JP7221812B2 - Flux-cored wire for Ar-CO2 mixed gas shielded arc welding of high-strength steel - Google Patents

Description

The present invention relates to a flux-cored wire for Ar—CO ₂ mixed gas shielded arc welding of 590-690 MPa class high-strength steel. - flux cored wire for _CO2 mixed gas shielded arc welding.

In recent years, steel structures such as buildings, bridges, and offshore structures have become larger and lighter, and automobiles and construction machinery have become lighter. 690 MPa class high-strength steel is widely used.

Welded structures using high-tensile steel with a tensile strength of 590 to 690 MPa have a small amount of diffusible hydrogen in the weld metal, excellent mechanical properties such as crack resistance and low temperature toughness, and a small amount of slag generation. There is a demand for Ar-CO ₂ mixed gas shielded arc welding that uses a metallic flux-cored wire capable of high-efficiency welding and generates less spatter.

Conventionally, for Ar-CO ₂ mixed gas shielded arc welding of high-strength steel, for example, Ar-CO ₂ mixed gas containing alloy components such as Ni, Cr, and Mo disclosed in Patent Document 1 and Patent Document 2 Solid wire for shielded arc welding is used. However, the solid wires for Ar—CO ₂ mixed gas shielded arc welding described in Patent Documents 1 and 2 contain many alloy components and the wires themselves are hard and have high rigidity. Wire feeding becomes unstable and the arc becomes unstable, resulting in a large amount of spatter.

On the other hand, as a flux-cored wire for Ar-CO ₂ mixed gas shielded arc welding of high-strength steel, Patent Document 3 describes an Ar-CO ₂ mixed high-strength steel with good welding workability and excellent crack resistance. A flux cored wire for gas shielded arc welding is disclosed. According to the flux-cored wire described in Patent Document 3, a high-strength weld metal can be obtained, and good welding workability such as a stable arc and a small amount of spatter generated can be obtained. However, the flux-cored wire for Ar—CO ₂ mixed gas shielded arc welding described in Patent Document 3 contains a large amount of slag-forming agent, so the amount of oxygen in the weld metal increases, and the low-temperature toughness of the weld metal is stabilized. hard to get.

As a technique for obtaining excellent low-temperature toughness by reducing the oxygen content in the weld metal, Patent Document 4 discloses that the content of metal fluoride in the wire is increased to reduce the oxygen content in the weld metal. A flux-cored wire for Ar--CO ₂ mixed gas shielded arc welding is disclosed. However, according to the Ar—CO ₂ mixed gas shielded arc welding wire described in Patent Document 4, although a weld metal having excellent low-temperature toughness can be obtained, since the flux wire contains a large amount of metal fluoride, arc becomes unstable and the amount of spatter generated is large, resulting in poor welding workability.

Furthermore, Patent Documents 5 and 6 disclose techniques related to metal-based flux-cored wires containing a large amount of alloy powder. There was a problem that it was difficult to achieve both satisfactory welding workability, such as a good bead shape, a stable arc, and a small amount of spatter.

JP-A-57-124594 Japanese Patent Application Laid-Open No. 2000-301379 Japanese Patent Application Laid-Open No. 2006-281223 Japanese Unexamined Patent Application Publication No. 2011-20154 JP 2007-144516 A JP 2008-93715 A

The present invention has been made to solve the above problems, and in welding 590 to 690 MPa class high-strength steel, a weld metal having appropriate strength and good and stable toughness in a low temperature region can be obtained. The purpose is to provide a flux-cored wire for Ar-CO ₂ mixed gas shielded arc welding of high-strength steel which is excellent in arc stability, bead appearance and bead shape, and has excellent welding workability such as less spatter generation. do.

The gist of the present invention is a flux-cored wire for Ar-CO ₂ mixed gas shielded arc welding of high-tensile steel in which the steel sheath is filled with flux. C: 0.05-0.15%, Si: 0.5-2.0%, Mn: 1.5-3.0%, Ti: 0.01-0.20%, Cu: 0.01-0.20%. 05 to 0.50%, S: 0.005 to 0.020%, and further, SiO ₂ : 0.05 to 0.20%, oxygen content in the flux in terms of mass% with respect to the total mass of the wire 0.30% or less iron powder: 3 to 8%, fluorine compounds: 0.005 to 0.080% in total of F conversion values, Na compounds and K compounds: Na ₂ O conversion values and K ₂ O conversion values 0.02 to 0.15% in total, and the balance is composed of the Fe content of the steel outer shell, the Fe content of the iron alloy powder, and unavoidable impurities.

It is also characterized by containing Mo: 0.1 to 0.5% in terms of mass % relative to the total mass of the wire, in terms of the total of the steel sheath and flux.

Furthermore, it is characterized by containing B: 0.0015 to 0.0150% in mass % with respect to the total mass of the wire as the total of the steel sheath and flux.

A flux-cored wire for Ar- _CO mixed gas shielded arc welding of high-strength steel, characterized in that the joints of the formed steel skin are welded to eliminate seams in the steel skin. be.

According to the flux-cored wire for Ar-CO ₂ mixed gas shielded arc welding of high-strength steel to which the present invention is applied, the stability of the arc during welding, the bead appearance and bead shape are excellent, and the amount of spatter generation is small. A flux-cored wire for Ar-CO ₂ mixed gas shielded arc welding of high-strength steel that has good toughness, strength of 590 to 690 MPa class, good low-temperature toughness, and high quality weld metal without defects. can provide.

In order to solve the above-mentioned problems, the present inventors obtained a weld metal having appropriate strength and low-temperature toughness in Ar—CO ₂ mixed gas shielded arc welding of 590-690 MPa class high-strength steel, Component composition of flux-cored wire for Ar-CO ₂ mixed gas shielded arc welding of high-strength steel with stable arc, less spatter, good bead appearance and good bead shape. was examined in detail.

As a result, the stability of the arc and the reduction in the amount of spatter generated depend on the total amount of C in the wire, the content of iron powder in the flux, the Na compound and the K compound converted to Na ₂ O and K ₂ O converted values, and It has been found that it is effective to optimize the total amount of F-equivalent values of fluorine compounds. In addition, the bead appearance and bead shape can be improved by adjusting the content of Si in the wire, SiO ₂ in the flack and iron powder in appropriate amounts, and the slag removability can be improved by adjusting the content of S in an appropriate amount. I found out. Furthermore, high temperature cracking is prevented by specifying the upper limits of the contents of C, Cu, S and B in the wire, and cold cracking is prevented by eliminating the seam of the wire skin, and Si, Mn, It has also been found that welding defects such as slag entrainment can be prevented by specifying the upper limit of the total amount of SiO ₂ and Na compounds and K compounds in terms of Na ₂ O and K ₂ O.

In addition, in order to achieve an appropriate strength of the weld metal and a stable improvement in low-temperature toughness at the same time, it is necessary to reduce oxides, which are slag-forming agents, in the wire as much as possible, and It has been found that optimization of each Cu content is effective. They also found that the low temperature toughness of the weld metal can be further improved by reducing the oxygen content of the iron powder in the wire.

Furthermore, the inventors have found that adding an appropriate amount of Mo to the wire makes it possible to further increase the strength, and adding an appropriate amount of B makes it possible to further improve the low-temperature toughness of the weld metal.

The flux-cored wire for Ar—CO ₂ mixed gas shielded arc welding of high-strength steel to which the present invention is applied is made possible by the synergistic effect of each component composition alone and coexistence. The reason for adding the composition and the reason for limitation will be described. In the following description, the chemical composition of the flux-cored wire is represented by % by mass, which is the ratio to the total mass of the wire, and the description of % by mass is simply described as %.

[Total C of steel skin and flux: 0.05 to 0.15%]
C is an element necessary for improving the strength of the weld metal by solid solution strengthening. Also, C has the effect of stabilizing the arc. If C is less than 0.05%, the effect cannot be sufficiently obtained, and the strength of the weld metal cannot be obtained. If the C content is less than 0.05%, the arc becomes unstable and the amount of spatter generated increases. On the other hand, if C exceeds 0.15%, the strength of the weld metal becomes excessively high and the toughness decreases. Moreover, when C exceeds 0.15%, hot cracking is likely to occur. Therefore, the total content of C in the steel skin and flux should be 0.05 to 0.15%. C can be added from metal powder, alloy powder, etc. from the flux, in addition to the components contained in the steel outer sheath.

[Si: 0.5 to 2.0% in total of steel skin and flux]
Si is added for deoxidizing the weld metal. Si also has the effect of improving the strength of the weld metal and adjusting the viscosity of the molten metal to improve the bead appearance and bead shape. If the Si content is less than 0.5%, the weld metal is deoxidized insufficiently, and the toughness of the weld metal is lowered, and the strength of the weld metal cannot be obtained. On the other hand, if the Si content is less than 0.5%, the weld bead appearance and bead shape become poor. On the other hand, if Si exceeds 2.0%, the strength of the weld metal becomes excessively high, and the toughness of the weld metal cannot be stably obtained. On the other hand, if the Si content exceeds 2.0%, the amount of slag generated during welding increases, and welding defects such as slag entrainment tend to occur. Therefore, the total Si content of the steel sheath and flux should be 0.5 to 2.0%. Si can be added from metal Si, Fe--Si, Fe--Si--Mn or other alloy powder from the flux, in addition to the components contained in the steel outer sheath.

[Total Mn of steel skin and flux: 1.5 to 3.0%]
Mn is added to ensure the low temperature toughness of the weld metal and to improve the strength. If the Mn content is less than 1.5%, the strength of the weld metal will be low and sufficient low temperature toughness cannot be ensured. On the other hand, if the Mn content exceeds 3.0%, the yield of Mn in the weld metal increases, the strength becomes excessively high, and the low-temperature toughness of the weld metal cannot be stably obtained. On the other hand, if Mn exceeds 3.0%, the amount of slag generated during welding increases, and welding defects such as slag entrainment tend to occur. Therefore, Mn is set to 1.5 to 3.0% in total of the steel sheath and flux. Mn can be added from metal Mn, Fe--Mn, Fe--Si--Mn, and other alloy powders from the flux, in addition to components contained in the steel outer sheath.

[Ti: 0.01 to 0.20% in total of steel skin and flux]
Ti acts as a deoxidizing agent and also has the effect of producing fine oxides of Ti in the weld metal and improving the low-temperature toughness of the weld metal. If Ti is less than 0.01%, the effect cannot be sufficiently obtained, and the low temperature toughness of the weld metal cannot be obtained. On the other hand, when Ti exceeds 0.20%, solid solution Ti in the weld metal increases and the low temperature toughness decreases. Therefore, the total amount of Ti in the steel skin and flux should be 0.01 to 0.20%. Note that Ti is added from metal Ti from the flux, alloy powder such as Fe—Ti, etc., in addition to components contained in the steel outer sheath.

[Cu: 0.05 to 0.50% in total of steel skin and flux]
Cu has a precipitation-strengthening effect, and has the effect of lowering the transformation temperature, refining the structure of the weld metal, and stabilizing the toughness. If Cu is less than 0.05%, stable toughness of the weld metal cannot be obtained. On the other hand, when Cu exceeds 0.50%, precipitation embrittlement occurs, the toughness of the weld metal is lowered, and hot cracking is likely to occur. Therefore, the total content of Cu in the steel sheath and flux should be 0.05 to 0.50%. Cu can be added from metal Cu from flux, alloy powder such as Fe--Si--Cu, in addition to components contained in the steel skin and Cu plating applied to the surface of the steel skin.

[Total S of steel skin and flux: 0.005 to 0.020%]
S has the effect of reducing the crystallinity of slag and improving the slag releasability. If the S content is less than 0.005%, the effect is not sufficiently obtained, and the slag removability deteriorates. On the other hand, if the S content exceeds 0.020%, the toughness of the weld metal is lowered, and hot cracks are likely to occur. Therefore, S is set to 0.005 to 0.020% in total of the steel skin and the flux. In addition, S can be added from alloy powder such as FeS from flux in addition to components contained in the steel outer sheath.

[SiO ₂ content in flux: 0.05 to 0.20%]
SiO ₂ has the effect of improving conformability of the bead toe and improving bead appearance and bead shape. If the SiO ₂ content is less than 0.05%, the conformability of the bead toe of the weld bead is poor, and the bead appearance and bead shape are poor. On the other hand, if SiO ₂ exceeds 0.20%, the amount of oxygen in the weld metal increases and the toughness decreases. On the other hand, if the SiO ₂ content exceeds 0.20%, the amount of slag on the bead surface increases and welding defects such as slag entrainment tend to occur. Therefore, the SiO ₂ content in the flux should be 0.05 to 0.20%. SiO ₂ can be added from silica sand from the flux, solid components of water glass comprising sodium silicate and potassium silicate, and the like.

[Iron powder with an oxygen content of 0.30% or less in the flux: 3 to 8%]
Iron powder has the effect of softening the arc state and stabilizing the arc. In addition to securing the high weldability that is a feature of metal-based flux-cored wires, it is added from the flux to adjust the composition. If the iron powder content is less than 3%, the arc becomes unstable, the amount of spatter is increased, and the welding amount is insufficient, resulting in poor bead appearance and bead shape. On the other hand, if the iron powder content exceeds 8%, the flux filling rate fluctuates in the longitudinal direction of the wire during the wire drawing process in manufacturing, resulting in an unstable arc and an increased amount of spatter. Therefore, the content of iron powder in the flux should be 3-8%.

In the flux-cored wire for Ar--CO ₂ mixed gas shielded arc welding of the present invention, hydrogen-reduced iron powder or atomized iron powder having an oxygen content of 0.30% or less is used. By using these low-oxygen iron powders, the amount of oxygen in the weld metal can be suppressed to 0.05% or less without adding strong deoxidizers such as Al, Mg, and Zr, which increase the amount of slag generated. Therefore, it is possible to further improve the low temperature toughness of the weld metal.

[Fluorine compound contained in flux: 0.005 to 0.080% in total of F conversion value]
The fluorine compound has the effect of concentrating and stabilizing the arc. If the total F conversion value of the fluorine compound is less than 0.005%, the effect is not sufficiently obtained, the arc becomes unstable, and the amount of spatter generation increases. On the other hand, if the total F conversion value of the fluorine compound exceeds 0.080%, the arc becomes unstable and the amount of spatter generation increases. Therefore, the fluorine compound contained in the flux should be 0.005 to 0.080% in terms of total F conversion value. The fluorine compound can be added from CaF ₂ , NaF, LiF, MgF ₂ , K ₂ SiF ₆ , Na ₃ AlF ₆ , AlF ₃ etc. from the flux, and the F conversion value is the total amount of F contained in them. .

[Na compound and K compound contained in flux: 0.02 to 0.15% in total of Na ₂ O converted value and K ₂ O converted value]
The Na compound and K compound have the effect of softening and stabilizing the arc state. If the sum of the Na compound and K compound converted to Na ₂ O and converted to K ₂ O is less than 0.02%, the arc becomes unstable and the amount of spatter generated increases. On the other hand, when the sum of the Na compound and the K compound converted to Na ₂ O and converted to K ₂ O exceeds 0.15%, the arc becomes excessively strong, the amount of spatter generated increases, and the bead toe portion The familiarity of the bead deteriorates, resulting in poor bead appearance and bead shape. Furthermore, if the sum of these Na ₂ O converted values and K ₂ O converted values exceeds 0.15%, the amount of slag on the bead surface increases, and welding defects such as slag entrainment tend to occur. Therefore, the sum of the Na compound and K compound contained in the flux in terms of Na ₂ O and K ₂ O should be 0.02 to 0.15%. The Na compound and K compound can be added from the solid component of water glass consisting of sodium silicate and potassium silicate from the flux, and powders such as NaF, K ₂ SiF ₆ and Na ₃ AlF ₆ .

[Total Mo of steel skin and flux: 0.1 to 0.5%]
Mo has the effect of improving the strength of the weld metal. If Mo is less than 0.1%, the effect of improving the strength of the weld metal cannot be sufficiently obtained. On the other hand, if Mo exceeds 0.5%, the strength of the weld metal becomes excessively high, and low temperature toughness cannot be stably obtained. Therefore, Mo is set to 0.1 to 0.5% in total in the steel outer sheath and flux. In addition, Mo can be added from the metal Mo powder from flux other than the component contained in steel outer coverings.

[Total B of steel skin and flux: 0.0015 to 0.0150%]
B has the effect of refining the structure of the weld metal and improving the low temperature toughness. If B is less than 0.0015%, the effect of improving the low temperature toughness of the weld metal cannot be sufficiently obtained. On the other hand, when B exceeds 0.0150%, the grain boundaries of the weld metal become brittle and the low temperature toughness decreases. Moreover, when B exceeds 0.0150%, hot cracking tends to occur. Therefore, B is set to 0.0015 to 0.0150% in total of the steel skin and the flux. B can be added from alloy powders such as Fe--Si--B and Fe--Mn--B in addition to components contained in the steel outer shell.

[Elimination of seams in the steel skin by welding the joints of the molded steel skin]
The flux-cored wire for Ar--CO ₂ mixed gas shielded arc welding of high-strength steel according to the present invention has a structure in which a steel outer sheath is formed into a pipe and the interior of which is filled with flux. As for the types of wire, there are two types of wire: a wire with no seams in the steel skin obtained by welding the seams of the formed steel skin, and a wire with no seams in the steel skin without welding the seams on the steel skin. It can be broadly divided into wires with In the present invention, a wire having any cross-sectional structure can be used. However, a wire having a seam on a steel skin is susceptible to cold cracking when the strength of the weld metal increases. need to use. On the other hand, a wire with a seamless steel sheath can be heat-treated to reduce the total amount of hydrogen in the wire. can be reduced and the cold cracking resistance can be improved. For this reason, it is more preferable to use a seamless cross-sectional structure by welding seams of the steel skin.

The remainder of the flux-cored wire for Ar—CO ₂ mixed gas shielded arc welding of high-strength steel of the present invention is composed of the Fe content of the steel sheath and the Fe content of iron alloy powder such as Fe—Si, Fe—Mn, and Fe—Ti alloys. and unavoidable impurities. Inevitable impurities are not particularly limited, but from the viewpoint of prevention of hot cracking, P is preferably 0.010% or less.

Although the flux filling rate is not particularly limited, it is preferably 8 to 20% with respect to the total mass of the wire from the viewpoint of productivity.

The shielding gas is a mixed gas of Ar and _CO.sub.2 , and the mixed amount of _CO.sub.2 is in the range of 5 to 25% by volume to reduce the amount of oxygen in the weld metal. Also, the flow rate of the shielding gas is preferably 20 to 35 liters/minute for the purpose of defect resistance and prevention of contamination of nitrogen from the atmosphere.

EXAMPLES Hereinafter, the effects of the present invention will be specifically described with reference to examples.

A steel plate specified in JIS G 3141 is used as the steel skin, and in the process of forming the steel skin, it is formed into a U shape, filled with flux at a filling rate of 8 to 20%, and formed into a C shape, A seamless wire with welded joints of steel skins and a wire with gaps that were not welded were formed into pipes and drawn, and flux-cored wires with various compositions shown in Table 1 were experimentally produced. The wire diameter was 1.2 mm. In addition, the wire without a seam welded at the seam of the steel skin was annealed during wire drawing, but the wire with the seam of the steel skin was dried before filling the flux, and after the wire was manufactured, it was annealed. In order to prevent the flux from absorbing moisture, it was sealed in a vinyl bag and stored in that state until just before welding.

Welding workability, weld metal performance and crack resistance were investigated using the prototype flux-cored wire.

Welding workability and deposited metal performance were evaluated by performing a deposited metal test under the welding conditions shown in Table 2 according to JIS Z 3111 using a steel plate having a thickness of 20 mm specified in JIS G 3106 SM570.

Welding workability was investigated by visually confirming the quality of arc stability, amount of spatter, slag peelability, bead appearance and bead shape during the deposited metal test.

In the weld metal test, A0 tensile test pieces and impact test pieces were taken from the plate thickness center of the welded weld metal portion to investigate the mechanical performance.

In strength evaluation, a tensile strength of 590 to 680 MPa was considered good. For evaluation of toughness, a Charpy impact test at −40° C. was performed three times each, and an average value of absorbed energy of 47 J or more and a minimum value of 30 J or more were considered good.

For the evaluation of weld defects, the welded specimens were subjected to an X-ray transmission test in accordance with JIS Z 3106 to investigate the presence or absence of slag entrainment in the weld metal.

For the cracking resistance test, a U-shaped weld cracking test was carried out under the welding conditions shown in Table 2 in accordance with JIS Z 3157 using a steel plate having a thickness of 40 mm specified in JIS G 3106 SM570. After visually confirming the presence or absence of hot cracking in the test piece immediately after welding, the presence or absence of cold cracking in surface cracks and cross-section cracks (5 cross sections) of the test piece 48 hours after welding was checked in accordance with JIS Z 2343. It was investigated by a flaw detection test. These results are summarized in Table 3.

Wire symbols W1 to W17 in Tables 1 and 3 are examples of the present invention, and wire symbols W18 to W31 are comparative examples. The wire symbols W1 to W17, which are examples of the present invention, are suitable for C, Si, Mn, Ti, Cu, and S in the flux-cored wire, and the sum of the F conversion values of SiO ₂ , iron powder, and fluorine compounds in the flux, The sum of the Na compound and K compound converted to Na ₂ O and K ₂ O converted values is appropriate, and the oxygen content in the iron powder is appropriate. The appearance and bead shape were good, no hot cracks or cold cracks occurred, no welding defects, and the tensile strength of the deposited metal and the average and minimum values of absorbed energy were all good.

In addition, wire symbols W1, W4, W7, W8, W11 and W14, W17 have a proper amount of B added, so that the average value of the absorbed energy of the weld metal is even better, and the results are extremely satisfactory.

Wire symbols W4, W6, W10, W11, and W15 had seams in the steel skin, but the tensile strength of the weld metal was appropriate, so cold cracks did not occur.

Wire symbol W18 among the comparative examples had a low tensile strength of the weld metal due to a small amount of C. In addition, since the amount of C was small, the arc was unstable and a large amount of spatter was generated. Furthermore, since the amount of Cu was small, the minimum value of absorbed energy of the deposited metal was low.

Wire symbol W19 had a large amount of C, so the weld metal had a high tensile strength, a low average value and a minimum value of absorbed energy, and crater cracking occurred. Furthermore, since the amount of SiO ₂ was small, the bead appearance and bead shape were poor.

Wire symbol W20 had a low Si content, so the weld metal had a low tensile strength, and the average and minimum values of absorbed energy were low. In addition, since the amount of Si was small, the bead appearance and bead shape were unsatisfactory. Furthermore, since the amount of iron powder was large, the arc was unstable and a large amount of spatter was generated.

Wire symbol W21 had a large amount of Si, so the weld metal had a high tensile strength, a low minimum value of absorbed energy, and slag entrainment occurred. Furthermore, since the steel skin had seams, cold cracks occurred. In addition, since the total F conversion value of the fluorine compound was small, the arc was unstable and a large amount of spatter was generated.

Wire symbol W22 had a low Mn content, so the tensile strength of the weld metal was low, and the average and minimum values of absorbed energy were low. In addition, since the total F conversion value of the fluorine compound was large, the arc was unstable and the amount of spatter generated was large.

Wire symbol W23 had a large amount of Mn, so the weld metal had a high tensile strength and a low minimum value of absorbed energy. Moreover, since there was much Mn, slag entrainment occurred. Furthermore, since the steel skin had seams, cold cracks occurred. In addition, since the sum of Na compound and K compound converted to Na ₂ O and converted to K ₂ O was small, the arc was unstable and a large amount of spatter was generated.

Wire symbol W24 had a small amount of Ti, so the average value and minimum value of the absorbed energy of the weld metal were low. In addition, since the amount of S was small, the slag removability was poor.

Wire symbol W25 had a large amount of Ti, so the average value and minimum value of absorbed energy of the weld metal were low. In addition, since the total amount of Na compound and K compound converted to Na ₂ O and K ₂ O is large, the arc is strong, the amount of spatter generated is large, the bead appearance and bead shape are poor, and slag is involved. There has occurred.

Wire symbol W26 has a large amount of Cu, so the average value and minimum value of absorbed energy of the weld metal are low, and crater cracking occurs.

Wire symbol W27 has a large amount of S, so the average value and minimum value of the absorbed energy of the weld metal are low, and crater cracking occurs.

Wire symbol W28 has a large amount of SiO ₂ , so the average value and minimum value of absorbed energy of the weld metal are low, and slag entrainment occurs.

With wire symbol W29, the amount of iron powder was small, so the arc was unstable, the amount of spatter generated was large, and the bead appearance and bead shape were unsatisfactory.

With the wire symbol W30, the oxygen content in the iron powder was high, so the average value and minimum value of absorbed energy of the weld metal were low.

Wire symbol W31 had a small amount of Ti, so the average value and minimum value of absorbed energy of the weld metal were low. Also, since the amount of B was small, the effect of improving the low temperature toughness of the weld metal could not be obtained.

Wire symbol W32 was poor in bead appearance and bead shape due to low SiO ₂ content. In addition, since the amount of B was large, the average value and minimum value of absorbed energy of the weld metal were low, and crater cracking occurred.

As in Example 1, a steel plate specified in JIS G 3141 was used as the steel outer skin, the steel outer skin was formed into a U shape, and the flux was filled at 8 to 20% to form a C shape. , welded joints of the steel outer skins, made pipes, and wire-drawn to fabricate flux-cored wires of various compositions shown in Table 4. The wire diameter was 1.2 mm.

Using the flux-cored wires shown in Table 4, welding workability, deposited metal performance and crack resistance were investigated.

Welding workability and weld metal performance were evaluated by using a steel plate having a thickness of 20 mm specified in JIS G 3128 SHY685 and carrying out a weld metal test under the welding conditions shown in Table 2 according to JIS Z 3111.

Welding workability was investigated by visually confirming the stability of the arc during the deposited metal test, the increase or decrease in the amount of spatter generated, the slag peelability, the appearance of the bead, and the shape of the bead.

In the weld metal test, a No. 0 tensile test piece and an impact test were taken from the plate thickness center of the welded weld metal portion to investigate the mechanical performance.

In strength evaluation, a tensile strength of 690 to 830 MPa was considered good. For evaluation of toughness, a Charpy impact test at −40° C. was performed three times each, and an average value of absorbed energy of 47 J or more and a minimum value of 30 J or more were considered good.

For the evaluation of weld defects, the welded specimens were subjected to an X-ray transmission test in accordance with JIS Z 3106 to investigate the presence or absence of slag entrainment in the weld metal.

For the cracking resistance test, a U-shaped weld cracking test was carried out under the welding conditions shown in Table 2 according to JIS Z 3157 using a steel plate having a thickness of 40 mm specified in JIS G 3128 SHY685. After visually confirming the presence or absence of hot cracks on the test piece immediately after welding, the test piece 48 hours after welding was subjected to a penetrant inspection test in accordance with JIS Z 2343 for the presence or absence of cold cracks in surface cracks and cross-sectional cracks (5 cross sections). Investigated by These results are summarized in Table 5.

Wire symbols W33 to W35 in Tables 4 and 5 are examples of the present invention, and wire symbols W36 and W37 are comparative examples. Wire symbols W33 to W35, which are examples of the present invention, are suitable for C, Si, Mn, Ti, Cu, and S in the flux-cored wire, and the sum of the F conversion values of SiO ₂ , iron powder, and fluorine compounds in the flux, The sum of the Na compound and K compound converted to Na ₂ O and the K ₂ O converted value is appropriate, and the oxygen content in the iron powder is appropriate, so the arc is stable and the amount of spatter generated is small. The appearance and bead shape were good, no cracks occurred, no welding defects, and the tensile strength of the deposited metal and the average and minimum values of absorbed energy were both good.

Also, since an appropriate amount of Mo was added, the tensile strength of the weld metal was extremely good at 700 MPa or more. In addition, the wire symbols W33 and W35 had an appropriate amount of B added, so that the average value of the absorbed energy of the weld metal was even better, and the results were extremely satisfactory.

Wire symbol W36 had a small amount of C, so the arc was unstable, the amount of spatter generated was large, and the tensile strength of the deposited metal was low. Moreover, since the Mo content is small, the effect of improving the strength of the weld metal cannot be obtained.

With wire symbol W37, the total F conversion value of the fluorine compound was small, so the arc was unstable and the amount of spatter generated was large. In addition, since the Mo content was large, the tensile strength of the weld metal was high, and the average and minimum values of absorbed energy were low.

Claims (4)

A flux-cored wire for Ar-CO ₂ mixed gas shielded arc welding of high-strength steel, in which the steel sheath is filled with flux,
% by mass of the total mass of the wire, the sum of the steel sheath and flux,
C: 0.05 to 0.15%,
Si: 0.5 to 2.0%,
Mn: 1.5-3.0%,
Ti: 0.01 to 0.20%,
Cu: 0.05-0.50%,
S: contains 0.005 to 0.020%,
In addition, in mass % with respect to the total mass of the wire, in the flux,
SiO ₂ : 0.05-0.20%,
Iron powder with an oxygen content of 0.30% or less: 3 to 8%,
Fluorine compound: 0.005 to 0.080% in total F conversion value,
Na compound and K compound: 0.02 to 0.15% in total of Na ₂ O conversion value and K ₂ O conversion value,
A flux-cored wire for Ar-CO ₂ mixed gas shielded arc welding of high-strength steel, wherein the balance is composed of the Fe content of the steel skin, the Fe content of the iron alloy powder and unavoidable impurities. % by mass of the total mass of the wire, the sum of the steel sheath and flux,
The flux-cored wire for Ar—CO ₂ mixed gas shielded arc welding of high-strength steel according to claim 1, further containing Mo: 0.1 to 0.5%. % by mass of the total mass of the wire, the sum of the steel sheath and flux,
3. The flux-cored wire for Ar--CO ₂ mixed gas shielded arc welding of high-strength steel according to claim 1 or 2, further comprising B: 0.0015 to 0.0150%. 4. The high-strength steel Ar--CO ₂ according to any one of claims 1 to 3, characterized in that seams of the formed steel skin are welded to eliminate seams in the steel skin. Flux cored wire for mixed gas shielded arc welding.