 590-770 MPa was considered good; and an average absorbed energy value of 3 at 20°C of 27 J or more was considered good.

Furthermore, after preparing the specimens for the weld metal test, the excess metal and backing metal were ground down to the steel plate surface to make it smooth, and then an X-ray transmission test in accordance with JIS Z 3104 was conducted to check for the presence or absence of welding defects. These results are summarized in Table 3.

Welding workability was evaluated based on arc stability, slag coverage, and bead shape during welding in the weld metal test.

Arc stability was evaluated by measuring voltage fluctuations for 10 seconds during welding and measuring the magnitude of the voltage. Setting a threshold of ±4V relative to the average voltage, the arc was deemed stable when the voltage fluctuation exceeded the threshold for 80% or less of the 10-second period, and unstable when the voltage fluctuation exceeded the threshold for more than 20% of the 10-second period. Slag coverage was evaluated by estimating the visually visible area of the bead where no slag was present, and an area of 10% or less where no slag was present was deemed good. Bead shape was deemed good when the healthy part of the weld bead had no undercuts or overlaps that required rework.

In Tables 1 and 3, wire symbols 1, 4, 7 , 11, and 12 are examples of the present invention, and wire symbols 13 to 24 are comparative examples. Wires symbol 1, 4, 7 , 11, and 12, which are examples of the present invention, had appropriate C, Mn, Al, and Si in the flux-cored wires for self-shielded arc welding, and appropriate Mg, the total of Si oxides converted into _SiO2 , the total of metal fluorides converted into F, the total of metal carbonates, and the total of Na oxides and K oxides converted into _Na2O and _K2O , so the arc was stable, the slag coverage and bead shape were good, no welding defects such as pits, blowholes, or slag entrapment occurred, and the tensile strength and absorbed energy of the deposited metal were both good.

In addition, since the amounts of Ni added were appropriate for wires 1 and 12, high-strength weld metal of 700 MPa or more was obtained, and absorbed energy was 50 J or more. Furthermore, since the amounts of Mo and Ni added for wire 11 were appropriate, high-strength weld metal of 750 MPa or more was obtained, and absorbed energy was 50 J or more.

In the comparative example, wire number 13 had a low carbon content, which resulted in blowholes in the weld. Also, because of the low carbon content, the tensile strength of the deposited metal was low. Furthermore, because of the high magnesium content, the arc became excessively strong and unstable. Furthermore, because of the low nickel content, the effect of improving the tensile strength and absorbed energy of the deposited metal was not achieved.

Wire No. 14 had a high C content, so the deposited metal had high tensile strength and low absorbed energy. In addition, the total _SiO2 equivalent value of silicon oxide was low, so the slag encapsulation was poor.

Wire No. 15 had a low Mn content, resulting in low tensile strength and absorbed energy in the deposited metal. Also, because the total F-equivalent value of the metal fluorides was low, the arc became unstable. Also, because the total F-equivalent value of the metal fluorides was low, blowholes occurred in the weld. Furthermore, because the Mo content was low, the effect of improving the tensile strength of the deposited metal was not achieved.

Wire No. 16 had a high Mn content, resulting in high tensile strength of the deposited metal and low absorbed energy. Also, because the total F-equivalent value of the metal fluorides was high, the arc was excessively strong and unstable.

Wire number 17 had a low Al content, resulting in blowholes in the weld.

Wire No. 18 had a high Al content, so the absorbed energy of the deposited metal was low. Also, the total of the Na ₂ O equivalent values and K ₂ O equivalent values of Na oxides and K oxides was low, so the arc became unstable.

Wire number 19 had a low Mg content, which resulted in blowholes in the weld. Also, because of the low Mg content, the absorbed energy of the deposited metal was low.

Wire No. 20 had a high Si content, so the absorbed energy of the weld metal was low. In addition, the total of the Na ₂ O equivalent values and K ₂ O equivalent values of Na oxides and K oxides was high, so the bead toe did not conform well and the bead shape was poor.

Wire No. 21 had a high total _SiO2 equivalent of silicon oxides, so the absorbed energy of the weld metal was low. Also, because the total metal carbonates were high, the arc was excessively strong and unstable, causing slag inclusion in the weld.

Wire number 22 had a low total metal carbonate content, so the absorbed energy of the weld metal was low. Also, because the total metal carbonate content was low, blowholes occurred in the weld.

Wire number 23 had a high total F-equivalent value of metal fluorides, which resulted in an excessively strong and unstable arc. Also, because it contained a high amount of Mo, the tensile strength of the deposited metal was high and the absorbed energy was low.

Wire No. 24 had a low total Na ₂ O equivalent value and K ₂ O equivalent value of Na oxide and K oxide, resulting in an unstable arc. In addition, because it contained a large amount of Ni, the tensile strength of the deposited metal was high and the absorbed energy was low.

Claims (3)

In a flux-cored wire for self-shield arc welding, which is made by filling a steel sheath with flux, the total mass of the steel sheath and flux is, in mass %,
C: 0.05-0.25%,
Mn: 0.5-1.5%,
Al: 1.0-3.0%,
Si: 0.8% or less;
Furthermore, in the flux, in mass % relative to the total mass of the wire,
Mg: 1.0-3.0%,
Total _SiO2 equivalent value of Si oxides: 0.1 to 0.5%,
Total F-equivalent value of metal fluorides: 1.3 (excluding 1.3) to 5%,
Total of one or more metal carbonates of calcium carbonate, barium carbonate, and lithium carbonate : 0.5 to 1.5%,
The total of one or both of Na oxides and K oxides in terms of Na ₂ O and K ₂ O is 0.02 to 0.11%,
1. A flux-cored wire for self-shield arc welding, the balance of which consists of the iron content of the steel sheath, iron powder, the iron content of the iron alloy powder, and impurities. The flux-cored wire for self-shielded arc welding according to claim 1, characterized in that the total mass of the wire, including the steel sheath and flux, contains 0.5% or less Mo. The flux-cored wire for self-shielded arc welding according to claim 1 or 2, characterized in that the total mass of the steel sheath and flux contains 1.0% or less Ni.