[0001]The present invention relates to a spot welding method for a plurality of steel plates including galvanized steel sheet.
Background technique
[0002]
　In recent years, in the automotive field, lower fuel consumption and CO 2 in order to reduce emissions, and that the weight of the vehicle body, for the improvement of collision safety, are required to increase the strength of the vehicle body member. To satisfy these requirements, it is effective to use a high strength steel sheet such as a vehicle body member, various parts.
[0003]
　From the viewpoint of the vehicle body high proof Sabika, it is necessary to configure an excellent steel plate member corrosion resistance. Zinc-plated steel sheets, corrosion resistance are widely known to be good, in terms of weight reduction and strengthening described above, in the galvanized steel sheet used for automotive, high strength steel sheet be plated zinc-plated high-strength steel sheet used is used.
[0004]
　Such as the mounting of the vehicle body of the assembly and parts of motor vehicles, mainly, it is spot welding is used. However, when the spot welding galvanized high strength steel sheet, from the outer surface of the steel sheet in contact with the welding electrode, toward the nugget, there is a problem that cracking occurs. Figure 1 shows a schematic of a crack in the welded portion in the case of performing spot welding to the galvanized steel sheet. Doing spot welding galvanized steel sheet 1, it and cracking 3 to progress toward the nugget 2 from the outer surface of the steel sheet 1 in contact with the welding electrode, cracks 5 to progress from the electrode shoulder to the heat affected zone 4 occurs It has been known.
[0005]
　This cracking, pressure and steel thermal expansion of the electrode, the tensile stress due to shrinkage is applied to the welding point, zinc and melted at the surface of the steel sheet of the welded portion, the alloy of copper zinc and the electrode, the steel grains caused by reducing the grain boundary strength invade the field is said to be cracking due to the so-called liquid metal embrittlement. The automobile body, there is a problem that crack is significant and strength of the welded portion is reduced, by controlling the chemical composition and structure of the steel sheet, are known a technique for suppressing cracking of the welded portions.
[0006]
　For example, Patent Document 1, to adjust the component composition of the steel sheet, and the austenite phase generated during spot welding to fine crystal grains, and having a crystal grain and intricate metal structure of another phase it is, complicates the diffusion penetration path of molten zinc into the crystal grain boundaries, and difficult to penetrate the molten zinc, a technology for preventing liquid metal embrittlement cracks during spot welding is disclosed.
[0007]
　Further, Patent Document 2, only complicates the grain boundaries by the organization control of the steel sheet, it is described that may not be sufficiently suppressed cracking of the weld. Patent Document 2, to adjust the component composition of the steel sheet, the grain boundary oxidation depth of the hot rolled steel sheet and 5μm or less, to perform an Fe-based electroplating the cold-rolled steel plate prior galvannealed process by, by the intergranular corrosion depth of the galvannealed steel sheet to 5μm or less, thereby suppressing the generation of cracks in the welded portion of the galvannealed steel sheet is disclosed.
[0008]
　In Patent Document 3, when manufacturing the electric resistance welded steel pipe from the steel sheet plated such as zinc, in order to prevent liquid metal embrittlement, techniques are disclosed for removing the plating of the abutment of the strip ends ing.
CITATION
Patent Literature
[0009]
Patent Document 1: Japanese Patent 2006-265671 JP
Patent Document 2: Japanese Patent 2008-231493 JP
Patent Document 3: Japanese Patent Laid-Open 05-277552 discloses
Summary of the Invention
Problems that the Invention is to Solve
[0010]
　Despite these countermeasures cracking of spot welding points are discussed, in the spot welded joint of the part of galvanized steel sheet, or, in the spot welded joint of the part of the non-plated steel plate and zinc-plated steel sheet, still , was that the desired tensile strength can not be obtained. Therefore, the present inventors have investigated the reason for this.
[0011]
Figure 2 shows the welding spot after the spot welding galvanized steel sheet, a schematic diagram of the thickness direction of the cross section including the nugget. As shown in FIG. 2, the desired tensile strength in the Never spot welded joint obtained, corona bonding straight out of the crack 6 of overlapping surfaces of the steel plate had occurred. There was also the cracks 7 during nugget corona bond occurs.
[0012]
　Then, the inner surface at the position relationship between the welding electrode and the workpiece to be welded in the immediately preceding spot welding, when performing welding in a state where disturbance factors are present such as the following (a) ~ (d), the steel plate was superposed cracking during nugget corona bond straight outer and corona bond occurring in a side has been found to be liable to occur.
(A) and the axis of the welding electrode, the welding electrode and the perpendicular surface of the steel sheet in contact with the non-parallel state
and the distance from the tip of the (b) one of the welding electrode to the one surface of the steel sheet, other welding distance from the tip portion to the other surface of the steel sheet is different states of the electrodes
with respect to the extension line of the axis of the welding electrode one (c), a state where the axis of the other of said welding electrodes are displaced
(d) welding location state of having a gap between the overlapping surfaces of the
[0013]
　If a disturbance factor as described above exists, after the dwell completion of the welding electrode (after the electrode opening), when nugget corona bond straight outer and corona bonding, point the residual stress is increased occurs. This phenomenon, present inventors have found that the crystal grain boundaries of the steel sheet of the locations, molten galvanized invade lowers the grain boundary strength, generating cracks during nugget corona bond straight outer or corona bond It was considered to be allowed to.
[0014]
　Conventionally, after finishing the power between the welding electrodes, immediately I had a non-contact steel welding electrode. However, continuing the pressurization holding welding electrodes after power distribution end, it was solidifying the molten galvanized before electrode opening, that cracking during nugget corona bond straight outer or corona bonding may not occur heading was. Also, was the squeeze time until the start of supply of welding current from contacting the welding electrode against the steel sheet is a weld members to or greater than a predetermined time, even if there is a disturbance factor as described above, the corona bond straight cracking during nugget outer or corona bond was found that there may not occur.
Furthermore, the present inventors have found that when continuing the dwell, found the tendency of the joint strength decreases, it is possible to suppress cracking and was examined the time dwell not to drop even joint strength, pressure of a welding electrode holding by the time and squeeze time function of the total thickness it was found that sufficient joint strength can be obtained with a crack can be suppressed.
[0015]
　The present invention is, cracks in the plate sets of spot welding including galvanized steel sheet, even if the various disturbances factors are present, which occurs upon nugget corona bond straight outer and corona bonding of overlapping surfaces of the steel plate suppressed, and an object thereof is to provide a spot welding method capable of forming a high quality of the spot welded joint.
Means for Solving the Problems
[0016]
(1) Spot welding method according to an embodiment of the present invention is a method of spot welding the workpiece to be welded constituted by a plurality of steel plates, at least the welding portions are overlapped, said member being welded is at least the overlapping surface of the welding portion is covered with zinc-plated steel sheet comprises at least one of the total thickness of the plurality of steel plates t (mm) is not less than 1.35 mm, welding the to be welded member filled squeeze time St from contacting the electrode to the start of the welding current to the welding electrode (s) is the following (equation 1), and the work piece and the welding electrode from the time of welding operation ends between the welding electrodes retention time after welding until a non-contact and a member Ht (s) satisfy the following (equation 2).
　0.020 ≦ St · · · (Equation 1)
　0.015T 2 + 0.020 ≦ Ht · · · (Equation 2) According to the spot welding method having the above configuration, when a nugget corona bond straight outer and corona bond it can be cracked by the inhibition of, securing the joint strength.
[0017]
(2) According to another aspect of the present invention, in the spot welding method according to (1), the welding holding time Ht (s) and the squeeze time St (seconds) is the further following (Equation 3) it may be filled.
+ St ≦ ht 0.20T 2 -0.40T + 1.50 · · · (Equation 3)
In the above embodiment, not only can inhibit cracking during nugget corona bond straight outer and corona bonding, while ensuring productivity desired joint strength of can be obtained.
[0018]
According to another aspect of (3) The present invention, in the spot welding method according to (1) or (2), immediately before contacting the welding electrode to the workpiece to be welded, of the conditions below, one one or may be filled with two or more.
(A) and the axis of the welding electrode, the welding electrode and the perpendicular surface of the steel sheet in contact with the non-parallel state
and the distance from the tip of the (b) one of the welding electrode to the one surface of the steel sheet, other welding distance from the tip portion to the other surface of the steel sheet is different states of the electrodes
with respect to the extension line of the axis of the welding electrode one (c), a state where the axis of the other of said welding electrodes are displaced
(d) welding location state a gap between the overlapping surfaces of
the case where there is a disturbance factor as defined above embodiment, although cracks during nugget corona bond straight outer and corona bond becomes significant, the (1) or (2 According to the spot welding method), to suppress such cracking, and can be secured joint strength.
[0019]
According to another aspect of (4) the present invention, in the spot welding method according to any one of the above (1) to (3), at least one of the plurality of steel plates, tensile strength above 780MPa it is a high-strength steel sheet.
In the above embodiment, by tensile strength of the steel sheet is a high strength steel sheet is not less than 780 MPa, it becomes possible to lightweight, high strength of the welded members.
[0020]
(5) According to another aspect of the present invention, in the spot welding method according to any one of the above (1) to (4), prior to the welding operation, it performs further before the supply, the welding operation at the beginning it may be energized at the start of the pre-energization.
(6) According to another aspect of the present invention, in the spot welding method according to any one of the above (1) to (4), after the welding operation, performed post-energization Furthermore, the welding operation ends when it may be energized at the end of the post-energized.
(7) According to another aspect of the present invention, in the spot welding method according to any one of the above (1) to (4), prior to the welding operation, it performs further before the supply, the welding operation start is energized at the start of the pre-energizing, and after the welding operation, performed post-energization further, during the welding operation ends it may be energized at the end of the post-energized.
　In this manner, it is possible to secure a high strength and toughness than in the spot welded joint comprising a high-strength steel sheet.
Effect of the invention
[0021]
　According to this aspect of the present invention, the plate sets of spot welding including galvanized steel sheet, even if the disturbance factors are present, to suppress the cracking during nugget corona bond straight outer and corona bonding, fitting it is possible to ensure the strength.
BRIEF DESCRIPTION OF THE DRAWINGS
[0022]
FIG. 1 is a diagram showing an outline of cracks in the sheet thickness direction of the cross section of the welded portion subjected to spot welding galvanized steel sheet.
It is a schematic diagram illustrating a crack during nugget corona bond straight outer or corona bonding in the sheet thickness direction of the cross section of the welded portion subjected to spot welding in FIG. 2 galvanized steel sheet.
3 is a graph showing the relationship between the hit corners and squeeze time and stress change of a welding electrode during spot welding.
[4] relates to cracking during nugget corona bond straight outer or corona bonding is a graph showing post-weld holding time Ht relationship between SoitaAtsu t.
[5] cross tensile strength about (CTS), which is a graph showing the relationship between the sum of the holding time after welding Ht and squeeze time St for SoitaAtsu t.
[6] and the axis of the welding electrode is a schematic diagram for the normal of the steel sheet surface in contact with the welding electrode will be described condition is non-parallel.
[7] the distance from each tip of the opposing welding electrodes to each surface of the steel sheet is a schematic diagram for explaining the different states.
To the extension line of the axis of FIG. 8 one said welding electrode is a schematic view for explaining a state in which the axis of the other of said welding electrodes are displaced.
9 is a schematic view for explaining a state in which a gap between the overlapping surfaces of the welding location.
FIG. 10 is a photograph of a cross section of the welding place where the axis, and the vertical line of the steel sheet surface in contact with the welding electrode was spot welded in a state which is not parallel welding electrode.
11 is a cross-sectional photograph of a welding portion where the distance is carried out spot welding at different states from each tip of the opposing welding electrodes to each surface of the steel sheet.
[12] to the extension line of the axis of one said welding electrode in a cross-sectional photograph of the welded portion subjected to spot welding in a state in which the axis of the other of said welding electrodes are displaced.
13 is a cross-sectional photograph of the welded portion that was spot welded in a state in which a gap between the overlapping surfaces of the welding location.
DESCRIPTION OF THE INVENTION
[0023]
Hereinafter will be described an embodiment of the present invention, the present invention is not limited to these embodiments will be apparent.
[0024]
(First Embodiment)
spot welding method according to the present embodiment is a method of spot welding the workpiece to be welded constituted by a plurality of steel plates, at least the welding portions are overlapped. In the spot welding method, for at least one of the plurality of steel plates, overlapping surfaces of at least the welding portions are coated with a zinc-based plating. Further, in the spot welding method, the total thickness of the plurality of steel plates t (mm) is not less than 1.35 mm.
[0025]
　Spot welding method according to the present embodiment, the welding electrode squeeze time until the start of supply of welding current from in contact with the workpiece to be welded St (seconds) satisfy the following (Equation 1), and between the welding electrodes wherein said welding electrode from the time of welding operation ends after welding retention time until the non-contact with the welded members Ht (s) satisfy the following (equation 2).
　0.020 ≦ St · · · (Equation 1)
　0.015T 2 + 0.020 ≦ Ht · · · (Equation 2)
[0026]
　In this embodiment, the squeeze time St, means the time from the time of contacting the welding electrode on the surface of the steel sheet is a weld members, until the time of starting the supply of welding current. Time of starting the supply of welding current is superimposed faces of the steel plate to be welded contacts the welding point, pressure of the electrode means when elevated.
[0027]
　In the spot welding method according to the present embodiment, squeeze time St is in the range of more than 0.020 seconds, which is defined by (Equation 1). This expression is an expression that has been determined experimentally, so long as the squeeze time St is (Formula 1), or when the welding locations axis of the welding electrode has an inclination to the surface of the steel sheet even if there is a gap between the overlapping surfaces of the effect of reducing the effects of these disturbances factors is sufficient. As a result, the tensile stress at the time of releasing the welding electrode after welding can be alleviated, it is possible to suppress the cracking during nugget corona bond straight outer or corona bonding.
[0028]
Here, an experimental example of change in stress when performing spot welding by setting a punching angle of the welding electrode (the angle of the axis of the welding electrode relative to the normal of the surface of steel sheet) to 5 degrees as a disturbance factor in FIG. In Figure 3, and the absence of disturbance factors, there is a disturbance factor (striking angle 5 degrees), in the state of the squeeze time St is 0.01 seconds and 0.10 seconds, the stress before and after releasing the welding electrode after welding change is shown.
As can be seen from FIG. 3, although the tensile at the electrode release stress that there is a disturbance factor occurs, by a range defined by the squeeze time St (Equation 1), it is possible to relax the tensile stress .
[0029]
　Will now be described (Equation 2). In the spot welding method according to the present embodiment, as defined in equation (2), retention time Ht is defined as a function of the total thickness t of the plurality of steel plates.
SoitaAtsu t and welding after holding time defines the Ht relationship (Equation 2) is an expression that has been determined experimentally. Figure 4 relates to cracking during nugget corona bond straight outer or corona bonding, showing a post-weld holding time Ht relationship between SoitaAtsu t. 4, it was created from the experimental results in Examples and various plate set below. In the member being welded to less than SoitaAtsu 1.35mm, because the incidence of cracking during nugget corona bond straight outer or corona bonding is small, equation (1) is not less than SoitaAtsu 1.35mm It was assumed to be applied to.
[0030]
　Although detailed in Example, the results shown in Figure 4, when the spot welding by using two different types of steel of the steel sheet galvanized is coated, the total thickness t of the steel sheet, between the welding electrodes the welding electrode from the time of welding operation ends of and the post-weld holding time Ht until a non-contact and a member to be welded is obtained by implemented with various changes. Then, the welded joint obtained, is obtained by confirming the presence or absence of cracks during nugget corona bond straight outer or corona bonding. Incidentally, cracking during nugget corona bond straight outer or corona bond was confirmed by observing the thickness direction of the cross section including the nugget.
[0031]
　4, in the total plate thickness t and welding after holding time Ht circled markers, cracking during nugget corona bond straight outer or corona bond was not the welded joint. In contrast, in the total plate thickness t and welding after the retention time Ht markers squares, cracking during nugget corona bond straight outer or corona bond was observed. From this result, it was found that can be defined by a boundary cracking during nugget corona bond straight outer or corona bonding occurs (boundary markers circle markers and squares) (Equation 1).
[0032]
　Against SoitaAtsu t, the retention time after welding Ht by a range defined by (Equation 1), the reason that cracks during nugget corona bond straight outer or corona bond disappears, considered as follows It is.
[0033]
　First, place the welding residual stress during nugget corona bond straight outer and corona bonding is high, but in a compressed state in the process of being crushed by the welding electrode during spot welding, in tension and the welding electrode is separated from the surface of the steel sheet resulting becomes tensile stress in. This can be seen from FIG. 3 previously described. Cracking during nugget corona bond straight outer and corona bonds, galvanized metal melted at the corona bond straight outer is steel welding residual stress is high point during nugget corona bond straight outer and corona bonding after welding invade the grain boundaries is caused by decreasing the grain boundary strength.
[0034]
　The location where residual stress is increased occurs, and after the welding electrode is separated from the surface of the steel sheet for a (electrode opening after), (extended after welding retention time Ht) pressurizing continued pressure holding welding electrodes after application end , by solidifying the molten galvanized before electrode open, molten galvanized does not penetrate into the grain boundaries of the steel sheet having high residual stress locations can be suppressed cracking. Then, solidification of the zinc-based plating, the ease is the cooling of the steel sheet, that is, to relate to the total thickness t of the member to be welded, adjusting the post-weld holding time Ht welding electrode as a function of SoitaAtsu t it is possible to suppress the cracking during nugget corona bond straight outer or corona bonding.
[0035]
According to the spot welding method according to the above embodiment, to suppress the cracking during nugget corona bond straight outer and corona bond can be to secure the joint strength.
[0036]
　(Second Embodiment)
will be described another embodiment according to the present invention will now. Spot welding method according to this embodiment is basically the same as the first embodiment.
Spot welding method according to the present embodiment, a method for the spot welding workpiece to be welded constituted by a plurality of steel plates, at least the welding portions are overlapped, for at least one of the plurality of steel plates, at least the welding overlapping surfaces of portions is covered with zinc-plated, the total thickness of the plurality of steel plates t (mm) is not less than 1.35 mm, the said welding electrodes from contacting the welding electrode to the workpiece to be welded welding to squeeze time to start the supply of welding current St (seconds) satisfy the following (equation 1), and a non-contact with the workpiece to be welded and the welding electrode from the time of welding operation ends between the welding electrodes post holding time Ht (s) satisfies the following equation (2), the weld holding time Ht (s) and the squeeze time St (seconds) further satisfies the following (equation 3).
　0.020 ≦ St · · · (Equation 1)
　0.015T 2 + 0.020 ≦ Ht · · · (Equation 2) Ht + St ≦ 0.20T 2 -0.40T + 1.50 · · · (Equation 3)
[0037]
In the spot welding method according to the present embodiment, the sum of the holding time Ht and squeeze time St is as defined in (Equation 3), it is defined as a function of the total thickness t of the plurality of steel plates.
[0038]
Welding retention time Ht and defining the squeeze time St (Equation 3) is an equation which is experimentally determined. 5, the joint strength of the weld members after welding, showing the relationship of the sum of the holding time after welding Ht and squeeze time St for SoitaAtsu t. 5 has been created from the experimental results in Examples and various plate set below, various conditions are similar to those of the first embodiment. 5 shows, the resulting welded joint, illustrating a result of evaluation cross tensile strength is measured by a load tensile peeling direction (CTS). It should be noted that the cross tensile strength (CTS) was confirmed by the method prescribed in JIS Z3137.
[0039]
In the sum of the holding time after welding Ht and squeeze time St to the total thickness t of the marker square mark in Fig. 5, but cracks not during nugget corona bond straight outer or corona bonding in weld joints, reduction of the CTS There was relatively large. From this result, it was found to be defined with sufficient CTS is obtained boundary (boundary markers and circle markers squares) (Equation 3).
[0040]
　As described above, experimentally, if the sum of the holding time after welding Ht and squeeze time St for SoitaAtsu t satisfies the (Equation 3), it was found that reduction of the CTS is smaller. Against SoitaAtsu t, by a range defined the sum of the holding time after welding Ht and squeeze time St (Equation 3), why it is possible to obtain a sufficient CTS, as follows Conceivable.
[0041]
When squeeze time St is long, the effect of suppressing the disturbance factor increases. In this case, holding time Ht that is shortened, the cooling rate of the weld is not too fast, since the hardness of the nuggets and the heat affected zone is not increased, the cross tensile strength is not lowered. On the other hand, if the squeeze time St is short, with improved productivity, by increasing the holding time Ht, sufficient toughness can be obtained, it is possible to obtain sufficient joint strength.
That is, according to the spot welding method having the above structure, not only can inhibit cracking during nugget corona bond straight outer and corona bonding, while ensuring productivity also can obtain a desired joint strength.
[0042]
　Above the spot welding method according to the first embodiment or the second embodiment, immediately before contacting the welding electrode to the workpiece to be welded, of the conditions of the following disturbance factors (a) ~ (d), 1 or 2 or more even when conditions are met, inhibiting the cracking during nugget corona bond straight outer and corona bonding, and can be secured joint strength.
[0043]
Disturbance factor (a): the axis of the welding electrodes, the state perpendicular and are non-parallel surface of the steel sheet in contact with the welding electrode
in the spot welding, the shed welding electrode vertically is basic with respect to the surface of the steel sheet . However, there are multiple welds to be welded members, may require welding at various welding positions. In such a case, such as for spot welding machine of teaching poor and working time constraints, not allowing time to correct welding position, inclined with the axis of the welding electrodes, and the steel sheet surface in contact with the welding electrode from the vertical It was left there is if you are spot welding. In (a) of FIG. 6 shows the axis 9 of the welding electrodes 8, and the perpendicular 10 on the surface of the steel sheet 1 in contact with the welding electrode 8 is a schematic view of a state that is not parallel.
[0044]
As described above, the axis 9 of the welding electrodes 8, if the angle between the surface of the steel sheet 1 in contact with the welding electrodes 8 are welded in a state deviated from vertical, the steel plate 1 in contact with the welding electrodes 8 axis 9 of the welding electrode 8 with respect to the perpendicular 10 on the surface of an angle of hitting the corners. When the striking angle is contacted welding electrode 8 on the surface of the steel sheet 1 at greater than 0 degrees state (state that is not perpendicular axis 9 with respect to the surface of the steel sheet 1 of the welding electrode 8), shown in FIG. 6 (b) steel plate 1 is deformed as. Such deformation, relatively stress becomes point occurs higher in the vicinity of the corona bond, cracking is likely to occur at this point.
[0045]
When hitting the corners is more than three times, particularly prone to cracking during nugget corona bond straight outer and corona bonding, in the first embodiment or spot welding method of the second embodiment, the inhibitory effects of these cracks is there. Also, when hitting the corners is more than 5 degrees, but tends to decrease further joint strength remarkably cracked during nugget corona bond straight outer and corona bonding, the first or second embodiment Spot the welding method, such degradation can be suppressed.
[0046]
Disturbance factor (b): the distance from the tip of one of the welding electrode to the one surface of the steel sheet, the distance is different state from the tip portion of the other welding electrode to the other surface of the steel sheet
　in spot welding, the sandwiching welding electrodes against, in order to keep the central position of the held plate, it may be provided equalizing mechanism. When providing the equalizing mechanism, that much spot welding gun is large in size, it is necessary to the welding robot that, correspondingly, increases the cost of the welding robots. Therefore, there is a case to be welded without providing the equalizing mechanism in the spot welding gun.
[0047]
In this case, as shown in FIG. 7 (a), immediately before contacting the welding electrodes 8 to be welded member (steel plate 1), from the tip of one of the welding electrodes 8, nearest the steel plate 1 to the distal end and distance to the surface, the distance is different state from the tip portion of the other welding electrode 8 to the nearest steel plate 1 of the surface of the tip portion there. When make contact and pressure of the welding electrodes 8 in this state, as shown in FIG. 7 (b), in the welding position, one of the steel sheet 1 (in the case of FIG. 7, the upper steel plate 1) It is deformed to the other steel plate 1 side. Doing welding in a state in which thus deformed, local stress is generated in the vicinity of the corona bond. Then, cracking is likely to occur at this point.
[0048]
As described above, and when welding without providing the equalizing mechanism in the spot welding gun, if not properly set the distance between the structure on the welding electrode and the surface of the steel sheet member to be welded is the axial direction of the welding electrode in no positional deviation of the spot welding gun and the workpiece to be welded is absorbed, easily cracked during nugget corona bond straight outer or corona bonding occurs. Even in such a case, in the first embodiment or spot welding method of the second embodiment, there is a crack suppressing effect of the time nugget corona bond straight outer or corona bonding.
[0049]
Disturbance factor (c): to the extension line of the axis of one of the welding electrodes, the state is the axis of the other of said welding electrodes are displaced
　when welding multiple RBI, buckling or spot welding gun of the welding electrode axis due to wear of the movable portion of, as shown in FIG. 6 (a), with respect to the extension line of the axis 9 of one of the welding electrodes 8, are displaced axis 9 of the other of said welding electrode 8, the electrode there is that misalignment occurs. When such electrode misalignment performs welding while generating, joined by deformation as shown in FIG. 6 (b), there are cases where stress is generated in the vicinity of the corona bond.
[0050]
When the electrode misalignment of the welding electrode is 0.5mm or more, in particular, easily cracked during nugget corona bond straight outer or corona bonding occurs. However, in the first embodiment or spot welding method of the second embodiment, there are inhibitory effects of these cracks. The electrode misalignment of the welding electrode is at 1mm or more, even if a larger stress is generated, there is the effect of suppressing cracks by spot welding method of the first or second embodiment.
[0051]
Disturbance factor (d): a state having a clearance between the superposed surfaces of the welding location
　, etc. When, as shown in (a) of Figure 9, the other member 11 between the steel plates 1 are inserted, welded portions superposition surface gap between the steel sheet 1 (hereinafter also referred to as "Itasuki") sometimes spot welding while retaining a. When such a plate clearance for the contact and pressure in the presence, as shown in (b) of FIG. 9, in the welding position, local deformation of the steel sheet 1 is observed. Doing welding in a state in which thus deformed, local stress is generated in the vicinity of the corona bond. Then, cracking is likely to occur at this point.
In weld members, when the axial direction of the plate gap in the strike position of the welding electrode is 0.2mm or more, in particular, easily cracked during nugget corona bond straight outer or corona bonding occurs. Even in such a case, in the first embodiment or spot welding method of the second embodiment, there is a suppression effect of such cracking. The plate gap is 1mm or more, more is at 2mm or more, even if a larger stress is generated, there is the effect of suppressing cracks by spot welding method of the first or second embodiment.
[0052]
　Next, spot welding method according to the above embodiment, further, sequentially described necessary requirements and preferred features. First, the member to be welded, is described.
[0053]
(Weld members)
　in the spot welding method according to the above embodiment, the member to be welded, a plurality of steel plates welded portions are superimposed, at least one or more steel plates of which, zinc-plated is covered it is preferred to prepare what is steel sheet. For example, zinc-based plated is coated, tensile strength 780MPa or more, Ceq is 0.15% by mass or more, may be prepared two steel plates having a thickness of 0.5 ~ 3.0 mm.
[0054]
　When performing spot welded to weld members, by superimposing two steel plates, from both sides, so as to sandwich the two steel plates, are energized while pressing the electrode made of a copper alloy, forming a molten metal and, by heat conduction to the heat removal and the steel plate by water-cooled electrode after the end of energization, it solidified rapidly cooled molten metal, during steel sheet to form a nugget oval cross section.
[0055]
　As an example of the condition of the spot welding, the electrode assumes the Domurajiasu type tip diameter 6 ~ 8 mm, pressure 1.5 ~ 6.0kN, energizing time 5-50 cycles (power supply frequency 50 Hz), energizing current 4 ~ 15 kA it may be.
[0056]
Weld members to be spot welded is composed of a plurality of steel plates, at least the welding portions are overlapped, if the zinc plating is coated on the overlapped surfaces of at least one or more steel plates of which, limited not shall. For example, all or a plurality of steel plates zinc-based plating superimposed surface of the steel plate is coated to be spot welded, zinc-based plating is steel sheet and spot welding coated superimposed surface of steel sheet is spot welded including a plurality of steel plates including steel plates zinc-based plating is not coated on the steel sheet is illustrated.
[0057]
　Further, in the steel sheet galvanized on overlapping surfaces to be spot welded is coated, overlapping surface opposite to the surface of steel sheet spot welding, that is, the zinc-plated on the contact surface between the welding electrodes, coated be or may not be coated. However, considering the corrosion resistance of the spot welded joint, that is the contact surface to be galvanized with the welding electrode is coated preferred.
[0058]
　As a plurality of steel plates to be spot welded, in FIGS. 6-9, has been described an example of two steel plates, depending on the form of structural parts to be joined, be a three or more of a plurality of steel plates can. The thickness of each steel sheet spot welding, but the present invention is not particularly limited, it may be 0.5 ~ 3.0 mm. The total thickness t of the entire plurality of steel sheets, and more 1.35 mm, the upper limit is not specifically limited, and may be less 7.0mm the SoitaAtsu t.
The lower limit of a more preferable total thickness t is at 2.4mm or more, more preferably not less than 2.7 mm. A more preferred upper limit value of the total plate thickness t is at 4.0mm or less, and more preferably 3.2mm or less. The SoitaAtsu t in this range, suppressing cracking during nugget corona bond straight outer and corona bond, ensuring the joint strength, more attained weight reduction strength of the welded members.
[0059]
　Further, the steel sheet has a plate-shaped portion on at least a portion, as long as it has a portion where the plate-like portion is stacked superposed each other or even entirely without a plate. Further, the plurality of steel plates, not limited to those composed of separate steel plate, or a superposition of those molding a single steel plate into a predetermined shape, such as tubular.
[0060]
　Further, the steel sheet of the welded member is spot welded, or chemical composition, metal structure such as, not limited in particular. However, steel galvanized were coated on overlapping surfaces to be welded, or, with respect to steel plates zinc-based plating is superimposed over the steel sheet and the galvanized coating, especially in such TRIP steel sheet above cracks but is likely to occur, such as even when the Ceq in other high-strength steel sheets with 0.15 mass% or more of the steel sheet, cracks at the time nugget corona bond straight outer and corona bonding is likely to occur Therefore, in the welding method of the present invention, in particular, it may be subject to such a steel sheet.
[0061]
　Ceq is defined as follows (Equation 4).
Ceq = [C] + [Si ] / 30 + [Mn] / 20 + 2 [P] +4 [S] ··· ( Equation 4)
　however, [C], "Si], [Mn], [ P], [S ] is, C, a content of Si, P, and S (mass%).
　from the viewpoint of weight reduction and high strength, preferably Ceq is at least 0.18 wt%, and more preferably Ceq is 0. 20 mass% or more.
[0062]
　Also, zinc-based plating is coated on the steel sheet to be welded, as long as a plating including zinc, not particularly limited, for example, as a plating seed, galvannealed, galvanized, electro-galvanized, zinc-nickel plating is exemplified. It can also include plating of zinc-aluminum-magnesium system.
[0063]
(Before the supply and the rear conduction)
in the above according to the first embodiment or the second embodiment spot welding method, prior to the welding operation, it may be further pre-energization. In prior energized, before the welding operation, by superimposing two steel plates are welded member, so as to sandwich the two steel plates from both sides, is pressurized by welding electrodes, before passing a current value current I f and (kA), the current value before the supply current I f state before the supply time t (kA) f to energize before holding (msec). Before the supply current I f (kA) and before the supply time t f (msec) is not particularly limited, in order to suppress the occurrence of scattering, before the supply current I f (kA), the welding operation current I W (kA) of 0.4 times or more, the welding current of the current I W less than (kA), before energization time t f (msec) is more than 20msec is exemplified.
[0064]
Before the energization time t f if (msec) has elapsed, while maintaining the pressure of the welding electrodes, the current value to 0 (zero), after pre-energized state of the current value 0 (zero) cooling time t C (msec )Hold. Before After energizing the cooling time t C (msec) is less than 0 msec 250 msec is illustrated. Incidentally, at the start before the supply, the current value immediately before the supply current I f does not (kA), the current value before the supply current I f until (kA), After cooling, while maintaining the pressure of the welding electrodes, the current value welding energization current I W may perform welding in the (kA).
When performing before the supply, at the start of the welding current is energized at the start before the supply.
[0065]
Above the spot welding method according to the first or second embodiment, after the welding operation may be performed post-energizing further. In the post-energized, after the welding operation has been completed, while maintaining the pressure of the welding electrodes, the current value to 0 (zero), the current value is 0 (zero) state cooling time after welding current supply (clotting time ) t S (msec) and held, solidifying from the outer periphery of the molten metal to form a shell-like solidification range unsolidified region remains inside. Time of cooling is not particularly limited, in order to improve the toughness of the heat affected zone by energizing after that will be described later, 1 ~ 300 msec is illustrated. Cooling time after welding current t S when (msec) has elapsed, while maintaining the pressure of the welding electrodes, when the unsolidified region is present, the rear energizing current I a current value P and (kA), I P rear energization time t the state of (kA) P and (msec) holding, performs post-energization. Rear energization current I P (kA) and the rear energization time t P (msec) is not limited in particular, P significantly affect the toughness of the heat affected zone, in order to reduce the segregation of S and the like, rear energized current I P (kA), the welding operation of the current I W 0.6 times or more (kA), the welding current of the current I W (kA) hereinafter, rear energization time t P (msec) are exemplified 1 ~ 500 msec It is.
[0066]
Incidentally, to improve the continuous dotting property of the spot welding, the rear conduction and cooling for the electric current value to 0 (zero) as one step may be repeated twice or more. Is when two or more times, the cooling time t of the first step S , the rear energization current I P , the rear energization time t P and, when the same and these conditions in the subsequent steps, the working efficiency preferably. Each rear energization time t in a subsequent energization P total is preferably 80 ~ 2000 msec.
When performing post-energization, the end welding current is energized at the end of the post-energized. Above the spot welding method according to the first embodiment or the second embodiment may be carried out both before the supply and the rear conduction above.
[0067]
(Previously carried out spot welding)
　after the end welding operation between the welding electrodes to be welded member, immediately performs spot welding to the non-contact with the welding electrode and the workpiece to be welded in advance, corona bonding straight out of the crack and nugget corona bond cracks were confirmed when, when the cracks occur, since, in the welding for the welding parts, may implement the welding by spot welding method according to the first or second embodiment.
[0068]
　Previously performed spot-welded to the workpiece to be welded after completion welding operation between the welding electrodes, immediately to the welding electrode and the workpiece to be welded in a non-contact, without extending the holding time after welding Ht, similar to the above spot welding to, by superimposing two steel plates, from both sides, so as to sandwich the two steel plates, are energized while pressing the electrodes, to form a nugget oval cross section. Conditions of spot welding in this case, it is possible to employ the same conditions as described above.
[0069]
　Then, to check for the occurrence of nugget during cracking of the overlay plane corona bond straight out of the crack and the corona bonds. Confirmation of cracks, for example, to include a nugget, and cut in the thickness direction is performed to check the cross-section. As shown in FIG. 2, when checking cracking nugget when on at least one of the superposed surface corona bond straight outer and corona bonds, so as to satisfy the condition defined in the first or second embodiment a manner may be carried out again spot welding.
[0070]
　To check the occurrence of nugget during cracking of the corona bond straight outer and corona bonding spot welded joint, check method of occurrence of cracks is not particularly limited. And how to do by observing the thickness direction of the cross section including the nugget can be used a method that performs the determination by a tensile test of spot-welded joints in whether a predetermined tensile strength. Or, depending on the cutting position of the thickness direction of the cross section including the spot weld, since the cracks during nugget corona bond straight outer and corona bonds may not be observed, to confirm the crack by performing an X-ray transmission test it may be.
[0071]
　Cracking during nugget corona bond straight outer and corona bonding are superimposed surface of steel sheet galvanized is coated, or, in the steel sheet zinc-based plating is superimposed over the steel sheet and the galvanized coating generated on the surface. Then, among the surfaces of these steel sheets, generated in the corona bond and its vicinity. The corona bond within and near the one surface of the steel sheet, the nugget end of the corona bond is that up to approximately 1.2 times the corona bond diameter Dc.
Example
[0072]
　Next, a description will be given of an embodiment of the present invention, conditions in examples are an example of conditions adopted for confirming the workability and effects of the present invention, the present invention is, in this single condition example the present invention is not limited. The present invention does not depart from the gist of the present invention, as long as they achieve the object of the present invention, it is capable of adopting various conditions.
[0073]
(Example 1)
　Table 1 shows the steel sheet A ~ I used for the test. Steel shown in Table 1 A ~ F, H and I are both a galvannealed steel sheet, a zinc plating is coated on both sides. Steel A ~ F, the coating layer of the decorated with galvannealed to H and I, is alloyed after dipped in a zinc plating bath after annealing, or attached to hot stamp or galvanizing bath, after dipped in a zinc plating bath It was formed through a process called.
[0074]
[Table 1]

[0075]
　The combination of steel plate as shown in Table 2, in various welding conditions and the disturbance factors shown in Table 2, were spot welded using a welding machine of the servo gun type. Further, in all of the spot welding, the tip curvature radius in the copper Domurajiasu type 40 (mm), it was welded using the electrode tip diameter 8 (mm). It should be noted that, during pressurization was so as not to change the pressure.
[0076]
[Table 2]

[0077]
　Then, a welded joint obtained were each cut along the center line of the weld, by observing the cross section was determined the existence of cracks. Furthermore, in a defined manner in JIS Z 3137 under the same welding conditions, measured spot welded joint CTS (the cross tensile force). Table 3, post-weld holding time Ht lower limit of which is defined by Equation (2) (Ht min), the maximum value of the "sum of the holding time Ht and squeeze time St" defined by Equation (3) (Ht + St max), and shows the presence of cracks, results of CTS and welded joint strength ratio.
[0078]
　Welded joint strength ratio is the ratio relative to the CTS spot welding joint welding conditions and disturbances factors other than the holding time after welding Ht or squeeze time St was formed in the same. For example, the weld joint strength ratio of numbers 1a, except holding time after welding Ht at the same welding conditions by subtracting the CTS numbers 1a from CTS numbered 1 to form a spot welded joint, divided by the number 1 CTS , is a numerical value obtained by multiplying by 100. Welded joint strength ratio of similarly numbered 2b is a value calculated based on the CTS number 2. Incidentally, the weld joint strength ratio is not less than 30%, it is determined that CTS is lowered.
[0079]
[table 3]

[0080]
　As can be seen from Table 3, No. 1-16, the result of the numbers 1a ~ 6a and No. 10a ~ 16a, a crack in the nugget corona bond straight outer and corona bonding does not occur. This squeeze time St satisfies the (Equation 1), and is believed to be due to the relationship between the SoitaAtsu t and welding after holding time Ht satisfies the (Equation 2).
[0081]
　In contrast, in number 1b ~ 9b and number 12b ~ 16b, a crack in the nugget corona bond straight outer or corona bonding has occurred. This cracking, in some cases caused also reduction of CTS. This is considered to be because the relationship between SoitaAtsu t and welding after holding time Ht does not satisfy the equation (2).
　Also, number 1c, 3c, in 10c and 11c, cracking occurs during nugget corona bond straight outer or corona bonding, it is found that no sufficient CTS is obtained. This squeeze time St is believed to be due not satisfying the equation (1). In the experimental example No. 11c, restless squeeze time St is the (Equation 1), and the relationship between SoitaAtsu t and welding after holding time Ht is not satisfied (Equation 2).
[0082]
　In the results of the numbers 1a ~ 6a and No. 10a ~ 16a, although cracking during nugget corona bond straight outer or corona bond was not confirmed, the bonding strength because of high and low CTS compared to the numbers 1 to 16 There is a tendency. This is considered to be because the relationship between SoitaAtsu t as "post-weld holding time Ht + squeeze time St" does not satisfy the equation (3).
[0083]
(Example 2)
　shows the steel sheet A was subjected to the test in Table 4. Steel A is a galvannealed steel sheet, a zinc plating is coated on both sides. Coating layer has been subjected galvannealed steel sheet A is and composition of the coating layer is the same as in Example 1.
As shown in Table 5, the combination of the two steel plates A, at various welding conditions and the disturbance factors were spot welded using a welding machine of the servo gun type. Further, in all of the spot welding, the tip curvature radius in the copper Domurajiasu type 40 (mm), it was welded using the electrode tip diameter 8 (mm). It should be noted that, during pressurization was so as not to change the pressure.
[0084]
[Table 4]

　
[0085]
[table 5]

[0086]
　Then, in the same manner as in Example 1, was a confirmation of the presence or absence of cracks in the spot welded joint, the measurement of the CTS. Table 6, after welding the retention time Ht lower limit of which is defined by Equation (2) (Ht min), the maximum value of the "sum of the holding time Ht and squeeze time St" defined by Equation (3) (Ht + St max), and shows the presence or absence of cracks, the CTS and welded joint strength ratio. Incidentally, the weld joint strength ratio is not less than 30%, it is determined that CTS is lowered.
[0087]
[Table 6]

[0088]
　As can be seen from Table 6, No. 1 in the results of the 1-8 and No. 1a-8a, cracking during nugget corona bond straight outer and corona bonding does not occur. This squeeze time St satisfies the (Equation 1), and is believed to be due to the relationship between the SoitaAtsu t and welding after holding time Ht satisfies the (Equation 2).
[0089]
　In contrast, in number 1b ~ 8b, a crack in the nugget corona bond straight outer or corona bonding has occurred. This is considered to be because the relationship between SoitaAtsu t and welding after holding time Ht does not satisfy the equation (2).
Further, even in number 1c ~ 8c, cracking during nugget corona bond straight outer or corona bonding occurs. Squeeze time St is believed to be due not satisfying the equation (1). In the experimental example No. 8c, restless squeeze time St is the (Equation 1), and the relationship between SoitaAtsu t and welding after holding time Ht is not satisfied (Equation 2).
[0090]
　The numbers in the results of 1a ~ 8a, although cracking during nugget corona bond straight outer or corona bond was not confirmed, the bonding strength ratio is high, in CTS low tendency compared to the numbers 1-8. This is considered to be because the relationship between SoitaAtsu t as "post-weld holding time Ht + squeeze time St" does not satisfy the equation (3).
[0091]
　Then, according to the disturbance factor, it will be described variation of the welding location. 10, in the weld portion subjected to spot welding in punching angle 5 degrees, a cross-sectional photograph taken by an optical microscope thickness direction cross-section. By having a striking angle, it is found to be different manner of deformation of the steel sheet in the left and right of the nugget.
Figure 11 is a cross-sectional photograph in the case of providing a 1mm clearance between the steel plate and the electrode without providing the equalizing mechanism performing spot welding. In welding location, it can be seen that a state of deformation of the lower steel plate and an upper steel plate different.
[0092]
Figure 12 is a sectional photograph in the case of performing spot welding in a state where the axis of the welding electrode is displaced 1.5 mm. It can be seen that the vertical indentation is asymmetrical.
Figure 13 is a sectional photograph in the case of performing spot welding in the presence of a plate gap of 2mm height from RBI to the location of 20mm on one side between the overlapping surfaces of the welding location. State of deformation of the steel plate in the left and right of the nugget is seen to be different.
Industrial Applicability
[0093]
　According to the present invention, in the spot welding, so that the holding time after welding of the welding electrode as a function of the total thickness, that the securing of the suppression and joint strength of cracking during nugget corona bond straight outer and corona bond it can. In particular, the liquid intended for a plurality of steel plates directly affects the strength of the joint, occurs when nugget corona bond straight outer and corona bonding of overlapping surfaces of the steel sheet comprising a galvanized high strength steel sheet for automobiles It prevents metal cracking, to a spot welding method capable of ensuring the joint strength. Accordingly, the present invention has high industrial applicability.
DESCRIPTION OF SYMBOLS
[0094]
　1 steel plate
　2 nugget
　3 progresses from the surface toward the nugget steel cracking
　4 HAZ
　5 shoulder progresses toward the heat-affected zone from cracking
　6 corona bond straight out of the crack
　7 corona bond breakage during nugget
　8 Welding electrode
　9 shaft core
　10 perpendicular
　11 other members
　Dc corona bond diameter

claims

[Claim 1]A method of spot welding the workpiece to be welded constituted by a plurality of steel plates, at least the welding portions are overlapped,
　the welded member is superposed surfaces of at least the welding portions are coated with zinc-plated wherein at least one of steel, the total thickness of the plurality of steel plates t (mm) is not more than 1.35 mm,
　the squeeze time from contacting the welding electrode to the workpiece to be welded to the start of supply of welding current St (seconds) satisfy the following (equation 1), and wherein said from the end welding current between the welding electrodes welding electrode and the welding after holding time until the non-contact with the welded members Ht (in seconds) is the following spot welding method characterized by satisfying the equation (2).
　0.020 ≦ St · · · (Equation 1)
　0.015T 2 + 0.020 ≦ Ht · · · (Equation 2)
[Claim 2]
The weld holding time Ht (s) and the squeeze time St (seconds) is, spot welding method according to claim 1, characterized by further satisfying the following (Equation 3).
Tasu St ≦ Ht 0.20T 2 -0.40T Tasu 1.50 · · · (Equation 3)
[Claim 3]
　Just before contacting the welding electrode to the workpiece to be welded, of the conditions below (a) ~ (d), spot welding according to claim 1 or 2, characterized in that meet one or more Method.
And the axis of (a) the welding electrode, the state and the perpendicular surface of the steel sheet in contact with the welding electrode is non-parallel,
and the distance to one surface of the steel sheet from the tip of the welding electrode of one (b), the other of the distance from the tip of the welding electrode to the other surface of the steel sheet is different states,
with respect to the extension line of the axis of the welding electrode of one (c), a state where the axis of the other of said welding electrodes are displaced and
a state in which a gap between the surface superposition of; (d) welding spot
[Claim 4]
Wherein at least one of a plurality of steel plates, the tensile strength is spot welding method according to any one of claims 1 to 3, characterized in that the high-strength steel sheet is not less than 780 MPa.
[Claim 5]
　Before the welding operation, further pre performs energization, spot welding method according to any one of claims 1 to 4, characterized in that during the welding operation start is at the energization start of the pre-energization.
[Claim 6]
　After said welding operation, performed post-energization Further, spot welding method according to any one of claims 1 to 4, characterized in that during the supply of welding current terminates is energized after the clear after energization.
[Claim 7]
　Before the welding operation, performs further before the supply, the a at the start welding power energization start of the pre-energizing, and
after the welding operation, performed post-energization Further, during the welding operation completed after the it is energized at the end of the energization
spot welding method according to any one of claims 1 to 4, characterized in that.