JP2010240740A - Method of manufacturing resistance spot welded joint - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide a method of manufacturing a resistance spot welded joint by which a nugget with a required size is formed without adding an excessive process and without generating expulsion, even in a sheet assembly with a large sheet thickness ratio including a high strength steel sheet and by which a resistance spot welded joint having a superior tensile strength can be stably obtained. <P>SOLUTION: The method of manufacturing a resistance spot welded joint is characterized in that, in manufacturing a resistance spot welded joint by joining a sheet assembly with a plurality of superimposed metallic sheets by resistance spot welding, a sheet assembly having a sheet thickness ratio of 5 or higher is used as an object and the resistance spot welding is configured to be welding of three steps comprising a first, a second and a third steps, that the welding of the second step is designed to have a higher pressurizing force, a lower or equal electric current, and a longer or equal energizing time compared with the welding of the first step, and that a higher pressurizing force and higher current energization than in the second step are repeated in the third step. <P>COPYRIGHT: (C)2011,JPO&INPIT

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 to join metal plates that are overlapped with each other. 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.

  Also, in recent years, due to the increasing demand for improved 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 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, reinforcement and other high-strength and thickening have progressed, and a thin floor panel (thin plate) 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.

  When spot welding is performed in such a plate assembly with a large thickness ratio (= total thickness / thickness of the thinnest plate) as in the past, with the pressure and welding current kept constant. Is known to be 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 pressurization with an electrode.

  Moreover, when the thin plate arrange | positioned on the outermost side is a floor panel, since a moldability becomes more important than intensity | strength, the steel plate used often becomes 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 1 proposes a method of spot welding a plate assembly having a large plate thickness ratio in which a thin plate is further overlapped with at least one of two stacked thick plates. The technique described in Patent Document 1 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, and is low in 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.

  However, in the technique described in Patent Document 1, a nugget of a necessary size can be formed between a thin plate and a thick plate, but 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.

  In order to solve this problem, Patent Document 2 discloses that a plate assembly of metal plates is a plate assembly having a plate thickness ratio of 5 or more, in which a thin plate is superimposed on at least one of two or more thick plates that are overlapped, and resistance. Spot welding is a welding consisting of two stages, the first stage and the second stage. Compared to the first stage welding, the second stage welding has a higher pressure, lower current or the same current, longer energization time or the same energization time. By using welding, a nugget of a necessary size can be formed without adding an extra process and without generating a scatter even in a plate assembly having a large thickness ratio.

  However, the resistance spot welded joint obtained by the technique described in Patent Literature 2 may have the following problems when a high strength steel plate (high strength steel plate) is included in the plate assembly.

  In general, the tensile strength of the resistance spot welded joint obtained is determined by JIS as a strength index (strength when the tensile test is performed in the shear direction of the joint) and cross tensile strength (the tensile test is performed in the direction of peeling of the joint). Strength when) is important.

  The tensile shear strength of spot welds in high-strength steel sheets tends to increase as the tensile strength of steel sheets increases, while the cross tensile strength hardly increases regardless of the increase in tensile strength of steel sheets, but decreases. Sometimes. The reason for this is that, in order to achieve the strength of the high-strength steel sheet, the carbon equivalent Ceq represented by the following formula must be large, and in addition, since welding is a rapid and rapid quenching phenomenon, This is probably because the hardness increases in the affected area and the toughness decreases.

Ceq = C + 1/24 × Si + 1/6 × Mn (%)
Here, C, Si, and Mn indicate mass% of each.

  In order to solve this problem, from the viewpoint of the welding method, an increase in the number of hit points and an increase in the nugget diameter can be considered. However, an increase in the number of hit points requires a space, and the strength decreases even if the number of hit points is increased. There is a tendency. Furthermore, in order to enlarge the nugget diameter, the electrode must be enlarged or the applied pressure must be increased, which is also limited by the apparatus.

  Accordingly, various attempts have been made to energize (post-heat energization) after the main energization for forming the nugget in order to ensure the strength with the same nugget diameter as in the prior art.

As an example, in Patent Document 3, the product of the square of (It / To) and (Tt / To) is calculated using the energization time To and energization current Io in temper energization and the energization time Tt and energization current It in main energization. It is desirable to be in the range of 0.25 / 0.82.
In Patent Document 4, in a high-tensile steel sheet having a tensile strength of 35 kg / mm ² or more, in addition to the main energization exceeding the scattering occurrence limit current value, the temper energization is performed at a current value lower than the main energization to thereby improve the shear strength and fatigue. It is said that the strength can be improved.

  Instead of temper energization, pulsation energization has also been proposed in which a constant current is divided and added several times. For example, in Patent Document 5, pulsation energization is performed on a three-layer steel sheet after main energization. By doing so, it is said that a sufficient nugget diameter can be secured.

  In recent years, as seen in non-patent literature, a method called Spike-Tempering has been proposed in which energization is performed for an extremely short time after constant cooling to obtain the same effect as temper energization. The time required for energization is about 40 cycles (0.8 seconds).

  Further, in Patent Document 6, it is stated that the cross tensile strength of the high-tensile steel sheet can be improved by performing temper energization with a current value lower than the main energization after performing the main energization.

  However, in the energization methods as described in Patent Documents 3 to 5, the current range that can be used is narrow and small in order to select a range in which a predetermined resistance heat generation is possible with a current value lower than the main energization. It must be greatly affected by changes in the energization current and current time, and is stable when mounted in production sites where various disturbance factors exist (for example, a large current drop exceeding 50% of the main energization occurs). There is a problem that there is a small margin for performing general construction.

  In addition, the general martensite temper type energization method is temper energization in which tempering is performed by energizing after sufficient cooling, as implemented or described in the non-patent document. Cooling time (according to non-patent literature, about 40 cycles (0.8 seconds)) is required, and the total welding time (defined from the start of the first energization to the completion of the last energization) There was a problem of becoming longer.

  Further, Patent Document 6 is a method for securing a melted portion with respect to a set of three or more sheets, that is, an object of enlarging a nugget formed by main energization by post-heat energization. Conventionally, from the viewpoint that there is a close relationship between the nugget diameter and the joint strength, the joint strength has been arranged and evaluated with respect to the final nugget diameter regardless of the presence or absence of post-heat conduction. As described above, it is important to improve the strength with a specific nugget diameter. In addition, if the nugget or HAZ is cooled from the molten state, the joint strength cannot be improved.

JP 2003-071569 A Japanese Patent No. 3922263 JP 58-003792 A JP 58-003793 A JP 2002-103048 A JP 2008-093726 A

AISI / DOE Technology Roadmap Program, DE-FC36-97ID13554, B.R. Girvin et al., Development of Appropriate Spot Welding Practicing for Advanced High-Strength Steels, 2004

  The present invention has been made in view of the circumstances as described above, and even in a plate assembly with a large plate thickness ratio including a high-strength steel plate, without adding an extra step and without generating scattering. An object of the present invention is to propose a method of manufacturing a resistance spot welded joint in which a nugget of a necessary size is formed and a resistance spot welded joint having a good tensile strength can be stably obtained.

  In order to achieve the above-described problems, the inventors have performed first-stage welding and second-stage welding based on the technique described in Patent Document 2, thereby forming a nugget of a necessary size, and We considered securing the required joint strength by performing third-stage welding as heat conduction.

  Therefore, the present inventors have intensively studied post-heat energization capable of performing stable construction as third-stage welding for ensuring necessary joint strength.

  Conventionally, in order to improve the strength of the joint by tempering the joint after forming the joint, a method of applying a low current as post-heat conduction (temper conduction) has been used, whereas the main conduction current that forms the melted part on the contrary. We thought whether joint strength could be improved by applying a higher current than that. However, the addition of a high current causes scattering and electrode welding, and there is no change in joint strength due to remelting and rapid cooling, or it may decrease.

  Therefore, further studies were conducted to solve the problem.

  At that time, as shown in FIG. 8 and FIG. 9, a pair of two high strength steel plates (upper high strength steel plate 22 and lower high strength steel plate 23) that are superposed on each other is used as a pair of upper and lower electrode chips (upper side). The electrode tip 31 and the lower electrode tip 32) are joined together by pressurization and energization to obtain a resistance spot welded joint.

  First, there is a correlation between the cross tensile strength and fracture mode of resistance spot welded joints, and low-strength welded joints cause peeling fracture that breaks parallel to the steel sheet, and one steel sheet remains in a button shape as the strength increases. It changes to plug rupture that breaks so as to come off. In order to see the change in fracture mode at the same nugget diameter, a welded joint that was not subjected to temper energization and a welded joint that was subjected to temper energization were created. I understood that.

  That is, the fracture surface was a fracture surface with a cleavage plane and a brittle fracture, and the plug fracture surface was a smooth and ductile fracture. As shown in FIG. 8, if the portion surrounding the nugget 16 and the structure of which has been changed by heating is the heat-affected zone 17, the heat-affected zone 17 is hardened more than the inside of the nugget 16 in the case of peeling and breaking. Whereas the portion was seen, the heat-affected zone 17 was softened when the plug was broken. The softening of the heat-affected zone 17 is caused by tempering of the martensite structure caused by tempering. The softening allows plastic deformation at the outer periphery of the nugget 16 and alleviates stress concentration at the end of the nugget 16. Therefore, it was considered that the fracture mode was changed from the peeling fracture to the plug fracture.

  Therefore, the inventors decided to consider a completely new softening method for the heat affected zone 17 in order to obtain the same effect as described above when the post-heat energization is performed at a high current. That is, as shown in FIG. 8, the nugget 16 and the heat-affected zone 17 are not softened by one idea, but the heat-affected zone 17 is replaced with the heat-affected zone 17a on the electrodes 31 and 32 side as shown in FIG. And the heat-affected zone 17b on the softened zone 18 side, it was thought that it could be controlled separately. The heat affected zone 17b on the softened region 18 side also has heat transfer from the nugget 16, so the cooling rate is relatively slow, whereas the heat affected zone 17a on the electrode 31 and 32 side has a cooling rate due to heat dissipation to the electrodes. Because it is fast.

  The heat-affected zone 17 is hardened from the nugget 16 when the temper energization is not performed because the resistance spot welding concentrates heating to the center of the nugget 16, so that the electrodes 31 and 32 having a relatively low temperature or the softened region 18 are heated. It is considered that the heat affected zone 17 facing is caused by the cooling rate being higher than that inside the nugget 16.

  In view of this, in the post-heat energization, it was thought that the cooling rate of the heat affected zone 17b of the heat affected zone 17, particularly the softened zone 18 side, should be slowed down so as to give an appropriate energization that is comparable to the nugget 16. On the other hand, the heat affected zone 17a on the side of the electrodes 31 and 32 has a short time required for the temperature to sufficiently decrease, and it is considered that the same effect as the temper treatment can be obtained by heating after waiting for the time.

  Therefore, the present invention improves the tensile strength according to the following principle. That is, after the formation of the nugget 16 by main energization (first-stage welding and second-stage welding based on the technique described in Patent Document 2), as a post-heat energization (third-stage welding), a predetermined cooling time (pause) By applying a high current after a certain period of time, this heat input suppresses rapid cooling of the heat affected zone 17b on the nugget 16 and softening region 18 side and suppresses hardening, and at the same time, heat on the electrodes 31 and 32 side. The affected part 17a is tempered and softened by the temper effect. Unlike the temper energization and pulsation energization, the tempering effect is not given to the entire joint, but the effect is different in the joint portion, which is a significant difference from the prior art.

  In order to effectively establish the above principle, it is necessary to pay attention to the following points. In other words, the heat-affected zone 17a on the electrodes 31 and 32 side needs to be heated appropriately after being sufficiently cooled. For this purpose, the cooling time (resting time) needs to be longer than a predetermined time, but the purpose can be achieved in about 10 cycles at the longest. Furthermore, in the heat-affected zone 17b on the softened zone 18 side, if the energization time of the post-heat energization is too long, it will be heated more than necessary and will be re-quenched, causing conversely curing. However, it also causes scattering. Therefore, the energization time should be about 5 cycles at the longest. Also, the current value should be set for the same reason, and the current value up to about three times the current value in the main energization should be appropriately selected according to the object to be welded.

  And when these are applied to a high-tensile steel plate having a tensile strength of 590 MPa or more, which is markedly hardened by rapid cooling, a significant effect is exhibited.

  In addition, by increasing the applied pressure in the post-heat energization over the main energization, not only the current value margin in the post-heat energization is increased, but also scattering can be suppressed.

  In this way, in the post-heat energization in the third stage welding, the above-described high current is energized in a short time, preferably a plurality of times, and the applied pressure is increased over the main energization, so that scattering and electrode welding are performed. It succeeded in improving the joint strength stably.

  Based on the above, the present invention has the following features.

  [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 three-stage welding of the first stage, the second stage, and the third stage, and the second stage welding is the first stage. Compared to the welding of, welding with high pressure, low current or the same current, long energization time or the same energization time, and the third stage repeats high current energization with higher pressure than the second stage A method of manufacturing a resistance spot welded joint.

[2] 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. Thus, the welding satisfying the following formulas (1) to (3) is performed, and the welding of the second stage is performed with the welding pressure P _II , welding current I _II , and energization time T _II being the following (4) to (6 The method of manufacturing a resistance spot welded joint according to [1], wherein the welding satisfies the formula (1).
0.8 tm ≦ P _I ≦ 5 tm (1)
2 ≦ T _I ≦ 6 (2)
3tm + 5 ≦ I _I (3)
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)
Here, tm: plate 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 _I , T _II : energization time (cycles / 50 Hz)

[3] The pressure P _III , the cooling time Tc, the welding current I _III , and the energization time T _III in the third stage welding are the second stage pressure P _II , the welding current I _II , and the energization time T _II . In the above relationship, the following formulas (7) to (10) are satisfied. The method for producing a resistance spot welded joint according to the above [1] or [2].
P _II <P _III (7)
1 ≦ Tc ≦ 20 (8)
1 ≦ T _III ≦ 5 (9)
I _II ≦ I _III ≦ 3I _II (10)
Here, P _II and P _III : Applied pressure (kN)
I _II , I _III : Welding current (kA)
Tc: Cooling time (cycles / 50 Hz)
T _III : Energization time (cycles / 50 Hz)

[4] The method of manufacturing a resistance spot welded joint according to [3], wherein a cooling time Tc in the third stage welding satisfies the following expression (8a).
1 ≦ Tc ≦ 10 (8a)
Here, Tc: Cooling time (cycles / 50 Hz)

[5] the third stage, the the energization composed of cooling time Tc, and energization time _{T III} and the welding current _{I III,} and repeating the following five or more times [1] to [4] The manufacturing method of the resistance spot welding joint in any one of.

  In the present invention, a nugget of a necessary size is formed without adding an extra process and without generating scattering even in a large plate thickness ratio including a high-strength steel plate. Thus, a resistance spot welded joint having a good tensile strength can be obtained stably.

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 condition at the time of the 3rd 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. It is explanatory drawing which shows the heat affected zone and softening area | region in resistance spot welding. It is explanatory drawing which shows the principle of this invention.

  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, welded by resistance spot welding to be pressurized and energized to form a nugget of a necessary size, and necessary. Get joint strength.

  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.

  As an embodiment of the present invention, as shown in FIGS. 1 to 3, a case where spot welding is performed on a plate set of metal plates in which thin plates 11 are stacked on the outside of two or more stacked thick plates 12 and 13. An example will be described below.

  In this embodiment, the resistance spot welding is a three-stage welding of a first stage, a second stage, and a third stage. A pressure pattern in resistance spot welding of this embodiment is schematically shown in FIG. 4, and an energization pattern is schematically shown in FIG.

  First, the plate assembly is sandwiched between a pair of upper and lower electrodes 31 and 32 at a desired welding position, and pressurization is started. Start energization after pressure is applied. In the first stage of welding, a pressing force and a welding current are set so that contact resistance heat generation does not become small, and a nugget 16a (diameter N1) is formed between the thin plate 11 and the thick plate 12. 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 thin plate 11 and the thick plate 12 is narrow and the current density is high, and there is little influence of expansion of the current path due to melting of plating, etc., and the generated contact resistance heat is effectively applied to the nugget N1 formation. Will be able to.

In the first stage welding, the pressure P _I (kN) is related to the plate thickness tm of the thinnest metal plate among the plurality of metal plates (plate thickness of the metal plate 11 in FIG. 1). It is preferable to set so as to satisfy the following expression (1).
0.8 tm ≦ P _I ≦ 5 tm (1)

The pressure P _I in the welding of the first stage exceeds 5TM (kN), applied pressure becomes too high, heat generation due to contact resistance is reduced, the nugget is not formed between the thin sheet 11 and the thick sheet 12. On the other hand, if the pressure P _I is less than 0.8tm, the contact resistance between the electrode tip 31 and the thin plate 11 is increased, along with the spark is likely to occur, from between sheet 11 and the thick 12 Scattering is likely to occur.

In the first stage welding, it is preferable to set the energization time T _I (cycles / 50 Hz) so as to satisfy the following expression (2).
2 ≦ T _I ≦ 6 (2)

It is less than the energization time T _I is 2 cycles, because the energization time is too short, the desired size of the nugget between the thin plate 11 and the thick 12 is not formed. On the other hand, the energization time _{T I} is the longer beyond 6Cycles, expulsion 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). Therefore, it is preferable to set so as to satisfy the following expression (3).
3tm + 5 ≦ I _I (3)

Welding current I _I in the welding of the first stage, the (3tm + 5) less than a small current, can not be effectively utilized contact resistance heating, nugget is not formed between the thin sheet 11 and the thick sheet 12. In the first stage welding, it is preferable to pass a large current during the initial few 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 thin plate 11 and the thick plate 12.

  In this embodiment, 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. In this embodiment, during welding (after the completion of the first stage welding), the welding pressure 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, heat generation due to volume resistance heat generation is mainly performed, and a nugget is formed at the center between the electrodes, and as shown in FIG. 2, the nugget 16b (diameter) is formed between the thin plate 11 and the thick plate 12. Can form N2).

In the second stage welding, the pressure P _II is preferably set so as to satisfy the following expression (4) in relation to the pressure P _I in the first stage welding.
1.1 P _I ≦ P _II ≦ 10 P _I (4)

Further, the welding current I _II is preferably set so as to satisfy the following expression (5) in relation to the welding current I _I of the first stage welding.
0.5I _I ≦ I _II ≦ I _I (5)

Further, the energization time T _II, in relation to the energization time T _I of the first stage welding, it is preferable to set so as to satisfy the following equation (6).
_{T I ≦ T II ≦ 10T I} ......... (6)

  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.

  In this embodiment, the third stage welding is performed subsequent to the second stage welding described above. As described above, the third-stage welding is a post-heat energization for securing the joint strength, and the energizing force is increased more than the second-stage welding, and the high-current energization is repeated.

  As shown in FIG. 9, by applying a high current in this way, as shown in FIG. 3, the heat input suppresses rapid cooling of the heat-affected zone 17b on the nugget 16 and the softened region 18 side, thereby hardening. At the same time, the heat-affected zone 17a on the electrodes 31 and 32 side is tempered and softened by the temper effect, and the tensile strength of the joint is improved.

Welding of the third stage, in a pressurized state under a pressure P _III, the cooling time Tc is not energized the welding current, and a current time T _III to be energized by the welding current I _III.

At that time, as described above, the pressure P _III increases from pressure P _II of the second stage welding. That is, it is set so as to satisfy the following expression (7).
P _II <P _III (7)

It is desirable that the timing of increasing the applied pressure be completed before the first energization of the third stage welding (multi-stage energization) is applied so that the effect of the increase in the applied pressure can be obtained. Moreover, since an excessively high applied pressure may cause deterioration of the electrode, it is more preferable to keep the pressure in the range of P _II <P _III ≦ 3P _II .

The cooling time Tc is preferably set so as to satisfy the following equation (8).
6 ≦ Tc ≦ 20 (8)

Furthermore, the cooling time Tc is more preferably set so as to satisfy the following expression (8a).
1 ≦ Tc ≦ 10 (8a)

Further, the energization time T _III is preferably set so as to satisfy the following equation (9).
1 ≦ T _III ≦ 5 (9)

Further, the welding current I _III is preferably set so as to satisfy the following expression (10) in relation to the welding current I _II of the second stage welding.
I _II ≦ I _III ≦ 3I _II (10)

Then, the energization composed of cooling time Tc, and energization time _{T III} and the welding current _{I III,} it is preferable to repeat the following five or more times.

In a third stage welding, cooling time Tc as described above, the energization time T _III, taking the welding current I _III, since as cooling after the second stage welding progresses, the resistance value is low, the welding current I _III Must be higher than the welding current I _II of the second stage welding. However, if the current is applied before the cooling proceeds, the nugget 16 may be completely remelted or excessively raised to a high temperature. It may cause a decrease in strength. In addition, an energization time that is too long or a current value that is too high causes scattering and decreases the electrode life. Also, an excessively long cooling time leads to an increase in takt time, which is undesirable. Therefore, energization time _{T III} until five cycles, the cooling time Tc is up to 20 cycles, the welding current _{I III} is up to three times the welding current _{I II} of the second stage welding, is selected appropriately depending on the combination.

Incidentally, in view of the stability and scattering occurrence limit of at the construction surface, the energization time not too long, the cooling time is not too short, because it should be selected energization current is not too high, the energization time T _III 2-4 cycle, cooling time Tc is 6-10 cycles, the welding current _{I III} is that falls in the range of _{I II} <Ib ≦ _2I II, considered to be the most suitable.

However, it is conceivable that the cooling will be delayed due to the influence of the construction atmosphere such as temperature and humidity and the base material temperature. At this time, those cooling time Tc is even exceeded 20 cycles, the higher current than the welding current I _II of the second stage welding by adding two or more times, reduced the cooling rate without melting the entire nugget If so, it can be said to be within the scope of the present invention.

  Moreover, in the steel plate with a tensile strength of less than 590 MPa, from the viewpoint that sufficient joint strength can be achieved by ordinary welding, it is used for a plate set including a high strength steel plate with a tensile strength of 590 MPa to 1960 MPa. In particular, the effect can be obtained with a plate set including a high-tensile steel plate having a tensile strength of 980 MPa or more.

Furthermore, in order to achieve the effects of the present invention based on the principle of the present invention described above, the cooling time Tc, the energization time T _III and the welding current I _III in the third stage welding are not necessarily constant in each pulse. For example, if the cooling does not proceed sufficiently during the first cooling time, but the cooling proceeds too much during the second cooling time, the first cooling time may be longer than the second cooling time. It is done. Similarly, the first welding current value may be reduced or the energization time may be shortened. For these reasons, the cooling time Tc, the energization time T _III and the welding current I _III in the third stage welding are individually set. It does not depart from the intention of the present invention.

  And the manufacturing method of the resistance spot welding joint of this invention is not limited to the board assembly illustrated to FIGS. 1-3. Moreover, a steel plate can be illustrated as a metal plate which applies this invention as a to-be-welded material. As the steel plate, a case where a high-tensile steel plate is included is suitable, but it is not limited to a strength level (soft steel, high-tensile steel plate) or presence / 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 applied to a plate assembly having a plate thickness ratio (= total plate thickness mm / plate thickness mm of the thinnest plate) of 5 or more.

As shown in FIG. 7 (a) and Table 1 (plate assembly No. A), one sheet of a 270 MPa class mild steel plate having a thickness of 0.7 mm and a GA-plated steel plate having a thickness of 1.6 mm (original plate 750 MPa class high strength steel plate) , With a basis weight of 45/45 g / m ² ), a total of three thin steel plates in the order of 270 MPa grade mild steel plate (thin plate) 11, GA plated steel plate (thick plate) 12, GA plated steel plate (thick plate) 13. The combined plate assembly was resistance spot welded under the welding conditions shown in Table 2 to produce a resistance spot welded joint.

  Here, the example of the present invention is one in which resistance spot welding including the first stage, the second stage, and the third stage as shown in the above embodiment is performed. On the other hand, the comparative example performs resistance spot welding consisting of two stages, a first stage and a second stage.

  Resistance spot welding was performed using a stationary, servo-motor pressurization type single-phase AC resistance spot welding machine. The electrode used was an electrode tip of DR (tip diameter 6 mm) shown in FIG.

  About each obtained welded joint, the nugget diameter in each between the thin plate 11 and the thick plate 12 and between the thick plate 12 and the thick plate 13 is measured, and the case where each nugget diameter satisfy | fills 4√t or more is favorable ( (Circle)) and the case where it was not so was evaluated as bad (x). Note that t is the thickness (mm) of the thinner steel plate of the two adjacent steel plates.

  Moreover, about each obtained welded joint, the tensile strength between the thick plate 12 and the thick plate 13 was measured.

  Table 3 shows the conformity (○) and nonconformity (×) to the above equations (1) to (10) for each welded joint, and the obtained evaluation results (nugget, nugget diameter, between thick plates) Tensile strength).

  In Table 3, the quality of the nugget is judged based on whether a good nugget is formed between the thin plate and the thick plate. If there is no change compared to the comparative example, ○, by remelting or scattering occurrence When the change was remarkable, it was set as x. Regarding the tensile strength, the case where it exceeded 3 kN or more compared with the comparative example was rated as ◎, the case where it exceeded but less than 3 kN was marked as ◯, and the case where it was lower than x.

  As shown in Table 3, good nuggets are formed in both the inventive examples and the comparative examples. And in the example of this invention, the tensile strength between thick plates is improving compared with the comparative example.

As shown in FIG. 7 (b) and Table 1 (plate assembly No. B), two 270 MPa grade mild steel plates having a thickness of 0.7 mm and a GA plated steel plate having a thickness of 2.3 mm (original plate 270 MPa grade mild steel plate, the basis weight 45 ^/ 45g / ^m 2) 2 sheets in total four thin steel, 270 MPa grade mild steel plate (thin plate) 11, GA-plated steel plate (thick plate) 12, GA-plated steel plate (thick plate) 13,270MPa grade mild steel Resistance spot welding was performed under the welding conditions shown in Table 4 on the plate assembly in which the plates (thin plates) 14 were superposed in order, and a resistance spot welded joint was produced.

  Here, the example of the present invention is one in which resistance spot welding including the first stage, the second stage, and the third stage as shown in the above embodiment is performed. On the other hand, the comparative example performs resistance spot welding consisting of two stages, a first stage and a second stage.

  Resistance spot welding was performed using a stationary, servo-motor pressurization type single-phase AC resistance spot welding machine. The electrode used was an electrode tip of DR (tip diameter 6 mm) shown in FIG.

  About each obtained welded joint, the nugget diameter in each between the thin plate 11 and the thick plate 12, between the thick plate 12 and the thick plate 13, and between the thick plate 13 and the thin plate 14 is measured, and each nugget diameter is 4 A case satisfying √t or more was evaluated as good (◯), and a case other than that was evaluated as poor (×). Note that t is the thickness (mm) of the thinner steel plate of the two adjacent steel plates.

  Moreover, about each obtained welded joint, the tensile strength between the thick plate 12 and the thick plate 13 was measured.

  Table 5 shows the conformity (◯) and nonconformity (×) to the above formulas (1) to (10) for each welded joint, and the obtained evaluation results (nugget, nugget diameter, between thick plates) Tensile strength).

  In Table 5, the quality of the nugget is judged based on whether a good nugget is formed between the thin plate and the thick plate. The case where there is no change compared to the comparative example is ○, due to the occurrence of remelting and scattering. When the change was remarkable, it was set as x. Regarding the tensile strength, the case where it exceeded 3 kN or more compared with the comparative example was rated as ◎, the case where it exceeded but less than 3 kN was marked as ◯, and the case where it was lower than x.

  As shown in Table 5, good nuggets are formed in both the inventive examples and the comparative examples. And in the example of this invention, the tensile strength between thick plates is improving compared with the comparative example.

11 Metal plate (thin plate)
12 Metal plate (thick plate)
13 Metal plate (thick plate)
14 Metal plate (thin plate)
16 Nugget 16a Nugget between thin plate and thick plate 16b Nugget between thick plate and thick plate 17 Heat affected zone 17a Heat affected zone on electrode side 17b Heat affected zone on softened zone 18 Softened zone 22 High tensile strength steel plate (upper side)
23 High-tensile steel sheet (lower side)
31 Electrode tip (upper side)
32 Electrode tip (lower side)

Claims (5)

  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. In addition, the plate thickness ratio is 5 or more, and the resistance spot welding is a three-stage welding of the first stage, the second stage, and the third stage, and the second stage welding is the first stage welding. Compared to welding with high pressure, low current or the same current, long energization time or the same energization time, the third stage repeats high current energization with higher pressure than the second stage Manufacturing method of spot welded joint. 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. (1) the welding satisfies - (3), the welding of the second stage, pressure _{P II} of the welding, the welding current _{I II,} the energization time _{T II} is the following (4) to (6) The method of manufacturing a resistance spot welded joint according to claim 1, wherein the welding is satisfactory.
0.8 tm ≦ P _I ≦ 5 tm (1)
2 ≦ T _I ≦ 6 (2)
3tm + 5 ≦ I _I (3)
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)
Here, tm: plate 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 _I , T _II : energization time (cycles / 50 Hz) The pressure P _III , the cooling time Tc, the welding current I _III , and the energization time T _III in the third stage welding are related to the pressure P _II , the welding current I _II , and the energization time T _{II of} the second stage. The method of manufacturing a resistance spot welded joint according to claim 1 or 2, wherein the following expressions (7) to (10) are satisfied.
P _II <P _III (7)
1 ≦ Tc ≦ 20 (8)
1 ≦ T _III ≦ 5 (9)
I _II ≦ I _III ≦ 3I _II (10)
Here, P _II and P _III : Applied pressure (kN)
I _II , I _III : Welding current (kA)
Tc: Cooling time (cycles / 50 Hz)
T _III : Energization time (cycles / 50 Hz) The method of manufacturing a resistance spot welded joint according to claim 3, wherein a cooling time Tc in the third stage welding satisfies the following expression (8a).
1 ≦ Tc ≦ 10 (8a)
Here, Tc: Cooling time (cycles / 50 Hz) Of the third stage, the energization composed of cooling time Tc, and energization time T _III and the welding current I _III, according to any one of claims 1 to 4, characterized in that repeated below 5 times more than once A method of manufacturing a resistance spot welded joint.

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