Abstract:
The purpose of the present invention is, in gas shielded arc welding using an active gas and flux-cored wire with primer-coated steel plates as a base material, to suppress occurrences of poor appearance in welded parts caused by pores originating in the primer. A welding method for a welded article positions a first steel plate and a second steel plate formed from primer-coated steel plates in a T shape and welds each of corner parts formed in two locations. Using flux-cored wire ( 100 ), which is formed by filling the inside of a steel outside skin with flux, and a shielding gas (carbon dioxide gas), a welding pool ( 400 ) is formed in the corner part by supplying a welding current to the corner part via an arc from the flux-cored wire ( 100 ) by means of the shielding gas; additionally, an alternating magnetic field is applied to the welding pool, and the welding current and the alternating magnetic field are set such that the welding current (A) and the magnetic flux density (mT) for the alternating magnetic field have a relationship of 20000≦welding current×magnetic flux density≦30000.

Description:
TECHNICAL FIELD 
       [0001]    The present invention relates to a method of fabricating a weldment, a welding method, and a welding apparatus. 
       BACKGROUND ART 
       [0002]    Welding operation using a flux-cored wire is one of keys in a manufacturing process in a field of manufacturing ships and bridges. In such a field, gas shielded arc welding (mainly CO2 gas shielded arc welding) is used (see PTL 1), in which a shield gas mainly containing carbon dioxide gas is supplied from a welding torch, a flux-cored wire is fed into the welding torch while a welding current is supplied to the flux-cored wire, and arc is generated in the shield gas between the flux-cored wire and a base plate to perform welding. 
         [0003]    As is well known, in a field of manufacturing automobiles, gas shielded arc welding (mainly mixed-gas shielded arc welding) is also used, in which a shield gas mainly containing inert gas such as argon gas is supplied from a welding torch, a solid wire including no flux is fed into the welding torch while a welding current is supplied to the solid wire, and arc is generated between the solid wire and a base plate in the shield gas to perform welding. To decrease pores formed during welding of a galvanized steel sheet for use in automobile components, PTL 2 describes that arc welding is performed while the molten pool is stirred by applying a rectangular-wave AC magnetic field, having a magnetic flux density 3 to 8 MT, a duty ratio 30 to 70%, and a frequency 5 to 30 Hz, perpendicularly to a surface of a molten pool from a magnetic coil attached to a tip of a welding torch. 
       CITATION LIST 
     Patent Literature 
       [0004]    PTL 1: Japanese Unexamined Patent Application Publication No. 2006-95550. 
         [0005]    PTL 2: Japanese Unexamined Patent Application Publication No. 2007-98459. 
       SUMMARY OF INVENTION 
     Technical Problem 
       [0006]    In the field of manufacturing ships and bridges, a primer-coated steel plate is often used as a base plate to be welded. At least the front and the back of the primer-coated steel plate is subjected to surface treatment using a shop primer (primer), i.e., primary rust preventive paint, in order to suppress rust occurrence during a machining step such as cutting and welding and an assembly step. 
         [0007]    However, in a fillet weld joint that accounts for the most of weld joints in such a field, a primer applied on a surface of a steel plate is evaporated during welding and enters the molten pool, so that pores such as pits and blowholes are easily formed in the surface and the inside of a weld metal (weld bead). The term “pit” refers to a pore opened in a weld bead surface, and the term “blowhole” refers to a pore confirmed in the inside of the weld metal. If a pit is formed in the surface of the weld bead, the weld bead must be repaired, resulting in an increase in the number of steps. 
         [0008]    A base plate to be welded and thickness thereof, a shield gas to be used, a type of a welding wire to be used, a welding condition, and the like are each different between the field of manufacturing ships and bridges and the field of manufacturing automobiles. Hence, the above-described problem cannot be solved only by applying an AC magnetic field in the carbon dioxide gas shielded arc welding used in the field of manufacturing ships and bridges. 
         [0009]    To specifically describe, for example, in the field of manufacturing automobiles, since lap fillet welding of a thin plate (about 3.2 mm or less in thickness) is performed, a molten pool has a small size, and can be easily rotated by a small magnetic flux density (3 to 8 MT) as described in PTL 2. In addition, as described above, the mixed-gas shielded arc welding, which uses a solid wire and a mixed gas mainly containing inert gas such as argon gas, is performed in such a field. Hence, an extremely small amount of slug is formed on the molten pool, and constraint of the molten pool due to slug as described later is not necessary to be considered. 
         [0010]    On the other hand, for example, in the field of shipbuilding and bridges, a medium or thick plate having a thickness of 6 mm or more is often used, and a molten pool in fillet welding is likely to have a large size. It is therefore necessary to exert the optimum Lorentz force on a molten pool depending on size of the molten pool in order to rotationally stir the molten pool as a whole in welding with a primer steel plate as a base plate. If the Lorentz force does not satisfy the optimum condition, the rotational flow of the molten pool is merely disturbed. Thus, occurrence of pores cannot be suppressed by directly using the technique described in PTL 2. 
         [0011]    In the field of shipbuilding and bridges, an extremely beautiful bead shape is required in horizontal fillet welding of a T joint; hence, CO 2  gas shielded arc welding with a flux-cored wire is used to fix the bead shape thanks to bearing of a molten pool by slug. However, the welding with the flux-cored wire is different from the welding with a solid wire in that the surface of the molten pool is covered with the high-viscosity slug. Hence, simple application of a magnetic field may not result in sufficient stirring of molten metal because the molten metal is constrained by slug, and thus the pore suppression effect may not have been provided. 
         [0012]    An object of the invention is to suppress occurrence of poor appearance of a weld due to pores originating in a primer in the gas shielded arc welding using active gas and a flux-cored wire and using a primer-coated steel plate as a base plate. 
       Solution to Problem 
       [0013]    According to the present invention, there is provided a method of fabricating a weldment through welding of corners between a lower plate and a vertical plate, the lower plate being composed of a primer-coated steel plate, the vertical plate being composed of a steel plate and placed vertically on the lower plate, in which a molten pool is formed in each of the corners by supplying a welding current from a flux-cored wire to the corner via arc with the shield gas, the flux-cored wire including a steel sheath having a flux-filled inside, the shield gas mainly containing carbon dioxide gas, and an alternating magnetic field is applied to the molten pool, the welding current (A) and the magnetic flux density (mT) of the alternating magnetic field having a relationship 
         [0000]      20000≦welding current×magnetic flux density≦30000.
 
         [0014]    In the method of fabricating the weldment, the alternating magnetic field has a fundamental frequency of 2 to 5 Hz. The composition of the flux-cored wire has, relative to the total mass of the wire, a total Ti equivalent of metal Ti, Ti oxide, and Ti compounds: 1.5 to 3.5 mass %, a total Si equivalent of metal Si, Si oxide, and Si compounds: 0.6 to 2.0 mass %, a total Al equivalent of metal Al, Al oxide, and Al compounds: 0.2 to 1.0 mass %, a total Zr equivalent of metal Zr, Zr oxide, and Zr compounds: 0.6 to 1.0 mass %, and a total Mg equivalent of metal Mg, Mg oxide, and Mg compounds: 0.2 to 0.8 mass %. 
         [0015]    Furthermore, according to the invention, there is provided a method of welding a corner between primer steel plates or between a primer steel plate and another steel plate using a flux-cored wire and a shield gas, the flux-cored wire including a steel sheath having a flux-filled inside, the shield gas mainly containing carbon dioxide gas, in which a molten pool is formed in the corner by supplying a welding current from the flux-cored wire to the corner via arc with the shield gas, and an alternating magnetic field is applied to the molten pool, the welding current (A) and the magnetic flux density (mT) of the alternating magnetic field having a relationship 
         [0000]      20000≦welding current×magnetic flux density≦30000.
 
         [0016]    In the welding method, the alternating magnetic field has a fundamental frequency of 2 to 5 Hz. The composition of the flux-cored wire has, relative to the total mass of the wire, a total Ti equivalent of metal Ti, Ti oxide, and Ti compounds: 1.5 to 3.5 mass %, a total Si equivalent of metal Si, Si oxide, and Si compounds: 0.6 to 2.0 mass %, a total Al equivalent of metal Al, Al oxide, and Al compounds: 0.2 to 1.0 mass %, a total Zr equivalent of metal Zr, Zr oxide, and Zr compounds: 0.6 to 1.0 mass %, and a total Mg equivalent of metal Mg, Mg oxide, and Mg compounds: 0.2 to 0.8 mass %. 
         [0017]    Furthermore, according to the invention, there is provided a welding apparatus: including a shield gas supply unit that supplies a shield gas mainly containing carbon dioxide gas to the periphery of a flux-cored wire including a steel sheath having a flux-filled inside; a welding current supply unit that uses the flux-cored wire and the shield gas, and supplies a welding current from the flux-cored wire to a corner formed by a plurality of base plates including a primer-coated steel plate via arc with the shield gas; an alternating-magnetic-field application unit that applies an alternating magnetic field to the molten pool formed in the corner along with the supply of the welding current; and a control unit that controls the welding current supply unit and the alternating-magnetic-field application unit such that the welding current (A) and the magnetic flux density (mT) of the alternating magnetic field satisfy a relationship 
         [0000]      20000≦welding current×magnetic flux density≦30000.
 
         [0018]    In the welding apparatus, the alternating-magnetic-field application unit sets a fundamental frequency of the alternating magnetic field to 2 to 5 Hz. 
       Advantageous Effects of Invention 
       [0019]    According to the invention, it is possible to suppress occurrence of poor appearance of a weld due to pores originating in a primer in the gas shielded arc welding using active gas and a flux-cored wire and using a primer-coated steel plate as a base plate. 
     
    
     
       BRIEF DESCRIPTION OF DRAWINGS 
         [0020]      FIG. 1  illustrates a schematic configuration of a welding apparatus according to one embodiment of the invention. 
           [0021]      FIG. 2  is a sectional view for explaining a configuration of a welding torch provided in the welding apparatus. 
           [0022]      FIG. 3  is a block diagram illustrating a configuration of a control section provided in the welding apparatus. 
           [0023]      FIG. 4  is a diagram for explaining an exemplary configuration of a work (weldment) fabricated using the welding app aratus. 
           [0024]      FIG. 5  is a schematic view illustrating a relationship between the welding torch as well as a flux-cored wire and a molten pool formed in the work in a manufacturing method (welding method) of the embodiment. 
           [0025]      FIG. 6  is a graph with a horizontal axis as a product of welding current and magnetic flux density and a vertical axis as the number of blowholes of 3 mm or more in length in a bead. 
           [0026]      FIG. 7   a  is a diagram for explaining pores formed in a first weld (bead) in a work. 
           [0027]      FIG. 7   b  is a diagram for explaining pores formed in the first weld (bead) in a work. 
           [0028]      FIG. 7   c  is a diagram for explaining pores formed in the first weld (bead) in a work. 
           [0029]      FIG. 8  is a graph with a horizontal axis as a frequency of a coil current and a vertical axis as the number of blowholes of 3 mm or more in length in a bead. 
           [0030]      FIG. 9  illustrates a fracture of the first weld for the frequency of the coil current of 0.5 Hz. 
           [0031]      FIG. 10   a  is a diagram for explaining a fracture of one of beads provided in examples and comparative examples. 
           [0032]      FIG. 10   b  is a diagram for explaining a fracture of one of beads provided in the examples and the comparative examples. 
           [0033]      FIG. 10   c  is a diagram for explaining a fracture of one of beads provided in the examples and the comparative examples. 
           [0034]      FIG. 10   d  is a diagram for explaining a fracture of one of beads provided in the examples and the comparative examples. 
           [0035]      FIG. 10   e  is a diagram for explaining a fracture of one of beads provided in the examples and the comparative examples. 
       
    
    
     DESCRIPTION OF EMBODIMENTS 
       [0036]    Hereinafter, one embodiment of the invention will be described in detail with reference to the accompanying drawings.  FIG. 1  illustrates a schematic configuration of a welding apparatus  1  according to one embodiment of the invention. The welding apparatus  1  welds a work  200  by a carbon dioxide gas shielded arc welding technique using carbon dioxide gas as a shield gas among gas shielded arc welding techniques of a consumable electrode type. 
         [0037]    The welding apparatus  1  illustrated in  FIG. 1  includes a welding torch  10  for welding the work  200  using a flux-cored wire  100  (see  FIG. 2  described later), a welding source  20  that supplies a welding current to the welding torch  10 , a wire feeder  30  that sequentially feeds the flux-cored wire  100  to the welding torch  10 , a shield gas supply device  40  that supplies carbon dioxide gas as a shield gas to the welding torch  10 , and a magnetic-field application source  50  that supplies a coil current (described in detail later) for generation of an AC magnetic field to the welding torch  10 . 
         [0038]    In this embodiment, a combination of the shield gas supply device  40  and the welding torch  10  functions as a shield gas supply unit, a combination of the welding source  20  and the welding torch  10  functions as a welding current supply unit, and a combination of the magnetic-field application source  50  and the welding torch  10  functions as an alternating-magnetic-field application unit. 
         [0039]      FIG. 2  is a sectional view for explaining a configuration of the welding torch  10  provided in the welding apparatus  1  illustrated in  FIG. 1 . The welding torch  10  illustrated in  FIG. 2  includes a torch body  11 , a nozzle  12 , a tip base  13 , a contact tip  14 , a support  15 , a coil  17 , and a coil holder  18 . 
         [0040]    The nozzle  12  has a cylindrical shape, and is fitted in an opening side of the torch body  11  on the lower side in the drawing, and thereby fixed to the torch body  11 . The nozzle  12  is provided to jet carbon dioxide gas supplied from the shield gas supply device  40  (see  FIG. 1 ) to the work  200  (see  FIG. 1 ). 
         [0041]    The tip base  13  is composed of a conductor and has a cylindrical shape, and is disposed inside the torch body  11  and the nozzle  12  while being in contact with the inner circumferential face of the torch body  11 , and thereby fixed to the torch body  11 . A plurality of gas supply ports  13   a,  which each penetrate through the side face of the tip base  13 , are provided in a region of the tip base  13 , the region being opposed to the inner circumferential face of the nozzle  12 . 
         [0042]    The contact tip  14  is composed of a conductor and has a cylindrical shape, and is fitted in an opening side of the tip base  13  on the lower side in the drawing, and thereby fixed to the torch body  11  via the tip base  13  in the inside of the nozzle  12 . The contact tip  14  is detachably fitted in the tip base  13 , and thus if the contact tip  14  is consumed with long-term use, the contact tip  14  can be exchanged. 
         [0043]    The support  15  has a cylindrical shape, and is fitted in the tip base  13  projecting above the torch body  11  from the opening of the torch body  11  on the upper side in the drawing, and thereby fixed to the torch body  11  via the tip base  13 . An undepicted substrate is provided on the illustratively upper side of the support  15 , and bears the support  15 . 
         [0044]    The coil  17  is composed of a wire made of metal (for example, copper), and is wound on an outer side of the outer periphery of the nozzle  12 . The coil  17  is connected to a conducting wire so as to receive power from the magnetic-field application source  50  (see  FIG. 1 ). 
         [0045]    The coil holder  18  is composed of an insulator or a material covered with an insulator, and has a ring shape. The coil holder  18  is fixed to the torch body  11  via the nozzle  12  outside the outer periphery of the nozzle  12 , and accommodates the coil  17  therein. 
         [0046]    The welding torch  10  of this embodiment has a supply path for supplying the flux-cored wire  100  from the upper side to the lower side in the drawing through the support  15 , the tip base  13 , and the contact tip  14 . The inner diameter of a first part of the supply path provided inside the contact tip  14  is slightly larger than the diameter of the flux-cored wire  100 , and thus the flux-cored wire  100  passes through the supply path while being in contact with the contact tip  14 . On the other hand, the inner diameter of a second part of the supply path provided inside the support  15  and the tip base  13  is larger than the inner diameter of the first supply path part provided inside the contact tip  14 , and thus carbon dioxide gas is supplied from the upper side in the drawing to the inside of the nozzle  12  through a gap provided between the second supply path part and the flux-cored wire  100  and through the gas supply ports  13   a  provided in the tip base  13 . 
         [0047]    The welding torch  10  of this embodiment is designed such that power is supplied from the welding source  20  (see  FIG. 1 ) to the tip base  13  and in turn supplied from the tip base  13  to the flux-cored wire  100  via the contact tip  14 . 
         [0048]    The flux-cored wire  100  used in the welding apparatus  1  is now described. The flux-cored wire  100  of this embodiment includes a cylindrical steel sheath of which the inside is filled with flux described below. 
         [0049]    To describe more specifically, the flux-cored wire  100  of this embodiment has, relative to the total mass of the wire, a total Ti equivalent of metal Ti, Ti oxide, and Ti compounds: 1.5 to 3.5 mass %, a total Si equivalent of metal Si, Si oxide, and Si compounds: 0.6 to 2.0 mass %, a total Al equivalent of metal Al, Al oxide, and Al compounds: 0.2 to 1.0 mass %, a total Zr equivalent of metal Zr, Zr oxide, and Zr compounds: 0.6 to 1.0 mass %, and a total Mg equivalent of metal Mg, Mg oxide, and Mg compounds: 0.2 to 0.8 mass %, the remainder consisting of Fe and inevitable impurities. 
         [0050]    There are now described the reason for adding each component to the flux-cored wire  100  of this embodiment and the reason for limiting the composition of the flux-cored wire  100 . 
       Ti Equivalent: 1.5 to 3.5 Mass % 
       [0051]    TiO 2  increases viscosity of slug. Metal Ti and Ti compounds in the wire are each decomposed beneath the arc into ions to be combined with oxygen, leading to an effect equivalent to that of TiO 2 . The Ti equivalent of 1.5 mass % or more leads to good arc stability and good slag coverage during welding. The Ti equivalent of 3.5 mass % or more leads to high viscosity of slug, which reduces the effect of magnetically stirring the molten pool (as described later in detail). Hence, the content of Ti in the wire is preferably 1.5 to 3.5 mass % in the Ti equivalent. 
       Si Equivalent: 0.6 to 2.0 Mass % 
       [0052]    SiO 2  increases viscosity of slug, and lowers solidification temperature of slug. Metal Si and Si compounds in the wire are each decomposed beneath the arc into ions to be combined with oxygen, leading to an effect equivalent to that of SiO 2 . The Si equivalent of 0.6 mass % or more leads to good slag coverage. The Si equivalent of 2.0 mass % or more leads to high viscosity of slug, which reduces the effect of magnetically stirring the molten pool. Hence, the content of Si in the wire is preferably 0.6 to 2.0 mass % in the Si equivalent. 
       Al Equivalent: 0.2 to 1.0 Mass %  
       [0053]    Al 2 O 3  increases viscosity of slug, and lowers solidification temperature of slug as with SiO 2 . Metal Al and Al compounds in the wire are each decomposed beneath the arc into ions to be combined with oxygen, leading to an effect equivalent to that of Al 2 O 3 . The Al equivalent of 0.2 mass % or more leads to good slag coverage. The Al equivalent of 1.0 mass % or more leads to high viscosity of slug, which reduces the effect of magnetically stirring the molten pool. Hence, the content of Al in the wire is preferably 0.2 to 1.0 mass % in the Al equivalent. 
       Zr Equivalent: 0.6 to 1.0 Mass % 
       [0054]    ZrO 2  decreases viscosity of slug, and raises solidification temperature of slug. Metal Zr and Zr compounds in the wire are each decomposed beneath the arc into ions to be combined with oxygen, leading to an effect equivalent to that of ZrO 2 . The Zr equivalent of 0.6 mass % or more leads to low viscosity of slug and high slag fluidity, allowing a molten pool to be easily stirred by magnetic force. The Zr equivalent of 1.0 mass % or more leads to an excessive amount of slug, which reduces the effect of magnetically stirring the molten pool. Hence, the content of Zr in the wire is preferably 0.6 to 1.0 mass % in the Zr equivalent. 
       Mg Equivalent: 0.2 to 0.8 Mass % 
       [0055]    MgO decreases viscosity of slug, and raises solidification temperature of slug as with ZrO 2 . Metal Mg and Mg compounds in the wire are each decomposed beneath the arc into ions to be combined with oxygen, leading to an effect equivalent to that of MgO. The Mg equivalent of 0.2 mass % or more leads to low viscosity of slug and high slag fluidity, allowing a molten pool to be easily stirred by magnetic force. The Mg equivalent of 0.8 mass % or more leads to an excessive amount of slug, which reduces the effect of magnetically stirring the molten pool. Hence, the content of Mg in the wire is preferably 0.2 to 0.8 mass % in the Mg equivalent. 
         [0056]    The filling rate of flux in the flux-cored wire  100  (mass of flux relative to the total mass of the wire) is preferably, but not limited to, 10 to 25 mass % as with a typical flux-cored wire for horizontal fillet welding. 
       Remainder: Fe and Inevitable Impurities 
       [0057]    The remainder of the composition of the flux-cored wire  100  as a whole consists of Fe and inevitable impurities. In addition to the above-described wire components, the wire composition may contain, in the flux, a small amount of elements such as Ca and Li as a fine adjuster for deoxidation or the like, and a small amount of elements such as Cu, Co, and N as an additional hardener for weld metal. Such elements have no influence on the object of the invention. The flux further contains a small amount of alkali metal compounds including metal elements other than the above-described elements. Examples of the inevitable impurities may include C, B, Ni, Mo, Cr, Nb, and V in the contents of C: less than 0.1 mass %, B: less than 0.0003 mass %, Ni: less than 0.1 mass %, Mo: less than 0.01 mass %, Cr: less than 0.30 mass %, Nb: less than 0.10 mass %, and V: less than 0.10 mass %. The inevitable impurities however are not limited to such components and/or numerical values. 
       Others 
       [0058]    Examples of a method of manufacturing the flux-cored wire  100  include a method in which flux is spread in a longitudinal direction of a steel hoop, and then the steel hoop is formed into a circular section in a wrapping manner and drawn, and a method in which flux is filled in the inside of a large-diameter steel tube, and then the steel tube is drawn. However, the flux-cored wire  100  may be manufactured by any of the methods because each method has no influence on the invention. Furthermore, the flux-cored wire  100  may be of a seam type or a seamless type. Although the composition of the sheath is not necessary to be defined, mild steel material is typically used for the sheath in light of cost and drawability. The surface of the flux-cored wire  100  may be, but not necessarily, subjected to Cu plating. 
         [0059]    A control system for the welding apparatus  1  illustrated in 
         [0060]      FIG. 1  is now described.  FIG. 3  is a block diagram for explaining a configuration of a control section  70  that is provided in the welding source  20  illustrated in  FIG. 1 , and controls operation of each section as a component of the welding apparatus  1 . The control section  70  as an exemplary control unit includes a setting reception section  71  that receives various types of setting sent from an undepicted setting device (such as a computer device), a welding current setting section  72  that sets a magnitude of a welding current, which is supplied from the welding source  20  to the flux-cored wire  100  via the welding torch  10 , based on the setting received by the setting reception section  71 , and a coil current setting section  73  that sets a magnitude of a coil current, which is supplied from the magnetic-field application source  50  to the coil  17  provided in the welding torch  10 , based on the magnitude of the welding current set by the welding current setting section  72 . The control section  70  further includes a feed speed setting section  74  that sets feed speed of the flux-cored wire  100  to be fed from the wire feeder  30  to the welding torch  10  based on the setting received by the setting reception section  71 . 
         [0061]    The welding current setting section  72  sets a DC current value as the welding current. The coil current setting section  73  sets an AC current value as the coil current. A specific technique for setting each of the welding current and the coil current is described later. 
         [0062]      FIG. 4  is a diagram for explaining an exemplary configuration of the work  200  (weldment) fabricated using the welding apparatus  1  illustrated in  FIG. 1 . The work  200  illustrated in  FIG. 4  is a T joint, in which an end face of a first steel plate  201  as an exemplary vertical plate is placed on a surface of a second steel plate  202  as an exemplary lower plate, thereby such two base plates (the first steel plate  201  and the second steel plate  202 ) to be welded form a T-shape. In addition, the work  200  is a fillet joint having a first weld  301  and a second weld  302 , which are formed by performing horizontal fillet welding by the welding apparatus  1  on the first steel plate  201  and the second steel plate  202  disposed in the T shape at (two) corners between two planes intersecting at a substantially right angle of the steel plates. 
         [0063]    This embodiment employs a primer-coated steel plate as each of the first steel plate  201  and the second steel plate  202 . For the primer-coated steel plate, however, its end face may not be subjected to surface treatment using a shop primer. Examples of the shop primer used for the primer-coated steel plate include a nonorganic zinc primer, a wash primer, a zinc rich primer, and a non-zinc primer. A weldment fabrication method and a welding method described below may be applied to all the primer-coated steel plates. 
         [0064]    Each of the first steel plate  201  and the second steel plate  202  has a thickness of 6 mm or more. Such a steel plate having a thickness of 6 mm or more is called medium or thick plate, which is widely used in the field of manufacturing ships and bridges. 
         [0065]    A method of manufacturing the work  200  (method of welding the first steel plate  201  and the second steel plate  202 ) using the welding apparatus  1  of this embodiment is now described with reference to  FIGS. 1 to 4 . The first steel plate  201  and the second steel plate  202  are disposed so as to form a T-shape as illustrated in  FIG. 4  before start of the welding. 
         [0066]    First, the wire feeder  30  starts feed of the flux-cored wire  100  to the welding torch  10 , and the shield gas supply device  40  starts supply of carbon dioxide gas to the welding torch  10 . In addition, the welding source  20  starts supply of a voltage (welding voltage) to the welding torch  10  (flux-cored wire  100 ), and the magnetic-field application source  50  starts supply of a coil current to the welding torch  10  (coil  17 ). 
         [0067]    Subsequently, arc is generated between the flux-cored wire  100  and the work  200  to start welding. In addition, the tip of the flux-cored wire  100  is melted by the arc and moves toward the work  200 . In the work  200 , a target portion in each of the first steel plate  201  and the second steel plate  202  is also melted by the arc. As a result, such melted materials mixedly form a molten pool in a region of the work  200 , the region being opposed to the tip of the flux-cored wire  100 . In addition, slug come from the flux surfaces from the molten pool and covers the molten pool. The welding torch  10  is traveled from one end to the other end along a boundary, thereby the molten pool and the slug are sequentially formed along the boundary. 
         [0068]    However, while the tip of the flux-cored wire  100  passes through the molten pool along with the travel of the welding torch  10 , the molten pool is gradually less heated as it is more distant from the arc. Consequently, the molten pool is then gradually solidified as it is gradually cooled. This embodiment employs the flux-cored wire  100 . Hence, the molten pool being cooled is gradually shifted into a state, in which the solidified weld metal is covered with the slug including solidified nonmetallic substances. The weld metal corresponds to each of the first weld  301  and the second weld  302 . 
         [0069]    In this way, the first weld  301  and the second weld  302  are formed. Subsequently, the slug covering each of the first weld  301  and the second weld  302  is removed, thereby the work  200  illustrated in  FIG. 4  is produced. 
         [0070]      FIG. 5  is a schematic view illustrating a relationship between the welding torch  10  as well as the flux-cored wire  100  and a molten pool  400  formed in the undepicted work  200  in the manufacturing method (welding method) of this embodiment.  FIG. 5  does not show the slug on the molten pool  400 . 
         [0071]    On the tip side of the flux-cored wire  100  projecting from the welding torch  10 , the flux-cored wire  100  and the undepicted first and second steel plates  201  and  202  are each gradually melted as the welding current is supplied and thus arc is generated, so that the molten pool  400  is formed as described above. In addition, while the DC welding current flows from the flux-cored wire  100  attached to the welding torch  10  to the molten pool  400 , the welding current radially flows along a plane direction in the molten pool  400  (as shown by a broken line in  FIG. 5 ). 
         [0072]    In this embodiment, an AC coil current is supplied to the coil  17  provided in the welding torch  10 . The coil  17  accordingly generates an alternating magnetic field in a direction substantially perpendicular to the surface of the molten pool  400  (as shown by dash-dot line arrows in  FIG. 5 ). The alternating magnetic field causes Lorentz force that is exerted on the welding current radially spreading in the molten pool  400 . As a result, molten metal forming the molten pool  400  receives rotational force in each of forward and reverse directions as shown by empty arrows in  FIG. 5 , and the molten metal repeats forward and reverse rotations in a period corresponding to the frequency of the coil current. 
         [0073]    This embodiment employs the primer-coated steel plate as each of the first steel plate  201  and the second steel plate  202 . Hence, the shop primer applied on each of the first steel plate  201  and the second steel plate  202  (particularly the second steel plate  202 ) evaporates and enters the molten pool  400  during welding, and pores called pits or blowholes are easily formed in the surface and the inside of the solidified weld metal (the first weld  301  or the second weld  302 ). The term “pit” refers to a pore opened in the surface of a bead composed of weld metal. The term “blowhole” refers to a pore confirmed in the inside of weld metal. Such a pore originating in a gas (primer gas) come from the shop primer easily becomes large compared with a typical pore due to a defect such as insufficient shield of a shield gas. 
         [0074]    Such a large blowhole in the inside of a bead may lower tensile strength and fatigue strength of a weld. Such a large pit exposed on a bead surface requires repair after welding, leading to an increase in the number of steps. 
         [0075]    In this embodiment, the work  200  is fabricated such that the control section  70  (specifically a combination of the welding current setting section  72  and the coil current setting section  73 ) sets the welding current I and the coil current as a source of the magnetic flux density B in conjunction with each other such that the product of the welding current I and the magnetic flux density B satisfies 20000≦I×B≦30000, and preferably 20000≦I×B≦27000, where I represents the magnitude (average) of the welding current (A), and B represents the magnitude (effective value) of the magnetic flux density caused by the coil current (mT). The magnetic flux density B is defined by a value (effective value) measured with a gaussmeter (teslameter) at a position of the tip of the flux-cored wire  100  extending from the welding torch  10  (extension of the flux-cored wire  100 : a position 25 mm away from the welding torch  10  in this exemplary case). 
         [0076]    When the coil  17  is disposed around the flux-cored wire  100 , and when a magnetic field is applied substantially perpendicularly to the surface of the molten pool  400 , Lorentz force is exerted on the welding current radially spreading in the molten pool  400 , and the molten metal forming the molten pool  400  rotates. Using the alternating magnetic field periodically inverts the magnetic field, so that a convection direction of the molten metal is also periodically inverted. Consequently pores are less likely to grow along one direction. Hence, the primer gas evaporated beneath the arc is less likely to enter the close molten pool  400 . Alternatively, even if the primer gas enters such a molten pool  400 , the primer gas is less likely to grow into pores. The primer gas therefore comes around the front side of the molten pool  400 , the front side being not covered with the molten metal, and is discharged to the outside air. Hence, only small-diameter blowholes remain in the inside of a bead produced by welding, and a greatly grown blowhole, which will become a pit on the surface of the bead, is less likely to be formed. In this way, it is possible to suppress occurrence of poor appearance of a bead surface due to pores originating in the primer. In other words, it is possible to produce a beautiful bead shape. 
         [0077]    When the magnetic-field application source  50  supplies a DC current as the coil current, the molten metal continuously rotates in one direction. In such a case, a formation direction of the weld bead is biased to one side, and the primer gas entering the molten pool  400  grows pores in one direction along its rotational direction, which reduces the effect of suppressing growth of pores. To suppress pores, therefore, it is important to use an AC current as the coil current to periodically invert the convection direction of the molten metal. 
         [0078]    While any of waveforms such as a sine wave, a rectangular wave, and a chopping wave may be used as the waveform of the coil current (AC current) supplied to the coil  17 , the frequency (fundamental frequency) of the coil current is preferably set to 2 to 5 Hz. If the frequency f of the coil current is too low, the weld bead meanders in accordance with the rotational direction of the molten pool  400 , and thus beautiful bead appearance may not be obtained. If the frequency f of the coil current is too high, it is difficult to invert the convection direction of the molten pool  400  even if the magnetic field is inverted, and thus a sufficient stirring effect may not be obtained. 
         [0079]    In this embodiment, since the flux-cored wire  100  is used for welding, a large amount of high-viscosity slug is formed over the molten pool  400 . Hence, if an alternating magnetic field is simply applied by the coil  17 , the molten metal forming the molten pool  400  is constrained by the slug and is not sufficiently stirred, and thus the pore suppression effect is less likely to be provided. It is therefore important to lower the viscosity of each of the slug and the molten metal at high temperature. To lower the viscosity of the molten pool  400  at high temperature under the alternating magnetic field, it is preferred that among the components of the flux configuring the flux-cored wire  100 , Ti, Si, Al, Zr, and Mg are appropriately mixed so as to satisfy the above-described ranges relative to the total mass of the wire. 
         [0080]    Since formation tendency of the pores caused by the evaporated primer typically increases with welding speed, welding speed is controlled to be relatively low at the point of production, which is a large factor in disturbing improvement in production efficiency. The pores are therefore decreased by setting each of the welding current I and the magnetic flux density B to be within the above-described range. As a result, welding speed can be increased. 
       Examples 
       [0081]    Hereinafter, the invention is described further in detail according to Examples. However, the invention is not limited to the Examples within the scope without departing from the gist of the invention. 
         [0082]    The basic welding condition was as follows.
       First steel plate  201  and second steel plate  202 : JIS G3106 SM490A inorganic zinc primer-coated steel plate (coating thickness 30 μm), 12 mm thick, 75 mm wide, and 475 mm long.   Inclination angle of welding torch  10 : 45°.   Extension of flux-cored wire  100  from welding torch  10 : 25 mm.   Target position: Root.   Diameter of flux-cored wire  100 : 1.4 mm.       
 
       [Relationship Between Welding Current and Magnetic Flux Density] 
       [0088]    First, the inventors have made investigations on a relationship between the welding current I (average) supplied to the flux-cored wire  100  for formation of the molten pool  400 , and the magnetic flux density B (effective value) of the alternating magnetic field supplied to the molten pool  400 . Tables 1 to 4 show setting conditions in such investigations and obtained results. 
         [0000]    
       
         
               
               
               
               
               
               
               
               
               
               
             
               
               
               
               
               
               
               
               
               
               
             
           
               
                 TABLE 1 
               
               
                   
               
               
                   
                   
                   
                   
                   
                   
                   
                   
                   
                 The number 
               
               
                   
                   
                 Magnetic 
                 Feed 
                 Welding 
                 Welding 
                 Welding 
                 Leg 
                   
                 of BH 
               
               
                   
                 Frequency f 
                 flux B 
                 speed 
                 speed 
                 current I 
                 voltage 
                 length 
                 I × B 
                 (3 mm 
               
               
                 Number 
                 (Hz) 
                 (mT) 
                 (m/min) 
                 (cm/min) 
                 (A) 
                 (V) 
                 (mm) 
                 (A · mT) 
                 or more) 
               
               
                   
               
             
             
               
                   
               
             
          
           
               
                 1 
                 — 
                 0 
                 11.4 
                 60 
                 330 
                 33.3 
                 6.5 
                 0 
                 54 
               
               
                 2 
                 3 
                 28 
                 11.4 
                 60 
                 330 
                 33.3 
                 6.5 
                 9240 
                 53 
               
               
                 3 
                 3 
                 42 
                 11.4 
                 60 
                 330 
                 33.3 
                 6.5 
                 13860 
                 43 
               
               
                 4 
                 3 
                 58 
                 11.4 
                 60 
                 330 
                 33.3 
                 6.5 
                 19140 
                 13 
               
               
                 5 
                 3 
                 64 
                 11.4 
                 60 
                 330 
                 33.3 
                 6.5 
                 21120 
                 6 
               
               
                 6 
                 3 
                 68 
                 11.4 
                 60 
                 330 
                 33.3 
                 6.5 
                 22440 
                 8 
               
               
                 7 
                 — 
                 0 
                 11.4 
                 60 
                 330 
                 33.3 
                 6.5 
                 0 
                 56 
               
               
                 8 
                 3 
                 26 
                 11.4 
                 60 
                 330 
                 33.3 
                 6.5 
                 8700 
                 51 
               
               
                 9 
                 3 
                 43 
                 11.4 
                 60 
                 330 
                 33.3 
                 6.5 
                 14200 
                 47 
               
               
                 10 
                 3 
                 58 
                 11.4 
                 60 
                 330 
                 33.3 
                 6.5 
                 19300 
                 14 
               
               
                 11 
                 3 
                 63 
                 11.4 
                 60 
                 330 
                 33.3 
                 6.5 
                 20700 
                 8 
               
               
                 12 
                 3 
                 70 
                 11.4 
                 60 
                 330 
                 33.3 
                 6.5 
                 23100 
                 7 
               
               
                 13 
                 — 
                 0 
                 11.4 
                 60 
                 330 
                 33.3 
                 6.5 
                 0 
                 58 
               
               
                 14 
                 3 
                 27 
                 11.4 
                 60 
                 330 
                 33.3 
                 6.5 
                 8800 
                 53 
               
               
                 15 
                 3 
                 42 
                 11.4 
                 60 
                 330 
                 33.3 
                 6.5 
                 13900 
                 50 
               
               
                 16 
                 3 
                 56 
                 11.4 
                 60 
                 330 
                 33.3 
                 6.5 
                 18400 
                 30 
               
               
                 17 
                 3 
                 62 
                 11.4 
                 60 
                 330 
                 33.3 
                 6.5 
                 20400 
                 9 
               
               
                 18 
                 3 
                 72 
                 11.4 
                 60 
                 330 
                 33.3 
                 6.5 
                 23600 
                 5 
               
               
                   
               
             
          
         
       
     
         [0000]    
       
         
               
               
               
               
               
               
               
               
               
               
             
               
               
               
               
               
               
               
               
               
               
             
           
               
                 TABLE 2 
               
               
                   
               
               
                   
                   
                   
                   
                   
                   
                   
                   
                   
                 The number 
               
               
                   
                   
                 Magnetic 
                 Feed 
                 Welding 
                 Welding 
                 Welding 
                 Leg 
                   
                 of BH 
               
               
                   
                 Frequency f 
                 flux B 
                 speed 
                 speed 
                 current I 
                 voltage 
                 length 
                 I × B 
                 (3 mm 
               
               
                 Number 
                 (Hz) 
                 (mT) 
                 (m/min) 
                 (cm/min) 
                 (A) 
                 (V) 
                 (mm) 
                 (A · mT) 
                 or more) 
               
               
                   
               
             
             
               
                   
               
             
          
           
               
                 19 
                 — 
                 0 
                 15.2 
                 80 
                 390 
                 37.7 
                 6.5 
                 0 
                 26 
               
               
                 20 
                 3 
                 28 
                 15.2 
                 80 
                 390 
                 37.7 
                 6.5 
                 10920 
                 25 
               
               
                 21 
                 3 
                 42 
                 15.2 
                 80 
                 390 
                 37.7 
                 6.5 
                 16380 
                 13 
               
               
                 22 
                 3 
                 58 
                 15.2 
                 80 
                 390 
                 37.7 
                 6.5 
                 22620 
                 5 
               
               
                 23 
                 3 
                 64 
                 15.2 
                 80 
                 390 
                 37.7 
                 6.5 
                 24960 
                 0 
               
               
                 24 
                 3 
                 68 
                 15.2 
                 80 
                 390 
                 37.7 
                 6.5 
                 26520 
                 0 
               
               
                 25 
                 — 
                 0 
                 15.2 
                 80 
                 390 
                 37.7 
                 6.5 
                 0 
                 24 
               
               
                 26 
                 3 
                 24 
                 15.2 
                 80 
                 390 
                 37.7 
                 6.5 
                 9500 
                 26 
               
               
                 27 
                 3 
                 42 
                 15.2 
                 80 
                 390 
                 37.7 
                 6.5 
                 16500 
                 12 
               
               
                 28 
                 3 
                 57 
                 15.2 
                 80 
                 390 
                 37.7 
                 6.5 
                 22300 
                 6 
               
               
                 29 
                 3 
                 64 
                 15.2 
                 80 
                 390 
                 37.7 
                 6.5 
                 25000 
                 0 
               
               
                 30 
                 3 
                 69 
                 15.2 
                 80 
                 390 
                 37.7 
                 6.5 
                 27000 
                 0 
               
               
                 31 
                 — 
                 0 
                 15.2 
                 80 
                 390 
                 37.7 
                 6.5 
                 0 
                 22 
               
               
                 32 
                 3 
                 25 
                 15.2 
                 80 
                 390 
                 37.7 
                 6.5 
                 9700 
                 27 
               
               
                 33 
                 3 
                 44 
                 15.2 
                 80 
                 390 
                 37.7 
                 6.5 
                 17200 
                 13 
               
               
                 34 
                 3 
                 57 
                 15.2 
                 80 
                 390 
                 37.7 
                 6.5 
                 22100 
                 5 
               
               
                 35 
                 3 
                 64 
                 15.2 
                 80 
                 390 
                 37.7 
                 6.5 
                 24800 
                 0 
               
               
                 36 
                 3 
                 71 
                 15.2 
                 80 
                 390 
                 37.7 
                 6.5 
                 27500 
                 0 
               
               
                   
               
             
          
         
       
     
         [0000]    
       
         
               
               
               
               
               
               
               
               
               
               
             
               
               
               
               
               
               
               
               
               
               
             
           
               
                 TABLE 3 
               
               
                   
               
               
                   
                   
                   
                   
                   
                   
                   
                   
                   
                 The number 
               
               
                   
                   
                 Magnetic 
                 Feed 
                 Welding 
                 Welding 
                 Welding 
                 Leg 
                   
                 of BH 
               
               
                   
                 Frequency f 
                 flux B 
                 speed 
                 speed 
                 current I 
                 voltage 
                 length 
                 I × B 
                 (3 mm 
               
               
                 Number 
                 (Hz) 
                 (mT) 
                 (m/min) 
                 (cm/min) 
                 (A) 
                 (V) 
                 (mm) 
                 (A · mT) 
                 or more) 
               
               
                   
               
             
             
               
                   
               
             
          
           
               
                 37 
                 — 
                 0 
                 18.0 
                 95 
                 440 
                 40.6 
                 6.5 
                 0 
                 27 
               
               
                 38 
                 3 
                 28 
                 18.0 
                 95 
                 440 
                 40.6 
                 6.5 
                 12320 
                 28 
               
               
                 39 
                 3 
                 42 
                 18.0 
                 95 
                 440 
                 40.6 
                 6.5 
                 18480 
                 28 
               
               
                 40 
                 3 
                 58 
                 18.0 
                 95 
                 440 
                 40.6 
                 6.5 
                 25520 
                 7 
               
               
                 41 
                 3 
                 64 
                 18.0 
                 95 
                 440 
                 40.6 
                 6.5 
                 28160 
                 8 
               
               
                 42 
                 3 
                 68 
                 18.0 
                 95 
                 440 
                 40.6 
                 6.5 
                 29920 
                 19 
               
               
                 43 
                 — 
                 0 
                 18.0 
                 95 
                 440 
                 40.6 
                 6.5 
                 0 
                 33 
               
               
                 44 
                 3 
                 30 
                 18.0 
                 95 
                 440 
                 40.6 
                 6.5 
                 13000 
                 29 
               
               
                 45 
                 3 
                 43 
                 18.0 
                 95 
                 440 
                 40.6 
                 6.5 
                 19100 
                 25 
               
               
                 46 
                 3 
                 59 
                 18.0 
                 95 
                 440 
                 40.6 
                 6.5 
                 26000 
                 8 
               
               
                 47 
                 3 
                 63 
                 18.0 
                 95 
                 440 
                 40.6 
                 6.5 
                 27600 
                 13 
               
               
                 48 
                 3 
                 73 
                 18.0 
                 95 
                 440 
                 40.6 
                 6.5 
                 32200 
                 24 
               
               
                 49 
                 — 
                 0 
                 18.0 
                 95 
                 440 
                 40.6 
                 6.5 
                 0 
                 34 
               
               
                 50 
                 3 
                 31 
                 18.0 
                 95 
                 440 
                 40.6 
                 6.5 
                 13500 
                 23 
               
               
                 51 
                 3 
                 43 
                 18.0 
                 95 
                 440 
                 40.6 
                 6.5 
                 18700 
                 21 
               
               
                 52 
                 3 
                 58 
                 18.0 
                 95 
                 440 
                 40.6 
                 6.5 
                 25400 
                 6 
               
               
                 53 
                 3 
                 63 
                 18.0 
                 95 
                 440 
                 40.6 
                 6.5 
                 27900 
                 15 
               
               
                 54 
                 3 
                 75 
                 18.0 
                 95 
                 440 
                 40.6 
                 6.5 
                 33000 
                 29 
               
               
                 55 
                 3 
                 48 
                 18.0 
                 95 
                 440 
                 40.6 
                 6.5 
                 21000 
                 8 
               
               
                 56 
                 3 
                 50 
                 18.0 
                 95 
                 440 
                 40.6 
                 6.5 
                 22100 
                 9 
               
               
                 57 
                 3 
                 53 
                 18.0 
                 95 
                 440 
                 40.6 
                 6.5 
                 23500 
                 5 
               
               
                 58 
                 3 
                 55 
                 18.0 
                 95 
                 440 
                 40.6 
                 6.5 
                 24300 
                 5 
               
               
                 59 
                 3 
                 58 
                 18.0 
                 95 
                 440 
                 40.6 
                 6.5 
                 25300 
                 5 
               
               
                 60 
                 3 
                 60 
                 18.0 
                 95 
                 440 
                 40.6 
                 6.5 
                 26600 
                 3 
               
               
                 61 
                 3 
                 62 
                 18.0 
                 95 
                 440 
                 40.6 
                 6.5 
                 27300 
                 4 
               
               
                 62 
                 3 
                 70 
                 18.0 
                 95 
                 440 
                 40.6 
                 6.5 
                 31000 
                 25 
               
               
                 63 
                 3 
                 75 
                 18.0 
                 95 
                 440 
                 40.6 
                 6.5 
                 33000 
                 26 
               
               
                 64 
                 3 
                 73 
                 18.0 
                 95 
                 440 
                 40.6 
                 6.5 
                 32200 
                 32 
               
               
                 65 
                 3 
                 76 
                 18.0 
                 95 
                 440 
                 40.6 
                 6.5 
                 33500 
                 34 
               
               
                 66 
                 3 
                 78 
                 18.0 
                 95 
                 440 
                 40.6 
                 6.5 
                 34200 
                 29 
               
               
                 67 
                 3 
                 78 
                 18.0 
                 95 
                 440 
                 40.6 
                 6.5 
                 34500 
                 28 
               
               
                 68 
                 3 
                 79 
                 18.0 
                 95 
                 440 
                 40.6 
                 6.5 
                 34900 
                 27 
               
               
                 69 
                 3 
                 80 
                 18.0 
                 95 
                 440 
                 40.6 
                 6.5 
                 35100 
                 31 
               
               
                 70 
                 3 
                 85 
                 18.0 
                 95 
                 440 
                 40.6 
                 6.5 
                 37600 
                 33 
               
               
                 71 
                 3 
                 86 
                 18.0 
                 95 
                 440 
                 40.6 
                 6.5 
                 37900 
                 27 
               
               
                 72 
                 3 
                 87 
                 18.0 
                 95 
                 440 
                 40.6 
                 6.5 
                 38200 
                 29 
               
               
                 73 
                 3 
                 89 
                 18.0 
                 95 
                 440 
                 40.6 
                 6.5 
                 39200 
                 34 
               
               
                 74 
                 3 
                 87 
                 18.0 
                 95 
                 440 
                 40.6 
                 6.5 
                 38400 
                 37 
               
               
                 75 
                 3 
                 73 
                 18.0 
                 95 
                 440 
                 40.6 
                 6.5 
                 32100 
                 20 
               
               
                 76 
                 3 
                 71 
                 18.0 
                 95 
                 440 
                 40.6 
                 6.5 
                 31100 
                 19 
               
               
                 77 
                 3 
                 72 
                 18.0 
                 95 
                 440 
                 40.6 
                 6.5 
                 31500 
                 18 
               
               
                 78 
                 3 
                 72 
                 18.0 
                 95 
                 440 
                 40.6 
                 6.5 
                 31600 
                 21 
               
               
                 79 
                 3 
                 70 
                 18.0 
                 95 
                 440 
                 40.6 
                 6.5 
                 30900 
                 22 
               
               
                 80 
                 3 
                 68 
                 18.0 
                 95 
                 440 
                 40.6 
                 6.5 
                 29800 
                 28 
               
               
                   
               
             
          
         
       
     
         [0000]    
       
         
               
               
               
               
               
               
               
               
               
               
             
               
               
               
               
               
               
               
               
               
               
             
           
               
                 TABLE 4 
               
               
                   
               
               
                   
                   
                   
                   
                   
                   
                   
                   
                   
                 The number 
               
               
                   
                   
                 Magnetic 
                 Feed 
                 Welding 
                 Welding 
                 Welding 
                 Leg 
                   
                 of BH 
               
               
                   
                 Frequency f 
                 flux B 
                 speed 
                 speed 
                 current I 
                 voltage 
                 length 
                 I × B 
                 (3 mm 
               
               
                 Number 
                 (Hz) 
                 (mT) 
                 (m/min) 
                 (cm/min) 
                 (A) 
                 (V) 
                 (mm) 
                 (A · mT) 
                 or more) 
               
               
                   
               
             
             
               
                   
               
             
          
           
               
                 81 
                 3 
                 68 
                 12.1 
                 63 
                 345 
                 34.1 
                 6.5 
                 23460 
                 3 
               
               
                 82 
                 3 
                 68 
                 12.7 
                 67 
                 356 
                 34.8 
                 6.5 
                 24208 
                 4 
               
               
                 83 
                 3 
                 68 
                 13.3 
                 70 
                 365 
                 35.7 
                 6.5 
                 24820 
                 8 
               
               
                 84 
                 3 
                 68 
                 13.9 
                 73 
                 375 
                 36.3 
                 6.5 
                 25500 
                 0 
               
               
                 85 
                 3 
                 68 
                 14.5 
                 76 
                 385 
                 36.8 
                 6.5 
                 26180 
                 7 
               
               
                 86 
                 3 
                 58 
                 15.7 
                 82 
                 405 
                 38.0 
                 6.5 
                 23490 
                 4 
               
               
                 87 
                 3 
                 58 
                 16.3 
                 86 
                 418 
                 38.6 
                 6.5 
                 24244 
                 7 
               
               
                 88 
                 3 
                 58 
                 16.9 
                 89 
                 430 
                 39.2 
                 6.5 
                 24940 
                 2 
               
               
                 89 
                 3 
                 58 
                 17.5 
                 92 
                 434 
                 40.1 
                 6.5 
                 25172 
                 6 
               
               
                 90 
                 3 
                 58 
                 18.1 
                 95 
                 440 
                 40.8 
                 6.5 
                 25520 
                 2 
               
               
                 91 
                 3 
                 64 
                 13.1 
                 69 
                 369 
                 35.8 
                 6.5 
                 23600 
                 8 
               
               
                 92 
                 3 
                 64 
                 14.0 
                 73 
                 372 
                 36.1 
                 6.5 
                 23800 
                 7 
               
               
                 93 
                 3 
                 64 
                 14.1 
                 74 
                 373 
                 36.4 
                 6.5 
                 23900 
                 6 
               
               
                 94 
                 3 
                 64 
                 13.8 
                 72 
                 377 
                 36.5 
                 6.5 
                 24100 
                 2 
               
               
                 95 
                 3 
                 64 
                 14.4 
                 76 
                 383 
                 36.8 
                 6.5 
                 24500 
                 0 
               
               
                 96 
                 3 
                 64 
                 15.0 
                 79 
                 388 
                 37.2 
                 6.5 
                 24800 
                 0 
               
               
                 97 
                 3 
                 64 
                 15.2 
                 80 
                 389 
                 37.3 
                 6.5 
                 24900 
                 1 
               
               
                 98 
                 3 
                 64 
                 15.8 
                 83 
                 397 
                 37.8 
                 6.5 
                 25400 
                 1 
               
               
                 99 
                 3 
                 64 
                 15.5 
                 81 
                 392 
                 37.7 
                 6.5 
                 25100 
                 2 
               
               
                 100 
                 3 
                 64 
                 15.4 
                 81 
                 400 
                 37.9 
                 6.5 
                 25600 
                 3 
               
               
                 101 
                 3 
                 60 
                 17.4 
                 91 
                 435 
                 40.2 
                 6.5 
                 26100 
                 0 
               
               
                 102 
                 3 
                 60 
                 17.8 
                 93 
                 437 
                 40.5 
                 6.5 
                 26200 
                 5 
               
               
                 103 
                 3 
                 60 
                 17.9 
                 94 
                 438 
                 40.1 
                 6.5 
                 26300 
                 6 
               
               
                 104 
                 3 
                 60 
                 18.2 
                 96 
                 442 
                 41.0 
                 6.5 
                 26500 
                 4 
               
               
                 105 
                 3 
                 60 
                 18.5 
                 97 
                 453 
                 42.1 
                 6.5 
                 27200 
                 7 
               
               
                 106 
                 3 
                 60 
                 18.9 
                 99 
                 458 
                 42.3 
                 6.5 
                 27500 
                 3 
               
               
                 107 
                 3 
                 60 
                 19.0 
                 100 
                 465 
                 43.1 
                 6.5 
                 27900 
                 2 
               
               
                 108 
                 3 
                 62 
                 18.5 
                 97 
                 452 
                 41.8 
                 6.5 
                 28000 
                 2 
               
               
                 109 
                 3 
                 62 
                 18.9 
                 99 
                 453 
                 41.8 
                 6.5 
                 28100 
                 3 
               
               
                 110 
                 3 
                 62 
                 19.1 
                 100 
                 458 
                 42.3 
                 6.5 
                 28400 
                 4 
               
               
                 111 
                 3 
                 62 
                 12.4 
                 65 
                 342 
                 34.0 
                 6.5 
                 21200 
                 5 
               
               
                 112 
                 3 
                 62 
                 13.4 
                 70 
                 361 
                 35.1 
                 6.5 
                 22400 
                 6 
               
               
                 113 
                 3 
                 62 
                 14.2 
                 75 
                 371 
                 35.9 
                 6.5 
                 23000 
                 2 
               
               
                 114 
                 3 
                 62 
                 15.4 
                 81 
                 389 
                 37.8 
                 6.5 
                 24100 
                 3 
               
               
                 115 
                 3 
                 62 
                 16.2 
                 85 
                 410 
                 38.9 
                 6.5 
                 25400 
                 5 
               
               
                 116 
                 3 
                 62 
                 10.7 
                 56 
                 327 
                 32.7 
                 6.5 
                 20300 
                 2 
               
               
                 117 
                 3 
                 62 
                 11.5 
                 60 
                 346 
                 34.0 
                 6.5 
                 21450 
                 1 
               
               
                 118 
                 3 
                 62 
                 13.1 
                 69 
                 360 
                 34.8 
                 6.5 
                 22300 
                 2 
               
               
                 119 
                 3 
                 62 
                 14.9 
                 78 
                 385 
                 37.1 
                 6.5 
                 23900 
                 6 
               
               
                 120 
                 3 
                 62 
                 15.1 
                 79 
                 389 
                 37.3 
                 6.5 
                 24100 
                 2 
               
               
                   
               
             
          
         
       
     
         [0089]    Tables 1 to 4 each show sample number, frequency f (Hz) of coil current, magnetic flux density B (mT) generated by the coil current, feed speed (m/min) of the flux-cored wire  100 , welding speed (cm/min), welding current I (A) supplied to the flux-cored wire  100 , welding voltage (V) for the welding current I, leg length (mm) of a bead provided by welding, a product (A·mT) of welding current I and magnetic flux density B, and the number (the number of BH) of blowholes (mentioned as BH) of 3 mm or more in length in the bead provided by welding. In Tables 1 to 3, “—” in the column of frequency f represents that the coil current itself is not supplied. The number of blowholes shown in each of Tables 1 to 4 represents a result of measurement in a continuous 400 mm region of a stationary portion (intermediate portion) other than a start portion (a leading edge side of the welding operation) and an end portion (a trailing edge side of the welding operation) in a weld (weld bead) provided by welding operation. The number of blowholes is measured in the same manner in each of Tables 5 and 6 described later. In this investigation, the frequency f of the coil current is fixed to 3 Hz (sine wave) in all samples other than samples 1, 7, 13, 19, 25, 31, 37, 43, and 49, i.e., sample numbers 2 to 6, 8 to 12, 14 to 18, 20 to 24, 26 to 30, 32 to 36, 38 to 42, 44 to 48, and 50 to 120. 
         [0090]    Table 1 (sample numbers 1 to 18) shows a case where the magnetic flux density B is varied while the welding current I is fixed to 330 A, and the welding speed is fixed to 60 cm/min. Table 2 (sample numbers 9 to 36) shows a case where the magnetic flux density B is varied while the welding current I is fixed to 390 A, and the welding speed is fixed to 80 cm/min. Table 3 (sample numbers 37 to 80) shows a case where the magnetic flux density B is varied while the welding current I is fixed to 440 A, and the welding speed is fixed to 95 cm/min. In Tables 1 to 3, the product (I×B) of the welding current I and the magnetic flux density B include products that fall within the range of 20000 to 30000 and products that do not fall within such a range. Table 4 (sample numbers 81 to 120) shows a case where the product (I×B) of the welding current I and the magnetic flux density B is controlled to fall within the range of 20000 to 30000 while the welding current I and the magnetic flux density B are each varied. 
         [0091]      FIG. 6  is a graph generated based on Tables 1 to 4 while a horizontal axis represents the product (A·mT) of the welding current I and the magnetic flux density B, and a vertical axis represents the number of blowholes of 3 mm or more in length in a bead. In  FIG. 6 , samples listed in Tables 1, 2, 3, and 4 are plotted with “⋄”, “Δ”, “x”, and “∘”, respectively. 
         [0092]    Tables 1 to 4 and  FIG. 6  reveal that the number of blowholes that have grown into 3 mm or more in length can be decreased in the range where the product of the welding current I and the magnetic flux density B is 20000 to 30000 compared with the case where the product is less than 20000 or more than 30000. 
         [0093]      FIGS. 7   a  to  7   c  are diagrams for explaining pores formed in the first weld  301  (bead) in the work  200  illustrated in  FIG. 4 .  FIG. 7   a  illustrates longitudinal section views of the first steel plate  201 , the second steel plate  202 , and the first weld  301 .  FIG. 7   b  illustrates a fracture of the first weld  301  for the product of the welding current I and the magnetic flux density B of 20000 to 30000.  FIG. 7   c  illustrates a fracture of the first weld  301  for the product of the welding current I and the magnetic flux density B of less than 20000 or more than 30000. 
         [0094]    When the first steel plate  201  and the second steel plate  202  are formed into a T-fillet joint, as illustrated in  FIG. 7   a , there is a plate lap portion in which an end face of the first steel plate  201  is in contact with the second steel plate  202 . In the plate lap portion, the shop primer evaporated from the second steel plate  202  has its escape cut off and thus enters the first weld  301 , and is formed into a pore extending toward a bead surface of the first weld  301 . 
         [0095]    When welding is performed while the product of the welding current I and the magnetic flux density B is set to be within the range of 20000 to 30000, as illustrated in  FIG. 7   b , pores are less likely to grow, and large expansion of blowholes is suppressed, and formation of pits is also suppressed. 
         [0096]    On the other hand, when welding is performed while the product of the welding current I and the magnetic flux density B is set to be less than 20000 or more than 30000, as illustrated in  FIG. 7   c , pores easily grow, and blowholes are greatly expanded, and some of blowholes arrive at the bead surface and are formed into pits. 
       [Frequency of Coil Current] 
       [0097]    The inventors have made further investigations on the frequency of the alternating magnetic field supplied to the molten pool  400 , i.e., the frequency f of the coil current. Table 5 shows setting conditions in such investigations and obtained results. 
         [0000]    
       
         
               
               
               
               
               
               
               
               
             
               
               
               
               
               
               
               
               
             
           
               
                 TABLE 5 
               
               
                   
               
               
                   
                   
                   
                   
                   
                   
                   
                 The number 
               
               
                   
                   
                 Magnetic 
                 Welding 
                 Welding 
                 Welding 
                   
                 of BH 
               
               
                   
                 Frequency f 
                 flux B 
                 speed 
                 current I 
                 voltage 
                 I × B 
                 (3 mm 
               
               
                 Number 
                 (Hz) 
                 (mT) 
                 (cm/min) 
                 (A) 
                 (V) 
                 (A · mT) 
                 or more) 
               
               
                   
               
             
             
               
                   
               
             
          
           
               
                 121 
                 0 
                 0 
                 60 
                 330 
                 33.3 
                 0 
                 54 
               
               
                 122 
                 0 
                 68 
                 60 
                 330 
                 33.3 
                 22440 
                 42 
               
               
                 123 
                 0 
                 0 
                 80 
                 390 
                 37.7 
                 0 
                 26 
               
               
                 124 
                 0 
                 68 
                 80 
                 390 
                 37.7 
                 26520 
                 35 
               
               
                 125 
                 0 
                 0 
                 95 
                 440 
                 40.6 
                 0 
                 27 
               
               
                 126 
                 0 
                 68 
                 95 
                 440 
                 40.6 
                 29920 
                 24 
               
               
                 127 
                 3 
                 68 
                 80 
                 390 
                 37.7 
                 26520 
                 0 
               
               
                 128 
                 3.5 
                 68 
                 80 
                 390 
                 37.7 
                 26520 
                 3 
               
               
                 129 
                 2.5 
                 68 
                 80 
                 390 
                 37.7 
                 26520 
                 7 
               
               
                 130 
                 2 
                 68 
                 80 
                 390 
                 37.7 
                 26520 
                 15 
               
               
                 131 
                 1.5 
                 68 
                 80 
                 390 
                 37.7 
                 26520 
                 10 
               
               
                 132 
                 4 
                 68 
                 80 
                 390 
                 37.7 
                 26520 
                 5 
               
               
                 133 
                 4.5 
                 68 
                 80 
                 390 
                 37.7 
                 26520 
                 4 
               
               
                 134 
                 5 
                 68 
                 80 
                 390 
                 37.7 
                 26520 
                 8 
               
               
                 135 
                 5.5 
                 68 
                 80 
                 390 
                 37.7 
                 26520 
                 14 
               
               
                 136 
                 6 
                 68 
                 80 
                 390 
                 37.7 
                 26520 
                 8 
               
               
                 137 
                 8 
                 68 
                 80 
                 390 
                 37.7 
                 26520 
                 11 
               
               
                 138 
                 10 
                 68 
                 80 
                 390 
                 37.7 
                 26520 
                 15 
               
               
                 139 
                 1 
                 68 
                 80 
                 390 
                 37.7 
                 26520 
                 21 
               
               
                 140 
                 4 
                 68 
                 80 
                 390 
                 37.7 
                 26520 
                 0 
               
               
                 141 
                 5 
                 68 
                 80 
                 390 
                 37.7 
                 26520 
                 0 
               
               
                 142 
                 2 
                 68 
                 80 
                 390 
                 37.7 
                 26520 
                 0 
               
               
                 143 
                 3 
                 68 
                 80 
                 390 
                 37.7 
                 26520 
                 5 
               
               
                 144 
                 20 
                 68 
                 80 
                 390 
                 37.7 
                 26520 
                 59 
               
               
                 145 
                 15 
                 68 
                 80 
                 390 
                 37.7 
                 26520 
                 29 
               
               
                 146 
                 8 
                 68 
                 80 
                 390 
                 37.7 
                 26520 
                 24 
               
               
                 147 
                 12 
                 68 
                 80 
                 390 
                 37.7 
                 26520 
                 22 
               
               
                 148 
                 7 
                 68 
                 80 
                 390 
                 37.7 
                 26520 
                 15 
               
               
                 149 
                 1 
                 68 
                 80 
                 390 
                 37.7 
                 26520 
                 5 
               
               
                   
               
             
          
         
       
     
         [0098]    Table 5 shows sample number (sample numbers 121 to 149), frequency f (Hz) of coil current, magnetic flux density B (mT) generated by the coil current, welding speed (cm/min), welding current I (A) supplied to the flux-cored wire  100 , welding voltage (V) for the welding current I, leg length (mm) of a bead provided by welding, a product (A·mT) of welding current I and magnetic flux density B, and the number (the number of BH) of blowholes (mentioned as BH) of 3 mm or more in length in the bead provided by welding. In this exemplary case, the frequency f of the coil current is varied within a range of 0 Hz (the coil current is not supplied) to 20 Hz. In Table 5, a case where the column of frequency f shows “0” and the column of magnetic flux density B also shows “0” represents a case where the coil current itself is not supplied. A case where the column of frequency f does not show “0” and the column of magnetic flux density B also does not show “0” represents a case where a DC current is supplied as the coil current, thereby a DC magnetic field is generated. 
         [0099]      FIG. 8  is a graph generated based on Table 5 while a horizontal axis represents the frequency of the coil current, and a vertical axis represents the number of blowholes of 3 mm or more in length in a bead. In  FIG. 8 , the samples listed in Table 5 are plotted by “♦”. 
         [0100]    Table 5 and  FIG. 8  reveal that the number of blowholes that has grown into 3 mm or more in length can be decreased in the range where the frequency f of the coil current is 2 to 5 Hz compared with the case where the frequency f of the coil current is less than 2 Hz or more than 5 Hz. 
         [0101]      FIG. 9  illustrates a fracture of the first weld  301  for the frequency f of the coil current of 0.5 Hz. 
         [0102]    As illustrated in  FIG. 9 , if the frequency f of the coil current is too low, dilution in the first weld  301  (bead) periodically varies in accordance with the frequency f, and a large pore (arrowed in the drawing) grows in response to such a variation. 
       [Composition of Flux-Cored Wire] 
       [0103]    Furthermore, the inventors have made investigations on the composition of the flux-cored wire  100 . Table 6 shows mainly added elements of the flux-cored wire  100  in each of Examples 1 to 10 and comparative examples 1 to 10, and obtained results. 
         [0000]    
       
         
               
               
               
               
               
               
               
               
               
             
               
               
               
               
               
               
               
               
               
             
           
               
                   
                 TABLE 6 
               
               
                   
                   
               
               
                   
                   
                   
                   
                   
                   
                   
                   
                 The 
               
               
                   
                   
                   
                   
                   
                   
                   
                   
                 number 
               
               
                   
                   
                   
                   
                   
                   
                   
                   
                 of BH 
               
               
                   
                   
                   
                   
                   
                   
                   
                   
                 (3 mm 
               
               
                   
                 Number 
                 Ti 
                 Si 
                 Al 
                 Zr 
                 Mg 
                 Bead shape 
                 or more) 
               
               
                   
                   
               
             
             
               
                   
               
             
          
           
               
                 Example 
                 1 
                 1.5 
                 0.9 
                 0.5 
                 0.8 
                 0.4 
                 ◯ 
                 3 
               
               
                   
                 2 
                 3.5 
                 1.2 
                 0.7 
                 0.8 
                 0.6 
                 ◯ 
                 7 
               
               
                   
                 3 
                 2.2 
                 0.6 
                 0.8 
                 0.9 
                 0.6 
                 ◯ 
                 0 
               
               
                   
                 4 
                 2.5 
                 2 
                 0.6 
                 1 
                 0.3 
                 ◯ 
                 6 
               
               
                   
                 5 
                 3 
                 1.4 
                 0.2 
                 0.8 
                 0.7 
                 ◯ 
                 2 
               
               
                   
                 6 
                 1.8 
                 1 
                 1 
                 0.9 
                 0.7 
                 ◯ 
                 6 
               
               
                   
                 7 
                 1.9 
                 1.6 
                 0.6 
                 0.6 
                 0.5 
                 ◯ 
                 4 
               
               
                   
                 8 
                 2.7 
                 1.5 
                 0.3 
                 1 
                 0.6 
                 ◯ 
                 0 
               
               
                   
                 9 
                 3.2 
                 1.8 
                 0.7 
                 0.8 
                 0.2 
                 ◯ 
                 0 
               
               
                   
                 10 
                 3 
                 1.1 
                 0.7 
                 0.8 
                 0.8 
                 ◯ 
                 5 
               
               
                 Comparative 
                 1 
                 1.4 
                 1.1 
                 0.5 
                 0.6 
                 0.3 
                 X 
                 5 
               
               
                 example 
                 2 
                 3.5 
                 1 
                 0.9 
                 0.8 
                 0.4 
                 ◯ 
                 13 
               
               
                   
                 3 
                 2.2 
                 0.5 
                 0.2 
                 1.5 
                 0.7 
                 X 
                 6 
               
               
                   
                 4 
                 2.6 
                 2.2 
                 0.9 
                 0.9 
                 0.6 
                 ◯ 
                 17 
               
               
                   
                 5 
                 2 
                 0.8 
                 0.1 
                 1.2 
                 0.7 
                 X 
                 4 
               
               
                   
                 6 
                 2.9 
                 1.8 
                 1.1 
                 0.9 
                 0.4 
                 ◯ 
                 24 
               
               
                   
                 7 
                 2.3 
                 1.6 
                 0.3 
                 0.5 
                 0.3 
                 X 
                 0 
               
               
                   
                 8 
                 3 
                 2 
                 0.7 
                 1.1 
                 0.5 
                 ◯ 
                 18 
               
               
                   
                 9 
                 2.4 
                 0.9 
                 0.4 
                 0.6 
                 0.1 
                 X 
                 2 
               
               
                   
                 10 
                 3.2 
                 1.2 
                 0.6 
                 0.7 
                 0.9 
                 ◯ 
                 20 
               
               
                   
               
             
          
         
       
     
         [0104]    Table 6 shows numbers of Examples or comparative examples, the content (reduced mass percent in the total wire mass) of each of Ti, Si, Al, Zr, and Mg in the flux-cored wire  100 , a shape of a bead provided by welding, and the number (the number of BH) of blowholes (mentioned as BH) of 3 mm or more in length in the bead provided by welding. In this case, the welding current I was fixed to  380  (A) and the magnetic flux density B was fixed to 68 (mT), thereby the product of the welding current I and the magnetic flux density B was fixed to 25480 (A·mT). The frequency f of the coil current was fixed to 3 Hz. 
         [0105]    In each of the Examples 1 to 10, a good bead shape (∘) was provided by welding, and the number of blowholes was less than 10. 
         [0106]    In the comparative example 1 having a Ti equivalent of 1.4, although the number of blowholes was less than 10, dripping occurred in a bead provided by welding, resulting in a bad bead shape (x). In the comparative example 2 having a Ti equivalent of 3.6, although a good bead shape (∘) was provided by welding, the number of blowholes exceeded 10. 
         [0107]    In the comparative example 3 having a Si equivalent of 0.5, although the number of blowholes was less than 10, overlap occurred in a bead provided by welding, resulting in a bad bead shape (x). In the comparative example 4 having a Si equivalent of 2.2, although a good bead shape (∘) was provided by welding, the number of blowholes exceeded 10. 
         [0108]    In the comparative example 5 having an Al equivalent of 0.1, although the number of blowholes was less than 10, overlap occurred in a bead provided by welding, resulting in a bad bead shape (x). In the comparative example 6 having an Al equivalent of 1.1, although a good bead shape (∘) was provided by welding, the number of blowholes exceeded 10. 
         [0109]    In the comparative example 7 having a Zr equivalent of 0.5, although the number of blowholes was less than 10, dripping occurred in a bead provided by welding, resulting in a bad bead shape (x). In the comparative example 8 having a Zr equivalent of 1.1, although a good bead shape (∘) was provided by welding, the number of blowholes exceeded 10. 
         [0110]    In the comparative example 9 having an Mg equivalent of 0.1, although the number of blowholes was less than 10, dripping occurred in a bead provided by welding, resulting in a bad bead shape (x). In the comparative example 10 having an Mg equivalent of 0.9, although a good bead shape (∘) was provided by welding, the number of blowholes exceeded 10. 
         [0111]    As described above, it is preferred to use the flux-cored wire  100  having, relative to the total mass of the wire, a total Ti equivalent of metal Ti, Ti oxide, and Ti compounds: 1.5 to 3.5 mass %, a total Si equivalent of metal Si, Si oxide, and Si compounds: 0.6 to 2.0 mass %, a total Al equivalent of metal Al, Al oxide, and Al compounds: 0.2 to 1.0 mass %, a total Zr equivalent of metal Zr, Zr oxide, and Zr compounds: 0.6 to 1.0 mass %, and a total Mg equivalent of metal Mg, Mg oxide, and Mg compounds: 0.2 to 0.8 mass %, the remainder consisting of Fe and inevitable impurities. 
         [0112]      FIGS. 10   a  to  10   e  are diagrams for explaining fractures of beads provided in Examples and comparative examples.  FIG. 10   a  illustrates a fracture of a bead provided in the Example  1 ,  FIG. 10   b  illustrates a fracture of a bead provided in the Example  7 ,  FIG. 10   c  illustrates a fracture of a bead provided in the Example  9 ,  FIG. 10   d  illustrates a fracture of a bead provided in the comparative example 4, and  FIG. 10   e  illustrates a fracture of a bead provided in the comparative example 6. 
         [0113]    As described above, to suppress blowhole expansion or pit formation, it is important to control viscosity of the slug caused by the flux-cored wire  100  to be low, and when each component of the flux-cored wire  100  is controlled to be within the above-described range, pores can be reduced while a good bead shape is maintained. 
         [0114]    Although this embodiment has been exemplarily described with a carbon-dioxide-gas shielded arc welding technique employing carbon dioxide gas as the shield gas, available shield gas is not limited to carbon dioxide gas. For example, similar results are also obtained in the case of using a mixed gas including carbon dioxide gas as a main component (50% or more) and an inert gas (for example, argon gas) as an additive. 
         [0115]    Although this embodiment has been described with an exemplary case where the work  200  (weldment) is fabricated using the first steel plate  201  composed of a primer-coated steel plate and the second steel plate  202  composed of a primer-coated steel plate, if the second steel plate  202  is a primer-coated steel plate, the first steel plate  201  may not be a primer-coated steel plate. 
         [0116]    Furthermore, although this embodiment has been described with an exemplary case where a T joint is formed using the first steel plate  201  and the second steel plate  202 , the invention is not limited thereto. For example, the invention is also useful in the case where a lap joint or a corner joint is formed using such steel plates. 
       LIST OF REFERENCE SIGNS 
       [0117]      1  welding apparatus,  10  welding torch,  11  torch body,  12  nozzle,  13  tip base,  13   a  gas supply port,  14  contact tip,  15  support,  17  coil,  18  coil holder,  20  welding source,  30  wire feeder,  40  shield gas supply device,  50  magnetic-field application source,  70  control section,  71  setting reception section,  72  welding current setting section,  73  coil current setting section,  74  feed speed setting section,  100  flux-cored wire,  200  work,  201  first steel plate,  202  second steel plate,  301  first weld,  302  second weld,  400  molten pool, I welding current, B magnetic flux density, f frequency