JP6648791B2 - Resistance spot welding method - Google Patents

Description

  The present invention relates to a resistance spot welding method for joining plate materials.

  Transport equipment including automobiles, industrial equipment, and the like are composed of a plurality of structural components. In many cases, resistance spot welding (hereinafter, also simply referred to as “spot welding”) is used for manufacturing structural components.

  Usually, spot welding is performed as follows. Prepare a board as a material. The plate set has a laminated portion in which a plurality of metal plate materials are stacked. Next, the laminated portion of the plate set is sandwiched between a pair of electrodes. Then, a current is applied between the electrodes while the laminated portion is pressed by the electrodes. As a result, in the laminated portion of the plate set, adjacent metal plates come into contact with each other with the pressurization by the electrodes, and a current flows in this contact region. The contact area is melted by heat generated by electric resistance, and solidifies to form a nugget. Due to the formation of the nugget, the metal plates of the plate set are joined and joined, and a structural component is manufactured.

  2. Description of the Related Art In recent years, the weight of a vehicle body has been reduced in transportation equipment, and there has been a strong demand for weight reduction of structural components constituting the vehicle body. For this reason, the strength of steel sheets used has been increasing. On the other hand, in the development of steel sheet materials, the use of medium-high carbon steel with reduced rare metals as an environment-friendly next-generation material is being studied. This medium-high carbon steel positively contains C (carbon) instead of reducing rare metals to ensure material strength.

  Generally, a steel sheet material has a composition containing more elements as the strength increases. However, if the amount of C contained in the steel sheet is too large, the quenchability is improved, and the hardness of the structure (martensite) after quenching is also increased. For this reason, in the case of welding of middle and high carbon steels, rapid cooling from a high temperature causes a remarkable increase in hardness, makes the weld metal brittle and easily breaks brittlely at the interface, and deteriorates the properties of the welded joint. The joint properties referred to here include a fracture mode, tensile shear strength (TSS), and cross tensile strength (CTS).

  On the other hand, high alloy steels containing specific elements such as Cr, Ni and Mn such as various stainless steels, TWIP (Twinning-Induced Plasticity) steels, etc. containing several% to several tens% are the same. Deterioration of joint characteristics does not pose a problem in common material welding of materials. However, when a high alloy steel and a high alloy steel or a low alloy steel of the same type are welded, remarkable hardening of the weld metal is developed, which may cause a problem of deterioration of joint characteristics. These high-alloy steels have a high-temperature phase of iron stabilized at room temperature by a specific composition.However, when they melt with the same type of steel and change the composition of the weld metal, a hard and brittle martensitic structure is formed after cooling. Because it appears.

  In addition, a structure in which the same or the same kind of metal material is combined, such as high-C (carbon) -containing steel and high-C-containing steel, may be adopted as a structural component. In this case, a plurality of plate materials of the same material or the same material are stacked and joined by spot welding.

  When the same material or the same material is combined and joined by spot welding, there are the following problems. The weld metal may become extremely hardened and become brittle, failing to provide sufficient joint properties. This tendency is particularly apparent in spot welding between steels having a high carbon content. In the future, it is expected that spot welding of steel materials further containing C will be required. In that case, the formation of a fragile structure accompanying the hardening of the weld metal is promoted, and the deterioration of the joint characteristics becomes more apparent.

  JP-A-2002-103054 (Patent Document 1) and JP-A-2013-78782 (Patent Document 2) join a high-strength steel, that is, a steel having a relatively high C content and being easily hardened by a weld metal, by spot welding. Disclose the technology. In the spot welding methods disclosed in these Patent Documents 1 and 2, steel sheets are stacked, sandwiched between electrodes, current is applied between the electrodes to perform welding by melting and solidification, and then current is applied between the electrodes. Add. Such additional energization after welding is referred to as post-energization. The applied current of the post-energization is a current lower than the applied current at the time of welding or a current for a short application time, and is a current that does not cause melting of the steel sheet but generates heat. Thereafter, by energization, the hardened structure is softened at the end of the nugget, which is the stress concentration portion when a peeling load is applied to the joint, and segregation such as P is reduced. As a result, the structure of the weld metal is improved, and the resistance when the joint breaks increases.

JP 2002-103054 A JP 2013-77882 A

  The spot welding methods disclosed in Patent Documents 1 and 2 are intended for welding steel sheets having a C content of up to about 0.3% by mass. For this reason, the structure is improved at the nugget end, that is, at a very small portion of the weld metal, thereby improving the joint characteristics. However, for steel sheets containing more C, it is necessary to improve the structure throughout the nugget. Therefore, the effect of improving the joint characteristics is small in a short and local heat treatment such as post-energization.

  Furthermore, in welding of high alloy steels such as stainless steel and TWIP steel, if the composition of the weld metal changes by welding with the same kind of high alloy steel or low alloy steel, heat treatment such as post-energization is performed. Even the high temperature phase of iron can no longer be stabilized at room temperature. Therefore, the effect of improving the joint characteristics cannot be obtained.

It is an object of the present invention to provide a resistance spot welding method having the following properties:
・ Spot welding can be performed regardless of the combination of materials;
・ Sufficient joint characteristics can be obtained.

The resistance spot welding method according to an embodiment of the present invention includes:
A spot welding method for joining sheet materials,
A preparing step of preparing a plate set including a laminated portion in which plate materials are stacked on at least three layers;
A welding step of sandwiching the laminated portion of the plate set by a pair of electrodes, applying a current between the electrodes while pressing the laminated portion with the electrodes, and performing resistance spot welding on the laminated portion.
In the preparing step, as the laminated portion, a plate material of the same type of metal is arranged on the outermost layer, and a plate material of the same material as any one of the outermost plate materials on the inner layer. A plate having a through hole is arranged.
In the welding step, resistance spot welding is performed in a state where the positions of the through holes and the positions of the electrodes in the inner layer plate material are matched.

  In the case of this spot welding method, it is possible to adopt a configuration in which one of the outermost layer plate materials is a high C content steel and the other plate material is a low C content steel.

The above spot welding method can be changed to the following configuration:
The material of the plate material of the outermost layer is the same as each other,
The material of the inner layer plate is the same as the material of the outermost layer plate.

  In the case of this spot welding method, it is possible to adopt a configuration in which the material of the outermost layer plate is a low C-containing steel and the material of the inner layer plate is a high C-containing steel. Instead of this configuration, the outermost plate material may be a high alloy steel.

The above spot welding method can be changed to the following configuration:
The material of the outermost plate material is the same kind,
The material of the inner layer plate is the same as the material of the outermost layer plate.

  In any of the spot welding methods described above, the inner layer plate may have an insulating coating.

  In any of the spot welding methods described above, any one of the outermost layer plate members may be a backing plate.

  In any of the spot welding methods described above, it is preferable that the contour shape of the through-hole in the plate material of the inner layer is circular or square, and the diameter or the length of one side of the through-hole is larger than a target nugget diameter.

  In any of the spot welding methods described above, it is preferable that each thickness of the outermost layer plate material is 3.0 mm or less, and the entire thickness of the inner layer plate material is 2.4 mm or less.

The resistance spot welding method of the present invention has the following remarkable effects:
・ Spot welding can be performed regardless of the combination of materials;
・ Sufficient joint characteristics can be obtained.

FIG. 1A is a cross-sectional view schematically showing the procedure of the resistance spot welding method according to the present embodiment, and shows plate materials before they are stacked. FIG. 1B is a cross-sectional view schematically showing the procedure of the resistance spot welding method according to the present embodiment, and shows a state before welding of a laminated portion where plate materials are stacked. FIG. 1C is a cross-sectional view schematically showing a procedure of the resistance spot welding method according to the present embodiment, and shows a state in a middle stage of welding. FIG. 1D is a cross-sectional view schematically showing the procedure of the resistance spot welding method of the present embodiment, and shows a state at the end of welding. FIG. 1E is a cross-sectional view schematically showing the procedure of the resistance spot welding method according to the present embodiment, and shows the laminated portion after welding is completed.

  The spot welding method according to one embodiment of the present invention is used for joining plate materials, and includes a preparation step and a welding step. The preparation step is a step of preparing a plate set including a laminated portion in which plate materials are stacked in at least three layers. The welding step is a step of sandwiching the laminated portion of the plate set by a pair of electrodes, applying a current between the electrodes while pressing the laminated portion with the electrodes, and performing resistance spot welding on the laminated portion. In the preparation step, as the laminated portion of the plate set, a plate material of the same or the same kind of metal is arranged on the outermost layer, and a plate material having a through hole is arranged on the inner layer. In the welding process, resistance spot welding is performed in a state where the position of the through hole in the inner layer plate material and the position of the electrode are matched.

  Hereinafter, a specific mode of the resistance spot welding method according to the present embodiment will be described.

  1A to 1E are cross-sectional views schematically showing the procedure of the resistance spot welding method according to the present embodiment. In these figures, FIG. 1A shows the plate members before they are stacked. FIG. 1B shows a state before welding of a laminated portion in which plate materials are stacked. FIG. 1C shows a state in the middle stage of welding. FIG. 1D shows a state at the end of welding. FIG. 1E shows the laminated portion after welding is completed.

  The present embodiment is directed to a case where plate materials are combined into at least three layers and these plate materials are joined by spot welding. In particular, a case where a plate material of the same kind of metal is arranged in the outermost layer and a plate material of the same material as that of one of the outermost layers is arranged in the inner layer. In the following, first, as the outermost layer plate, a high-strength high C-containing steel plate and a low-strength low-C-containing steel plate are arranged, and as the inner layer plate, a high-strength high-C-containing steel plate is used. An example of the arrangement is shown.

  Here, the same kind of metal material means a metal material in which the element having the highest content in the chemical composition is the same. For example, high C content steel and low C content steel are of the same type. The same metal material means not only a metal material having exactly the same chemical composition but also a metal material having the same name in the standard. High C content steel refers to steel having a C content of 0.4% by mass or more. Low C content steel refers to steel having a C content of less than 0.4% by mass.

  Prepare a board as a material. As shown in FIG. 1A and FIG. 1B, the plate set includes three plate materials 1, 2, and 3, and includes a laminated portion 10 having a three-layer structure in which the plate materials 1, 2, and 3 are sequentially stacked. In the laminated portion 10, the material of the first-layer plate 1 of the first and third outermost layers 1 and 3 is high-C content steel. The material of the plate material 3 of the third layer is a low C content steel of the same type as the material of the plate material 1 of the first layer. The material of the plate material 2 of the second layer which is the inner layer is the same high C content steel as the material of the plate material 1 of the first layer.

  That is, the plate material 2 of the second layer of the high C content steel is sandwiched between the plate material 1 of the first layer of the high C content steel and the plate material 3 of the third layer of the low C content steel. The plate material 2 of the second layer is provided with a through hole 2a in advance. The outline shape of the through hole 2a may be circular or square. Practically, the through hole 2a is circular.

  Of the plate materials 1 and 3 of the first layer and the third layer (outermost layer), the plate material 3 of the third layer, which is the same kind but not the same as the plate material 2 of the second layer (inner layer) and is a low C content steel, , But does not form the essential shape of the structural component. That is, the first-layer plate 1 and the second-layer plate 2 which are the same material of the high C content steel form the essential shape of the structural component. In the preparation step, a plate set including the laminated unit 10 having such a configuration is prepared.

  After the preparation process, the process proceeds to a spot welding welding process. In the welding process, first, the laminated portion 10 of the plate set is sandwiched between the pair of electrodes 20, 20. At that time, as shown in FIG. 1B, the positions of the through holes 2a provided in the plate material 2 of the second layer (inner layer) and the positions of the electrodes 20, 20 are made to match. Subsequently, the laminated portion 10 is pressed by the electrodes 20 and 20. As a result, as shown in FIG. 1C, the plate materials 1 and 3 of the first layer and the third layer (outermost layer) are deformed and are in contact with each other in the area of the through hole 2a in the plate material 2 of the second layer (inner layer). become.

  Further, a current is applied between the electrodes 20 while the laminate 20 is pressed by the electrodes 20. As a result, a current flows through the contact areas of the first and third (outermost) plate materials 1 and 3. The contact region is melted and solidified by the resistance heat, and a nugget 4 is formed as shown in FIG. 1D. Around the nugget 4, the first and third (outermost) plate materials 1 and 3 that are in contact with each other are pressed against each other.

  Further, in the above-mentioned welding step, it may be changed so that a current is applied between the electrodes 20, 20 before the plate materials 1, 3 of the first layer and the third layer (outermost layer) contact each other. In this case, first, the plate materials 1 and 3 of the first layer and the third layer (outermost layer) come into contact with the plate material 2 of the second layer in the annular region around the through hole 2a in the plate material 2 of the second layer (inner layer). It will be in a state of having done. For this reason, an electric current flows in the annular contact area between the first and third plate materials 1 and 3 and the second layer plate material 2. As a result, the annular region generates heat, so that the plate materials 1 and 3 of the first and third layers are softened in and around the annular region. Further, when the laminate 20 is continuously pressed by the electrodes 20 while applying a current between the electrodes 20, the softened first and third (outermost) plates 1 and 3 are deformed. Then, they come into contact with each other in the region of the through hole 2a in the plate material 2 of the second layer (inner layer) (see FIG. 1C). As a result, a current also flows through the contact areas of the first and third layers (outermost layers) of the plate materials 1 and 3, and finally the nugget 4 is formed (see FIG. 1D).

  In this way, the laminated portions 10 are joined by spot welding, and a structural component is manufactured (see FIG. 1E). In the spot welding, the first layer (outermost layer) 1 that is a high C content steel and the third layer (outermost layer) 3 that is a low C content steel are formed by a second layer that is a high C content steel. They are joined via the through holes 2a in the (inner layer) plate 2. By this joining, the laminated portion 10 is in a state in which the inner layer plate member 2 is strongly sandwiched between the outermost layer plate members 1 and 3.

  As described above, according to the spot welding method of the present embodiment, the same material of the high C content steel can be combined and joined. Substantially joining by spot welding is performed between the outermost (first layer and third layer) plate materials 1 and 3 that are the same material of high C content steel and low C content steel. For this reason, the C content of the weld metal is lower than in the case where high C content steels are welded to each other. This is because the C content of the weld metal depends on the C content of each of the steel sheets to be welded, and is approximately the average of the C contents of both. Therefore, sufficient joint characteristics can be obtained since no fracture occurs due to the generation of a fragile structure due to the hardening of the weld metal.

  In the above-described spot welding method of the present embodiment, the materials of the outermost layers (first and third layers) of the plate materials 1 and 3 are the same kind of metal material, and the material of the inner layer (second layer) of the plate material 2 is The material is not limited as long as it is the same as the material of any one of the outermost plate materials 1 and 3. For example, in the above embodiment, the material of the plate material 2 of the second layer as the inner layer can be the same low C content steel as the material of the plate material 3 of the third layer as the outermost layer.

  For example, in the case of steel-based materials, low C-containing steel and high C-containing steel are metal materials of the same type. In addition, α-based (ferritic) stainless steel, γ-based (austenitic) stainless steel, TWIP (Twinning-Induced Plasticity) steel, and the like are metal materials of the same kind of high alloy steel. . Speaking of an Al-based material, Al and an Al alloy are the same type of metal material.

  However, the materials of the plate materials 1 and 3 of the outermost layer (first and third layers) are the same kind of metal material, and the material of the plate material 2 of the inner layer (second layer) is the material of the plate materials 1 and 3 of the outermost layer. The configuration may be changed to the same type as. Since substantial joining by spot welding is performed between the same type of metal materials, if the joining target includes a metal material with a low C content or alloying element content, the hardening of the weld metal will be reduced. Because there is no.

  For example, one of the outermost layer 1 of the first layer plate 1 and the inner layer 2 of the second layer plate 2 is made of high C-containing steel, and the other plate is made of the same high C-containing steel. The material of the plate material 3 of the third layer, which is another outermost layer, can be low C-containing steel. The material of the first layer plate 1 or the second layer plate 2 may be a plated steel plate.

  The materials of the plate materials 1 and 3 of the outermost layers (first and third layers) are the same metal material, and the material of the plate material 2 of the inner layer (second layer) is the same as the material of the plate materials 1 and 3 of the outermost layer. May be changed to the configuration. Since the actual joining by spot welding is performed between the same metallic materials, if the joining target includes metallic materials with low C content or alloying element content, the hardening of the weld metal is reduced. Because there is no. Also, in the welding of high alloy steels such as various stainless steels, TWIP steels, or the like, or the welding of Al alloys, there is no decrease in joint characteristics.

  For example, the plate materials 1 and 3 of the first and third layers as the outermost layers can be made of low C-containing steel, and the plate material 2 of the second layer which is the inner layer can be made of high C-containing steel. One of the first and third outermost plate materials 1 and 3 and the second innermost plate material 2 is α-type stainless steel, and the other layer is α-type stainless steel. Can be γ-based stainless steel. Similarly, of the outermost layer plate materials 1 and 3 and the inner layer plate material 2, any one of the plate materials may be Al and the other layer plate material may be an Al alloy.

  Further, the plate material 2 of the inner layer (second layer) may be a steel sheet provided with an insulating film having a surface coated with an insulating film such as a coating (base material: low C content steel, high C content steel). This is because the substantial joining by spot welding is performed between the outermost (first and third) layers of the plate materials 1 and 3 which are metal materials, and therefore, the conductivity is not essential as a characteristic of the inner layer plate material 2. .

  Table 1 below shows an example of combinations of the material of the plate material 1 of the first layer (outermost layer), the material of the plate material 2 of the inner layer, and the material of the plate material 3 of the third layer (outermost layer).

  When metal materials such as stainless steel and TWIP steel are combined, the outermost (first and third layers) plate materials 1 and 3 may be made of the same or similar metal material of the same or similar chemical composition among the same metal materials. It is preferable to employ a material. When a metal material containing an alloy element having a content of more than 6% by mass is combined, the outermost layers (first and third layers) of the plate materials 1 and 3 are preferably made of a metal material having the same alloy element (more preferably). Are preferably the same metal material).

  In the spot welding method of the present embodiment described above, of the outermost (first and third) plate materials 1 and 3, the first layer plate material 1 is used as a backing plate instead of the third layer plate material 3. Is also good. However, it is not always necessary to use one of the outermost plate materials 1 and 3 as a backing plate. That is, both the outermost plate materials 1 and 3 and the inner layer (second layer) plate material 2 may form the essential shape of the structural component.

  The lamination part 10 of the plate set may be any one in which plate materials are stacked in at least three layers. That is, the inner layer may have a configuration in which a plurality of plate members are stacked. In this case, a through hole may be provided coaxially in all of the inner layer plate members.

  Here, in the spot welding method of the present embodiment described above, the thickness of each of the outermost layers (the first layer and the third layer) of the plate materials 1 and 3 is set according to the design specification of the structural component. For example, the thickness of each of the outermost plate materials 1 and 3 is practically 0.3 mm to 3.0 mm. If the thickness is too thin, the strength cannot be secured. Therefore, a preferred lower limit of the thickness is 0.3 mm, more preferably 0.5 mm. On the other hand, if the thickness is too large, deformation due to pressure from the electrode during spot welding becomes difficult, and contact between the outermost plate materials 1 and 3 becomes insufficient. For this reason, the preferable upper limit of the thickness is 3.0 mm, more preferably 2.0 mm, and still more preferably 1.6 mm.

  The thickness of the plate material 2 of the inner layer (second layer) (when the inner layer has two or more layers, the thickness of the entire inner layer) is related to the thickness of the plate materials 1 and 3 of the outermost layers (first and third layers). Then, it is set according to the design specification of the structural component. For example, the overall thickness of the inner layer plate material 2 is practically 0.3 mm to 2.4 mm. If the thickness is too thin, the strength cannot be secured. Therefore, a preferred lower limit of the thickness is 0.3 mm, more preferably 0.5 mm. On the other hand, if the thickness is too large, the amount of deformation of the outermost layer plates 1 and 3 due to pressure from the electrodes during spot welding becomes excessive, and the outermost layer plates 1 and 3 are insufficiently contacted with each other. For this reason, the preferable upper limit of the thickness is 2.4 mm, more preferably 1.8 mm, and still more preferably 1.2 mm.

  The electrodes 20, 20 used for spot welding may be any of DR (dome radius) type, CF (center flat) type, and SR (single radius) type. The DR-type electrode has a substantially cylindrical shape with a tip protruding in a dome shape, and has a tip end surface formed as a convex curved surface having a large radius of curvature. The CF electrode has a substantially cylindrical shape with a tip portion protruding in a truncated cone shape, and has a tip surface formed into a flat surface. The SR type electrode has a substantially cylindrical shape, and its tip surface is formed as a convex curved surface having a constant radius of curvature.

  Here, in the spot welding method of the present embodiment described above, it is preferable that the diameter of the through hole 2a provided in the inner layer (second layer) of the plate member 2 is larger than the target nugget diameter. The reason is shown below.

Generally, in manufacturing a structural component by spot welding, a target nugget diameter (diameter of a nugget) ND _aim is determined by design specifications. That is, the nugget formed by spot welding is required to be _{equal to} or _larger than the target nugget diameter ND _aim . The target nugget diameter ND _aim is smaller than the diameter D of the electrode body.

Usually, the target nugget diameter ND _aim is represented by X√t [mm] using the thickness t [mm] of the plate material to be joined as an index. The coefficient X is determined by design specifications. The target nugget diameter ND _aim is, for example, 5 ° t.

On the other hand, in the spot welding method of the present embodiment, the nugget 4 penetrates through the contact area of the outermost (first and third) plate materials 1 and 3, that is, the inner layer (second layer) of the plate material 2. It is formed in the area of the hole 2a. Accordingly, in order to form the nugget 4 having a size equal to or larger than the target nugget diameter ND _{aim, the} diameter of the through hole 2a (when the through hole is circular) or the length of one side (when the through hole is square) is equal to the target. It is necessary to make it larger than the nugget diameter ND _aim . More safely, the diameter or the length of one side of the through hole 2a may be equal to or larger than the diameter D of the electrode body.

  However, if the diameter or the length of one side of the through-hole 2a is too large, it is not possible to sandwich the plate material 2 of the inner layer (second layer) by joining the plate materials 1 and 3 of the outer layer (first and third layers). Will be enough. For this reason, it is preferable that the diameter or the length of one side of the through hole 2a is not more than 2.5 times the diameter D of the electrode body.

  In order to confirm the effects of the present invention, the following welding tests were performed by applying the spot welding method of the present embodiment shown in FIGS. 1A to 1E.

[Example 1]
As a test plate, a non-plated high C-containing steel plate having a thickness of 1.6 mm (hereinafter, also referred to as a “high C steel plate”) and a non-plated low C-containing steel having a thickness of 1.6 mm are used. Many plate materials (hereinafter, also referred to as “low C steel plates”) were prepared. In some of the high-C steel plates, through holes having a diameter of 20 mm were formed by laser cutting. The C content of the high C steel sheet was 0.55% by mass. The C content of the low C steel sheet was less than 0.01% by mass.

  As Example 1 of the present invention, a plate set having a three-layer structure in which a high-C steel sheet without holes, a high-C steel sheet with holes, and a low-C steel sheet without holes was sequentially stacked was prepared. As Comparative Example 1, a plate having a two-layer structure in which a holeless high-C steel sheet and a holeless high-C steel sheet were sequentially stacked was prepared, and spot welding was performed.

In each spot welding, the welding current was variously changed. The common welding conditions are as follows.
・ Welding machine: Air pressurized spot welding machine with single layer AC power supply ・ Electrode: DR type electrode (tip surface diameter: φ6 mm, radius of curvature of tip surface: R40 mm, body diameter: φ16 mm)
・ Pressing force: 400 kgf (= 3.92 kN)
Energizing time: 40 cycles in a three-layer structure, 20 cycles in a two-layer structure (1 cycle means 1/60 second)

  The joint characteristics of each plate set after the spot welding were performed were evaluated. The evaluation of the joint properties was performed with a tensile shear strength (TSS) based on JIS Z3136. Further, each plate set was cut so as to pass through the center of the nugget, the cross section thereof was mirror-polished, and the Vickers hardness (Hv) of the nugget (weld metal) was measured. The Vickers hardness was measured according to JIS Z2244. The test load was 1 kgf, and the hardness was an average value of five measured points. Table 2 below shows the test conditions and test results.

  The results shown in Table 2 show the following. In Example 1 of the present invention, the Vickers hardness of the weld metal was about 600, and the plug was broken at the outer periphery of the nugget (weld heat affected zone) of the high C steel sheet of the first layer. On the other hand, in Comparative Example 1, the Vickers hardness of the weld metal was about 750, and the brittle fracture occurred in the weld metal (nugget), which was the joining interface between the high C steel plates. Therefore, it was found that according to the welding conditions of Example 1 of the present invention, the hardening of the weld metal can be suppressed and brittle fracture in the weld metal can be avoided.

[Example 2]
A number of high C steel sheets having a thickness of 1.6 mm and many low C steel sheets having a thickness of 1.4 mm were prepared as test plate materials. Through holes were formed in some of the high C steel sheets in the same manner as in Example 1 above. The C content of the high C steel sheet was 0.55% by mass. The C content of the low C steel sheet was 0.15% by mass.

  As Example 2 of the present invention, a three-layer plate assembly in which a low-C steel sheet without holes, a high-C steel sheet with holes, and a low-C steel sheet without holes were sequentially stacked was prepared. As Comparative Example 2, a two-layer structure plate set in which a holeless high-C steel plate and a holeless low-C steel plate were sequentially stacked was prepared, and spot welding was performed.

  In each spot welding, the welding current was variously changed in the same manner as in Example 1 described above. The common welding conditions and evaluation method are the same as in the first embodiment.

  The results shown in Table 3 show the following. In Example 2 of the present invention, the Vickers hardness of the weld metal was about 450, and the plug was broken at the outer periphery of the nugget of the low-C steel sheet of the first layer. On the other hand, in Comparative Example 2, the Vickers hardness of the weld metal was about 670, and the brittle fracture occurred in the weld metal, which was the joining interface between the high C steel sheet and the low C steel sheet. Therefore, according to the welding conditions of Example 2 of the present invention, it was found that hardening of the weld metal can be suppressed, and brittle fracture in the weld metal can be avoided.

[Example 3]
A number of TWIP steel plates having a thickness of 1.4 mm and low alloy steel plates having a thickness of 1.4 mm were prepared as test plate materials. In some of the low alloy plates, through holes were formed as in Example 1 above. For the TWIP steel sheet, the C content was 0.6% by mass, the Si content was 0.3% by mass, and the Mn content was 20%. For the low alloy steel sheet, the C content was 0.2% by mass, the Si content was 0.05% by mass, and the Mn content was 1.2% by mass.

  As Example 3 of the present invention, a plate set having a three-layer structure in which a non-perforated TWIP steel sheet, a perforated low alloy steel sheet, and a perforated TWIP steel sheet were sequentially stacked was prepared. As Comparative Example 3, a two-layer plate assembly in which a holeless TWIP steel plate and a holeless low alloy steel plate were sequentially stacked was prepared, and spot welding was performed.

  In each spot welding, the welding current was variously changed in the same manner as in Example 1 described above. The common welding conditions and evaluation method are the same as in the first embodiment.

  The results shown in Table 4 show the following. In Inventive Example 3, the Vickers hardness of the weld metal was about 410, and the plug was broken at the outer periphery of the nugget of the TWIP steel sheet of the first layer. On the other hand, in Comparative Example 3, the Vickers hardness of the weld metal was about 720, and brittle fracture occurred in the weld metal, which was the joining interface between the TWIP steel sheet and the low alloy steel sheet. Therefore, according to the welding conditions of Example 3 of the present invention, it was found that hardening of the weld metal could be suppressed and brittle fracture in the weld metal could be avoided.

  INDUSTRIAL APPLICABILITY The present invention can be effectively used for spot welding of any material.

1: plate material of first layer (outermost layer) 2: plate material of second layer (inner layer)
2a: through hole, 3: plate material of third layer (outermost layer), 4: nugget,
10: Laminated part of plate assembly, 20: Electrode

Claims (6)

A resistance spot welding method for joining plate materials,
A preparing step of preparing a plate set including a laminated portion in which plate materials are stacked on at least three layers;
A welding step of sandwiching the laminated portion of the plate set between a pair of electrodes, applying a current between the electrodes while pressing the laminated portion with the electrodes, and performing resistance spot welding on the laminated portion,
In the preparing step, as the laminated portion, a plate material of the same metal as each other is arranged in the outermost layer, and a plate material having the same kind of material as the material of the outermost layer plate material and having a through hole is arranged in the inner layer. ,
Wherein in the welding step, and facilities the position and the resistance spot welding in a state where the position-matched to the electrodes of the through-holes in the inner layer of the plate material,
The resistance spot welding method , wherein a material of the outermost layer plate material is a low C content steel, and a material of the inner layer plate material is a high C content steel . A resistance spot welding method for joining plate materials,
A preparing step of preparing a plate set including a laminated portion in which plate materials are stacked on at least three layers;
A welding step of sandwiching the laminated portion of the plate set between a pair of electrodes, applying a current between the electrodes while pressing the laminated portion with the electrodes, and performing resistance spot welding on the laminated portion,
In the preparing step, as the laminated portion, a plate material of the same metal as each other is arranged in the outermost layer, and a plate material having the same kind of material as the material of the outermost layer plate material and having a through hole is arranged in the inner layer. ,
In the welding step, resistance spot welding is performed in a state where the position of the through hole and the position of the electrode in the inner layer plate material are matched,
The resistance spot welding method, wherein a material of the outermost layer plate material is a high alloy steel. The resistance spot welding method according to claim 1 or 2 ,
A resistance spot welding method, wherein the inner layer plate has an insulating coating. The resistance spot welding method according to any one of claims 1 to 3 ,
A resistance spot welding method, wherein one of the outermost layer plate members is a backing plate. The resistance spot welding method according to any one of claims 1 to 4 ,
A resistance spot welding method, wherein a contour shape of the through hole in the inner layer plate material is circular or square, and a diameter or a side length of the through hole is larger than a target nugget diameter. The resistance spot welding method according to any one of claims 1 to 5 ,
The resistance spot welding method, wherein each thickness of the outermost layer plate is 3.0 mm or less, and an entire thickness of the inner layer plate is 2.4 mm or less.

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