Method for manufacturing gas-shielded arc welded joint

Abstract

The purpose of the present invention is to provide a method for manufacturing a gas-shielded arc welded joint. The present invention provides a method for manufacturing a gas-shielded arc welded joint using gas-shielded arc welding that causes a short circuit transfer. This welding is pulse welding, and the welding conditions of this welding are such that: 98 vol% or more of an Ar gas is used as the shielding gas; the average welding current IAVE is 160-250 A; the pulse base current IB is 40-130 A; the arc voltage V is 18.0-33.0 V; the wire feeding speed W is 100.0-145.0 mm / s; the sheet thickness t of a steel sheet is 1.2-4.2 mm; the pulse base current IB and the average welding current IAVE satisfy formula (1); and the pulse base current IB, the arc voltage V, the wire feeding speed W, and the sheet thickness t satisfy formula (2). (1): 0.24 ≤ IB / IAVE ≤ 0.62 (2): 2.38 × t ≤ (IB × V) / W ≤ 5.67 × t + 9.66

Description

Manufacturing method for gas shielded arc welded joints

[0001] This invention relates to a method for manufacturing a gas-shielded arc welded joint using gas-shielded arc welding.

[0002] In recent years, there has been a growing need for automobiles to achieve both increased strength and rigidity in the components used in the vehicle body to improve safety and reliability, and reduced weight to improve fuel efficiency. As a result, the use of high-strength steel plates is being used to thin the steel plates of components. On the other hand, among the various components used in automobiles, components in the suspension (such as lower arms) use thicker steel plates compared to the body due to their strength and rigidity. Therefore, if the steel plates used in suspension components are made stronger and thinner, further weight reduction of the vehicle body will be possible. This will enable improved fuel efficiency while maintaining component strength and rigidity.

[0003] Generally, components used in corrosive environments are treated with rust prevention measures such as chemical conversion treatment and electrodeposition coating after welding to ensure corrosion resistance. However, rust and corrosion may occur in and around the welds over time. As described above, corrosion that occurs in electrodeposited components tends to start from the welds and, over time, expands to a wide area around the welds, accompanied by blistering of the coating, and also progresses in the thickness direction of the plate. As corrosion progresses in this way, the thickness of the plate in and around the welds decreases, resulting in a decrease in the strength of the weld and, consequently, the strength of the component. In other words, if corrosion occurs and progresses in components where load is applied to the welds (for example, components in the undercarriage of an automobile), it may lead to the failure of the component.

[0004] When performing electrodeposition coating, a chemical conversion treatment (e.g., zinc phosphate treatment) is applied to the base steel plate and weld metal as a pretreatment to improve adhesion between the base steel plate and weld metal and the electrodeposited coating film before electrodeposition coating. Zinc phosphate treatment, a widely used example of chemical conversion treatment, is a technique that grows zinc phosphate crystals on the surface of the base steel plate and weld metal to improve the adhesion of the coating film in electrodeposition coating. However, with conventional techniques, even in components that have undergone chemical conversion treatment prior to electrodeposition coating, blistering of the coating film frequently occurs over time in and around the weld. In other words, with the technique of performing the above chemical conversion treatment as a pretreatment before electrodeposition coating, it is difficult to completely suppress the occurrence of corrosion starting from the weld.

[0005] Furthermore, when gas-shielded arc welding is performed on components using steel plates with a plating layer, the plating layer evaporates in the weld area, which is exposed to high heat by the arc plasma (hereinafter referred to as the arc), and the unplated area is locally exposed. Therefore, a significant improvement in corrosion resistance that would justify the increased cost associated with using steel plates with a plating layer cannot be expected.

[0006] As explained above, various manufacturing technologies have been developed to improve the corrosion resistance of components, but each has its own advantages and disadvantages. Furthermore, from the perspective of improving corrosion resistance while keeping manufacturing costs down, technologies are being considered to more effectively prevent the occurrence and progression of corrosion starting from welded joints.

[0007] Specifically, the following have been conventionally known as starting points for corrosion occurring in welds: (a) slag adhering to the weld (mainly the surface of the weld bead), (b) welding fumes adhering to the weld, and (c) oxides generated on the surface of the steel plate exposed to high temperatures by welding. Even when components with the above-mentioned adherings (a) and (b) or oxides (c) present in the weld are subjected to chemical conversion treatment, these adherings and products serve as starting points, and areas not covered by the chemical conversion treatment layer made of zinc phosphate crystals remain locally.

[0008] Furthermore, in areas not covered by the chemical conversion treatment layer, electrodeposition coating results in insufficient film formation and poor adhesion, leading to a significant decrease in corrosion resistance. This results in a reduction in plate thickness due to the occurrence and progression of corrosion. Therefore, the following patent documents 1 to 3 have been considered as technologies to prevent the formation of the deposits described in (a) and (b) and the oxides described in (c) above.

[0009] For example, Patent Document 1 discloses a technique in which a non-oxidizing acidic solution with a pH of 2 or less and a liquid temperature of 30 to 90°C is sprayed or immersed on the welded area and its vicinity after gas shielded arc welding and before electrodeposition coating. This technique removes the slag (a) mentioned above, the welding fumes (b), and the oxides (c) mentioned above by dissolving the weld bead and steel plate with a non-oxidizing solution.

[0010] Patent Document 2 discloses a technique for improving the corrosion resistance of the welded area and its vicinity after painting by reducing the total Si content of the welding wire and base material used in gas shielded arc welding, and increasing the total Mn content of the welding wire and base material.

[0011] Patent Document 3 discloses a technique for sufficiently forming a chemical conversion layer even in weld beads containing slag, welding fumes, and oxides, by adjusting the components of the treatment solution used in the chemical conversion treatment. This technique facilitates the formation of the chemical conversion layer by performing surface treatment using a surface conditioning solution containing zinc phosphate colloid. Furthermore, by performing the chemical conversion treatment using a zinc phosphate treatment solution with a fluorine concentration of 100 ppm by mass or more, slag, welding fumes, and oxides are dissolved and removed, thereby improving the adhesion of the coating film by electrodeposition coating.

[0012] JP-A-9-20994 JP-A-8-33997 Patent No. 5549615

[0013] However, the technology disclosed in Patent Document 1 requires washing away the acidic solution before electrodeposition coating, which complicates the manufacturing process of the components. Furthermore, since the components formed into the desired shape are made by overlapping and joining steel plates of various shapes, residual acidic solution in the gaps between the overlapping plates causes severe corrosion. In addition, because a large amount of acidic solution is used, the manufacturing equipment is exposed to a corrosive environment, making it prone to corrosion and malfunction. Moreover, it is necessary to prevent the scattering of fumes and ensure the safety of workers.

[0014] Furthermore, as described in Patent Document 2, if the Si content is intentionally reduced, the Mn content must be limited from the viewpoint of ensuring joint strength, requiring adjustment of the welding wire composition. Moreover, the Mn content of the welding wire needs to be adjusted according to the composition of the steel plate. In other words, the technology disclosed in Patent Document 2 requires the use of a special welding wire suited to the steel plate, and if a general-purpose welding wire is used, it may not be possible to achieve both corrosion resistance after painting and joint strength.

[0015] Furthermore, the technology disclosed in Patent Document 3 uses a zinc phosphate treatment solution containing fluorine, which is designated as a toxic substance. Therefore, when discharging the wastewater outside the factory, the fluorine content must be reduced to a level that meets environmental standards. Consequently, in addition to the manufacturing facilities for the components, large-scale wastewater treatment facilities are required.

[0016] This invention has been made in view of these problems, and aims to provide a method for manufacturing a gas-shielded arc welded joint that can prevent oxygen from being mixed into the weld metal and suppress the formation of slag without the use of special equipment, and can stably produce an arc welded joint with sufficient penetration.

[0017] The inventors investigated a manufacturing method to improve the corrosion resistance of the welded joint and to stably obtain a gas-shielded arc welded joint with sufficient penetration in the welded joint. As a result, they found that applying a pulse base current corresponding to the plate thickness of the steel plate to be welded and the average welding current, stably forming molten metal at the end of the welding wire during the pulse base period, and sufficiently heating the molten metal and molten pool is effective in solving the above problems.

[0018] To suppress the amount of slag generated, which affects the corrosion resistance of welded joints, it is effective to suppress the oxidation of Si, Mn, Ti, and other elements contained in the steel plate and welding wire. Furthermore, by using a shielding gas with a reduced content of oxidizing gases, the oxidation of these elements can be suppressed, thereby reducing the amount of slag generated.

[0019] However, in gas shielded arc welding with a high Ar gas ratio in the shielding gas, the arc tends to spread due to the small thermal pinch effect, and the arc becomes unstable because the cathode point crawls around on the steel plate surface after the oxide film on the steel plate surface is removed by the cleaning action. As a result, concentrated heat input cannot be obtained at the target joint location. Gas shielded arc welded joints fabricated under such conditions tend to have shallow penetration depth, resulting in a new problem of reduced joint strength.

[0020] Therefore, the inventors focused on utilizing short-circuit transfer, which is less affected by arc fluctuations. Based on this, they considered that by applying a pulse base current that matches the plate thickness and average welding current, stably forming molten metal at the end of the welding wire during the pulse base period, and sufficiently heating the molten metal and molten pool, sufficient penetration can be ensured even in gas-shielded arc welding with a high Ar gas ratio in the shielding gas.

[0021] The present invention is based on the above findings, and its gist is as follows: [1] A method for manufacturing a gas shielded arc welded joint using gas shielded arc welding which results in a short-circuit transition, wherein the gas shielded arc welding is pulse welding, and the welding conditions for the gas shielded arc welding are: Ar gas is used as the shielding gas at a concentration of 98 volume% or more, and the average welding current is I AVE The current is 160-250A, and the pulse base current is I B The current is 40 to 130 A, the arc voltage V is 18.0 to 33.0 V, the wire feed speed W is 100.0 to 145.0 mm / s, and the thickness t of the steel plate to be welded is 1.2 to 4.2 mm, and the pulse base current I B and the average welding current I AVEsatisfies the following formula (1), and the pulsed base current I B and the arc voltage V, the wire feeding speed W, and the plate thickness t of the steel plate satisfy the following formula (2). A method for manufacturing a gas shielded arc welding joint. 0.24 ≦ I B / I AVE ≦ 0.62 … (1) 2.38 × t ≦ (I B × V) / W ≦ 5.67 × t + 9.66 … (2) [2] The welding conditions further include a pulsed peak current I P of 450 to 650 A and a pulsed peak time T P of 1.5 to 3.5 ms. The method for manufacturing a gas shielded arc welding joint according to [1] above.

[0022] According to the present invention, it is possible to stably obtain a gas shielded arc welding joint having an extremely small amount of slag adhesion on the surface of the welded portion and sufficient penetration. Thereby, improvement of the corrosion resistance of the welded portion can be realized. Moreover, even when the welding apparatus used in conventional MAG welding using an oxidizing gas or MIG welding using a gas mainly composed of an inert gas is applied to the welding of the present invention, the above-described effects can be obtained without special specification changes.

[0023] FIG. 1 is a perspective view schematically showing an example of applying the present invention to fillet welding. FIG. 2 is a welded portion formed by the fillet welding of FIG. 1, and is a perspective view schematically showing the bead termination portion and the start and end portions of the welding bead. FIG. 3 is a cross-sectional view taken along line A - A of the welded portion shown in FIG. 2, and is a view schematically showing the bead termination portion and its periphery. FIGS. 4(A) and 4(B) are schematic views showing the state of droplet transfer by conventional MIG welding. FIGS. 5(A) and 5(B) are schematic views showing the state of short-circuit transfer according to the present invention.

[0024] Hereinafter, an embodiment of the method for manufacturing a gas shielded arc welding joint of the present invention will be described with reference to each drawing. Note that the present invention is not limited to this embodiment.

[0025] The method for manufacturing a gas shielded arc welding joint of the present invention includes a welding step of joining two or more steel plates by gas shielded arc welding to form a gas shielded arc welding joint.

[0026] Hereafter, an embodiment of the present invention applied to lap fillet welding will be described as an example of gas shielded arc welding. However, the present invention is not limited to lap fillet welding and can be applied to various welding techniques (for example, butt welding).

[0027] [Welding Process] In the welding process of the present invention, two or more prepared steel plates are overlapped, and the steel plates are filled-welded together using gas-shielded arc welding to form a gas-shielded arc welded joint. In the present invention, as described above, a short-circuit transfer that is less affected by arc fluctuations is utilized.

[0028] In other words, in this welding process, two or more steel plates are joined using gas shielded arc welding, which involves short-circuiting the welding wire and the base material (i.e., the steel plate), thereby creating a gas shielded arc welded joint.

[0029] The gas shielded arc welding method of this invention is pulse welding.

[0030] Furthermore, the welding conditions for the gas shielded arc welding shall be as follows: Ar gas shall be used as the shielding gas in an amount of 98% by volume or more. In addition, the average welding current I AVE The current is 160-250A, and the pulse base current is I B The current is 40 to 130 A, the arc voltage V is 18.0 to 33.0 V, the wire feed speed W is 100.0 to 145.0 mm / s, and the thickness t of the steel plate to be welded is 1.2 to 4.2 mm, and the pulse base current I B and the average welding current I AVE The pulse base current I satisfies equation (1) described below. B The arc voltage V, the wire feeding speed W, and the thickness t of the steel plate satisfy equation (2) described below.

[0031] First, the gas shielded arc welding method of the present invention will be explained with reference to Figures 1 to 3.

[0032] Figure 1 shows an example of two steel plates being fillet welded together using gas shielded arc welding. Figure 2 shows the weld formed by the fillet weld in Figure 1. Figure 3 shows a cross-sectional view of the weld shown in Figure 2 along line A-A in the thickness direction.

[0033] As shown in Figure 1, the welding wire 1, which is fed through the center of the welding torch 2, is used as the anode, and the steel plate 3 is used as the cathode, and a welding voltage is applied from a welding power source (not shown). The welding wire 1 is continuously fed to the welding line, which is formed by the corner 4 of a step created by stacking two base materials (hereinafter referred to as base material 3), which are steel plates 3. A portion of the Ar shielding gas (not shown) supplied from inside the welding torch 2 is ionized and turned into plasma, forming an arc 5 between the welding wire 1 and the base material 3. In addition, the portion of the Ar shielding gas that flows from the welding torch 2 to the base material 3 without ionization serves to shield the molten pool (not shown in Figure 1), which is formed by the melting of the arc 5 and the base material 3, from the outside air. Due to the heat of the arc 5, the tip of the welding wire 1 melts and becomes a droplet, and this droplet is transported to the molten pool by electromagnetic force, gravity, etc. This phenomenon occurs continuously as the welding torch 2 or base material 3 moves, causing the molten pool to solidify behind the weld line and forming a weld bead 6. This achieves the joining of the two steel plates, resulting in a welded joint with a welded section as shown in Figure 2.

[0034] In gas-shielded arc welded joints joined in this manner, if the penetration of the weld is insufficient (i.e., if the weld does not have sufficient penetration), stress concentration in the weld increases, resulting in a new problem: a decrease in joint strength.

[0035] In this invention, "sufficient penetration" refers to the penetration shape when the penetration depth (d) and the steel plate thickness (t) satisfy a predetermined relationship in a weld with a weld bead width of 5 mm or more, as shown in the cross-sectional view of the weld toe in Figure 3. The "weld toe" refers to the boundary between the weld metal and the unmelted base steel plate in a direction perpendicular to the weld line of the weld bead. The "penetration depth" can be measured by the method described in the examples. In this invention, the ratio (d / t) of the penetration depth (d) and the steel plate thickness (t) measured by this method is 0.15 or more, in which case the weld is evaluated as having "sufficient penetration."

[0036] Furthermore, when two or more steel plates to be welded have different thicknesses, the "steel plate thickness (t)" used in the above ratio (d / t) shall be the thickness of the lower plate. In the case of butt welding, the "steel plate thickness (t)" shall be the thickness of the steel plate with the smaller thickness (i.e., the thickness of the thinner plate).

[0037] This problem, in gas shielded arc welding using Ar gas at 98% volume or more as the shielding gas, cannot always be solved by simply suppressing arc fluctuations or controlling droplet transfer to make it periodic in order to stabilize the welding, as this may not provide sufficient heat input.

[0038] Therefore, in order to solve this problem, the present invention focuses on applying pulse base current and arc voltage that are matched to the plate thickness and wire feeding speed. By doing so, it is thought that sufficient melting can be ensured by stably forming molten metal at the wire end during the pulse base period, controlling Joule heating and the amount of heat retained in the molten metal, and optimizing the amount of heat input transferred to the molten pool during short-circuit transfer.

[0039] Next, with reference to Figures 4 and 5, the reason for using gas shielded arc welding with short-circuit transfer in the present invention will be explained. Figures 4 and 5 show a state in which two steel plates are overlapped and fillet welded.

[0040] Here, we consider an example where carbon steel is used for the steel plate (base material 3) to be joined by gas shielded arc welding. When dealing with carbon steel, conventional MIG welding presents the problem of extremely unstable welding.

[0041] MIG welding and MAG welding are reverse polarity welding processes where the electrode (wire) is the anode. As a result, the cathode is formed starting from a location on the base metal surface where the work function is low, such as an oxide, and electron emission is likely to occur. In aluminum alloys with a strong oxide film on the base metal surface, the cathode is stably formed starting from the oxide film on the weld line, allowing for good welding. However, in carbon steel with a relatively thin oxide film or no oxide film at all, and in MIG welding where oxides derived from O2 or CO2 are not generated, unlike MAG welding, the cathode is not fixed and moves violently around the base metal surface in search of a location with a low work function. Therefore, the welding is unstable, and the weld bead becomes meandering or wavy.

[0042] In response to this phenomenon, the inventors conducted welding experiments to observe the arc behavior. As a result, they concluded that the main cause of the meandering and wavy shape of the weld bead, which is a problem in MIG welding of carbon steel, is unstable droplet transfer.

[0043] Figures 4(A) and 4(B) show cross-sectional views in the thickness direction illustrating the droplet transfer process in conventional MIG welding, and as an example, they show the state of MIG welding under conditions where the welding current is 300A or less. Under these conditions, as shown in Figure 4(A), the wire 1 melts and is continuously transported from a long, slender liquid column to the molten pool 8, and as shown in Figure 4(B), a large droplet 7 is generated at the tip of the wire 1 and transported to the molten pool 8 by falling or short-circuiting. To suppress this unstable droplet transfer, it is considered effective to regularly detach the droplet 7 from the tip of the wire 1, but in the case of Ar shielding gas, the electromagnetic pinch force acting on the wire 1 is small, making it difficult to detach the droplet 7.

[0044] Therefore, the inventors have found that completing the bonding process between the tip of the wire 1 and the base material 3 by short-circuit transfer as shown in Figure 5 is an effective means of stabilizing droplet transfer.

[0045] Figure 5 shows a cross-sectional view in the thickness direction illustrating the short-circuit migration of the present invention, with Figure 5(A) showing the non-short-circuit state and Figure 5(B) showing the short-circuit state. The above-mentioned "short-circuit migration" refers to the regular repetition of this non-short-circuit state and short-circuit state, during the short-circuit state, when the molten droplet 7 migrates to the base material 3.

[0046] Next, we will explain why the gas shielded arc welding is replaced with pulse welding in this invention.

[0047] As described above, the present invention uses pulse welding. The purpose of using pulsed current is to (i) promote stable droplet growth while suppressing arc fluctuations by using a low current during the base period, and (ii) promote short circuits by pushing the grown droplets down into the molten pool with electromagnetic force and the shear force of Ar shielding gas, rather than detaching the grown droplets from the wire, during the fall period.

[0048] Furthermore, (iii) during the base period, the current density is concentrated at the wire end to effectively heat the molten metal, and during the peak period, the increased current increases the heat flux from the arc to the molten pool and the short-circuiting of the molten metal grown during the base period provides heat input, which also contributes to achieving sufficient penetration.

[0049] Furthermore, in this invention, it is not necessary to cause one short circuit in each cycle of the pulsed current; it is sufficient to cause one short circuit every one to several pulses. As long as one short circuit can be generated every one to several pulses, the pulse frequency of the pulsed current is not particularly limited.

[0050] Next, the welding conditions for gas shielded arc welding in the present invention will be described.

[0051] As described above, in gas shielded arc welding with a high Ar gas ratio in the shielding gas, the arc tends to spread and become unstable, preventing concentrated heat input. Therefore, in this invention, a pulse current is applied that matches the plate thickness and wire feeding speed in order to stably obtain arc welded joints with sufficient penetration in the weld area. In addition, it is also important to control the welding conditions of gas shielded arc welding as follows.

[0052] [Shielding gas: Ar gas of 98% by volume or more] If the Ar gas ratio of the shielding gas is less than 98% by volume, the chemical reaction between the oxidizing gas in the shielding gas and the alloying elements contained in the molten metal is accelerated, resulting in an increase in the amount of slag produced, which causes a decrease in corrosion resistance. For this reason, the Ar gas ratio of the shielding gas should be 98% by volume or more. Preferably, the Ar gas ratio of the shielding gas is 99% by volume or more. The upper limit of this ratio is not particularly limited, and the Ar gas ratio may be 100%.

[0053] Furthermore, if the Ar gas ratio is 98% by volume or more and less than 100% by volume, the remainder is an oxidizing gas, and this oxidizing gas shall be greater than 0% by volume and less than or equal to 2% by volume. Examples of oxidizing gases include CO ２ , O ２ These are some examples.

[0054] [Average welding current I AVE (Unit: A): 160-250 A] Average welding current I AVE This is the time-averaged value of the periodically changing welding current. Average welding current I AVE If the average welding current is too low, it will not be able to suppress the instability of the MIG arc, and the required penetration depth will not be achieved. AVE The average welding current I shall be 160A or higher. AVE It is preferable that the average welding current I AVE If the average welding current is too high, the heat input will be excessive, which can cause burn-through, a welding defect. AVE The average welding current I shall be 250A or less. AVE It is preferable that the current is 240A or less.

[0055] [Pulse base current I B (Unit: A): 40-130 A] Pulse base current I B If the pulse base current is too low, the arc discharge during the pulse base period becomes unstable, which can result in short-circuit instability or poor penetration. B The pulse base current I shall be 40A or more. B It is preferable that the current be 50A or more. On the other hand, the pulse base current I B If it is excessive, it will cause melting, and the pulse peak current I P and pulse base current I B The difference between the two cannot be sufficiently secured. As a result, the effect of pushing the molten droplet formed on the tip of the welding wire down into the molten pool and short-circuiting it is not sufficiently obtained, and welding may not be stable. Therefore, pulse base current I B The current shall be 130A or less. Pulse base current I B It is preferable that the current be 120A or less.

[0056] [Arc voltage V (unit: V): 18.0 to 33.0 V] If the arc voltage V is less than 18.0 V, sufficient heat input cannot be secured and the predetermined penetration depth cannot be obtained. For this reason, the arc voltage V should be 18.0 V or higher. Preferably, the arc voltage V should be 20.0 V or higher. On the other hand, if the arc voltage V is greater than 33.0 V, the arc spread becomes excessive, and the behavior of the molten droplet formed at the wire end becomes unstable, resulting in deterioration of the bead and penetration shape. For this reason, the arc voltage V should be 33.0 V or lower. Preferably, the arc voltage V should be 30.0 V or lower.

[0057] [Wire feed speed W (unit: mm / s): 100.0 to 145.0 mm / s] If the wire feed speed W is less than 100.0 mm / s, the resistance heating of the wire and the heat input from the arc to the wire increase, causing the wire to melt excessively. This increases the arc length, which is the distance between the base material and the wire tip, and destabilizes the arc, reducing the heat input to the base material and preventing the desired penetration depth from being achieved. Therefore, the wire feed speed W should be 100.0 mm / s or more. Preferably, the wire feed speed W should be 105.0 mm / s or more. On the other hand, if the wire feed speed W is greater than 145.0 mm / s, the size of the droplets formed on the tip of the welding wire becomes excessive, which may cause instability of short circuits or deterioration of the bead and penetration shape. Therefore, the wire feed speed W should be 145.0 mm / s or less. Preferably, the wire feed speed W should be 140.0 mm / s or less.

[0058] [Steel plate thickness t (unit: mm) to be welded: 1.2 to 4.2 mm] If the plate thickness t is less than 1.2 mm, the heat input will be excessive within the range of average welding current conditions required to achieve periodic short-circuit transfer, which may result in burn-through, a welding defect. For this reason, the plate thickness t should be 1.2 mm or more. It is preferable that the plate thickness t be 2.0 mm or more. On the other hand, if the plate thickness t is greater than 4.2 mm, the heat input will be more easily dispersed, and the required penetration depth may not be obtained. For this reason, the plate thickness t should be 4.2 mm or less. It is preferable that the plate thickness t be 3.6 mm or less.

[0059] Furthermore, if the thickness of the two or more steel plates to be welded differs, it is sufficient that each steel plate falls within the above numerical range.

[0060] [Pulse base current I B and average welding current I AVE [Relationship] 0.24 ≤ I B / I AVE ≤ 0.62 …(1) Pulse base current I B and average welding current I AVEIn addition to controlling each of these values ​​within the above numerical ranges, it is also important to satisfy equation (1). Equation (1) is a relation derived by the inventors. This relation expresses the magnitude of the electromagnetic force during the pulse base period, when ensuring arc stability is most difficult, and the pulse base current I, which is an indicator of the amount of heat in the molten metal formed at the wire end. B And, from the perspective of achieving both arc stability and penetration depth, the average welding current I AVE Pulse base current I B It specifies the proportion.

[0061] "I B / I AVE If the value calculated by "I" is less than 0.24, the arc is likely to become unstable, the heat input during the pulse base period will be insufficient, and as a result, the predetermined penetration depth cannot be obtained. B / I AVE The value of "I" should be 0.24 or greater. B / I AVE The value of "I" is preferably 0.25 or higher. B / I AVE If the value of "I" is greater than 0.62, the heat input during the pulse base period will be excessive, which may result in burn-through, a welding defect. B / I AVE The value of "" shall be 0.62 or less. B / I AVE The value of " is preferably 0.60 or less.

[0062] [Pulse base current I B [Relationship between arc voltage V, wire feed speed W, and steel plate thickness t] 2.38 × t ≤ (I B ×V) / W ≤ 5.67×t + 9.66 …(2) Pulse base current I BIn addition to controlling the arc voltage V, wire feeding speed W, and steel plate thickness t within the above numerical ranges, it is also important that equation (2) is satisfied. Equation (2) is also a relational expression derived by the inventors. In this relational expression, from the viewpoint of controlling the amount of penetration, the pulse base current I represents the amount of heat of the molten metal formed at the wire end, which is the main factor determining the penetration. B Focusing on the arc voltage V, the amount of heat in the molten metal is specified according to the plate thickness t and wire feeding speed W.

[0063] When performing gas shielded arc welding using steel plates of different thicknesses, the thickness of the lower plate shall be used for "t" in equation (2). Furthermore, when applying the present invention to, for example, butt welding and using steel plates of different thicknesses, the thickness of the thinner steel plate may be used for "t" in equation (2).

[0064] (I B If the value calculated by "(I × V) / W" is less than the value calculated by "2.38 × t", the heat input during the pulse base period will be insufficient, and as a result, the predetermined penetration depth cannot be obtained. B The value of "×V) / W" shall be "2.38 × t" or greater. B The value of "×V) / W" is preferably "2.50 × t" or greater. On the other hand, "(I B If the value of "(V) / W" is greater than the value calculated by "5.67 × t + 9.66", the heat input during the pulse base period will be excessive, which may result in burn-through, a welding defect. Therefore, "(I B The expression "×V) / W" should be less than or equal to "5.67×t+9.66". B It is preferable that "×V) / W" is less than or equal to "5.39 × t + 9.18".

[0065] In the present invention, by controlling the welding conditions of gas shielded arc welding by short-circuit transfer as described above, a gas shielded arc welded joint having the above-described characteristics can be obtained. Furthermore, from the viewpoint of obtaining the effects of the present invention more effectively, the following welding conditions may be specified in addition to the above-described welding conditions.

[0066] Specifically, the welding conditions of the present invention further include a pulse peak current IP The current is between 450A and 650A, and the pulse peak time T P It is preferable that the interval be between 1.5 ms and 3.5 ms.

[0067] [Pulse peak current I P (Unit: A): 450-650 A] Pulse peak current I P If the pulse peak current is less than 450A, the molten droplet formed at the tip of the welding wire cannot be pushed down into the molten pool, which can lead to an unstable short circuit or insufficient heat input, resulting in an inability to achieve the desired penetration depth. P It is preferable that the current be 450A or higher. P It is more preferable that the current be 480A or higher. On the other hand, the pulse peak current I P If the pulse peak current is greater than 650A, the instantaneous heat input becomes excessive, resulting in welding defects such as burn-through, increased slag generation due to poor shielding, or increased spatter. Burn-through, a welding defect, means that the base materials are not properly joined, and as a result, a gas-shielded arc welded joint with sufficient joint strength cannot be obtained. Therefore, the pulse peak current I P It is preferable that the pulse peak current I be 650A or less. P It is more preferable to keep it below 620A. Pulse peak current I P It is even more preferable that the current be 600A or less.

[0068] [Pulse peak time T] P (Unit: ms): 1.5 to 3.5 ms] Pulse peak time T P If the pulse peak time T is less than 1.5 ms, the heat input becomes insufficient, similar to the pulse peak current, resulting in a decrease in the penetration depth. P It is preferable that the pulse peak time T be 1.5 ms or longer. P It is more preferable that the pulse peak time T be 2.0 ms or longer. PIf the pulse peak time exceeds 3.5 ms, the heat input becomes excessive, resulting in welding defects such as burn-through, increased slag generation due to poor shielding, or increased spatter. Therefore, the pulse peak time T P It is preferable that the pulse peak time T be 3.5 ms or less. P It is more preferable that the interval be 3.0 ms or less.

[0069] The arc welding method (MIG welding) described above in the present invention does not require the supply of oxygen or the addition of special elements. Therefore, by using solid wire, which is less expensive than flux-cored wire, as the welding wire, the cost of the process can be reduced.

[0070] In this invention, the wire composition of the solid wire used (hereinafter sometimes referred to as "wire component composition") is not particularly limited. Suitable solid wires include, for example, a wire composition containing C: 0.020 to 0.150 mass%, Si: 0.20 to 1.00 mass%, Mn: 0.50 to 2.50 mass%, P: 0.020 mass% or less, S: 0.03 mass% or less, with the remainder being Fe and unavoidable impurities. The reason for specifying this numerical range for the wire composition of the solid wire will be explained below.

[0071] Carbon (C) is an element necessary for ensuring the strength of the weld metal and has the effect of reducing the viscosity of the molten metal and improving its fluidity. However, if the C content is less than 0.020 mass%, the strength of the weld metal cannot be ensured. On the other hand, if the C content exceeds 0.150 mass%, the toughness of the weld metal decreases. Therefore, a C content of 0.020 to 0.150 mass% is preferable.

[0072] Si (silicon) is an element that has a deoxidizing effect, and when added in appropriate amounts, it enhances the hardenability of the weld metal, contributing to improved toughness and strength. In MIG welding, the incorporation of oxygen into the weld metal can be suppressed by Ar shielding gas. While the deoxidizing effect of Si is not particularly necessary, if the Si content is less than 0.20 mass%, the molten droplets and molten pool will oscillate during welding, resulting in a large amount of spatter. On the other hand, if the Si content exceeds 1.00 mass%, the toughness of the weld metal will decrease. Therefore, a Si content of 0.20 to 1.00 mass% is preferable.

[0073] Mn, like Si, has a deoxidizing effect and is an element that improves the mechanical properties of weld metal. However, if the Mn content is less than 0.50 mass%, the amount of Mn remaining in the weld metal is insufficient, and sufficient strength and toughness cannot be obtained. On the other hand, if the Mn content exceeds 2.50 mass%, the toughness of the weld metal decreases. Therefore, a Mn content of 0.50 to 2.50 mass% is preferable.

[0074] P is an element that is introduced into steel as an impurity during the steelmaking and casting processes. It reduces the high-temperature cracking resistance of the weld metal, and it is preferable to reduce its content as much as possible. In particular, if the P content exceeds 0.020 mass%, the high-temperature cracking resistance of the weld metal decreases significantly. Therefore, a P content of 0.020 mass% or less is preferable.

[0075] S is an impurity that is inevitably contained in steel wire and is an element that reduces the high-temperature cracking resistance of the weld metal, so it is preferable to reduce it as much as possible. In particular, if the S content exceeds 0.03 mass%, high-temperature cracking of the weld metal becomes more likely. Therefore, it is preferable that the S content be 0.03 mass% or less.

[0076] In addition, N and Cu are unavoidable impurities in the wire composition.

[0077] Nitric oxide (N) is an impurity that inevitably gets mixed in during the steel melting and steel wire manufacturing stages, and it adversely affects the toughness of the weld metal. For this reason, it is preferable to keep the N content below 0.01% by mass.

[0078] Cu is an impurity that is inevitably present in steel wire and is an element that reduces the toughness of the weld metal. In particular, if the Cu content exceeds 3.0 mass%, the toughness of the weld metal decreases significantly. For this reason, a Cu content of 3.0 mass% or less is preferable.

[0079] Furthermore, in addition to the wire composition described above, the solid wire may optionally contain one or more elements selected from Ni, Cr, Ti, and Mo.

[0080] Ni is an element that increases the strength and improves the weather resistance of weld metal. Since it can be included as needed, the Ni content may be as low as 0% by mass. However, if the Ni content is less than 0.02% by mass, these effects may not be achieved. On the other hand, if the Ni content exceeds 3.50% by mass, it leads to a decrease in the toughness of the weld metal. Therefore, when adding Ni, a Ni content of 0.02% to 3.50% by mass is preferable.

[0081] Cr, like Ni, is an element that increases the strength of weld metal and improves its weather resistance. It can be included as needed, and the Cr content may be as low as 0% by mass. However, if the Cr content is less than 0.01% by mass, these effects may not be obtained. On the other hand, if the Cr content exceeds 1.50% by mass, it leads to a decrease in the toughness of the weld metal. Therefore, when adding Cr, the Cr content is preferably between 0.01% and 1.50% by mass.

[0082] Ti acts as a deoxidizing agent and is an element that improves the strength and toughness of the weld metal. Ti also stabilizes the arc and reduces spatter. However, if the Ti content exceeds 0.15% by mass, not only do molten droplets become coarser during welding, resulting in large spatter particles, but the toughness of the weld metal also decreases significantly. Therefore, when adding Ti, a Ti content of 0.15% by mass or less is preferable. Note that the Ti content can be 0% by mass, as it can be included as needed. A Ti content of 0% by mass or more is more preferable.

[0083] Mo is an element that improves the strength of weld metal, but if its content exceeds 0.8% by mass, the toughness of the weld metal decreases. Therefore, when adding Mo, the Mo content is preferably 0.8% by mass or less. The Mo content may be 0% by mass, as it can be included as needed. However, a Mo content of 0% by mass or more is more preferable.

[0084] With the wire composition described above, by appropriately adjusting the components, it can be applied to gas shielded arc welding of a wide range of steel types, from mild steel to ultra-high tensile steel. Examples include YGW15, YGW16, YGW17, and YGW19 as described in JIS Z 3312 (2009). The diameter of the solid wire is preferably 0.8 to 1.6 mm.

[0085] [Welding speed: 5.0 to 16.7 mm / s] If the welding speed is less than 5.0 mm / s, the heat input and deposition amount will be excessive, which may result in burn-through, a welding defect. For this reason, the welding speed should be 5.0 mm / s or higher. A welding speed of 8.3 mm / s or higher is preferable. On the other hand, if the welding speed is greater than 16.7 mm / s, the heat input will be insufficient, the arc will become unstable, and the heat input will be easily dispersed, making it impossible to obtain the required penetration depth. For this reason, the welding speed should be 16.7 mm / s or lower. A welding speed of 15.0 mm / s or lower is preferable.

[0086] As described above, the welding process of the present invention is gas shielded arc welding using short-circuit transfer. However, if the volume of the molten droplet 7 generated from the tip of the welding wire 1 is too large or too small, the welding becomes unstable. Therefore, it is desirable that the volume of the molten droplet 7 transported to the molten pool 8 in a single short circuit be approximately the volume of a sphere with the same diameter as the wire diameter. For this reason, in addition to the welding conditions described above, the present invention may also control the frequency of short-circuit transfer (i.e., the short-circuit frequency) F (unit: Hz). Specifically, it is desirable that the short-circuit frequency F be 30 to 130 Hz.

[0087] [Short-circuit frequency F: 30 to 130 Hz] If the short-circuit frequency F is less than 30 Hz, the amount of molten droplets formed at the wire end becomes excessive, widening the current path at the wire end, causing the arc to become unstable, and reducing the amount of heat transported to the molten pool, thus preventing the attainment of the required penetration depth. For this reason, the short-circuit frequency F should be 30 Hz or higher. It is more preferable that the short-circuit frequency F be 50 Hz or higher. On the other hand, if the short-circuit frequency F is greater than 130 Hz, the formation and growth of molten droplets at the wire end cannot keep up with the short-circuit period, resulting in a large amount of spatter being generated during the short circuit. Due to this spatter scattering, some of the heat contained in the molten droplets is transported to areas other than the molten pool, reducing the actual heat input to the molten pool, thus preventing the attainment of the required penetration depth. For this reason, the short-circuit frequency F should be 130 Hz or lower. It is more preferable that the short-circuit frequency F be 120 Hz or lower.

[0088] Here, the short-circuit frequency F (Hz) can be measured, for example, by monitoring the arc voltage during welding using an oscilloscope, counting the number of times it becomes zero, and dividing the count by the monitoring time to obtain the count per second. The monitoring time should preferably be 10 seconds or longer, as too short a time will result in large variations in the count. By adjusting the welding conditions so that the measured value of the short-circuit frequency F is the target value, 30 ≤ F (Hz) ≤ 130 can be achieved.

[0089] By controlling the short-circuit frequency F, regular droplet transfer can be achieved even in gas-shielded arc welding using Ar shielding gas, resulting in welded joints with sufficient penetration.

[0090] The operation and effects of the present invention will be described below using examples. However, the present invention is not limited to the following examples.

[0091] First, steel plates having the components shown in Table 1 were prepared, and these steel plates were gas-shielded arc welded to create gas-shielded arc welded joints. Here, two of these steel plates were overlapped, and lap fillet welding was performed using, for example, the method shown in Figure 1. In addition to the elements shown in Table 1, the steel plates used contained Fe and alloying elements such as Ni, Cr, Ti, and Mo. The thickness of the two steel plates was the same, and the thickness t of each steel plate is shown in Table 2.

[0092] The gas shielded arc welding was carried out under the welding conditions shown in Table 2. As the welding wire, YGW16 described in JIS Z 3312 was used. The diameter of the welding wire was 1.2 mm. The short circuit frequency F was appropriately controlled so as to be within the above-mentioned numerical range (30 to 130 Hz).

[0093] Next, using the obtained gas shielded arc welded joint, the following test methods were followed to evaluate (1) the slag covering area ratio (S RATIO ), and (2) the penetration depth (d).

[0094] (1) Slag covering area ratio The slag covering area ratio refers to the ratio of the slag adhering to the welded part (mainly the surface of the welding bead) (see Figure 2). Figure 2 shows a perspective view of a welded joint including a welding bead 6 to which slag 7 adheres. As shown in Figure 2, the bead surface area S BEAD and the slag surface area S SLAG were calculated by photographing the surface of the welding bead 6 directly from above in the area excluding the bead start and end portions 10 (each with a length of 15 mm) of the welding bead 6, and measuring the projected areas from the upper surfaces of the welding bead 6 and the slag 7. The slag surface area S SLAG was taken as the total surface area of the slag 7 on the welding bead 6. The above-mentioned photographing was performed at an equal magnification.

[0095] When the length of the welding bead 6 is less than 130 mm, the entire surface excluding the bead start and end portions 10 was photographed. On the other hand, when the length of the welding bead 6 is 130 mm or more, the surface of the welding bead at an arbitrary site (with a length of 100 mm) excluding the bead start and end portions 10 was photographed.

[0096] The calculated value of the slag surface area S SLAG was divided by the value of the bead surface area S BEAD to obtain the slag covering area ratio S RATIO . That is, it was obtained using the formula "S RATIO = S SLAG / S BEAD ". The obtained slag covering area ratios S RATIO are shown in Table 2 respectively. In this example, S RATIO being 30% or less was regarded as passing.

[0097] (2) Penetration depth The measurement of the penetration depth d was carried out in the same manner as in (1) above, with the region excluding the bead start and end portions (each with a length of 15 mm) of the weld bead 6 being the measurement target. In the region of the measurement target, as shown in FIG. 3, cross-sections in the plate thickness direction perpendicular to the weld line were observed at five arbitrary positions on the weld bead 6. However, the five arbitrary positions were set to be at least 5 mm apart from each other. The above-mentioned "perpendicular to the weld line" means a direction parallel to the A-A line shown in FIG. 2.

[0098] The weld bead was cut in the plate thickness direction perpendicular to the weld line at five arbitrary positions, and the penetration depth at each position was determined. The average value of these was taken as the "penetration depth d (mm)". The measurement of the penetration depth was carried out using an optical microscope (magnification 10 times).

[0099] Using the obtained values of "penetration depth d" and "plate thickness t of the steel plate", "d / t" was determined, and the obtained values are shown in Table 2 respectively. In the case of fillet welding, the plate thickness of the lower plate was taken as the above-mentioned plate thickness t. In this embodiment, a d / t of 0.15 or more was considered acceptable. That is, as described above, it was evaluated that "the welded portion has sufficient penetration".

[0100] Here, the evaluation of the welded joint was carried out according to the following criteria. <Evaluation Criteria> - S RATIO is 30% or less and d / t is 0.25 or more: Evaluation A (particularly excellent) - S RATIO is 30% or less and d / t is 0.15 or more and less than 0.25: Evaluation B (excellent) - S RATIO is greater than 30%, d / t is less than 0.15, and at least one of the occurrence of spatter is satisfied: Evaluation F (unqualified) The evaluation results of the obtained welded joints are shown in Table 2 respectively.

[0101]

[0102]

[0103] As shown in Table 2, for the welding conditions No. 1 to 12 which are inventive examples, S RATIOThe conditions of 30% or less and d / t of 0.15 or more were satisfied. In other words, in the welded joint of the inventive example, a weld with sufficient penetration depth was obtained while suppressing slag. In particular, in welding conditions No. 1 to 8 of the above inventive example, d / t was 0.25 or more, resulting in an even better penetration depth.

[0104] In contrast, in comparative welding conditions No. 13 to 22, "S RATIO The welded joint in the comparative example met one or more of the following conditions: "the slag was greater than 30%", "d / t was less than 0.15", and "burn-through occurred". In other words, the welded joint in the comparative example did not yield a slag-free weld with sufficient penetration depth.

[0105] 1. Welding wire 2. Welding torch 3. Base metal 4. Corner 5. Arc 6. Weld bead 7. Drip 8. Molten pool 9. Slag 10. Bead start / end

Claims

1. A method for manufacturing a gas-shielded arc welded joint using gas-shielded arc welding that results in a short-circuit transition, wherein the gas-shielded arc welding is pulse welding, and the welding conditions for the gas-shielded arc welding are: Ar gas is used as the shielding gas at a concentration of 98 volume% or more, and the average welding current is I AVE The current is 160-250A, and the pulse base current is I B The current is 40 to 130 A, the arc voltage V is 18.0 to 33.0 V, the wire feed speed W is 100.0 to 145.0 mm / s, and the thickness t of the steel plate to be welded is 1.2 to 4.2 mm, and the pulse base current I B and the average welding current I AVE The following equation (1) is satisfied, and the pulse base current I B A method for manufacturing a gas-shielded arc welded joint, wherein the arc voltage V, the wire feeding speed W, and the thickness t of the steel plate satisfy the following equation (2). 0.24 ≤ I B / I AVE ≦ 0.62...(1) 2.38×t ≦ (I B ×V) / W≦5.67×t+9.66…(2) 2. The welding conditions further include a pulse peak current I P of 450 to 650 A, and a pulse peak time T P of 1.5 to 3.5 ms. The method for manufacturing a gas shielded arc welded joint according to claim 1.

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