JP2019147188A - Spot welding method and steel sheet component - Google Patents

Abstract

To inhibit sheet separation between high-strength steel plates without making the cost of welding equipment high, when the plurality of high-strength steel plates are joined together by spot welding.SOLUTION: A plurality of stacked high-strength steel plates (thick plates 32 and 33) are joined together by being energized in the state of being sandwiched and pressurized between a pair of electrodes 12a and 12b. In this case, a sacrificial plate 40 is interposed between the thick plate 33 and the electrode 12b.SELECTED DRAWING: Figure 1

Description

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

  In spot welding, two or more metal plates (steel plates) are pressed between a pair of electrodes, and a high current is applied between the pair of electrodes to partially melt the metal plate by resistance heat generation. And the molten portion is solidified while the damage of the electrode is reduced by the cooling water flowing through the electrode.

  As shown in FIG. 7, when a plate assembly composed of a plurality of metal plates 101, 102 is sandwiched and pressed by a pair of electrodes 110, 120, the recesses 101 a, 102 a are formed in the metal plates 101, 102 by the pressure of the electrodes 110, 120. At the same time, the regions around the recesses 101a and 102a are lifted from the adjacent metal plates, and a gap G is formed between the two metal plates 101 and 102 (this phenomenon is also referred to as “sheet separation”). For example, in a part constituting a car body of a car, if a large gap is formed between metal plates, the ride quality of the car may be affected.

  For example, when joining mild steel plates, the mild steel plate can be locally recessed by pressurization of electrodes, so that the lift around the recess is relatively small. However, when the metal plates 101 and 102 are high-tensile steel plates, as shown by the dotted lines in FIG. 7, the metal plates 101 and 102 are not easily bent by the pressurization of the electrodes 110 and 120. It floats up greatly and the gap G between the metal plates 101 and 102 becomes large.

  For example, Patent Document 1 shown below suppresses the occurrence of sheet separation by sandwiching a region around an electrode in an overlapped metal plate with a ring-shaped member.

International Publication No. 2015/137512

  However, providing the ring-shaped member as described above in the welding apparatus increases the cost.

  Then, when joining a some high strength steel plate by spot welding, this invention aims at suppressing the sheet separation between high strength steel plates, without causing the high cost of a welding apparatus.

  In order to solve the above-mentioned problem, the present invention is a spot welding method in which a plurality of stacked high-tensile steel plates are joined by energizing them while being sandwiched and pressed by a pair of electrodes, and the plurality of high-tensile steel plates The spot welding method is characterized in that a sacrificial plate is interposed between the electrode and the high-tensile steel plate arranged on the outermost side in the thickness direction.

  In this way, a sacrificial plate is interposed between the outermost high-strength steel plate and the electrode, whereby 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-tensile steel plate is not directly pressed by the electrode, deformation (warpage) associated with the formation of the recess is suppressed. As a result, sheet separation between the high-tensile steel plates is suppressed, and their adhesion is enhanced. The “sacrificial plate” refers to a metal plate whose main purpose is to avoid direct contact between the high-tensile steel plate and the electrode, and which has substantially no other function.

  According to the spot welding method, a plurality of high-tensile steel plates joined via nuggets, and a sacrificial plate joined via nuggets to the surface on one side in the thickness direction of the plurality of high-tensile steel plates. Steel plate parts are obtained. Since this steel plate part has high adhesiveness between high-tensile steel plates, it can be suitably used for components of 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-tensile steel plate and the electrode, without incurring high cost of the welding apparatus, Sheet separation can be suppressed.

(A)-(D) are sectional drawings which show a mode that a plate assembly is welded with the spot welding method which concerns on embodiment of this invention. It is a graph which shows an example of a pressurization energization pattern. It is sectional drawing which shows the board assembly which concerns on other embodiment. (A) is a perspective view which shows the board assembly which concerns on other embodiment, (B) is the same sectional drawing. It is sectional drawing which shows the board assembly which concerns on other embodiment. It is sectional drawing which shows the board assembly which concerns on other embodiment. It is sectional drawing which shows the conventional spot welding method.

  Hereinafter, embodiments of the present invention will be described with reference to the drawings.

  In this embodiment, a plate assembly as shown in FIG. The steel plate component made of this plate assembly is, for example, a component of a car body of an automobile, and specifically, a thin plate 31 that functions as an outer plate or an inner plate panel (a side outer panel, a side inner panel, a floor panel, etc.), and a skeleton 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 specifically, a hot dip galvanized steel plate is used. The thick plates 32 and 33 are thicker than the thin plate 31 and are superposed on one side (downward in the figure) of the thin plate 31. As the thick plates 32 and 33, a high-tensile steel plate having a tensile strength of 490 MPa or more, particularly an ultra-high-tensile steel plate having a tensile strength of 980 MPa or more is used. In the present embodiment, the thick plates 32 and 33 are high-tensile steel plates made of the same material and equal in thickness, and specifically, ultra-high-strength steel plates made of cold-rolled steel plates.

  In this plate assembly, the sacrificial plate 40 is disposed on the outer side (lower side) of the high-tensile steel plate (in the illustrated example, the lower thick plate 33) disposed on the outermost side in the thickness direction. The sacrificial plate 40 is made of a metal plate that is 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, and is made of, for example, 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 symmetric 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 (total plate thickness T of the plate assembly / plate thickness T1 of the thin plate 31) is 4 or more.

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

[Up slope, step W1]
When the pressure reaches P1, the pressure is maintained at the pressure P1, and the current value is gradually increased until reaching the value I1 according to a predetermined time (upslope: UPSL). When the current value reaches I1, the current value I1 is held. In this way, a predetermined time (for example, 2 to 4 cycles) is maintained in a state where a constant pressure P1 and a current value I1 are applied to the welded portion of the plate assembly. 1 cycle = 1/60 seconds.

  The pressure P1 and the current value I1 in step W1 are 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, and can be stably generated by, for example, a servo motor (not shown) that drives one of the electrodes 12a, and the steel plates 31 to 33 and the sacrificial plate 40. The minimum pressure at which these can be reliably brought into contact with each other is controlled. Thus, the contact area between the steel plates, in particular, 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 by setting the pressure P1 to a small value. As a result, the current density at the contact portion between the thin plate 31 and the thick plate 32 and between the sacrificial plate 40 and the thick plate 33 is increased, and this portion generates heat preferentially to form the nuggets N1 and N2 {FIG. (See (A)). The energizing time in step W1 is set so that a nugget N1 having a predetermined diameter is obtained.

[Step W2]
Thereafter, the current value is increased from I1 to I2, and the applied pressure is increased from P1 to P2 in synchronism with this, so that the center of the plate assembly 30 including the contact portion between the thick plates 32 and 33 in the thickness direction A melted portion (nugget N3) due to resistance heat generation is formed in the vicinity (see FIG. 1B). Nuggets N1, N2, and N3 are provided at the same position in the direction orthogonal to the plate thickness direction. Thus, after forming the nuggets N1 and N2 in step W1, energy is continuously supplied to the welded portion of the plate assembly 30 by continuously increasing the current value from I1 to I2, so that the steel plate 30 The contact resistance between each other and the base metal resistance of each steel plate do not change suddenly, and the 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 pressure (especially the effective applied pressure: the actual applied pressure by the pair of electrodes 12a and 12b) are preferably matched, but need not be matched completely. However, it is only necessary to secure a sufficient time for holding the pressures P1 and P2 in the respective steps W1 and W2 before and after that.

  At this time, concave portions 31a and 40a are formed in the thin plate 31 and the sacrificial plate 40 in direct contact with the electrodes 12a and 12b. Accordingly, regions around the thin plate 31 and the recesses 31a and 40a of the sacrificial plate 40 are lifted from the adjacent thick plates 32 and 33, and between the thin plate 31 and the thick plate 32, and between the sacrificial plate 40 and the thick plate. Gap G <b> 1 and G <b> 2 are respectively formed between

[Step W3]
Thereafter, the current value is slightly reduced from I2 to I3 while maintaining the applied pressure at P2, and the current value I3 and the applied pressure P2 are maintained for a predetermined time (for example, 2 to 4 cycles). Thereby, the nugget N3 can be further grown while preventing the occurrence of sputtering.

[Steps W4 to W7]
In step W4, the current value I3 in step W3 is increased to a current value I4 larger than the current value I2 in step W2. In step W5, the current value is decreased from I4 to I5. Current value I5 is larger than current value I3 in step W3. Further, in step W6, the current value I5 in step W5 is increased to a current value I6 larger than the current value I4 in step W4. Then, in step W7, the current value is decreased from I6 to I7. Current value I7 is larger than current value I5 in step W5. In this embodiment, when shifting from step W3 to step W4, the applied pressure is increased from P2 to P3 in synchronization with the timing of increasing the current value from I3 to I4. In subsequent steps, the pressure is maintained 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 increased while suppressing the occurrence of sputtering by increasing the current value stepwise. In addition, the nugget N3 can be grown in the plate thickness direction (vertical direction in FIG. 1). In particular, in the present embodiment, when the current value is increased from I3 to I4, the applied pressure is increased from P2 to P3, so that the growth of the nugget N3 is further promoted. Thus, the nugget N3 can be integrated with the nuggets N1 and N2 {see FIG. 1C}. Note that the nuggets N that join the plate assemblies are not necessarily integrated, and the nuggets N1, N2, and N3 may be separated.

  As the nugget N3 expands, the electrodes 12a and 12b further push the thin plate 31 and the sacrificial plate 40, the recesses 31a and 40a become deeper, and the gap G1 between the thin plate 31 and the thick plate 32 and the sacrificial plate Gap G2 between 40 and thick plate 33 is increased. On the other hand, since the thick plates 32 and 33 are not directly pressurized by the electrodes 12a and 12b, the concave portions are hardly formed and the region around the concave portions hardly rises. Therefore, the thick plates 32 and 33 are mutually connected. Maintained in good contact. In this way, the sacrificial plate 40 is disposed on the outer side (lower side in the drawing) of the thick plate 33 so that the thick plate 33 is not directly pressed by the electrode 12b, thereby suppressing the deformation of the thick plate 33 and increasing the The adhesion between the thick plates 32 and 33 made of a tensile steel plate can be enhanced.

  In this state, the energization is stopped and cooled while maintaining the applied pressure at P3 (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 | cures in the state melt | dissolved in 40, and a board assembly is joined {refer FIG.1 (D)}.

  In the steel plate component composed of the plate assembly thus joined, the sacrificial plate 40 is joined to the surface of one side in the thickness direction (lower side in the figure) of the steel plates 32 and 33 joined via the nugget N via the nugget N. Has been. This steel plate component is assembled to the vehicle body with the sacrificial plate 40 being joined. However, in this plate assembly, the sacrificial plate 40 is a member whose main purpose is to suppress the sheet separation by avoiding direct contact between the thick plate 33 and the lower electrode 12b. It has substantially no function (function as an outer plate or inner plate panel or function as a skeleton frame). That is, even if the sacrificial plate 40 is not provided in the above plate assembly, the performance (strength and the like) required as a component part of the automobile body is satisfied.

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

  Further, the sacrificial plate 40 and the thin plate 31 may be provided integrally. Specifically, as shown by a chain line in FIGS. 4A and 4B, a part of the thin plate 31 is extended, and the extended portion 40 ′ is bent and disposed below the thick plate 33. This portion can function as the sacrificial plate 40. In this case, since it is not necessary to separately form the sacrificial plate 40, the cost can be reduced.

  Moreover, as shown in FIG. 5, when joining the plate assembly which consists only of the thick plates 32 and 33 which consist of a high-tensile steel plate, you may arrange | position the sacrificial plate 40 on the both sides of the thick plates 32 and 33. FIG. Or as shown in FIG. 6, when joining the plate assembly which consists only of the thick plates 32 and 33 which consist of a high-tensile steel plate, you may distribute the sacrificial plate 40 only to one side. In this case, since the lower thick plate 33 is directly pressed by the electrode 12b, a recess is formed and is separated from the upper thick plate 32. However, the upper thick plate 32 is provided with the sacrificial plate 40. Since the formation of the recess is avoided, the gap between the thick plates 32 and 33 can be reduced as compared with the case where the thick plates 32 and 33 are directly pressed by the electrodes.

12a, 12b electrode 31 Thin plate 32, 33 Thick plate (high tensile steel plate)
40 Sacrificial plates G1, G2 Gap N, N1, N2, N3 Nugget

Claims (2)

A spot welding method of joining by energizing a plurality of superposed high-tensile steel plates sandwiched and pressed between a pair of electrodes,
A spot welding method, wherein a sacrificial plate is interposed between the electrode and the high-tensile steel plate arranged on the outermost side in the thickness direction among the plurality of high-tensile steel plates.   A steel plate component comprising: a plurality of high-tensile steel plates joined via nuggets; and a sacrificial plate joined via a nugget to one surface in the thickness direction of the plurality of high-tensile steel plates.

Images