JP2013010139A - Arc spot-welded joint excellent in joint strength and method for manufacturing the same - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide an arc spot-welded joint which can secure both cross tensile strength and shear tensile strength, and excellent in joint strength; and to provide a method for manufacturing the same.SOLUTION: This arc spot-welded joint is manufactured as follows: two high-strength steel plates 1 with ≥0.07 mass% carbon content in the steel plate component are superposed and arc spot-welded to form a weld bead 3 of almost circular shape in planar view; the weld bead 3 is formed by melting until rear surface 1d side of a second steel plate 1B so as to project from respective surfaces of a first steel plate A and the second steel plate B; when the bead diameter at the surface 1a of the first steel plate 1A is W1 (mm), the bead diameter at the rear surface 1d of the second steel plate 1B is W2 (mm), and the plate thickness of the high-strength steel plates 1 (1A, 1B) is t (mm), the relationships among the bead diameters W1, W2 and the plate thickness t are defined in a proper range, respectively. Furthermore the relationship between base material hardness Hv (BM) and welding metal hardness Hv (WM) of the weld bead is defined in a proper range.

Description

  The present invention relates to an arc spot welded joint and a method for producing the same, and more particularly, to an arc spot welded joint having excellent joint strength obtained by arc spot welding with two high-strength steel sheets stacked and a method for producing the same. Is.

In recent years, in the automotive field, high tensile strength has been achieved even with a thin plate thickness in order to reduce vehicle weight and improve collision safety for the purpose of reducing fuel consumption and reducing carbon dioxide (CO ₂ ) emissions. There is a growing need to use high tensile strength steel sheets for vehicle bodies and parts. By designing an automobile member using such a high-tensile steel plate, it is possible to reduce the thickness of various structural members such as a vehicle body and a frame because the same strength can be ensured compared to the case of using a general steel plate. It becomes possible. In addition, since the high strength steel sheet has a higher specific strength than a general steel sheet, it is possible to reduce the weight as compared with the case where an aluminum alloy sheet is used, and there is an advantage that the cost is low. . On the other hand, spot welding is mainly used in processes such as vehicle body assembly and component mounting (see, for example, Patent Documents 1 to 3).

However, the following problems occur when spot-welding a high-tensile steel plate.
Examples of quality indicators for spot welds include tensile strength (static strength) and fatigue strength. The tensile strength of such a welded joint includes tensile shear strength (TSS) measured by applying a tensile load in the shear direction and cross tensile strength (CTS) measured by applying a tensile load in the peeling direction. There is. Here, conventionally, when high-strength steel sheets having a steel sheet strength of 980 MPa class or higher are spot-welded, the toughness and ductility of the spot welded portion (welded metal) are reduced due to the improvement of the hardenability of the base material. It is known that tensile steel sheets tend to concentrate stress because of their low work hardening coefficient. Moreover, since the spot welding method is a method of sandwiching stacked steel plates with electrodes from both sides, the steel plates are thinner than the surroundings with the weld metal interposed therebetween. As described above, when the shape that is partially thin as compared with the surroundings becomes remarkable, the stress level rises with respect to the load. Since these overlap, when a high-tensile steel plate is joined by spot welding, there exists a problem that the cross tensile strength of a welding part falls.

  The graph of FIG. 5 shows the relationship between the tensile strength (base material) of a high-strength steel plate and the cross tensile strength and shear tensile strength of a spot welded joint. As shown in FIG. 5, the shear tensile strength of the spot welded joint is improved as the tensile strength of the steel plate is increased. On the other hand, the cross tensile strength of the spot welded joint has a characteristic that the tensile strength of the steel plate is saturated at around 590 MPa, and conversely decreases as the tensile strength of the steel plate becomes higher. I understand that.

  Here, for example, it is conceivable to increase the joint strength by forming a spot welded portion having a larger nugget diameter by increasing the current using an electrode of a spot welder having a large tip diameter. However, in general, it is practical to adopt large tip diameter electrodes and high current due to limitations due to the shape and size of the members, the plate thickness, and the power supply capacity of the device in the process of automobile members and the like where high-tensile steel plates are used. There was a problem of not.

  Further, in spot welding of steel sheets having general tensile strength, a method using an arc spot welding method in which the stacked steel sheets are joined by forming a weld bead by arc welding has been proposed (for example, patents). References 4 and 5). Although the arc spot welding method described in Patent Documents 4 and 5 is slightly longer in process time, it can be welded from one side of the steel plate to the spot welding method in which the electrodes are pressed from both sides of the steel plate, It has the merit that it can respond to welding of a large-sized member or a member having a complicated shape.

  However, the method described in Patent Document 4 is a blowhole in which zinc gas vaporized by welding heat remains in the weld metal when arc-spot welding is performed on a galvanized steel sheet having a general tensile strength and a beautiful appearance. It suppresses welding defects such as. For this reason, even when the method described in Patent Document 4 is applied and arc spot welding is performed on a high-tensile steel sheet, it is difficult to prevent a decrease in cross tensile strength due to a decrease in toughness of the weld as described above. is there.

  In addition, the method described in Patent Document 5 is provided with a surface treatment material removing step and a main welding step having different welding conditions when performing arc spot welding on a steel plate that has been subjected to a surface treatment such as galvanization. After discharging the steam, etc., the main welding is performed. According to Patent Document 5, by adopting the above-described process, it is said that a weldability deterioration at the time of arc spot welding of a steel plate that has been subjected to plating treatment is suppressed, and stable welding can be performed. However, even when arc spot welding is performed on a high-tensile steel sheet by applying the method described in Patent Document 5, it is difficult to prevent a decrease in cross tensile strength due to a decrease in toughness of the welded portion, as in Patent Document 4. there were.

JP 11-279682 A Japanese Patent Publication No. 6-45827 JP 2002-103048 A JP 2001-121262 A JP-A-7-266055

  As described above, when a high strength steel sheet having a tensile strength of 980 MPa or more is welded by a conventional spot welding method, it is difficult to obtain high joint strength, and the shape and size of applicable members are limited. There was a problem. Further, when the conventional arc spot welding method is applied to the lap welding of high-tensile steel sheets having a tensile strength of 980 MPa or more, there is a problem that it is difficult to obtain high joint strength as described above.

  The present invention has been made in view of the above problems, and in an arc spot welded joint obtained by arc spot welding by superposing high-tensile steel plates, both high cross tensile strength and shear tensile strength are ensured. It is possible to provide an arc spot welded joint that can be applied to members of various shapes and sizes, and has excellent joint strength, and a method for manufacturing the same.

As a result of extensive research conducted by the present inventors to solve the above-mentioned problems, in a joint in which high-strength steel plates are overlapped and arc spot welded, first, by melting to the back side of the superimposed high-strength steel plates. The overall strength of the joint can be improved by controlling the relationship between the thickness of the high-tensile steel plate and the bead diameter of the weld bead on the surface of each of the two steel plates to an appropriate range while making it thicker than the surroundings. I found out. Furthermore, by controlling the relationship between the base metal hardness of the high-tensile steel sheet and the weld metal hardness within an appropriate range, the strength of the weld metal can be secured, high cross tensile strength can be obtained, and the arc has excellent joint strength. The present inventors have found that a spot welded joint can be obtained and completed the present invention.
That is, the gist of the present invention is as follows.

[1] An arc spot welded joint in which two high-strength steel plates having a carbon content of 0.07% by mass or more in a steel plate component are superposed and a weld bead having a substantially circular shape in plan view is formed by arc spot welding. In the high-tensile steel plate, the side irradiated with the arc is the first steel plate, the other is the second steel plate, and is melted to the back side of the second steel plate, and the first steel plate and The weld bead formed so as to protrude from each surface of the second steel plate has a bead diameter on the surface of the first steel plate of W1 (mm), a bead diameter on the back surface of the second steel plate of W2 (mm), When the thickness of the high-tensile steel plate is t (mm), the relationship between the bead diameters W1 and W2 and the plate thickness t satisfies the following (1) to (3), and the base material of the high-tensile steel plate Hardness Hv (BM) and the weld bead (weld metal) An arc spot welded joint excellent in joint strength, characterized in that the relationship between the genus) and the hardness Hv (WM) satisfies the following formula (4).
2t (mm) ≤ W2 (mm) (1)
W2 (mm) <W1 (mm) ≦ 12 t (mm) {provided that W2> 5 t} (2)
5t (mm) ≦ W1 (mm) ≦ 12t (mm) {provided that 2t ≦ W2 ≦ 5t} (3)
0.7 ≦ Hv (WM) / Hv (BM) ≦ 1.2 (4)
{However, in the above formulas (1) to (4), W1: bead diameter (mm) on the surface of the first steel plate, W2: bead diameter (mm) on the surface of the second steel plate, t: plate thickness of the high-tensile steel plate (Mm): When the plate thickness of the first steel plate is different from that of the second steel plate, the plate thickness on the thinner side, Hv (BM): base material hardness (Vickers hardness) of high-tensile steel plate, Hv (WM): welding Indicates the hardness (Vickers hardness) of the bead (welded metal). }
[2] The arc spot welded joint having excellent joint strength according to the above [1], wherein a thickness t of the high-tensile steel plate is in a range of 0.5 to 3.0 mm.

[3] A method for producing an arc spot welded joint according to [1] or [2] above, wherein two high-tensile steel plates having a carbon content of 0.07% by mass or more in a steel plate component are overlapped, When forming a weld bead having a substantially circular shape in plan view by arc spot welding, the side irradiated with the arc is the first steel plate and the other is the second steel plate, and the thickness of these high strength steel plates is t (mm), when the diameter of the through hole formed in the first steel plate before welding is d (mm), the relationship between the plate thickness t and the diameter d of the through hole is expressed by the following equation (5). Further, Ar gas or a mixed gas of Ar and CO ₂ having an Ar volume concentration of 70% or more and less than 100% is used as a shielding gas at the time of welding, and the wire target position at the time of welding is set to the through hole. Or the penetration Central and wherein the swinging between the hole edge from the arc spot welded joint manufacturing method which is excellent in joint strength.
d (mm) / t (mm) = 3-12 (5)
{However, in the above formula (5), d: diameter of the through hole formed in the first steel plate (mm), t: plate thickness of the high-tensile steel plate (mm); thickness of the first steel plate and the second steel plate If they are different, the plate thickness on either side is shown. }
[4] As the high-tensile steel plate, a steel plate in which at least one or both of the first steel plate and the second steel plate is subjected to molten or alloyed zinc plating treatment or aluminum plating treatment on the surface thereof. The joint according to the above [3], wherein the gap C between the first steel plate and the second steel plate used during welding is in a range satisfying the relationship represented by the following expression (6). A method of manufacturing an arc spot welded joint with excellent strength.
0.1 (mm) ≦ C (mm) ≦ 0.5 t (mm) (6)
{However, in the above formula (6), C: the gap between the first steel plate and the second steel plate, t: the plate thickness (mm) of the high-tensile steel plate; The thickness of the thin side is shown. }
[5] A method for manufacturing an arc spot welded joint according to [1] or [2] above, wherein the high-tensile steel plate is at least one or both of the first steel plate and the second steel plate. In addition, using a steel plate with a surface that has been subjected to galvanization treatment such as melting or alloying, two high-strength steel plates having a carbon content of 0.07% by mass or more are superposed, and the plan view is omitted by arc spot welding. When forming a circular weld bead, the side irradiated with the arc is the first steel plate and the other is the second steel plate, and the thickness of these high strength steel plates is t (mm), before welding. When the diameter of the through hole formed in advance in the first steel plate is d (mm), the relationship between the plate thickness t and the diameter d of the through hole is in the range of the following formula (5), and further during welding as shielding gas, the volume concentration of O ₂ is 2 _20% volume concentration of O 2 + CO ₂ is in a range of 35% or less, or Ar, CO _2, and with a mixed gas of _{O 2,} the wire aiming position during welding, the center of the through hole Alternatively, the method of manufacturing an arc spot welded joint having excellent joint strength, wherein the rocking is performed between the center of the through hole and the hole end.
d (mm) / t (mm) = 3-12 (5)
{However, in the above formula (5), d: diameter of the through hole formed in the first steel plate (mm), t: plate thickness of the high-tensile steel plate (mm); thickness of the first steel plate and the second steel plate If they are different, the plate thickness on either side is shown. }
[6] Further, in the above [5], the gap C between the first steel plate and the second steel plate to be overlapped at the time of welding is in a range satisfying a relationship represented by the following expression (6): The manufacturing method of the arc spot welded joint excellent in the described joint strength.
0.1 (mm) ≦ C (mm) ≦ 0.5 t (mm) (6)
{However, in the above formula (6), C: the gap between the first steel plate and the second steel plate, t: the plate thickness (mm) of the high-tensile steel plate; The thickness of the thin side is shown. }

  In addition, the weld bead demonstrated in this invention means the weld metal formed by one welding.

  According to the arc spot welded joint having excellent joint strength according to the present invention, as described above, it is melted to the back side of the superposed high-strength steel plates and protrudes from the respective surfaces of the first steel plate and the second steel plate. With respect to the weld bead formed in the above, the relationship between the thickness t of the high-tensile steel plate and the bead diameters W1 and W2 of the weld bead on the surface of each of the two steel plates is within an appropriate range, and the base material hardness Hv (BM) A configuration is adopted in which the relationship with the hardness Hv (WM) of the weld bead (welded metal) is controlled within an appropriate range. Thereby, both high cross tensile strength and shear tensile strength can be obtained without lowering the toughness and ductility of the welded portion, and an arc spot welded joint with excellent joint strength can be realized.

  According to the method for manufacturing an arc spot welded joint having excellent joint strength according to the present invention, when two high strength steel plates are overlapped and arc spot welded, the thickness t of the high strength steel plate and one steel plate before welding are preliminarily set. The relationship with the diameter d of the through hole to be formed and the composition of the shielding gas during welding are controlled within an appropriate range, and the wire target position during welding is set as the center of the through hole, or the center of the through hole The method of swinging between the hole ends is adopted. As a result, it is possible to ensure both high cross tensile strength and shear tensile strength without reducing the toughness and ductility of the welded portion, and it is possible to manufacture an arc spot welded joint with excellent joint strength.

Therefore, for example, by applying the present invention in the process of manufacturing automobile parts, assembling the vehicle body, etc., it is possible to reduce fuel consumption by reducing the weight of the entire vehicle body, to reduce carbon dioxide (CO ₂ ) emissions, and to improve collision safety. It is possible to fully enjoy the merits such as the improvement of the society and its social contribution is immeasurable.

BRIEF DESCRIPTION OF THE DRAWINGS It is a figure which illustrates typically one Embodiment of the arc spot welded joint excellent in the joint strength concerning this invention, and its manufacturing method, and shows the arc spot welded joint formed by welding two high-tensile steel plates It is sectional drawing. BRIEF DESCRIPTION OF THE DRAWINGS It is a figure which illustrates typically one Embodiment of the arc spot welded joint excellent in the joint strength which concerns on this invention, and its manufacturing method, and shows the procedure at the time of performing arc spot welding by superimposing two high-tensile steel plates. It is process drawing. BRIEF DESCRIPTION OF THE DRAWINGS It is a figure which illustrates typically one Embodiment of the arc spot welded joint excellent in the joint strength concerning this invention, and its manufacturing method, and is the schematic which shows the test method of the cross tensile strength of a welded joint. BRIEF DESCRIPTION OF THE DRAWINGS It is a figure which illustrates typically one Embodiment of the arc spot welded joint excellent in the joint intensity | strength which concerns on this invention, and its manufacturing method, and is the schematic which shows the test method of the shear tensile strength of a welded joint. BRIEF DESCRIPTION OF THE DRAWINGS FIG. 1 is a diagram schematically illustrating an embodiment of an arc spot welded joint with excellent joint strength according to the present invention and a method for producing the same, and a tensile strength of a high-tensile steel plate, a cross tensile strength and a shear tensile strength of a spot welded joint. It is a graph which shows the relationship with strength. BRIEF DESCRIPTION OF THE DRAWINGS It is a figure which illustrates typically one Embodiment of the arc spot welded joint excellent in the joint strength concerning this invention, and its manufacturing method, and the welding in the arc spot welded joint formed by welding two high-tensile steel plates It is a figure which shows the definition of the cross-sectional area of a bead (extension).

  Hereinafter, an embodiment of an arc spot welded joint excellent in joint strength of the present invention and a manufacturing method thereof will be described with reference to FIGS. The present embodiment will be described in detail in order to make the purpose of the arc spot welded joint excellent in joint strength of the present invention and the manufacturing method thereof better understood. Therefore, the present invention is limited unless otherwise specified. Not what you want.

In recent years, especially in the automobile field, the strength of steel sheets has been maintained in the car body and parts, etc. in order to reduce the weight of the car body and improve collision safety for the purpose of reducing fuel consumption and reducing carbon dioxide (CO ₂ ) emissions. However, there is a growing need to use high-strength steel sheets that can be made thinner. Further, when assembling a vehicle body or mounting parts using such a high-tensile steel plate, a spot welding method is mainly used. Here, as shown in the graph of FIG. 5, when spot welding is performed by the conventional method, the shear tensile strength (TSS) of the welded portion increases as the strength (tensile strength) of the steel plate (base material) increases. However, the cross tensile strength (CTS) is that the tensile strength of the steel sheet is 590 MPa class due to factors such as a decrease in the toughness and ductility of the weld metal, stress concentration at this part, and the joint shape becoming concave. When it is around, it starts to decline. For this reason, when spot-welding a high-tensile steel plate, there is a great need for a method that can ensure both the shear tensile strength and the cross tensile strength of the joint and realize a high joint strength. For such needs, in the present invention, as described above, the melting range by arc welding, the relationship between the plate thickness of the high-tensile steel plate and the bead diameter of the weld bead, the relationship between the base metal hardness and the weld metal hardness. Each is defined within the appropriate range. Thereby, both high cross tensile strength and shear tensile strength can be obtained without lowering the toughness and ductility of the welded portion, and an arc spot welded joint with excellent joint strength can be realized.

As illustrated in FIGS. 1A and 1B, the arc spot welded joint 10 of the present embodiment is a high-tensile steel plate 1 (first steel plate) in which the carbon content in the steel plate component is 0.07% by mass or more. 1A and the 2nd steel plate 2B) are piled up, and the welding bead 3 of planar view substantially circular shape is formed by arc spot welding. The arc spot welded joint 10 includes a high-strength steel plate 1 (1A, 1B) on which the arc is irradiated as a first steel plate 1A and the other as a second steel plate 1B, and the back surface 1d side of the second steel plate 1B. W1 (mm) is the bead diameter of the surface 1a of the first steel plate 1A of the weld bead 3 that is formed by melting up to and protrudes from the surfaces of the first steel plate 1A and the second steel plate 1B. When the bead diameter on the back surface 1d of the second steel plate 1B is W2 (mm) and the plate thickness of the high-tensile steel plate 1 (1A, 1B) is t (mm), each of the bead diameters W1, W2 and the plate thickness t. However, it is set as the structure which satisfy | fills following (1)-(3). Furthermore, the arc spot welded joint 10 has a relationship between the base material hardness Hv (BM) of the high-tensile steel plate 1 (1A, 1B) and the hardness Hv (WM) of the weld bead (welded metal) according to the following formula (4). It is configured to satisfy.
2t (mm) ≦ W2 (mm) (1)
W2 (mm) <W1 (mm) ≦ 12t (mm) {provided that W2> 5t} (2)
5t (mm) ≦ W1 (mm) ≦ 12t (mm) {provided that 2t ≦ W2 ≦ 5t} (3)
0.7 ≦ Hv (WM) / Hv (BM) ≦ 1.2 (4)
However, in said Formula (1)-(4), W1: Bead diameter (mm) in the surface 1a of the 1st steel plate 1A, W2: Bead diameter (mm) in the surface 1d of the 2nd steel plate 1B, t: High-tensile steel plate 1 (1A, 1B) plate thickness (mm); if the plate thickness of the first steel plate 1A and the second steel plate 1B is different, the plate thickness on the thinner side, Hv (BM): high-tensile steel plate 1 (1A, 1B) ) Base material hardness (Vickers hardness), Hv (WM): Indicates the hardness (Vickers hardness) of the weld bead (welded metal).

"Arc spot welding method"
FIGS. 2A to 2C are schematic diagrams for explaining an arc spot welding method used for welding the high-tensile steel plate 1 in the present invention.
The arc spot welding method used in the present invention is a kind of arc welding method defined in JIS Z 3001, and is heated while supplying a welding wire from one of the superposed steel plates that are to be welded. This is a welding method in which superposed steel sheets are fused in a dot shape. Examples of such arc spot welding methods include covered arc spot welding, TIG (Tungsten Inert Gas) spot welding, MIG (Metal Inert Gas) spot welding, and the like.

  In the arc spot welding of the present invention, for example, as in the example shown in FIGS. 2A to 2C, two high-tensile steel plates 1 (first steel plate 1A and second steel plate 2B) that are materials to be welded are used. Superposition and arc spot welding is performed with the through hole 1 formed in the first steel plate 1A as a target position. At this time, arc discharge is performed while supplying the welding wire 55 toward the through hole 1 using the torch 5, thereby melting the base material (high-tensile steel plate 1) and the welding wire 55 to form a weld metal. Thereby, the arc spot welded joint 10 joined by the weld bead 3 is obtained.

"High tensile steel plate"
The high-strength steel plate 1 (first steel plate 1A, second steel plate 1B), which is an object to be welded, used for the arc spot welded joint 10 of the present invention has a steel plate composition containing 0.07% by mass or more of carbon in particular, The material has a high tensile strength of 980 grade or higher.

  The steel type of the high-tensile steel sheet 1 used in the present invention is not particularly limited. For example, a two-phase structure type (for example, a structure including ferrite and martensite, a structure including ferrite and bainite), a work-induced transformation type (ferrite and Any type of steel sheet may be used, such as a structure containing residual austenite) or a fine crystal type (ferrite main structure). By applying the present invention to any high-tensile steel plate made of any steel type, an arc spot welded joint having both high cross tensile strength and shear tensile strength can be obtained.

  The thickness t of the high-tensile steel plate 1 used in the present invention is not particularly limited, but is preferably in the range of 0.5 to 3.0 mm. If the thickness of the high-tensile steel plate is less than 0.5 mm, the joint strength is largely governed by the plate thickness. Therefore, the effect of improving the joint strength is obtained by applying the present invention to improve the weld quality of the weld bead. It becomes difficult to be. Further, in the arc spot welded joint 10 of the present invention, particularly when considering application to the automobile field, the strength and rigidity of the member cannot be secured if the plate thickness of the high-tensile steel plate is less than 0.5 mm, so this range is excluded. did. On the other hand, if the thickness of the high-tensile steel plate exceeds 3.0 mm, it will be out of the range for achieving both high strength and thinning in realizing further weight reduction in the automobile field. did.

Note that the application of the present invention is not limited to the combination of the same type and thickness of the high-tensile steel plate 1, and can be the same type of different thickness, different type of different thickness, or a combination of different types of different thickness. Moreover, in this invention, when the 1st steel plate 1A and 2nd steel plate 1B from which plate | board thickness each differs are used as the high-tensile steel plate 1, it is preferable that the plate | board thickness of any steel plate is in the said range.
Further, the components of the steel sheet are not particularly limited with respect to the components other than the above-described carbon, and may be set as appropriate in consideration of characteristics after arc spot welding and the like.

Further, in the present invention, in addition to the high-tensile steel plate not subjected to surface treatment such as plating, at least one of the surfaces 1a, 1b, 1c, and 1d is subjected to molten or alloyed galvanizing treatment 1 It is also possible to adopt. Such galvanizing treatment may be any plating layer such as Zn-based, Zn-Fe-based, Zn-Ni-based, Zn-Al-based, Zn-Mg-based, and the like. Also, for example, a high-tensile steel plate having an aluminum plating treatment such as an Al—Si system on the surface may be used. Further, an inorganic or organic film (for example, a lubricating film) may be applied to the surface layer of the plating layer. Also, the basis weight of these plating layers is not particularly limited, but the basis weight on both sides is preferably 100 g / m ² or less. If the plating weight per unit area exceeds 100 g / m ² , the plating layer may become an obstacle during welding.

  In addition, when performing arc spot welding of a steel plate, it is necessary to form a through hole 11 in advance by drilling the first steel plate 1A irradiated with the arc. For this reason, it is difficult to replace the spot welding method with the arc spot welding method in assembly welding on the body of an automobile, etc., but for example, it is applied only to parts that require severe peel strength characteristics as performance during impact resistance. It is also possible to do.

"Weld bead"
In the present invention, for the weld bead 3 formed by arc spot welding, the relationship between the melting range, the thickness t of the high-tensile steel plate 1 and the bead diameters W1 and W2 of the weld bead 3, the base metal hardness of the high-tensile steel plate 1 and The relationship with the weld metal hardness is defined in the range described in detail below. Here, the bead diameter defined in the present invention is a bead diameter in a direction that is approximately maximum in plan view. For example, when an elliptical bead is formed in plan view, the bead diameter is the largest. The bead diameter in the ellipse direction.

(Dimensions and shapes)
In the present invention, the weld bead 3 formed by arc spot welding has a substantially circular shape in plan view as shown in FIGS. 1 (a) and 1 (b) and extends to the back surface 1d side of the second steel plate 1B. It is formed by melting. In the present invention, regarding the dimensions of the weld bead 3 having the above shape, the bead diameter on the front surface 1a of the first steel plate 1A is W1 (mm), the bead diameter on the back surface 1d of the second steel plate 1B is W2 (mm), and the high-tensile steel plate 1 is used. When the plate thickness is t (mm), a configuration is adopted in which the relationship between the average bead widths W1 and W2 and the plate thickness t satisfies the relationships represented by the following (1) to (3).
2t (mm) ≦ W2 (mm) (1)
W2 (mm) <W1 (mm) ≦ 12t (mm) {provided that W2> 5t} (2)
5t (mm) ≦ W1 (mm) ≦ 12t (mm) {provided that 2t ≦ W2 ≦ 5t} (3)
However, in the above formulas (1) to (3), W1: bead diameter (mm) on the surface 1a of the first steel plate 1A, W2: bead diameter (mm) on the surface 1d of the second steel plate 1B, t: high-tensile steel plate 1 shows the plate thickness (mm). In addition, when the plate | board thickness of the 1st steel plate 1A and the 2nd steel plate 1B differs, the plate | board thickness t of the thin side is used.

  In the present invention, in order to improve the cross tensile strength (CTS) of the arc spot welded joint, first, it is essential to secure a bead diameter in which the relationship with the plate thickness t of the high-tensile steel plate 1 is within an appropriate range. Further, in the present invention, as will be described in detail later, it is essential to control the hardness and toughness of the welded portion (weld bead 3) of the high-strength steel plate 1 that is generally hardened.

  Furthermore, in the present invention, a configuration in which the weld bead 3 (welded metal) is melted to the back surface 1d side of the second steel plate 1B is essential. As a result, a bead diameter of a predetermined value or more can be secured on the overlapping surfaces 1b and 1c, which mainly govern the joint strength, and whether or not the bead diameter is secured can be recognized in appearance. . Further, the weld bead 3 is formed by melting to the back surface 1d side of the second steel plate 1B, so that the cross-sectional shape of the bead is a straight shape, that is, from the front surface 1a of the first steel plate 1A to the back surface 1d of the second steel plate 1B. It is possible to reduce the welding deformation by approaching the shape with the bead diameter being approximately the same.

  First, in the present invention, as represented by the above formula (1), the bead diameter W2 on the back surface 1d of the second steel plate 1B is defined to be 2 t (mm) or more. When the bead diameter W2 on the back surface 1d side is less than 2 t (mm), high joint strength cannot be ensured.

  Next, as represented by the above formula (2), when the two-layer steel plates are arc spot welded, the bead diameter of the weld bead 3 on the surface 1a of the first steel plate 1A on the side irradiated with the arc is usually obtained. W1 is larger than the bead diameter W2 on the back surface 1d side of the second steel plate 1B. In the present invention, as expressed by the above equation (2), when the relationship between the bead diameter W2 and the plate thickness t is in the range of the following expression {5t <W2}, the lower limit of the bead diameter W1 is set to exceed the bead diameter W2. And the upper limit of the bead diameter W1 is 12t. It is impossible to form a weld bead with the bead diameter W1 being equal to or smaller than the bead diameter W2. Moreover, if the weld bead is formed with a bead diameter W1 exceeding 12 times the plate thickness t, it is necessary to set a large swing range of the torch, resulting in a long process time and a decrease in productivity and arc irradiation. Increase in range and total heat input may result in significant welding deformation.

  In the present invention, as expressed by the above formula (3), when the relationship between the bead diameter W2 and the plate thickness t is in the range of the following formula {W2 ≦ 5t}, the lower limit of the bead diameter W1 is 5t. To do. When the weld bead is formed with the bead diameter W1 being less than 5 times the plate thickness t, there is a possibility that sufficient joint strength cannot be obtained. Also in this case, the upper limit of W1 is set to 12t as described above. In this case, the lower limit of W2 is not particularly provided, but is preferably 3t or more.

As the shape of the weld bead 3 on the front surface 1a of the first steel plate 1A and the back surface 1d of the second steel plate 1B, it protrudes from each surface of the first steel plate 1A and the second steel plate 1B from the viewpoint of obtaining excellent joint strength. Such a shape, specifically, a convex shape is essential. By making the shape of the weld bead 3 on the front surface 1a and the back surface 1d convex, the joint strength can be further improved. Here, the convex shape described in the present embodiment may include a depression in a part of a bead (excess) as illustrated in FIG. In this embodiment, even if it is a recessed part, if the bottom of the hollow is a position outside the steel plate surface, it is called a convex shape. There is no particular limitation on the height of such a convex shape (convex portion; extra portion = a portion of the bead that protrudes outward from the steel plate surface). However, in the weld cross section as shown in FIG. 6, the average thickness (= area of the surplus portion / bead diameter, ie, S1) of the surplus with the smaller cross section of the surplus on the front side and the back side. If / W1, S2 / W2) is in the range of 0.4 times or more of the steel plate thickness t, particularly high joint strength can be obtained. Here, the area of the surplus portion can be obtained by an image analysis device after cutting a portion having a bead diameter in a direction that is approximately maximum in plan view and taking a cross-sectional photograph as shown in FIG.
As a mechanism for obtaining the above-described effects, the following actions can be considered.

  Generally, when a peel test such as a cross tensile test is performed, tensile stress acts on the position of the overlapping surfaces 1b and 1c of the first steel plate 1A and the second steel plate 1B with respect to the welded portion (weld bead 3). Moreover, compressive stress acts on the front surface 1a and the back surface 1d side. In this embodiment, the shape of the weld bead 3 on the front surface 1a and the back surface 1d is made convex so that the same effect as that of increasing the plate thickness, that is, the stress load area is expanded (stress reduction). It is considered that the effect of reducing the tensile stress level at the positions of the surfaces 1b and 1c is obtained, and the joint strength is improved.

(Weld bead: hardness of weld metal)
In the present invention, the base material hardness Hv (BM) of the high-tensile steel plate 1 (first steel plate 1A, second steel plate 1B) forming the arc spot weld joint 10 and the hardness Hv (WM) of the weld bead 3 (welded metal). However, it is set as the structure which satisfy | fills the relationship represented by following (4) Formula.
0.7 ≦ Hv (WM) / Hv (BM) ≦ 1.2 (4)
However, in the above formula (4), Hv (BM): base material hardness (Vickers hardness) of high-tensile steel plate 1 (1A, 1B), Hv (WM): hardness of weld bead 3 (welded metal) (Vickers hardness) A).

In the present invention, for the purpose of further improving the joint strength, the relationship between the base material hardness Hv (BM) and the hardness Hv (WM) of the weld bead 3 (welded metal) is defined in the above range.
Here, if Hv (WM) / Hv (BM) is less than 0.7, the strength of the weld metal is insufficient, so that the weld metal is broken and the joint strength is reduced. Further, if Hv (WM) / Hv (BM) is greater than 1.2, the toughness and ductility of the weld metal will be poor, and even if a sufficient bead diameter is secured to form a weld bead, sufficient The joint strength may not be obtained. The absolute value of the hardness Hv (WM) of the weld bead 3 (weld metal) is not particularly specified, but the Vickers hardness is preferably Hv 420 or less.

  Further, in order to satisfy the range of Hv (WM) / Hv (BM) represented by the above formula (4), the base material (high-tensile steel plate 1) and the welding wire 55 (FIGS. 2A to 2C) are used. It is preferable to control the carbon content of the weld metal (weld bead 3) formed by melting and mixing in the range of 0.07 to 0.25%. That is, although details will be described later, it is preferable to employ a welding wire 55 having a component capable of controlling the amount of carbon in the weld bead (welded metal) 3 within the above range.

  The arc spot welding method employed in the present invention is a spot welding method that does not use a welding wire in that the welding metal hardness Hv (WM) can be controlled to a desired characteristic by the selection of the welding wire 55 and the dilution conditions during welding. It is different from laser welding. In addition, the arc spot welding method suppresses the generation of 100% martensite having poor toughness in the hardened structure because the cooling rate during welding is slower than the spot welding method or laser welding method. It is possible to further improve the joint strength.

"Production method of arc spot welded joint (welding method)"
Each condition and procedure for manufacturing the arc spot welded joint 10 according to the present invention as described above will be described in detail below mainly with reference to FIGS.

In the production method of the present invention, two high-strength steel plates 1 each having a carbon content of 0.07% by mass or more in a steel plate component are overlapped, and a weld bead 3 having a substantially circular shape in plan view is formed by arc spot welding. This is a method for manufacturing the arc spot welded joint 10. At this time, among the high-strength steel plates 1, the side irradiated with the arc is the first steel plate 1A, the other is the second steel plate 1B, the plate thickness of these high-tensile steel plates 1 is t (mm), and the first steel plate before welding. When the diameter of the through hole 11 formed in advance in 1A is d (mm), the relationship between the plate thickness t and the diameter d is defined in the range of the following equation (5). Further, Ar gas or a mixed gas of Ar and CO ₂ having an Ar volume concentration of 70% or more and less than 100% is used as a shield gas at the time of arc spot welding, and the wire target position at the time of welding is set at the through hole 11. The center 11a is used, or a method of swinging between the center 11a of the through hole 11 and the hole end 11b is adopted.
d (mm) / t (mm) = 3-12 (5)
However, in said (5) type | formula, d: Diameter (mm) of the through-hole 11 formed in the 1st steel plate 1A, t: Plate thickness (mm) of the high-tensile steel plate 1; 1st steel plate 1A and 2nd steel plate 1B If the plate thickness is different, the plate thickness on either side is shown.

(Relationship between plate thickness t and through hole diameter d)
Since an arc is a heat source with a lower energy density than a laser or the like, when arc spot welding is performed under conventional conditions, melting in the plate thickness direction of the steel plate (penetration in the plate thickness direction of the steel plate) is difficult. There is a risk of becoming. At this time, if the welding heat input is increased by increasing the welding current / voltage, the melting / penetrating ability in the plate thickness direction of the steel sheet can be improved. However, for example, when arc spot welding is performed on a thin high-tensile steel sheet having a thickness of about 0.5 mm to 3.0 mm, which is used for an automobile body or the like, an excessive welding heat input causes There is a problem that burn-off occurs.
Therefore, in the present invention, before performing the arc spot welding, the through hole 11 is formed in the first steel plate 1A on the side irradiated with the arc in advance. This eliminates the need for an increase in penetration capability due to an increase in welding current / voltage, and lowers heat input, thereby preventing burn-out during welding.

  In the present invention, in manufacturing the arc spot welded joint 10 having the above-described configuration, the thickness t (mm) of the high-strength steel plate 1 and the diameter d (mm) of the through-hole 11 formed on the first steel plate 1A side. The relationship is defined in the range represented by the above equation (5). Thereby, it is possible to stably perform arc spot welding while penetrating to the back surface 1d side of the second steel plate 1B, and it is possible to manufacture the arc spot welded joint 10 having the above size and shape. It becomes.

Here, if d / t in the above formula (5) is less than 3, the diameter d of the through hole formed in the first steel plate is insufficient with respect to the plate thickness t, and the second steel plate by arc welding The penetration to the back side becomes unstable. For this reason, it may be difficult to ensure the bead width at the positions of the overlapping surfaces 1b and 1c of the first steel plate 1A and the second steel plate 1B.
On the other hand, when d / t in the above formula (5) exceeds 12, it is necessary to swing the torch with a large swing width in order to sufficiently melt the hole end of the through hole. For this reason, process time may become long and productivity may fall, and since the total amount of arc irradiation range and welding heat input increases, there exists a possibility that the welding deformation mentioned above may become remarkable.

  In addition, in arc spot welding, it is desirable to weld over the entire circumference of the through hole from the viewpoint of exhibiting high joint strength. However, when the position where high stress is applied to the weld bead is known in advance, it is desirable to more reliably melt the through hole at the high stress position, and to penetrate the low stress position. It does not matter as a condition for leaving the holes to melt.

(Shielding gas)
In the manufacturing method of the present invention, Ar gas or a mixed gas of Ar and CO ₂ having an Ar volume concentration of 70% or more and less than 100% is used as a shielding gas during arc spot welding. By arc spot welding the high strength steel sheet 1 with the shield gas composition in the above range, it is possible to suppress the occurrence of excessive oxidation and defects in the welded portion, and to produce an arc spot welded joint 10 having excellent joint characteristics. It becomes possible. Here, when a mixed gas of Ar and CO ₂ having an Ar volume concentration of less than 70% is used as the shielding gas, it becomes difficult to obtain the effect of suppressing defects in the welded portion.

  Further, in the production method of the present invention, the flow rate of the shielding gas is not particularly limited, and can be appropriately set according to the welding conditions, but sufficient shielding properties can be secured, welding defects, etc. It is more preferable to set it as 5-30 (L / min) as a range from which the effect which suppresses is acquired.

(Aim and swing of welding wire)
In the manufacturing method of the present invention, the wire aiming position at the time of welding in FIG. 2A, that is, the aiming position of the tip 55 a of the welding wire 55 is set to the center 11 a of the through-hole 11, or the through-hole 11 is used. A method of rocking, such as rotating or reciprocating, between the center 11a and the hole end 11b is adopted.
As described above, by setting the target position of the tip 55a of the welding wire 55 to the center 11a of the through hole 11, the weld bead 3 having a uniform shape can be stably formed.

  In particular, when the diameter d (mm) of the through hole 11 is large, the torch 5 is operated so that the tip 55a of the welding wire 55 swings between the center 11a of the through hole 11 and the hole end 11b. It is preferable to make it. Thereby, even if it is a case where the large through-hole 11 is provided in the range with which the said (5) Formula is satisfy | filled according to the plate | board thickness t of the high-tensile steel plate 1, especially the fusion | melting becomes inadequate in the hole end 11b. It becomes possible to form the uniform weld bead 3 without generating a location.

(surface treatment)
In the present invention, as described above, in addition to the high-tensile steel plate not subjected to surface treatment such as plating, at least one of the surfaces 1a, 1b, 1c, and 1d, that is, the first steel plate 1A and the second steel plate. It is also possible to employ the high-tensile steel plate 1 that has been subjected to melting or alloying galvanizing treatment on at least one or both of the steel plates 1B. Such galvanizing treatment may be any plating layer such as Zn-based, Zn-Fe-based, Zn-Ni-based, Zn-Al-based, Zn-Mg-based, and the like. Also, for example, a high-tensile steel plate having an aluminum plating treatment such as an Al—Si system on the surface may be used.

Further, in the present invention, when the high-tensile steel sheet 1 subjected to galvanizing treatment of melting or alloying on at least one of the surfaces 1a, 1b, 1c, and 1d as described above is used, an arc spot is further provided. as a shielding gas during welding, _{O 2} volume concentration of _2~20%, O ₂ + CO 2 volume concentration is in the range of less 35%, Ar, CO _2, and using a mixed gas of _{O 2,} This is preferable from the viewpoint that welding defects caused by plating can be reduced.
Usually, when performing arc spot welding, the 1st steel plate 1A and the 2nd steel plate 1B are piled up and welded. Here, when a galvanized steel sheet is used, the zinc in the plating that has reached the boiling point of the weld zone evaporates and penetrates into the molten metal, and in the weld metal when it cannot be dissipated into the atmosphere during the subsequent solidification process. In other words, so-called pore defects remain. For this reason, when using a high-tensile steel sheet that has been subjected to molten or alloyed galvanizing treatment, it is preferable to mix O _{2 in the} shielding gas from the viewpoint of reducing pore defects. Here, when the volume concentration of O ₂ in the shielding gas is less than 2%, the above effect cannot be obtained. On the other hand, if the volume concentration of O ₂ exceeds 30%, the above effect is rather reduced, and there is a problem that bead shape disorder, blowhole generation, excessive formation of slag, and the like are induced.

The mechanism that can reduce the pore defects in the weld metal as described above is not necessarily clear, but by including an appropriate amount of O ₂ in the shield gas, the viscosity of the molten metal is lowered, and the rising speed of the zinc vapor is increased. It is thought that it will improve and dissipate into the atmosphere.

Further, in the production method of the present invention, the above-described high-tensile steel plate 1 in which at least one of the surfaces 1a, 1b, 1c, and 1d is subjected to molten or alloyed galvanizing treatment or aluminum plating treatment is employed. In this case, it is possible to set the gap C between the first steel plate 1A and the second steel plate 1B to be overlapped at the time of arc spot welding to a range satisfying the relationship represented by the following formula (6). It is preferable from the point that it becomes possible to reduce.
0.1 (mm) ≦ C (mm) ≦ 0.5 t (mm) (6)
However, in the above formula (6), C: the gap between the first steel plate and the second steel plate, t: the thickness of the high-tensile steel plate (mm); The thickness of the thin side is shown.

  When the first steel plate 1A and the second steel plate 1B are overlapped and welded, in the case of a galvanized steel plate, the above-described pores are generated, and pore defects may remain in the weld metal. On the other hand, in the case of an aluminum-plated steel sheet, the aluminum in the plating layer melts and penetrates into the molten metal, leading to coarsening of ferrite grains, and the fatigue strength of the joint may be greatly reduced. For this reason, it is effective to provide a gap C between the first steel plate 1A and the second steel plate 1B from the viewpoint of preventing characteristic deterioration of these galvanized steel plates and aluminum plated steel plates. By providing such a gap C, zinc vapor resulting from the plating layer is dissipated into the atmosphere (gap) to reduce pore defects, and aluminum contained in the molten plating layer penetrates into the molten metal. Therefore, it is possible to suppress the coarsening of the ferrite grains. Here, if the gap C is less than 0.1 mm, the above effect is insufficient, and if it exceeds 0.5 t (mm), welding is performed at the positions of the overlapping surfaces 1b and 1c of the first steel plate 1A and the second steel plate 1B. The bead shape may be disturbed, and it becomes difficult to stably penetrate and melt the second steel plate during welding.

(Welding wire)
In the present invention, the components, diameters, etc. of the welding wire 55 used for arc spot welding are not particularly limited. For example, conventionally known ones such as welding wires defined by JIS Z 3312, JIS Z 3313, etc. Can be used without any limitation.
In addition, as described above, the welding wire 55 used in the present invention takes into account that the hardness Hv (WM) of the weld bead (welded metal) 3 is controlled to a desired characteristic, and the components and dilution conditions during welding are determined. It is desirable to decide the hiring while taking into consideration.

Further, the diameter of the welding wire 55 is not particularly limited, but is preferably in the range of 0.9 to 1.4 mm.
If the diameter of the welding wire is smaller than the above range, the rigidity of the wire becomes small and buckling is likely to occur, the welding target position at the tip of the wire becomes unstable, deviates from the arc irradiation range, or the molten state is not uniform. Therefore, the weld bead may not have a uniform shape and size. On the other hand, when the diameter of the welding wire 55 exceeds the above range, since the wire is too thick, the current density is lowered, the arc becomes unstable, and there is a possibility that the welding wire 55 is not completely melted.

  Further, the supply speed of the welding wire 55 is not particularly limited, and can be set as appropriate according to the size of the workpiece and the target welding bead. For example, when the high-strength steel plates 1 having a thickness of 0.5 to 3 mm, which are frequently used in automobile bodies, are overlapped and arc spot welding is performed, the supply speed of the welding wire 55 is set to about 30 to 100 mm / sec. be able to. If the supply speed of the welding wire 55 is within the above range, arc spot welding of the high-tensile steel plate 1 having a thickness of 0.5 to 3 mm, which is optimal for an automobile body, etc., in a stable state without welding defects or the like. It becomes possible to do.

(Welding energization conditions)
In the production method of the present invention, the energization conditions for arc spot welding are not particularly limited, and any conventionally known energization pattern and welding current / voltage conditions used for arc spot welding of steel plates are not limited at all. It is possible to adopt without. Moreover, as a welding apparatus, it is possible to employ | adopt an arc spot welding apparatus provided with the torch 5 as shown to Fig.2 (a)-(c) without any limitation. Also, the welding power source supplied to the torch 5 is not particularly limited in terms of its control method and the like, and any direct current or alternating current power source may be used.

  In addition, when the high-strength steel sheets 1 are overlapped and arc spot welded under the conditions specified in the present invention, the welding current may be set to about 100 to 300 (A) from the viewpoint of securing sufficient joint strength. preferable. Further, from the viewpoint of productivity and cost, it is more preferable that the welding time and the welding current are set to a level sufficient to form the weld bead 3 and not to be excessive.

(Arc spot welding procedure)
In the arc spot welding of the present invention, as shown in FIG. 2A, first, two high-tensile steel plates 1, that is, a first steel plate 1A and a second steel plate 1B are overlapped. At this time, as in the illustrated example, the first steel plate 1A is previously formed with a through hole 11 in which a weld metal is formed after arc spot welding.
Next, as shown in FIG. 2B, arc spot welding is performed using a torch 5 with the through hole 1 formed in the first steel plate 1A as a target position. At this time, the welding wire 55 is supplied from the torch 5 toward the through hole 1 and arc discharge is performed to melt the base material (high-tensile steel plate 1) and the welding wire 55.

  And as shown in FIG.2 (c), a molten metal is produced | generated in the position corresponding to the through-hole 11 in the inside of the through-hole 11 formed in 1 A of 1st steel plates, and the 2nd steel plate 1B. Thereafter, the molten metal is cooled and solidified to form a weld metal (weld bead) having a cross-sectional shape having a bulge on the surface 1a or surface 1d side as illustrated. By such a process, an arc spot welded joint 10 is obtained in which two high-tensile steel plates 1 (first steel plate 1A and second steel plate 2B) are overlapped and spot-welded and joined by a weld bead 3.

  In the present invention, as shown in FIGS. 2A to 2C and the like, it is most preferable that the welding posture is a downward horizontal in which the torch 5 faces downward. It is also possible to adopt a welding posture as appropriate. At this time, in the case where arc spot welding is continuously performed on a plurality of places of the superposed high-tensile steel plates 1, the moving direction may be any direction.

  As described above, according to the arc spot welded joint excellent in joint strength according to the present invention, as described above, it is melted to the back side of the superposed high-tensile steel sheet, and the thickness t of the high-tensile steel sheet is The relationship between the bead diameters W1 and W2 of the weld bead on the surface of each of the two steel sheets is within an appropriate range, and the relationship between the base material hardness Hv (BM) and the hardness Hv (WM) of the weld bead 3 (weld metal). The configuration is controlled within the proper range. Thereby, both high cross tensile strength and shear tensile strength can be obtained without lowering the toughness and ductility of the welded portion, and an arc spot welded joint with excellent joint strength can be realized.

  In addition, according to the method for manufacturing an arc spot welded joint having excellent joint strength according to the present invention, when two high-tensile steel plates are overlapped and arc spot welding is performed, the thickness t of the high-tensile steel plate and one steel plate before welding In addition to controlling the diameter of the through-hole formed in advance to d and the composition of the shielding gas during welding to an appropriate range, the wire target position during welding is set to the center of the through-hole, or through A method of swinging between the hole center and the hole end is adopted. As a result, it is possible to ensure both high cross tensile strength and shear tensile strength without reducing the toughness and ductility of the welded portion, and it is possible to manufacture an arc spot welded joint with excellent joint strength.

Therefore, for example, by applying the present invention in the process of manufacturing automobile parts, assembling the vehicle body, etc., it is possible to reduce fuel consumption by reducing the weight of the entire vehicle body, to reduce carbon dioxide (CO ₂ ) emissions, and to improve collision safety. It is possible to fully enjoy the merits such as the improvement of the society and its social contribution is immeasurable.

  EXAMPLES Hereinafter, examples of the arc spot welded joint having excellent joint strength according to the present invention and a method for manufacturing the same will be described, and the present invention will be described more specifically. However, the present invention is not limited to the following examples. However, the present invention can be carried out with appropriate modifications within a range that can meet the gist of the preceding and following descriptions, all of which are included in the technical scope of the present invention.

"Production of specimens"
In this example, first, as shown in Table 1 below, a high-tensile steel plate (Japan Iron and Steel Federation standard: CR1470, GI-HP) having a thickness of 1.0 to 2.3 mm and a tensile strength of 1410 to 1520 MPa is used. Using. Among these, hot-dip galvanized high-tensile steel plates were used as the steel plate symbols A1 and A2, and aluminum-plated high-tensile steel plates were used as the steel plate symbol A3.

  Next, based on the cross-tension test method (JIS Z3137) for spot-welded joints, the test pieces were superposed in a cross shape as shown in FIG. The test pieces were welded to each other by an arc spot welding method under the conditions, and a cross tensile test piece in which each sample piece was joined by a welded joint was produced. At this time, YGW17 described in JIS Z3312 was used as a welding wire. Further, Table 2 below shows heat input (current, voltage, time when arc-cross welded with a cross tensile specimen and a shear tensile specimen. Also determined).

  Similarly, using the above test pieces for strength test, the test pieces were superposed in parallel as shown in FIG. 4 based on the spot-welded joint shear tensile test method (JIS Z3136), and shown in Table 2 below. Test specimens were welded to each other by an arc spot welding method under each condition, and shear tensile test specimens were prepared by joining the sample specimens with welded joints.

  Further, strength test specimens prepared in the same manner are overlapped with a combination of the same steel types as shown in FIG. 3, and the test specimens are welded together by a conventionally known spot welding method under the conditions shown in Table 2 below. A sample piece was prepared. Under the present circumstances, unlike the test piece joined by the said arc spot welding method, the through-hole to the 1st steel plate side was not formed.

Further, the bead diameters W1 and W2 of the weld beads on the front surface side and the back surface side were measured, and the dimensions are shown in Table 2 below. In addition, the bead diameter shown in following Table 2 measured the bead diameter of the direction which is a rough maximum in planar view. For test numbers 1, 7, 15, and 19, the diameter of the nugget formed by spot welding was measured, and the dimensions are shown in Table 2 below.
As for the average thickness (average height) of the surplus portion, first, a portion of the bead diameter in the direction that is approximately maximum in plan view is cut, and a cross-sectional photograph as shown in FIG. 6 is taken, and then image analysis is performed. It calculated | required by calculating | requiring the area of the surplus part with an apparatus and dividing this value by bead diameter. And the ratio with respect to the plate | board thickness t of this steel plate of the average thickness of this extra-banking part was shown in following Table 3.

"Evaluation method"
About each test piece after arc spot welding obtained by the above procedure, first, the weld bead shape on the front surface side (first steel plate) and the back surface side (second steel plate) is confirmed visually, and the bead shape defect and welding are also confirmed. The presence or absence of deformation was visually confirmed, and the results are shown in Table 3 below. In Table 3 below, the results are indicated by “convex” and “concave” for the weld bead shape, and the results are indicated by “OK” and “NG” for the presence or absence of the bead shape defect and weld deformation. . Here, the “convex” of the weld bead shape refers to a state in which the surplus on both the front and back sides is equal to or higher than the position of the steel plate surface throughout the weld cross section, and the “concave” means The state where at least part of the front and back surpluses is thinner than the position on the surface of the steel sheet is indicated. Table 3 below shows the results of evaluating the ratio of the average thickness of the thinner one of the inner and outer surpluses (= area of surplus part / bead diameter) and the plate thickness in the weld cross section. .

Next, a cross tension test was performed on the obtained cross tension test piece based on a cross tension test method for spot welded joints (JIS Z3137). At this time, by applying a load in the peeling direction, that is, as shown by reference numeral 6 in FIG. 3, the upper test piece is upward and the lower test piece is downward. A cross tensile test was performed and the cross tensile strength (CTS) was measured. The results are shown in Table 3 below.
Similarly, a shear tensile test was carried out on the obtained shear tensile test piece based on the spot welded joint shear tensile test method (JIS Z3136). At this time, as shown by the reference numeral 7 in FIG. 4, a shear tensile test was performed by applying a load in the shear direction to each test piece in the left-right direction and the shear tensile strength. (TSS) was measured and the results are shown in Table 3 below.

Moreover, in the present Example, the Vickers hardness of the weld bead (welded metal) and the base material was measured for each test piece after arc spot welding obtained by the above procedure. At this time, according to a conventionally known measurement method, first, a pyramid-shaped indenter made of a regular pyramid diamond is pushed into the weld metal surface of the test piece with a load F (N), and after unloading, the indenter is removed from the portion. Vickers hardness is calculated by calculating the surface area S (mm ² ) from the length d (mm) of the diagonal line of the dent that has moved and indented and dividing the load F (N) by the surface area S (mm ² ). (Hv) was determined. Moreover, the Vickers hardness (Hv) was calculated | required also about the base material of the high strength steel plate using the same method, and these results were shown in Table 3 below.

"Evaluation results"
Among the results shown in Table 3, test numbers 2, 3, 5, 9, 10, 17, 20, 21 are examples of the present invention, and test numbers 1, 4, 6-8, 11-16, 18, 19, 22 is a comparative example. Among these, the test numbers 1 to 14 are a high-tensile steel plate (CR1470) in which the surface of steel type number B has not been plated, and a high-tensile steel plate (GI) in which the surface of steel type number A1 has been subjected to hot dip galvanizing treatment. -HP). Moreover, the test numbers 15-18 are the examples which joined high-tensile steel plate (GI-HP) by which the hot dip galvanization process was given to the surface of steel type number A2. Moreover, the test numbers 19-22 are the high-tensile steel plate (GI-HP) by which the surface of steel type number A2 was hot-dip galvanized, and the aluminum plating high-tensile steel plate (GI-HP) on the surface of the steel plate symbol A3. Is an example of joining. Moreover, the test numbers 23-25 are the examples which joined the high-tensile steel plate (CR-1470) to which the hot dip galvanization process is not given to the surface of the steel type number B. Test numbers 1, 7, 15, and 19 are examples of joining using a conventionally known spot welding method.

  As shown in the results of Table 3, in the present invention example in which high-strength steel sheets having carbon amounts defined in the present invention were overlapped and arc spot welding was performed under each condition defined in the present invention, from any steel type Even when a high-tensile steel plate is used, it was revealed that both the cross tensile strength and the shear tensile strength exhibited excellent characteristics and the joint strength was excellent. Further, in these examples of the present invention, no large welding deformation or the like was observed in the welded portion (weld bead).

  On the other hand, in the comparative example in which arc spot welding was performed under a welding condition outside the range specified in the present invention, either a cross tensile strength or a shear tensile strength was used when a high-tensile steel plate of any steel type was used. Is inferior, or welding deformation occurs in the weld bead.

Since the comparative examples of Test Nos. 1, 7, and 19 were obtained by welding high-tensile steel plates by the conventional resistance spot welding method, all resulted in inferior cross tensile strength.
The comparative example of test number 15 is obtained by welding a high-tensile steel plate by a conventional resistance spot welding method, and although the cross tensile strength is secured because of the thick plate thickness, it is practical for welding automobile bodies and the like. This is an example in which a large nugget needs to be formed using a large electrode that is not.
In the comparative examples of Test Nos. 4 and 14, since the shielding gas is a component outside the range specified in the present invention, the surface-side weld bead shape is disordered, specifically, the shape is circular. No bead diameter was measured because no pits were seen. Furthermore, in Test Nos. 4 and 14, no bead was formed on the back surface side, and the results were inferior in cross tensile strength.

In the comparative examples of test numbers 6, 8, 22, and 25, arc spot welding was performed without providing a through hole in the first steel plate, and through-melting was not possible because the welding conditions were outside the specified range of the present invention. Since no weld bead was formed on the back side, the bead shape was evaluated as “NG”, and either the cross tensile strength or the shear tensile strength was inferior.
In the comparative examples of Test Nos. 11 and 18, since the through hole formed in the first steel plate was too large, the bead diameter W1 on the surface side became too large and welding deformation occurred.
In the comparative example of test number 12, since the conditions such as the selection of the welding wire were not appropriate, the weld metal hardness (Hv) was too high, and the cross tensile strength was inferior.

In the comparative example of Test No. 13, the shape of the weld bead on the front side and the back side was a concave shape, so that the cross tensile strength was inferior.
In the comparative example of test number 16, since the diameter of the through hole formed in the first steel plate is too small, the bead diameter on the surface side becomes too small, and both the cross tensile strength and the shear tensile strength are inferior. It was.

  From the results of the examples described above, by using the arc spot welded joint excellent in joint strength of the present invention and the manufacturing method thereof, particularly, a high-tensile steel sheet having a tensile strength of 980 MPa class or more and a thin plate thickness is arc spotted. It has been clarified that an arc spot welded joint having high joint strength and excellent joint strength can be obtained even in the case of welding.

According to the present invention, when spot-welding high-tensile steel plates used for automobile bodies and parts, etc., while ensuring good welding workability, the weld metal toughness and ductility are prevented from being lowered, and high cross tensile strength is achieved. Since both the tensile strength and the shear tensile strength can be ensured, the joint strength can be improved. Therefore, by using high-strength steel sheets in the automotive field, etc., you can fully enjoy the benefits of lower fuel consumption, carbon dioxide (CO ₂ ) emissions, and improved collision safety associated with weight reduction of the entire vehicle body. And its social contribution is immeasurable.

1 ... high-tensile steel plate,
1A ... 1st steel plate (high-tensile steel plate),
11 ... Through hole 11a ... Center (through hole),
11b ... hole end (through hole),
1B ... 2nd steel plate (high-tensile steel plate)
3 ... weld bead (welded metal),
5 ... Torch,
55 ... welding wire,
10: Arc spot welded joint,
t: Thickness (high-tensile steel plate),
W1 ... Bead diameter (first steel plate),
W2 ... Bead diameter (second steel plate),
d: Diameter (through hole),

Claims (6)

An arc spot welded joint in which two high-tensile steel plates having a carbon content of 0.07% by mass or more in a steel plate component are superposed and a weld bead having a substantially circular shape in plan view is formed by arc spot welding. Of the high-tensile steel plate, the side irradiated with the arc is the first steel plate, the other is the second steel plate, and is melted to the back side of the second steel plate, and the first steel plate and the second steel plate The weld bead formed so as to protrude from each surface of the steel plate has a bead diameter W1 (mm) on the surface of the first steel plate, a bead diameter on the back surface of the second steel plate W2 (mm), and the high-tensile steel plate. When the plate thickness is t (mm), the relationship between each of the bead diameters W1 and W2 and the plate thickness t satisfies the following (1) to (3), and the base material hardness Hv ( BM) and the weld bead (welded metal) Relationship between the degree Hv (WM), characterized in that satisfies the following equation (4), the arc spot welded joint having excellent joint strength.
2t (mm) ≤ W2 (mm) (1)
W2 (mm) <W1 (mm) ≦ 12 t (mm) {provided that W2> 5 t} (2)
5t (mm) ≦ W1 (mm) ≦ 12t (mm) {provided that 2t ≦ W2 ≦ 5t} (3)
0.7 ≦ Hv (WM) / Hv (BM) ≦ 1.2 (4)
{However, in the above formulas (1) to (4), W1: bead diameter (mm) on the surface of the first steel plate, W2: bead diameter (mm) on the surface of the second steel plate, t: plate thickness of the high-tensile steel plate (Mm): When the plate thickness of the first steel plate is different from that of the second steel plate, the plate thickness on the thinner side, Hv (BM): base material hardness (Vickers hardness) of high-tensile steel plate, Hv (WM): welding Indicates the hardness (Vickers hardness) of the bead (welded metal). }   2. The arc spot welded joint with excellent joint strength according to claim 1, wherein a thickness t of the high-tensile steel plate is in a range of 0.5 to 3.0 mm. A method for producing an arc spot welded joint according to claim 1 or 2,
When two high-strength steel plates having a carbon content of 0.07% by mass or more in a steel plate component are overlapped and a weld bead having a substantially circular shape in plan view is formed by arc spot welding, The side to be irradiated is the first steel plate, the other is the second steel plate, the plate thickness of these high-tensile steel plates is t (mm), and the diameter of the through-hole formed in the first steel plate before welding is d (mm). The relationship between the plate thickness t and the diameter d of the through hole is within the range of the following formula (5),
Furthermore, Ar gas or a mixed gas of Ar and CO ₂ having an Ar volume concentration of 70% or more and less than 100% is used as a shielding gas at the time of welding, and the wire target position at the time of welding is defined as the center of the through hole. Or a method of manufacturing an arc spot welded joint having excellent joint strength, characterized by rocking between the center of the through hole and the hole end.
d (mm) / t (mm) = 3-12 (5)
{However, in the above formula (5), d: diameter of the through hole formed in the first steel plate (mm), t: plate thickness of the high-tensile steel plate (mm); thickness of the first steel plate and the second steel plate If they are different, the plate thickness on either side is shown. } As the high-tensile steel plate, at least one or both of the first steel plate and the second steel plate is welded using a steel plate that has been subjected to galvanizing treatment or alloying treatment on the surface or aluminum plating. The arc excellent in joint strength according to claim 3, wherein the gap C between the first steel plate and the second steel plate that are sometimes overlapped is set to a range satisfying a relationship represented by the following expression (6). Manufacturing method of spot welded joint.
0.1 (mm) ≦ C (mm) ≦ 0.5 t (mm) (6)
{However, in the above formula (6), C: the gap between the first steel plate and the second steel plate, t: the plate thickness (mm) of the high-tensile steel plate; The thickness of the thin side is shown. } A method for producing an arc spot welded joint according to claim 1 or 2,
As the high-tensile steel plate, at least one or both of the first steel plate and the second steel plate, a steel plate that has been subjected to melting or alloying galvanizing treatment on the surface,
When two high-strength steel plates having a carbon content of 0.07% by mass or more in a steel plate component are overlapped and a weld bead having a substantially circular shape in plan view is formed by arc spot welding, The side to be irradiated is the first steel plate, the other is the second steel plate, the plate thickness of these high-tensile steel plates is t (mm), and the diameter of the through-hole formed in the first steel plate before welding is d (mm). The relationship between the plate thickness t and the diameter d of the through hole is within the range of the following formula (5),
Furthermore, as the shielding gas during welding, _2-20% volume concentration of _{O 2,} the volume concentration of O 2 + CO ₂ is in a range of less 35%, Ar, CO _2, and with a mixed gas of _{O 2} The manufacture of an arc spot welded joint with excellent joint strength, characterized in that the aiming position of the wire at the time of welding is set to the center of the through hole or from the center of the through hole to the end of the hole. Method.
d (mm) / t (mm) = 3-12 (5)
{However, in the above formula (5), d: diameter of the through hole formed in the first steel plate (mm), t: plate thickness of the high-tensile steel plate (mm); thickness of the first steel plate and the second steel plate If they are different, the plate thickness on either side is shown. } Furthermore, the joint C according to claim 5, wherein the gap C between the first steel plate and the second steel plate that are overlapped at the time of welding is within a range that satisfies a relationship represented by the following expression (6). A method of manufacturing an arc spot welded joint that excels in resistance.
0.1 (mm) ≦ C (mm) ≦ 0.5 t (mm) (6)
{However, in the above formula (6), C: the gap between the first steel plate and the second steel plate, t: the plate thickness (mm) of the high-tensile steel plate; The thickness of the thin side is shown. }

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