JP3922263B2 - Method of manufacturing resistance spot welded joint - Google Patents

Description

  The present invention relates to a resistance spot welding method which is a kind of lap resistance welding method, and in particular, a plate assembly in which a plurality of thin metal plates (materials to be welded) are overlapped is desired by a resistance spot welding method without causing any scattering. The present invention relates to a resistance spot welded joint manufacturing method in which a size nugget is formed to form a resistance spot welded joint.

  In general, a resistance spot welding method, which is a kind of a lap resistance welding method, is used for joining metal plates that are overlapped. For example, in the manufacture of automobiles, several thousand spots are welded per vehicle. This welding method is a method in which two or more metal plates are overlapped, and the surface is directly sandwiched between the upper and lower electrodes and a large current is applied between the upper and lower electrodes for a short time to join them. is there. A spot-like weld is obtained by utilizing resistance heat generated by passing a large welding current. This spot-like weld is called a nugget and is a part where both metal plates melt and solidify at the contact points of both metal plates when current is passed through both metal plates. Are joined together.

  Since the joint strength of resistance spot welds depends on the nugget diameter, it is important to ensure a nugget diameter of a predetermined diameter or greater, especially when high joint strength is required for automobile parts and the like. . In general, when the applied pressure and energization time are constant, the nugget diameter gradually increases as the welding current increases, but when the value exceeds a certain value, a phenomenon of scattering of molten metal between the metal plates occurs. . The occurrence of scatter is dangerous and scatters around the weld and deteriorates the appearance, causing variations in the nugget diameter and joint tensile strength, resulting in unstable joint quality.

  In order to deal with such a problem, for example, in Patent Literature 1, two or more thin steel plates are sandwiched between rod-shaped electrodes at an arbitrary pressure, and the previous energization is performed to apply a current that causes one or more scatterings. After that, a thin steel sheet lap resistance spot welding method has been proposed in which an arbitrary cooling time is given and subsequently energized late using a current that does not cause scattering. The technique described in Patent Document 1 accepts the occurrence of scattering and compresses and shrinks the generated cavity during the latter energization, but the problems due to the occurrence of scattering remain.

  Patent Document 2 proposes a resistance spot welding method that prevents the occurrence of a doughnut-shaped nugget in which a nugget exists only on the outer periphery of the weld. In the technique described in Patent Document 2, the applied pressure and energization amount (welding current value x energization time) are set so that the minimum thickness of the welded portion after welding does not fall below 80% of the total thickness of the stacked steel plates. Although resistance spot welding is performed after adjustment, the problem due to the occurrence of scattering cannot be solved.

  Further, in recent years, due to the increasing demand for improving the collision safety of automobile bodies, for example, a structure in which reinforcement is sandwiched between a floor panel and a member constituting a floor portion of a vehicle has been adopted. Yes. In this structure, unlike the conventional simple two-ply steel plate that is spot-welded, it is required that three or more steel plates be superposed and spot-welded.

  Furthermore, recently, with the demand for further improvement in the collision safety of the car body, the reinforcement and the thickening of reinforcement etc. have progressed, and a floor panel (thin plate) with a thin plate thickness has been placed on the outside, and the plate on the inside It is often necessary to spot weld a plate assembly that combines thick members and reinforcements (thick plates). In addition, suppose that a thin plate is described as a thin plate, and a relatively large thickness is described as a thick plate among metal plates assembled in a plate, and the following is also used. The same description is used.

  In such a plate assembly having a large plate thickness ratio (= total thickness (A) / thickness of the thinnest plate (B)), the pressure and welding current are kept constant as in the conventional case. It is known that when spot welding is performed, it is difficult to form a nugget of a necessary size between the thin plate and the thick plate on the outermost side (the side in contact with the electrode tip). This tendency is particularly strong when the plate thickness ratio exceeds 5.

  This is because the temperature hardly rises between the outermost thin plate and the thick plate due to cooling by the electrode tip. Nugget is formed by volume resistance heat generation due to the specific resistance of steel material from the center between the electrodes, but before the nugget grows to a thin plate, between the thick plate and the thick plate located near the center between the electrodes The nugget grows at a large distance, and it is difficult to obtain a nugget of a necessary size between the thin plate and the thick plate without the occurrence of scattering, because the nugget grows greatly and cannot be suppressed by pressing with an electrode.

  Further, when the thin plate disposed on the outermost side is a floor panel, formability is more important than strength, so that the steel plate used is often mild steel. On the other hand, a thick steel plate is a strength reinforcing member, and a high-tensile steel plate is often used. In such a plate assembly, the position where heat is generated is biased toward the high-tensile steel plate having a high specific resistance, so that nuggets are more difficult to be formed between the thick plate and the thin plate (mild steel). Moreover, when the steel plate used becomes a plated steel plate, the plating layer melted at a low temperature expands the current-carrying path between the steel plates, thereby reducing the current density and making it more difficult to form a nugget on the thin plate side.

In order to deal with such a problem, for example, Patent Document 3 proposes a method of spot welding a plate assembly having a large plate thickness ratio in which a thin plate is further overlapped on at least one of two stacked thick plates. The technique described in Patent Document 3 forms a seat surface that is one step higher than the general part at the portion to be welded of the thin plate, and forms an electrode that opposes the thin plate with a spherical tip at the initial stage of welding. This is a spot welding method in which a thin plate and an adjacent thick plate are welded so as to crush the seating surface of the thin plate with a pressing force, and then the two thick plates are welded with a high pressing force. According to this technique, a nugget of a necessary size can be formed between a thin plate and a thick plate without causing scattering.
JP 58-23579 A JP 2003-251470 A JP 2003-71569 A

  In the technique described in Patent Document 3, a nugget of a necessary size can be formed between a thin plate and a thick plate. However, a seat surface that is one step higher than a general portion is formed in advance on a portion to be welded by a press or the like. There is a problem that a process is required, the process becomes complicated, and productivity is lowered.

  The present invention advantageously solves the above-mentioned problems of the prior art, and can form a nugget of a necessary size without adding an extra step even in a plate assembly having a large plate thickness ratio, and without causing scattering. It aims at proposing the manufacturing method of a resistance spot welded joint.

In order to achieve the above-described problems, the present inventors diligently studied various factors affecting nugget formation in resistance spot welding. As a result, a nugget of a necessary size is formed between the thin plate and the adjacent thick plate and between the thick plate and the thick plate in a plate assembly (spot weld joint) having a large plate thickness ratio in which the thin plate is disposed on the outside. (1) In order to form a nugget between a thin plate and a thick plate, the contact resistance heat generation between the metal plates should be used effectively in the initial stage of welding.
(2) In order to suppress the occurrence of scattering, it has been found that it is important to change the conditions of applied pressure and current during welding.

  The present invention has been completed based on the above findings and further studies.

That is, the gist of the present invention is as follows.
(1) In manufacturing a resistance spot welded joint by welding a plate assembly in which a plurality of metal plates are overlapped by resistance spot welding, the plate assembly is thinned on at least one of the two or more thick plates overlapped. And the resistance spot welding is a two-stage welding of the first stage and the second stage, and the second stage welding is the first stage welding. A method of manufacturing a resistance spot welded joint, characterized in that welding is performed with a high applied pressure, a low current or the same current, a long energization time or the same energization time.
(2) In (1), the welding of the first stage is performed with the welding metal pressure P _I , welding current I _I , and energization time T _I of the thinnest metal plate among the plurality of metal plates. The following formulas (1) to (3) in relation to the thickness tm
0.8tm ≦ P _I ≦ 5tm (1)
_{2 ≦ T I ≦ 6 ......... (} 2)
3tm + 5 ≦ I _I (3)
(Where tm is the thickness (mm) of the thinnest metal plate among a plurality of metal plates, P _{I is the} applied pressure (kN), I _{I is the} welding current (kA), T _{I is the} energization time (cycles / 50Hz))
The welding in the second stage is the welding pressure P _II , welding current I _II , energization time T _II of the following equations (4) to (6):
1.1 P _I ≦ P _II ≦ 10 P _I (4)
0.5I _I ≦ I _II ≦ I _I (5)
_{T I ≦ T II ≦ 10T I} ......... (6)
(Where tm is the thickness (mm) of the thinnest metal plate among the plurality of metal plates, P _I and P _{II are the} applied pressure (kN), I _I and I _{II are the} welding current (kA), T _I, T _II: energizing time (cycles / 50Hz))
A method of manufacturing a resistance spot welded joint, characterized in that the welding satisfies the above .

  According to the present invention, even in a large plate assembly in which a thin plate is overlaid on at least one of two or more thick plates that are overlaid and the plate thickness ratio exceeds 5, without adding an extra step, A spot-welded joint having a nugget of a necessary size can be easily produced without generating scatter, and a remarkable industrial effect can be obtained.

  In the present invention, a plate assembly in which a plurality of metal plates are overlapped is sandwiched between a pair of upper and lower electrode tips and welded and joined by resistance spot welding that is pressurized and energized to form a nugget of a required size to form a resistance spot welded joint And

  A welding apparatus that can be suitably used in the present invention includes a pair of upper and lower electrode tips, and a pressurizing mechanism (such as an air cylinder or a servo motor), as long as the pressurizing force and welding current can be arbitrarily controlled during welding, The type (stationary, robot gun, etc.), electrode shape, etc. are not particularly limited.

  In the present invention, the resistance spot welding is a welding consisting of two stages of a first stage and a second stage.

  An example in which spot welding is performed on a metal plate assembly in which thin plates 11 are superimposed on the outside of two or more thick plates 12 and 13 that are superimposed as shown in FIG. 1 will be described below.

  First, the plate assembly is sandwiched between a pair of upper and lower electrodes at a desired welding position, and pressurization is started. Start energization after pressure is applied. In the first stage welding, the nugget N1 is formed between the metal plates 11 and 12 by setting the pressure and welding current so that the contact resistance heat generation does not become small. In the first stage welding, it is preferable to apply a large welding current in a short time with a low pressure. As a result, the current path between the metal plates 11 (thin plate) and 12 (thick plate) is narrow and the current density is high, and there is little effect of expansion of the current path due to melting of the plating, etc., and the generated contact resistance heat is effectively nuggeted. It becomes possible to act on N1 formation.

In the first stage welding, the pressure P _I (kN) is related to the thickness tm of the thinnest metal plate among the plurality of metal plates (in FIG. 1, the thickness of the metal plate 11: mm). Next formula (1)
_{0.8tm ≦ P I ≦ 5tm ......... (} 1)
It is preferable to set so as to satisfy the above. The pressure P _I in the welding of the first stage is 5TM (kN) or more, pressure becomes too high, heat generation due to contact resistance is reduced, not nugget is formed between the metal plates 11. On the other hand, if the pressure P _I is less 0.8tm the contact resistance between the electrode tip and the metal plate 11 is increased, along with the spark is likely to occur, expulsion from between the metal plate 11 and 12 It tends to occur.

Moreover, in the welding of the first stage, the energization time _{T I} (cycles / 50Hz), the following equation (2)
2 ≦ T _I ≦ 6 (2)
It is preferable to set so as to satisfy the above. Energization time T _I is, for too short energization time is less 2 cycles, the desired size of the nugget is not formed between the metal plates 11. On the other hand, when longer than 6 cycles, scattering occurs.

In the first stage welding, the welding current I _I (kA) is related to the thickness tm of the thinnest metal plate among the plurality of metal plates (the thickness of the metal plate 11 in FIG. 1 is mm). Then, the following equation (3)
3tm + 5 ≦ I _I (3)
It is preferable to set so as to satisfy the above. Welding current I _I in the welding of the first stage, the (3tm + 5) following a small current, can not be effectively utilized contact resistance heating, no longer nugget is formed between the metal plates 11. In the first stage welding, it is preferable to pass a large current for the initial several cycles.

For this reason, in the welding of the first stage, the above-mentioned (1), (2), (3) the pressure P _I to satisfy the _equation, the welding current I _I, setting the energization time T _I Is preferable for forming a nugget of a desired size between the metal plates 11 and 12.

  In the present invention, the second stage welding is performed following the first stage welding described above. The second stage welding is a welding with a high pressure, a low current, and a long energization time as compared with the first stage welding. The pressurization pattern in resistance spot welding of the present invention is schematically shown in FIG. 3, and the energization pattern is schematically shown in FIG. In the present invention, during welding (after the completion of the first stage welding), the pressurizing force is increased, the welding current is decreased, and the energization time is lengthened as compared with the first stage welding. As a result, the occurrence of scattering is suppressed, and heat generation by volume resistance heat generation is mainly performed, and a nugget is formed at the center between the electrodes. As shown in FIG. N2 can be formed.

In the second stage welding, the pressure P _II is related to the pressure P _{I in} the first stage welding.
1.1 P _I ≦ P _II ≦ 10 P _I (4)
(Where P _I , P _II : pressure (kN))
It is preferable to set so as to satisfy the above. Further, the welding current I _II is related to the welding current I _I of the first stage welding.
0.5I _I ≦ I _II ≦ I _I (5)
It is preferable to set so as to satisfy the above. Further, the energization time T _II, in relation to the energization time T _I of the first stage welding, the following equation (6)
_{T I ≦ T II ≦ 10T I} ......... (6)
It is preferable to set so as to satisfy the above. If the conditions of the second stage welding are out of the above range, it will be difficult to prevent the occurrence of scattering and obtain a nugget diameter of a predetermined size, and if the applied pressure is excessively increased, the heat mark and lift will increase. Also occurs.

Method for producing a resistance spot welded joint of the present invention is not limited to the illustrated plate assembled in FIG. Moreover, a steel plate can be illustrated as a metal plate which applies this invention as a to-be-welded material. The steel plate is not limited to the strength level (soft steel, high-tensile steel plate) or the presence or absence of surface treatment (no surface treatment, plated steel plate). The present invention is applicable to any type of steel sheet. Further, the present invention is a plate thickness ratio (= SoitaAtsu mm / thickness mm of the thinnest plate) is applied to the case of 5 or more Itakumi.

Example 1
Resistance spot welding was carried out under the welding conditions shown in Table 2 on the plate assembly in which 2 to 4 thin steel sheets shown in Table 1 were overlapped to produce a resistance spot welded joint. Resistance spot welding was performed using a stationary, servo-motor pressurization type single-phase AC resistance spot welding machine. The electrode used was a DR type (tip diameter: 6 mm) electrode tip (FIG. 5).

  For each of the obtained welded joints, the welded portion was cut, the cross section (see FIG. 2) was etched, and then observed with an optical microscope, the nugget diameter d (mm) was measured, and the nugget diameter d was 4√t or more (t : The case of the thickness (mm) of the thinner steel plate of two adjacent steel plates was evaluated as good (◯). A nugget diameter d of less than 4√t was regarded as defective (x). The obtained results are also shown in Table 2. However, when the number of plate assemblies is 3 or more, for example, a plurality of nugget diameters are measured as N1 and N2 in FIG. In this case, the case where all nugget diameters satisfy 4√t or more was evaluated as “Good”.

In all of the examples of the present invention, there is no occurrence of scattering, a nugget having a diameter of 4√t or more is obtained, and a good spot welded joint is obtained. In the example of the present invention, spot welding having a nugget of a necessary size can be performed without generating scattering even when the plate thickness ratio is 5 or more. On the other hand, in the comparative example outside the scope of the present invention, scattering occurs or a nugget having a sufficient size is not formed.
(Example 2)
About the welding conditions evaluated as ○ in Example 1, the appropriate welding current range was investigated. In the appropriate welding current range, the welding conditions of the first stage or the second stage are fixed, the welding current in the second stage or the first stage is changed, and the nugget diameter d is 4√t or more (t: adjacent) Obtain the limit current value (lower limit current value) and the scatter occurrence limit current value (upper limit current value) that will be the thickness (mm) of the thinner one of the two steel sheets, and calculate the difference between the appropriate welding current range did.

  The obtained results are shown in Table 3 (Appropriate welding current range for second stage welding) and Table 4 (Appropriate welding current range for first stage welding).

  The resistance spot welding conditions (examples of the present invention) shown in Example 1 both have an appropriate welding current range of 2 kA or more even in the first stage and second stage welding, and resistance spot welding work is facilitated. I understand.

It is explanatory drawing which shows typically the formation condition of the nugget at the time of the 1st stage welding in the resistance spot welding of this invention. It is explanatory drawing which shows typically the formation condition of the nugget at the time of the second stage welding in the resistance spot welding of this invention. It is explanatory drawing which shows typically the pressurization pattern in the resistance spot welding of this invention. It is explanatory drawing which shows typically the electricity supply pattern in the resistance spot welding of this invention. It is sectional drawing which shows the shape of an electrode tip typically. It is sectional drawing which shows typically the board assembly used in the Example.

Explanation of symbols

11, 14 Metal plate (thin plate)
12, 13 Metal plate (thick plate)

Claims (2)

In manufacturing a resistance spot welded joint by welding a plate assembly in which a plurality of metal plates are overlapped by resistance spot welding, the plate assembly is overlapped with at least one of two or more thick plates overlapped. The plate thickness ratio is 5 or more, and the resistance spot welding is a two-stage welding of the first stage and the second stage, and the second stage welding is higher than the first stage welding. A method of manufacturing a resistance spot welded joint, wherein welding is performed with a pressurizing force, a low current or the same current, a long energization time or the same energization time. In the first-stage welding, the welding pressure P _I , welding current I _I , and energization time T _I are related to the thickness tm of the thinnest metal plate among the plurality of metal plates. The welding satisfying the expressions (1) to (3), and the welding of the second stage, the welding pressure P _II , the welding current I _II and the energization time T _II are the following expressions (4) to (6) The method of manufacturing a resistance spot welded joint according to claim 1, wherein the welding is satisfactory.
Record
0.8tm ≦ P _I ≦ 5tm (1)
2 ≦ T _I ≦ 6 (2)
3tm + 5 ≦ I _I (3)
_{1.1P I ≦ P II ≦ 10P I} ......... (4)
0.5I _I ≦ I _II ≦ I _I (5)
_{T I ≦ T II ≦ 10T I} ......... (6)
Here, tm: the thickness (mm) of the thinnest metal plate among the plurality of metal plates,
P _I , P _II : Applied pressure (kN),
I _I , I _II : welding current (kA),
T _II : Energizing time (cycles / 50Hz)

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