JP7638595B2 - Low hydrogen iron powder covered electrode - Google Patents

Description

The present invention relates to an iron powder low-hydrogen covered metal arc welding rod that has good arc stability, a tensile strength of 780 MPa or more, and good low-temperature toughness and bending performance of the welded joint, particularly in arc welding using a DC power source.

Low-hydrogen covered electrodes have good arc stability, excellent crack resistance, and low-temperature toughness of the weld metal, so they are widely used in areas with strong restraints and for welding high-tensile steel.

On the other hand, with the recent increase in size of welded structures, there is a demand for higher strength steel materials to be used. Also, in order to ensure safety in large marine structures and spherical tanks aimed at the development of natural resources, it is important to further improve the low-temperature toughness of the weld metal and ensure excellent mechanical performance, such as by ensuring sufficient strength. However, since achieving both high strength and low-temperature toughness in weld metal are generally contradictory properties, there is a need to develop new methods that achieve both high strength and low-temperature toughness.

In response to this situation, several proposals have been made to improve the mechanical performance of weld metal. For example, Patent Document 1 discloses technology related to a covered metal arc welding rod that aims to obtain a weld metal with excellent low-temperature toughness even when the Ni content is 1 mass% or less. However, the technology described in Patent Document 1 has the problem that when welding is performed using a DC power source, magnetic blow and one-sided melting of the coating are likely to occur, making it difficult to ensure sufficient welding workability.

Patent Document 2 discloses a low-hydrogen covered electrode technology that, when multi-layer welding is performed on the circumference of a steel pipe of 490 MPa class or more using a DC power source, provides excellent arc stability and excellent weld metal strength and low-temperature toughness. However, the technology disclosed in Patent Document 2 is applied to multi-layer welding of the circumference of a steel pipe of 490 MPa class or more, and is a technology for welding materials with a tensile strength of 550 to 650 MPa. For this reason, sufficient characteristics cannot be obtained with welding materials of 780 MPa or more.

Patent Document 3 also discloses technology for low-hydrogen covered metal arc welding rods that ensure stable mechanical performance of the weld metal when the tensile strength of the deposited metal is 590 MPa or more, good welding workability in all positions, and low-temperature toughness. However, the technology disclosed in Patent Document 3 has the problem that the welding rod is expensive because Ni is added to the steel core wire.

JP 2014-151338 A JP 2016-203253 A JP 2010-253495 A

Low-hydrogen covered electrodes are generally designed for use in Japan with AC power sources for welding, but recently there has been a demand for the development of low-hydrogen covered electrodes that can be used with DC power sources, which are cheaper power sources, for on-site welding of spherical tanks and marine structures overseas.

When low-hydrogen covered metal arc welding rods are welded using a DC power source, magnetic blow and one-sided melting of the coating material can occur, making the arc unstable and preventing the production of a sound bead. For this reason, there is a need for the development of an iron powder low-hydrogen covered metal arc welding rod that has excellent arc stability and good mechanical properties of the weld metal, even when using a DC power source.

The present invention was devised in consideration of these problems, and aims to provide an iron powder, low hydrogen type covered metal arc welding rod for 780 MPa class high tensile steel, which has good welding workability such as arc stability, and which produces welded joints with excellent low temperature toughness and bending workability when used in multi-layer welding using a DC power source with higher strength 780 MPa class high tensile steel.

The present invention relates to an iron powder low hydrogen type covered electrode for multi-pass welding of 780 MPa class high tensile steel using a DC power source , in which a steel core wire is coated with a covering agent, the covering rate of the covering agent being 30 to 45% in mass % relative to the total mass of the welding electrode, the composition of the covering agent being, in mass % relative to the total mass of the covering agent, 3.0 to 6.0% Mn, 3.5 to 7.5% Ni, 0.3 to 1.3% Mo, 0.2 to 1.0% Cr, 2.0 to 5.0% Si, a total of _SiO2 converted values of Si _oxides : 3.5 to 7.5%, 0.2 to 1.5% Ti, a total of Ti oxides in TiO2 converted values: 1.0 to 5.0%, a total of one or more metal carbonates: 25 to 45%, a total of one or more metal fluorides: 5 to 15%, and the Al oxide _Al2O the total of Mn, Ni, Mo and Cr in terms of formula ( ₁ ) is 150-300, Fe is 20-30%, and the total of the remainder consisting of impurities is 1.0% or less.

2.3 × [Mn] + 38.5 × [Ni] + 4.2 × [Mo] + 2.1 × [Cr] ... formula (1)
Here, the brackets [ ] indicate the mass % of each component relative to the total mass of the coating agent.

The coating material is also characterized in that the total CaO equivalent value of Ca oxides is 0.5% or less, expressed as a percentage by mass relative to the total mass of the coating material.

The present invention further provides an iron powder low hydrogen type covered metal arc welding electrode, characterized in that the coating material has a total content of Zr oxide calculated as _ZrO2 of 1.5% or less, expressed as mass% based on the total mass of the coating material.

The low-hydrogen iron powder covered metal arc welding electrode of the present invention achieves strength equal to or greater than that of steel in multi-layer welding of 780 MPa-class high-tensile steel using a DC power source, provides good welding workability such as arc stability, and provides stable toughness at low temperatures, resulting in good bending performance in the welded joints produced. Therefore, the reliability of welded joints for various steel structures can be significantly improved.

In order to solve the above-mentioned problems, the inventors conducted detailed research into an iron powder low-hydrogen covered metal arc welding electrode, and in particular the component composition of its coating, that can provide a strength equal to or greater than this strength, good welding workability such as arc stability, stable low-temperature toughness, and good bending performance of the welded joint after welding when using a DC power source for multi-layer welding of 780 MPa-class high-tensile steel.

As a result, the following findings were obtained, and the present invention was developed by adjusting the various ingredients according to these findings.

(1) By adjusting the content of Mn, Ni, Mo, and Cr to an appropriate amount, it is possible to ensure the strength of the weld metal and good bending performance of the welded joint.

(2) The toughness of the weld metal at low temperatures can be improved by adjusting the metal carbonate, Si, and Ti content and the coverage rate of the steel core wire.

(3) In order to stabilize the arc and reduce the amount of spatter generated with respect to welding workability, the amount of Si, Ti, Ti oxide (converted into TiO2 ₎ , metal carbonate, metal fluoride, and one or a total of two or more of Na oxide, Na fluoride, K oxide, and K fluoride can be adjusted to an appropriate amount.

(4) The bead shape and bead appearance can be improved by adjusting the total content of silicon oxide ( _SiO2 equivalent), titanium oxide ( _TiO2 equivalent), metal carbonates, metal fluorides, and magnesium oxide (MgO equivalent) to appropriate amounts.

(5) The slag removability and encapsulation properties can be improved by adjusting the total amount of silicon oxide ( _SiO2 equivalent), metal carbonate, metal fluoride, and aluminum oxide ( _Al2O3 equivalent ₎ to an appropriate amount.

(6) Rod burn, where the welding rod itself becomes red hot, can be prevented by adjusting the Fe content to an appropriate amount, one-sided melting of the protective tube of the welding rod can be prevented by adjusting the metal fluoride and Fe content to an appropriate amount, and productivity of the welding rod, such as the paintability of the coating agent, can be improved by adjusting the total amount of one or more of Na oxide, Na fluoride, K oxide, and K fluoride to an appropriate amount.

(7) By setting the total amount of Ca oxides in terms of CaO to an appropriate amount, arc stabilization and reduction in the amount of spatter generation can be further improved, and by setting the total amount of Zr oxides in terms of _ZrO2 to an appropriate amount, the bead shape and bead appearance can be further improved.

The reasons for limiting the range of each composition in the coating of the low-hydrogen iron powder covered metal arc welding electrode of the present invention will be explained in detail below. Note that the content of each component in the coating of the low-hydrogen iron powder covered metal arc welding electrode is expressed as mass% relative to the total mass of the coating, and when expressing the mass%, it is simply written as %.

[Covering rate: 30-45% by mass relative to the total mass of the welding rod]
The coverage rate of the coating material on the outer circumference of the steel core wire has a large effect on the shielding resistance during welding. If the coverage rate is less than 30% by mass of the coating material relative to the total mass of the iron powder low hydrogen type covered metal arc welding electrode (hereinafter simply referred to as %), the coating material itself becomes less, resulting in insufficient shielding, and the N content in the weld metal increases, reducing the toughness of the weld metal. On the other hand, if the coverage rate of the coating material exceeds 45%, the amount of slag increases too much and the arc becomes unstable. Therefore, the coverage rate is set to 30 to 45%.

[Mn: 3.0 to 6.0%]
Mn is added from metal Mn, Fe-Mn, Fe-Si-Mn, etc., and is an element necessary as a deoxidizer for the weld metal, and has the effect of refining the weld metal structure and increasing the low-temperature toughness and strength of the weld metal. If Mn is less than 3.0%, the strength and low-temperature toughness of the weld metal decrease. Also, if Mn is less than 3.0%, deoxidation is insufficient and blowholes are likely to occur in the weld metal. On the other hand, if Mn exceeds 6.0%, the strength of the weld metal becomes excessively high and the toughness decreases. Therefore, Mn is set to 3.0 to 6.0%.

[Ni: 3.5-7.5%]
Ni is added from metallic Ni and has the effect of improving the strength and low-temperature toughness of the weld metal. If Ni is less than 3.5%, the strength and low-temperature toughness of the weld metal decrease. On the other hand, if Ni exceeds 7.5%, the strength of the weld metal becomes excessively high. Therefore, Ni is set to 3.5 to 7.5%.

[Mo: 0.3-1.3%]
Mo is added from metal Mo, Fe-Mo, etc., and has the effect of further improving the strength of the weld metal. If Mo is less than 0.3%, the strength of the weld metal decreases. On the other hand, if Mo exceeds 1.3%, the strength of the weld metal becomes excessively high and the toughness decreases. Therefore, Mo is set to 0.3 to 1.3%.

[Cr: 0.2-1.0%]
Cr is added from metallic Cr, Fe-Cr, etc., and has the effect of further improving the strength of the weld metal. If Cr is less than 0.2%, the strength of the weld metal decreases. On the other hand, if Cr exceeds 1.0%, the strength of the weld metal becomes excessively high and the toughness decreases. Therefore, Cr is set to 0.2-1.0%.

[Si: 2.0 to 5.0%]
Silicon is added from metal silicon, Fe-Si, Fe-Si-Mn, etc., and is used for the purpose of deoxidizing the weld metal. If the silicon content is less than 2.0%, deoxidation is insufficient, making blowholes more likely to occur in the weld metal and making the arc unstable. On the other hand, if the silicon content exceeds 5.0%, low-melting-point oxides are precipitated at the grain boundaries of the weld metal, reducing the low-temperature toughness of the weld metal. Therefore, the silicon content is set to 2.0-5.0%.

[Total of Si oxides converted into _SiO2 : 3.5 to 7.5%]
Silicon oxide is added from silica sand, zircon sand, potassium feldspar, solid components of water glass such as sodium silicate and potassium silicate, wollastonite, etc., and has the effect of increasing the viscosity of the molten slag, ensuring slag with appropriate viscosity, and improving the bead shape. If the _SiO2 equivalent value of silicon oxide is less than 3.5%, the viscosity of the molten slag will be low and the bead shape will be poor. On the other hand, if the _SiO2 equivalent value of silicon oxide exceeds 7.5%, the slag will become glassy and the slag removability will be poor. Therefore, the _SiO2 equivalent value of silicon oxide is set to 3.5 to 7.5%.

[Ti: 0.2 to 1.5%]
Ti is added from metallic Ti, Fe-Ti, etc., and is effective as a deoxidizer, and at the same time, has the effect of lowering the potential gradient of the arc and stabilizing the arc. In addition, it has the effect of refining the weld metal structure and improving the low-temperature toughness of the weld metal. If Ti is less than 0.2%, this effect cannot be obtained and the arc becomes unstable. Also, if Ti is less than 0.2%, the amount of oxygen in the weld metal increases and the microstructure of the weld metal is not refined, so that the low-temperature toughness of the weld metal decreases. On the other hand, if Ti exceeds 1.5%, the precipitation of Ti oxide in the weld metal increases, and the low-temperature toughness of the weld metal decreases. Therefore, Ti is set to 0.2 to 1.5%.

[Total of Ti oxides converted into _TiO2 : 1.0 to 5.0%]
Ti oxide is added from rutile, titanium oxide, titanium slag, calcium titanate, etc., and has the effect of stabilizing the arc and adjusting the viscosity of the molten slag to improve the bead shape. If the _TiO2 equivalent of Ti oxide is less than 1.0%, the arc becomes unstable and the bead shape becomes poor. On the other hand, if the _TiO2 equivalent of Ti oxide exceeds 5.0%, the viscosity of the molten slag increases and the fluidity of the slag deteriorates, resulting in a convex bead shape. Therefore, the _TiO2 equivalent of Ti oxide is set to 1.0 to 5.0%.

[One or more metal carbonates in total: 25-45%]
Metal carbonates are added from calcium carbonate, magnesium carbonate, barium carbonate, etc., and decompose with the heat of the arc to generate _CO2 gas, which has the effect of protecting the weld metal from the atmosphere. If the total of one or more metal carbonates is less than 25%, the shielding effect is insufficient and blowholes are likely to occur. If the total of one or more metal carbonates is less than 25%, nitrogen from the atmosphere is mixed into the weld metal, reducing low-temperature toughness. On the other hand, if the total of one or more metal carbonates exceeds 45%, the arc becomes unstable, the bead shape becomes convex, and slag removability is also poor. Therefore, the total of one or more metal carbonates is set to 25-45%.

[Metal fluorides (one or more): 5-15%]
Metal fluorides are added from fluorite, magnesium fluoride, aluminum fluoride, lithium fluoride, sodium fluoride, potassium silicofluoride, etc., and have the effect of adjusting the fluidity of the molten slag and improving the bead appearance. If the total content of one or more metal fluorides is less than 5%, the fluidity of the molten slag is poor, the slag encapsulation is poor, and the bead appearance is poor. On the other hand, if the total content of one or more metal fluorides exceeds 15%, the shape of the covering tube becomes incomplete, resulting in a one-sided melting state and an unstable arc. Therefore, the total content of one or more metal fluorides is set to 5-15%.

[Total of Al oxide converted into Al ₂ O ₃ : 2.0% or less]
Aluminum oxide is added from alumina, potassium feldspar, etc., and has the effect of stabilizing the arc and improving the bead shape. However, if the total content of aluminum oxide ( _{Al2O3 )} exceeds 2.0%, the slag becomes glassy and the slag removability becomes poor. Therefore, the total content of aluminum oxide ( _{Al2O3 )} is set to 2.0% or less.

[Total of Mg oxides converted into MgO: 0.8% or less]
Mg oxide is added from magnesium oxide, magnesia clinker, etc., and has excellent heat resistance and is effective in suppressing the one-sided melting of the coating material. However, if the total of Mg oxide converted into MgO exceeds 0.8%, the viscosity of the molten slag increases, and the bead shape becomes convex. Therefore, the total of Mg oxide converted into MgO is set to 0.8% or less.

[Total of one or more of Na oxide, Na fluoride, K oxide and K fluoride: 1.5 to 3.0% in terms of Na and K]
Na is added from Na oxides such as sodium silicate and other solid components of water glass, and Na fluorides such as sodium fluoride, and K is added from potassium silicate and other solid components of water glass, K oxides such as potassium feldspar, and K fluorides such as potassium silicofluoride, and these have the effect of improving the paintability during the manufacture of the welding rod and the stability of the arc during welding. If the total of one or more of Na oxides, Na fluorides, K oxides, and K fluorides is less than 1.5% in terms of Na and K, the arc becomes unstable. If the total of Na and K is less than 1.5%, the paintability during production is poor, and cracks are likely to occur on the coating surface during the manufacture of the welding rod, thereby reducing the productivity of the covered arc welding rod. On the other hand, if the total of Na and K exceeds 3.0%, the arc blow becomes strong and the amount of spatter generation increases. Therefore, the total content of one or more of Na oxide, Na fluoride, K oxide and K fluoride is 1.5 to 3.0% in terms of Na and K. The Na and K values include 0%.

[Mn, Ni, Mo and Cr are 150 to 300 in formula (1)]
2.3 × [Mn] + 38.5 × [Ni] + 4.2 × [Mo] + 2.1 × [Cr] ... formula (1) (where [ ] indicates the mass% of each component with respect to the total mass of the coating agent)

By optimizing the values obtained by formula (1) for the above-mentioned Mn, Ni, Mo, and Cr, good bending performance of the welded joint can be obtained. If the value of formula (1) is less than 150, good bending performance of the welded joint cannot be obtained. On the other hand, if the value of formula (1) exceeds 300, the austenitic structure after welding becomes excessive, and bending performance deteriorates. Therefore, the value of formula (1) is set to 150 to 300.

[Fe:20-30%]
Fe is added from iron powder, iron alloy powder such as Fe-Mn and Fe-Mo, or iron oxide, and has the effect of reducing the potential gradient of the arc, shortening the arc length, and preventing one-sided melting of the coating material, and is the most important raw material, especially in welding using a DC power source. If Fe is less than 20%, the arc length becomes long and one-sided melting of the coating material is likely to occur. On the other hand, if Fe exceeds 30%, the thermal conductivity becomes high, and in welding with a covered metal arc welding rod, the covered metal arc welding rod itself becomes red hot (hereinafter referred to as rod burn) in the latter half of welding, making welding difficult. Therefore, Fe is set to 20-30%.

[Total of Ca oxides converted into CaO: 0.5% or less]
Ca oxide is added from calcium titanate, wollastonite, etc., and is effective in stabilizing the arc and reducing spatter generation. If the total of Ca oxide converted into CaO exceeds 0.5%, the arc becomes weak and unstable, and welding defects such as poor fusion are likely to occur. Therefore, the total of Ca oxide converted into CaO is set to 0.5%.

[Total of Zr oxide converted into _ZrO2 : 1.5% or less]
Zr oxide is added from zircon sand, zirconia, etc., and has a high melting point of 2700°C, and has stable fire resistance even when the coating material and steel core wire are overheated, and is effective in suppressing one-sided melting of the coating material. If the total of Zr oxide converted into _ZrO2 exceeds 1.5%, the viscosity of the molten slag increases, the fluidity of the slag deteriorates, and the bead shape becomes convex. Therefore, the total of Zr oxide converted into _ZrO2 is set to 1.5% or less.

The remainder of the coating material in the iron powder low hydrogen covered metal arc welding electrode of the present invention is impurities contained in the paint and alloy powder. Hectorite, mica, etc. are used as the paint material. Impurities contained in the alloy powder include P, S, Cu, Nb, V, B, etc., and P and S in particular both form compounds with low melting points that reduce the toughness of the weld metal, so the total amount of impurities is adjusted to 1.0% or less.

The steel core wire used is preferably JIS G3523:1980 SWY11, but does not have to be SWY11. The C content of the steel core wire is preferably 0.08% or less, and the C content can be appropriately adjusted from the coating material to adjust the strength. P in the steel core wire reduces toughness, so it is preferable that it is 0.010% or less, and S reduces the fluidity of the slag, so it is preferable that it is 0.010% or less.

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

A JIS G3523:1980 SWY11 steel core wire (C: 0.08%, Si: 0.02%, Mn: 0.46%, P: 0.009%, S: 0.006%) having a diameter of 4.0 mm and a length of 400 mm was coated with a coating agent having the composition shown in Table 1 at the coverage shown in Tables 1 and 2, and then dried to produce various iron powder low hydrogen type covered metal arc welding rods. In this case, the composition of the coating agent was investigated using the ICP optical emission spectrometry method described in JIS K0116 :2014. The coating agent of the prototype welding rod was peeled off, recovered, and pulverized to obtain a powder sample. The powder sample was decomposed using an acid decomposition method or an alkali fusion method. A part of the solution was sprayed and introduced into the argon plasma of an ICP optical emission spectrometry device, and the emission intensity of the analytical line of the quantitative component or the ratio of the emission intensity of the analytical line of the quantitative component to the emission intensity of the internal standard element was measured. The concentration of the element to be measured in the sample solution was determined from the obtained emission intensity using the calibration curve method. X-ray diffraction analysis was also used for quantitative analysis of metals and oxides, such as Si and _SiO2 . For example, the amount of Si in the coating agent was specified using ICP emission spectroscopy, and the metal Si and oxide Si were measured using the same coating agent by X-ray diffraction analysis to obtain the quantitative analysis values for each. In addition, in order to improve the analytical accuracy, it is desirable to crush the coating agent into fine particles of 10 μm or less using an agate mortar or the like. The procedure for filling the sample was to place a glass sample plate on a flat glass plate with a size of 10 cm square and a thickness of about 5 mm, hold it down with a glass plate in a groove with a depth of about 0.5 mm, finish it so that the surface of the sample plate and the sample surface are on the same plane, and perform the analysis with an X-ray diffraction analysis device.

Using the above various prototype welding rods, weld metal test specimens were prepared by forming a 20 mm thick steel plate with the composition shown in Table 3 into a backed groove with a groove angle of 20° and a root gap of 16 mm, and using a DC power source with a welding current of 170 A, welding heat input of 19 kJ/cm, and preheat/interpass temperature of 95-120°C. Furthermore, a 20 mm thick steel plate with the composition shown in Table 3 was formed into a butt V-groove with a groove angle of 60°C, root face: 2 mm, and root gap: 2.5 mm, and welded and back-ground using a DC power source with a welding current of 80-140 A, welding heat input of 24 kJ/cm, and preheat/interpass temperature of 95-120°C, and welded under the same conditions to prepare welded joints.

Welding workability was evaluated using each prototype welding rod, and arc stability, amount of spatter generation, bead shape and appearance, slag removability, and the presence or absence of one-sided melting and rod burn were visually inspected during the above welding. After welding was completed, an X-ray transmission test was performed in accordance with AWS A5.5 to check for the presence or absence of welding defects. Tensile test pieces (AWS B4.0:2007) and V-notch impact test pieces (AWS B4.0:2007) were taken from the center of the plate thickness of the weld metal test pieces. In the tensile test, a tensile strength of 810 to 920 MPa was considered good, and toughness was evaluated by performing a Charpy impact test at a test temperature of -50°C, and an average of 40 J or more for the three out of five absorbed energy tests, excluding the lowest and highest values, was considered good. To evaluate bending performance, bending test pieces (AWS B4.0:2007) were taken from the welded joints, and a total crack length of 3.2 mm or less was considered to be pass, while a total crack length of more than 3.2 mm was considered to be fail. The test results are summarized in Table 5.

[Welding workability]
(Arc stability)
If the arc was stable during welding and did not disappear, it was rated as good, and if the arc disappeared even once, it was rated as poor.

(Amount of spatter generated)
It is preferable that the amount of spatter generated during welding is small. Specifically, the weight of spatter generated during one minute of welding was measured using a copper collection box to determine the value per unit time (g/min). The spatter was measured five times under the welding conditions shown in Table 4, and the average value was determined, with 2.0 g/min or less being considered good.

(Bead shape and appearance)
It is preferable that the bead waveform of the deposited metal is uniform and free of disturbance, and that there is no undercut or overlap that requires rework. It is preferable that the deposited metal has a bead shape with excellent uniformity in the excess height and bead width. Specifically, a case where the bead waveform of the deposited metal is disturbed or where undercut or overlap that requires rework occurs on the bead surface of the deposited metal is determined as defective.

(Slag removability)
It is preferable that the solidified slag on the weld bead surface can be easily removed after welding. After welding, the solidified slag on the weld bead surface was struck with a chipping hammer (total length 300 mm, weight 350 g) by swinging it down lightly in an arc around the handle. If the slag cracked and could be easily removed, it was rated as good, and if the slag did not crack, it was rated as poor.

(Partial dissolution of coating material)
It is preferable that the arc is generated parallel to the welding rod without any chipping of the coating material during welding, and that the arc spread is uniform and stable. If part of the coating material is chipped during welding, the arc is deflected in a direction other than parallel to the welding rod, and the arc spread is non-uniform, it is rated as defective.

(stick burn)
It is preferable that the coating does not fall off during welding because the welding rod becomes red hot. A welding rod that does not change color during welding is rated as good, and a welding rod that becomes red hot and turns red is rated as bad.

[Welding defects]
Specifically, an X-ray examination was performed in accordance with AWS A5.5 to judge whether there were any defects in the weld metal. A specimen without any defects was rated as good, and a specimen with even one defect was rated as poor.

Welding rods No. 1 to No. 13 in Tables 1, 2 and 5 are examples of the present invention, and welding rods No. 14 to No. 28 are comparative examples. In Example 13, the coating agent had appropriate amounts of Mn, Ni, Mo, Cr, Si, Si oxide in _SiO2 equivalent, Ti, Ti oxide in _TiO2 equivalent, metal carbonate, metal fluoride, total Al oxide in _Al2O3 equivalent, total Mg oxide in MgO equivalent, Na oxide, Na fluoride, total Na and K values of one _or more of K oxide and K fluoride, Fe, and coverage, so that the arc condition was good, the amount of spatter generation was small, the condition of the protective tube was good, no rod burn occurred, the bead appearance, bead shape, slag removability and slag encapsulation were good, and so the welding workability was good, the productivity was good, there were no welding defects, and the tensile strength and absorbed energy of the weld metal were good. Since the value of the following formula (1) was appropriate, the bending performance of the weld metal was also good, and the results were extremely satisfactory.

2.3 × [Mn] + 38.5 × [Ni] + 4.2 × [Mo] + 2.1 × [Cr] ... formula (1)
Here, the brackets [ ] indicate the mass % of each component relative to the total mass of the coating agent.

In addition, since No. 6, No. 10, and No. 11 have an appropriate total CaO equivalent value of Ca oxide, the arc condition was better than the other inventive examples, and there was less spatter.

In addition, since No. 1, No. 3, and No. 12 have an appropriate total amount of Zr oxide converted into _ZrO2 , the one-sided dissolution of the coating material was suppressed more than in the other invention examples.

Among the comparative examples, welding rod No. 14 had a high Mn content and a low Si content, so the tensile strength of the deposited metal was high, the absorbed energy was low, the arc was unstable, and blowholes occurred.

Welding rod No. 15 had a high coverage, low Mn, and low _SiO2 equivalent value of silicon oxide, so the deposited metal had low tensile strength, low absorbed energy, poor bead shape, unstable arc, and blowholes.

Welding rod No. 16 had a large amount of Ni, a large total amount of Al oxide _in terms of _Al2O3 , and a large total amount of Na and K, so the tensile strength of the weld metal was high, the slag removability was poor, the arc was strong, and there was a lot of spatter. Furthermore, the value of the following formula (1) was large, so the bending test of the weld metal was unsuccessful.

2.3 × [Mn] + 38.5 × [Ni] + 4.2 × [Mo] + 2.1 × [Cr] ... formula (1)
Here, the brackets [ ] indicate the mass % of each component relative to the total mass of the coating agent.

Welding rod No. 17 had a low Ni content and a high total metal carbonate content, so the tensile strength and absorbed energy of the weld metal were low, the slag removability was poor, the bead shape was convex, and the arc was unstable. Furthermore, because the value of the following formula (1) was small, the weld metal failed the bending test.

2.3 × [Mn] + 38.5 × [Ni] + 4.2 × [Mo] + 2.1 × [Cr] ... formula (1)
Here, the brackets [ ] indicate the mass % of each component relative to the total mass of the coating agent.

Welding rod No. 18 contains a lot of Mo and a large amount of metal fluorides in total, so the tensile strength of the weld metal is high, the absorbed energy is low, the arc is unstable, and one-sided melting occurs.

Welding rod No. 19 had a low Mo content and a low total metal fluoride content, so the tensile strength of the deposited metal was low, the slag encapsulation was poor, and the bead appearance was poor.

Welding rod No. 20 had a large amount of Cr and a small amount of Ti oxide converted into _TiO2 , so the tensile strength of the weld metal was high, the absorbed energy was low, the bead shape was poor, and the arc was unstable.

Welding rod No. 21 had a low Cr content and a low total Na and K content, so the tensile strength of the deposited metal was low, the arc was unstable, and productivity was poor.

Welding rod No. 22 has a high Si and Fe content, so it absorbed less energy and rod burn occurred.

Welding rod No. 23 had a small amount of Ti, a large amount of Ti oxide converted into _TiO2 , and a small amount of Fe, so that the absorbed energy was low, the bead shape was convex, the arc was unstable, and one-sided melting occurred.

Welding rod No. 24 had a large amount of Si oxide in terms of _SiO2 and a large amount of Ti, so the absorbed energy was low and the slag removability was poor. Furthermore, Ca oxide was added, but the total amount of Ca oxide in terms of CaO was high, so the arc was unstable and poor fusion occurred.

Welding rod No. 25 had a low total amount of metal carbonates, so the absorbed energy was low and blowholes occurred. Furthermore, Zr oxide was added, but the total amount of Zr oxide converted to _ZrO2 was high, so the bead shape was convex.

Welding rod No. 26 had a low coverage rate and a high total MgO equivalent value of MgO oxide, so the absorbed energy was low and the bead shape was convex.

Claims (3)

In a low-hydrogen iron powder covered metal arc welding electrode for multi-layer welding using a DC power source for 780 MPa class high tensile steel, the steel core wire is coated with a coating material,
The coverage of the coating material is 30 to 45% by mass relative to the total mass of the welding rod;
The composition of the coating agent is expressed as a mass % based on the total mass of the coating agent,
Mn: 3.0 to 6.0%,
Ni: 3.5-7.5%,
Mo: 0.3-1.3%,
Cr: 0.2-1.0%,
Si: 2.0 to 5.0%,
Total of Si oxides converted into _SiO2 : 3.5 to 7.5%,
Ti: 0.2 to 1.5%,
Total of Ti oxides converted into _TiO2 : 1.0 to 5.0%,
Total of one or more metal carbonates: 25-45%;
One or more metal fluorides in total: 5 to 15%;
The total value of Al _oxide calculated as _Al2O3 : 2.0% or less,
The total amount of magnesium oxides converted into MgO is 0.8% or less.
One or more of Na oxide, Na fluoride, K oxide, and K fluoride in total: 1.5 to 3.0% in terms of Na and K,
and the Mn, Ni, Mo and Cr are 150 to 300 in formula (1);
Further, Fe: 20 to 30% is contained,
1. An iron powder low hydrogen type covered metal arc welding electrode, characterized in that it is coated with a coating material having a total impurity balance of 1.0% or less.
2.3 × [Mn] + 38.5 × [Ni] + 4.2 × [Mo] + 2.1 × [Cr] ... formula (1)
Here, [ ] indicates the mass % of each component relative to the total mass of the coating agent. The coating agent is, in mass % based on the total mass of the coating agent,
2. The low hydrogen iron powder covered metal arc welding electrode according to claim 1, characterized in that the total amount of Ca oxides calculated as CaO is 0.5% or less. The coating agent is, in mass % based on the total mass of the coating agent,
3. The low hydrogen iron powder covered metal arc welding electrode according to claim 1, wherein the total amount of Zr oxide calculated as _ZrO2 is 1.5% or less.