JP7453540B2 - Welded joints, automobile parts, and building material parts - Google Patents

Description

The present invention relates to welded joints, automobile parts, and building material parts.

Electrodeposition coating is a means for improving the corrosion resistance of mechanical structural parts, such as automobile parts and building material parts. Electrodeposition coating is a process in which the object to be coated and an electrode are placed in a tank containing electrodeposition paint, and an electric potential is generated between the two to cause electrophoresis of the coating film components, thereby coating the surface of the object. This is a coating method that deposits a film.

When improving the post-coating corrosion resistance of mechanical structural parts by electrocoating, poor electrocoating at welded areas poses a problem. Welding slag may adhere to welded parts, and this slag causes defects in electrodeposition coating and impairs corrosion resistance after coating of defective areas.

Patent Document 1 describes C: 0.03 to 0.15%, Si: 0.2 to 0.5%, and Mn: 0.3 to 0.8% in mass % based on the total mass of the wire including plating. , P: 0.02% or less, S: 0.02% or less, Al: 0.1 to 0.3%, Ti: 0.001 to 0.2%, Cu: 0 to 0.5%, Cr: 0 to 2.5%, Nb: 0 to 1.0%, V: 0 to 1.0%, and the remainder consists of Fe and impurities, and the value of the following X is 1.5 to 3 in mass%. Solid wire for gas shielded arc welding is disclosed in the range of .5%. According to this solid wire, when gas shielded arc welding is performed on steel plates such as zinc or zinc alloy coated steel plates, it is possible to reliably prevent both the occurrence of blowholes in the weld metal and the generation of slag on the surface of the weld metal after solidification. And it is said that it can be suppressed stably. However, as shown in the example of Patent Document 1, even this technique cannot completely suppress the generation of slag.

Patent Document 2 describes C: 0.15% by weight or less, Si: 0.5 to 2.5% by weight, Mn: 1.0% by weight or less, and one or more selected from Zr, Ti, and Al. A welding wire is disclosed that contains two or more kinds in a total amount of 0.05 to 1.0% by weight, with the balance being Fe and impurities. According to this welding wire, the slag attached to the surface of the weld metal after welding can be easily removed, and the slag can be removed extremely easily. However, it is difficult to separate all the slag due to thermal stress at the interface between the steel plate and the slag. Even if the technique of Patent Document 2 is used, it is necessary to carry out a step of peeling off the slag before electrodeposition coating, resulting in a decrease in production efficiency and an increase in production cost.

Patent Document 3 describes a consumable electrode type gas shielded arc welding method for arc welding two steel plates using a welding torch having a consumable electrode, in which a shielding gas having an oxygen potential α of 1.5% to 5% is used. Arc welding is performed while supplying from the welding torch toward the consumable electrode, and the oxygen potential β is 15% with respect to the weld bead and weld toe formed by the arc welding and which are in a state of 700° C. or higher. A consumable electrode type gas shield arc welding method is disclosed in which an oxidation-promoting gas of ~50% is sprayed at a flow rate of 1 to 3 m/sec. According to this welding method, the surface of the weld bead and the weld bead toe can be covered with conductive iron oxide slag, so it is said that insulating Si, Mn-based slag will not appear on the surface. Ru. Therefore, even if a structural member including a welded part is electrodeposited, no defects in the electrodeposition will occur in the welded part, and therefore it is said that the corrosion resistance of the structural member can be improved. However, in the technique of Patent Document 3, it is necessary to spray the oxidation-promoting gas onto the weld bead and the weld toe, which increases production costs.

Patent Document 4 discloses a gas-shielded metal arc welding wire for a carbon steel base material that is electrodeposited after welding, and contains Si: 0.4% or less and Ti: 0.05 to 1.0%. , further characterized in that Si and Ti satisfy [Si(%)/Ti(%)]≦4, and gas shield metal arc welding having excellent corrosion resistance after electrodeposition coating in and near the welding part. A steel wire for industrial use is disclosed. The steel wire of Patent Document 4 can obtain a sufficient deoxidizing effect by using Ti instead of Si as a strong deoxidizing element, and the Ti-based slag produced by the deoxidizing reaction is electrodeposited. It is said to have good paintability. However, in Patent Document 4, only the components of the steel wire are studied in detail. The composition and phase composition of the slag should be influenced by both the composition of the steel wire and the composition of the carbon steel matrix. However, Patent Document 4 does not discuss the carbon steel base material at all, and does not disclose detailed analysis results of slag. Therefore, the technique of Patent Document 4 may be applicable only to carbon steel base materials with specific components.

Patent Document 5 discloses a gas-shielded arc welding wire for joining a plurality of thin steel plates by gas-shielded arc welding, in which C: 0.06 to 0.15% in mass % based on the total mass of the wire; Si: more than 0 to 0.18%, Mn: 0.3 to 2.2%, Ti: 0.06 to 0.30%, Al: 0.001 to 0.30%, B: 0.0030 to 0 .0100%, P: more than 0 to 0.015%, S: more than 0 to 0.030%, Sb: 0 to 0.10%, Cu: 0 to 0.50%, Cr: 0 to 1.5% , Nb: 0 to 0.3%, V: 0 to 0.3%, Mo: 0 to 1.0%, Ni: 0 to 3.0%, the balance being iron and impurities, Si, A solid wire for gas shielded arc welding is disclosed in which Mn, Ti, and Al satisfy Si×Mn≦0.30 and (Si+Mn/5)/(Ti+Al)≦3.0. According to the solid wire disclosed in Patent Document 4, it is said that it is possible to improve electrodeposition coating properties by suppressing the generation of Si-based slag and generating conductive Ti-based slag on the surface of the weld bead. However, even in Patent Document 5, there is no detailed study regarding the components and phase structure of the slag. Therefore, similar to Patent Document 4, the technique of Patent Document 5 may also be applicable only to steel sheets with specific components.

In this way, in the prior art, as a means to improve the electrodeposition coating property of welded parts, (1) suppressing the amount of slag generation (for example, Patent Document 1), (2) improving slag removability (for example, Patent Document 2) , (3) control of slag generation form (for example, Patent Document 3), and (4) slag modification (for example, Patent Documents 4 and 5) have been proposed. However, with either technique, it is difficult to improve both the productivity of welded joints and the electrocoatability of welded areas.

Re-publication WO2014/126246 publication Japanese Unexamined Patent Publication No. 62-124095 Re-publication WO2017/126657 publication Japanese Patent Application Publication No. 8-103884 International Publication No. 2019/124305

In view of the above circumstances, it is an object of the present invention to provide welded joints, automobile parts, and building material parts that have high production efficiency and high electrodeposition coating properties. In addition, "welded joints with high electrocoatability" refers to welded joints that are unlikely to cause coating defects when subjected to electrocoating, if the welded joints do not have an electrocoated film. If it has an electrodeposited coating, it means a welded joint with few coating defects.

The gist of the invention is as follows.
(1) A welded joint according to one aspect of the present invention is a welded joint comprising a steel material, a weld bead, and a welding slag attached to the surface of the welding bead, the welding slag containing a conductive oxide phase. The chemical components of the conductive oxide phase excluding oxygen are mass% based on the total mass of the conductive oxide phase excluding oxygen, Si: 0 to 5%, Mn: 0 to 65%. , Ti: 0.3 to 100%, Al: 0 to 30%, and Fe: 0 to 65%, and the chemical components of the conductive oxide phase excluding the oxygen are expressed by formula 1 and formula 2, and the area ratio of the conductive oxide phase in the cross section of the welding slag is 20% or more.
3×Si+Mn/4-4×Ti+Al/4<0...Formula 1
Si+Mn+Ti+Al+Fe≧70...Formula 2
However, Si, Mn, Ti, Al, and Fe in the above formulas 1 and 2 mean the content of each element in mass % with respect to the total mass of the conductive oxide phase excluding the oxygen. do.
(2) The welded joint according to (1) above may further include an electrodeposited coating film provided on the surfaces of the steel material, the weld bead, and the weld slag.
(3) In the welded joint according to (1) or (2) above, the area ratio of the conductive oxide phase in the cross section of the welding slag may be 25% or more.
(4) In the welded joint according to any one of (1) to (3) above, the chemical component of the conductive oxide phase excluding oxygen further includes the conductive oxide excluding oxygen. It is characterized by containing one or more types selected from the group consisting of Zr: 10% or less (including 0) and Cr: 5% or less (including 0) in mass % based on the total mass of the phase, and the remainder consists of impurities. shall be.
(5) An automobile component according to another aspect of the present invention includes the welded joint according to any one of (1) to (4) above.
(6) A building material component according to another aspect of the present invention includes the welded joint according to any one of (1) to (4) above.

According to the present invention, it is possible to provide welded joints, automobile parts, and building material parts with high production efficiency and high electrodeposition coating properties.

FIG. 3 is a cross-sectional view of an example of a welded joint. FIG. 3 is a cross-sectional view of an example of a welded joint. 1 is a SEM photograph of an example of welding slag having a conductive oxide phase. 1 is a current image of an example of a welding slag having a conductive oxide phase. These are an electron micrograph showing an overview of the observation position of a welding slag cross section, and an EDS mapping image of Si, Mn, Ti, Al, and Fe at the observation position.

The present inventors have intensively studied means for obtaining a welded joint with excellent electrodeposition coating properties. As a result, it was found that it is difficult to completely suppress the generation of slag. Furthermore, it is not preferable to include an additional step for slag removal in the method for manufacturing a welded joint, considering production efficiency. Therefore, the present inventors investigated a method of improving electrodeposition coating properties while leaving slag on the weld bead.

The present inventors have also found that by setting the area ratio of the conductive oxide phase in the cross section of the welding slag to 20% or more, it is possible to form an electrodeposition coating film on the slag. Here, the conductive oxide phase is defined as an oxide phase having a predetermined chemical component, which will be described later. The conductive oxide phase has high electrical conductivity due to its chemical composition. Thereby, the electrodeposition coating properties can be improved while the slag remains on the weld bead. This is presumed to be because the conductive oxide phase forms a current path in the slag and promotes the formation of an electrodeposited film on the slag.

One of the advantages of the means discovered by the present inventors is that as long as the area ratio of the conductive oxide phase is 20% or more, the other phase components of the slag may be insulating components. It is in. For example, even if the base steel plate contains a lot of Si and the phase other than the conductive oxide phase in the slag is insulating Si or Mn-based slag, even if the area ratio of the conductive oxide phase is 20%. An electrodeposition coating film can be formed.

A welded joint 1 according to one aspect of the present invention obtained from the above findings includes a steel material 11, a weld bead 12, and a welding slag 13 attached to the surface of the weld bead 12, as shown in FIG. 1A. , the welding slag 13 includes a conductive oxide phase 131, and the chemical components of the conductive oxide phase 131 excluding oxygen are mass % with respect to the total mass of the conductive oxide phase 131 excluding oxygen, Si: 0 to 5%, Mn: 0 to 65%, Ti: 0.3 to 100%, Al: 0 to 30%, and Fe: 0 to 65%, excluding oxygen in the conductive oxide phase 131. The chemical components satisfy Formulas 1 and 2, and the area ratio of the conductive oxide phase 131 in the cross section of the welding slag 13 is 20% or more.
3×Si+Mn/4-4×Ti+Al/4<0...Formula 1
Si+Mn+Ti+Al+Fe≧70...Formula 2
However, Si, Mn, Ti, Al, and Fe in Formulas 1 and 2 mean the content of each element in mass % with respect to the total mass of the conductive oxide phase excluding oxygen. Hereinafter, the welded joint 1 according to this embodiment will be described in detail.

The welded joint 1 according to this embodiment is composed of a steel material 11 and a weld bead 12 that joins the steel material 11, like a normal welded joint. When the welded joint 1 includes two or more steel materials 11, the chemical compositions of the plurality of steel materials 11 may be different.

As shown in FIG. 1B, the welded joint 1 may further include an electrodeposited coating film 14 provided on the surfaces of the steel material 11, the welding bead 12, and the welding slag 13. The steel material 11 may further be provided with surface treatment such as zinc-based plating. Further, the type of welded joint 1 is not particularly limited either. Although a butt joint is illustrated in FIGS. 1A and 1B, other shapes may be applied to the welded joint 1 according to this embodiment. For example, the welded joint 1 may be a lap fillet welded joint, a T-shaped fillet welded joint, or the like.

Welded joint 1 further includes welding slag 13 attached to the surface of weld bead 12 . In normal welded joints, welding slag 13 causes poor electrodeposition coating, so it is preferable that it be as small as possible. However, removing the welding slag 13 increases the manufacturing cost of the welded joint 1. Therefore, in the welded joint 1 according to this embodiment, the welding slag 13 is left.

In order to improve the electrodeposition coating property of the welded joint 1 while leaving the welding slag 13, the welding slag 13 of the welded joint 1 according to the present embodiment includes an oxide phase having a predetermined component, and this oxide The phase content is greater than or equal to a predetermined value. This oxide phase is hereinafter referred to as a conductive oxide phase 131. The chemical components of the conductive oxide phase 131 excluding oxygen will be explained below. Note that the reason why the chemical components of the conductive oxide phase 131 are defined by the content of each element except oxygen is that it is difficult to ensure the measurement accuracy of oxygen. For example, when the chemical components of an oxide phase are analyzed by EDS, the measured values of elements with small atomic weights (so-called light elements) such as O have large analytical errors. Furthermore, according to experiments conducted by the present inventors, it was sufficiently possible to identify an oxide phase with high conductivity by evaluating chemical components excluding oxygen.

The unit "%" for the content of elements constituting the conductive oxide phase 131 means mass % with respect to the total mass of the conductive oxide phase excluding oxygen. For example, the components of the conductive oxide phase include 50% by mass of O, 30% by mass of Fe, 10% by mass of Mn, and 10% by mass of Ti, based on the total mass of the conductive oxide phase. When the conductive oxide phase contains Mn, the amount of Mn in the conductive oxide phase is 20% (=10÷(100-50)) in mass % based on the total mass of the conductive oxide phase excluding oxygen.

(Si: 0-5%)
Si oxide is an insulator. Therefore, in the conductive oxide phase 131, the amount of Si is set to 5% or less. It is estimated that a phase containing more than 5% of Si does not have sufficient electrical conductivity and is unlikely to become a current path during electrodeposition coating. The amount of Si in the conductive oxide phase 131 may be 0% since it is preferable to have a small amount from the viewpoint of electrodeposition coating properties.

However, Si may be included in the chemical components of the steel material 11. Further, Si may be included in the welding material as a deoxidizing element. Si of 5% or less is considered not to impair the conductivity of the conductive oxide phase 131, and therefore is allowed in the conductive oxide phase 131 of the welded joint 1 according to the present embodiment.

(Mn: 0-65%)
Mn promotes deoxidation of the molten pool during arc welding and improves the mechanical properties of the weld bead 12. Moreover, Mn increases the tensile strength of the weld bead 12. Further, although Mn oxide is an insulator depending on the type, the adverse effect of Mn oxide on electrodeposition coating properties is smaller than that of Si oxide. Therefore, in manufacturing the welded joint 1 according to the present embodiment, Mn is used as one of the deoxidizing agents for the molten pool. As a result, the conductive oxide phase 131 of the welded joint 1 according to this embodiment may contain Mn.

However, from the viewpoint of ensuring conductivity, the Mn content of the conductive oxide phase may be 0%. On the other hand, it is estimated that a phase having a Mn content of more than 65% does not have sufficient conductivity and is difficult to become a current path during electrodeposition coating. Therefore, the upper limit of the Mn content of the conductive oxide phase 131 is set to 65%.

(Ti: 0.3-100%)
Ti promotes deoxidation of the molten pool during arc welding and improves the mechanical properties of the weld bead 12. Furthermore, since Ti oxide is a conductor, it improves the electrical conductivity of welding slag. Therefore, in manufacturing the welded joint 1 according to the present embodiment, Ti is used as one of the deoxidizing agents for the molten pool. As a result, the conductive oxide phase 131 of the welded joint 1 according to this embodiment contains 0.3% or more of Ti.

(Al: 0-30%)
Al oxide is an insulator. From the viewpoint of conductivity, the conductive oxide phase 131 does not need to contain Al oxide. Therefore, the lower limit of the amount of Al in the conductive oxide phase 131 is 0%. Further, a phase in which the amount of Al oxide is 30% or more cannot ensure electrodeposition coating properties. Therefore, the upper limit of the amount of Al in the conductive oxide phase 131 is set to 30%.

However, Al has a high deoxidizing ability and has the effect of reducing the amount of slag. Therefore, in manufacturing the welded joint 1 according to the present embodiment, Al may be used as one of the deoxidizing agents for the molten pool, and the conductive oxide phase 131 may contain Al.

(Fe: 0-65%)
Fe oxide is derived from the steel material 11 and is contained in the welding slag 13. However, in the experiments conducted by the present inventors, conductive oxide phases 131 not containing Fe were also found here and there. This is presumed to be because oxygen is more likely to combine with other alloying elements (such as the above-mentioned Si and Ti) than with Fe. Therefore, in the conductive oxide phase 131, Fe may be 0%. On the other hand, Fe oxide has an effect of improving conductivity. However, in order to obtain a phase with an Fe content of more than 65%, it is necessary to manufacture a weld bead while supplying an oxidation-promoting gas using a separate part, as in the technique described in Patent Document 3. This is not desirable in terms of production efficiency. Therefore, the upper limit of the Fe content of the conductive oxide phase 131 is set to 65%.

Note that the conductive oxide phase 131 may contain elements other than the components listed above. For example, automobile steel sheets may contain elements such as Zr and Cr. By applying such an automobile steel plate to the steel material 11, the conductive oxide phase 131 contains elements such as Zr and Cr. For example, the chemical components of the conductive oxide phase 131 excluding oxygen are selected from the group consisting of Zr: 10% or less (including 0) and Cr: 5% or less (including 0), in addition to the above-mentioned elements. It may contain at least one type of impurities, with the remainder being impurities. The unit of the amount of Zr and the amount of Cr is mass % based on the total mass of the conductive oxide phase 131 excluding oxygen.

Furthermore, the chemical components of the conductive oxide phase 131, excluding oxygen, satisfy Formulas 1 and 2.
3×Si+Mn/4-4×Ti+Al/4<0...Formula 1
Si+Mn+Ti+Al+Fe≧70...Formula 2
Si, Mn, Ti, Al, and Fe in Formula 1 and Formula 2 mean the content of each element in mass % with respect to the total mass of the conductive oxide phase excluding oxygen.

The present inventors produced weld beads using steel plates with various components and welding materials with various components. Then, the oxide phase of the slag formed on the weld bead was analyzed, and the electrodeposition coating properties were investigated. As a result, in the left side of Equation 1, "3×Si+Mn/4−4×Ti+Al/4," the coefficients of Si, Mn, and Al are positive numbers, and the coefficient of Ti is a negative number. It has been statistically revealed that the conductive oxide phase 131 in which "3×Si+Mn/4-4×Ti+Al/4" increases to 0 or more has no effect of improving conductivity.

Furthermore, from the above experiment, it is necessary that the total mass percentage of Si, Mn, Ti, Al, and Fe be 70% or more in order for the conductive oxide phase 131 to have the effect of improving electrodeposition coating properties. I gained knowledge. Therefore, the conductive oxide phase 131 needs to satisfy not only formula 1 but also formula 2. However, as long as Formula 2 is satisfied, the chemical components (excluding oxygen) of the conductive oxide phase 131 may contain elements other than Si, Mn, Ti, Al, and Fe (for example, the above-mentioned Zr and Cr). unhindered.

The welding slag 13 of the welded joint 1 according to the present embodiment contains a predetermined amount or more of the conductive oxide phase 131 defined by the above-mentioned chemical components, Formula 1, and Formula 2. Specifically, the area ratio of the conductive oxide phase 131 in the cross section of the welding slag 13 is set to 20% or more. Thereby, a current path is secured inside the welding slag 13, and it becomes possible to form an electrodeposited coating film covering the welding slag 13. The more conductive oxide phase 131 there is, the more preferable it is, and its area ratio may be set to 25% or more, 30% or more, or 40% or more. The upper limit of the area ratio of the conductive oxide phase 131 is not particularly specified, and may be 100%. The area ratio of the conductive oxide phase 131 may be 95% or less, 90% or less, or 85% or less.

FIG. 2 shows an example of welding slag 13 in which the area ratio of conductive oxide phase 131 is 20% or more. FIG. 2 is an SEM photograph of a cross section of the welded joint 1 according to the present embodiment, taken with the weld bead 12, weld slag 13, and electrodeposited coating film 14 included in the field of view. A light-colored phase with a dendrite-like shape contained in the welding slag 13 is a conductive oxide phase 131. Since the conductive oxide phase 131 acts as a current path, the electrodeposited coating film 14 could be formed on the surface of the welding slag 13 in the welded joint 1 shown in FIG.

FIG. 3 shows an example of a current image of welding slag 13 having conductive oxide phase 131. The current image is an image created using an SPM (Scanning Probe Microscope) that visualizes the ease with which current flows. In the current image in Figure 3, areas with high brightness indicate areas where current is difficult to flow (i.e., areas with high resistance values), and areas with low brightness indicate areas where current flows easily (i.e., areas with low resistance values). . It can be seen that the conductive oxide phase 131 having a dendrite shape has a low brightness and therefore a low resistance value. A welded joint 1 having a welding slag 13 as shown in FIG. 3 has good electrodeposition coating properties.

The method for measuring the area ratio of the conductive oxide phase 131 is as follows.

(1) Cut the weld joint 1 perpendicularly to the longitudinal direction of the weld bead 12. During cutting, it is necessary to prevent the welding slag 13 attached to the welding bead 12 from falling off. When the welded joint 1 has the electrodeposited coating 14, no special consideration is required. On the other hand, when the welded joint 1 does not have the electrodeposited coating 14, it is preferable to apply an adhesive to the surface of the weld bead 12 before cutting the welded joint 1.

(2) Obtain the element distribution in the cross section of welding slag 13. The elemental distribution is obtained using EDS or EPMA attached to a scanning electron microscope. The observation magnification is 500 to 1000 times.

Normally, welding slag 13 has one or more oxide phases with uniform composition. When the element distribution of welding slag 13 is obtained, a uniform region is often observed as shown in FIG. 4. FIG. 4 shows a backscattered electron image of a cross section of the slag and a concentration mapping image of Si, Mn, Ti, Al, and Fe in the cross section of the slag. By comparing these images, an oxide phase with uniform components can be identified. The chemical composition of each of these regions (phases) is measured and recorded.

(3) Identify the region corresponding to the conductive oxide phase 131 in the cross section of the welding slag 13. The region corresponding to the conductive oxide phase 131 is a region in which the chemical components excluding oxygen are within the above-mentioned range.

(4) Measure the area of welding slag 13 and the area of conductive oxide phase 131 in the measurement field of view. Based on these values, the area ratio of the conductive oxide phase 131 in the cut surface of the welding slag 13 formed in (1) is calculated.

(5) Perform the steps (1) to (4) above at 10 locations on the welded joint 1. Then, the average value of the area ratios obtained at each of the ten locations is regarded as the area ratio of the conductive oxide phase 131 in the welded joint 1.

Note that welding slag may not adhere to the cut surface. In this case, the measurement on the cut surface is not performed, and another cut surface is produced. For example, when one cut surface to which no welding slag is attached is formed, 11 cut surfaces are formed and measurements are performed at 10 locations. On the other hand, if there is a cut surface to which welding slag is attached but the slag does not contain a conductive oxide phase, the area ratio of the conductive oxide phase on that cut surface is considered to be 0%, and (5 ) is included in the calculation of the average value.

Further, when measurement is performed using the above-described method, two or more types of phases satisfying the requirements for the conductive oxide phase 131 may be observed. In this case, the total area ratio of the two types of conductive oxide phases 131 is regarded as the area ratio of the conductive oxide phases 131 in the welded joint 1.

The method of manufacturing the welded joint 1 according to this embodiment is not particularly limited. The components of the steel material 11 and the welding material (for example, solid wire, etc.) may be adjusted as appropriate so that the components of the slag fall within the above-mentioned range. The steel material 11 may be a hot-rolled steel plate or a cold-rolled steel plate. The steel material 11 may be a plated steel plate. The thickness of the steel material 11 is exemplified to be about 1.0 mm to 3.6 mm. The diameter of the solid wire is exemplified to be about 0.9 mm to 2.0 mm. An example of the chemical composition of the steel material 11 suitable for obtaining the welded joint 1 according to the present embodiment is, in mass %, C: 0.05 to 0.30%, Si: 0.20% or less, Mn: 0. 3 to 3.0%, P: 0.02% or less, S: 0.03% or less, Ti: 0.02 to 0.30%, B: 0.01% or less, Al: 0.30% or less, Cr: 0.5% or less, Nb: 0.5% or less, V: 0.3% or less, Mo: 1.0% or less, Ni: 3.0% or less, Zr: 0.2% or less, and the balance : Iron and impurities. An example of a suitable chemical composition of the solid wire is, in terms of mass %, C: 0.05 to 0.20%, Si: 0.15% or less, Mn: 0.3 to 2.5%, P: 0 .02% or less, S: 0.04% or less, Ti: 0.02 to 0.20%, B: 0.012% or less, Al: 0.22% or less, Cr: 0.5% or less, Nb: 0.3% or less, V: 0.3% or less, Mo: 1.0% or less, Ni: 3.0% or less, Zr: 0.200% or less, Cu: 0.5% or less, and balance: iron and impurities. The welding method is also not particularly limited, and can be appropriately selected depending on the shape of the welded joint 1. From the viewpoint of reducing the amount of welding slag 13, it is preferable to manufacture the welded joint 1 according to the present embodiment by gas-shielded arc welding using a solid wire, for example.

Next, an automobile part according to another aspect of the present invention will be explained. An automobile component according to another aspect of the present invention includes a welded joint according to this embodiment. Thereby, the automobile part according to the present embodiment has high electrodeposition coating properties in both the steel material 11 and the weld bead 12. Therefore, the automobile part according to the present embodiment has excellent corrosion resistance after electrodeposition coating (corrosion resistance after coating) in both the steel material 11 and the weld bead 12.

The type of automobile parts is not particularly limited. Examples of the automobile parts according to this embodiment include a lower arm, a subframe, a suspension member, and a battery case. The members constituting the automobile floor may be the automobile parts according to this embodiment.

Next, a building material component according to another aspect of the present invention will be described. A building material component according to another aspect of the present invention includes the welded joint according to this embodiment. As a result, the building material component according to this embodiment has high electrodeposition coating properties on both the steel material 11 and the weld bead 12. Therefore, the building material component according to this embodiment has excellent corrosion resistance after electrodeposition coating (corrosion resistance after coating) in both the steel material 11 and the weld bead 12.

The type of building material parts is not particularly limited. An example of the building material component according to this embodiment is a balcony.

Steel materials A to E (plate thickness 2.6 mm) having the chemical components listed in Table 1 and solid wires a to e (diameter 1.2 mm) having the chemical components listed in Table 2 were Various welded joints were created by combining the two. The remainder of the chemical components disclosed in Tables 1 and 2 were iron (Fe) and impurities. The welding method was gas shielded arc welding. The welding conditions were as follows.

<Welding conditions>
・Welding current: 200A
・Welding voltage: 22.0V
・Welding speed: 80cm/min
・Shield gas type: Ar-20% _CO2
・Shield gas flow rate: 20l/min
・Wire protrusion length: 15mm
・Joint shape: overlapped fillet (overlap allowance: 10mm)
・Welding mode: pulse

In the welded joints 1 to 14 thus obtained, the coating failure area rate was investigated using the following procedure. First, the welded joint was degreased and chemically treated. Next, electrodeposition coating was performed so that the film thickness was 20 μm. Then, the weld bead portion after the electrodeposition coating was photographed, and the photograph was image analyzed to measure the ratio of the area of the defective electrodeposition portion to the area of the weld bead. This ratio is listed in Table 4 as the percentage of defective coating area. Note that the length of the weld bead was 120 mm. The length of this weld bead excluding the weld start end (area up to 15 mm from the end of the weld bead on the welding start side) and the termination end (area up to 15 mm from the end of the weld bead on the weld end side) The above-mentioned measurements were performed on a 90 mm area. Gray paint was used for electrodeposition painting. Since yellow-brown slag is exposed in areas where the electrodeposition coating is defective, if the electrodeposition coating is performed using a gray paint, the electrodeposition coating failure areas can be easily identified. A welded joint with a coating failure area ratio of 5% or less was judged to be a welded joint with excellent electrodeposition coating properties.

Furthermore, in the welded joint that had been electrodeposition coated in the above-described procedure, 10 cuts were made between the weld beads of 90 mm, and the cross sections were polished. The presence or absence of conductive oxide and its area ratio were investigated using EDS and SPM. The investigation procedure was as described above. As a result, one to four types of phases were confirmed in the welding slag of all welded joints. Here, the "phase" is a region composed of oxides whose components are uniform. Table 4 lists the phase components confirmed in each welded joint. Note that the unit "mass%" for the content of each element contained in the phase indicates the ratio of the mass of each element to the total mass of the phase excluding oxygen. Although phases containing elements other than Si, Mn, Ti, Al, Fe, Zr, and Cr were found here and there, the description of the content of elements other than these elements was omitted. Furthermore, for phases that meet the requirements for conductive oxides, their area ratios are also listed in Table 4.

Welded joints 1 to 6 contained one or more phases of conductive oxide, and the total area ratio was 20% or more. Although these welded joints 1 to 6 were manufactured without any special treatment such as slag removal, the defectiveness of the electrodeposition coating was suppressed to 5% or less. Therefore, these welded joints 1 to 6 can be said to be welded joints with high production efficiency and high electrodeposition coating properties.

On the other hand, welded joints 7 to 13 contained no conductive oxide, and welded joint 14 had an area ratio of conductive oxide of less than 20%. The electrodeposition coating properties of these welded joints were significantly inferior to those of welded joints 1 to 6.

According to the present invention, it is possible to provide a welded joint with high production efficiency and high electrodeposition coating properties. If the present invention is applied to, for example, automobile parts or building material parts, a car body having high corrosion resistance can be manufactured at low cost. Therefore, the present invention has high industrial applicability.

1 Welded joint 11 Steel material 12 Weld bead 13 Weld slag 131 Conductive oxide phase 14 Electrodeposition coating film

Claims (6)

steel and
a weld bead;
A welded joint comprising a welding slag attached to the surface of the welding bead,
the welding slag includes a conductive oxide phase,
The chemical components of the conductive oxide phase excluding oxygen are mass % based on the total mass of the conductive oxide phase excluding oxygen,
Si: 0-5%,
Mn: 0-65%,
Ti: 0.3-100%,
Al: 0-30%, and Fe: 0-65%
Contains
The chemical components of the conductive oxide phase excluding the oxygen satisfy Formulas 1 and 2,
A welded joint, wherein the area ratio of the conductive oxide phase in the cross section of the welding slag is 20% or more.
3×Si+Mn/4-4×Ti+Al/4<0...Formula 1
Si+Mn+Ti+Al+Fe≧70...Formula 2
However, Si, Mn, Ti, Al, and Fe in the above formulas 1 and 2 mean the content of each element in mass % with respect to the total mass of the conductive oxide phase excluding the oxygen. do. The welded joint according to claim 1, further comprising an electrodeposited coating provided on the surfaces of the steel material, the weld bead, and the weld slag. The welded joint according to claim 1 or 2, wherein the area ratio of the conductive oxide phase in the cross section of the welding slag is 25% or more. The chemical components of the conductive oxide phase excluding the oxygen are Zr: 10% or less (including 0), Cr: 5% by mass relative to the total mass of the conductive oxide phase excluding oxygen. The welded joint according to any one of claims 1 to 3, characterized in that the welded joint contains at least one type selected from the group consisting of % or less (including 0), and the remainder consists of impurities. An automobile part comprising the welded joint according to any one of claims 1 to 4. A building material component comprising the welded joint according to any one of claims 1 to 4.

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