ARC WELDED JOINT AND ARC WELDING METHOD

Abstract

Provided are an arc welded joint and an arc welding method. The arc welded joint has a slag-coverage area ratio S.sub.RATIO (%) of 15% or less, and a weld bead width ratio W.sub.RATIO (%) of 60% or more. The S.sub.RATIO is calculated by using an equation S.sub.RATIO=100×S.sub.SLAG/S.sub.BEAD. In this equation, an area of a surface of a weld bead formed by performing arc welding on a steel sheet is defined as a weld bead surface area S.sub.BEAD (mm.sup.2) and, of the weld bead surface area S.sub.BEAD, an area of a region covered with slag is defined as a slag surface area S.sub.SLAG (mm.sup.2). The W.sub.RATIO is calculated by using an equation W.sub.RATIO=100×W.sub.MIN/W.sub.MAX from a maximum value W.sub.MAX (mm) and a minimum value W.sub.MIN (mm) of a weld bead width in a direction perpendicular to a welding line of the weld bead.

Claims

1. An arc welded joint having: a slag-coverage area ratio S.sub.RATIO (%) of 15% or less as calculated by using equation (1):
S.sub.RATIO=100×S.sub.SLAG/S.sub.BEAD   (1), where: S.sub.BEAD (mm.sup.2) is a weld bead surface area defined as an area of a surface of a weld bead formed by performing arc welding on a steel sheet, and S.sub.SLAG (mm.sup.2) is a slag surface area defined as an area of a region of the weld bead surface area S.sub.BEAD that is covered with slag; and a weld bead width ratio W.sub.RATIO (%) of 60% or more as calculated by using equation (2):
W.sub.RATIO=100×W.sub.MIN/W.sub.MAX   (2), where: W.sub.MAX (mm) is a maximum value of a weld bead width in a direction perpendicular to a welding line of the weld bead, and W.sub.MIN (mm) is a minimum value of the weld bead width.

2. The arc welded joint according to claim 1, wherein: a cleaning region, in which oxides formed on a surface of the steel sheet have been removed by the arc welding, is adjacent to a weld bead toe, and a minimum value M.sub.MIN (mm) of a distance M (mm) in the direction perpendicular to the welding line between an outer edge of the cleaning region and the weld bead toe is 0.5 mm or more.

3. An arc welding method comprising: transferring a welding current from a welding wire supported by a contact tip to a steel sheet to form an arc welded joint between the steel sheet and another steel member, the arc welded joint having: a slag-coverage area ratio S.sub.RATIO (%) of 15% or less as calculated by using equation (1):
S.sub.RATIO=100×S.sub.SLAG/S.sub.BEAD   (1), where: S.sub.BEAD (mm.sup.2) is a weld bead surface area defined as an area of a surface of a weld bead formed by performing the arc welding on the steel sheet, and S.sub.SLAG (mm.sup.2) is a slag surface area defined as an area of a region of the weld bead surface area S.sub.BEAD that is covered with slag; and a weld bead width ratio W.sub.RATIO (%) of 60% or more as calculated by using equation (2):
W.sub.RATIO=100×W.sub.MIN/W.sub.MAX   (2), where: W.sub.MAX (mm) is a maximum value of a weld bead width in a direction perpendicular to a welding line of the weld bead, and W.sub.MIN (mm) is a minimum value of the weld bead width

4. The arc welding method according to claim 3, wherein: the arc welding is performed with reverse polarity, a cleaning region, in which oxides formed on a surface of the steel sheet are removed during the arc welding due to formation of a cathode spot, is formed so as to be adjacent to a weld bead toe, and a minimum value M.sub.MIN (mm) of a distance M (mm) in the direction perpendicular to the welding line between an outer edge of the cleaning region and the weld bead toe is 0.5 mm or more.

5. The arc welding method according to claim 3, wherein during the arc welding: a short circuit intermittently occurs between the steel sheet and the welding wire, and the short circuit occurs at an average short circuit frequency F.sub.AVE (Hz) of 20 Hz to 300 Hz with a maximum short circuit cycle T.sub.CYC (s) of 1.5 s or less.

6. The arc welding method according to claim 4, wherein during the arc welding: a short circuit intermittently occurs between the steel sheet and the welding wire, and the short circuit occurs at an average short circuit frequency F.sub.AVE (Hz) of 20 Hz to 300 Hz with a maximum short circuit cycle T.sub.CYC (s) of 1.5 s or less.

7. The arc welding method according to claim 3, wherein: the welding current is a pulse current, and X (A.Math.s/m) as calculated by using equation (3) satisfies a relational expression 50≤X≤250:
X=(I.sub.PEAK×t.sub.PEAK/L)+(I.sub.PEAK+I.sub.BASE)×(t.sub.UP+t.sub.DOWN)/(2×L)   (3), where: I.sub.PEAK (A) is a peak current of the pulse current, I.sub.BASE (A) is a base current of the pulse current, t.sub.PEAK (ms) is a peak time of the pulse current, t.sub.UP (ms) is a rise time of the pulse current, t.sub.DOWN (ms) is a fall time of the pulse current, and L (mm) is a distance between the steel sheet and the contact tip.

8. The arc welding method according to claim 4, wherein: the welding current is a pulse current, and X (A.Math.s/m) as calculated by using equation (3) satisfies a relational expression 50≤X≤250:
X=(I.sub.PEAK×t.sub.PEAK/L)+(I.sub.PEAK+I.sub.BASE)×(t.sub.UP+t.sub.DOWN)/(2×L)   (3), where: I.sub.PEAK (A) is a peak current of the pulse current, I.sub.BASE (A) is a base current of the pulse current, t.sub.PEAK (ms) is a peak time of the pulse current, t.sub.UP (ms) is a rise time of the pulse current, t.sub.DOWN (ms) is a fall time of the pulse current, and L (mm) is a distance between the steel sheet and the contact tip.

9. The arc welding method according to claim 5, wherein: the welding current is a pulse current, and X (A.Math.s/m) as calculated by using equation (3) satisfies a relational expression 50≤X≤250:
X=(I.sub.PEAK×t.sub.PEAK/L)+(I.sub.PEAK+I.sub.BASE)×(t.sub.UP+t.sub.DOWN)/(2×L)   (3), where: I.sub.PEAK (A) is a peak current of the pulse current, I.sub.BASE (A) is a base current of the pulse current, t.sub.PEAK (ms) is a peak time of the pulse current, t.sub.UP (ms) is a rise time of the pulse current, t.sub.DOWN (ms) is a fall time of the pulse current, and L (mm) is a distance between the steel sheet and the contact tip.

10. The arc welding method according to claim 6, wherein: the welding current is a pulse current, and X (A.Math.s/m) as calculated by using equation (3) satisfies a relational expression 50≤X≤250:
X=(I.sub.PEAK×t.sub.PEAK/L)+(I.sub.PEAK+I.sub.BASE)×(t.sub.UP+t.sub.DOWN)/(2×L)   (3), where: I.sub.PEAK (A) is a peak current of the pulse current, I.sub.BASE (A) is a base current of the pulse current, t.sub.PEAK (ms) is a peak time of the pulse current, t.sub.UP (ms) is a rise time of the pulse current, t.sub.DOWN (ms) is a fall time of the pulse current, and L (mm) is a distance between the steel sheet and the contact tip.

11. The arc welding method according to claim 3, wherein at least one of following conditions (A) and (B) is satisfied: (A) the arc welding is performed in the presence of Ar gas as a shielding gas, and (B) the welding wire is a solid wire.

12. The arc welding method according to claim 4, wherein at least one of following conditions (A) and (B) is satisfied: (A) the arc welding is performed in the presence of Ar gas as a shielding gas, and (B) the welding wire is a solid wire.

13. The arc welding method according to claim 5, wherein at least one of following conditions (A) and (B) is satisfied: (A) the arc welding is performed in the presence of Ar gas as a shielding gas, and (B) the welding wire is a solid wire.

14. The arc welding method according to claim 6, wherein at least one of following conditions (A) and (B) is satisfied: (A) the arc welding is performed in the presence of Ar gas as a shielding gas, and (B) the welding wire is a solid wire.

15. The arc welding method according to claim 7, wherein at least one of following conditions (A) and (B) is satisfied: (A) the arc welding is performed in the presence of Ar gas as a shielding gas, and (B) the welding wire is a solid wire.

16. The arc welding method according to claim 8, wherein at least one of following conditions (A) and (B) is satisfied: (A) the arc welding is performed in the presence of Ar gas as a shielding gas, and (B) the welding wire is a solid wire.

17. The arc welding method according to claim 9, wherein at least one of following conditions (A) and (B) is satisfied: (A) the arc welding is performed in the presence of Ar gas as a shielding gas, and (B) the welding wire is a solid wire.

18. The arc welding method according to claim 10, wherein at least one of following conditions (A) and (B) is satisfied: (A) the arc welding is performed in the presence of Ar gas as a shielding gas, and (B) the welding wire is a solid wire.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

[0045] FIG. 1 is a schematic perspective diagram illustrating an example in which an embodiment of the disclosed embodiments is used for lap fillet welding.

[0046] FIG. 2 is a schematic perspective diagram illustrating an example of a weld bead formed by performing lap fillet welding illustrated in FIG. 1.

[0047] FIG. 3(a) and FIG. 3(b) are enlarged schematic cross-sectional diagrams illustrating a welding wire and a portion in the vicinity of the wire illustrated in FIG. 1 and a manner in which a short circuit transfer occurs.

[0048] FIG. 4 is a graph illustrating a pulse current waveform of current applied as welding current.

[0049] FIG. 5 is a schematic perspective diagram illustrating a weld bead toe and weld bead start/finish end portions formed by performing lap fillet welding illustrated in FIG. 1.

DETAILED DESCRIPTION

[0050] Hereafter, with reference to FIGS. 1 to 5, an example in which an embodiment of the disclosed embodiments is used for lap fillet welding will be described. However, the disclosed embodiments may be used for not only lap fillet welding but also various welding techniques (for example, butt welding and the like).

[0051] Here, the present embodiment is intended for arc welding performed on at least two steel sheets, and FIG. 1 illustrates one example in which two steel sheets are welded.

[0052] In the disclosed embodiments, for example, as illustrated in FIG. 1, by using a welding wire 1, which is continuously fed through the center of a welding torch 2 from the welding torch 2 to steel sheets (base materials) 3 (in detail, for example, a welding line corresponding to the corner of a step formed by two steel sheets 3 as base materials, overlapped with each other), and the steel sheets 3 as electrodes, a welding voltage is applied from a welding power source (not illustrated). As a result of a portion of shielding gas (not illustrated) fed from inside the welding torch 2 being ionized to form plasma, an arc 5 is formed between the welding wire 1 and the steel sheets 3. In addition, the other portion of the shielding gas, which is not ionized and which flows from the welding torch 2 to the steel sheets 3, has a role in sealing the arc 5 and a weld pool (not illustrated in FIG. 1), which is formed due to the steel sheet 3 being melted, from outside air. The front end of the welding wire 1 is melted by the heat of the arc 5 to form a droplet, and the droplet is transported to the weld pool by an electromagnetic force, gravity, and the like. As a result of such a phenomenon occurring continuously due to the movement of the welding torch 2 or the steel sheets 3, the weld pool is solidified to form a weld bead 6 on the rear side of the welding line. Consequently, the joining of the two steel sheets is completed.

[0053] As illustrated in FIG. 1, when two steel sheets 3 are overlapped with each other to perform lap fillet welding by using an arc welding method, since O.sub.2 or CO.sub.2 mixed in the shielding gas is heated by the arc 5, a reaction expressed by formula (4) or formula (5) progresses.

O.sub.2->2[O]  (4)

CO.sub.2->CO+[O]  (5)

[0054] When oxygen generated due to such a decomposition reaction is dissolved in a molten metal 7 or a weld pool 8 (refer to FIG. 3(a) and FIG. 3(b)), the oxygen is retained in a weld metal in the form of bubbles when the molten metal or the weld pool 8 is cooled and solidified to form the weld metal. In addition, since an oxidation reaction between oxygen and iron progresses, there may be a case of a deterioration in the mechanical properties of the weld metal.

[0055] To solve such a problem, a welding wire 1 and a steel sheet 3 to which non-ferrous metals such as Si, Mn, Ti, and the like are added as deoxidizing agents are used. That is, by discharging oxygen which is generated due to a reaction expressed by formula (4) or formula (5) in the form of slag formed of SiO.sub.2, MnO, TiO.sub.2, and the like, a reaction between oxygen and iron is inhibited.

[0056] Slag which has been discharged to the surface of the weld pool 8 is aggregated in a subsequent cooling process, allowed to adhere to the surface of the weld bead 6 and a weld bead toe 9 (that is, a weld bead) (refer to FIG. 5), and solidified. In the case of an arc welded joint in which slag adheres to a weld bead in such a manner, a chemical conversion coating layer is not sufficiently formed even when chemical conversion coating is performed on the arc welded joint. Moreover, since slag is a nonconductor, it is difficult to form a uniform electrodeposition coating film. Therefore, it is necessary to inhibit the formation of slag while preventing a deterioration in the mechanical properties of a weld metal by using a welding wire 1 and a steel sheet 3 containing deoxidizing agents.

[0057] Here, the weld bead toe 9 and the weld bead start/finish end portions 10 will be described with reference to FIG. 5. As illustrated in FIG. 5, in the disclosed embodiments, the term “weld bead start/finish end portions” denotes a weld bead start end portion and a weld bead finish end portion. The term “weld bead start end portion” denotes a region of the weld bead from a weld bead start end position (welding start position) to a point on the welding line located 15 mm toward a weld bead finish end position (welding finish position), and the term “weld bead finish end portion” denotes a region of the weld bead from the weld bead finish end position to a point on the welding line located 15 mm toward the weld bead start end position. In the disclosed embodiments, the term “weld bead toe” denotes a boundary in a direction perpendicular to the welding line of the weld bead between the weld metal and the unmelted base steel sheet.

[0058] Therefore, in the disclosed embodiments, by using a shielding gas containing mainly Ar gas, there is a decrease in the amount of O.sub.2 and CO.sub.2 mixed in, which results in the formation of slag being inhibited. Specifically, when the area of the surface of the weld bead 6 is defined as a weld bead surface area S.sub.BEAD (mm.sup.2) and, of the weld bead surface area S.sub.BEAD, the area of the region covered with slag is defined as a slag surface area S.sub.SLAG (mm.sup.2), a slag-coverage area ratio S.sub.RATIO (%) calculated by using equation (1) is set to be 15% or less. Moreover, since the aggregation of slag on the surface of the weld bead 6 is inhibited in the case where there is a decrease in the amount of slag formed, it is preferable that the slag-coverage area ratio S.sub.RATIO be 9% or less or more preferably 5% or less.

[0059] Furthermore, the lower the amount of non-conducting slag formed, the better the chemical conversion coatability and the electrodeposition coatability. Therefore, since it is preferable that the slag-coverage area ratio S.sub.RATIO be as small as possible, there is no particular limitation on the lower limit of the slag-coverage area ratio S.sub.RATIO. It is preferable that the slag-coverage area ratio S.sub.RATIO be 0.1% or more.

S.sub.RATIO=100×S.sub.SLAG/S.sub.BEAD   (1)

[0060] To prevent slag from being non-uniformly distributed on the surface of the weld bead 6, it is necessary to stabilize the shape of the weld bead 6.

[0061] Therefore, in the disclosed embodiments, a weld bead width ratio W.sub.RATIO (%) calculated by using equation (2) from the maximum value W.sub.MAX (mm) and the minimum value W.sub.MIN (mm) of a weld bead width (refer to FIG. 2) in a direction perpendicular to a line parallel to the welding direction of the weld bead 6 (hereinafter, referred to as “welding line”) is set to be 60% or more. By decreasing a variation in weld bead width (that is, by decreasing a difference between W.sub.MAX and W.sub.MIN), the shape of the weld bead 6 becomes stable. As a result, it is possible to keep heat input to the weld bead constant. That is, it is possible to form a weld bead 6 having uniform surface quality. Therefore, it is possible to obtain a uniform chemical conversion coating layer formed by performing chemical conversion coating and a uniform coating film formed by performing electrodeposition coating. Moreover, as a result of a difference between W.sub.MAX and W.sub.MIN being decreased, it is possible to inhibit the formation of a local treatment solution pool at a position corresponding to W.sub.MIN when chemical conversion coating or electrodeposition coating is performed. Therefore, it is preferable that the weld bead width ratio W.sub.RATIO be 70% or more or more preferably 80% or more.

[0062] Here, there is no particular limitation on the upper limit of the weld bead width ratio W.sub.RATIO. It is preferable that the weld bead width ratio W.sub.RATIO be 100% or less.

W.sub.RATIO=100×W.sub.MIN/W.sub.MAX   (2)

[0063] It is preferable that arc welding be performed with the steel sheet 3 being set at the cathode and with the welding wire 1 being set at the anode (that is, with so-called reverse polarity). By using the reverse polarity, since a cathode spot, which is an electron-emitting source, is formed on the steel sheet 3, a region 4 (a so-called cleaning region), in which oxides (for example, mill scale formed in the manufacturing process of the steel sheet 3, oxides formed due to heat input when welding is performed, and the like) formed on the surface of the steel sheet 3 are removed, is formed.

[0064] In the case where a distance in a direction perpendicular to the welding line between the outer edge of the cleaning region 4 and the toe of the weld bead 6, that is, a cleaning width M (refer to FIG. 2), is excessively small, oxides remain in the vicinity of the toe of the weld bead 6. Consequently, since a chemical conversion coating layer formed by performing chemical conversion coating and a coating film formed by performing electrodeposition coating become non-uniform, corrosion tends to progress in the weld bead. Therefore, it is preferable that the minimum value M.sub.MIN (mm) of the cleaning width M (mm) be 0.5 mm or more, more preferably 2.0 mm or more, or even more preferably 4.0 mm or more.

[0065] On the other hand, in the original portion of a steel sheet which is not affected by welding heat, it is not possible to expect that there is an improvement in chemical conversion coatability or electrodeposition coatability due to a cleaning function. In addition, in the case where a region in which cathode spot formation occurs is wide, arc discharge becomes unstable. Therefore, it is preferable that the maximum value M.sub.MAX (mm) of the cleaning width M (mm) be 8.0 mm or less.

[0066] By performing arc welding with reverse polarity, the welding wire 1 is set at the anode, and the steel sheet 3 is set at the cathode. Then, as a result of a welding voltage being applied through the welding wire 1, which is continuously fed through the center of the welding torch 2 to the steel sheets 3, a portion of shielding gas, which is fed from inside the welding torch 2, is ionized to form plasma. Consequently, the arc 5 is formed between the welding wire 1 and the steel sheets 3. The remaining shielding gas (that is, the portion of the gas, which is not ionized and which flows from the welding torch 2 to the steel sheets 3) seals the arc 5, the molten metal 7, and the weld pool 8 from outside air (refer to FIG. 3). Consequently, oxygen incorporation (that is, the formation of slag) and nitrogen incorporation (that is, the formation of blow holes) are prevented.

[0067] The front end of the welding wire 1 is melted by the heat of the arc 5 to form molten metal 7, and the droplet of the molten metal 7 is transported to the weld pool 8 by an electromagnetic force, gravity, and the like. At this time, a state in which the molten metal 7 is separated from the weld pool 8 (refer to FIG. 3(a)) and a state in which the molten metal 7 is in contact with the weld pool 8, that is, a short circuit state, (refer to FIG. 3(b)) are alternately repeated regularly. Then, as a result of such a phenomenon occurring continuously while the welding wire 1 is moved in the welding line direction, the weld pool 8 is solidified to form a weld bead 6 on the rear side of the welding line.

[0068] In the case of arc welding utilizing Ar gas as a shielding gas, since the amount of oxygen which is mixed in the molten metal 7 and the weld pool 8 is significantly small, the effect of preventing the formation of slag is realized. However, since a cathode spot severely moves around, there is a disadvantage in that the weld bead 6 tends to meander or to have a wavy shape. Here, the chemical composition of the Ar gas described above is a chemical composition containing Ar in an amount of 99.0% or more in terms of volume fraction. Such a shielding gas containing mainly the Ar gas described above is also referred to as an “Ar shielding gas”.

[0069] To eliminate such a disadvantage, in arc welding, the cycle at which a short circuit occurs between the welding wire 1 and the steel sheet 3 (hereinafter, referred to as “short circuit cycle”) and the frequency with which such a short circuit occurs (hereinafter, referred to as “short circuit frequency”) are specified in the disclosed embodiments. Specifically, it is preferable that the maximum value of the short circuit cycle T.sub.CYC (s) be 1.5 s or less and that the average value of the short circuit frequency (average short circuit frequency) F.sub.AVE (Hz) be 20 Hz to 300 Hz.

[0070] By specifying the maximum value of the short circuit cycle and the average short circuit frequency to realize stable droplet transfer, since it is possible not only to inhibit the formation of slag but also to realize stable arc discharge, it is possible to form a weld bead 6 in which the slag-coverage area ratio S.sub.RATIO and the weld bead width ratio W.sub.RATIO are within the ranges described above.

[0071] In the case where the volume of the droplet formed from the front end of the welding wire 1 is excessively large or small, the weld pool 8 becomes unstable. Specifically, in the case where the average short circuit frequency F.sub.AVE is less than 20 Hz, large droplets are transferred to the weld pool 8, or droplet transfer modes other than a short circuit transfer mode (for example, streaming transfer mode and the like) are mixed irregularly. In addition, in the case where the average short circuit frequency F.sub.AVE is more than 300 Hz, although the size of droplets is small, arc reignition due to a short circuit occurs excessively often. For such reasons, in any of such cases, since the weld pool 8 is disturbed, it is difficult to eliminate the meandering or wavy shape of the weld bead. That is, by controlling the average short circuit frequency F.sub.AVE to be 20 Hz to 300 Hz, it is possible to control the volume of a droplet which is transferred to the weld pool 8 in one short circuit cycle to be about the same as that of a sphere having a diameter equal to that of the welding wire 1. As a result, it is possible to stabilize droplet transfer.

[0072] To eliminate a variation in the volume of a droplet which is transferred to the weld pool 8 in one short circuit cycle so that there is an improvement in the uniformity of the weld bead, it is more preferable that the average short circuit frequency F.sub.AVE be 35 Hz or more or even more preferably 50 Hz or more. In addition, in the case where the average short circuit frequency F.sub.AVE is large, there may be a case where droplets having small volumes are scattered in the form of a large number of spatters at the times of a short circuit and reignition. Therefore, it is more preferable that the average short circuit frequency F.sub.AVE be 250 Hz or less or even more preferably 200 Hz or less.

[0073] In addition, in the case where the maximum short circuit cycle T.sub.CYC is more than 1.5 s, since droplet transfer becomes unstable, the weld bead width and a penetration depth become unstable. That is, by controlling the maximum short circuit cycle T.sub.CYC to be 1.5 s or less, it is possible to form a weld bead 6 having a good shape. The term “maximum short circuit cycle T.sub.CYC” denotes the maximum value of a short circuit cycle in a welding pass for forming an arc welded joint. This means that any of the short circuit cycles in a welding pass does not exceed 1.5 s.

[0074] By specifying the average short circuit frequency F.sub.AVE and the maximum short circuit cycle T.sub.CYC as described above, regular and stable droplet transfer is possible in arc welding utilizing an Ar shielding gas. Here, to control the average short circuit frequency F.sub.AVE described above to be 20 Hz or more, it is more preferable that the maximum short circuit cycle T.sub.CYC be 1.0 s or less or even more preferably 0.2 s or less. In addition, it is sufficient that the maximum short circuit cycle T.sub.CYC be within a range in which the average short circuit frequency F.sub.AVE becomes 300 Hz or less, and it is preferable that the maximum short circuit cycle T.sub.CYC be 0.004 s or more.

[0075] The term “average short circuit frequency F.sub.AVE” denotes the average value of a short circuit frequency in a welding pass for forming an arc welded joint. That is, when a change in the arc voltage in a welding pass is measured by using a measuring device (for example, oscilloscope and the like) to count the number of times that the arc voltage becomes zero, the average short circuit frequency F.sub.AVE is defined as a value obtained by dividing the number of times by the time (s) required for the welding pass (number/s=Hz).

[0076] Here, examples of preferable welding conditions include welding current: 150 A to 300 A, arc voltage: 20 V to 35 V, Ar gas flow rate: 15 Liter/min to 25 Liter/min, distance L between the steel sheet 3 and a contact tip (hereinafter, referred to as “CTWD”): 5 mm to 30 mm, and the like. Here, the welding current and the arc voltage are represented by their respective average values in a welding pass.

[0077] Moreover, there is no particular limitation on the methods used for controlling the average short circuit frequency and the maximum short circuit cycle to be within the ranges described above. For example, by performing current waveform control utilizing pulse current as illustrated in FIG. 4, when a peak current is defined as I.sub.PEAK (A), a base current is defined as I.sub.BASE (A), a peak time is defined as t.sub.PEAK (ms), a rise time is defined as t.sub.UP (ms), a fall time is defined as t.sub.DOWN (ms), and CTWD is defined as L (mm), as a result of X (A.Math.s/m) calculated by using equation (3) below satisfying the relational expression 50≤X≤250, it is possible to more effectively realize the effect of the disclosed embodiments.

X=(I.sub.PEAK×t.sub.PEAK/L)+(I.sub.PEAK+I.sub.BASE)×(t.sub.UP+t.sub.DOWN)/(2×L)   (3)

[0078] In the case where the value of X (A.Math.s/m) calculated by using equation (3) is excessively small, there may be a case where the arc 5 sways and/or droplet transfer becomes unstable. On the other hand, in the case where the value of X is excessively large, there may be a case where the welding wire 1 plunges in the weld pool 8 or a case where a grown droplet flies apart at the time of a short circuit, resulting in a deterioration in weld bead shape, spatter adhesion, and the like. Therefore, it is preferable that the value of X satisfy the relational expression 50≤X≤250 or more preferably 60≤X≤230. It is even more preferable that the value of X be 80 or more and that the value of X be 200 or less. Here, “s” used in the unit of X (A.Math.s/m) denotes “seconds”, and the unit of t.sub.PEAK, t.sub.UP, and t.sub.DOWN (ms) is “milliseconds”(= 1/1000 seconds).

[0079] In addition, in the case where the value of the distance L between the steel sheet 3 and the contact tip is excessively small, since severe wear occurs in the welding torch 2, welding becomes unstable. In the case where the value of the distance L between the steel sheet 3 and the contact tip is excessively large, the arc 5 sways. Therefore, it is preferable that the value of L be 5 mm to 30 mm or more preferably 8 mm to 20 mm.

[0080] In the case where the value of I.sub.PEAK is excessively small, since it is not possible to achieve sufficient heat input, there is a deterioration in weld bead shape. In the case where the value of I.sub.PEAK is excessively large, burn through occurs, and there is an increase in the number of spatters. Therefore, it is preferable that I.sub.PEAK be 250 A to 600 A. It is more preferable that I.sub.PEAK be 400 A or more and that I.sub.PEAK be 500 A or less.

[0081] In the case where the value of I.sub.BASE is excessively small, arc becomes unstable. In the case where the value of I.sub.BASE is excessively large, burn through occurs. Therefore, it is preferable that I.sub.BASE be 30 A to 120 A. It is more preferable that I.sub.BASE be 40 A or more and that I.sub.BASE be 100 A or less.

[0082] In the case where the value of is excessively small, it is not possible to achieve sufficient heat input. In the case where the value of t.sub.PEAK is excessively large, burn through occurs. Therefore, it is preferable that t.sub.PEAK be 0.1 ms to 5.0 ms. It is more preferable that t.sub.PEAK be 1.0 ms or more and that t.sub.PEAK be 4.5 ms or less.

[0083] In the case where t.sub.UP or t.sub.DOWN is excessively small, arc sway is induced. In the case where t.sub.UP or t.sub.DOWN is excessively large, there is a deterioration in weld bead shape. Therefore, it is preferable that each of t.sub.UP and t.sub.DOWN be 0.1 ms to 3.0 ms. It is more preferable that each of t.sub.UP and t.sub.DOWN be 0.5 ms or more and that each of t.sub.UP and t.sub.DOWN be 2.5 ms or less.

[0084] When the base time of the pulse current is defined as t.sub.BASE (ms), although t.sub.BASE is not used in equation (3), which is used for calculating the value of X, in the case where t.sub.BASE is excessively small, there is an excessive decrease in the size of a droplet. In the case where t.sub.BASE is excessively large, there is an excessive increase in the size of a droplet. In any of such cases, welding becomes unstable. Therefore, it is preferable that t.sub.BASE be 0.1 ms to 10.0 ms. It is more preferable that t.sub.BASE be 1.0 ms or more and that t.sub.BASE be 8.0 ms or less.

[0085] Furthermore, in the disclosed embodiments, it is not necessary that a short circuit occur in every cycle of the pulse current. It is sufficient that a short circuit occur once in one to several pulses. In addition, as long as a short circuit occurs once in one to several pulses, there is no particular limitation on the pulse frequency of the pulse current.

[0086] In the disclosed embodiments, the purpose of using the pulse current is (1) to promote the stable growth of the droplet in the base time while inhibiting the arc from swaying by applying lower current and (2) to promote a short circuit in the peak time and the fall time by pushing down the grown droplet to the weld pool by using electromagnetic force and the shearing force of the Ar shielding gas without separating the grown droplet from the wire.

[0087] In the disclosed embodiments, it is not necessary to feed oxygen or to add special elements. Therefore, by using a solid wire, which is less expensive than a flux-cored wire, as a welding wire, it is possible to realize a decrease in process costs.

[0088] Here, the solid wire which can preferably be used in the disclosed embodiments has a wire chemical composition containing C: 0.020 mass % to 0.250 mass %, Si: 0.05 mass % to 1.50 mass %, Mn: 0.50 mass % to 3.0 mass %, P: 0.020 mass % or less, S: 0.03 mass % or less, and a balance of Fe and incidental impurities. It is preferable that the diameter of the solid wire be 0.4 mm to 2.0 mm.

EXAMPLES

[0089] The arc welded joint and arc welding method according to the disclosed embodiments will be described in detail in accordance with examples.

[0090] By performing lap fillet welding (refer to FIG. 1) on two steel sheets (having a thickness of 2.6 mm each) having one of the chemical compositions given in Table 1, an arc welded joint was formed. The welding conditions are given in Table 2. The chemical compositions of the welding wires (having a diameter of 1.2 mm each) denoted by wire codes given in Table 2 are given in Table 4. Here, the remainder which was different from the constituents given in Table 1 or Table 4 was incidental impurities. After having performed alkaline degreasing, surface conditioning, and zinc phosphate-based chemical conversion coating on the formed arc welded joint, cation electrodeposition coating was performed under a condition in which the film thickness on the flat base steel sheet other than the weld was 15 μm. Subsequently, a corrosion test in accordance with SAE J 2334 was performed for 60 cycles.

[0091] Here, the weld bead surface area S.sub.BEAD and the slag surface area S.sub.SLAG were derived by taking the image of the surface of the region of the weld bead 6 excluding the weld bead start/finish end portions 10 (having a length of 15 mm each) from directly above and by measuring the projected areas of the weld bead and slag viewed from above. In the case of a weld bead 6 having a length of less than 130 mm, the image of the surface of the full length of the weld bead 6 excluding the weld bead start/finish end portions 10 was taken. In the case of a weld bead 6 having a length of 130 mm or more, the image of the surface of a portion (having a length of 100 mm) of the weld bead 6 excluding the weld bead start/finish end portions 10 was taken. The slag-coverage area ratio S.sub.RATIO was derived by using equation (1) above from the weld bead surface area S.sub.BEAD and the slag surface area S.sub.SLAG derived as above. The derived slag-coverage area ratio S.sub.RATIO is given in Table 3.

[0092] Similarly, the maximum value W.sub.MAX and minimum value W.sub.MIN of the weld bead width were measured by taking the image of the surface of the region of the weld bead 6 excluding the weld bead start/finish end portions 10 (having a length of 15 mm each) and by analyzing the taken image. In the case of a weld bead 6 having a length of less than 130 mm, the image of the surface of the full length of the weld bead 6 excluding the weld bead start/finish end portions 10 was taken. In the case of a weld bead 6 having a length of 130 mm or more, the image of the surface of a portion (having a length of 100 mm) of the weld bead 6 excluding the weld bead start/finish end portions 10 was taken. The weld bead width ratio W.sub.RATIO was derived by using equation (2) above from the maximum value W.sub.MAX and minimum value W.sub.MIN of the weld bead width measured as above. The derived weld bead width ratio W.sub.RATIO is given in Table 3.

[0093] In addition, similarly, the maximum value M.sub.MAX and the minimum value M.sub.MIN of the cleaning width were measured by taking the image of the surface of the region of the weld bead 6 excluding the weld bead start/finish end portions 10 (having a length of 15 mm each) and by analyzing the taken image. In the case of a weld bead 6 having a length of less than 130 mm, the image of the surface of the full length of the weld bead excluding the weld bead start/finish end portions 10 was taken. In the case of a weld bead 6 having a length of 130 mm or more, the image of the surface of a portion (having a length of 100 mm) of the weld bead 6 excluding the weld bead start/finish end portions 10 was taken. Each of the measured maximum value M.sub.MAX and the minimum value M.sub.MIN of the cleaning width is given in Table 3.

[0094] The evaluation of “corrosion resistance” given in Table 3 was performed as follows. First, after having removed the electrodeposition coating layer by immersing the arc welded joint which had been subjected to the corrosion test in a removing solution, the corrosion product was removed in accordance with ISO 8407. Subsequently, in the case where the weld bead start/finish end portions 10 (having a length of 15 mm each) of the weld bead 6 were included, the image of the surface of the region excluding the weld bead start/finish end portions 10 was taken, and the maximum corrosion width H.sub.MAX from the weld bead toe 9 was measured by analyzing the taken image. The evaluation of corrosion resistance was made in accordance with the following criteria, and the evaluation results are denoted by reference signs A to C and F.

[0095] Here, in Table 3, “reference sign A” denotes a case of “a maximum corrosion width H.sub.MAX from the weld bead toe of less than 3.0 mm”. “Reference sign B” denotes a case of “a maximum corrosion width H.sub.MAX from the weld bead toe of 3.0 mm or more and less than 4.5 mm”. “Reference sign C” denotes a case of “a maximum corrosion width H.sub.MAX from the weld bead toe of 4.5 mm or more and less than 6.0 mm”. “Reference sign F” denotes a case of “a maximum corrosion width H.sub.MAX from the weld bead toe of 6.0 mm or more”. The rank denoted by reference sign A is the highest followed by those denoted by reference signs B and C in this order, and the rank denoted by reference sign F is the lowest.

[0096] Here, as illustrated in FIG. 5, of the term “weld bead start/finish end portions”, the term “weld bead start end portion” denotes a region of the weld bead from a weld bead start end position (welding start position) to a point on the welding line located 15 mm toward a weld bead finish end position (welding finish position), and the term “weld bead finish end portion” denotes a region of the weld bead from the weld bead finish end position to a point on the welding line located 15 mm toward the weld bead start end position. The term “weld bead toe” denotes a boundary in a direction perpendicular to the welding line of the weld bead between the weld metal and the unmelted base steel sheet.

[0097] The evaluation results are given in Table 3.

TABLE-US-00001 TABLE 1 Tensile Strength Chemical Composition of Steel Sheet (mass %) of Steel Sheet C Si Mn P S 980 MPa 0.060 0.71 1.80 0.006 0.001 440 MPa 0.055 0.02 1.35 0.011 0.001

TABLE-US-00002 TABLE 2 Tensile Strength Welding Arc Welding of Steel Droplet Current Voltage Speed L Shielding Sheet Wire Transfer F.sub.AVE No. A V cm/min mm Gas MPa Code Mode Hz 1 160 22.0 70 15 Ar—20%CO.sub.2 980 W1 Spray — 2 157 21.6 70 15 Ar—20%CO.sub.2 980 W1 Short Circuit 53 3 245 19.6 90 10 100% Ar 440 W1 Short Circuit 23 4 268 23.8 70 15 100% Ar 980 W1 Short Circuit 43 5 189 22.8 70 15 Ar—5%CO.sub.2 980 W1 Spray — 6 171 20.3 50 10 Ar—5%CO.sub.2 980 W2 Short Circuit 91 7 190 22.8 70 15 Ar—5%CO.sub.2 440 W2 Spray — 8 197 23.0 70 15 Ar—3%CO.sub.2 980 W1 Spray — 9 254 21.9 120 15 Ar—3%CO.sub.2 980 W2 Spray — 10 219 22.0 70 10 Ar—3%CO.sub.2 440 W2 Spray — 11 220 21.5 70 10 Ar—1%CO.sub.2 980 W1 Spray — 12 209 22.1 50 10 Ar—1%CO.sub.2 980 W1 Spray — 13 237 27.1 70 10 100% Ar 980 W1 Short Circuit 47 14 295 25.6 70 10 100% Ar 980 W1 Short Circuit 70 15 226 25.4 70 10 100% Ar 980 W1 Short Circuit 87 16 213 20.8 70 15 100% Ar 980 W1 Short Circuit 91 17 235 21.0 70 10 100% Ar 980 W2 Short Circuit 88 T.sub.CYC I.sub.PEAK I.sub.BASE t.sub.PEAK t.sub.UP t.sub.DOWN t.sub.BASE X*.sup.1 No. s Pulse A A ms ms ms ms A .Math. s/m Note 1 — With 450 50 1.5 1.0 1.0 5.6 78.3 Comparative Example 2 0.05 without — — — — — — — Comparative Example 3 0.48 without — — — — — — — Comparative Example 4 1.59 without — — — — — — — Comparative Example 5 — with 450 50 1.5 1.0 1.0 3.7 78.3 Example 6 0.01 without — — — — — — — Example 7 — with 450 50 1.5 1.0 1.0 3.6 78.3 Example 8 — with 450 50 1.5 1.0 1.0 3.3 78.3 Example 9 — with 500 50 2.0 0.8 0.8 2.6 96.0 Example 10 — with 450 50 1.5 1.0 1.0 2.4 117.5 Example 11 — with 450 50 1.5 1.0 1.0 2.4 117.5 Example 12 — with 500 50 1.5 1.0 1.0 3.1 130.0 Example 13 0.06 with 450 50 1.5 1.0 1.0 1.9 117.5 Example 14 0.04 with 450 50 1.5 1.0 1.0 0.6 117.5 Example 15 0.01 with 550 50 2.0 1.0 1.0 4.5 170.0 Example 16 0.01 with 450 80 1.5 1.0 1.0 2.6 78.3 Example 17 0.01 with 450 50 3.0 2.0 2.0 3.8 235.0 Example polarity: direct current with reverse polarity gas flow rate: 15 L/min *.sup.1X = (I.sub.PEAK × t.sub.PEAK/L) + (I.sub.PEAK + I.sub.BASE) × (t.sub.UP + t.sub.DOWN)/(2 × L)

TABLE-US-00003 TABLE 3 S.sub.RATIO W.sub.RATIO M.sub.MAX M.sub.MIN H.sub.MAX No. % % mm mm mm Evaluation *2 Note 1 36.1 89 0.1 0.1 8.5 F Comparative Example 2 20.1 92 0.1 0.1 8.1 F Comparative Example 3 1.7 56 6.5 2.9 6.3 F Comparative Example 4 1.0 40 9.3 3.0 6.9 F Comparative Example 5 12.0 90 0.1 0.1 4.8 C Example 6 14.6 85 0.2 0.2 5.6 C Example 7 8.5 96 1.1 1.0 4.1 B Example 8 8.3 90 0.6 0.5 4.4 B Example 9 6.1 82 0.6 0.6 3.6 B Example 10 2.5 91 1.8 1.4 2.9 A Example 11 1.3 87 2.5 2.0 2.7 A Example 12 1.9 90 2.4 2.2 2.7 A Example 13 1.0 73 5.5 4.0 2.0 A Example 14 1.0 82 6.1 4.1 0.5 A Example 15 1.0 64 6.5 4.0 2.5 A Example 16 1.0 66 7.8 6.7 2.4 A Example 17 1.0 85 5.6 3.2 1.9 A Example *2 Evaluation A denotes a case of a maximum corrosion width HMAX from the weld bead toe of less than 3.0 mm. B denotes a case of a maximum corrosion width HMAX from the weld bead toe of 3.0 mm or more and less than 4.5 mm. C denotes a case of a maximum corrosion width HMAX from the weld bead toe of 4.5 mm or more and less than 6.0 mm. F denotes a case of a maximum corrosion width HMAX from the weld bead toe of 6.0 mm or more.

TABLE-US-00004 TABLE 4 Wire Chemical Composition of Welding Wire (mass %) Code C Si Mn P S W1 0.068 0.57 1.06 0.006 0.006 W2 0.054 0.90 1.37 0.005 0.015

[0098] As indicated by Tables 2 and 3, it is clarified that, in the case of welding Nos. 5 to 15, which were examples of the disclosed embodiments, since S.sub.RATIO was 15% or less, and since W.sub.RATIO was 60% or more, arc welded joints excellent in terms of corrosion resistance were obtained.

[0099] Of these examples of the disclosed embodiments, in the case of welding Nos. 7 to 15, since M.sub.MIN was 0.5 mm or more, arc welded joints excellent in terms of corrosion resistance at a higher level were obtained.

[0100] In contrast, in the case of welding Nos. 1 and 2 where S.sub.RATIO was more than 15%, and in the case of welding Nos. 3 and 4 where W.sub.RATIO was less than 60%, that is, in the case of the comparative examples, there was a deterioration in phosphatability and electrodeposition coatability, which resulted in a deterioration in the corrosion resistance of the arc welded joints.

[0101] In addition, as indicated by the data of welding Nos 5 to 15, which were the examples of the disclosed embodiments, it is clarified that arc welded joints excellent in terms of corrosion resistance were obtained regardless of whether a welding wire for an ultra-high tensile strength steel sheet (wire code W1 in Table 4) or a welding wire for a mild steel sheet (wire code W2 in Table 4) was used.