JP2023040930A - Weld joint and method for manufacturing the same - Google Patents

Abstract

To provide a weld joint having high joint strength by superimposing four steel plates including a soft steel plate and a high strength steel plate and performing spot welding.SOLUTION: A weld joint is obtained by superimposing four steel plates and performing spot welding, wherein the four steel plates include at least one soft steel plate having tensile strength of 270 MPa or more and less than 590 MPa, and at least two high strength steel plates having tensile strength of 980 MPa or more. The first soft steel plate is arranged on the outermost side of the four steel plates, the high strength steel plate is superimposed and arranged on one side of the first soft steel plate, a nugget formed by spot welding satisfies an expression (1) and a plate thickness retention rate defined by an expression (2) is 84% or more. Expression (1): dN/√t1≥3.0. In the expression, dN(mm) is a nugget diameter on a contact surface where the soft steel plate and the high strength steel plate are brought into contact with each other, and t1(mm) is a plate thickness of the soft steel plate brought into contact with the high strength steel plate. Expression (2): plate thickness retention rate (%)=(minimum thickness of weld joint)/(total plate thickness of four steel plates)×100.SELECTED DRAWING: Figure 1

Description

The present disclosure relates to welded joints and methods of making same.

Since automobile bodies are formed by joining parts made of steel plates to form a structure, the joining technology of steel plates is a very important technology. As a method for joining steel plates, spot welding is used, in which overlapping steel plates are sandwiched between electrodes and the steel plates are melted and joined by Joule heat generated by applying an electric current.

For example, Patent Document 1 discloses a lap resistance spot welding method in which a plate assembly in which three or more steel plates are superimposed is sandwiched and pressed by a pair of welding electrodes, and current is applied to weld the contact points of each steel plate. Are listed. The method of Patent Document 1 includes a step of assembling and welding the thinnest one of the steel plates so that it is in contact with one of the pair of welding electrodes, and at least the plate A phosphate treatment film, a chromate treatment film, an organic film or an inorganic film is formed on the overlapping surfaces of the thinnest steel sheet and the steel sheet adjacent to the thinnest steel sheet.

Patent Literature 2 describes a manufacturing method for manufacturing a resistance spot welded joint by welding and joining a plate assembly in which a plurality of metal plates are superimposed by resistance spot welding. In the manufacturing method of Patent Document 2, the plate assembly is a plate assembly having a plate thickness ratio of 5 or more, in which a thin plate is superimposed on at least one of two or more thick plates that are superimposed. In addition, the resistance spot welding is welding consisting of three stages of the first stage, the second stage, and the third stage, and the second stage welding has a higher applied pressure, a lower current, or a higher pressure than the first stage welding. Welding is performed with the same current, long welding time, or the same welding time, and the third stage repeats the welding with a higher current than the second stage.

Patent Document 3 discloses a method for manufacturing a resistance spot welded joint similar to Patent Document 2, except that the third stage welding is repeated with a higher pressure and a higher current than the second stage. A manufacturing method similar to that of document 2 is described.

Patent Document 4 describes a steel plate lap welding method and a steel plate lap weld joint in which two or more steel plates are overlapped and welded. In the lap welding method of Patent Document 4, the two or more steel plates include a surface-side steel plate having a plate thickness in the range of 0.3 to 1 mm, and one or more high-thickness steel plates having a plate thickness larger than the surface-side steel plate. and are included. Also, the ratio of the plate thickness of the surface-side steel plate to the total plate thickness of the surface-side steel plate and the one or more high-thickness steel plates (total plate thickness/plate thickness of the surface-side steel plate) is set to 5 or more. Then, in a state in which the two or more steel plates are superimposed so that the surface-side steel plate is arranged on the surface side, laser welding and spot welding are used in combination to weld them.

Patent Literature 5 describes a spot welding method in which two or more metal plates are joined together by applying a welding current to the overlapped portion. In the method of Patent Document 5, a part of the outer peripheral portion of a pair of electrodes for applying current to the overlapped portion is pressurized using a pressurizing member separate from the electrodes, so that even if the welding current is increased, dust is not removed. The generation can be suppressed, and a large nugget diameter can be realized.

Patent Document 6 discloses a weld joint in which two steel plates having thicknesses of _t1 and _t2 are superimposed and spot-welded, and the diameter of the nugget formed by spot welding is 3√{( _t1 + _t2 )/2} (the nugget diameter, _t1 and _t2 are all in mm), and the radius of curvature of the nugget at both ends in the thickness direction is 0.3 ( _t1 + _t2 ) Thus, a spot welded joint is disclosed.

Japanese Patent No. 4884958 JP 2010-240739 A JP 2010-240740 A JP 2010-264503 A JP 2014-28392 A JP 2014-180686 A

In general, spot welding is only acceptable for welding up to three steel plates. For example, when spot-welding four steel plates, the overlapping surfaces of the second and third steel plates are preferentially melted. On the other hand, the overlapping surfaces of the first and second steel plates and the overlapping surfaces of the third and fourth steel plates are hard to melt because they are strongly affected by cooling by the electrode tip. Therefore, the welding of the overlapped surfaces was insufficient, and the joint strength of the welded joint was insufficient.

In particular, when considering the structure of the car body, there are cases where multiple steel plates with high strength that can withstand collisions are placed inside the car body, and a mild steel plate (for example, galvanized steel plate) is placed on the outermost surface of the car body to cover the car body. be. In such a combination of steel plates, the internal high-strength steel plate has a higher electric resistance than the outermost mild steel plate, so it heats and melts preferentially. On the other hand, a mild steel plate, which has a relatively low electrical resistance, is less likely to generate heat and melt.
In this way, the mild steel plate placed on the outermost surface is not sufficiently welded to the adjacent high-strength steel plate due to the cooling effect of the electrode tip and the lack of heat generation due to the relatively low electrical resistance. easy to become

It is considered effective to increase the welding current for spot welding in order to promote welding of the outermost mild steel plate. However, if the welding current becomes too high, a phenomenon called "splashing" occurs, in which the molten metal is scattered. If flash occurs, the amount of metal in the spot-welded portion is reduced, which may result in insufficient joint strength. In addition, the scattering of molten metal due to the generation of dust contaminates the surroundings of the welded portion, and the molten metal may damage the welding device and workers. Therefore, the welding current can be increased only to the extent that flash does not occur, and the welding of the mild steel plate on the outermost surface may not be sufficiently improved.

Patent Documents 1 to 6 do not specifically disclose spot welding of four layers of mild steel plates and high-strength steel plates, and consideration is given to the need to improve the welding of the outermost mild steel plate. not

Therefore, in one embodiment of the present invention, four steel plates including a mild steel plate arranged on at least one outermost surface and two or more high-strength steel plates arranged on one side of the mild steel plate are superimposed. An object of the present invention is to provide a welded joint formed by spot welding and having a high joint strength. Another embodiment of the invention aims to provide a manufacturing method by which such welded joints can be manufactured.

Aspect 1 of the present invention is
A welded joint in which four steel plates are superimposed and spot-welded,
The four superimposed steel plates include at least one mild steel plate with a tensile strength of 270 MPa or more and less than 590 MPa, and at least two high-strength steel plates with a tensile strength of 980 MPa or more,
The first mild steel plate is arranged on the outermost side of the four steel plates,
The at least two high-strength steel plates are superimposed on one side of the first mild steel plate,
A nugget formed by spot welding satisfies the following formula (1) and has a plate thickness retention rate defined by the following formula (2) of 84% or more.

d _N /√t ₁ ≧3.0 (1)

Here, d _N (mm) is the nugget diameter at the contact surface where the mild steel plate and the high-strength steel plate contact,
t ₁ (mm) is the plate thickness of the mild steel plate in contact with the high-strength steel plate.

Plate thickness retention rate (%) = (minimum thickness of welded joint) / (total plate thickness of four steel plates) x 100 (2)

Aspect 2 of the present invention is
The welded joint according to aspect ₁ , wherein the thickness t1 of the mild steel plate is smaller than the thickness of the high-strength steel plate in contact with the mild steel plate.

Aspect 3 of the present invention is
Aspect 1 or 2, wherein the four steel plates are composed of the first mild steel plate and three high-strength steel plates stacked on one side of the first mild steel plate. It is a welded joint.

Aspect 4 of the present invention is
A method for manufacturing a welded joint by spot welding overlapping portions of four steel plates,
A step of superposing four steel plates including at least one mild steel plate having a tensile strength of 270 MPa or more and less than 590 MPa and at least two high-strength steel plates having a tensile strength of 980 MPa or more,
The four steel plates are arranged so that the first mild steel plate is arranged on the outermost side of the four steel plates, and the at least two high-strength steel plates are arranged on one side of the first mild steel plate. a step of superimposing steel plates;
A step of pressurizing and holding the overlapped portion of the steel plate by a pair of electrodes and a pair of pressure members provided so as to surround the electrode tip of each electrode,
The pressurizing member is configured to pressurize a range of 180° or more in a circumferential direction centering on the tip of the electrode,
the separation distance between the pressure tip of the pressure member and the tip of the electrode is 0.1 mm or more and 10.0 mm or less;
The overlapping portion is pressed by the electrode tip with an electrode load of 1.5 kN or more, and is pressed by the pressing tip with a peripheral load of 0.1 to 1.0 times the electrode load. , a step of pressurizing and clamping;
and welding the overlapping portion by applying a welding current of 5 kA or more and 15 kA or less to the pair of electrodes.

Aspect 5 of the present invention is
The method for manufacturing a welded joint according to aspect 4, wherein the thickness _t1 of the mild steel plate is smaller than the thickness of the high-strength steel plate in contact with the mild steel plate.

According to the welded joint according to one embodiment of the present invention, a welded joint having high joint strength can be realized. Moreover, according to the method for manufacturing a welded joint according to another embodiment of the present invention, the welded joint according to one embodiment can be manufactured.

1(a) is a bottom view and FIG. 1(b) is a cross-sectional view taken along line AA in FIG. 1(a); FIG. . Fig. 2(a) and Fig. 2(b) are schematic cross-sectional views of an electrode for spot welding and a pressure terminal; FIG. 3(a) is a schematic perspective view showing a state in which four steel plates are welded, and FIG. 3(b) is a schematic cross-sectional view of a cross section taken along line XX of FIG. 3(a). .

The inventors found that when spot-welding a plate set in which four steel plates including a mild steel plate and a high-strength steel plate are superimposed to form a welded joint, when the mild steel plate is arranged on the outermost surface, the mild steel plate and the adjacent steel plate are formed. We focused on the fact that poor welding occurs between steel and the high-strength steel plate, or that the desired joint strength cannot be obtained, and investigated the cause. As a result, when trying to expand the molten pool in the thickness direction so as to melt the mild steel plate placed on the outermost surface, the molten pool spreads in the lateral direction and melts from the gap before the molten pool expands sufficiently in the thickness direction. It turned out that the metal spouted out and dust was generated. When spot welding is performed, welding is performed at an electrode current that does not generate dust, so it has been difficult to expand the molten pool in the thickness direction. The inventors conducted extensive research to expand the molten pool in the thickness direction while suppressing the generation of dust, and found that by applying an appropriate pressure to the overlapped portion around the electrode for spot welding, the molten metal It is possible to suppress the generation of dust by obstructing the route from which the is ejected, and to efficiently concentrate the heat generated by the energization between the electrodes. The present inventors have completed the embodiments of the present invention by discovering that it is possible to promote melting in all directions.

A method for manufacturing a welded joint according to an embodiment of the present invention spot-welds overlapping portions of four steel plates,
A step of superposing four steel plates including at least one mild steel plate having a tensile strength of 270 MPa or more and less than 590 MPa and at least two high-strength steel plates having a tensile strength of 980 MPa or more,
The four steel plates are arranged so that the first mild steel plate is arranged on the outermost side of the four steel plates, and the at least two high-strength steel plates are arranged on one side of the first mild steel plate. a step of superimposing steel plates;
A step of pressurizing and holding the overlapped portion of the steel plate by a pair of electrodes and a pair of pressure members provided so as to surround the electrode tip of each electrode,
The pressurizing member is configured to pressurize a range of 180° or more in a circumferential direction centering on the tip of the electrode,
the separation distance between the pressure tip of the pressure member and the tip of the electrode is 0.1 mm or more and 10.0 mm or less;
The overlapping portion is pressed by the electrode tip with an electrode load of 1.5 kN or more, and is pressed by the pressing tip with a peripheral load of 0.1 to 1.0 times the electrode load. , a step of pressurizing and clamping;
and welding the overlapping portion by applying a welding current of 5 kA or more and 15 kA or less to the pair of electrodes.

In the method for manufacturing a welded joint, the pressure range of the pressure member is 180° or more in the circumferential direction (50% or more of the entire circumference), the electrode load is 1.5 kN or more, and the outer peripheral load is 0.1 of the electrode load. By setting it to 1.0 times, spot welding of four steel plates including a mild steel plate on the outermost surface, which could not be sufficiently welded by the conventional spot welding method, became possible.
The welded joint obtained by this manufacturing method has the following characteristics.

A welded joint according to an embodiment of the present invention is formed by welding a mild steel plate with a tensile strength (TS) of 270 MPa or more and less than 590 MPa and a high strength steel plate with a tensile strength of 980 MPa or more. Here, the reason why the lower limit of the TS of the mild steel plate is set to 270 MPa is that the TS of the general mild steel plate is specified to be 270 MPa or more, and the upper limit is set to less than 590 MPa because the high-strength steel plate is not included. It is the purpose. In addition, the reason why the lower limit of the TS of the high-strength steel sheet is set to 980 MPa is to target high-strength steel sheets with a high strength level that is difficult to spot weld among high-strength steel sheets, and the upper limit is particularly limited. Although it is not, it is about 1900 MPa at present.

A welded joint according to an embodiment of the present invention is formed by superimposing four steel plates and spot-welding them. When four steel plates are superimposed, one mild steel plate (referred to as the “first mild steel plate”) is placed on the outermost side of the four steel plates, and on one side of the first mild steel plate, At least two high-strength steel plates are placed one on top of the other. In addition, another steel plate may be arranged between the first mild steel plate and at least two high-strength steel plates.

The four steel plates consist of four steel plates including at least one mild steel plate and at least two high-strength steel plates. As a specific combination of steel plates, four steel plates consist of one mild steel plate and three high-strength steel plates (pattern 1), and two mild steel plates and two high-strength steel plates. There may be a case (pattern 2).
A specific example of the order of stacking four steel plates (plate set) will be described in detail for pattern 1 and pattern 2. FIG.

(Pattern 1: when four steel plates are composed of one mild steel plate and three high-strength steel plates)
In pattern 1, one type of board group (board group (1-1)) is assumed.
[Itagumi (1-1)]
One mild steel plate (first mild steel plate) is arranged on the outermost side of the four steel plates, and three high-strength steel plates are superimposed on one side of the first mild steel plate. By stacking the four steel plates in this manner, the four steel plates are stacked in the order of (i) first mild steel plate - (ii) high strength steel plate - (iii) high strength steel plate - (iv) high strength steel plate.

(Pattern 2: when the four steel plates consist of two mild steel plates and two high-strength steel plates)
In pattern 2, two types of board groupings (board grouping (2-1) and board grouping (2-2)) are assumed.
[Itagumi (2-1)]
Two high-strength steel plates are sandwiched between two mild steel plates. In other words, one mild steel plate (first mild steel plate) is arranged on the outermost side of the four steel plates, and the other mild steel plate (this is called a “second mild steel plate”) is placed in the four steel plates. It is arranged on the outermost side opposite to the first mild steel plate. Then, between the first mild steel plate and the second mild steel plate, two high-strength steel plates are stacked and arranged. When the four steel plates are stacked in this way, the four steel plates are stacked in the order of (i) the first mild steel plate - (ii) the high strength steel plate - (iii) the high strength steel plate - (iv) the second mild steel plate. Become.

[Itagumi (2-2)]
Two mild steel plates are superimposed, and two high-strength steel plates are superimposed on one side of one of the mild steel plates. In other words, the first mild steel plate is arranged on the outermost side of the four steel plates, the two high-strength steel plates are superimposed on one side of the first mild steel plate, and the first mild steel plate and Another mild steel plate (second mild steel plate) is arranged between the two high-strength steel plates. When superimposed in this way, the four steel plates are stacked in the order of (i) the first mild steel plate - (ii) the second mild steel plate - (iii) the high strength steel plate - (iv) the high strength steel plate. Become. Since the second mild steel plate exists between the first mild steel plate and the high-strength steel plate, the first mild steel plate and the high-strength steel plate do not directly contact each other.

In the welded joint according to the embodiment of the present invention, the nugget diameter (mm) at the contact surface between the mild steel plate and the high-strength steel plate adjacent to the mild steel plate satisfies the formula (1).

d _N /√t ₁ ≧3.0 (1)

Here, d _N (mm) is the nugget diameter at the contact surface where the mild steel plate and the high-strength steel plate contact,
t ₁ (mm) is the plate thickness of the mild steel plate in contact with the high-strength steel plate.

In general, a joint formed by welding is considered to be a better welded joint (for example, a higher joint strength) as d _N /√t ₁ is larger. In the present embodiment, by specifying d _N /√t ₁ to be 3 or more as in Equation (1), a welded joint having sufficiently high cross tensile strength can be obtained.

The nugget diameter (the diameter of the melted and solidified portion (nugget) generated in the molten part), which is the most important factor for the weld strength (joint strength) of the weld, is generally several times the square root of the plate thickness. is necessary. In the embodiment of the present invention, considering that it is particularly important to control the nugget diameter formed on the surface of a mild steel plate that is difficult to melt, It was specified that the nugget diameter d _N (mm) at the contact surface (overlapping surface) is 3√t ₁ or more (t ₁ (mm) is the thickness of the mild steel plate), that is, the expression (1) is satisfied. The nugget diameter is preferably 4√t ₁ or more, more preferably 5√t ₁ or more, and even more preferably 6√t ₁ or more.

The "nugget diameter" conforms to the method for measuring the nugget diameter in two-layer or three-layer welds described in "JIS Z3139: 2009 Spot, projection and seam weld section test methods". A method for measuring the nugget diameter in the embodiment of the present invention will be described in detail with reference to FIGS. 3(a) and 3(b).
FIG. 3(a) is a schematic perspective view showing an example of a plate set including one mild steel plate and three high-strength steel plates (corresponding to [plate set (1-1)] above). In this figure, four steel plates, one mild steel plate P1 and three high-strength steel plates P2, P3, and P4 stacked in this order from the top, are welded together by a weld joint WJ. FIG. 3(b) is a schematic cross-sectional view of a plane (a cross section along line XX) passing through the vicinity of the center of the welded joint WJ in FIG. 3(a). As shown in FIG. 3(b), the diameter of the nugget N (width of the nugget N) at the contact surface (or interface) between the mild steel plate P1 and the high-strength steel plate P2 is measured and defined as the nugget diameter _dN. .

Furthermore, the “nugget diameter d _N ” for another set of plates will be described.
In the case of the plate set (2-1), there are two contact surfaces where the mild steel plate and the high-strength steel plate contact (a contact surface between the first mild steel plate and the high-strength steel plate, and a contact surface between the second mild steel plate and the high-strength steel plate. contact surface with steel plate). In this case the nugget diameter _dN is measured on both two contact surfaces. In the embodiment of the present invention, a welded joint with sufficiently high cross tensile strength can be obtained by satisfying the formula (1) for the nugget diameter d _N at any contact surface.

In the case of the plate set (2-2), there is one contact surface where the mild steel plate and the high strength steel plate are in contact (contact surface between the second mild steel plate and the high strength steel plate). The nugget diameter _dN is measured at the contact surface between the second mild steel plate and the high strength steel plate.

The welded joint according to the embodiment of the present invention has a plate thickness retention rate of 84% or more defined by the formula (2).

Plate thickness retention rate (%) = (minimum thickness of the welded joint) / (total plate thickness of the four steel plates) x 100 (2)

The "minimum thickness of the welded joint" in formula (2) is the thinnest portion of the welded joint WJ in a cross section passing through the welded joint WJ, and is the thickness indicated by symbol t _WJ in FIG. 3(b). point to

The “total thickness of the four steel plates” in the formula (2) means, in the example of the plate set ( _1-1 ) shown in FIG. It is the total (=t ₁ +t ₂ +t ₃ +t ₄ ) of the respective thicknesses t ₂ , t ₃ and t ₄ of P3 and P4.
In the case of the plate set (2-1) and the plate set (2-2) as well, the “total thickness of the four steel plates” refers to the total thickness of each of the four steel plates.

As described above, in general spot welding, up to three steel plates are overlapped. Therefore, the welding current must be increased as much as possible to fabricate a welded joint by stacking four steel plates. If an excessively high welding current is applied, the strength of the steel sheet is excessively reduced during welding, and large plastic deformation occurs in the weld zone due to the pressure applied by the electrode. In some cases, flash is generated from the welded portion, and the plate thickness is significantly reduced at the welded portion immediately below the electrode. As a result, the volume of the weld nugget that contributes to the joint strength is reduced, which is presumed to be a factor in joint strength reduction and variations (instability) in the joint strength.

In the embodiment of the present invention, the ratio of the plate thickness of the welded portion (= the minimum thickness of the spot welded joint) to the total plate thickness before welding (= total plate thickness of the steel plate before welding) is expressed as a “plate thickness retention ratio”. Defined in (2), if the plate thickness retention rate is 84% or more, it is determined that the welded joint is sound.
The plate thickness retention rate is preferably 85% or more, more preferably 87% or more.

The thickness of each of the four steel plates to be superimposed is arbitrary, but for example, the thickness _t1 of the first mild steel plate provided on the outermost surface of the vehicle body of an automobile should be smaller than the high-strength steel plate inside the vehicle body. There is Even in the case of steel plates of the same grade, if spot welding is performed by stacking them in the order of thin plate-thick plate-thick plate-thick plate, the overlapping surface between the thick plate and the thick plate melts preferentially, causing the thin plate-thick plate to melt. Melting is less likely to occur at the overlapping surfaces between them. This is because the thin plate is more strongly affected by the cooling by the electrode tip than the thick plate. Therefore, when the mild steel plate is thin and the high-strength steel plate is thick, in the example of the set of plates (1-1), “(i) first mild steel plate of thin plate-(ii) high-strength steel plate of thick plate-(iii) Thick high-strength steel plate - (iv) thick high-strength steel plate” are stacked in this order. When such laminated steel plates are welded by conventional spot welding, the welding at the contact surfaces between (i) the thin first mild steel plate and (ii) the thick high strength steel plate may be insufficient.

However, the welded joint according to the embodiment of the present invention satisfies the above formula (1) and has a plate thickness retention rate defined by the formula (2) of 84% or more, thereby achieving sufficient joint strength. Therefore, even when the plate thickness _t1 of the first mild steel plate on the outermost surface is thin, a welded joint having sufficient joint strength can be obtained. Therefore, the welded joint according to the embodiment of the present invention is also suitable for welding laminated steel plates including thin mild steel plates.

Regarding the definition of formula (2), the effect of the welded joint in the case of plate set (1-1) has been explained. Since the welded joints of the plate assembly (2-1) and the plate assembly (2-2) also have the first mild steel plate provided on the outermost surface, the same effect can be obtained by satisfying expression (2). can play

A preferred method of manufacturing the welded joint will now be described.
A method of manufacturing a welded joint by spot welding overlapping portions of four steel plates,
A step of superimposing four steel plates including at least one mild steel plate having a tensile strength of 270 MPa or more and less than 590 MPa and at least two high-strength steel plates having a tensile strength of 980 MPa or more;
a step of pressing and holding the overlapped portion of the steel plate by a pair of electrodes and a pair of pressure members provided so as to surround the electrode tip of each electrode;
and welding the overlapping portion by applying a welding current of 5 kA or more and 15 kA or less to the pair of electrodes.

(1. Step of superimposing)
Four steel plates including at least one mild steel plate with a tensile strength of 270 MPa or more and less than 590 MPa and at least two high-strength steel plates with a tensile strength of 980 MPa or more are prepared. One mild steel plate (first mild steel plate) is arranged on the outermost side of the four steel plates, and at least two high-strength steel plates are superimposed on one side of the first mild steel plate, Overlap four steel plates. As a result, a superimposed portion of four steel plates having a mild steel plate disposed on at least one of the outermost surfaces can be formed.
The thickness of each of the four overlapping steel plates is arbitrary, but the thickness _t1 of the first mild steel plate provided on the outermost surface is made smaller than the high-strength steel plate in contact with the first mild steel plate. may

(2. Step of pressure-sandwiching the overlapped portion)
A pair of electrodes and a pair of pressing members provided so as to surround the electrode tip of each electrode press and hold the overlapped portion of the steel plate.
The four superimposed steel sheets are energized while being pressed by electrodes from above and below, so that the interfaces between the steel sheets are melted by Joule heat between the sheets. As a preparation, the steel plate is fixed under pressure in the welding device before the current is applied. The steel plate is pressurized and sandwiched between the electrodes and the pressurizing member. By applying pressure around the electrode, the pressure member can suppress the lateral growth of the molten pool and promote the growth of the molten pool in the plate thickness direction.

The electrodes and pressure member will be described in detail with reference to FIGS. 1 and 2. FIG.
FIG. 1 is a diagram showing a schematic configuration of an electrode 1 and a pressure member 2 for spot welding, where (a) is a bottom view and (b) is a cross-sectional view taken along line AA in FIG. 1(a). is.
Note that FIG. 1 shows only the electrode 1 and the pressure member 2 arranged on the upper side of the superimposed steel plates. A pressure member 2 is also arranged on the underside of the superimposed steel plates. As will be described later, the overlapping portions of the steel plates are pressurized between the upper and lower electrodes and between the upper and lower pressure members.

The electrode 1 shown in FIG. 1 is a cylindrical electrode with a smooth tip, and the diameter of the tip 1x of the electrode is D. As shown in FIG. The electrode 1 is composed of a pair of upper and lower electrodes, which are composed of an upper electrode shown in FIG. 1(b) and a lower electrode (not shown). A pair of electrodes sandwiches the overlapped portion of the four steel plates from both sides, and an electric current is passed through the overlapped portion. As for the shape of the electrode 1, in addition to the smooth tip type as shown in FIG.

The pressure member 2 is a member provided so as to surround the electrode tip 1x of the electrode 1 . A pressing tip 2x of the pressing member 2 contacts and presses the stacked steel plates. The pressing tip 2x may be ring-shaped, for example, or may have another shape. As long as the steel plate is not deformed locally when the steel plate is softened during welding, the pressure tip 2x may be formed by removing a ring-shaped portion to provide a non-pressure portion. For example, the pressurizing tip 2x has a C-shape in which one portion is cut in a bottom view, or a shape in which two or more portions are cut and divided into a plurality of arcs (for example, as shown in FIG. 1(a), three can be divided into arcs).

Hereinafter, the form of the pressure member 2 shown in FIGS. 1(a) and 1(b) will be described in detail.
As shown in FIG. 1( a ), the pressure member 2 is composed of three pressure members 21 , 22 and 23 . The three pressurizing parts 21, 22, 23 are arranged so as to surround the electrode tip 1x of the electrode 1. As shown in FIG. The pressurizing parts 21, 22, 23 each have a fan-shaped pressurizing tip 2x with central angles a1, a2, and a3 in the circumferential direction around the electrode tip 1x and a thickness t. Adjacent pressing parts 21, 22, and 23 are separated from each other by gaps having central angles b1, b2, or b3 in the circumferential direction about the electrode tip 1x.

The pressurizing member 2 is designed to pressurize a range of 180° or more (50% of the entire circumference) in the circumferential direction around the electrode tip 1x. Here, “180° or more” means that, as shown in FIG. It means that the sum of the central angles a1, a2, a3 of the pressing tips 2x of 21, 22, 23 is 180° or more. In other words, the sum of the central angles b1, b2, and b3 of the gaps is 180° or less.
When the angle range that can be applied by the pressure member 2 is 180° or more, even when a relatively large welding current is applied to weld four steel plates, the generation of dust can be suppressed. The angle range in which pressure can be applied by the pressure member 2 is preferably 270° or more (75% or more of the entire circumference), more preferably 360° (100% of the circumference). 2x is ring-shaped).

As shown in FIG. 1B, the pressing tip 2x of each of the pressing parts 21, 22, and 23 and the electrode tip 1x of the electrode 1 are separated by a clearance CL. In the embodiment of the present invention, the distance CL is 0.1 mm or more and 10.0 mm or less. If the separation distance CL between the pressure tip 2x and the electrode tip 1x is too narrow, the electrode 1 and the pressure member 2 will come into contact with each other, and current will flow from the pressure member 2 to the electrode 1, causing the liquid to flow in the lateral direction of the molten pool. It is not possible to obtain the effect of suppressing the spread of If the separation distance CL is too large, the effect of pressing the overlapped portion around the electrode 1 with the pressing member 2 is reduced, and the effect of suppressing the generation of dust cannot be sufficiently exhibited. The lower limit of the clearance CL is preferably 0.5 mm, more preferably 1.0 mm, and the upper limit of the clearance CL is preferably 8.0 mm, more preferably 6.0 mm.

The overlapping portion of the steel plates is pressed between the electrode tips 1 x of the pair of upper and lower electrodes 1 . Further, in the overlapped portion, the periphery of the portion pressed by the electrode tip 1x is pressed between the pressing tips 2x of the pressing members 2 arranged above and below.

When the load applied to the overlapped portion by the electrode 1 (this is referred to as "electrode load") is high, the effect of suppressing the deformation of the steel plate due to heating and melting during welding, and the effect of suppressing the generation of dust. is high. However, if the electrode load is too high, the welded portion may be crushed and deformed during welding, reducing the plate thickness retention rate. If the electrode load is too low, the amount of heat generated in the welded portion during welding is dissipated through the electrode 1, and the amount of heat removed is reduced, which tends to cause surface flaking.
The lower limit of the electrode load is 1.5 kN, preferably 2.0 kN. The electrode load should not be excessively high, and its upper limit is preferably 7.0 kN, more preferably 6.0 kN.

The load applied to the overlapped portion by the pressing member 2 (this is referred to as "peripheral load") is set to 0.1 times or more and 1.0 times or less of the electrode load. If the peripheral load is too low, the effect of suppressing dust cannot be obtained. Note that there is an upper limit to the total load of the peripheral load and the electrode load due to limitations of the welding equipment. If the peripheral load is increased, it is necessary to reduce the electrode load, which reduces the amount of heat removed through the electrode 1 and tends to cause surface dust. Therefore, it is necessary to properly set the peripheral load so that the electrode load can be sufficiently secured.
The lower limit of the peripheral load is preferably 0.2 times the electrode load, more preferably 0.3 times. The upper limit of the peripheral load is preferably 0.8 times the electrode load, more preferably 0.5 times.

As the material of the electrode 1, electrode materials commonly used in spot welding, such as pure copper, chromium copper, and alumina-dispersed copper, can be used.
The material of the pressure member 2 may be either a non-conductor or a conductor. Also, the material of the pressing member 2 needs to have strength to the extent that it is not plastically deformed when pressurized.

(3. Process of welding overlapped portions)
A welding current of 5 A or more and 15 kA or less is applied to the pair of electrodes to weld the overlapping portions. The upper limit of welding current is preferably 13 kA, more preferably 10 kA.

- Welded specimen A 270 MPa grade GA steel sheet with a thickness of 0.6 mm was prepared as a mild steel sheet, and a 1180 MPa grade non-plated steel sheet (CR steel sheet) with a thickness of 1.4 mm was prepared as a high strength steel sheet. A mild steel plate and a high-strength steel plate were laminated in the same manner as shown in Table 1 to prepare a welded specimen (a laminate of four steel plates).

・Welding method A DC inverter spot welder having an air cylinder type pressure mechanism was used as the welder. As the electrode 1, an electrode tip made of chromium copper and having a DR shape (electrode diameter 13.0 mm, tip diameter 6.0 mm) shown in FIGS. 2(a) and 2(b) was used.
As the pressure member 2, a pipe-shaped pressure member covering the periphery of the electrode 1 was used. By using the pipe-shaped pressure member 2, the peripheral load can be applied in a range of 360° in the circumferential direction around the electrode tip 1x. In addition to the pressurizing mechanism for the electrode 1, an air cylinder for pressurizing the pressurizing member 2 was prepared.
A pipe-shaped pressure member 2 was pressed against the overlapped portion of the specimen to be welded to apply a peripheral load around the electrode tip 1x.

In order to confirm the influence of the separation distance (clearance) CL between the electrode 1 and the pressure member 2 on the welded joint, two types of pressure members 2 having different shapes of the pressure tip 2x were used (Fig. 2 (a), (b)).
The pressurizing member 201 shown in FIG. 2A has a diameter that decreases in the vicinity of the pressurizing tip 2x toward the tip. The dimensions of the pressure member 2 were 14.0 mm for the inner diameter of the parallel portion (the portion whose diameter was not reduced), 10.0 mm for the inner diameter of the pressure tip 2x, and 1.0 mm for the thickness of the pressure member. . Also, the clearance CL between the pressurizing tip 2x and the electrode tip 1x was 2.0 mm.
The pressure member 202 shown in FIG. 2(b) has a constant diameter as a whole. That is, the pressing member 2 does not have a reduced diameter in the vicinity of the pressing tip 2x. The pressure member 2 had an inner diameter of 14.0 mm and a wall thickness of 1.0 mm. Also, the clearance CL between the pressurizing tip 2x and the electrode tip 1x was 4.0 mm.

The plate assembly specimens to be welded shown in Table 1 were applied under the conditions shown in Table 2 (electrode load by electrode 1, peripheral load by pressure member 2, and clearance CL between pressure tip 2x and electrode tip 1x). clamped. The clearance CL of "2.0 mm" means that the pressure member 201 shown in FIG. 2(a) is used, and the clearance CL of "4.0 mm" means that the pressure member shown in FIG. 202 is used. In C1 to C4, the pressure member 2 was not used.
After that, a welding current shown in Table 2 was applied between the electrodes to perform spot welding. The energization time was 320 ms (milliseconds).

The nugget diameter d _N and the minimum thickness t _WJ of the welded joint were measured for the welded specimens after welding. The nugget diameter _dN conformed to the method for measuring the nugget diameter in two- or three-layer welding described in JIS Z3139:2009. Specifically, as shown in FIG. 3(b), the diameter of the nugget N (the width of the nugget N) at the interface between the outermost mild steel plate P1 and the high-strength steel plate P2 was measured, and was taken as the nugget diameter _dN. .
The minimum weld joint thickness _tWJ is the thinnest portion of the weld joint WJ in a cross section through the weld joint WJ. The cross-sectional macro was observed, and the thickness of the thinnest part at the center of the welded joint was measured to determine the minimum thickness _tWJ of the welded joint (Fig. 3(b)).

Using the measured value of the nugget diameter _dN and the value of the thickness _t1 of the mild steel plate, it was verified whether or not each example satisfied the formula (1).

d _N /√t ₁ ≧3.0 (1)

Here, d _N (mm) is the nugget diameter at the contact surface where the mild steel plate and the high-strength steel plate contact,
t ₁ (mm) is the plate thickness of the mild steel plate in contact with the high-strength steel plate.

Table 2 lists the values of d _N /√t ₁ . When this value was 3.0 or more (that is, the formula (1) was satisfied), it was determined as "good", and when it was 4.0 or more, it was determined as "excellent".

In addition, the thickness retention rate defined by the formula (2) was obtained by considering the total thickness of the test piece to be welded=the total thickness of the four steel sheets, and using the measured minimum thickness of the welded joint.

Plate thickness retention rate (%) = (minimum thickness of the welded joint) / (total plate thickness of the four steel plates) x 100 (2)

A plate thickness retention rate of 84% or more was determined as "accepted".

In addition, a cross tension test was performed on some welded specimens in order to evaluate the joint strength of the welded joint between the outermost surface (mild steel plate) and the second sheet (high strength steel plate). A cross tension test was performed according to JIS Z3140:2017.
Regarding the cross tension strength, 2.1 kN or more was judged as "good", and 3.0 kN or more was judged as "excellent".

Table 2 lists the measurement results. In addition, in Table 2, the underlined numerical values deviate from the ranges specified in the embodiments of the present invention.

Consider the results in Table 2.
The welded joint satisfies the manufacturing conditions of the embodiment of the present invention. In D-3, E-1, E-2, F-2 and F-3, the d _N /√t ₁ of the welded joint was 3.0 or more and the plate thickness retention was 84% or more. As a result, it is considered that the joint strength increases. No. on which the cross tension test was actually performed. In D-3 and F-3, the cross tensile strength of the welded joint was high.
In particular, No. 1 welded joints with d _N /√t ₁ of 4.0 or more. E-2, F-2 and F-3 are considered to have even higher joint strength. No. on which the cross tension test was actually performed. In F-3, No. The cross tensile strength of the welded joint was higher than that of D-3.

No. In C-1 to C-4, the pressure member 2 was not used, so the steel plate was not clamped under pressure during welding. Therefore, d _N /√t ₁ was less than 3.0 and/or thickness retention was less than 84%. As a result, the joint strength is considered to be low. No. on which the cross tension test was actually performed. In C-1 and C-3, the cross tension strength of the welded joint was low.

No. D-1 and D-2 had d _N /√t ₁ of less than 3.0. As a result, the joint strength is considered to be low.
No. D-4 had a plate thickness retention rate of less than 84%. As a result, the joint strength is considered to be low.
No. E-3 and E-4 had a plate thickness retention rate of less than 84%. As a result, the joint strength is considered to be low.
No. F-1 had d _N /√t ₁ less than 3.0. As a result, the joint strength is considered to be low.
No. F-4 had a plate thickness retention rate of less than 84%. As a result, the joint strength is considered to be low.

REFERENCE SIGNS LIST 1 electrode 2 pressure terminal P1 mild steel plate P2, P3, P4 high-strength steel plate WJ welded joint N nugget

Claims (5)

A welded joint in which four steel plates are superimposed and spot-welded,
The four superimposed steel plates include at least one mild steel plate with a tensile strength of 270 MPa or more and less than 590 MPa, and at least two high-strength steel plates with a tensile strength of 980 MPa or more,
The first mild steel plate is arranged on the outermost side of the four steel plates,
The at least two high-strength steel plates are superimposed on one side of the first mild steel plate,
A welded joint, wherein a nugget formed by spot welding satisfies the following formula (1) and has a plate thickness retention rate defined by the following formula (2) of 84% or more.

d _N /√t ₁ ≧3.0 (1)

Here, d _N (mm) is the nugget diameter at the contact surface where the mild steel plate and the high-strength steel plate contact,
t ₁ (mm) is the plate thickness of the mild steel plate in contact with the high-strength steel plate.

Plate thickness retention rate (%) = (minimum thickness of welded joint) / (total plate thickness of four steel plates) x 100 (2)
Welded joint according to claim ₁ , wherein the thickness t1 of the mild steel plate is less than the thickness of the high-strength steel plate in contact with the mild steel plate. 3. The four steel plates according to claim 1 or 2, wherein the four steel plates are composed of the first mild steel plate and three high-strength steel plates stacked on one side of the first mild steel plate. welded joints. A method for manufacturing a welded joint by spot welding overlapping portions of four steel plates,
A step of superposing four steel plates including at least one mild steel plate having a tensile strength of 270 MPa or more and less than 590 MPa and at least two high-strength steel plates having a tensile strength of 980 MPa or more,
The four steel plates are arranged so that the first mild steel plate is arranged on the outermost side of the four steel plates, and the at least two high-strength steel plates are arranged on one side of the first mild steel plate. a step of superimposing steel plates;
A step of pressurizing and holding the overlapped portion of the steel plate by a pair of electrodes and a pair of pressure members provided so as to surround the electrode tip of each electrode,
The pressurizing member is configured to pressurize a range of 180° or more in a circumferential direction centering on the tip of the electrode,
the separation distance between the pressure tip of the pressure member and the tip of the electrode is 0.1 mm or more and 10.0 mm or less;
The overlapping portion is pressed by the electrode tip with an electrode load of 1.5 kN or more, and is pressed by the pressing tip with a peripheral load of 0.1 to 1.0 times the electrode load. , a step of pressurizing and clamping;
a welding current of 5 kA or more and 15 kA or less being applied to the pair of electrodes to weld the overlapped portion. The method for producing a welded joint according to claim 4, wherein the thickness _t1 of the mild steel plate is smaller than the thickness of the high-strength steel plate in contact with the mild steel plate.

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