JP2017047475A - Spot welding method - Google Patents

Abstract

The present invention provides a welding method capable of forming a high-quality spot welded joint by suppressing cracks that occur immediately outside the corona bond and on the corona bond nugget of the overlapping surface of a plate assembly including a zinc-based plated steel sheet.
A spot welding method is performed on a member to be welded composed of a plurality of steel plates on which at least welded portions are overlapped, and at least one of the plurality of steel plates is overlapped. The surface is covered with zinc-based plating, the total thickness t (mm) of the plurality of steel plates is 1.35 mm or more, and the welding electrode and the welded member are A spot welding method, wherein a post-weld holding time Ht (seconds) until non-contact is within the range of the following formula. 0.015t ² + 0.020 ≦ Ht ≦ 0.16t ² −0.40t + 0.70
[Selection] Figure 3

Description

  The present invention relates to a spot welding method for a plurality of steel sheets including galvanized steel sheets.

In recent years, in the automobile field, it has been required to reduce the weight of a vehicle body in order to reduce fuel consumption and reduce CO ₂ emissions, and to increase the strength of vehicle body members in order to improve collision safety. In order to satisfy these requirements, it is effective to use high-strength steel sheets for vehicle body members and various parts.

  Moreover, it is necessary to comprise a member with the steel plate excellent in corrosion resistance from a viewpoint of the high rust prevention of a vehicle body. Zinc-based galvanized steel sheets are widely known to have good corrosion resistance. From the viewpoints of lightening and increasing the strength described above, zinc-based galvanized steel sheets used for automobiles are made of high-strength steel sheets. The used zinc-based plated high-strength steel sheet is used.

  Spot welding is mainly used in the assembly of automobile bodies and parts. However, when spot welding is performed on a zinc-based plated high-strength steel plate, there is a problem that cracks occur from the outer surface of the steel plate in contact with the welding electrode toward the nugget. In FIG. 1, the outline of the crack of the welding location at the time of performing spot welding to a zinc-based plated steel plate is shown. When spot welding is performed on the galvanized steel sheet 1, a crack 3 that progresses from the outer surface of the steel sheet 1 in contact with the welding electrode toward the nugget 2 and a crack 5 that progresses from the electrode shoulder to the heat affected zone 4 occur. It has been known.

  This crack is caused by the applied pressure of the electrode, the thermal expansion and contraction of the steel sheet, and the tensile stress due to the shrinkage applied to the welded part. It is said to be a crack caused by so-called liquid metal embrittlement caused by entering the boundary and lowering the grain boundary strength. In automobile bodies, there is a problem that the strength decreases when cracks in the welded portion are significant, and a technique for suppressing cracks in the welded portion by controlling the composition and structure of the steel sheet is known.

  For example, Patent Document 1 adjusts the component composition of a steel sheet, changes the austenite phase generated during spot welding to fine crystal grains, and has a metal structure that is complicated with the crystal grains of other phases. Thus, a technique has been disclosed in which the diffusion and penetration path of molten zinc into the crystal grain boundary is complicated to make it difficult for the molten zinc to enter and suppress liquid metal embrittlement cracking during spot welding.

  Further, in Patent Document 2, the composition of the steel sheet is adjusted and the hot-rolled steel sheet is adjusted, assuming that the occurrence of cracks in the welded part may not be sufficiently suppressed only by making the grain boundaries complicated by controlling the structure of the steel sheet. The grain boundary oxidization depth of the alloyed hot-dip galvanized steel sheet is reduced to 5 μm or less by performing Fe-based electroplating on the cold-rolled steel sheet before the alloying hot-dip galvanizing process. Thus, a technique for suppressing the occurrence of cracks at the welded portion of the galvannealed steel sheet is disclosed.

  Patent Document 3 discloses a technique for removing plating at a butt portion of a strip end portion in order to suppress embrittlement of liquid metal when an ERW steel pipe is manufactured from a steel plate plated with zinc or the like. ing.

JP 2006-265671 A JP 2008-231493 A Japanese Patent Laid-Open No. 05-277552

Although measures for cracking of spot welds have been studied in this way, in some spot welded joints of galvanized steel sheets, or in spot welded joints of some non-plated steel sheets and galvanized steel sheets, In some cases, the desired bonding strength cannot be obtained.
It is an object of the present invention to provide a spot welding method capable of suppressing cracking at the corona bond directly outside and at the corona bond nugget and ensuring the joint strength.

(1) A spot welding method according to an aspect of the present invention is a method of spot welding a welded member composed of a plurality of steel plates in which at least welding locations are overlapped, and at least of the plurality of steel plates. For one, at least the overlapping surface of the welded portions is coated with zinc-based plating, the total thickness t (mm) of the plurality of steel plates is 1.35 mm or more, and when the welding energization between the welding electrodes is completed The holding time after welding Ht (seconds) until the welding electrode and the member to be welded are brought into non-contact is set within the range of the following formula (1).
0.015t ² + 0.020 ≦ Ht ≦ 0.16t ² −0.40t + 0.70 (1)
According to the spot welding method having the above-described configuration, it is possible to suppress the cracking of the corona bond directly outside and the corona bond at the time of the nugget and ensure the joint strength.

(2) According to another aspect of the present invention, in the spot welding method of (1), the post-weld holding time Ht (seconds) further satisfies the following expression (2).
Ht ≦ 0.10t ² −0.40t + 0.80 (2)
In the above aspect, not only can the cracks immediately outside the corona bond and the nuggets of the corona bond be suppressed, but also desired joint strength can be obtained while ensuring productivity.

(3) According to another aspect of the present invention, in the spot welding method of (1) or (2), the post-weld holding time Ht (seconds) further satisfies the following expression (3).
0.015 t ² + 0.110 ≦ Ht (3)
In the above aspect, it is possible to obtain a desired joint strength while suppressing variations due to electrode materials and cooling performance and ensuring productivity.

(4) According to another aspect of the present invention, in the spot welding method according to any one of (1) to (3) above, the total thickness t (mm) of the plurality of steel plates is 2.4 mm or more. 2 mm or less.
In the above aspect, in addition to ensuring the effect of suppressing cracking and securing the joint strength, the welded member can be reduced in weight and strength.

(5) According to another aspect of the present invention, in the spot welding method according to any one of (1) to (4) above, at least one of the plurality of steel plates has a high strength steel plate having a tensile strength of 780 MPa or more. It is.
In the said aspect, the weight reduction and high intensity | strength of a to-be-welded member are attained by making a steel plate into a high strength steel plate whose tensile strength is 780 Mpa or more.

(6) According to another aspect of the present invention, in the spot welding method according to any one of (1) to (5), immediately before the welding electrode is brought into contact with the member to be welded, the following (a) to ( One or more of the conditions of d) are satisfied.
(A) a state in which the axis of the welding electrode and the perpendicular to the surface of the steel sheet in contact with the welding electrode are not parallel;
(B) a state in which the distance from the tip of one welding electrode to the steel plate surface closest to the tip is different from the distance from the tip of the other welding electrode to the steel plate surface closest to the tip;
(C) a state in which there is no axial center of the other welding electrode on an extension line of the axial center of the one welding electrode; and
(D) A state in which there is a gap between the overlapping surfaces of the welded portions.
When there is a disturbance factor defined in the above aspect, cracks in the nugget of corona bond directly outside and corona bond become prominent, but according to the spot welding method of any one of (1) to (5) above, Such cracks can be suppressed and joint strength can be ensured.

(7) According to another aspect of the present invention, in the spot welding method according to any one of (1) to (6) above, immediately after the end of welding energization between the welding electrodes to the welded member, Spot welding is performed in which the welding electrode and the member to be welded are not in contact with each other, and the presence or absence of cracks immediately outside the corona bond on the overlapped surface of the steel sheet and the crack at the time of corona bond nugget is confirmed. When at least one of the crack in the nugget of the corona bond and the crack in the corona bond is generated, the welding is further performed within the range of the retention time after welding Ht (seconds).
In the said aspect, since the spot welding method in any one of said (1) to (6) is applied with respect to the welding location which the crack generate | occur | produced, it is efficient.

  According to the present invention, in spot welding of a plate assembly including a galvanized steel sheet, even when a disturbance factor is present, cracks at the corona bond directly outside and at the corona bond nugget are suppressed, and the joint strength is ensured. can do.

It is a figure which shows the outline of the crack in the cross section of the plate | board thickness direction of the welding location which spot-welded the zinc-based plated steel plate. It is the schematic which shows the crack at the time of the corona bond right outside or corona bond nugget in the cross section of the plate | board thickness direction of the welding location which spot-welded the zinc-based plated steel plate. It is a figure which shows the relationship of the retention time after welding Ht with respect to the total board thickness t in the crack and joint strength in the case of a corona bond right outside or corona bond. It is the schematic for demonstrating the state where the axial center of a welding electrode and the perpendicular of the steel plate surface which contacts this welding electrode are not parallel. It is the schematic for demonstrating the state from which each distance from each front-end | tip part of the welding electrode which opposes to each steel plate surface differs. It is the schematic for demonstrating the state which has no axial center of the other said welding electrode on the extension line of the axial center of one said welding electrode. It is the schematic for demonstrating the state which had the clearance gap between the overlapping surfaces of a welding location. It is a cross-sectional photograph of the welding location which performed spot welding in the state where the axial center of a welding electrode and the perpendicular of the steel plate surface which contacts this welding electrode are not parallel. It is a cross-sectional photograph of the welding location which performed spot welding in the state from which each distance from each front-end | tip part of the opposing welding electrode to each steel plate surface differs. It is a cross-sectional photograph of the welding location which performed spot welding in the state where there is no axial center of the other said welding electrode on the extension line of the axial center of one said welding electrode. It is a cross-sectional photograph of the welding location which performed spot welding in the state which had the clearance gap between the overlapping surfaces of a welding location.

  The inventors of the present invention investigated the reason why a desired joint strength could not be obtained in a spot welded joint. In FIG. 2, the schematic of the cross section of the plate | board thickness direction containing a nugget in the welding location after performing spot welding to a zinc-based plated steel plate is shown. As shown in FIG. 2, in the spot welded joint where the desired tensile strength cannot be obtained, cracks 6 immediately outside the corona bond on the overlapping surface of the steel plates were generated. Moreover, the crack 7 at the time of the corrugated bond nugget sometimes occurred.

And in the positional relationship between the welding electrode and the member to be welded immediately before spot welding, the inner surface on which the steel plates are overlapped when welding is performed in the presence of disturbance factors such as the following (a) to (d) It was found that cracks occurred immediately outside the corona bond and on the side of the corona bond nugget.
(A) A state in which the axis of the welding electrode is not parallel to the perpendicular to the surface of the steel sheet in contact with the welding electrode (b) The distance from the tip of one welding electrode to the surface of the steel plate closest to the tip, and the other (C) State in which the distance from the tip of the welding electrode to the surface of the steel plate closest to the tip is different (c) State in which the axis of the other welding electrode is not on the extension line of the axis of the welding electrode (d) A state with a gap between the overlapping surfaces of the welds

  When the disturbance factors as described above are present, after the pressurization and holding of the welding electrode (after the electrode is opened), there are places where the residual stress becomes high immediately outside the corona bond and when the corona bond is nuggeted. About this phenomenon, the present inventors, the molten zinc-based plating penetrates into the grain boundary of the steel sheet of the relevant location, reduces the grain boundary strength, and generates cracks at the corona bond directly outside or corona bond nugget I thought that I was letting.

  Conventionally, the steel sheet and the welding electrode are not contacted immediately after the energization between the welding electrodes is completed. However, when the welding electrode is held under pressure after the end of energization and the molten zinc-based plating is solidified before the electrode is opened, cracks may not occur on the corona bond directly outside or on the corona bond nugget. I found it. In addition, when pressure holding was continued, the joint strength was found to decrease, and cracking was suppressed and the pressure holding time during which the joint strength was not reduced was examined. It was found that by using the thickness as a function, cracks can be suppressed and sufficient joint strength can be obtained.

  According to the spot welding method according to the present embodiment, in spot welding of a plate assembly including a galvanized steel sheet, cracks occurring at the time of corona bond directly outside the corona bond and on the corona bond nugget are suppressed. Quality spot welded joints can be formed.

  Although embodiments of the present invention will be described below, it is obvious that the present invention is not limited to these embodiments.

  The spot welding method according to the present embodiment is a method of spot welding to a member to be welded composed of a plurality of steel plates on which at least welding locations are overlapped, and at least about at least one of the plurality of steel plates. The overlapping surface of the welded portions is covered with zinc-based plating, and the total thickness t (mm) of the plurality of steel plates is 1.35 mm or more. And holding time after welding Ht (seconds) until no contact is made within the range of the following equation (1).

0.015t ² + 0.020 ≦ Ht ≦ 0.16t ² −0.40t + 0.70 (1)

Next, the expression (1) will be described. This expression is an expression obtained experimentally.
FIG. 3 shows the relationship of the post-weld holding time Ht to the total plate thickness t in the crack and joint strength immediately outside the corona bond or in the corona bond nugget. FIG. 3 is created from the results of experiments in later-described examples and various plate assemblies. In addition, in a member to be welded having a total plate thickness of less than 1.35 mm, since the occurrence rate of cracks at the corona bond directly outside or at the corona bond nugget is small, equation (1) is the case where the total plate thickness is 1.35 mm or more. Applicable to.

  As will be described in detail in the examples, this result shows that when spot welding is performed using two steel sheets of various steel types coated with zinc-based plating, the total thickness t of the steel sheet and the end of welding energization between the welding electrodes are completed. The post-welding holding time Ht from when the welding electrode and the member to be welded are brought into non-contact is variously changed. And about the obtained welded joint, the presence or absence of the crack at the time of the corona bond right outside or corona bond nugget, and the cross tensile strength (CTS) measured by applying a tensile load in the peeling direction were confirmed.

  Cracks at the outside of the corona bond or the corona bond nugget were confirmed by observing a cross section in the thickness direction including the nugget, and the cross tensile strength (CTS) was confirmed by the method defined in JIS Z3137.

In the total thickness t of the triangular or round marker shown in FIG. 3 and the retention time Ht after welding, the welded joint had no cracks immediately outside the corona bond or corona bond nugget, and had a sufficient CTS. . On the other hand, the total thickness t of the square markers located below the line A in FIG. 3 and the retention time Ht after welding had cracks directly outside the corona bond or at the time of the corona bond nugget. Accordingly, the boundary (the boundary between the triangular or round marker and the square marker) where the crack occurs immediately outside the corona bond or at the corona bond nugget is the line A (Ht = 0.015t ² +0.020). It has been found.

  By setting the retention time Ht after welding to a value equal to or greater than the line A (greater than or equal to the left side of the formula (1)) with respect to the total thickness t, cracks directly outside the corona bond or when the corona bond is nuggeted. The reason for disappearance is considered as follows.

  First, the part where the welding residual stress increases immediately outside the corona bond and during the corona bond nugget is in a compressed state in the process of being crushed by the welding electrode during spot welding, but is in a tensile state when the welding electrode leaves the steel plate surface. And tensile stress occurs. The occurrence of cracks in the corona bond directly outside and on the corona bond nugget is caused by the fact that the zinc-plated metal melted directly outside the corona bond has a high residual welding stress in the corona bond directly outside and on the corona bond nugget after welding. It is caused by intruding into the crystal grain boundary and lowering the grain boundary strength.

  Since the location where the residual stress increases occurs after the welding electrode is separated from the steel plate surface (after the electrode is opened), the welding electrode is kept under pressure after energization (holding time after welding Ht is extended). By solidifying the molten zinc-based plating before opening the electrode, the molten zinc-based plating does not enter the crystal grain boundaries of the steel sheet at the location where the welding residual stress is high, and cracks can be suppressed. Since the solidification of the zinc-based plating is related to the ease of cooling the steel sheet, that is, the total thickness t of the member to be welded, the post-welding holding time Ht of the welding electrode is adjusted as a function of the total thickness t. Thereby, the crack at the time of the nugget of corona bond right outside or a corona bond can be suppressed.

Further, in the total thickness t of the square marker located above the line B in FIG. 3 and the retention time Ht after welding, there is no crack at the welded joint immediately outside the corona bond or at the corona bond nugget, but the CTS. The drop in was large. Thus, the boundary (a boundary between the square marker and the triangle or circle marker) at which sufficient CTS is obtained may be a line B (Ht = 0.16t ² −0.40t + 0.70) shown in FIG. found.

  The reason why a sufficient CTS can be obtained by setting the post-weld holding time Ht to a value not more than the line B (below the right side of the equation (1)) with respect to the total thickness t is as follows. It is considered that the cross tensile strength does not decrease because the cooling rate of the material does not become too high and the hardness of the nugget and the heat affected zone does not increase.

  Therefore, when the total plate thickness t (mm) is used as a function and the post-weld holding time Ht (seconds) is set to be not less than the left side of the equation (1) and not more than the right side of the equation (1), It was found that cracking during bond nugget can be suppressed and sufficient CTS can be obtained, and the above formula (1) was found.

In the spot welding method according to the embodiment, it is more preferable that the post-weld holding time Ht (seconds) further satisfies the following expression (2).
Ht ≦ 0.10t ² −0.40t + 0.80 (2)

  When the post-weld holding time Ht satisfies the condition of equation (2), which is a function of the total thickness t, a higher cross tensile strength (CTS) can be obtained, and a more reliable spot welded joint is provided. it can.

Furthermore, in the spot welding method according to the embodiment, it is more preferable that the post-weld holding time Ht further satisfies the following expression (3).
0.015 t ² + 0.110 ≦ Ht (3)

  Cooling by the welding electrode varies depending on the material of the electrode and the amount of cooling water, and the cooling mode may vary. When the post-weld holding time Ht satisfies the condition of the expression (3), which is a function of the total plate thickness t, a desired joint strength is obtained while suppressing variations due to electrode materials and cooling performance and ensuring productivity. be able to.

In the spot welding method according to the above embodiment, it is further preferable that the post-weld holding time Ht further satisfies the following expression (4).
0.015t ² + 0.110 ≦ Ht ≦ 0.10t ² −0.40t + 0.80 (4)

  In the spot welding according to the above embodiment, two steel plates are overlapped and energized while pressing an electrode made of a copper alloy or the like so as to sandwich the two steel plates from both sides to form a molten metal, After the completion of the process, the molten metal is rapidly cooled and solidified by heat removal by the water-cooled electrode or heat conduction to the steel sheet itself, and a nugget having an elliptical cross section is formed between the steel sheets.

  The spot welding conditions at this time are not particularly limited as long as the post-welding holding time Ht with respect to the total sheet thickness t satisfies the above-described expression (1). For example, when the total thickness t is 4.0 mm, the post-weld holding time Ht is 0.24 to 1.66 seconds, the electrode is a dome radius tip having a diameter of 6 to 8 mm, and the applied pressure is 1.5 to It may be 6.0 kN, energization time 5 to 50 cycles (power supply frequency 50 Hz), and energization current 4 to 15 kA.

  As a result, the molten zinc-based plating does not invade the place where the welding residual stress is high, so that the occurrence of liquid metal cracks can be prevented immediately outside the corona bond and at the corona bond nugget, and sufficient CTS can be obtained. it can.

  In the spot welding method according to the present embodiment, immediately before the welding electrode is brought into contact with the member to be welded, one or two or more conditions among the following disturbance factors (a) to (d) are satisfied. In addition, it is possible to suppress cracks immediately outside the corona bond and at the time of corona bond nugget, and to secure joint strength. Therefore, it is preferable to perform the spot welding method according to the first to fifth aspects of the present invention when the following disturbance factors are present.

Disturbance factor (a): A state in which the axis of the welding electrode and the perpendicular to the surface of the steel sheet contacting the welding electrode are not parallel (non-parallel) In spot welding, the welding electrode is applied perpendicularly to the surface of the steel sheet. Is the basic. However, there are a plurality of welding locations on the member to be welded, and welding in various welding postures may be required. In such a case, due to poor teaching of the spot welder or restrictions on working time, the time for correcting the welding posture cannot be secured, and the axis of the welding electrode and the steel plate surface in contact with the welding electrode are inclined from the vertical. Spot welding may be performed as is. FIG. 4A is a schematic view showing a state in which the axis 9 of the welding electrode 8 and the perpendicular 10 on the surface of the steel plate 1 in contact with the welding electrode 8 are not parallel.

  As described above, when welding is performed in a state where the angle between the axis 9 of the welding electrode 8 and the surface of the steel plate 1 in contact with the welding electrode 8 deviates from vertical, the steel plate 1 in contact with the welding electrode 8. The axis 9 of the welding electrode 8 has an angle called a striking angle with respect to the normal 10 on the surface. If the welding electrode 8 is brought into contact with the surface of the steel plate 1 in a state where the angle of attack is greater than 0 degrees (a state where the axis 9 of the welding electrode 8 is not perpendicular to the surface of the steel plate 1), FIG. Thus, the steel plate 1 is deformed. Due to such deformation, a portion where the stress is relatively high is generated in the vicinity of the corona bond, and a crack is easily generated in this portion.

  When the striking angle is 3 ° or more, cracks are particularly likely to occur immediately outside the corona bond and during the corona bond nugget, but the spot welding method of this embodiment has an effect of suppressing these cracks. In addition, when the striking angle is 5 degrees or more, there is a tendency that the joint strength further decreases due to cracks in the corona bond directly outside and the corona bond nugget, but in the spot welding method of the present embodiment, Quality deterioration can also be suppressed. In addition, by sandwiching a pressure sensitive paper between the welding electrode 8 and the steel plate 1 and punching the welding electrode 8 (only applying pressure without energization), the shaft core 9 of the welding electrode 8 and the It is possible to detect whether or not the angle with the surface of the steel plate 1 in contact with the welding electrode 8 deviates from the vertical.

Disturbance factor (b): State in which the distance from the tip of one welding electrode to the steel plate surface closest to the tip is different from the distance from the tip of the other welding electrode to the steel plate surface closest to the tip During welding, an equalizing mechanism may be provided to maintain the center position of the held plate against the sandwiching of the welding electrode. When the equalizing mechanism is provided, the spot welding gun is increased in size by that amount, and the welding robot must be adapted to the size, which increases the cost of the welding robot. Therefore, the spot welding gun may be welded without providing an equalizing mechanism.

  In this case, as shown in FIG. 5 (a), immediately before the welding electrode 8 is brought into contact with the member to be welded (steel plate 1), the steel plate 1 on the side close to the tip from the tip of one welding electrode 8 is provided. The distance to the surface is different from the distance from the tip of the other welding electrode 8 to the surface of the steel plate 1 on the side close to the tip. When contact / pressurization of the welding electrode 8 is performed in such a state, as shown in FIG. 5B, one steel plate 1 (in the case of FIG. 5, the upper steel plate 1) is present at the welding location. The other steel plate 1 is deformed. When welding is performed in such a deformed state, local stress is generated in the vicinity of the corona bond. And it becomes easy to produce a crack in this location.

As mentioned above, when welding without providing an equalizing mechanism to the spot welding gun, or when the distance between the welding electrode and the steel plate surface cannot be set appropriately due to the construction of the welded member, the axial direction of the welding electrode In this case, the positional deviation between the spot welding gun and the member to be welded is not absorbed, and cracks are likely to occur immediately outside the corona bond or at the time of the corona bond nugget.
In the case where there is no equalizing mechanism and the clearance is 1 mm or more, cracks are particularly likely to occur immediately outside the corona bond and when the corona bond is nuggeted. It has the effect of suppressing cracks on the outside of the bond and the corona bond nugget.

Disturbance factor (c): State in which there is no axial center of the other welding electrode on the extension line of the axial center of the one welding electrode (with respect to the extension line of the axial center of the one welding electrode, (Shaft is misaligned)
When a large number of hit points are welded, the other of the welding electrodes 8 on the extension line of the axis 9 of the welding electrode 8 as shown in FIG. The axial center 9 of the welding electrode 8 may be absent, and a deviation (hereinafter referred to as “electrode core deviation”) may occur. When welding is performed with such electrode misalignment, deformation as shown in FIG. 6B is applied, and stress may occur in the vicinity of the corona bond.

  When the electrode misalignment of the welding electrode is 0.2 mm or more, cracks are particularly likely to occur immediately outside the corona bond or at the time of the corona bond nugget. However, the spot welding method of this embodiment has an effect of suppressing these cracks. Further, even when the electrode misalignment of the welding electrode is 1 mm or more and a larger stress is generated, there is an effect of suppressing cracking by the spot welding method of the present embodiment.

Disturbance factor (d): state in which there is a gap between the overlapped surfaces of the welded portions FIG. 7A shows an example in which there is a large gap on the right side of the two steel plates 1. The gap may be unbalanced on the left and right. These phenomena occur, for example, when a springback occurs during press molding, or when a single steel plate partially rises when spot welding is not performed in order at adjacent positions. FIG. 7A shows a case where another member 11 is inserted between the steel plates 1. Spot welding may be performed with a gap (hereinafter also referred to as “plate gap”) between the steel plates 1 on the overlapped surface of the welded portions. When contact / pressurization is performed in a state where such a sheet gap exists, local deformation of the steel sheet 1 is observed at the welded portion, as shown in FIG. When welding is performed in such a deformed state, local stress is generated in the vicinity of the corona bond. And it becomes easy to produce a crack in this location.
In the member to be welded, when the plate gap in the axial center direction at the hitting position of the welding electrode is 1 mm or more, cracks are particularly likely to occur immediately outside the corona bond or at the corrugated nugget. Even in such a case, the spot welding method of this embodiment has an effect of suppressing such cracks.

  Next, regarding the spot welding method according to the embodiment, necessary requirements and preferable requirements will be sequentially described. First, the member to be welded will be described.

(Welded member)
In the spot welding method according to the above-described embodiment, the welded members are a plurality of steel plates on which the welding locations are overlapped, and at least one of the steel plates is a steel plate coated with zinc-based plating. Prepare. For example, two steel plates having a tensile strength of 780 MPa or more, a Ceq of 0.15 mass% or more, and a plate thickness of 0.5 to 3.0 mm are prepared. Ceq is shown in the following formula (5).

Ceq = [C] + [Si] / 30 + [Mn] / 20 + 2 [P] +4 [S] (5)
However, [C], “Si], [Mn], [P], and [S] are the contents (mass%) of C, Si, P, and S.
From the viewpoint of weight reduction and strength enhancement, the preferred Ceq is 0.18% by mass or more, more preferably 0.20% by mass or more.

  A member to be welded to be spot welded is particularly limited as long as it is composed of a plurality of steel plates on which at least welded portions are overlapped, and at least one of the steel plates is covered with zinc-based plating. It is not something. For example, spot welding is performed with a plurality of steel plates in which zinc-plating is coated on the overlapping surfaces of all spot-welded steel plates or steel plates in which zinc-plating is coated on the overlapping surfaces of spot-welded steel plates. Examples of the steel sheet include a steel sheet that is not coated with zinc-based plating.

  In addition, in a steel plate with a zinc-plated coating on the overlapped surface to be spot-welded, the surface opposite to the overlapped surface of the steel plate to be spot-welded, that is, the contact surface with the welding electrode is coated with zinc-based plating. It may or may not be coated. However, in consideration of the corrosion resistance of the spot welded joint, it is preferable that the contact surface with the welding electrode is also coated with zinc-based plating.

  In FIG. 2 and the like, two steel plates are described as the plurality of steel plates to be spot-welded, but three or more steel plates can be used depending on the form of the structural parts to be joined. The plate | board thickness of each steel plate spot-welded is not specifically limited, It can be 0.5-3.0 mm. The total thickness t of the plurality of steel plates is 1.35 mm or more, and the upper limit is not particularly limited, and the total thickness t can be 7.0 mm or less.

  A more preferable range of the total thickness t is 2.4 mm or more and 3.2 mm or less, and a more preferable range is 2.7 mm or more and 3.2 mm or less. By setting the total sheet thickness t within this range, cracks at the outside of the corona bond and at the time of the corona bond nugget can be suppressed, the joint strength can be ensured, and further the weight and strength of the welded member can be increased.

  Moreover, the steel plate should just have a plate-shaped part in at least one part, and the said plate-shaped part should have a part by which each other is piled up, and the whole may not be a board. Further, the plurality of steel plates are not limited to those composed of separate steel plates, and may be a superposition of a single steel plate formed into a predetermined shape such as a tubular shape.

  Moreover, the steel plate of the member to be welded that is spot-welded is not particularly limited, such as a component composition or a metal structure. However, in the case of a TRIP steel plate, etc., especially with respect to a steel plate in which the overlapped surface to be welded is coated with zinc-based plating, or a steel plate coated with zinc-based plating and a steel plate overlapped via zinc-based plating. However, when using other high-strength steel sheets with a Ceq of 0.15% by mass or more, cracks are likely to occur on the outside of the corona bond and on the corona bond nugget. Therefore, in the welding method of the present invention, such a steel plate may be particularly targeted.

  Further, the zinc-based plating coated on the steel plate to be welded is not particularly limited as long as it is zinc-containing plating. For example, as a plating type, alloyed hot dip galvanizing, hot dip galvanizing, electrogalvanizing, Zinc / nickel plating is exemplified. Also, zinc / aluminum / magnesium plating can be included.

(Preliminary spot welding)
The spot welding method according to the above-described embodiment is efficient and desirable to be applied to a weld location where at least one of the cracks immediately outside the corona bond and the crack at the corona bond nugget occurs. Therefore, immediately after the end of the welding energization between the welding electrodes to the member to be welded, spot welding is performed in advance so that the welding electrode and the member to be welded are not in contact with each other. After confirming that a crack has occurred, it is preferable to perform spot welding with the post-weld retention time within the range of the above formula (1) in the welding to the welded portion.

  The spot welding to be performed on the welded member in advance is the same as the above-mentioned spot welding without making the welding electrode and the welded member non-contact immediately after the end of the welding energization between the welding electrodes and extending the holding time Ht after welding. In addition, the two steel plates are overlapped and energized while pressing the electrodes so as to sandwich the two steel plates from both sides, thereby forming a nugget having an elliptical cross section. The conditions similar to the above can be adopted as the spot welding conditions at this time.

  And the presence or absence of the generation | occurrence | production of the crack in the corona bond right outside of the overlapping surface and the crack at the time of corona bond nugget is confirmed. For example, the crack is confirmed by cutting in the thickness direction so as to include the nugget and confirming the cross section. As shown in FIG. 2, when a crack is confirmed in at least one of the corona bond immediately outside the corona bond and the nugget of the corona bond, the post-weld holding time Ht with respect to the total thickness t satisfies the above formula (1). Thus, the subsequent spot welding is performed.

  In particular, immediately after the end of the welding energization between the welding electrodes, spot welding is performed in advance so that the welding electrode and the member to be welded are not in contact with each other, and at least one of the cracks immediately outside the corona bond and the crack at the nugget of the corona bond is cracked. It is efficient and desirable to use the welding method of the present invention for a welded portion where cracks have occurred.

  The spot welding to the welded member to be performed in advance is performed immediately after the end of the energization between the welding electrodes, except that the welding electrode and the welded member are not in contact with each other and the holding time Ht after welding is not extended. Perform welding with the same welding conditions. For example, welding conditions are not particularly limited, and normal welding conditions can be employed. For example, the electrode may have a dome radius type tip diameter of 6 to 8 mm, a pressing force of 1.5 to 6.0 kN, an energization time of 5 to 50 cycles, and an energization current of 4 to 15 kA.

  Moreover, immediately after the end of the welding energization between the welding electrodes, the welding electrode and the member to be welded are not in contact with each other. Contact.

(Confirmation of cracks directly outside the corona bond and corona bond nuggets)
When confirming the occurrence of cracks on the corona bond directly outside the corona bond and on the corona bond nugget obtained by spot welding performed in advance, The confirmation method is not particularly limited, and is determined by observing a cross-section in the thickness direction including the nugget or by performing a tensile test of the spot welded joint to obtain a predetermined tensile strength. Can be used. Or, depending on the cutting position of the cross section in the plate thickness direction including the spot welded portion, cracks may not be observed directly outside the corona bond and when corona bond is nuggeted. Also good.

  Cracks on the corona bond directly outside and corona bond nugget are caused by the overlapping surface of the steel plate coated with zinc-based plating or the steel plate coated with zinc-based plating and the steel plate overlapped via zinc-based plating. Occurs on the surface. And it generate | occur | produces in the corona bond and its vicinity among the surfaces of these steel plates. In the corona bond and in the vicinity thereof, the surface of the steel sheet is from the nugget side end of the corona bond to a range approximately 1.2 times the corona bond diameter Dc.

  When there is one or more disturbance factors from (a) to (d), when performing the spot welding method of the above embodiment (spot welding method in which the retention time after welding is within an appropriate range), or The case where the spot welding method of the above embodiment (the spot welding method in which the retention time after welding is in an appropriate range) after performing spot welding in advance has been described. However, in actual production lines, it is troublesome to measure the angle of attack, to measure the clearance, to measure the deviation of the axis of the electrode, or to measure the gap between the steel plates. There is. If the spot welding method of the above embodiment is performed from the beginning without these preconditions, it is more preferable because it can suppress cracks immediately outside the corona bond and at the time of corona bond nugget.

  Next, examples of the present invention will be described. The conditions in the examples are one example of conditions used for confirming the feasibility and effects of the present invention, and the present invention is based on this one example of conditions. It is not limited. The present invention can adopt various conditions as long as the object of the present invention is achieved without departing from the gist of the present invention.

Example 1
Table 1 shows the steel plates A to I subjected to the test. Steel plates A to F, H, and I shown in Table 1 are all alloyed hot-dip galvanized steel plates, and both surfaces thereof are coated with galvanizing.
The coating layer of the alloying hot dip galvanizing applied to the steel plates A to F, H, and I was formed by annealing in a galvanizing bath after annealing.

  About the combination of the steel plates as shown in Tables 2-5, spot welding was performed using a servo gun type welding machine under various welding conditions and disturbance factors shown in Tables 2-5. In all spot welding, welding was performed using an electrode having a tip diameter of 8 (mm) with a copper dome radius type having a radius of curvature of the tip of 40 (mm). During pressurization, the applied pressure was not changed.

  Each of the obtained welded joints was cut along the center line of the welded portion, and the presence or absence of cracks was determined by observing the cross section. Further, the CTS (cross tensile force) of the spot welded joint was measured by the method specified in JIS Z 3137 under the same welding conditions. Tables 2 to 5 show the results of the presence or absence of cracks, the CTS, and the weld joint strength ratio.

  The weld joint strength ratio is a ratio based on the CTS of a spot weld joint formed with the same disturbance factors as the welding conditions other than the retention time after welding Ht. For example, the strength ratio of the welded joint of No. 1a is obtained by subtracting the CTS of No. 1a from the CTS of No. 1 and forming the spot welded joint under the same welding conditions except for the retention time after welding Ht, and dividing by the CTS of No. 1 , 100 is a numerical value obtained by multiplying by 100. Similarly, the weld joint strength ratio of No. 2b is a value calculated on the basis of the CTS of No. 2. In addition, when the weld joint strength ratio was 30 or more, it was determined that CTS was lowered. In Tables 2 to 5, the clearance of 1 mm means a state in which an equalizing mechanism is not provided and a clearance of 1 mm is generated between the electrode and the steel plate.

As can be seen from Tables 2 and 3, in the results of Nos. 1 to 26 and Nos. 1a to 22a, no cracks occurred on the corona bond directly outside or on the corona bond nugget. This is considered to be because the relationship between the total thickness t and the post-weld holding time Ht satisfies the equation (1).
Further, it can be seen that the experimental example in Table 2 tends to have a relatively high CTS than the experimental example in Table 3. This is presumably because the relationship between the total thickness t and the post-weld holding time Ht satisfies the equation (2).

  On the other hand, in the numbers 1b to 22b in Table 4, cracks occurred at the outside of the corona bond or at the corona bond nugget. In some cases, the crack also caused a decrease in CTS. This is considered because the relationship between the total thickness t and the post-weld holding time Ht does not satisfy the relationship on the left side of the equation (1).

  Moreover, in the numbers 1c-22c of Table 5, although the above cracks were not seen, sufficient CTS was not obtained. This is considered because the relationship between the total thickness t and the post-weld holding time Ht does not satisfy the relationship on the right side of the equation (1).

Next, the deformation | transformation aspect of the welding location by each disturbance factor is demonstrated. FIG. 8 is a cross-sectional photograph in which a cross section in the plate thickness direction is photographed with an optical microscope at a welded spot where spot welding is performed at a strike angle of 5 degrees. It can be seen that by having the hitting angle, the deformation state of the steel sheet is different on the left and right sides of the nugget.
FIG. 9 is a cross-sectional photograph when spot welding is performed with a clearance of 1 mm between the steel plate and the electrode without providing an equalizing mechanism. It can be seen that the state of deformation of the lower steel plate and the upper steel plate is different at the weld location. In the case of FIG. 9, the distance from the tip of the welding electrode to the steel sheet surface is smaller on the upper side than on the lower side.

FIG. 10 is a cross-sectional photograph when spot welding is performed in a state where the axis of the opposing welding electrode is shifted by 1.5 mm. It can be seen that the indentation above and below the nugget is asymmetric.
FIG. 11 is a cross-sectional photograph in the case where spot welding is performed in a state where there is a plate gap of 2 mm in height at a location 20 mm from the striking point on one side between the overlapping surfaces of the welded portions. It can be seen that the deformation of the steel sheet is different on the left and right sides of the nugget.

  According to the present invention, in the spot welding, since the retention time after welding of the welding electrode is a function of the total plate thickness, it is possible to suppress cracking at the outside of the corona bond and the nugget of the corona bond and ensure the joint strength. . Especially for multiple steel sheets including zinc-plated high-strength steel sheets for automobiles, which directly affects the strength of the joints. The liquid generated directly outside the corona bond and the corona bond nugget on the overlapping surface of the steel sheets. The present invention relates to a spot welding method capable of preventing metal cracking and ensuring joint strength. Therefore, the present invention has high industrial applicability.

DESCRIPTION OF SYMBOLS 1 Steel plate 2 Nugget 3 Crack which progresses from the surface of steel plate toward nugget 4 Heat-affected zone 5 Crack which progresses from shoulder to heat-affected zone 6 Crack directly outside corona bond 7 Crack at corona bond nugget 8 Welding Electrode 9 Axis core 10 Perpendicular line 11 Other member A Boundary line where crack generation does not occur in the relationship between total plate thickness and retention time after welding B Boundary line that provides sufficient CTS in the relationship between total plate thickness and retention time after welding Dc Coronabond diameter

(1) A spot welding method according to an aspect of the present invention is a method of spot welding a welded member composed of a plurality of steel plates in which at least welding locations are overlapped, and at least of the plurality of steel plates. One is a high-strength steel plate having a tensile strength of 780 MPa or more. For at least one of the plurality of steel plates, at least the overlapping surface of the welded portions is coated with zinc-based plating, and the total of the plurality of steel plates is A plate thickness t (mm) is 1.35 mm or more, and a post-welding holding time Ht (seconds) from the end of welding energization between the welding electrodes until the welding electrode and the welded member are brought into non-contact with each other. Within the range of the following formula ( 4 ).
0.015 t ² + 0.110 ≦ Ht ≦ 0.10 t ² −0.40 t + 0.80 ( 4 )
According to the spot welding method having the above-described configuration, it is possible to suppress the cracking of the corona bond directly outside and the corona bond at the time of the nugget and ensure the joint strength.

( 2 ) According to another aspect of the present invention, in the spot welding method of (1) , a total thickness t (mm) of the plurality of steel plates is 2.4 mm or more and 3.2 mm or less.
In the above aspect, in addition to ensuring the effect of suppressing cracking and securing the joint strength, the welded member can be reduced in weight and strength.

( 3 ) According to another aspect of the present invention, in the spot welding method of the above (1) or (2) , immediately before the welding electrode is brought into contact with the welded member, the following (a) to (d): Satisfy one or more of the conditions.
(A) a state in which the axis of the welding electrode and the perpendicular to the surface of the steel sheet in contact with the welding electrode are not parallel;
(B) a state in which the distance from the tip of one welding electrode to the steel plate surface closest to the tip is different from the distance from the tip of the other welding electrode to the steel plate surface closest to the tip;
(C) a state in which there is no axial center of the other welding electrode on an extension line of the axial center of the one welding electrode; and
(D) A state in which there is a gap between the overlapping surfaces of the welded portions.
When there is a disturbance factor stipulated in the above aspect, cracks at the corona bond directly outside and corona bond nugget become prominent, but according to the spot welding method of (1) or (2) above, Cracks can be suppressed and joint strength can be ensured.

( 4 ) According to another aspect of the present invention, in the spot welding method according to any one of (1) to ( 3 ), immediately after the end of welding energization between the welding electrodes to the welded member, Spot welding is performed in which the welding electrode and the member to be welded are not in contact with each other, and the presence or absence of cracks immediately outside the corona bond on the overlapped surface of the steel sheet and the crack at the time of corona bond nugget is confirmed. When at least one of the crack in the nugget of the corona bond and the crack in the corona bond is generated, the welding is further performed within the range of the retention time after welding Ht (seconds).
In the said aspect, since the spot welding method in any one of said (1) to ( 3 ) is applied with respect to the welding location which the crack generate | occur | produced, it is efficient.

Claims (7)

It is a method of spot welding to a member to be welded composed of a plurality of steel plates on which at least welded portions are superimposed,
For at least one of the plurality of steel plates, at least the overlapping surface of the welded portions is coated with zinc-based plating, and the total thickness t (mm) of the plurality of steel plates is 1.35 mm or more,
The post-weld holding time Ht (seconds) from the end of welding energization between the welding electrodes to the time when the welding electrode and the member to be welded are brought into contact with each other is set within the range of the following formula (1). Spot welding method to do.
0.015t ² + 0.020 ≦ Ht ≦ 0.16t ² −0.40t + 0.70 (1) The spot welding method according to claim 1, wherein the post-weld holding time Ht (seconds) further satisfies the following expression (2).
Ht ≦ 0.10t ² −0.40t + 0.80 (2) The spot welding method according to claim 1 or 2, wherein the post-weld holding time Ht (seconds) further satisfies the following expression (3).
0.015 t ² + 0.110 ≦ Ht (3)   4. The spot welding method according to claim 1, wherein a total plate thickness t (mm) of the plurality of steel plates is 2.4 mm or more and 3.2 mm or less. 5.   The spot welding method according to any one of claims 1 to 4, wherein at least one of the plurality of steel plates is a high-strength steel plate having a tensile strength of 780 MPa or more. 6. The method according to claim 1, wherein one or more of the following conditions (a) to (d) are satisfied immediately before the welding electrode is brought into contact with the member to be welded: The spot welding method described in 1.
(A) a state in which the axis of the welding electrode and the perpendicular to the surface of the steel sheet in contact with the welding electrode are not parallel;
(B) a state in which the distance from the tip of one welding electrode to the steel plate surface closest to the tip is different from the distance from the tip of the other welding electrode to the steel plate surface closest to the tip;
(C) a state in which there is no axial center of the other welding electrode on an extension line of the axial center of the one welding electrode; and
(D) A state having a gap between the overlapping surfaces of the welding locations In the spot welding method according to any one of claims 1 to 6,
In advance, after the welding energization between the welding electrodes to the welded member is completed, spot welding is performed so that the welding electrode and the welded member are not in contact with each other. And when the occurrence of cracks in the corona bond nugget is confirmed, when at least one of the cracks immediately outside the corona bond and the crack in the corona bond nugget occurs,
A spot welding method, wherein welding is further performed within the range of the post-welding holding time Ht (seconds).