JP2008161878A - Lap resistance spot welding method - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide a lap resistance spot welding method capable of obtaining requested penetration even on a thinner steel sheet side and free from occurrence of scattering even under the constant pressure during the welding when performing the lap resistance spot welding of three or more steel sheets. <P>SOLUTION: An assembly of three or more lapped steel sheets is held by a pair of welding electrodes, and contact parts of the steel sheets are welded by running the current under the pressure. The thinnest steel sheet 3 is a bare or plated steel sheet having the thickness of 0.5-1.0 mm and the strength of 270-980 MPa while a phosphate-treated coating film, a chromate-treated coating film, an organic coating film or an inorganic coating film is deposited at least on a lap surface, the contact resistance is 50-500 mΩ, the mean roughness Ra of a lapped surface is 1.0-3.0 μm, or 1-10 projections per 10 mm<SP>2</SP>having the height of 0.1-1.2 mm are provided on the lapped surface. The steel sheets are set so that the thinnest steel sheet 3 is brought into contact with one electrode. <P>COPYRIGHT: (C)2008,JPO&INPIT

Description

  The present invention relates to a lap resistance spot welding method in which a plurality of steel plates are overlapped and spot-welded by resistance welding, and in particular, a plate assembly in which three or more steel plates are overlapped is sandwiched between a pair of electrodes and pressed. The present invention relates to a lap resistance spot welding method in which energization is performed while welding.

  The lap resistance spot welding method is a welding method in which a pair of electrodes is sandwiched between a pair of electrodes and energized while being pressed with this pair of electrodes to join the materials to be joined together. Due to the resistance heat generated by energization, a spot-like melted portion (nugget) is formed at the contact portion of the material to be joined.

  Welding machines that perform such lap resistance spot welding methods include power supply systems, single-phase AC systems, single-phase and three-phase rectification systems, inverter DC systems, and capacitor systems. From the viewpoint of power saving, inverter type DC resistance spot welding is becoming mainstream. However, the direct current resistance welding method has a problem that the electrode life is shorter than the alternating current resistance welding method. Therefore, conventionally, in order to extend the electrode life, when the zinc-based single-side surface-treated steel sheets are overlapped and welded together, the direct current is arranged in such a manner that the positive electrode is in contact with the zinc-based surface-treated surface and the negative electrode is in contact with the non-surface-treated surface. A method of performing resistance spot welding has been proposed (see, for example, Patent Document 1).

  However, when three or more materials to be joined are welded by the conventional lap resistance spot welding method as described in Patent Document 1, when a thin plate is arranged on the outermost side, the thin plate and the adjacent thick plate There is a problem that a good nugget is not formed between them and a sufficient bonding strength cannot be obtained. Further, if the welding conditions are set so that the nugget grows to the outermost thin plate, the nugget formed between the thick plates on the inner side grows too much and scattering occurs.

  Therefore, conventionally, a method has been proposed for obtaining sufficient joint strength when three or more materials to be joined are subjected to resistance spot welding (see, for example, Patent Documents 2 to 6). For example, in Patent Document 2, when a thin plate is overlapped and welded on two thick plates, a convex portion is formed at a portion to be welded of the thin plate, and the convex portion is crushed with a low pressure in the initial stage of welding. A spot welding method is disclosed in which a thin plate and a thick plate are welded, and then two thick plates are welded with high pressure. Further, in the spot welding method described in Patent Document 3, when a thin plate with low rigidity is superimposed on a thick plate with high rigidity and welded, the tip diameter of the electrode tip that contacts the thin plate is brought into contact with the thick plate. By making it smaller than the tip diameter of the electrode tip, the contact area between the thin plate and the electrode tip is made smaller than the contact area between the thick plate and the electrode tip, thereby improving the welding strength.

  On the other hand, in the spot welding method described in Patent Document 4, the bonding strength is improved by changing the applied pressure between the thin plate side and the thick plate side. In order to prevent the occurrence of this, spot welding is performed in two stages, and the second stage welding is performed with a higher applied pressure, lower current or the same current, longer energization time or the same energization time than the first stage welding. Further, in the resistance spot welding methods described in Patent Documents 5 and 6, even if the plate pressure ratio is a plate assembly, an electrode tip that contacts a thick metal plate for the purpose of forming a nugget of a required size without occurrence of scattering. The tip of the electrode is a curved surface having a radius of curvature larger than the radius of curvature of the tip of the electrode tip in contact with a flat or thin metal plate, spot welding is performed in two stages, and the second stage welding is higher than the first stage welding. It is done with pressure.

Conventionally, cold rolled steel sheets and aluminized steel sheets that have specified surface roughness have been proposed for the purpose of improving weldability in spot welding (see, for example, Patent Documents 7 to 9). In the cold-rolled steel sheet, the relationship between the surface roughness (SRa) and the yield stress (YS) is SRa ≧ (32.4 / YS) −1.1, and the convex area ratio of the surface The SSr is 60% or less, and the average area SGr per protrusion is 2 × 10 ⁴ μm ² or more. Further, the aluminum-plated steel sheet described in Patent Document 8, the surface roughness of the plating layer is 3μm or less, by performing further post-treatment coating 2 g / m ² or less, and the contact resistance to 0.05~12mΩ . Further, in the aluminum-plated steel sheet described in Patent Document 9, the surface roughness Ra of the intermetallic compound layer formed on the surface is set to 0.5 μm or more.

JP-A-4-94877 JP 2003-71569 A JP 2003-251468 A JP 2003-251469 A JP 2005-262259 A JP 2006-55898 A Japanese Unexamined Patent Publication No. Sho 63-317648 JP 2000-273609 A JP 2004-2932 A

  However, the above-described conventional technology has the following problems. That is, in the spot welding method described in Patent Document 2 described above, it is necessary to provide a convex portion in advance at the location to be welded on the thin plate side, the welding location is limited thereby, and more precisely at the center of the convex portion. There is a problem in that the manufacturing cost increases because the accuracy of contacting the electrodes is required. Further, in the spot welding method described in Patent Document 3, since the contact area of the electrode in contact with the thin plate side is small, the current density is relatively large, so that the sheet separation (plate lift) on the thin plate side is large. This causes the problem that the finished accuracy of the product is deteriorated. Furthermore, since the electrode on the thin plate side has a high current density, the electrode is significantly fouled and worn. As a result, the electrode must be frequently dressed or replaced, resulting in problems such as production delays and increased costs. Occur frequently. Further, in the spot welding method described in Patent Document 4, the applied pressure of the electrode that contacts the thin plate side is made smaller than the applied pressure of the electrode that contacts the thick plate side, thereby reducing the contact resistance value on the thick plate side. The contact resistance value on the side is controlled so as to decrease, and heat generation is promoted.To that end, a servo motor and a gun controller that operates the servo motor are used to apply a force that pushes up the gun body from the bottom. In addition, as for the welding machine, a stationary spot welding machine cannot be applied, and it is limited to a robot type spot gun. There is a problem that the cost increases.

  On the other hand, in the spot welding methods described in Patent Documents 5 and 6, in order to carry out a welding process consisting of a first stage and a second stage, the spot welding machine is generally not provided in a short time. There is a problem that it is necessary to provide a variable pressure mechanism that operates, and the equipment becomes expensive. Also, in these welding methods, the nugget is formed on the thin plate side in the first stage, and then the nugget is formed on the thick plate side in the second stage without being scattered. It is necessary to apply the conditions and to apply a long-time energization or the same energization time condition at a low current or the same current. For this reason, although the welding has already been completed in the first stage welding, a large pressure and excessive heat input and load are generated in the second stage welding on the thin plate side which is still in a high temperature state. As a result, the indentation on the thin plate side becomes large, and there is a problem that the degree of deformation as a product becomes large. In particular, in the welding method described in Patent Document 6, this tendency increases because the radius of curvature of the thin plate side electrode is small. Further, in the welding methods described in Patent Documents 5 and 6, the electrode on the thin plate side is heavily soiled and worn, and frequent dressing or replacement of the electrode is required. .

  Moreover, the steel plate described in Patent Document 7 is limited to a cold-rolled steel plate and is difficult to apply to a plated steel plate. Furthermore, regarding the steel sheet characteristics, in the range of the technique of increasing the yield stress of the steel sheet and increasing the surface roughness, when the plate assembly is such that the thinnest steel sheet contacts the electrode in multiple stacks of three or more, An effective nugget formation effect on the thinnest steel plate cannot be expected, and Patent Document 7 does not mention or disclose such cases and effects.

The technique described in Patent Document 8 is an aluminum-plated steel sheet having a surface roughness of 3 μm or less, a post-treatment film of 2 g / m ² or less, and a contact resistance value of 0.05 to 12 mΩ. The technology described in 1 is a technology in which the surface roughness Ra of a metal compound mainly composed of aluminum is 0.5 μm or less, both of which are limited to aluminum-plated steel plates, and cannot be applied to cold-rolled steel plates. It is. In general, aluminum-plated steel sheets have a large plating thickness and large electrical conductivity, making it difficult to perform multi-layer welding of three or more layers. In this case, the effect of forming an effective nugget cannot be expected at all. In addition, Patent Documents 8 and 9 do not describe anything about application to multi-layer welding of three or more sheets. Therefore, in the steel plates described in Patent Documents 8 and 9, when three or more steel plates are overlapped and resistance spot welded, if a thin steel plate is arranged on the outermost side, a good nugget is not formed between adjacent steel plates, There is a problem that sufficient bonding strength cannot be obtained.

  The present invention has been made in view of the above-described problems. When three or more steel plates are subjected to resistance spot welding, even if the welding pressure is constant, the thin steel plate side is also provided. It is an object of the present invention to provide a lap resistance spot welding method in which necessary penetration is obtained and no scatter occurs.

  The lap resistance spot welding method according to the first invention of the present application is a method in which a plate assembly in which three or more steel plates are overlapped is sandwiched between a pair of welding electrodes and energized while being pressed to weld the contact points of each steel plate. In the resistance spot welding method, the steel plate having the thinnest thickness among the steel plates is assembled and welded so as to be in contact with one of the electrodes. Bare steel plate having a contact resistance value of 50 to 500 mΩ, having a thickness of 270 to 980 MPa, a phosphate treatment film, a chromate treatment film, an organic film or an inorganic film formed on at least the overlapping surface Or it is set as a plated steel plate.

  In the lap resistance spot welding method according to the second invention of the present application, a plate assembly in which three or more steel plates are overlapped is sandwiched between a pair of welding electrodes, and a contact point of each steel plate is welded by energizing while pressing. In the resistance spot welding method, the steel plate having the thinnest thickness among the steel plates is assembled and welded so as to be in contact with one of the electrodes. .5 to 1.0 mm, strength is 270 to 980 MPa, average roughness Ra of the overlap surface is 1.0 to 3.0 μm, and at least the overlap surface is a phosphate-treated film and chromate treatment. A film, an organic film, or an inorganic film is formed.

The lap resistance spot welding method according to the third invention of the present application is a method in which a plate assembly in which three or more steel plates are overlapped is sandwiched between a pair of welding electrodes and energized while being pressed to weld the contact points of each steel plate. In the resistance spot welding method, the steel plate having the thinnest thickness among the steel plates is assembled and welded so as to be in contact with one of the electrodes. 0.5 to 1.0 mm, strength is 270 to 980 MPa, and a phosphate treatment film, chromate treatment film, organic film or inorganic film is formed on at least the overlapping surface, and the height is 0.1 to 1 A bare steel plate or a plated steel plate having 1 to ² projections of ² mm per 10 mm ² is characterized.

  The protrusion in the third invention of the present application described above can be, for example, a button shape, a cone shape, an elliptical shape, or a square shape.

  In the first to third inventions of the present application described above, welding may be performed by pulsation energization or multistage energization.

According to the present invention, when the steel sheet having the thinnest plate thickness is assembled and welded so as to contact one of the electrodes, the overlapping surface of the steel plates having the thinnest plate thickness has a contact resistance value of 50 to 500 mΩ or an average. The roughness Ra is set to 1.0 to 3.0 μm, or 1 to 10 protrusions having a height of 0.1 to 1.2 mm are provided per 10 mm ² on the overlapping surface, and further, the phosphate treatment film is provided on the overlapping surface. In addition, since the chromate-treated film, organic film or inorganic film is formed, it is possible to spot weld three or more steel plates without generating scattering even if the applied pressure during welding is constant. It is possible to form a penetration enough to obtain a sufficient bonding strength on the thin steel plate side.

  The best mode for carrying out the present invention will be described below in detail with reference to the accompanying drawings. First, a lap resistance spot welding method (hereinafter also simply referred to as a welding method) according to a first embodiment of the present invention will be described. FIG. 1 is a diagram schematically showing the lap resistance spot welding method of the present embodiment. As shown in FIG. 1, in the lap resistance spot welding method of the present embodiment, first, the three steel plates 1, 2, 3 having different thicknesses are arranged so that the steel plate 3 having the smallest thickness is on the outside. That is, the thin steel plates 3 are overlapped so as to contact the electrodes 5 or 6. The plate assembly 4 composed of the three steel plates 1, 2, 3 is held between the welding electrodes 5, 6 and is energized and welded while being pressed.

  At this time, the thinnest steel plate 3 has a thickness t of 0.5 to 1.0 mm, a strength of 270 to 980 MPa, and a phosphate treatment film, a chromate treatment film, an organic film, an inorganic film or the like on at least the overlapping surface. The surface coating 3a made of the steel plate is formed, and the contact resistance value with the adjacent steel plate 2 is 50 to 500 mΩ. In addition, when the surface film 3a is not formed on the overlapping surface, the contact resistance value cannot be in the range of 50 to 500 mΩ.

  Moreover, the contact resistance value prescribed | regulated here is the value measured by the method shown below. FIG. 2 is a diagram schematically showing a method for measuring contact resistance. As shown in FIG. 2, the contact resistance value is obtained when a plate current 4 is sandwiched between CF-type electrodes 15 and 16 having an electrode tip diameter of 4 mm, and a direct current of 1 A is applied while pressing with a pressure of 1.96 kN. It calculated | required by calculation from the voltage V between the steel plates 2 and 3 of this. When the contact resistance measured under these conditions is less than 50 mΩ, a good nugget cannot be obtained at the joint between the steel plate 3 and the steel plate 2, and when the welding conditions are set so that the nugget grows on the steel plate 3, The nugget formed at the joint between the steel plate 1 and the steel plate 2 grows too much and scattering occurs. On the other hand, if the contact resistance value exceeds 500 mΩ, the electrodes 5 and 6 are not energized and cannot be welded.

  When the usual spot welding technique is applied, the thinnest steel plate 3 is cooled by the electrode in contact with it, and it is difficult to form a sufficiently large nugget on the steel plate 3 side. In the form of the overlap resistance spot welding method, a phosphate treatment film, a chromate treatment film, an organic film or an inorganic film is formed on the overlap surface of the thinnest steel plate 3, and the contact resistance value is 50 to 500 mΩ. The thinnest steel plate 3 can be easily cooled by the electrode, that is, even if the thinnest steel plate 3 is arranged so as to be in contact with the electrode 5 or 6, a good nugget can be formed on the thinnest steel plate 3. It becomes possible. Furthermore, a nugget can be formed in which sufficient bonding strength can be obtained without causing scattering even if the welding pressure is not changed.

  In the lap resistance spot welding method of the embodiment, the welding of a plate assembly in which three steel plates are overlapped is described as an example, but the present invention is not limited to this, and three or more sheets are used. If it is the lap resistance spot welding method of this steel plate, the effect mentioned above will be acquired. Further, the thinnest steel plate 3 may have a surface film such as a phosphate-treated film, a chromate-treated film, an organic film, or an inorganic film formed on both surfaces. Further, the welding method is not particularly limited, and welding can be performed by pulsation energization and multistage energization.

  Next, a lap resistance spot welding method according to the second embodiment of the present invention will be described. In the lap resistance spot welding method of the present embodiment, as the steel plate having the thinnest thickness, the plate thickness is 0.5 to 1.0 mm, the strength is 270 to 980 MPa, and the average roughness Ra of one surface is 1.0 to 1.0. For a bare steel plate or plated steel plate having a thickness of 3.0 μm, a surface coating such as a phosphate-treated film, a chromate-treated film, an organic film, or an inorganic film is formed on at least one surface. Then, this one surface is used as an overlapping surface, and the thinnest steel plate is arranged on the outside so as to be in contact with the electrode, as in the welding method of the first embodiment shown in FIG. It is energized and welded while pressing with a welding electrode.

  Thus, in the bare steel plate or the plated steel plate before the surface coating is formed, the average roughness Ra of the surface (one surface) to be the overlapping surface is 1.0 to 3.0 μm, and further, this overlapping surface is obtained. By forming the surface film on the surface (one surface), the contact resistance value between the thinnest steel plate and the steel plate adjacent thereto can be set to 50 to 500 mΩ. When the average roughness Ra of the surface (one surface) to be the overlapping surface is less than 1.0 μm, the overlapping surface after the formation of the surface film is smooth, and the concentrated resistance in the thinnest steel sheet is reduced, so the heat generation effect is reduced. Reduce. As a result, a good nugget is not formed on the thinnest steel plate. Further, when the welding conditions are set so that the nugget grows on the thinnest steel plate, the nugget formed at the joint portion of the other steel plate grows too much and the scattering occurs. On the other hand, when the average roughness Ra of the surface to be the overlapping surface (one surface) exceeds 3.0 μm, the roll surface may be crushed in a normal process and equipment, so that it can be stably formed. Have difficulty. For this reason, it is necessary to provide a process and an installation separately, and a manufacturing cost increases.

  In addition, the method of making surface roughness Ra of the thinnest steel plate into the range mentioned above is not specifically limited, Well-known methods, such as laser processing and roll processing, can be applied. Moreover, the surface roughness of surfaces other than the overlapping surface in the thinnest steel plate is not particularly limited and can be set as appropriate. Also, the energization method during welding is not particularly limited, and for example, a multistage energization method, pulsation energization, or the like can be applied.

  As described above, in the lap resistance spot welding method of the present embodiment, for the thinnest steel plate, the average roughness Ra of the surface that becomes the overlapping surface in the steel plate before the surface coating is formed is 1.0 to 3.0 μm. In addition, a phosphate-treated film, chromate-treated film, organic film, or inorganic film with a large electrical resistance is formed on the surface to be the superposed surface, facilitating resistance heat generation on the thinnest steel plate side. Sufficient penetration can be obtained. As a result, it is possible to form a nugget having a size that can provide sufficient joint strength without causing scattering even if the pressure during welding is not changed.

  The configuration and effects other than those described above in the lap resistance spot welding method of the present embodiment are the same as those of the welding method of the first embodiment described above. Moreover, in the welding method of this embodiment, the average roughness Ra of the overlapping surface of the thinnest steel sheet is set to 1.0 to 3.0 μm, and further, a phosphate treatment film, a chromate treatment film, an organic film, or an inorganic film is applied. Although it is formed, the effect of promoting nugget growth in the thinnest steel sheet can be obtained if either of the conditions of average roughness Ra or surface coating is satisfied. However, in order to obtain the effect of the present invention that a nugget having a sufficient size can be formed without causing scattering without changing the welding pressure, these conditions are satisfied. It is necessary to satisfy both.

Next, a lap resistance spot welding method according to the third embodiment of the present invention will be described. The lap resistance spot welding method of the present embodiment is a thin steel plate having a thickness of 0.5 to 1.0 mm and a strength of 270 to 980 MPa. A surface coating such as a treatment coating, an organic coating, or an inorganic coating is formed, and a bare steel plate or a plated steel plate provided with 1 to 10 protrusions having a height of 0.1 to 1.2 mm per 10 mm ² is used. Then, the surface on which the protrusions are formed is an overlapping surface, and, like the welding method of the first embodiment shown in FIG. This plate assembly is sandwiched between a pair of welding electrodes and energized while being pressurized and welded.

Thus, the contact resistance value between the thinnest steel plate and the steel plate adjacent to the thinnest steel plate can be set to 50 to 500 mΩ by forming the surface film and the protrusion on the overlapping surface of the thinnest steel plate. Here, when the height of the protrusion is less than 0.1 mm, if the welding is performed with a normal pressure, the protrusion is crushed before energization, and the steel plates are brought into contact with each other in the front surface, so that local effective heat generation cannot be obtained. On the other hand, when the height of the protrusion exceeds 1.2 mm, the protrusion is not sufficiently crushed by a normal pressure. As a result, when welding heat input (for example, welding current) is small, protrusions remain after welding and gaps are formed between the steel plates. When welding heat input is large, severe scattering occurs before the protrusions are crushed. Appropriate welding conditions do not exist or conditions selection becomes difficult. On the other hand, when the number of protrusions per 10 mm ² exceeds 10, the contacts are dispersed and the heat generation effect is reduced. Therefore, the protrusions formed on the thinnest overlapping surface of the steel plates have a height of 0.1 to 1.2 mm and 1 to 10 per 10 mm ² . At that time, if the bottom area per protrusion is less than 0.5 mm ² , the strength of the protrusion itself is lowered, and a sufficient effect may not be obtained. For this reason, it is desirable that the bottom area per protrusion is 0.5 mm ² or more.

  The specific shape of the protrusion can be, for example, a button shape, a cone shape, an elliptical shape, or a square shape. Moreover, the method of forming such protrusions on the steel plate is not particularly limited, and known methods such as punching, coining, roll forming and upsetting can be applied.

As described above, in the lap resistance spot welding method of the present embodiment, since 1 to 10 protrusions having a height of 0.1 to 1.2 mm are provided per 10 mm ² on the overlapping surface of the thinnest steel plate, The welding current concentrates on the protrusions, and the resistance heat generation on the thinnest steel plate side is effectively promoted, and sufficient nugget penetration can be obtained. As a result, it is possible to form a nugget having a size that can provide sufficient joint strength without causing scattering even if the pressure during welding is not changed.

The configuration and effects other than those described above in the lap resistance spot welding method of the present embodiment are the same as those of the welding method of the first embodiment described above. Further, in the welding method of the present embodiment, 1 to 10 protrusions having a height of 0.1 to 1.2 mm are formed per 10 mm ² on the overlapping surface of the thinnest steel plates, and the phosphate treatment film and chromate are formed. Although the treatment film, the organic film or the inorganic film is formed, the effect of promoting the nugget growth in the thinnest steel sheet can be obtained if any of the conditions of the protrusion or the surface film is satisfied. However, in order to obtain the effect of the present invention that a nugget having a size capable of obtaining a sufficient joint strength can be formed without causing scattering even if the pressure during welding is not changed, these conditions are satisfied. It is necessary to satisfy both.

Hereinafter, the effects of the present invention will be specifically described with reference to examples of the present invention and comparative examples that are out of the scope of the present invention. In this example, three steel plates having different thicknesses and strengths were spot-welded by the method shown in FIG. 1, and the presence / absence of scattering and the size of the nugget diameter of the joint were examined. In this case, the steel sheet 1 is an alloyed hot-dip galvanized steel sheet in which the steel sheet 1 is galvanized with an adhesion amount of 45 g / m ² on one side on both sides of a high-strength steel sheet having a thickness of 1.6 mm and a tensile strength of 780 MPa. As the steel plate 2, an alloyed hot-dip galvanized steel plate that was galvanized with an adhesion amount of 45 g / m ² per side on both sides of a high-tensile steel plate with a plate thickness of 2.3 mm and a tensile strength of 590 MPa was used. . The steel plate 3 is an alloyed hot-dip galvanized steel plate that is galvanized on both sides of a steel plate having a thickness of 0.7 mm and a tensile strength of 270 MPa, with an adhesion amount per side of 45 g / m ^2. The surface film shown in Table 1 was formed and the surface treatment shown in Table 1 below was applied. Table 1 below also shows the contact resistance value of each steel plate measured by the method shown in FIG.

On the other hand, as welding conditions, a single-phase AC power source is used as a welding power source, and the electrode has an electrode diameter D of 16 mm, a tip diameter of 6 mm, and a tip R of 40. The pressure was 6.0 kN. Other welding conditions are shown in Table 2 below. Further, as a comparative example, a surface film was formed on a steel sheet 3 by galvanizing an galvanized steel sheet galvanized on both surfaces of a steel sheet having a thickness of 0.7 mm and a tensile strength of 270 MPa with an adhesion amount per side of 45 g / m ^2. In addition, the same method and conditions as in the above-described embodiment were used without applying surface treatment and using a rust-preventive oil coated on both sides and a surface formed with protrusions outside the scope of the present invention. Welding was performed to check for the occurrence of scattering and the size of the nugget diameter at the joint. The above results are also shown in Table 2 below.

As shown in Table 2, as the thinnest steel plate 3, a chromate-treated film or a zinc phosphate-treated film is formed on the surface of a galvanized steel sheet having a surface roughness Ra in the range of 1.0 to 3.0 μm. Example No. using 1-No. 3 test materials, and zinc to form a zinc phosphate coating on the surface of the plated steel sheet, Example No of using what its overlapping faces three of height 0.5mm protrusions / was 10 mm ² provided . In all of the test materials of No. 4, there was no occurrence of scattering, and a sufficiently large nugget was obtained at the joint portion of each steel plate. On the other hand, as the thinnest steel plate 3, the surface roughness Ra is less than the range of the present invention by applying rust preventive oil to both sides of the galvanized steel plate without performing surface film formation and surface processing. Comparative Example No. 5 using Comparative Example No. 5 and Comparative Example No. 5 using a galvanized steel sheet in which a projection having a height of 1.4 mm deviating from the scope of the present invention was formed on the overlapping surface. In the test material No. 6, a sufficiently large nugget was formed between the steel plate 1 and the steel plate 2, but no nugget was formed between the steel plate 2 and the steel plate 3, and scattering occurred.

It is a figure which shows typically the resistance spot welding method which concerns on the 1st Embodiment of this invention. It is a figure which shows typically the measuring method of contact resistance.

Explanation of symbols

1, 2, 3 Steel plate 3a Surface coating 4 Board assembly 5, 6, 15, 16 Electrode

Claims (5)

In a lap resistance spot welding method in which a plate assembly in which three or more steel plates are overlapped is sandwiched between a pair of welding electrodes and energized while being pressed to weld the contact points of each steel plate,
The steel plate having the thinnest thickness is assembled and welded so as to be in contact with one electrode,
The thinnest steel plate has a thickness of 0.5 to 1.0 mm and a strength of 270 to 980 MPa, and a phosphate treatment film, a chromate treatment film, an organic film or an inorganic film is formed on at least the overlapping surface. A lap resistance spot welding method, characterized in that the contact resistance value is a bare steel plate or a plated steel plate having a contact resistance value of 50 to 500 mΩ. In a lap resistance spot welding method in which a plate assembly in which three or more steel plates are overlapped is sandwiched between a pair of welding electrodes and energized while being pressed to weld the contact points of each steel plate,
The steel plate having the thinnest thickness is assembled and welded so as to be in contact with one electrode,
This thin steel plate is a bare steel plate or a plated steel plate having a thickness of 0.5 to 1.0 mm, a strength of 270 to 980 MPa, and an average roughness Ra of the overlapping surface of 1.0 to 3.0 μm. A lap resistance spot welding method characterized in that a phosphate treatment film, a chromate treatment film, an organic film, or an inorganic film is formed on at least the overlapping surface. In a lap resistance spot welding method in which a plate assembly in which three or more steel plates are overlapped is sandwiched between a pair of welding electrodes and energized while being pressed to weld the contact points of each steel plate,
The steel plate having the thinnest thickness is assembled and welded so as to be in contact with one electrode,
This steel plate having the thinnest plate thickness is 0.5 to 1.0 mm and the strength is 270 to 980 MPa, and at least the overlapping surface has a phosphate treatment film, a chromate treatment film, an organic film or an inorganic film. A lap resistance spot welding method, characterized by being formed as a bare steel plate or a plated steel plate having 1 to 10 protrusions having a height of 0.1 to 1.2 mm per 10 mm ² .   The lap resistance spot welding method according to claim 3, wherein the protrusion is a button shape, a cone shape, an elliptical shape, or a rectangular shape.   The lap resistance spot welding method according to any one of claims 1 to 4, wherein welding is performed by pulsation energization or multistage energization.

Images