JP2020121334A - Resistance spot welding method - Google Patents

Abstract

To provide a resistance spot welding method which can suppress the generation of LME when laminating and joining galvanized high-tensile materials.SOLUTION: When laminating and joining galvanized super high-tensile materials W1, W2, an adhesive A added with an element (at least one of Ti, Ni, V, Mo, Nb and Al) which is selected in advance as an element for contributing to an improvement of the toughness of a joining part is charged between the super high-tensile materials W1, W2, and after that, resistance spot welding is performed. By this constitution, the hardness of the inside of a welding nugget can be lowered, the toughness of the joining part can be improved, and as a result, an LME crack can be suppressed.SELECTED DRAWING: Figure 3

Description

The present invention relates to a resistance spot welding method. In particular, the present invention relates to an improvement in a resistance spot welding method in which galvanized high-tensile materials (including super-high-tensile materials) are superposed on each other and joined.

Conventionally, resistance spot welding has been known as welding used for joining a plurality of steel plates to each other in the manufacture of automobile bodies. High joint strength is required for the joints between steel sheets obtained by this resistance spot welding.

Patent Document 1 discloses, as a joining method for obtaining a high joining strength at a joined portion, a weld bond method in which joining by an adhesive and joining by resistance spot welding are combined. More specifically, in Patent Document 1, resistance spot welding is performed in a state in which an adhesive and a carbon supply material are applied to overlapping portions of a plurality of steel plates, thereby increasing the amount of carbon inside the welding nugget. To increase the strength of the weld nugget.

Japanese Patent No. 6048622

As disclosed in Patent Document 1, when resistance spot welding is performed in a state in which an adhesive and a carbon supply material are applied to a superposed portion of a plurality of steel plates, a joint portion (a portion that becomes a welding nugget) is formed. Although the tensile strength of the joint can be improved by supplying carbon, dislocation of atoms in the joint is less likely to occur, the toughness decreases as the hardness of the weld nugget increases, and liquid metal embrittlement (Liquid There is a problem that cracks are likely to occur due to Metal Embrittlement (hereinafter referred to as “LME”). This cracking due to LME occurs especially when welding galvanized high-tensile steel materials, and during this welding, the zinc in the galvanized layer is melted by the heat generated, and the crystal grains of the steel sheet structure of the joint portion. It occurs because molten zinc penetrates into the boundary and tensile stress acts in that state. When cracks (hereinafter, sometimes referred to as LME cracks) due to such LMEs occur, the strength of the welded joint is reduced.

The inventors of the present invention have noted that the cause of LME cracking is caused by a combination of a material having low toughness, zinc, and heat and stress due to resistance spot welding.

The present invention has been made in view of the above points, and an object thereof is resistance spot welding capable of suppressing the occurrence of LME when joining galvanized high-tensile materials on top of each other. To provide a method.

The solution means of the present invention for achieving the above-mentioned object is premised on a resistance spot welding method in which galvanized high-tensile materials are superposed on each other and joined. Then, this resistance spot welding method includes an adhesive supplying step of interposing between the high-tensile materials an adhesive to which an element selected in advance as contributing to improving the toughness of the joint is interposed, and the adhesive supplying. After the step, there is provided a resistance spot welding step of sandwiching each of the high-tensile materials with a pair of electrodes and energizing the electrodes to melt and join the high-tensile materials together.

The elements selected in advance to contribute to the improvement of the toughness of the joint portion include Ti (titanium), Ni (nickel), V (vanadium), Mo (molybdenum), Nb (niobium), Al (aluminum). Is mentioned. One of these elements may be selected and added to the adhesive, or a plurality of these elements may be combined and added to the adhesive. In addition, the high-tensile steel here is a concept including a high-tensile steel plate and an ultra-high-tensile steel plate (super-high-tensile steel).

Due to this specific matter, when the high-tensile materials are superposed and joined to each other, the above-mentioned element (element added to the adhesive) causes the crystal grains of the structure of the joint to be refined, and The hardness can be reduced and the toughness of the joint can be improved. Thereby, LME cracking can be suppressed.

In the present invention, when the galvanized high-tensile materials are superposed and joined to each other, an adhesive agent added with an element selected in advance to contribute to the improvement of the toughness of the joint is interposed between the high-tensile materials. After this, resistance spot welding is performed. Therefore, the hardness inside the weld nugget can be reduced, the toughness of the joint can be improved, and as a result, LME cracking can be suppressed.

It is a schematic structure figure showing a welding gun of a resistance spot welding device concerning an embodiment. It is a figure which shows schematic structure of the control apparatus of a welding gun. It is sectional drawing which shows the state in which the resistance spot welding which concerns on embodiment is performed. It is a sectional view of a work joined by resistance spot welding concerning an embodiment. It is a sectional view showing the state where resistance spot welding concerning a conventional technology is performed. It is a sectional view of the work joined by resistance spot welding concerning a prior art. It is a schematic diagram which shows the state of the welding nugget when the resistance spot welding which concerns on a prior art is performed. It is a schematic diagram which shows the state of the welding nugget when the resistance spot welding which concerns on embodiment is performed.

Embodiments of the present invention will be described below with reference to the drawings. The present embodiment will be described by taking as an example a case where resistance spot welding (hereinafter, also simply referred to as welding) is performed on two galvanized ultra-high tensile steel materials. As an example, there is a case of resistance spot welding two super high tensile steel materials to each other in the manufacture of a car body.

-Structure of resistance spot welding device-
Before describing the resistance spot welding method, an outline of a resistance spot welding apparatus for carrying out the resistance spot welding method will be described.

FIG. 1 is a schematic configuration diagram showing a welding gun G of the resistance spot welding apparatus according to the present embodiment. Further, FIG. 2 is a diagram showing a schematic configuration of the control device 10 used for controlling the welding gun G.

The welding gun G is a motor body that is held by the robot arm RA, an upper electrode 2, a lower electrode 3 that is erected on the lower portion 1a of the gun body 1, and an electric type that holds the upper electrode 2 and moves it up and down. An upper electrode lifting device (hereinafter, simply referred to as an electrode lifting device) 4, an electrode position detection device 5, and a welding current value (hereinafter sometimes simply referred to as a current value) flowing between the upper electrode 2 and the lower electrode 3 The current adjusting device 6 for adjustment is used as a main component. In addition, in FIG. 1, W1 and W2 are super-high-tensile materials as works. The super-high-tensile steel materials W1 and W2 are super-high-tensile steel materials (super-high-strength steel sheets) that are galvanized, and have a zinc-plated layer on the surface. It does not matter whether the galvanized layer is formed by electroplating or hot dipping. That is, the resistance spot welding method according to the present invention may be applied to any of the hot-dip galvanized steel sheets, galvannealed steel sheets, galvanized galvanized steel sheets, and galvanized galvanized steel sheets as the ultra-high-tensile steel materials W1 and W2. Is possible.

As shown in FIG. 1, the gun body 1 is a generally U-shaped member, and a lower electrode 3 is detachably erected on the upper surface of a lower portion 1a thereof. An electrode lifting device 4 is attached to the tip of the upper portion 1b of the gun body 1.

The electrode lifting device 4 includes a servo motor 41 mounted on the tip of the upper portion 1b of the gun body 1, and a lifting member 42 connected to a drive shaft (not shown) of the servo motor 41. The upper electrode 2 is detachably attached to the lower end portion 42a of the elevating member 42.

The electrode position detecting device 5 is composed of, for example, an encoder and is attached to the upper end portion 41a of the servo motor 41. Then, the detected value is transmitted to the control device 10.

The current adjusting device 6 adjusts the value of the current flowing between the upper electrode 2 and the lower electrode 3 according to the current command value transmitted from the control device 10. As the current adjusting device 6, a well-known device such as one having a variable resistor or one having a converter is applied.

The control device 10 determines the electrode position by the input unit 11 that acquires information from the input device 7 (see FIG. 2) that inputs the plate thickness and the like of the ultra high tensile strength materials W1 and W2, and the detection value of the electrode position detection device 5. An electrode position calculation unit 12 for calculating, a current value calculation unit 13 for calculating a current value when energizing between the upper electrode 2 and the lower electrode 3, and a pressing force necessary for welding (the upper electrode 2 and the lower electrode). 3, a pressing force setting unit 14 for setting the pressing force to the super high tensile strength materials W1, W2), information on the current value calculated by the current value calculating unit 13 and the pressing force set by the pressing force setting unit 14. And an output unit 15 for outputting the information of 1.

The control device 10 is realized by storing a program corresponding to the above functions in a ROM, which is mainly provided with a CPU, a ROM, a RAM, an input/output interface, and the like. Further, the RAM temporarily stores information such as the detection value from the electrode position detection device 5 and the plate thickness. The other configurations of the control device 10 are the same as those conventionally used for the welding gun G, and thus detailed description thereof will be omitted.

-Resistance spot welding method-
Next, the resistance spot welding method, which is a feature of this embodiment, will be described.

In this resistance spot welding method, the adhesive supply step and the resistance spot welding step are sequentially performed.

Conventionally, when joining galvanized ultra-high-tensile materials by resistance spot welding, the zinc in the galvanized layer is melted by the heat generated during this resistance spot welding, and molten zinc is formed in the grain boundaries of the steel sheet structure at the joint. There was a possibility that LME cracking may occur due to the penetration of tensile stress in that state. When such LME cracking occurs, the strength of the welded joint is reduced.

In the present embodiment, in the adhesive supplying step, the adhesive to which the element selected in advance as contributing to the improvement of the toughness of the joint is added is interposed between the super high tensile strength materials W1 and W2. .. Then, in a resistance spot welding process performed after the adhesive supply process, each of the super high tensile strength materials W1 and W2 in which the adhesive is interposed is sandwiched by a pair of electrodes 2 and 3, and the electrodes 2 and 3 are sandwiched. The super high tensile strength materials W1 and W2 are melted and joined to each other by energizing the materials. FIG. 3 is a sectional view showing a state in which resistance spot welding according to the present embodiment is being performed. Hereinafter, each step will be specifically described.

In the adhesive supply step, before the super high-tensile steel W1 and W2 are superposed on each other, the super high-tensile steel W2 located on the lower side is provided with the adhesive A (elements selected in advance as those that contribute to improving the toughness of the joint portion) Is applied). The method for applying the adhesive A is not particularly limited, but an adhesive applying robot is installed adjacent to the welding robot having the robot arm RA and is provided for this adhesive applying robot. An adhesive application nozzle is attached to the tip of the robot arm, and the adhesive A is applied from the adhesive application nozzle to the super high tensile strength material W2. Alternatively, the adhesive A may be applied to the super high tensile strength material W2 by a manual operation of an operator. In this case, the application position of the adhesive A on the super high tensile strength material W2 is a peripheral region including a portion sandwiched by the electrodes 2 and 3 (a portion which becomes a joint by forming a welding nugget). .. In addition, the amount of the adhesive A applied is a predetermined size (specified based on experiments, etc.) in which the thickness dimension of the adhesive layer when the super high tensile strength materials W1 and W2 are sandwiched by the electrodes 2 and 3 is specified. It is set according to the viscosity and the like of the adhesive A so as to have a predetermined dimension).

Further, the adhesive A applied to the super high tensile strength material W2 in the adhesive supplying step is not particularly limited, but, for example, an epoxy resin adhesive, a modified acrylic adhesive, a polyurethane adhesive, or the like is used. Applicable.

Then, the adhesive A is added with an element selected in advance as contributing to improving the toughness of the joint. Examples of this element include Ti (titanium), Ni (nickel), V (vanadium), Mo (molybdenum), Nb (niobium), and Al (aluminum). Then, one of these elements may be selected and added to the adhesive A, or a plurality of these elements may be combined and added to the adhesive A.

When one of these elements is selected and added to the adhesive A, the addition amount (the addition amount represented by mass percentage [mass%] with respect to the adhesive A) is appropriately set within the following range.
・When the added element is Ti or Al... 0.1 to 40 [mass%]
・When the added element is any of Ni, V, Mo, and Nb... 0.01 to 40 [mass%]
In addition, the addition amount (total addition amount of each element) when a plurality of the above-mentioned elements are combined and added to the adhesive A is appropriately set in the range of 0.01 to 40 [mass%].

After the adhesive A having such a configuration is applied to the super high tensile strength material W2, the super high tensile strength material W1 is overlaid on the super high tensile strength material W2, and the resistance spot welding process is started in this state. Become.

In the resistance spot welding process, the super high tensile strength materials W1 and W2 are applied between the upper electrode 2 and the lower electrode 3 by a predetermined pressing force (pressing force set by the pressing force setting unit 14) by the operation of the electrode lifting device 4. In the sandwiched state, energization is performed with the current value calculated by the current value calculation unit 13. Along with this energization, a part of the super high tensile strength materials W1 and W2 are melted and a welding nugget having a predetermined shape is formed. The pressing force and current value at this time are, for example, the same values as the pressing force and current value in the conventional resistance spot welding.

At this time, the element added to the adhesive A is selected in advance as contributing to the improvement of the toughness of the joint, and the structure of the joint is made fine. As a result, the hardness inside the weld nugget after the resistance spot welding process is completed (after the energization is completed and the weld nugget is hardened) can be reduced, and the toughness of the joint can be improved. Thereby, LME cracking can be suppressed.

FIG. 4 is a cross-sectional view of works (super high tensile strength materials W1 and W2) joined by resistance spot welding according to the embodiment. In the weld nugget N formed over each of the ultra-high tensile strength materials W1 and W2, for example, the crystal grains in the central portion (the area indicated by reference numeral N1 in FIG. 4) are made fine, and the hardness is particularly high in this portion. Is low.

FIG. 5 is a cross-sectional view showing a state in which resistance spot welding according to a conventional technique is performed. Further, FIG. 6 is a cross-sectional view of a work (each super high tensile strength material W1, W2) joined by resistance spot welding according to a conventional technique. In these figures, the same parts as those in FIGS.

As shown in FIG. 5, resistance spot welding was performed without the presence of an adhesive (adhesive containing an element selected in advance as a component that contributes to improving the toughness of the joint) between the super-high-tensile steel W1 and W2. In the above, the welding nugget N formed over each of the ultra-high tensile strength materials W1 and W2 has a high hardness due to melting of zinc in the galvanized layer over the whole thereof. FIG. 7 is a schematic diagram showing the state of the welding nugget N when resistance spot welding according to this conventional technique is performed. As shown in FIG. 7, when the resistance spot welding according to the conventional technique is performed, the zinc in the galvanized layer is melted by the heat generated during the resistance spot welding, so that the crystal grain boundaries of the steel sheet structure of the joint part are melted. LME cracking (indicated by reference character CR in FIG. 7) occurs due to the fact that molten zinc penetrates and tensile stress acts in that state.

On the other hand, when the resistance spot welding according to the present embodiment is performed, as shown in FIG. 8 (schematic diagram showing the state of the welding nugget N when the resistance spot welding according to the present embodiment is performed). In addition, the above-mentioned element (the element added to the adhesive A) makes the crystal grains of the structure of the joint finer, the hardness inside the weld nugget N can be reduced, and the toughness of the joint is improved. Can be made. As a result, LME cracking is suppressed.

-Other embodiments-
It should be noted that the present invention is not limited to the above-described embodiment, and all modifications and applications included in the scope of the claims and a range equivalent to the scope of the claims are possible.

For example, in the above-described embodiment, the case where the present invention is applied to resistance spot welding for welding two ultra high-tensile steel materials W1 and W2 has been described. The present invention is not limited to this, and can also be applied as resistance spot welding for welding three or more super high tensile strength materials. In this case, it is preferable to apply the adhesive A (adhesive containing an element selected in advance to contribute to the improvement of the toughness of the joint portion) between all the ultra-high tensile materials. It should be noted that it is also within the scope of the technical idea of the present invention that the adhesive A is applied only between some of the super high tensile strength materials.

Further, the present invention is not limited to the case of welding high-tensile materials, but can be applied to the case of welding high-tensile materials.

Further, the resistance spot welding method according to the present invention can be applied not only to joining a plurality of steel plates to each other in the manufacture of an automobile body, but also to joining other steel plates.

INDUSTRIAL APPLICABILITY The present invention is applicable to a resistance spot welding method in which galvanized ultra-high tensile steel materials are superposed on each other and joined.

2 Upper electrode 3 Lower electrode 13 Current value calculation unit 14 Pressurization force setting unit A Adhesives W1 and W2 Super high tensile strength material (galvanized high tensile strength material)

Claims (1)

In the resistance spot welding method of joining the galvanized high-tensile materials by overlapping each other,
An adhesive supplying step of interposing an adhesive containing an element selected in advance as contributing to the toughness improvement of the joint between the high-tensile materials,
After the adhesive supply step, each high-tensile material is sandwiched by a pair of electrodes, and a resistance spot welding step of melting and joining the high-tensile materials with each other by applying an electric current between the electrodes is provided. Characteristic resistance spot welding method.

Images