JP7810884B2 - Welded joints - Google Patents

Description

The present invention relates to a welded joint.

Automotive components, including automobile undercarriage components, and various building materials are often manufactured using welded joints made by welding multiple steel materials together. Because these automotive and building materials are used while exposed to a variety of environments, the welded joints produced are expected to have excellent corrosion resistance. Therefore, various types of zinc-based plated steel sheets, including galvannealed steel sheets, are used as materials for such welded joints.

A particular problem that arises when manufacturing welded joints by welding galvanized steel sheets is a decrease in corrosion resistance due to blowholes formed near the "toe" defined in JIS Z3001 (2018) as a result of Zn evaporation from the plating during welding.

Various proposals have been made to solve the problem of blowhole formation as described above. For example, Patent Document 1 below proposes a plated steel material that includes a steel sheet and a coating layer that is disposed on the surface of the steel sheet and includes a Zn—Al—Mg alloy layer, in which, in a cross section of the Zn—Al—Mg alloy layer, the area fraction of the MnZn ₂ phase is 45 to 75%, the total area fraction of the MgZn ₂ phase and the Al phase is 70% or more, and the area fraction of the Zn—Al—MgZn ₂ ternary eutectic structure is 0 to 5%, and the coating layer has a predetermined chemical composition.

WO 2018/139620

Here, it is possible to solve the problem of blowhole formation by using the plated steel material proposed in Patent Document 1. However, after extensive research, the inventors have discovered that there is still room for improvement in the technology proposed in Patent Document 1, and that when areas other than the vicinity of the toe are used as the back surface, for example, the surface opposite the weld bead in a welded joint, further improvements can be expected in the corrosion resistance of the heat-affected zone on the back surface.

The present invention was made in consideration of the above problems, and its object is to provide a welded joint that can further improve the corrosion resistance of the heat-affected zone on the back surface of the welded joint.

As a result of extensive research conducted by the inventors to solve the above problems, they discovered that the decrease in corrosion resistance of the heat-affected zone on the backside of a welded joint due to welding is caused by the Zn in the plating evaporating or oxidizing during welding, resulting in no plating components remaining, and as a result, iron scale forms on the surface of the heat-affected zone on the backside. Therefore, they discovered that if the formation of iron scale on the surface of the heat-affected zone on the backside could be suppressed, it would be possible to further improve the corrosion resistance of the heat-affected zone on the backside.

Based on this finding, the present inventors conducted further studies and found that by studying improvements to the plated steel sheet used as a raw material, it is possible to allow the plating components to remain even after arc welding, thereby further improving the corrosion resistance of the heat-affected zone on the back surface.
The gist of the present invention, which was completed based on these findings, is as follows.

(1) A welded joint formed by arc welding between a first steel plate and a second steel plate, the weld joint comprising the first steel plate and the second steel plate, a weld bead formed by the arc welding, and a heat-affected zone located around the weld bead, wherein when a portion of the first steel plate and the second steel plate that is not thermally affected by the welding is defined as a non-heat-affected zone, at least one of the first steel plate or the second steel plate has a plating layer located on at least a part of a surface of a base steel, and an oxide layer located on the plating layer, in the non-heat-affected zone, the plating layer contains, in mass %, Al: 1.00 to 80.00%, Mg: 1.00 to 20.00%, Fe: 0.01 to 15.00%, Si: 0 to 10.00%, Ca: 0 to 4.00%, and optionally Sb: 0 to 0.50%. %, Pb: 0-0.50%, Cu: 0-1.00%, Sn: 0-1.00%, In: 0-1.00%, Bi: 0-1.00%, Ti: 0-1.00%, Cr: 0-1.00%, Nb: 0-1 .00%, Zr: 0-1.00%, Ni: 0-1.00%, Mn: 0-1.00%, V: 0-1.00%, Mo: 0-1.00%, Ag: 0-1.00%, Li: 0-1.00%, La: 0 a welded joint comprising 0 to 5.00% of Fe: 0 to 0.50%, Ce: 0 to 0.50%, B: 0 to 0.50%, Y: 0 to 0.50%, and Sr: 0 to 0.50%, with a total content of 0 to 5.00%, and the balance consisting of 5.00 mass% or more of Zn and impurities, wherein when the surface of the welded joint on which the weld bead portion does not exist is defined as the back surface, the thickness of Fe scale located on the heat-affected zone on the back surface is 0 to 5.0 μm.
(2) The welded joint according to (1), wherein the thickness of the Fe scale is 0 to 1.0 μm.
(3) The welded joint according to (1) or (2), wherein, when a position at a depth of 5 nm from the outermost surface of the oxide layer is observed by X-ray photoelectron spectroscopy (XPS), the intensity ratio ([Al—O]+[Mg—O])/[Zn—O] calculated from the intensities of peaks respectively assigned to Al—O bonds, Mg—O bonds, and Zn—O bonds is 5.0 or more.
(4) The welded joint according to (3), wherein the strength ratio ([Al—O]+[Mg—O])/[Zn—O] is 10.0 or more.
(5) The welded joint according to any one of (1) to (4), wherein the plating layer contains at least 18.00 to 60.00 mass% of Al and 5.00 to 15.00 mass% of Mg.
(6) The welded joint according to any one of (1) to (5), wherein the plating layer contains at least 35.00 to 60.00 mass% of Al and 7.00 to _{15.00 mass% of Mg, wherein a Mg32(Al,Zn)49 phase is present} in the plating layer, and the Mg content [Mg], Zn content [Zn], and Al content [Al] (each unit: atomic %) in the Mg32(Al,Zn)49 phase satisfy the relationship 0.50≦[Mg]/([Zn]+[Al])≦0.83.

As explained above, the present invention makes it possible to further improve the corrosion resistance of the heat-affected zone on the back surface of the welded joint.

1 is an explanatory diagram schematically illustrating an example of the structure of a welded joint according to an embodiment of the present invention. FIG. FIG. 10 is an explanatory diagram schematically illustrating an example of the structure of a welded joint according to another embodiment. FIG. 10 is an explanatory diagram schematically illustrating an example of the structure of a welded joint according to another embodiment. 1B is an explanatory diagram for explaining the welded joint according to the embodiment shown in FIG. 1A. FIG. 3 is an explanatory diagram for explaining a welded joint according to the same embodiment. FIG. 3 is an explanatory diagram for explaining a welded joint according to the same embodiment. FIG. FIG. 1 is an explanatory diagram for explaining peak intensities in XPS measurement results.

Preferred embodiments of the present invention will be described in detail below with reference to the accompanying drawings. Note that in this specification and drawings, components having substantially the same functional configuration will be designated by the same reference numerals, and redundant explanations will be omitted.

(Regarding welded joints)
First, the overall configuration of a welded joint according to an embodiment of the present invention will be described with reference to Fig. 1A. Fig. 1A is an explanatory diagram that schematically shows an example of the structure of a welded joint according to this embodiment.

For convenience, the following explanation will be given using the coordinate system shown in Figure 1A. Figure 1A illustrates an example of a welded joint formed by arc welding two steel plates.

Figure 1A is a schematic diagram showing the overall structure of a welded joint obtained by lap fillet welding a first steel plate and a second steel plate by arc welding, showing a cross section of the welded joint perpendicular to the extension direction of the weld bead. As shown schematically in Figure 1A, the welded joint 1 according to this embodiment has a first steel plate 10, a second steel plate 20, a weld bead 30, and a heat-affected zone 40.

Here, it is preferable to use various types of plated steel sheets as the material for at least one of the first steel sheet 10 and the second steel sheet 20 that constitute the welded joint 1, and it is more preferable to use plated steel sheets having a plating layer as described in detail below as the material for both the first steel sheet 10 and the second steel sheet 20 that constitute the welded joint 1.

The weld bead 30 is a region formed by arc welding, and interdiffusion of constituent elements occurs between the welding wire, which is used as needed during welding, and the first steel plate 10 and second steel plate 20. The weld bead 30 is formed by the oxidation of these diffusing elements. Therefore, in FIG. 1A, the joint interface between the weld bead 30 and the first steel plate 10 or the second steel plate 20 is shown as a smooth curve or line for convenience of illustration, but the actual joint interface is a complex curved surface. The weld bead 30 extends along the Y-axis direction in the figure, and the first steel plate 10 and the second steel plate 20 are joined by the weld bead 30.

The components of the weld bead portion 30 vary depending on the type of welding wire used and the chemical compositions of the first steel sheet 10 and second steel sheet 20 used as raw materials, making it difficult to uniquely define components that cover all possibilities. However, the weld bead portion 30 generally contains, as its main component, an oxide of an easily oxidized element from among the various elements that make up the plated steel sheet used as raw material. Examples of such easily oxidized elements include Al and Mg.

Furthermore, to identify the portion of the welded joint 1 of interest that corresponds to the weld bead 30, measurements can be performed, for example, as follows. That is, a sample having the weld bead 30 is prepared, cut along a plane (X-Z plane in FIG. 1A) perpendicular to the welding direction (Y-axis direction in FIG. 1A), and then embedded in resin and polished so that the cross section of the weld bead 30 (X-Z cross section in FIG. 1A) can be observed. After polishing, the cross section of the weld bead 30 is observed using a scanning electron microscope (SEM). SEM-EDS is used to obtain element distribution images of various elements (Zn, Al, Mg, Fe, Cr, Ni, Ti, etc.), which allows the slag layer present on the weld bead to be identified. The portion located closer to the steel plate than the slag layer identified in this way is the weld bead.

Oxides produced during welding are broadly divided into two types: scale and slag. Scale, excluding oxygen, contains 50% or more Fe by mass, with the remainder consisting of easily oxidized elements and impurities. Slag, excluding oxygen, contains 50% or more easily oxidized elements by mass, with the remainder consisting of less than 50% Fe and impurities. Here, "easily oxidized elements" are metal elements that are more easily oxidized than Fe on the Ellingham diagram and that can be added to the plating layer. Specific examples of such easily oxidized metal elements include Ca, In, Bi, Cr, Zr, Li, La, Ce, Sr, Y, Si, Mn, Al, and Ti.

In JIS Z3001 (2018), the point where the surface of the base material and the surface of the weld bead intersect is defined as the "toe." In the weld joint 1 shown in Figure 1A, the point where the surface of the weld bead portion 30 intersects with the surface of the first steel plate 10 or the second steel plate 20 corresponds to this "toe." The weld joint 1 according to this embodiment focuses on corrosion resistance in the vicinity of this toe T.

This "toe T" is not only defined in lap fillet weld joints such as those shown in Figure 1A, but is also defined in the same way in butt weld joints such as those shown in Figure 1B and T-shaped weld joints such as those shown in Figure 1C.

In addition, heat input during arc welding is typically applied from one side of the weld joint 1, and the weld bead 30 is exposed on the surface of the base steel plate on the heat input side of the arc welding, with the size of the weld bead 30 narrowing in the direction in which the heat input from the arc welding propagates. Therefore, the heat input direction during arc welding can be identified by noting whether or not there is a weld bead 30 exposed on the surface and the extent of the weld bead 30 (for example, the extent when focusing on a cross section such as that shown in Figure 1B).

Furthermore, in this embodiment, in joints in which two steel plates at least partially overlap, such as the lap fillet weld joint shown in Figure 1A or the T-weld joint shown in Figure 1C, the steel plate located on the heat input side of the arc welding as described above is referred to as the first steel plate 10, and the steel plate located on the side in which the heat input from the arc welding propagates is referred to as the second steel plate 20.

As shown schematically in Figures 1A to 1C, a heat-affected zone 40 is formed around the weld bead portion 30. This heat-affected zone 40 is formed when the metal structure of the first steel plate 10 and the second steel plate 20 is altered as a result of heat input to the first steel plate 10 and the second steel plate 20 by arc welding. The size of the heat-affected zone 40 depends on the amount of heat input during arc welding, and generally, the greater the amount of heat input, the larger the size of the heat-affected zone 40. Furthermore, due to the alteration of the metal structure of the first steel plate 10 and the second steel plate 20, the appearance (appearance) of the altered metal structure differs visually from the unaltered portions of the first steel plate 10 and the second steel plate 20, making it easy to identify the portion corresponding to the heat-affected zone 40.

<Non-heat-affected zone>
Next, the configuration of a portion of the welded joint 1 according to this embodiment that is not affected by heat due to welding will be described in detail with reference to Fig. 2. Fig. 2 is a diagram schematically showing a cross section of the welded joint 1 perpendicular to the extension direction of the weld bead portion 30.

In the following description, the portion of the welded joint 1 that is not thermally affected by welding (in other words, the portion that is not the weld bead 30 or the heat-affected zone 40) will be referred to as the "non-heat-affected zone." In the welded joint 1 shown in Figure 2, for example, the region R1, which is sufficiently separated from the vicinity of the toe T as surrounded by the dashed line in the figure, corresponds to this non-heat-affected zone. The position of this non-heat-affected zone can be considered to be, for example, a region separated from the toe T shown in Figure 2 by 10 mm or more in a direction perpendicular to the extension direction of the weld bead 30 (the Y-axis direction in Figure 2) and away from the toe T (the X-axis direction in Figure 2).

Fig. 3 is a diagram schematically showing a portion of a cross section parallel to the plate thickness direction in the non-heat-affected zone R1. As schematically shown in Fig. 3, the non-heat-affected zone R1 in at least either the first steel sheet 10 or the second steel sheet 20 has a base steel 101, a plating layer 103 located on at least a portion of the surface of the base steel 101, and an oxide layer 105 located on the plating layer 103. In the welded joint 1 according to this embodiment, the plating layer 103 and the oxide layer 105 may be present on one surface of the base steel 101, but it is more preferable that they be present on both surfaces of the base steel 101.
The base steel 101, the plating layer 103, and the oxide layer 105 will be described in detail below.

<About Chitetsudo 101>
In the welded joint 1 according to this embodiment, the base steel 101 corresponding to the base material of the plated steel sheet that is the raw material is not particularly limited. Various steel sheets can be used as the base steel 101 depending on the mechanical strength (e.g., tensile strength) required of the welded joint 1. Examples of such steel sheets include various steel sheets such as various types of Al-killed steel, extra-low carbon steel containing Ti, Nb, etc., and high-strength steel in which extra-low carbon steel further contains strengthening elements such as P, Si, Mn, etc.

Furthermore, the thickness of the base steel 101 is not particularly limited and is set appropriately depending on the mechanical strength required of the weld joint 1, etc.

<Regarding plating layer 103>
3, the plating layer 103 is provided on at least a portion of the surface of the base steel 101, and more preferably over the entire surface of the base steel 101. Such plating layer 103 originates from the plating layer that is present in the plated steel sheet that is the material of the welded joint 1.

The plating layer 103 is preferably provided on the surface where the weld bead portion 30 of the steel plate (first steel plate 10) located on the heat input side of the arc welding in the weld joint 1 is exposed (e.g., the surface on the positive Z-axis side in Figure 1A, etc.), and on the surface where the heat-affected zone 40 of the steel plate (second steel plate 20) on the heat propagation side of the arc welding is exposed (e.g., the surface on the negative Z-axis side in Figure 1A, etc.). It is preferable that the plating layer 103 described below be provided on surfaces of the first steel plate 10 and the second steel plate 20 other than those mentioned above, but the plating layer 103 does not have to be provided.

Below, we will first provide a detailed explanation of the chemical composition of the plating layer 103.

Regarding the chemical composition of plating layer 103, the chemical composition of plating layer 103 according to this embodiment contains, in mass%, 1.00 to 80.00% Al, 1.00 to 20.00% Mg, 0.01 to 15.00% Fe, 0 to 10.00% Si, and 0 to 4.00% Ca, with the balance consisting of 5.00 mass% or more Zn and impurities. In other words, in the chemical composition of plating layer 103 according to this embodiment, the contents of Al, Mg, Fe, Si, and Ca are within the above ranges and the total of these contents is less than 100 mass%, with the balance consisting of 5.00 mass% or more Zn and impurities.

These ingredients and their contents are explained in detail below.

[Al: 1.00 to 80.00% by mass]
Al is an element necessary for constituting the main phase (Zn—Al—Mg alloy phase) of the plating layer 103 according to this embodiment, and is contained in a certain amount or more to ensure corrosion resistance in the non-heat-affected zone. If the Al content in the plating layer 103 is less than 1.00 mass%, the corrosion resistance of the non-heat-affected zone as described above cannot be ensured. Therefore, in the plating layer 103 according to this embodiment, the Al content is 1.00 mass% or more. The Al content is preferably 18.00 mass% or more, and more preferably 35.00 mass% or more. By ensuring the Al content within the above range, it is possible to ensure corrosion resistance in the non-heat-affected zone.

On the other hand, if the Al content in the plating layer 103 exceeds 80.00 mass%, the Al phase, which functions as a cathode when placed in a corrosive environment, will increase excessively, facilitating corrosion of the base steel, and the corrosion resistance of the non-heat-affected zone cannot be ensured. Therefore, in the plating layer 103 according to this embodiment, the Al content is 80.00 mass% or less. The Al content is preferably 60.00 mass% or less, and more preferably 50.00 mass% or less.

[Mg: 1.00 to 20.00% by mass]
Mg is an element necessary for constituting the main phase (Zn—Al—Mg alloy phase) of the plating layer 103 according to this embodiment, and is contained in a certain amount or more to ensure corrosion resistance of the non-heat-affected zone. Therefore, the Mg content in the plating layer 103 according to this embodiment is 1.00 mass% or more. The Mg content is preferably 5.00 mass% or more, and more preferably 7.00 mass% or more. By ensuring the Mg content in the above range, it is possible to ensure corrosion resistance of the non-heat-affected zone.

On the other hand, if the Mg content in the plating layer 103 exceeds 20.00% by mass, anodic dissolution of the plating layer is more likely to occur when placed in a corrosive environment, making it impossible to ensure corrosion resistance in the non-heat-affected zone. Therefore, in the plating layer 103 according to this embodiment, the Mg content is 20.00% by mass or less. The Mg content is preferably 15.00% by mass or less, and more preferably 13.00% by mass or less. By keeping the Mg content within the above range, it is possible to reliably ensure corrosion resistance in the non-heat-affected zone.

[Fe: 0.01 to 15.00% by mass]
Elements constituting the base steel 101 may be mixed into the coating layer 103 from the base steel 101, which is the base material. In particular, when the coating layer 103 is formed by hot-dip galvanizing, elements constituting the base steel 101 are more likely to be mixed into the coating layer 103 due to interdiffusion of elements caused by solid-liquid reactions between the base steel 101 and the coating layer 103. Due to this mixing of elements, a certain amount of Fe is contained in the coating layer 103, and this content is generally 0.01 mass% or more. If this interdiffusion is promoted, the adhesion between the base steel 101 and the coating layer 103 is improved. From the viewpoint of improving the adhesion between the base steel 101 and the coating layer 103, the Fe content in the coating layer 103 is preferably 0.20 mass% or more.

Furthermore, Fe may be intentionally added to the plating bath used to produce the plating layer 103, provided that the effects of the present invention are not impaired. However, if the Fe content in the plating layer 103 is 15.00% by mass or more, high-melting-point intermetallic compounds of Fe and Al will form in the plating bath, and these high-melting-point intermetallic compounds will adhere to the plating layer as dross, significantly reducing the appearance quality, which is undesirable. From this perspective, the Fe content in the plating bath is adjusted so that the Fe content in the plating layer 103 is 15.00% by mass or less. The Fe content in the plating layer 103 is more preferably 10.00% by mass or less.

[Si: 0 to 10.00% by mass]
Si is an element that can suppress the excessive growth of an Fe-Al intermetallic compound phase that forms at the interface between the coating layer and the base steel, thereby improving the adhesion between the coating layer and the base steel. To suppress the excessive growth of the Fe-Al intermetallic compound phase, the Si content is preferably 0.05 mass% or more, and more preferably 0.20 mass% or more. On the other hand, if the Si content exceeds 10.00 mass%, an intermetallic compound phase with Mg that increases the melting point is excessively formed, inhibiting the formation of an Al-Mg oxide film that has the effect of suppressing Zn evaporation, making it difficult to suppress Zn evaporation during welding.

On the other hand, if the Si content in the plating bath used to produce the plating layer 103 is too high, the viscosity of the plating bath may increase more than necessary, resulting in reduced operability. Therefore, from the perspective of operability, the Si content in the plating bath is adjusted so that the Si content in the plating layer 103 is 10.00 mass% or less. The Si content in the plating layer 103 is preferably 5.00 mass% or less, and more preferably 2.00 mass% or less.

[Ca: 0 to 4.00% by mass]
When contained in the plating layer 103, Ca forms an intermetallic compound phase with Al and Zn. Furthermore, when Si is contained together with Ca in the plating layer 103, Ca forms an intermetallic compound phase with Si. These intermetallic compound phases have a high melting point and a stable structure, making it possible to suppress the formation of blowholes caused by Zn evaporation during welding of plated steel sheets and LME. The effect of suppressing the formation of blowholes and LME during welding is achieved by setting the Ca content to 0.01% by mass or more. The Ca content in the plating layer 103 is more preferably 0.10% by mass or more.

On the other hand, if the Ca content in the plating layer 103 exceeds 4.00 mass%, the corrosion resistance of the non-heat-affected zone will decrease. From this perspective, the Ca content in the plating layer 103 is 4.00 mass% or less. The Ca content in the plating layer 103 is preferably 2.50 mass% or less, and more preferably 1.50 mass% or less.

In the plating layer 103, the balance of the above-mentioned Al, Mg, Fe, Si, and Ca is 5.00 mass % or more of Zn and impurities.
Zn is an element necessary for constituting the main phase (Zn—Al—Mg alloy phase) of the plating layer 103 according to this embodiment, and is an important element for improving the corrosion resistance of the non-heat-affected zone. The effect of improving the corrosion resistance of the non-heat-affected zone is realized when the Zn content is 5.00 mass% or more, so the Zn content is set to 5.00 mass% or more.

In addition, the plating layer 103 according to this embodiment further selectively replaces a portion of the remaining Zn with Sb: 0-0.50%, Pb: 0-0.50%, Cu: 0-1.00%, Sn: 0-1.00%, In: 0-1.00%, Bi: 0-1.00%, Ti: 0-1.00%, Cr: 0-1.00%, Nb: 0-1.00%, Zr: 0- The plating layer 103 according to this embodiment may contain at least one of the following elements as optional additives: Sb, Pb, Cu, Sn, In, Bi, Ti, Cr, Nb, Zr, Ni, Mn, V, Mo, Ag, Li, La, Ce, B, Y, and Sr, in a total amount of 0 to 5.00% by mass within the above ranges and in a total amount of 5.00% by mass or less.

By setting the total content of the above optional additional elements to 5.00% by mass or less, it becomes possible to enjoy the effects exerted by the addition of each optional additional element as described in detail below without impairing each other. The total content of the above optional additional elements is preferably 1.00% by mass or less, and more preferably 0.20% by mass or less.
The content of each optional added element will be described in detail below.

[Sb: 0 to 0.50% by mass]
[Pb: 0 to 0.50% by mass]
[Sr: 0 to 0.50% by mass]
When at least one of Sb, Pb, and Sr is contained in the plating layer 103, spangles are formed on the surface of the plating layer 103, making it possible to improve metallic luster. Therefore, from the viewpoint of improving the design of the plated steel sheet, it is preferable that at least one of Sb, Pb, and Sr is contained in the plating layer 103. Such an effect of improving the design is realized when the content of at least one of Sb, Pb, and Sr is 0.05 mass% or more. Therefore, when at least one of Sb, Pb, and Sr is contained in the plating layer 103, it is preferable that the content of each of these elements is independently 0.05 mass% or more.

On the other hand, if a coating layer 103 is formed in which any one of the Sb, Pb, and Sr contents exceeds 0.50 mass%, the amount of dross generated in the coating bath used to form the coating layer 103 increases, making it impossible to produce a coated steel sheet with good coating properties. Therefore, the contents of Sb, Pb, and Sr in the coating layer 103 are each independently set to 0.50 mass% or less. The contents of Sb, Pb, and Sr are each independently set to preferably 0.20 mass% or less.

[Cu: 0 to 1.00% by mass]
[Ti: 0 to 1.00% by mass]
[Cr: 0 to 1.00% by mass]
[Nb: 0 to 1.00% by mass]
[Ni: 0 to 1.00% by mass]
[Mn: 0 to 1.00% by mass]
[V: 0 to 1.00% by mass]
When at least one of Cu, Ti, Cr, Nb, Ni, Mn, and V is contained in the plating layer 103, these elements are incorporated into the Al—Fe alloy layer produced by welding when the plated steel sheet is welded, thereby improving the corrosion resistance of the weld bead 30. This effect of improving the corrosion resistance of the weld is realized when the content of any of Cu, Ti, Cr, Nb, Ni, Mn, and V in the plating layer 103 is 0.005 mass% or more. Therefore, when at least one of Cu, Ti, Cr, Nb, Ni, Mn, and V is contained in the plating layer 103, it is preferable that the content of each of these elements is independently 0.005 mass% or more.

On the other hand, if a plating layer 103 is formed in which any one of the contents of Cu, Ti, Cr, Nb, Ni, Mn, and V exceeds 1.00 mass%, these elements tend to form various intermetallic compound phases in the plating bath used to form the plating layer 103. This increases the viscosity of the plating bath, making it impossible to produce a plated steel sheet with good plating properties. Therefore, the contents of Cu, Ti, Cr, Nb, Ni, Mn, and V in the plating layer 103 are each independently set to 1.00 mass% or less. The contents of Cu, Ti, Cr, Nb, Ni, Mn, and V are each independently set to 0.20 mass% or less, preferably.

[Sn: 0 to 1.00% by mass]
[In: 0 to 1.00% by mass]
[Bi: 0 to 1.00% by mass]
Sn, In, and Bi are elements that increase the Mg dissolution rate when the plating layer 103 containing Zn, Al, and Mg is placed in a corrosive environment. When the Mg dissolution rate increases, Mg ions are supplied to the exposed base steel, improving sacrificial corrosion protection. On the other hand, excessive addition of Sn, In, or Bi may excessively accelerate the Mg dissolution rate, potentially reducing the corrosion resistance of the non-heat-affected zone. This increase in the Mg dissolution rate becomes significant when any of the Sn, In, and Bi contents exceeds 1.00 mass%, so the Sn, In, and Bi contents are each independently 1.00 mass% or less. The Sn, In, and Bi contents are each independently preferably 0.20 mass% or less. On the other hand, the lower limits of the Sn, In, and Bi contents are not particularly specified and may each independently be 0 mass %, but when Sn, In, and Bi are contained, their contents are preferably each independently set to 0.005 mass % or more, which makes it possible to improve the sacrificial corrosion protection of the plating layer 103.

[Zr: 0 to 1.00% by mass]
When Zr is contained in the plating layer 103, it is possible to improve plating properties. Such an effect of improving plating properties is realized when the Zr content is 0.01 mass% or more. Therefore, when Zr is contained, the content is preferably 0.01 mass% or more.

On the other hand, when forming a plating layer 103 with a Zr content exceeding 1.00% by mass, a large amount of dross is likely to be generated in the plating bath used to form the plating layer 103. Therefore, the Zr content is 1.00% by mass or less. The Zr content is preferably 0.10% by mass or less.

[Mo: 0 to 1.00% by mass]
When Mo is contained in the plating layer 103, it is possible to improve corrosion resistance. The effect of improving corrosion resistance is realized when the Mo content is 0.01 mass% or more. Therefore, when Mo is contained, the content is preferably 0.01 mass% or more.

On the other hand, when forming a plating layer 103 with a Mo content exceeding 1.00% by mass, a large amount of dross is likely to be generated in the plating bath used to form the plating layer 103. Therefore, the Mo content is 1.00% by mass or less. The Mo content is preferably 0.05% by mass or less.

[Ag: 0 to 1.00% by mass]
When Ag is contained in the plating layer 103, it is possible to improve plating properties. Such an effect of improving plating properties is realized when the Ag content is 0.01 mass% or more. Therefore, when Ag is contained, the content is preferably 0.01 mass% or more.

On the other hand, when forming a plating layer 103 with an Ag content exceeding 1.00% by mass, a large amount of dross is likely to be generated in the plating bath used to form the plating layer 103. Therefore, the Ag content is 1.00% by mass or less. The Ag content is preferably 0.05% by mass or less.

[Li: 0 to 1.00% by mass]
When Li is contained in the plating layer 103, it is possible to improve plating properties. Such an effect of improving plating properties can be achieved when the Li content is 0.01 mass% or more. Therefore, when Li is contained, the content is preferably 0.01 mass% or more.

On the other hand, when forming a plating layer 103 with a Li content exceeding 1.00% by mass, a large amount of dross is likely to be generated in the plating bath used to form the plating layer 103. Therefore, the Li content is 1.00% by mass or less. The Li content is preferably 0.05% by mass or less.

[La: 0 to 0.50% by mass]
[Ce: 0 to 0.50% by mass]
[Y: 0 to 0.50% by mass]
La, Ce, and Y are elements that exhibit almost the same effect as Ca, suppressing blowhole formation and LME during welding. This is because the atomic radius of each element is close to that of Ca. When these elements are contained in the plating layer 103, they substitute for Ca. Therefore, these elements are detected at the same position as Ca in EDS. Furthermore, even when these elements become oxides, the oxides of these elements are detected at the same position as CaO.

The effect of suppressing blowhole formation and LME during welding is achieved by ensuring that the content of these elements is independently 0.01% by mass or more. Therefore, the content of La, Ce, and Y in the plating layer 103 is more preferably independently 0.05% by mass or more.

On the other hand, if the La, Ce, and Y contents in the plating bath used to produce the plating layer 103 are too high, the viscosity of the plating bath may increase more than necessary, resulting in reduced operability. Therefore, from the perspective of operability, the La, Ce, and Y contents in the plating bath are adjusted so that the La, Ce, and Y contents are each independently 0.50 mass% or less. The La, Ce, and Y contents are preferably each independently 0.10 mass% or less.

[B: 0 to 0.50% by mass]
When contained in the coating layer 103, B has the effect of suppressing LME. This is presumably because, when B is contained in the coating layer 103, it combines with at least one of Zn, Al, Mg, and Ca to form various intermetallic compound phases. Furthermore, the presence of B in the coating layer 103 is thought to cause B to diffuse from the coating layer 103 to the steel substrate 101, strengthening the grain boundaries and suppressing LME in the steel substrate 101. Furthermore, because the various intermetallic compounds formed with B have extremely high melting points, they are presumed to also act to suppress Zn evaporation during welding. These improving effects are achieved by including 0.05% or more of B. Therefore, the B content in the coating layer 103 is more preferably 0.05% or more of B.

On the other hand, if excessive B is added to the plating bath in order to incorporate B into the plating layer 103, a rapid rise in the plating melting point will occur, reducing plating operability and making it impossible to produce plated steel sheets with excellent plating properties. This reduction in operability becomes significant when the B content exceeds 0.50 mass%, so the B content is 0.50 mass% or less. The B content is preferably 0.10 mass% or less.

[Method for measuring chemical components]
The chemical composition of the plating layer 103 can be measured using ICP-AES (Inductively Coupled Plasma Atomic Emission Spectrometry) or ICP-MS (Inductively Coupled Plasma Mass Spectrometry). ICP-AES is used to analyze chemical compositions down to 0.1% by mass, while ICP-MS is used to analyze trace amounts of chemical compositions less than 0.1% by mass. A target area of the non-heat-affected zone is immersed in 10% hydrochloric acid containing an inhibitor for approximately 1 minute to remove the plating layer, and a solution in which the plating layer is dissolved is prepared. The resulting solution is analyzed by ICP-AES or ICP-MS to obtain the overall average chemical composition of the plating layer.

More Preferred Chemical Composition of Plating Layer 103 The plating layer 103 according to this embodiment has the chemical composition described above, but a more preferred chemical composition is as follows.
That is, the plating layer 103 according to this embodiment preferably contains, as a chemical composition, at least 18.00 to 60.00 mass% Al and 5.00 to 15.00 mass% Mg, and further contains the above-mentioned optional additional elements as required.

Furthermore, the plating layer 103 according to this embodiment has a chemical composition containing at least 35.00 to 60.00 mass% Al and 7.00 to 15.00 mass% Mg, and further contains the above-mentioned optional additional elements as necessary. It is even more preferable that the plating layer 103 further contains a Mg ₃₂ (Al, Zn) ₄₉ phase.

Here, the _Mg32 (Al,Zn) ₄₉ phase is defined as a phase in which the Mg content [Mg], Zn content [Zn], and Al content [Al] contained within the grains of the _Mg32 (Al,Zn) ₄₉ phase satisfy, in atomic percent, 0.5≦[Mg]/([Zn]+[Al])≦0.83. That is, it is defined as a crystalline phase or quasicrystalline phase in which the ratio of Mg atoms to the sum of Zn atoms and Al atoms, Mg:(Zn+Al), is 3:6 to 5:6. The chemical composition of _{the Mg32} (Al,Zn) ₄₉ phase is preferably measured using quantitative analysis by TEM-EDX (Transmission Electron Microscope-Energy Dispersive X-ray Spectroscopy). The Mg ₃₂ (Al, Zn) ₄₉ phase may be detected as both a crystalline phase and a quasicrystalline phase. In the case of a crystalline phase, it is possible to identify the crystal structure as the Mg ₃₂ (Al, Zn) ₄₉ phase from an electron diffraction image in TEM observation. Note that, when the Mg ₃₂ (Al, Zn) ₄₉ phase is a quasicrystalline phase, it can be confirmed by taking an electron diffraction image by TEM and checking whether a 5-fold symmetric crystal structure is observed in the electron diffraction image. The 5-fold symmetric crystal structure can be identified by obtaining an electron diffraction image called a Penrose pattern.

The _Mg32 (Al,Zn) ₄₉ phase exhibits sacrificial corrosion protection properties for plated steel sheets, thereby suppressing corrosion of the base steel from cuts and welds where the base steel is exposed, and improving red rust resistance. In addition, the _Mg32 (Al,Zn) ₄₉ phase itself has excellent corrosion resistance, and because the _Mg32 (Al,Zn) ₄₉ phase has a slow corrosion rate even in a corrosive environment, it also has the effect of suppressing under-film corrosion and improving corrosion resistance after painting in terms of the width of blistering of the paint film.

Regarding the deposition weight of the plating layer 103, the deposition weight of the plating layer 103 described above is not particularly specified, but it is preferably, for example, about 15 to 250 g/ ^m2 per side of the base steel 101. When the deposition weight of the plating layer 103 is within the above range, the non-heat-affected zone of the welded joint 1 according to this embodiment can exhibit sufficient corrosion resistance.

The deposition weight of the plating layer 103 can be determined by cutting a 30 mm x 30 mm sample (with tape sealed on the back side to prevent dissolution) from the plated steel sheet and measuring its mass in advance. The sample is then immersed in a 10% HCl aqueous solution containing an inhibitor to remove the plating layer by pickling, and the mass of the sample after pickling is measured. The deposition weight of the plating layer 103 can be determined from the change in mass of the sample before and after pickling.

<Regarding the oxide layer 105>
Next, the oxide layer 105 in the non-heat-affected zone of the welded joint 1 according to this embodiment will be described in detail.
As shown schematically in Fig. 3, an oxide layer 105 is located on the surface of the above-described plating layer 103. This oxide layer 105 originates from the oxide layer of the plated steel sheet that is the material of the welded joint 1.

This oxide layer 105 is formed when easily oxidized elements that make up the plating layer 103 react with oxygen in the heat treatment atmosphere during the cooling process that solidifies the plating layer during the production of the plated steel sheet.

As described above, this oxide layer 105 is primarily composed of oxides of the elements that make up the plating layer 103, and therefore its chemical composition varies depending on the elements contained in the plating layer 103. It is presumed that this oxide layer 105 contains a total of 50 mass% or more of Zn oxide, Mg oxide, and Al oxide, and may also contain hydroxides of these elements, oxides or hydroxides of other elements, impurities, etc.

The oxide layer 105 according to this embodiment exists in the following specific state as a result of undergoing a specific heat treatment process, as described in detail below, during the production of the plated steel sheet that serves as the base material. This state will be described in detail below with reference to Figures 4 and 5. Figure 4 is a schematic diagram showing a portion of a cross section of the oxide layer parallel to the sheet thickness direction. Figure 5 is an explanatory diagram for explaining the peak intensity in the XPS measurement results.

By undergoing a specific heat treatment process, as described in detail below, during the production of the plated steel sheet that serves as the base material, the oxide layer 105 according to this embodiment is a dense coating in which the total amount of at least one of Al oxide and hydroxide and at least one of Mg oxide and hydroxide is greater than the amount of at least one of Zn oxide and hydroxide. This will be explained in more detail below.

As shown schematically in FIG. 4, attention is now focused on a position 5 nm deep from the outermost surface of the oxide layer 105 (position A in FIG. 4). In the oxide layer 105 according to this embodiment, when this position is observed by X-ray photoelectron spectroscopy (XPS), it is preferable that the intensity ratio ([Al-O] + [Mg-O])/[Zn-O] calculated from the intensities of the peaks respectively attributed to Al-O bonds, Mg-O bonds, and Zn-O bonds is 5.0 or greater.

Here, the outermost surface of the oxide layer 105 may be contaminated with oils and grease. Therefore, it is desirable to perform the above-mentioned XPS measurement in a state where such contaminants are not present. From this perspective, the surface of the oxide layer 105 is subjected to a process such as ultrasonic cleaning in ethanol to remove contaminants, and the surface obtained by this process is used as the "outermost surface of the oxide layer 105" when performing the above-mentioned XPS measurement.

Then, the oxide layer 105 obtained as described above is removed from the outermost surface to a depth of 5 nm by Ar ion etching, and the surface of the obtained oxide layer 105 is measured by XPS. Here, the XPS measurement conditions may be, for example, as follows.
X-ray source: mono-Al Kα (1486.6eV)
X-ray diameter: 50 to 200 μm
Measurement area: 100-700μm x 100-700μm
Degree of vacuum: 1×10 ⁻¹⁰ to 1×10 ⁻¹¹ torr (1 torr is 133.32 Pa)
Acceleration voltage: 1 to 10 kV

In this embodiment, we focus on the peaks attributable to Al-O bonds, Mg-O bonds, and Zn-O bonds in the obtained XPS measurement results. These bonds are characteristic of oxides and hydroxides of Al, Mg, and Zn, and it can be considered that the intensity of the peaks attributable to these bonds is positively correlated with the amount of at least one of the oxides and hydroxides of Al, Mg, and Zn present.

Here, the peaks attributable to Al-O bonds are those observed in the range of 72 to 76 eV in the XPS spectrum focusing on Al 2p3/2. The peaks attributable to Mg-O bonds are those observed in the range of 48 to 52 eV in the XPS spectrum focusing on Mg 2p3/2. The peaks attributable to Zn-O bonds are those observed in the range of 1018 to 1024 eV in the XPS spectrum focusing on Zn 2p3/2.

Furthermore, the intensity of the peak attributed to each bond is determined by subtracting the intensity Ib of the baseline from the intensity _Ip of the peak of interest in the XPS spectrum as schematically shown in FIG. ₅ (i.e., " _Ip - _Ib ").

A more detailed method for calculating the intensity ratio is as follows.
That is, XPS is measured as described above at any location on the surface corresponding to a position 5 nm deep from the outermost surface (the surface at "Position A" in FIG. 4 ), and the intensity ratio ([Al—O]+[Mg—O])/[Zn—O] is calculated. This measurement and calculation process is performed at any five locations on the surface corresponding to "Position A," and the average of the five obtained intensity ratios is defined as the intensity ratio ([Al—O]+[Mg—O])/[Zn—O] in the oxide layer 105 according to this embodiment.

In the oxide layer 105 according to this embodiment, a dense coating is formed such that the intensity ratio is 5.0 or greater, thereby suppressing Zn evaporation during welding and preventing the formation of blowholes caused by Zn evaporation. If the intensity ratio is less than 5.0, the density required for the oxide layer 105 may be insufficient. The intensity ratio is more preferably 10.0 or greater. Meanwhile, the upper limit of the intensity ratio ([Al-O] + [Mg-O])/[Zn-O] is not particularly specified, but a practical upper limit is approximately 100.0.

The thickness (more specifically, the average thickness) of the oxide layer 105 described above is not particularly limited, but is preferably, for example, approximately 0.05 to 2.00 μm per side of the base steel 101. By ensuring that the thickness of the oxide layer 105 falls within the above range, the non-heat-affected zone according to this embodiment can adequately suppress the formation of blowholes due to Zn evaporation during welding. Furthermore, an oxide layer 105 of the above thickness can be achieved by controlling the steel sheet threading speed within an appropriate range during the production of the plated steel sheet used as the base material, and by subjecting the steel sheet to a heat treatment process as described in detail below.

The thickness of this oxide layer 105 can be measured using XPS. XPS measurements are performed in the depth direction at intervals of 1 to 3 nm from the surface of the plated steel sheet, and the depth at which the maximum oxygen intensity reaches 1/20 of the maximum intensity at the outermost surface is defined as the thickness of the oxide layer. The XPS measurement conditions may be the same as those described above.

The non-heat-affected zone in the welded joint 1 according to this embodiment has been described in detail above with reference to FIGS.
The non-heat-affected zone of the welded joint 1 according to this embodiment may further have one or more of various coating layers on the oxide layer 105. Examples of such coatings include a chromate coating, a phosphate coating, a chromate-free coating, and an organic resin coating.

<About the heat-affected zone on the back side>
As mentioned previously, in the welded joint 1 according to this embodiment, the surface on which the weld bead 30 is present (e.g., the surface on the positive Z-axis side in FIG. 1A ) will be referred to as the "front surface side of the welded joint 1," and the surface opposite the surface on which the weld bead 30 is present will be referred to as the "rear surface side of the welded joint 1." Here, the "front surface side of the welded joint 1" can be understood as the front surface of the component that is exposed to public view when the welded joint 1 according to this embodiment is used as a component such as an automobile suspension part, and the "rear surface side of the welded joint 1" can also be understood as the rear surface of the component that is not exposed to public view. For example, in the welded joint 1 shown in FIGS. 1A to 1C , the surface on the negative Z-axis side of the second steel plate 20 corresponds to the above-mentioned "rear surface side of the welded joint 1."

In the welded joint 1 according to this embodiment, it is important that a plated steel sheet having the base steel 101, plating layer 103, and oxide layer 105 described above is used as the material for at least the second steel sheet 20, and that the plated layer 103 and oxide layer 105 described above are provided at least on the surface that will become the back side of the second steel sheet 20 after welding. By using such a plated steel sheet in this state, the plating components can be sufficiently retained in the heat-affected zone 40 on the back side of the welded joint 1 obtained after arc welding, even after arc welding. As a result, the formation of iron scale (not shown) associated with arc welding can be sufficiently suppressed on the surface of the heat-affected zone 40 on the back side (region R2 in Figure 2). This enables the welded joint 1 according to this embodiment to have improved corrosion resistance in the heat-affected zone on the back side.

Specifically, in the welded joint 1 according to this embodiment, the thickness (average thickness) of the Fe scale (not shown) located on the heat-affected zone 40 on the back side is 0 to 5.0 μm. If the Fe scale thickness exceeds 5.0 μm, this means that the plating components were not sufficiently retained during arc welding, and the corrosion resistance of the heat-affected zone on the back side of the welded joint 1 cannot be ensured. The thickness of the Fe scale in the heat-affected zone 40 on the back side is preferably 0 to 1.0 μm. Furthermore, the thinner the Fe scale thickness in the heat-affected zone 40 on the back side, the better, with the lower limit being 0 μm (in other words, a state in which no Fe scale is present), as described above.

The thickness of the Fe scale described above can be measured as follows. Specifically, a cross-sectional SEM image is obtained, and point composition analysis is performed using an EPMA to identify the Fe scale portion in an arbitrary field of view (size: 64 μm x 48 μm, equivalent to 2000x magnification). The thickness of the Fe scale is measured at five random locations within the field of view, and the average value of the Fe scale thickness within that field of view is calculated. Similar measurements are performed on four other arbitrary fields of view, and the average value for the total of five fields of view is used as the Fe scale thickness of the sample of interest.

In the welded joint 1 according to this embodiment, even in the heat-affected zone on the back side, where Fe scale is likely to occur, sufficient plating components remain in the heat-affected zone 40, so the thickness of the Fe scale is within the range of 0 to 5.0 μm. Therefore, in other areas where sufficient plating components are present (for example, non-heat-affected zones), the thickness of the Fe scale is naturally kept within the range of 0 to 5.0 μm.

The welded joint 1 according to this embodiment has been described in detail above with reference to Figures 1A to 6. The welded joint 1 according to this embodiment as described above can be suitably used, for example, as an automobile suspension part.

(Regarding the manufacturing method of the plated steel sheet used as the raw material)
Next, an example of a method for manufacturing the plated steel sheet that serves as the material for the welded joint 1 as described above will be described.
The plated steel sheet that is the raw material for the welded joint 1 according to this embodiment is manufactured by using a steel sheet made of the above-described base steel 101 as a base material and forming a plating layer 103 and an oxide layer 105 on the surface of the base steel 101.

Here, the plating layer 103 can be formed by hot-dip plating, thermal spraying, cold spraying, sputtering, vapor deposition, electroplating, and other methods. However, hot-dip plating is most preferable from a cost perspective when forming a plating layer of a thickness generally used in automobiles, etc.

Then, the resulting plated steel sheet (a plated steel sheet consisting of the base steel 101 and the plated layer 103) is subjected to a specific heat treatment process as described below, thereby forming an oxide layer 105 on the surface of the plated layer 103. This allows the production of a plated steel sheet that can be used as the material for the welded joint 1 according to this embodiment.

An example of a manufacturing method for obtaining a plated steel sheet according to this embodiment using a hot dip galvanizing method will be described in detail below.
In the manufacturing process of such a plated steel sheet, first, a steel sheet made of a base steel 101 used as a base material is rolled by the Sendzimir method to a desired thickness, and then wound into a coil and placed in a hot-dip galvanizing line.

In the hot-dip galvanizing line, the steel sheet is continuously fed from the coil and threaded through the line. At this time, the steel sheet is subjected to a heat reduction treatment at 800°C in an _N2-5 % _H2 gas atmosphere in a non-oxidizing environment with an oxygen concentration of 20 ppm or less, for example, using annealing equipment installed on the line. The steel sheet is then air-cooled with _N2 gas to a temperature about 20°C higher than the bath temperature of the subsequent galvanizing bath, and then immersed in the galvanizing bath.

A molten plating alloy having the aforementioned chemical composition is prepared in the plating bath. The temperature of the plating bath is set to be above the melting point of the plating alloy (e.g., approximately 460-600°C). When preparing the plating alloy material, it is preferable to use pure metals (purity of 99% or higher) as the alloy material. First, predetermined amounts of alloy metals are mixed to achieve the plating layer composition described above, and then completely melted to form an alloy using a high-frequency induction furnace or arc furnace under vacuum or inert gas replacement conditions. Next, the alloy mixed with the predetermined components (the plating layer composition described above) is melted in the atmosphere, and the resulting molten material is used as the plating bath.

In addition, there is no particular restriction on using pure metals when making the plating alloys described above; existing Zn alloys, Mg alloys, and Al alloys can also be melted and used. In this case, there is no problem as long as an alloy with the specified composition containing few impurities is used.

After immersing the steel sheet in the above-described plating bath, it is pulled up at a predetermined pulling speed. During this process, the coating weight is controlled, for example, by _N2 wiping gas, so that the resulting plating layer 103 has a desired thickness. Here, general plating operating conditions may be applied for all conditions other than the bath temperature, and no special equipment or conditions are required.

Then, the molten plating alloy located on the steel sheet is subjected to the first and second cooling processes described below, converting the molten plating alloy into a plating layer 103 and forming an oxide layer 105 on the surface of the plating layer 103. The first and second cooling processes are described in detail below.

The first cooling process is carried out when the temperature of the plating alloy is within the range of the bath temperature to 250°C. The plated steel sheet within this temperature range is rapidly cooled at an average cooling rate of 10°C/second or more in an atmosphere with a dew point of -20°C or lower. If hot-dip plating is used in the plating process, this first cooling process is carried out immediately after the steel sheet leaves the plating bath. This solidifies the plating alloy present on the surface of the steel sheet, forming a plating layer.

Then, when the temperature of the plated alloy (plated layer) is within the range of 250 to 50°C, a second cooling process is carried out. This second cooling process involves slowly cooling the plated steel sheet, which is in the temperature range of 250 to 50°C, at an average cooling rate of 10°C/second or less in an atmosphere with a dew point of 0°C or higher. This controls the state of the oxides formed on the surface of the plated layer, resulting in the formation of the desired oxide layer.

As described above, by undergoing a two-stage cooling process in which the plating layer is rapidly cooled in the temperature range of from the bath temperature to 250°C and slowly cooled in the temperature range of 250 to 50°C, a dense oxide layer 105 is formed on the surface of the plating layer 103, which satisfies specific conditions in the XPS measurement results.

Here, the interval between the end of the first cooling step and the start of the second cooling step is preferably within 3 seconds, and it is preferable to start the second cooling step immediately after the end of the first cooling step. If the interval between the end of the first cooling step and the start of the second cooling step exceeds 3 seconds, an unintended cooling process will occur, and the desired oxide layer 105 will not be achieved.

In the first cooling step, the lower limit of the dew point is not particularly specified, but for example, approximately -90°C is a practical lower limit. Furthermore, the average cooling rate is more preferably 40°C/sec or higher. The upper limit of the average cooling rate is not particularly specified, but for example, approximately 90°C/sec is a practical upper limit.

In addition, in the second cooling step, the upper limit of the dew point is not particularly specified, but a practical upper limit is, for example, about 20°C. Furthermore, the average cooling rate is more preferably 4°C/second or less.

Note that if either the first cooling process or the second cooling process described above is not performed, the desired oxide layer 105 cannot be achieved. By performing both the first cooling process and the second cooling process described above, the oxide layer 105 according to this embodiment can be achieved.

Furthermore, if an alloying heat treatment process (e.g., a heat treatment process involving heating to a sheet temperature of approximately 480-550°C), which is commonly performed in the manufacture of alloyed hot-dip galvanized steel sheet, is performed after the second cooling process described above, the oxide formation state controlled by the first and second cooling processes will be disrupted, resulting in excessive oxide growth, and the Zn evaporation suppression effect that is the focus of this embodiment will not be achieved. From this perspective, it is important not to perform a heat treatment process after the second cooling process.

Here, in the cooling process as described above, commonly known methods such as _N2 gas cooling, mist cooling, submersion in water, etc. In addition to _N2 gas, gases with high heat removal effects, such as He gas and hydrogen gas, may also be used as the cooling gas.

The temperature of the coating layer can be measured using, for example, a contact thermocouple (K-type). By attaching a contact thermocouple to the base steel sheet, the average temperature of the entire coating layer can be constantly monitored. Furthermore, by mechanically controlling various speeds and thicknesses and standardizing various operating conditions such as the steel sheet preheating temperature and the hot-dip coating bath temperature, it becomes possible to monitor the temperature of the entire coating layer at that time under those manufacturing conditions with near-accuracy. This makes it possible to precisely control the cooling treatments in the first and second cooling steps. The surface temperature of the coating layer can also be measured using a non-contact radiation thermometer, although this is not as accurate as a contact thermocouple.

The relationship between the surface temperature of the plating layer and the average temperature of the entire plating layer can also be determined by a simulation that performs heat conduction analysis. Specifically, the surface temperature of the plating layer and the average temperature of the entire plating layer are determined based on various manufacturing conditions, such as the preheating temperature of the steel sheet, the temperature of the hot-dip plating bath, the rate at which the steel sheet is pulled out of the plating bath, the thickness of the steel sheet, the thickness of the plating layer, the amount of heat exchanged between the plating layer and the manufacturing equipment, and the amount of heat dissipated from the plating layer. The obtained results can then be used to determine the relationship between the surface temperature of the plating layer and the average temperature of the entire plating layer. In this way, by actually measuring the surface temperature of the plating layer during the manufacturing of the plated steel sheet, it is possible to estimate the average temperature of the entire plating layer at that time under those manufacturing conditions. As a result, it becomes possible to precisely control the cooling treatments in the first and second cooling processes.

The above provides a specific explanation of an example of a method for manufacturing plated steel sheets according to this embodiment.

In the method for producing plated steel sheet according to this embodiment, after the second cooling step, a further treatment may be carried out to form one or more layers of various coatings. Examples of such treatments include chromate treatment, phosphate treatment, chromate-free treatment, and organic resin coating formation treatment.

Chromate treatments include electrolytic chromate treatments, which form a chromate film through electrolysis; reactive chromate treatments, which form a film by reacting with the material and then rinse away excess treatment solution; and paint-on chromate treatments, which apply a treatment solution and then dry it without rinsing to form a film. Any of these chromate treatments may be used.

Examples of electrolytic chromate treatments include those using chromic acid, silica sol, resin (phosphate resin, acrylic resin, vinyl ester resin, vinyl acetate acrylic emulsion, carboxylated styrene butadiene latex, diisopropanolamine-modified epoxy resin, etc.), and hard silica.

Examples of phosphate treatments include zinc phosphate treatment, zinc calcium phosphate treatment, and manganese phosphate treatment.

Chromate-free treatments are particularly suitable because they do not place a burden on the environment. Examples of such chromate-free treatments include electrolytic chromate-free treatments, which form a chromate-free film through electrolysis; reactive chromate-free treatments, which form a film by utilizing a reaction with the material and then rinse away excess treatment solution; and paint-on chromate-free treatments, which apply a treatment solution and then dry it without rinsing to form a film. Any of these chromate-free treatments may be used.

The organic resin used in the organic resin film formation process is not limited to a specific resin, and various resins can be used, such as polyester resin, polyurethane resin, epoxy resin, acrylic resin, polyolefin resin, and modified versions of these resins. Here, modified versions refer to resins in which reactive functional groups contained in the structure of these resins have been reacted with other compounds (e.g., monomers, crosslinking agents, etc.) that contain functional groups in their structure that can react with the reactive functional groups.

As the organic resin, one of the above may be used alone, or two or more organic resins (unmodified) may be mixed and used. Also, organic resins obtained by modifying at least one other organic resin in the presence of at least one other organic resin may be used alone or in a mixture of two or more. Furthermore, organic resins that have been made water-based by dissolving or dispersing them in water may also be used. Furthermore, various color pigments and rust-preventive pigments may be contained in such organic resin coatings.

(Regarding the manufacturing method of welded joints)
The welded joint according to this embodiment is manufactured by using the plated steel sheet manufactured as described above as the material for at least one of the first steel sheet and the second steel sheet when manufacturing a welded joint, arranging the first steel sheet and the second steel sheet to form the desired shape of the welded joint, and welding the first steel sheet and the second steel sheet together.

The first and second steel plates can be welded together using arc welding. By performing the welding under the welding conditions described below, it is possible to achieve the above-described condition of the backside heat-affected zone.

More specifically, the first steel plate and the second steel plate may be arc-welded under the following welding conditions, for example.
Welding current: 220 A, welding voltage: 25.2 V, welding speed: 80 cm/min. Welding gas: 20% CO ₂ +Ar, gas flow rate: 20 L/min. Welding wire: YGW16 manufactured by Nippon Steel Welding Industry Co., Ltd., φ1.2 mm
(C: 0.1% by mass, Si: 0.80% by mass, Mn: 1.5% by mass, P: 0.015% by mass, S: 0.008% by mass, Cu: 0.36% by mass)
Welding torch tilt angle: 45°

The above describes an example of a method for manufacturing a welded joint according to this embodiment.

The following provides a detailed explanation of the welded joint according to the present invention, showing examples and comparative examples. Note that the examples shown below are merely examples of the welded joint according to the present invention, and the welded joint according to the present invention is not limited to the examples shown below.

In the examples and comparative examples shown below, a 3.2 mm thick hot-rolled steel plate (0.05 mass% C-0.007 mass% Si-0.25 mass% Mn, manufactured by Nippon Steel Corporation) was used as the base steel plate. This hot-rolled steel plate was cut into 100 mm x 200 mm pieces to prepare test specimens.

Each of the plating baths was prepared to realize the plating layer compositions shown in Table 1 below, and each was installed in a batch-type hot-dip galvanizing test apparatus manufactured by the company, and the test specimens were plated. The temperature of the test specimens was measured using a thermocouple spot-welded to the center of the test specimen. Furthermore, before immersion in the plating bath, the test specimens were subjected to a heat reduction treatment at 800°C in an N ₂ -5% H ₂ gas atmosphere in a furnace with an oxygen concentration of 20 ppm or less. After the heat reduction treatment, the test specimens were air-cooled with N ₂ gas, and after the temperature of the test specimens reached the bath temperature + 20°C, they were immersed in the plating bath of the hot-dip galvanizing test apparatus for approximately 3 seconds.

After immersion in the plating bath, the test specimen was pulled up at a pulling speed of 20 to 200 mm/sec. During pulling, the desired coating weight was controlled using _N2 wiping gas. In the following examples and comparative examples, the coating weight was controlled so that the coating weight per side of the test specimen after drying would be 15 to 250 g/ ^m2 . After pulling the test specimen from the plating bath, the test specimen was cooled from the plating bath temperature to room temperature under the conditions shown in Table 1 below. In the following examples and comparative examples, the second cooling step was started immediately after the completion of the first cooling step (i.e., the interval between the completion of the first cooling step and the start of the second cooling step was 0.2 seconds or less).

Here, a 30 mm x 30 mm steel plate was cut from the test piece plated as described above, and the steel plate was immersed in a 10% HCl aqueous solution containing an inhibitor to remove the plating layer by pickling. The composition of the plating layer was then measured by ICP analysis of the elements dissolved in the aqueous solution.

Furthermore, electron beam diffraction images of the obtained plating layers were taken by TEM, and the presence or absence of the Mg ₃₂ (Al, Zn) ₄₉ phase was confirmed based on whether or not a 5-fold symmetric crystal structure was observed in the electron beam diffraction image.

Furthermore, the XPS spectrum of the obtained oxide layer was measured according to the above-mentioned method, and the intensity ratio ([Al—O] + [Mg—O])/[Zn—O] was calculated. The obtained intensity ratio was evaluated based on the following criteria.
<Evaluation Criteria>
Rating "A": Intensity ratio value is 10.0 or more "B": Intensity ratio value is 5.0 or more and less than 10.0 "C": Intensity ratio value is less than 5.0

Furthermore, a steel plate measuring 150 mm x 50 mm was cut from the obtained test piece and designated as the first steel plate, and a steel plate measuring 150 mm x 30 mm was cut from the obtained test piece and designated as the second steel plate. The long sides of these steel plates were overlapped and welded by arc welding (lap fillet welding) to form a welded joint.

Here, the welding conditions for the arc welding are as follows.
Welding current: 220 A, welding voltage: 25.2 V, welding speed: 80 cm/min. Welding gas: 20% CO ₂ +Ar, gas flow rate: 20 L/min. Welding wire: YGW16 manufactured by Nippon Steel Welding Industry Co., Ltd., φ1.2 mm
(C: 0.1% by mass, Si: 0.80% by mass, Mn: 1.5% by mass, P: 0.015% by mass, S: 0.008% by mass, Cu: 0.36% by mass)
Welding torch tilt angle: 45°
Overlap: 10 mm
Steel plate size: Upper plate (first steel plate) 150 x 50 mm, lower plate (second steel plate) 150 x 30 mm
Gap: 0mm

<Thickness of Fe scale in the heat-affected zone on the back side>
The thickness of iron scale in the heat-affected zone on the back side of the welded joint obtained as described above was measured by the method previously described. The results are summarized in Table 1 below.

<Corrosion resistance of weld beads>
The welded joints obtained as described above were subjected to an automotive phosphate conversion treatment (Zn phosphate treatment, SD5350 system: standard manufactured by Nippon Paint Industrial Coating Co., Ltd.) and electrodeposition coating (PN110 Powernics Gray: standard manufactured by Nippon Paint Industrial Coating Co., Ltd.). In this case, the electrodeposition film thickness was 20 μm. The samples after electrodeposition coating were subjected to a combined cyclic corrosion test in accordance with JASO (M609-91) to evaluate the timing of red rust formation at the toe. The evaluation criteria were as follows. The results obtained are summarized in Table 1 below.
<Evaluation Criteria>
Rating "AAA": Red rust occurs more than 60 cycles. "AA": Red rust occurs more than 30 cycles and up to 60 cycles. "A": Red rust occurs more than 15 cycles and up to 30 cycles. "B": Red rust occurs up to 15 cycles.

The above describes in detail preferred embodiments of the present invention with reference to the accompanying drawings, but the present invention is not limited to such examples. It is clear that a person with ordinary skill in the technical field to which the present invention pertains can conceive of various modifications or alterations within the scope of the technical ideas set forth in the claims, and it is understood that these also naturally fall within the technical scope of the present invention.

REFERENCE SIGNS LIST 1 weld joint 10 first steel plate 20 second steel plate 30 weld bead portion 40 heat-affected zone 101 base steel 103 plating layer 105 oxide layer T toe

Claims (5)

A welded joint in which a first steel plate and a second steel plate are welded by arc welding,
The first steel plate and the second steel plate;
a weld bead portion formed by the arc welding;
a heat-affected zone located around the weld bead;
It has
When a portion of the first steel plate and the second steel plate that is not thermally affected by the welding is defined as a non-heat-affected zone, at least one of the first steel plate or the second steel plate has, in the non-heat-affected zone, a plating layer located on at least a part of a surface of a base steel, and an oxide layer located on the plating layer,
The plating layer comprises, in mass %,
Al: 1.00-80.00%,
Mg: 1.00-20.00%,
Fe: 0.01-15.00%,
Si: 0 to 10.00%,
Ca: 0-4.00%,
and optionally further comprising
Sb: 0 to 0.50%,
Pb: 0 to 0.50%,
Cu: 0 to 1.00%,
Sn: 0-1.00%,
In: 0 to 1.00%,
Bi: 0-1.00%,
Ti: 0 to 1.00%,
Cr: 0-1.00%,
Nb: 0 to 1.00%,
Zr: 0 to 1.00%,
Ni: 0 to 1.00%,
Mn: 0 to 1.00%,
V: 0-1.00%,
Mo: 0-1.00%,
Ag: 0-1.00%,
Li: 0 to 1.00%,
La: 0 to 0.50%,
Ce: 0 to 0.50%,
B: 0 to 0.50%,
Y: 0 to 0.50%,
Sr: 0 to 0.50%,
The balance is 5.00 mass% or more of Zn and impurities,
When a position at a depth of 5 nm from the outermost surface of the oxide layer is observed by X-ray photoelectron spectroscopy (XPS), the value of the intensity ratio ([Al—O]+[Mg—O])/[Zn—O] calculated from the intensities of peaks respectively assigned to an Al—O bond, an Mg—O bond, and a Zn—O bond is 5.0 or more,
A welded joint, wherein when the surface of the welded joint on which the weld bead portion does not exist is defined as the back surface, the thickness of Fe scale located on the heat-affected zone on the back surface is 0 to 5.0 μm. The welded joint described in claim 1, wherein the thickness of the Fe scale is 0 to 1.0 μm. The welded joint according to claim 1 or 2 , wherein the strength ratio ([Al—O] + [Mg—O])/[Zn—O] is 10.0 or more. The plating layer is
Al: 18.00 to 60.00% by mass,
Mg: 5.00 to 15.00% by mass,
The welded joint according to any one of claims 1 to 3 , comprising at least The plating layer is
Al: 35.00 to 60.00% by mass,
Mg: 7.00 to 15.00% by mass,
and
The plating layer contains an Mg ₃₂ (Al, Zn) ₄₉ phase,
5. The welded joint according to claim 1, wherein the Mg content [Mg], the Zn content [Zn], and the Al content [Al] (each unit: atomic %) in the _Mg32 (Al,Zn) ₄₉ phase satisfy the relationship 0.50≦[Mg]/([Zn]+[Al])≦ 0.83 .