JP7173736B2 - Spot welding method and steel plate parts - Google Patents

Description

The present invention relates to a spot welding method and a steel plate component.

In spot welding, two or more metal plates (steel plates) are sandwiched between a pair of electrodes and pressurized, and a high current is applied between the pair of electrodes to partially melt the metal plates by resistance heat generation. In this joining method, the melted portion is solidified while the damage to the electrode is mitigated by cooling water flowing through the electrode.

As shown in FIG. 7, when a plate assembly composed of a plurality of metal plates 101 and 102 is sandwiched and pressed between a pair of electrodes 110 and 120, recesses 101a and 102a are formed in the metal plates 101 and 102 by the pressure applied by the electrodes 110 and 120. At the same time, the areas around the recesses 101a and 102a are lifted from the adjacent metal plates, forming a gap G between the metal plates 101 and 102 (this phenomenon is also called "sheet separation"). For example, if a large gap is formed between metal plates in the parts that form the vehicle body of an automobile, it may affect the ride comfort of the automobile.

For example, when joining soft steel plates together, the soft steel plates can be locally depressed by applying pressure from the electrode, so that the area around the recess is relatively little lifted. However, when the metal plates 101 and 102 are high-strength steel plates, as indicated by the dotted lines in FIG. The gap G between the metal plates 101 and 102 becomes large because the metal plates 101 and 102 are greatly lifted.

For example, Japanese Patent Laid-Open No. 2002-200001 discloses that sheet separation is suppressed by sandwiching a region around an electrode in a superimposed metal plate with a ring-shaped member.

WO2015/137512

However, providing the ring-shaped member as described above in the welding device results in an increase in cost.

SUMMARY OF THE INVENTION Accordingly, an object of the present invention is to suppress sheet separation between high-tensile steel plates without increasing the cost of a welding apparatus when joining a plurality of high-tensile steel plates by spot welding.

In order to solve the above-mentioned problems, the present invention provides a spot welding method for joining a plurality of high-tensile steel plates that are superimposed by sandwiching and pressurizing a pair of electrodes by energizing them, wherein the plurality of high-tensile steel plates Among them, a spot welding method is provided in which a sacrificial plate is interposed between the electrode and the high-strength steel plate arranged on the outermost side in the thickness direction.

Thus, by interposing the sacrificial plate between the outermost high-tensile steel plate and the electrode, a recess is formed in the sacrificial plate, and a gap (sheet separation) is formed between the sacrificial plate and the high-tensile steel plate. ) occurs. In this case, since the high-strength steel sheet is not directly pressed by the electrodes, deformation (warping) due to the formation of the recesses is suppressed. As a result, sheet separation between high-strength steel sheets is suppressed, and adhesion between them is enhanced. The "sacrificial plate" is a metal plate whose main purpose is to avoid direct contact between the high-strength steel plate and the electrode, and which does not substantially have other functions.

According to the above spot welding method, a plurality of high-tensile steel plates joined via nuggets, and a sacrificial plate joined to one surface in the thickness direction of the plurality of high-strength steel plates via nuggets are provided. A steel plate part is obtained. Since the high-strength steel sheets have high adhesiveness, the steel sheet parts can be suitably used for components such as automobile bodies.

As described above, when joining a plurality of high-strength steel plates by spot welding, by interposing a sacrificial plate between the high-strength steel plates and the electrodes, the high-strength steel plates can be welded together without increasing the cost of the welding equipment. Sheet separation can be suppressed.

(A) to (D) are cross-sectional views showing how a plate assembly is welded by the spot welding method according to the embodiment of the present invention. It is a graph which shows an example of a pressurization energization pattern. It is a sectional view showing board combination concerning other embodiments. (A) is a cross- sectional view showing a board assembly according to another embodiment, and (B) is a perspective view of the same. It is a sectional view showing board combination concerning other embodiments. It is a sectional view showing board combination concerning other embodiments. It is a cross-sectional view showing a conventional spot welding method.

BEST MODE FOR CARRYING OUT THE INVENTION Hereinafter, embodiments of the present invention will be described with reference to the drawings.

In this embodiment, a plate assembly as shown in FIG. 1(A) is welded. The steel plate part made of this plate assembly is, for example, a component part of the vehicle body of an automobile. It has a plurality of thick plates (two thick plates 32 and 33 in the illustrated example) that function as frames (side members, reinforcement, etc.). As the thin plate 31, for example, a mild steel plate having a tensile strength of 300 MPa or less is used, and more specifically, a hot-dip galvanized steel plate is used. The thick plates 32 and 33 are thicker than the thin plate 31 and overlap one side of the thin plate 31 (lower side in the figure). As the thick plates 32 and 33, a high-tensile steel plate with a tensile strength of 490 MPa or more, especially an ultra-high-tensile steel plate with a tensile strength of 980 MPa or more is used. In this embodiment, the thick plates 32 and 33 are high-strength steel plates made of the same material and having the same plate thickness, specifically, ultra-high-strength steel plates made of cold-rolled steel plates.

In this plate combination, a sacrificial plate 40 is arranged further outside (below) the high-strength steel plate (lower thick plate 33 in the illustrated example) arranged on the outermost side in the thickness direction. The sacrificial plate 40 is made of a metal plate thinner than the thick plates 32 and 33 . The sacrificial plate 40 is made of a material having a lower tensile strength than the thick plates 32 and 33, such as a mild steel plate. The thickness and material of the sacrificial plate 40 are preferably set so that the composition of the entire plate assembly is symmetrical in the thickness direction. In this embodiment, the sacrificial plate 40 is made of a mild steel plate having the same material and thickness as the thin plate 31 . The plate thickness ratio of the plate assembly (the total plate thickness T of the plate assembly/the plate thickness T1 of the thin plate 31) is 4 or more.

The plate assembly is joined by energizing between the pair of electrodes 12a and 12b while the plate assembly is sandwiched and pressurized by the pair of electrodes 12a and 12b. In this embodiment, welding is performed according to the pressure pattern P (see the solid line) and the energization pattern I (see the chain line) set on the same time axis shown in FIG. Each step shown in FIG. 2 will be described in detail below.

[Up slope, step W1]
When the applied pressure reaches P1, the applied pressure is maintained at P1, and the current value is gradually increased (up slope: UPSL) until it reaches I1 in accordance with a predetermined time. Then, when the current value reaches I1, the current value is held at I1. In this way, the parts to be welded of the plate assembly are held for a predetermined time (for example, 2 to 4 cycles) while applying a constant pressure P1 and a constant current value I1. Note that 1 cycle = 1/60 second.

The pressure P1 and the current value I1 in step W1 are set to values suitable for joining the thin plate 31 and the thick plate 32 and the sacrificial plate 40 and the thick plate 33 together. Specifically, the pressure P1 is set to a value as small as possible. It is the minimum pressure that can suppress the variation of the pressure and bring them into contact with each other. By setting the pressure P1 to a small value in this way, the contact area between the steel plates, particularly the contact area between the thin plate 31 and the thick plate 32 and between the sacrificial plate 40 and the thick plate 33, is reduced. As a result, the current density increases at the contact portions between the thin plate 31 and the thick plate 32 and between the sacrificial plate 40 and the thick plate 33, and these portions preferentially generate heat, forming nuggets N1 and N2 (FIG. 1). (A)}. The energizing time in step W1 is set so as to obtain a nugget N1 with a predetermined diameter.

[Step W2]
After that, by increasing the current value from I1 to I2 and synchronously increasing the pressure from P1 to P2, the center of the thickness direction of the entire plate assembly 30 including the contact portion between the thick plates 32 and 33 is increased. A melted portion (nugget N3) is formed in the vicinity due to resistance heat generation {see FIG. 1(B)}. The nuggets N1, N2, N3 are provided at the same position in the direction perpendicular to the plate thickness direction. In this way, after the nuggets N1 and N2 are formed in step W1, the current value is continuously increased to I2 without decreasing from I1. The contact resistance between the steel plates and the base material resistance of each steel plate do not suddenly change, and a nugget N3 is easily formed at the contact portion of the thick plates 32 and 33, and the nugget N3 is easy to grow. The timing for increasing the current value and the timing for increasing the applied force (especially the effective applied force: the actual applied force by the pair of electrodes 12a and 12b) are preferably matched, but do not necessarily have to be completely matched. Instead, it is sufficient to ensure sufficient time for holding the pressures P1 and P2 in steps W1 and W2 before and after.

At this time, recesses 31a and 40a are formed in the thin plate 31 and the sacrificial plate 40 which are in direct contact with the electrodes 12a and 12b due to the pressing force of the electrodes 12a and 12b. Along with this, the areas around the recesses 31a and 40a of the thin plate 31 and the sacrificial plate 40 are lifted from the adjacent thick plates 32 and 33, and the areas between the thin plate 31 and the thick plate 32 and between the sacrificial plate 40 and the thick plate are lifted. 33, gaps G1 and G2 are formed respectively.

[Step W3]
After that, the current value is slightly decreased from I2 to I3 while the pressure is maintained at P2, and this current value I3 and pressure P2 are maintained for a predetermined time (for example, 2 to 4 cycles). As a result, the nugget N3 can be further grown while preventing the occurrence of sputtering.

[Steps W4 to W7]
At step W4, the current value I3 at step W3 is increased to a current value I4 larger than the current value I2 at step W2. At step W5, the current value is decreased from I4 to I5. Current value I5 is greater than current value I3 at step W3. Further, in step W6, the current value I5 in step W5 is increased to a current value I6 that is greater than the current value I4 in step W4, and then the current value is decreased from I6 to I7 in step W7. Current value I7 is greater than current value I5 at step W5. In this embodiment, when shifting from step W3 to step W4, the pressing force is increased from P2 to P3 in synchronization with the timing of increasing the current value from I3 to I4. In subsequent steps, the applied pressure is held at a constant pressure P3.

As described above, the diameter of the nugget N3 (the dimension in the direction perpendicular to the plate thickness, the dimension in the left-right direction in FIG. 1) is expanded while suppressing the occurrence of spatter by increasing the current value step by step. At the same time, the nugget N3 can be grown in the plate thickness direction (vertical direction in FIG. 1). In particular, in this embodiment, when the current value is increased from I3 to I4, the pressure is increased from P2 to P3, which further promotes the growth of the nugget N3. As described above, the nugget N3 can be integrated with the nuggets N1 and N2 {see FIG. 1(C)}. It should be noted that the nuggets N for joining the board assembly do not necessarily need to be integrated, and the nuggets N1, N2, and N3 may be separated.

As the nugget N3 expands, the electrodes 12a and 12b push the thin plate 31 and the sacrificial plate 40 further, deepen the recesses 31a and 40a, and increase the gap G1 between the thin plate 31 and the thick plate 32 and the sacrificial plate. The gap G2 between 40 and the thick plate 33 increases. On the other hand, since the thick plates 32 and 33 are not directly pressed by the electrodes 12a and 12b, almost no recesses are formed, and the areas around the recesses are hardly lifted. Maintains good adhesion. In this way, by disposing the sacrificial plate 40 outside the thick plate 33 (lower side in the figure) and preventing the electrode 12b from directly pressing the thick plate 33, the deformation of the thick plate 33 is suppressed and the height is increased. Adhesion between the thick plates 32 and 33 made of tension steel plates can be enhanced.

In this state, while the applied pressure is maintained at P3, the current is stopped and cooled (see "HOLD" in FIG. 2), so that the integrated nugget N becomes the thin plate 31, the thick plates 32 and 33, and the sacrificial plate. It hardens in a state in which it melts into 40, and the board assembly is joined {see FIG. 1(D)}.

In the steel plate component joined in this way, the sacrificial plate 40 is joined through the nugget N to the surface on one side in the thickness direction (lower side in the drawing) of the steel plates 32 and 33 joined through the nugget N. It is This steel plate component is assembled to the vehicle body with the sacrificial plate 40 still attached. However, in this plate assembly, the sacrificial plate 40 is a member whose main purpose is to suppress sheet separation by avoiding direct contact between the thick plate 33 and the lower electrode 12b. It does not substantially have any function (function as an outer plate or inner plate panel, or as a skeleton frame). In other words, even if the sacrificial plate 40 is not provided in the above-described plate assembly, the performance (strength, etc.) required as a component of the vehicle body of an automobile is satisfied.

The invention is not limited to the above embodiments. For example, as shown in FIG. 3, the sacrificial plate 40 may be partially arranged only at the locations where spot welding is to be performed. In this case, for example, a sacrificial plate 40 is provided for each spot welding point (nugget N).

Also, the sacrificial plate 40 and the thin plate 31 may be provided integrally. Specifically, as indicated by the dashed lines in FIGS. 4A and 4B, a part of the thin plate 31 is extended, and the extended portion 40′ is bent to be placed under the thick plate 33. , this portion can function as the sacrificial plate 40 . In this case, since there is no need to separately form the sacrificial plate 40, cost reduction can be achieved.

Further, as shown in FIG. 5, sacrificial plates 40 may be arranged on both sides of the thick plates 32 and 33 when joining a plate combination consisting of only the thick plates 32 and 33 made of high-strength steel. Alternatively, as shown in FIG. 6, a sacrificial plate 40 may be arranged only on one side when joining a plate assembly consisting of only thick plates 32 and 33 made of high-strength steel. In this case, since the lower thick plate 33 is directly pressed by the electrode 12b, a concave portion is formed and separates from the upper thick plate 32. Since the formation of recesses is avoided, the gap between the thick plates 32 and 33 can be made smaller than when the thick plates 32 and 33 are directly pressed by the electrodes.

12a, 12b electrodes 31 thin plates 32, 33 thick plates (high-tensile steel plates)
40 sacrificial plates G1, G2 gaps N, N1, N2, N3 nuggets

Claims (2)

A spot welding method in which a plurality of superimposed high-tensile steel plates are sandwiched and pressed between one electrode and the other electrode and energized to join them,
A sacrificial plate having a lower tensile strength than the high-tensile steel plate is interposed between the plurality of high-tensile steel plates and the one electrode,
A metal plate having a lower tensile strength than the high-tensile steel plate is interposed between the plurality of high-tensile steel plates and the other electrode ,
A spot welding method in which a portion of the metal plate is bent and interposed between the plurality of high-tensile steel plates and the one electrode, and this portion is used as the sacrificial plate . A plurality of high-tensile steel plates bonded via nuggets, and a sacrificial plate having a tensile strength lower than that of the high-tensile steel plates, which is bonded via a nugget to the surfaces on one side in the thickness direction of the plurality of high-tensile steel plates. , a metal plate having a lower tensile strength than the high-tensile steel plate, which is bonded to the surface of the plurality of high-tensile steel plates on the other side in the thickness direction via a nugget ,
A steel plate component in which a portion of the metal plate is bent and arranged on one side in the thickness direction of the plurality of high-tensile steel plates, and this portion is used as the sacrificial plate .

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