JP5766918B2 - Spot welding method for thin steel sheet - Google Patents

Description

  The present invention relates to a spot welding method for thin steel sheets.

In recent years, there has been a growing demand for the use of thin, high-strength steel sheets in the body and parts of automobiles in order to reduce vehicle weight and improve collision safety for the purpose of reducing fuel consumption and reducing CO ² emissions. Yes.
On the other hand, the spot welding method is mainly used for assembling the vehicle body and attaching parts, but the following problems arise when spot-welding a high-strength steel sheet.
Examples of quality indicators for spot welds (welded joints) include tensile strength and fatigue strength. The former is very important as a parameter for determining the strength of a member. The tensile strength of the welded joint includes a tensile shear strength (TSS) measured by applying a tensile load in the shear direction and a cross tensile strength (CTS) measured by applying a tensile load in the peeling direction.

In general, the tensile shear strength of a welded joint increases with the tensile strength of the steel plate, but the cross tensile strength shows a peak when the tensile strength of the steel plate is around 780 MPa, and decreases thereafter.
Further, parts assembled by spot welding of high-strength steel plates have a problem that tensile residual stress is generated in the welded portion, resulting in a decrease in fatigue strength and dimensional accuracy due to thermal deformation, that is, assembly accuracy.

  Conventionally, as a method of improving the cross tensile strength (CTS) of high-strength steel plate welded joints, a method of adjusting the strength of the spot welded part (nugget part) and the heat-affected part by controlling the energization pattern of spot welding (patent) Document 1, Patent Document 2, and Non-Patent Document 1), a method of increasing the number of spot welding points (number of nuggets), a method of using a steel plate with a low carbon equivalent, and the like are generally performed. Furthermore, Patent Document 3 discloses a method of spot welding by pressing a welding electrode against a recess formed on the surface of a metal plate.

  Further, as a method for reducing the residual stress in the welded portion, there is a method for relaxing the stress by controlling the energization pattern described above.

  Further, as a method of reducing thermal deformation due to spot welding, a method of adjusting the order of spot hitting points (see Patent Document 4) is disclosed.

JP 2002-103048 A JP 2009-241086 JP 2009-101399 A JP 2003-290932 A

Improvement of fatigue strength of spot welded joints in high-strength steel "Iron and Steel 68th (1982) No. 9 P318-325"

  However, the method of controlling the energization pattern is that the energization time is generally prolonged, such as the temper energization that reduces the hardness by annealing the spot welded part (nugget part) and the heat-affected zone, and the productivity is reduced. There is a problem that the strength varies due to the influence of the metal material and the welding electrode wear.

  Moreover, the method of increasing the spot welding point has a problem of reducing productivity.

  In addition, the use of a material having a low carbon equivalent has a problem that the material cost increases because an alloy having a high unit price is used in order to ensure high strength.

  Also, the method of spot welding by pressing the welding electrode against the concave shape previously applied to the surface of the metal plate has a problem that the shape of the welding electrode changes due to wear or the like, and stable energization cannot be obtained. .

  Any of these methods for improving the strength of spot welded joints generates tensile residual stress in the spot welded part (nugget part), which reduces the fatigue strength of the welded part and increases the dimensional accuracy due to thermal deformation of parts assembled at multiple points. There are problems such as lowering.

  On the other hand, the method of adjusting the spot hitting order has a problem that, for example, in the assembly line of an automobile, the moving distance of the welder head (spot gun) becomes long and the productivity is lowered.

  Therefore, the present invention has been made in view of the above circumstances, and it is possible to realize excellent tensile strength at a spot welded portion and prevent thermal deformation by spot welding without impairing productivity. It aims at providing the spot welding method of a thin steel plate.

  The gist of the present invention aimed at solving the above problems is as follows.

[1] A spot welding method for a pair of thin steel plates, the first step of forming an embossed stretched portion by embossing over the one steel plate of the pair of thin steel plates, and the embossed stretch by pushing the molding unit ulcer Succoth, have a circular protrusion, and a second step of compressive residual stress to form the indentations applied, on the side where the projecting portion of the one steel sheet protrudes The other thin steel plate is overlapped, and at the position where the indentation is formed, the pair of thin steel plates is spot-welded by applying current while pressing the pair of spot welding electrodes so as to sandwich the pair of thin steel plates from above and below. A method of spot welding a thin steel sheet, comprising:
[2] A spot welding method of the pair of thin steel sheet, have a circular protrusion, and the one of the thin steel sheet having a indentations compressive residual stress is applied, the protrusion of the one steel sheet The other thin steel plate is overlaid on the protruding side, and at the position where the indentation portion is formed, the pair of thin steel plates are energized while pressing the pair of spot welding electrodes so as to sandwich the pair of thin steel plates from above and below. A spot welding method for thin steel sheets, characterized by spot welding steel sheets.
[3] The spot welding method for a thin steel sheet according to [1], wherein the shape of the embossed overhanging portion is a substantially spherical head overhanging shape.
[4] The thin steel plate according to [1] or [3], wherein the first step and the second step are performed simultaneously with a plurality of press forming steps for the one thin steel plate. Spot welding method.

ADVANTAGE OF THE INVENTION According to this invention, the spot welding method of a pair of thin steel plate which is excellent in the tensile strength in a spot weld part, and can implement | achieve the thermal deformation prevention by spot welding can be provided, without inhibiting productivity.
Further, according to the present invention, in spot welding of a high-strength steel plate used for mounting automobile parts and assembling a vehicle body, the cross tensile strength of the welded joint can be improved while ensuring good welding workability. .
By applying the present invention, it is possible to fully enjoy the safety improvement by applying a high-strength steel sheet in the automobile field and the like, the low fuel cost by weight reduction, the merit of CO ² emission reduction, etc., and the social contribution is great.

It is process drawing explaining the spot welding method of a pair of thin steel plate which is embodiment of this invention. It is a cross-sectional schematic diagram of the metal mold | die used at the 1st process of the spot welding method of a pair of thin steel plate which is embodiment of this invention. It is a cross-sectional schematic diagram which shows the indentation part of one steel plate obtained at the 2nd process of the spot welding method of a pair of thin steel plate which is embodiment of this invention. It is a perspective view explaining the cross tension test method of a resistance spot welded joint.

  Hereinafter, although the spot welding method of the thin steel plate of this invention is demonstrated, referring drawings, this invention is not limited to the following embodiment.

  FIG. 1 is a process diagram for explaining a method of spot welding a thin steel plate according to the present invention. As shown in FIG. 1 (a), the spot welding method for thin steel sheets according to the present embodiment is embossed with respect to one steel sheet of two thin steel sheets (a pair of thin steel sheets) that are materials to be joined. A first step of forming an embossed overmolded portion having a substantially spherical head shape by embossing overmolding with a metal mold, and forming in a first step with a flat plate mold as shown in FIG. 1 (b) A second step of crushing the embossed overmolded portion to form an indented portion having an annular projection, and the other steel plate on the side from which the projection of one steel plate protrudes as shown in FIG. And a third step of spot welding a pair of steel plates. In addition, FIG.1 (d) shows the cross section of a pair of thin steel plate joined after electricity supply.

  In the first step, as shown in FIG. 1 (a), emboss overhanging of a substantially spherical head overhang shape is performed on one of the thin steel plates 11 and 12 with the embossing mold 20 for one of the thin steel plates 11. Do.

  In the first step, an embossing mold 20 shown in FIG. 2 is preferably used. An embossing mold 20 shown in FIG. 2 includes a die 21 and a punch 22.

When viewed in plan, the die 21 is provided with a through hole 21a having a round hole shape having a diameter φDd. A radius of curvature is Rd between the inner surface 21b of the through hole 21a and the lower surface 21c of the die 21. A taper 21d is formed.
The upper surface 22a of the punch 22 is provided with a spherical protrusion 22b having a radius of curvature Rp, and the position of the spherical protrusion 22b and the position of the through hole 21a of the die 21 overlap. It is configured as follows. The diameter φDp of the spherical head-shaped convex protrusion 22b is preferably designed to be the same as or smaller than φDd.

  By embossing and forming one thin steel plate 11 between the punch 22 and the die 21 shown in FIG.

  Further, as shown in FIG. 1 (c), the tip 41 of the spot welding electrode 40 has a generally spherical shell shape. Therefore, in order to obtain the nugget 60 having an elliptical cross section, the shape of the embossed overmolded portion 11a is substantially the same. It is desirable to have a spherical head protruding shape.

  Next, in the second step, as shown in FIG. 1 (b), one flat steel plate 11 that has been embossed and stretched in the first step is sandwiched between two flat plate dies 30, and an embossed stretched portion is formed. 11a is crushed to form an indented portion 11b having an annular protrusion 11c.

  The protrusion diameter (φDe) in the cross section of the indentation portion 11b is desirably about a predetermined nugget diameter. One of the thin steel plates 11 is crushed after being stretched to the plastic region by embossed stretch forming, whereby compressive residual stress is applied to the indented portion 11b. In FIG. 3, the direction of the arrow 90 is the embossing extension direction.

  Note that the spot welding method of the present embodiment is a case where the spot welding of the present embodiment is applied to these parts after performing press forming on a steel sheet a plurality of times to form parts of a predetermined shape. Is assumed. In such a case, the first step and the second step are preferably performed simultaneously with the press molding step. Thereby, it is not necessary to carry out the first process and the second process separately from the molding of the parts, and the efficiency of the process can be improved.

Next, in the third step, first, the other smooth steel plate 12 is superposed on the side of the one thin steel plate 11 on which the annular protrusion 11c projects. Next, electricity is applied while pressing the spot welding electrode 40 made of copper or the like at a position sandwiching the indentation portion 11 b, and a molten metal portion is formed between the pair of thin steel plates 11 and 12. At this time, the annular protrusion 11c of the indentation portion 11b is in contact with the other steel plate 12, and is energized. Since the electrical resistance of this contact portion is large, when the energization is started, heat is preferentially generated from the contact portion. The molten metal portion is melted and rapidly cooled and solidified by heat removal to the water-cooled electrode or heat conduction of the thin steel plate after the end of the spot welding energization, and between the pair of thin steel plates 11 and 12, FIG. A nugget (welded metal) 60 having an elliptical cross section shown in (d) is formed, and the pair of thin steel plates 11 and 12 are joined.
A heat affected zone 61 is formed around the nugget 60. The nugget 60 and the heat affected zone 61 form a welded portion 70.

  As described above, according to the spot welding method of the present embodiment, the current density distribution in the vicinity of the contact portion changes and the efficiency is improved by making the shape of the contact portion of the pair of thin steel plates 11 and 12 appropriate. The larger nugget 60 can be obtained by exothermic to melting. That is, the indentation portion 11b has an annular protrusion 11c as shown in FIG. 1 (b), and this annular protrusion 11c comes into contact with the other steel plate 12, and preferentially generates heat to melt. Therefore, a larger nugget 60 can be obtained as compared with spot welding between flat plates.

  Further, when the molten metal portion is thermally shrunk with solidification shrinkage and cooling, the surroundings are constrained by the material, and therefore, a tensile residual stress is locally applied after cooling to room temperature. Normally, this tensile residual stress becomes a driving force for deforming the component and causes a decrease in the dimensional accuracy of the component and a decrease in the fatigue strength. Therefore, the tensile residual stress after spot welding is relaxed by this compressive residual stress, and the total residual stress after welding can be reduced.

  In addition, a first step of embossing and forming the embossed overmolded portion 11a on one of the thin steel plates 11 and 12, and a second step of crushing the embossed overmolded portion 11a. This step can be carried out without an additional step, for example, by performing it in a press molding process of the part carried out before the welding / assembly process. That is, in the press molding process, in order to form a complicated part shape, the press is usually performed in a plurality of processes, for example, a molding process for forming a rough shape and a restructuring process for forming a detailed shape and finishing it to a predetermined dimension. Embossing overhanging and crushing shapes can be incorporated into the mold in the pressing process.

  Examples The effects of the present invention will be described below with reference to examples, but the present invention is not limited to the conditions used in the following examples.

  Various test specimens for spot welding according to the method of the present invention were prepared using alloyed hot-dip galvanized (GA) steel sheets having a thickness of 1.2 mm and a tensile strength of 980 MPa. After embossing overmolding using the embossing mold 20 shown in FIG. 2, the embossed overmolding part was crushed with a flat plate mold to form an indented part having an annular protrusion. Subsequently, a steel sheet of the same thickness and the same steel type is superposed on the protruding portion of the indented portion, a spot welding electrode is arranged at a position sandwiching the protruding portion of the indented portion, and a cross tension test method for a resistance spot welded joint (JIS Z3137: “ISO / DIS14272: Specimen dimensions and procedure for cross tension testing resistance spot and embossed projection welds are the same except for some provisions”). The welding conditions are as shown in Table 1.

  Next, with respect to the obtained welded joint, based on the cross tension test method (JIS Z3137) of the resistance spot welded joint, as shown in FIG. 4, the peeling direction (the upper steel plate as shown by reference numeral 5 in FIG. A cross tension test was performed by loading the lower steel plate in the direction of peeling to the lower side.

  The main dimensions of the embossing mold used for embossing overmolding were φDp = φDd = 10 mm, Rp = Rd = 6 mm, and H = 3 mm.

  Table 1 shows the results. Condition 1 is an example in which spot welding is performed by superimposing the other steel plate on the protruding side of the protrusion of one steel plate. Condition 2 is an example in which an indented portion is not formed on one steel plate, the plate is left flat, and the other steel plate is overlapped with this to perform spot welding. Condition 3 is an example in which spot welding is performed by superimposing the other steel plate on the side opposite to the protruding side of the protrusion of one steel plate. In Table 1, the joint surfaces of a pair of steel plates under conditions 1, 2 and 3 are expressed as convex, flat plate and concave, respectively. The spot welding specimen prepared under condition 1 has a larger nugget diameter at the same current density and improved cross tensile strength (CTS) than any of the spot welding specimens prepared under conditions 2 and 3. Was.

  In addition, the same results can be obtained in tests of various steel types and different plate thicknesses, and the effects according to the present invention are the same in cold-rolled steel plates, and also in experiments in which the plating type and basis weight are changed. there were.

ADVANTAGE OF THE INVENTION According to this invention, the cross tensile strength of a welded joint can be improved, ensuring favorable welding workability | operativity in the spot welding of the high-strength steel plate used for the attachment of the components for motor vehicles, the assembly of a vehicle body, etc. Therefore, this makes it possible to fully enjoy the benefits of improving safety by applying high-strength steel sheets, reducing fuel costs by reducing weight, and reducing CO ² emissions in the automotive field and the like.

  DESCRIPTION OF SYMBOLS 11 ... One thin steel plate, 11a ... Embossing overmolding part, 11b ... Indentation part, 11c ... Annular projection part, 12 ... The other thin steel plate, 20 ... Embossing metal mold, 21 ... Dies, 21a ... Through-hole, 21b: inner surface of through hole, 21c: lower surface of die, 21d: taper, 22 ... punch, 22a ... upper surface of punch, 22b ... spherical head-shaped convex protrusion of punch, 30 ... flat plate mold, 40 ... for spot welding Electrode, 41: tip of electrode for spot welding, 60: nugget, 61 ... heat-affected zone, 70 ... weld zone, 80 ... load direction in cross tension test, 90 ... emboss projection direction.

Claims (4)

A spot welding method for a pair of thin steel plates,
A first step of forming an embossed overmolded part by embossing overmolding one of the pair of thin steel sheets,
The ulcer Succoth press the embossed stretch forming portion, a second step of have a circular protrusion, and forming a dent portion for compressive residual stress is applied,
The other thin steel plate is overlapped on the side where the protruding portion of the one thin steel plate protrudes, and a pair of spot welding electrodes are pressed so as to sandwich the pair of thin steel plates from above and below at the formation position of the indentation portion. A third step of spot welding the pair of thin steel plates while energizing while,
A method of spot welding a thin steel sheet, comprising: A spot welding method for a pair of thin steel plates,
Have a protruding portion of the annular, and the one of the thin steel sheet having a indentations compressive residual stress is applied, the protrusion of the one of the thin steel sheet is superposed the other steel sheet on the side that protrudes, Spot welding of a thin steel plate, wherein the pair of thin steel plates is spot-welded by energizing the pair of thin steel plates while pressing the pair of spot welding electrodes so as to sandwich the pair of thin steel plates from above and below at the formation position of the indentation portion. Method.   The spot welding method for thin steel sheets according to claim 1, wherein the shape of the embossed overhang forming portion is a substantially spherical head overhang shape.   The method of spot welding a thin steel plate according to claim 1 or 3, wherein the first step and the second step are performed simultaneously with a plurality of press forming steps for the one thin steel plate.

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