ARC WELDED JOINT AND ARC WELDING METHOD

Abstract

An arc-welded joint and an arc-welding method are provided.An arc-welded joint according to the present invention is an arc-welded joint having a slag-to-coverage area ratio SRATIO (%) of 15% or less, wherein SRATIO is calculated using an equation SRATIO = 100 x SSLAG / SBEAD, where an area of ​​a weld bead surface formed by arc welding on a steel sheet is defined as a weld bead surface area SBEAD (mm2) and, of the weld bead surface area SBEAD, an area of ​​a region covered with slag is defined as a slag surface area SSLAG (mm2), and having a weld bead width ratio WRATIO (%) of 60% or more, wherein WRATIO is calculated using an equation WRATIO = 100 x WMIN / WMAX from a maximum value WMAX (mm) and a minimum value WMIN (mm) of a weld bead width in a direction perpendicular to a weld line of the weld bead.

Description

ARC WELDED JOINT AND ARC WELDING METHOD FIELD OF INVENTION The present invention relates to an arc-welded joint that is excellent in terms of corrosion resistance and can preferably be used for automobile chassis members and the like, and to a method of arc welding for forming the joint. BACKGROUND OF THE INVENTION Today, with regard to the components used in a car body, there is a growing need not only for increased strength and rigidity to improve the safety and reliability of the vehicle body, but also for reduced weight to improve fuel efficiency. As a result, the thickness of steel sheets used for these components is reduced by using high-strength steel. On the other hand, for several components used in a car, and particularly for chassis members (e.g., lower control arms and similar components), a thicker steel sheet is used from the perspective of strength and rigidity, compared to the steel sheet used for the car body.Therefore, by increasing the strength of a steel sheet used for chassis members, thereby further reducing the thickness of the steel sheet, it is possible to achieve a further reduction in the weight of the car body. Consequently, it is possible to achieve improved fuel efficiency while maintaining the strength and rigidity of the members. Generally, components used in corrosive environments undergo rust prevention treatments such as chemical conversion coating and electrodeposition to achieve corrosion resistance after welding. However, there may be cases where rust or corrosion is observed on a weld and in a portion of the surrounding area over time. In the case of a component that has undergone electrodeposition as described above, corrosion tends to start at a weld, spread within the weld and into a wide region surrounding it, while blistering of the coating film occurs over time, and also progresses in the direction of thickness.If corrosion progresses in this way, there is a decrease in the thickness of the weld and in the surrounding area, resulting in a reduction in the strength not only of the weld but also of the member. In other words, if corrosion occurs in members whose welds are subjected to a load (for example, car chassis members and similar components) and progresses, the member may fracture. In the case of electrodeposition coating, after a chemical conversion coating (e.g., zinc phosphate treatment or similar) is applied to a base steel sheet and weld metal as a pretreatment to improve the adhesion of the electrodeposition coating film to the base steel sheet and weld metal, the electrodeposition coating is performed. Zinc phosphate treatment, widely used as an example of a chemical conversion coating, is a technique in which zinc phosphate crystal grains are grown on the surface of the base steel sheet and weld metal to improve the adhesion of a coating film during electrodeposition.However, in the case of conventional techniques, even when a chemical conversion coating is applied to a member before the coating is applied. LQ / zLn / zznz / e / viAi Electrodeposition often causes blistering of the coating film on a weld and in a wide band surrounding the weld over time. This means that, in the case of the technique where the electrodeposition coating is performed after the chemical conversion coating described above has been carried out as a pretreatment, it is difficult to completely inhibit corrosion of a weld. Therefore, research is being conducted on a technique that uses a steel sheet with a plated layer (e.g., a zinc-based coating). However, in the case of a plated steel sheet, an increase in manufacturing costs compared to an ordinary steel sheet is unavoidable due to the additional costs associated with the plating treatment, which is expected given the improved corrosion resistance provided by the plated layer. However, even when using steel sheets with a plated layer, arc welding is used as the joining method, just as in conventional applications. Therefore, in a weld subjected to high temperatures due to the plasma arc (hereafter referred to as the arc), which is a heat source, the plated layer vaporizes, exposing a localized area without plating. Consequently, a significant improvement in corrosion resistance commensurate with the increased cost of using a plated steel sheet cannot be expected. As described above, although several manufacturing techniques have been developed to improve the corrosion resistance of components, all of these techniques have both advantages and disadvantages. Therefore, from the perspective of improving corrosion resistance while inhibiting an increase in manufacturing costs, research is being conducted regarding a technique to more effectively prevent corrosion from occurring and progressing at a weld. Examples of a conventionally known starting point in a weld where corrosion occurs include the following. (a) slag adhering to a weld (mainly the surface of a weld bead) (b) welding fume adhering to a weld (c) oxide formed on the surface of a steel sheet subjected to a high temperature due to welding. Even if a member has the adherent substance described in subsection (a) or (b) above or the oxide described in subsection (c) above in a weld that is subjected to a chemical conversion coating, the formation of a region that is not coated with the chemical conversion coating layer formed by zinc phosphate crystal grains begins in the adherent substance or the formed substance, and such a region is retained locally. Furthermore, in the region not coated with a chemically converted coating, even when electrodeposition is used, insufficient coating film formation and inadequate adhesion result in significant corrosion resistance. Consequently, corrosion decreases due to the development and progression of the coating. Therefore, research is underway to develop a technique to prevent the formation of the adherent substance described in sections (a) or (b) above, or the oxide described in section (c) above. For example, patent literature 1 describes a technique in which, after performing the LCWZLn / ZZnZ / B / YILI arc welding and prior to electrodeposition coating, a weld and a portion in the vicinity of the weld are subjected to a spray treatment or an immersion treatment using a non-oxidizing acidic solution having a pH of 2 or less at a temperature of 30°C to 90°C. This technique is a technique for removing the slag described in paragraph (a) above, the welding fume described in paragraph (b) above, and the oxide described in paragraph (c) above by dissolving a weld bead and a steel sheet with the non-oxidizing solution. However, in the case of the technique described in Patent Literature 1, since it is necessary to wash off the acid solution before performing the electrodeposition coating, the manufacturing process of the members becomes complex. Furthermore, since a member with a desired shape is formed from steel sheets of various shapes that are overlapped and welded together, any acid solution retained in the spaces between the overlapping steel sheets causes severe corrosion. Moreover, since a large quantity of acid solution is used, breakage and corrosion of the manufacturing equipment tend to occur because the equipment is exposed to a corrosive environment, and it is necessary to ensure the safety of the operators by preventing the dispersion of fumes. Patent bibliography 2 describes a technique in which the amounts of oxidizing gases (i.e., CO2 and O2) contained in a shielding gas (hereinafter referred to as shielding gas) are reduced during arc welding. This technique inhibits slag formation during welding, oxidation of the heat-affected weld zone, and welding fume adhesion. However, if the amount of oxidizing gases in a shielding gas decreases, the weld bead becomes unstable because the arc becomes unstable, resulting in poor weld penetration. Since this type of welding defect reduces the joint's strength, it is difficult to apply the technique described in Patent 2 to components such as chassis members, which must have sufficient strength and rigidity. Patent literature 3 describes a technique in which slag formation is inhibited by decreasing the total Si and Mn content in a welding wire used in arc welding and the Si and Mn content in a used steel sheet. However, if the Si and Mn content is reduced to inhibit slag formation, a decrease in the strength of the steel sheet is unavoidable. In other words, in the case of the technique described in Patent Literature 3, since a steel sheet must be thicker to achieve sufficient strength in the members, it is difficult to reduce the weight of a car body. Patent literature 4 describes a technique in which, even in the case of a weld bead containing slag, welding fumes, and oxides, a chemical conversion coating layer is sufficiently formed by controlling the chemical composition of a treatment solution used in the chemical conversion coating. In this technique, the formation of a chemical conversion coating layer is facilitated by performing a surface treatment with a surface conditioning solution containing a zinc phosphate colloid. Furthermore, by performing a chemical conversion coating with a zinc phosphate treatment solution containing F in an amount of 100 ppm by mass or more, the slag, welding fumes, and oxides are dissolved and LQ / ZLn / ZZnZ / Β / YILI remove, resulting in an improvement in the adhesion property of an electrodeposition coating film. However, in the case of the technique described in Patent Literature 4, since a zinc phosphate treatment solution containing fluorine, which is known to be a poisonous substance, is used, it is necessary to reduce the fluorine content in the liquid waste generated from the treatment solution to a level that meets environmental standards when the liquid waste is discharged from a factory. Therefore, a large liquid waste disposal system is required in addition to the manufacturing equipment. List of References Patent literature PTL 1: Publication of Unexamined Japanese Patent Application No. 9-20994 PTL 2: Publication of Unexamined Japanese Patent Application No. 8-33982 PTL 3: Publication of Unexamined Japanese Patent Application No. 8-33997 PTL 4: Japanese Patent No. 5549615. BRIEF DESCRIPTION OF THE INVENTION Technical Problem The present invention has been completed to solve the problems of conventional techniques, and one object of the present invention is to provide an arc-welded joint with excellent corrosion resistance that can preferably be used for various steel members (e.g., automobile chassis members and the like) that undergo electroplating prior to use, and a method of arc welding to form the joint. Solution to the Problem To solve the problems described above, the inventors of this application carried out investigations into the reasons for the deterioration of corrosion resistance in a weld of a member made of steel that has been subjected to an electrodeposition coating (hereinafter referred to as the “coated steel member”). A decrease in the corrosion resistance of the weld of a coated steel member is caused by welding slag and fumes adhering to the weld (i.e., the weld bead and heat-affected zone) and oxides forming on the surface of the steel sheet, which is subjected to high temperatures during arc welding. Generally, if a zinc phosphate treatment is applied as a chemical conversion coating before electrodeposition on a member fabricated from a worked steel sheet, the steel sheet dissolves due to the etching effect of the zinc phosphate treatment solution.Furthermore, whenever there is a local increase in pH at a solid-liquid interface due to the consumption of hydrogen ions by the dissolution of the steel sheet, zinc phosphate crystal grains (i.e., a chemical conversion coating layer) precipitate onto the steel sheet surface. However, the presence of slag, welding fumes, and other oxides in the steel sheet weld reduces the solubility of the zinc phosphate treatment solution. As a result, precipitation of the zinc phosphate crystal grains is difficult. Therefore, since a sufficient chemical conversion coating layer does not form on a weld, it is not possible to achieve sufficient adhesion properties of a film. LQ / ZLn / ZZnZ / B / YILI coating occurs when subsequent electrodeposition coating is performed. This is the reason why a deterioration in the corrosion resistance of the weld of a coated steel member occurs. In other words, in the case where it is possible to sufficiently precipitate the zinc phosphate crystal grains in a weld, since it is possible to improve the adhesion property of a coating film by electrodeposition, it is possible to improve the corrosion resistance of the weld. Therefore, the inventors of this document conducted research on a technique for improving corrosion resistance by densely precipitating zinc phosphate crystal grains onto a weld. They found that reducing the amount of slag adhering to the weld is the most effective way to improve corrosion resistance. However, in the case of a member fabricated from high-strength steel sheet and high-strength welding wire, the increased content of Si, Mn, Ti, and similar elements due to the member's high-alloy chemical composition leads to the problem of increased slag formation on the weld. By inhibiting the oxidation of Si, Mn, Ti, and similar elements contained in steel sheets and welding wire, this problem can be solved. That is, by using a shielding gas containing reduced amounts of oxidizing gases, the amount of slag formed can be decreased because the oxidation of these elements can be inhibited. However, if the oxidizing gas content in the shielding gas is reduced, the arc becomes unstable because the cathode point moves considerably during arc welding. This results in further problems with oxygen incorporation into the weld pool due to atmospheric air infiltration and deterioration of the weld bead shape. On the other hand, if the aim is to increase the strength of a member made of coated steel, it is not possible to avoid an increase in the contents of Si, Mn, Ti, and similar elements. Based on the results of research conducted by the inventors of this document, inhibiting the formation of slag that adheres to the weld bead to sufficiently precipitate a chemical conversion coating layer while maintaining arc stability to form a weld bead with good shape is effective in improving the corrosion resistance of a component. Furthermore, it is possible to further improve corrosion resistance by promoting the precipitation of a chemical conversion coating film as a result of the following. (A) decrease the amount of welding fumes that adhere to the foot of the weld bead; (B) decrease the amounts of oxides formed on the surface of a steel sheet due to welding; (C) removing the oxides (so-called mill scale) formed on the surface of a steel sheet in a process for manufacturing the steel sheet. The present invention has been completed on the basis of the discoveries described above. That is, the present invention is an arc-welded joint having a slag-to-coverage area ratio (Sratio) of 15% or less, where Sratio is calculated using equation (1), where the surface area of ​​a weld bead formed by arc welding on a steel sheet is defined as a weld bead surface area Sbead (mm2) and, of the surface area of ​​the LcwzLn / zznz / e / γΐΛΐ weld bead Sbead, an area of ​​a region covered with slag is defined as a slag surface area Ssu\g (mm2) , and with a weld bead width ratio Wratio (%) of 60% or more, where Wratio is calculated using equation (2) from a maximum value Wmax (mm) and a minimum value Wmin (mm) of a weld bead width in a direction perpendicular to a weld line of the weld bead. In the arc-welded joint according to the present invention, it is preferable that a cleaning region, in which the oxides formed on a surface of the steel sheet due to the formation of a cathode spot when arc welding is performed are removed, be formed so that it is adjacent to the toe of the weld bead and that a minimum value Mmin (mm) of a distance M (mm) in the direction perpendicular to the weld line between an outer edge of the cleaning region and the toe of the weld bead (hereinafter referred to as the cleaning width) is 0.5 mm or greater. Furthermore, the present invention is an arc welding method for making an arc welded joint, the arc welded joint having a slag-to-coverage area ratio Sratio (%) of 15% or less, wherein Sratio is calculated using equation (1), where a surface area of ​​a weld bead formed by arc welding on a steel sheet is defined as a weld bead surface area Sbead (mm2) and, of the weld bead surface area Sbead, an area of ​​a region covered with slag is defined as a slag surface area Sslag (mm2), and having a weld bead width ratio Wratio (%) of 60% or more, wherein Wratio is calculated using equation (2) from a maximum value Wmax (mm) and a minimum value Wmin (mm) of a weld bead width in a direction perpendicular to a weld line of the weld bead. In the arc welding method according to the present invention, it is preferable that the arc welding be performed with reverse polarity, as in the case of conventional CO2 welding and MAG (gas metal arc welding), that a cleaning zone, in which oxides form on the surface of the steel sheet and are removed due to the formation of a cathode spot (a source of electron emission), be formed adjacent to the toe of the weld bead, and that a minimum cleaning width M (mm) of 0.5 mm or more be established. Furthermore, it is preferable that argon gas (Ar) be used as the shielding gas. Furthermore, it is preferable that a short circuit occur intermittently between the steel sheet and the welding wire, and that this short circuit occur at an average short-circuit frequency Fave (Hz) of 20 Hz to 300 Hz with a maximum short-circuit cycle Tcyc (s) of 1.5 s or less. Furthermore, it is preferable that the pulse current be used as the welding current for the arc welding, and that X (As / m) be calculated using equation (3) from the peak current Ipeak (A), the base current Ibase (A), the peak time Ipeak (ms), the rise time tup (ms), and the fall time Idown (ms) of the pulse current, and that a distance L (mm) between the steel sheet and a contact tip satisfy a relational expression 50 < X ​​< 250. In the arc welding method according to the present invention, a solid wire may be used as the welding wire. Sratio = 100 x Ssi_ag / Sbead ·· (1) Wratio = 100 x Wmin / Wmax ” (2) X = (Ipeak x Ípeak / L) + (Ipeak + Ibase) x (tup + Ídown) / (2 x L) (3) Here, the cleaning region described above is a region in which, when performing arc welding with the steel sheet being placed on the cathode and with the welding wire being With LQ / ZLn / ZZnZ / B / YILI placed at the anode (i.e., with the so-called reverse polarity), a cathode point, which is a source of electron emission, forms on the steel sheet, and a phenomenon (called cleaning) occurs, in which the oxides formed on the surface of the steel sheet are removed due to the electron emission that occurs in this way. Furthermore, s used in the unit X (As / m) denotes seconds, and the units peak, tup, and down (ms) are milliseconds (= 1 / 1000 seconds). Advantageous effects of the invention According to the present invention, since it is possible to improve the corrosion resistance of welds on various types of members, such as chassis members, it is also possible to improve the rust prevention performance of members made from high-strength steel sheet and members used in highly corrosive environments. According to the present invention, it is possible to manufacture various types of members using high-strength steel sheet with a tensile strength of, for example, 440 MPa or higher (e.g., a 440 MPa class steel sheet, a 590 MPa class steel sheet, and a 980 MPa class steel sheet) and improve their corrosion resistance, which has a significant impact on industry. Furthermore, by using high-strength steel sheet, it is also possible to reduce the thickness of the members. BRIEF DESCRIPTION OF THE FIGURES Fig. 1 is a schematic perspective diagram illustrating an example in which the present invention is used for lap welding. Fig. 2 is a schematic perspective diagram illustrating an example of a weld bead formed by performing a lap weld illustrated in Fig. 1. Fig. 3a and Fig. 3b are schematic cross-sectional and enlarged diagrams illustrating a welding wire and a portion in the vicinity of the wire illustrated in Fig. 1 and the manner in which a short-circuit transfer occurs. Fig. 4 is a graph illustrating a pulse current waveform of the applied current as welding current. Fig. 5 is a schematic perspective diagram illustrating the weld bead toe and start / end weld bead portions formed when performing the lap weld illustrated in Fig. 1. DETAILED DESCRIPTION OF THE INVENTION An example of the use of the present invention for lap welding will now be described with reference to Figures 1 to 5. However, the present invention can be used not only for lap welding but also for various welding techniques (e.g., butt welding and the like). Herein, the present invention is oriented to arc welding performed on at least two steel sheets, and Fig. 1 illustrates an example in which two steel sheets are welded together. In the present invention, for example, as illustrated in Fig. 1, a welding wire 1 is continuously fed through the center of a welding torch 2 from the welding torch 2 to steel sheets (base materials) 3 (specifically, for example, a weld line corresponding to the corner of a step formed by two overlapping steel sheets 3 as base materials), and the steel sheets 3 serve as electrodes. A welding voltage is applied from a welding power source (not illustrated). As a result of the ionization of a portion of the shielding gas (not illustrated) fed from inside the welding torch 2, a welding arc is formed. LcwzLn / zznz / e / γΐΛΐ plasma, an arc 5 is formed between the welding wire 1 and the steel sheets 3. Furthermore, the other portion of the shielding gas, which is non-ionized and flows from the welding torch 2 to the steel sheets 3, plays a role in sealing the arc 5 and the weld pool (not illustrated in Fig. 1), which forms due to the melting of the steel sheet 3 from the surrounding air. The leading end of the welding wire 1 melts from the heat of the arc 5 to form a droplet, and this droplet is carried into the weld pool by electromagnetic forces, gravity, and similar factors. As a result of this continuous process, caused by the movement of the welding torch 2 or the steel sheets 3, the weld pool solidifies to form a weld bead 6 on the back side of the weld line. Consequently, the joining of the two steel sheets is completed. As illustrated in Fig. 1, when two steel sheets 3 are overlapped to perform a lap weld using an arc welding method, since the arc 5 heats the O2 or CO2 mixed in the shielding gas, a reaction expressed by formula (4) or formula (5) occurs. O2—>2[O]... (4) CO2-> CO + [O]... (5) When the oxygen generated by this decomposition reaction dissolves in molten metal 7 or a weld pool 8 (refer to Fig. 3a and Fig. 3b), the oxygen is retained in the weld metal as bubbles when the molten metal or weld pool 8 cools and solidifies to form the weld metal. Furthermore, since an oxidation reaction between oxygen and iron is ongoing, a deterioration of the weld metal's mechanical properties may occur. To solve this problem, a welding wire 1 and a steel sheet 3 are used, to which non-ferrous metals such as Si, Mn, Ti, and similar materials are added as deoxidizing agents. That is, by discharging the oxygen generated due to a reaction expressed by formula (4) or formula (5) in the form of slag formed by SiO2, MnO, TiO2, and similar materials, a reaction between oxygen and iron is inhibited. The slag discharged onto the surface of the weld pool 8 is added during a subsequent cooling process, allowing it to adhere to the surface of the weld bead 6 and the toe of the weld bead 9 (i.e., a weld bead) (refer to Fig. 5), and solidify. In the case of an arc-welded joint where the slag adheres to a weld bead in this manner, a sufficient chemical conversion coating layer does not form, even when chemical conversion coating is applied to the arc-welded joint. Furthermore, since the slag is non-conductive, it is difficult to form a uniform coating film by electrodeposition. Therefore, it is necessary to inhibit slag formation while preventing the deterioration of the mechanical properties of the weld metal by using welding wire 1 and steel foil 3 containing deoxidizing agents. Herein, the foot of the weld bead 9 and the start / end portions of the weld bead 10 will be described with reference to Fig. 5. As illustrated in Fig. 5, in the present invention, the term start / end portions of the weld bead denotes a start portion of the weld bead and an end portion of the weld bead. LcwzLn / zznz / e / γΐΛΐ The term "weld start end portion" indicates a region of the weld bead from a weld start end position (weld start position) to a point on the weld line located 15 mm toward the weld end end position (weld end position), and the term "weld end portion" indicates a region of the weld bead from the weld end position to a point on the weld line located 15 mm toward the weld start end portion. In the present invention, the term "weld toe" indicates a boundary in a direction perpendicular to the weld line of the weld bead between the weld metal and the unmelted base steel sheet. Therefore, in the present invention, by using a shielding gas that contains primarily Ar gas, there is a decrease in the amount of O2 and CO2 mixed together, resulting in the inhibition of slag formation. Specifically, when the surface area of ​​the weld bead 6 is defined as the weld bead surface area Sbead (mm2), and, of the weld bead surface area Sbead, the area of ​​the region covered with slag is defined as the slag surface area Sslag (mm2), a slag-to-coverage area ratio (%) calculated using equation (1) is set at 15% or less. Furthermore, since slag aggregation on the surface of the weld bead 6 is inhibited when there is a decrease in the amount of slag formed, it is preferable that the slag-to-coverage area ratio be 9% or less, or more preferably 5% or less. Furthermore, the lower the amount of non-conductive slag formed, the better the chemical conversion coating capacity and the electrodeposition coating capacity. Therefore, since it is preferable for the slag-to-Sratio coverage area ratio to be as small as possible, there is no particular limitation on the lower limit of the slag-to-Sratio coverage area ratio. It is preferable for the slag-to-Sratio coverage area ratio to be 0.1% or higher. Sratio = 1 00 x Sslag / Sbead ... (1) To prevent the slag from being distributed unevenly on the surface of weld bead 6, it is necessary to stabilize the shape of weld bead 6. Therefore, in the present invention, a weld bead width ratio Wratio (%) calculated using equation (2) from the maximum value Wmax (mm) and the minimum value Wmin (mm) of a weld bead width (refer to Fig. 2) in a direction perpendicular to a line parallel to the welding direction of weld bead 6 (hereinafter referred to as the weld line) is set at 60% or more. By decreasing the variation in the weld bead width (i.e., by decreasing the difference between Wmax and Wmin), the shape of weld bead 6 becomes stable. As a result, it is possible to maintain a constant heat input to the weld bead. That is, it is possible to form a weld bead 6 with a uniform surface quality.Therefore, it is possible to obtain a uniform chemical conversion coating layer formed by chemical conversion coating and a uniform coating film formed by electrodeposition coating. Furthermore, as a result of reducing the difference between Wmax and Wmin, it is possible to inhibit the formation of a localized treatment solution pool at the Wmin position when performing chemical conversion or electrodeposition coating. Therefore, the bead width ratio is preferable. LQ / ZLn / ZZnZ / B / YILI welding Wratio should be 70% or more or preferably 80% or more. There is no particular limitation on the upper limit of the weld bead width ratio (Wratio). It is preferable for the weld bead width ratio (Wratio) to be 100% or less. Wratio = 100 x Wmin / Wmax ... (2) It is preferable that arc welding be carried out with the steel sheet 3 fixed at the cathode and the welding wire 1 fixed at the anode (i.e., with so-called reverse polarity). When using reverse polarity, once a cathode spot is formed, which is a source of electron emission, a region 4 (called the cleaning region) is formed on the steel sheet 3, in which oxides (e.g., mill scale formed during the manufacturing process of the steel sheet 3, oxides formed due to heat input during welding, and the like) are formed on the surface of the steel sheet 3. If the distance perpendicular to the weld line between the outer edge of the cleaning zone 4 and the toe of the weld bead 6—that is, the cleaning width M (refer to Fig. 2)—is too small, oxides remain in the vicinity of the toe of the weld bead 6. Consequently, since the chemical conversion coating layer formed during chemical conversion and the coating film formed during electrodeposition become non-uniform, corrosion tends to progress along the weld bead. Therefore, it is preferable that the minimum value Mmin (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. Furthermore, in the original portion of a steel sheet unaffected by the welding heat, no improvement in chemical conversion coating capacity or electrodeposition coating capacity can be expected due to a cleaning function. Additionally, where the cathode spot formation region is wide, the arc discharge becomes unstable. Therefore, it is preferable for the maximum value Mmax (mm) of the cleaning width M (mm) to be 8.0 mm or less. When performing reverse polarity arc welding, welding wire 1 is placed at the anode, and steel sheet 3 is placed at the cathode. Then, as a result of applying a welding voltage through the welding wire 1, which is continuously fed through the center of the welding torch 2 to the steel sheet 3, a portion of the shielding gas, fed from inside the welding torch 2, is ionized to form plasma. Consequently, arc 5 is formed between the welding wire 1 and the steel sheet 3. The remaining shielding gas (i.e., the portion of the gas that is not ionized and that flows from the welding torch 2 to the steel sheet 3) seals the arc 5, the molten metal 7, and the weld pool 8 from the outside air (refer to Fig. 3a and 3b).Consequently, the incorporation of oxygen (i.e., slag formation) and the incorporation of nitrogen (i.e., blowhole formation) are avoided. The leading end of the welding wire 1 melts from the heat of the arc 5 to form molten metal 7, and the droplet of molten metal 7 is transported to the weld pool 8 by electromagnetic force, gravity, and similar means. At this point, there is a state in which the molten metal 7 is separated from the weld pool 8 (refer to Fig. 3a) and a state in which the molten metal LcwzLn / zznz / e / γΐΛΐ is in contact with the weld pool 8, i.e., a short-circuit state (refer to Fig. 3b), which is repeated alternately and regularly. Then, as a result of such a phenomenon occurring continuously while the welding wire 1 moves in the direction of the weld line, the weld pool 8 solidifies to form a weld bead 6 on the back side of the weld line. In the case of arc welding using argon gas (Ar) as a shielding gas, since the amount of oxygen mixed into the molten metal 7 and the weld pool 8 is significantly small, the effect of preventing slag formation is achieved. However, since the cathode point moves around too much, there is the disadvantage that the weld bead 6 tends to meander or have a wavy shape. At this point, the chemical composition of the argon gas described above is one that contains argon in an amount of 99.0% or more in terms of volume fraction. Such a shielding gas, which contains primarily the argon gas described above, is also called argon shielding gas. To eliminate this disadvantage, in arc welding, the cycle in which a short circuit occurs between the welding wire 1 and the steel sheet 3 (hereinafter referred to as the short-circuit cycle) and the frequency at which this short circuit occurs (hereinafter referred to as the short-circuit frequency) are specified in the present invention. Specifically, it is preferable that the maximum value of the short-circuit cycle Tcyc(s) be 1.5 or less and that the average value of the short-circuit frequency (average short-circuit frequency) Fave (Hz) be from 20 Hz to 300 Hz. By specifying the maximum short-circuit cycle value and the average short-circuit frequency to achieve stable droplet transfer, since it is possible not only to inhibit slag formation but also to achieve a stable arc discharge, it is possible to form a weld bead 6 in which the slag-to-coverage area ratio Sratio and the weld bead width ratio Wratio are within the ranges described above. If the droplet volume formed from the leading end of welding wire 1 is excessively large or small, the weld pool 8 becomes unstable. Specifically, if the average short-circuit frequency Fave is below 20 Hz, large droplets are transferred to the weld pool 8, or droplet transfer modes other than short-circuit transfer (e.g., transmission transfer) are mixed unevenly. Furthermore, if the average short-circuit frequency Fave is above 300 Hz, even if the droplet size is small, arc reignition due to short circuits occurs too frequently. Therefore, in any of these cases, since the weld pool 8 is disturbed, it is difficult to eliminate the wavy or serpentine shape of the weld bead.That is, by controlling the average short-circuit frequency Fave to be from 20 Hz to 300 Hz, it is possible to control the volume of a droplet that is transferred to the welding bath 8 in a short-circuit cycle so that it is approximately the same as that of a sphere that has a diameter equal to that of the welding wire 1. As a result, it is possible to stabilize the droplet transfer. To eliminate variations in the volume of a droplet transferred to the weld pool 8 during a short-circuit cycle, thereby improving weld bead uniformity, it is preferable that the average short-circuit frequency (Fave) be 35 Hz or higher, or even more preferably 50 Hz or higher. Furthermore, in the case of a short-circuit frequency... LQ / ZLn / ZZnZ / B / YILI If the average short-circuit frequency (Fave) is large, it's possible that droplets with small volumes will be dispersed as numerous splashes during short-circuit and restart events. Therefore, it's preferable for the average short-circuit frequency (Fave) to be 250 Hz or less, or even better, 200 Hz or less. Furthermore, if the maximum short-circuit cycle (Tcyc) exceeds 1.5 s, droplet transfer becomes unstable, resulting in unstable weld bead width and penetration depth. Therefore, by controlling the maximum short-circuit cycle (Tcyc) to 1.5 s or less, it is possible to form a weld bead with a good shape. The term maximum short-circuit cycle (Tcyc) denotes the maximum value of a short-circuit cycle in a welding pass required to form an arc-welded joint. This means that no single short-circuit cycle in a welding pass should exceed 1.5 s. By specifying the average short-circuit frequency Fave and the maximum short-circuit cycle Tcyc as described above, regular and stable droplet transfer is possible in arc welding using an Ar shielding gas. To control the average short-circuit frequency Fave described above and ensure it is 20 Hz or higher, it is preferable that the maximum short-circuit cycle Tcyc be 1.0 Hz or lower, or even more preferably 0.2 Hz or lower. Furthermore, it is sufficient that the maximum short-circuit cycle Tcyc be within a range where the average short-circuit frequency Fave is 300 Hz or lower, and it is preferable that the maximum short-circuit cycle Tcyc be 0.004 Hz or higher. The term “Fave average short-circuit frequency” indicates the average short-circuit frequency in a welding pass to form an arc weld. That is, when measuring the change in arc voltage during a welding pass using a measuring device (e.g., an oscilloscope) to count the number of times the arc voltage reaches zero, the Fave average short-circuit frequency is defined as the value obtained by dividing the number of times by the time (s) required for the welding pass (number / s = Hz). Here, examples of preferred welding conditions include welding current: 150 A to 300 A, arc voltage: 20 V to 35 V, gas flow rate (Ar): 15 liters / min to 25 liters / min, distance L between the steel sheet 3 and a contact tip (hereafter referred to as CTWD): 5 mm to 30 mm, and the like. Here, the welding current and arc voltage are represented by their respective average values ​​over a single welding pass. Furthermore, there is no particular limitation on the methods used to control the average short-circuit frequency and the maximum short-circuit cycle time to keep them within the ranges described above. For example, when controlling the current waveform using pulse current as illustrated in Fig. 4, where a peak current is defined as Ipeak (A), a base current is defined as Ibase (A), a peak time is defined as δpeak (ms), a rise time is defined as tup (ms), a fall time is defined as Idown (ms), and CTWD is defined as L (mm), as a result of X (As / m) calculated using the following equation (3) satisfying the relational expression 50 < X ​​< 250, it is possible to more effectively achieve the effect of the present invention. X = (Ipeak x Ípeak / L) + (Ipeak + Ibase) x (tup + Ídown) / (2 x L) · (3) In the event that the value of X (As / m) calculated using equation (3) is excessively small, there may be a case where arc 5 oscillates and / or droplet transfer becomes LcwzLn / zznz / e / γΐΛΐ unstable. On the other hand, if the value of X is too large, there may be a case where the welding wire 1 becomes immersed in the welding bath 8, or a case where a droplet that has grown is ejected at the moment of a short circuit, resulting in a deterioration in the shape of the weld bead, 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 as the unit of X (As / m) denotes seconds, and the unit of δpeak, δup, and δdown (ms) is milliseconds (= 1 / 1000 seconds). Furthermore, if the distance L between the steel sheet 3 and the contact tip is too small, the welding torch 2 will experience significant wear and tear, resulting in an unstable weld. Conversely, if the distance L between the steel sheet 3 and the contact tip is too large, the arc 5 will oscillate. Therefore, a distance of L between 5 mm and 30 mm, or preferably between 8 mm and 20 mm, is preferable. If the Ipeak value is too low, insufficient heat input will result in a deterioration of the weld bead shape. If the Ipeak value is too high, burning will occur, and spattering will increase. Therefore, an Ipeak value of 250 A to 600 A is preferable. An Ipeak value of 400 A or higher is preferable to one of 500 A or lower. If the base current (Ibase) is too small, the arc becomes unstable. If the base current (Ibase) is too large, a burn-out will occur. Therefore, a base current (Ibase) of 30 A to 120 A is preferable. A base current (Ibase) of 40 A or more is preferable to a base current (Ibase) of 100 A or less. If the peak time (ipeak) value is too small, sufficient heat input cannot be achieved. If the peak time (ipeak) value is too large, burns will occur. Therefore, an ipeak value between 0.1 ms and 5.0 ms is preferable. An ipeak value of 1.0 ms or higher is preferable to an ipeak value of 4.5 ms or lower. If the up or down time (tup or ídown) is too small, arc oscillation is induced. If the up or ídown time is too large, the weld bead shape deteriorates. Therefore, it is preferable for each of the up and ídown times to be between 0.1 ms and 3.0 ms. It is even more preferable for each of the up and ídown times to be 0.5 ms or more, and for each of the up and ídown times to be 2.5 ms or less. When the base time of the pulse current is defined as Ibase (ms), even though Ibase is not used in equation (3), which is used to calculate the value of X, if Ibase is too small, there is an excessive decrease in droplet size. If Ibase is excessively large, there is an excessive increase in droplet size. In either case, the weld becomes unstable. Therefore, it is preferable for Ibase to be between 0.1 ms and 10.0 ms. It is even more preferable for Ibase to be 1.0 ms or more and for Ibase to be 8.0 ms or less. Furthermore, in the present invention, it is not necessary for a short circuit to occur in each cycle of the pulse current. It is sufficient for a short circuit to occur once in one or more pulses. Moreover, provided that a short circuit occurs once in one or more pulses, there is no particular limitation on the pulse frequency of the pulse current. In the present invention, the purpose of using pulse current is (1) to promote the LcwzLn / zznz / e / γΐΛΐ stable droplet growth in base time while inhibiting arc oscillation by applying a lower current and (2) promoting a short circuit in peak time and fall time by pushing down the grown droplet into the weld pool by using electromagnetic force and the cutting force of the Ar shielding gas without separating the grown droplet from the wire. In the present invention, it is not necessary to supply oxygen or add special elements. Therefore, by using a solid wire, which is less expensive than a flux-cored wire, as welding wire, it is possible to reduce process costs. Herein, the solid wire that may be used preferably in the present invention has a chemical composition containing C: 0.020% by mass to 0.250% by mass, Si: 0.05% by mass to 1.50% by mass, Mn: 0.50% by mass to 3.0% by mass, P: 0.020% by mass or less, S: 0.03% by mass or less, and a balance of Fe and incidental impurities. It is preferable that the diameter of the solid wire be from 0.4 mm to 2.0 mm. EXAMPLES The arc-welded joint and the arc-welding method according to the present invention will be described in detail according to the examples. By performing a lap weld (see Fig. 1) on two steel sheets (each 2.6 mm thick) 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 (each 1.2 mm in diameter) indicated by the wire codes given in Table 2 are given in Table 4. The remainder, which differed from the constituents given in Table 1 or Table 4, consisted of iron and incidental impurities. After alkaline degreasing, surface conditioning, and zinc phosphate-based chemical conversion coating on the arc-welded joint, cationic electrodeposition was performed to achieve a film thickness of 15 μm on the flat base steel sheet other than the weld.Subsequently, a corrosion test was performed in accordance with SAE J 2334 for 60 cycles. Here, the weld bead surface area Sbead and the slag surface area Sslag were derived by taking a surface image of the weld bead region 6, excluding the start / end portions 10 of the weld bead (each 15 mm long), directly from above and measuring the projected areas of the weld bead and slag as viewed from above. For a weld bead 6 shorter than 130 mm, the surface image was taken of the entire length of the weld bead 6, excluding the start / end portions 10 of the weld bead. For a weld bead 6 130 mm or longer, the surface image was taken of a portion (100 mm long) of the weld bead 6, excluding the start / end portions 10 of the weld bead.The slag-coverage area ratio (Sratio) was derived using equation (1) above from the weld bead surface area (Sbead) and the derived slag surface area (Sslag) as previously indicated. The derived slag-coverage area ratio (Sratio) is given in Table 3. Similarly, the maximum value Wmax and the minimum value Wmin of the weld bead width were measured by taking the image of the surface of the weld bead region 6 excluding the start / end portions 10 of the weld bead (which have a length LcwzLn / zznz / e / γALA of 15 mm each) and analyzing the captured image. In the case of a weld bead 6 with a length of less than 130 mm, the surface image was taken of the entire length of the weld bead 6, excluding the start / finish end portions 10 of the weld bead. In the case of a weld bead 6 with a length of 130 mm or more, the surface image was taken of a portion (having a length of 100 mm) of the weld bead 6, excluding the start / finish end portions 10 of the weld bead. The weld bead width ratio Wratio was derived using equation (2) above from the maximum value Wmax and the minimum value Wmin of the weld bead width measured as described above. The derived weld bead width ratio Wratio is provided in Table 3. Furthermore, the maximum Mmax and minimum Mmin values ​​of the cleaning width were measured by imaging the surface of the weld bead region 6, excluding the start / end portions 10 of the weld bead (each 15 mm long), and analyzing the resulting image. For weld bead 6 less than 130 mm long, the surface was imaged along the entire length of the weld bead, excluding the start / end portions of weld bead 10. For weld bead 6 130 mm or longer, the surface was imaged along a 100 mm portion of weld bead 6, excluding the start / end portions of weld bead 10. The maximum Mmax and minimum Mmin values ​​of the cleaning width are given in Table 3. The corrosion resistance assessment given in Table 3 was performed as follows. First, after removing the electrodeposition coating by immersing the arc-welded joint that had been subjected to the corrosion test in a removal solution, the corrosion product was removed according to ISO 8407. Subsequently, if the start / end portions 10 of the weld bead (each 15 mm long) were included, an image of the surface region excluding the start / end portions 10 of the weld bead was taken, and the maximum corrosion width Hmax from the foot 9 of the weld bead was measured by analyzing the image. The corrosion resistance assessment was performed according to the following criteria, and the assessment results are indicated by the reference symbols A to C and F. Here in Table 3, reference symbol A denotes a case of a maximum corrosion width Hmax from the toe of the weld bead of less than 3.0 mm. Reference symbol B denotes a case of a maximum corrosion width Hmax from the toe of the weld bead of 3.0 mm or more and less than 4.5 mm. Reference symbol C denotes a case of a maximum corrosion width Hmax from the toe of the weld bead of 4.5 mm or more and less than 6.0 mm. Reference symbol F denotes a case of a maximum corrosion width Hmax from the toe of the weld bead of 6.0 mm or more. The range indicated by reference symbol A is the highest, followed by those indicated by reference symbols B and C in that order, and the range indicated by reference symbol F is the lowest. Here, as illustrated in Fig. 5, of the term weld bead start / end portions, the term weld bead start end portion denotes a region of the weld bead from a weld bead start end position (weld start position) to a point on the weld line located 15 mm towards a position of LQ / ZLn / ZZnZ / B / YILI end of the weld bead (weld end position), and the term weld end end portion denotes a region of the weld bead from the weld end end portion to a point on the weld line located 15 mm toward the weld start end position. The term “weld bead toe” indicates a boundary in a direction perpendicular to the weld line of the weld bead between the weld metal and the unfused base steel sheet. The results of the evaluation are given in Table 3. LcwzLn / zznz / e / γΐΛΐ Table 1 Tensile strength of steel sheet Chemical composition of steel sheet (%mass) C Si Mn PS 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 2 1. X = (IpEAK X IpEAk / L) + (IpEAK + IbASe) Table 3 No. Ratio Wratio Mmax Mmin Hmax Evaluation *2 Grade % % mm mm mm 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 foot of the weld bead of less than 3.0 mm. B denotes a case of a maximum corrosion width Hmax from the toe of the weld bead of 3.0 mm or more and less than 4.5 mm. C denotes a case of a maximum corrosion width Hmax from the toe of the weld bead of 4.5 mm or more and less than 6.0 mm. F denotes a case of a maximum corrosion width Hmax from the toe of the weld bead of 6.0 mm or more. LQ / zLn / zznz / e / γΐΛΐ Table 4 Cable code Chemical composition of welding wire (%mass) C Si Mn PS W1 0.068 0.57 1.06 0.006 0.006 W2 0.054 0.90 1.37 0.005 0.015 As indicated in Tables 2 and 3, it is clarified that, in the case of welds Nos. 5 to 17, which were examples of the present invention, since the Sratium was 15% or less, and since the Wratium was 60% or more, excellent arc-welded joints were obtained in terms of corrosion resistance. From these examples of the present invention, in the case of welds numbered 7 to 17, since Mmin was 0.5 mm or more, excellent arc-welded joints were obtained in terms of superior corrosion resistance. In contrast, in the case of welds No. 1 and 2 where the ratio was greater than 15%, and in the case of welds No. 3 and 4 where the ratio was less than 60%, that is, in the case of the comparative examples, there was a deterioration in the phosphatability and the capacity for coating by electrodeposition, which resulted in a deterioration in the corrosion resistance of the arc-welded joints. Furthermore, as indicated by the data from welds Nos. 5 to 17, which served as examples for the present invention, it is clear that excellent arc-welded joints were obtained in terms of corrosion resistance, regardless of whether the welding wire used was for ultra-high tensile strength steel sheet (wire code W1 in Table 4) or for mild steel sheet (wire code W2 in Table 4). List of reference numbers Welding wire Welding torch Steel sheet (base material) Cleaning region Bow Weld bead Molten metal (drop) Welding bath Welding bead foot Start / end portion of weld bead

Claims

1. An arc-welded joint, characterized in that it has a slag-to-coverage area ratio Sratio (%) of 15% or less, where Sratio is calculated by equation (1), where the area of ​​a surface of a weld bead formed by performing an arc weld on a steel sheet is defined as a weld bead surface area Sbead (mm2) and, of the weld bead surface area Sbead, an area of ​​a region covered with slag is defined as a slag surface area Sslag (mm2), and having a weld bead width ratio Wratio (%) of 60% or more, where Wratio is calculated by equation (2) from a maximum value Wmax (mm) and a minimum value Wmin (mm) of a weld bead width in a direction perpendicular to a weld line of the weld bead: Sratio = 100 x Sslag / Sbead · (1) Wratio = 100 x Wmin / wmax (2).

2. The arc-welded joint according to claim 1, further characterized in that a cleaning region, in which the oxides formed on a surface of the steel sheet are removed by arc welding, is formed so as to be adjacent to the toe of the weld bead, and wherein a minimum value Mmin (mm) of a distance M (mm) in the direction perpendicular to the weld line between an outer edge of the cleaning region and the toe of the weld bead is 0.5 mm or more.

3. An arc welding method for producing an arc-welded joint, characterized in that the arc-welded joint has a slag-to-coverage area ratio Sratio (%) of 15% or less, where Sratio is calculated using equation (1), where the area of ​​a weld bead surface formed by arc welding on a steel sheet is defined as a weld bead surface area Sbead (mm2) and, of the weld bead surface area Sbead, an area of ​​a region covered with slag is defined as a slag surface area Sslag (mm2), and with a weld bead width ratio Wratio (%) of 60% or more, where Wratio is calculated using equation (2) from a maximum value Wmax (mm) and a minimum value Wmin (mm) of a weld bead width in a direction perpendicular to a weld line of the weld bead: Sratio = 1 00 x Sslag / Sbead · (1) Wratio = 1 00 x Wmin / Wmax · (2).

4. The arc welding method according to claim 3, further characterized in that the arc welding is performed with reverse polarity, wherein a cleaning region, in which oxides formed on a surface of the steel sheet are removed due to the formation of a cathode spot, which is an electron-emitting source, is formed adjacent to a toe of the weld bead, and wherein a minimum value Mmin (mm) of a distance M (mm) in the direction perpendicular to the weld line between an outer edge of the cleaning region and the toe of the weld bead is 0.5 mm or more. LQ / zLn / zznz / e / γΐΛΐ 5. The arc welding method according to claim 3 or 4, further characterized in that Ar gas is used as a shielding gas in arc welding.

6. The arc welding method according to any of claims 3 to 5, further characterized in that a short circuit occurs intermittently between the steel sheet and a welding wire, and wherein said short circuit occurs at an average short-circuit frequency Fave (Hz) of 20 Hz to 300 Hz with a maximum short-circuit cycle Tcyc (s) of 1.5 s or less.

7. The arc welding method according to any of claims 3 to 6, further characterized in that a pulse current is used as the welding current of the arc welding, and wherein X (As / m) is calculated using equation (3) from the peak current Ipeak (A), the base current Ibase (A), the peak time ípeak (ms), the rise time tup (ms), and the fall time toowu (ms) of the pulse current and a distance L (mm) between the steel sheet and a contact tip that satisfies a relational expression 50 < X ​​< 250: X = (Ipeak x Ípeak / L) + (Ipeak + Ibase) x (tup + Ídown) / (2 x L) ··· (3).

8. The arc welding method according to any of claims 3 to 7, further characterized in that a solid wire is used as the welding wire in the arc welding.

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