KR20100074805A - Resistance welding method for high strength steel sheet - Google Patents

Abstract

PURPOSE: A resistance welding method for a high strength steel sheet is provided to reduce a difference in melt generation time due to a difference of junction resistance and improve welding efficiency between relatively thin outer sheet and reinforcement sheet. CONSTITUTION: A resistance welding method for a high strength steel sheet comprises the steps of: placing an outer sheet(100) with relatively small contact resistance and an electrode(600) with a relatively large tip section adjacent to each other, arranging an inner sheet(300) with relatively large contact resistance and an electrode(500) with a relatively small tip section adjacent to each other, applying current to the electrodes so as to weld the outer and inner sheets and a reinforcement sheet(200) between the outer and inner sheets.

Description

Resistance welding method for high strength steel sheet

The present invention relates to a welding method of a high strength steel sheet member having a tensile strength of 60 kgf or more, and more particularly, to a resistance welding method of a high strength steel sheet for adjusting contact resistance between welding materials when three welding materials are resistance welded.

In general, when a current flows through a conductor, heat loss occurs due to the electric resistance inside the conductor. Resistance welding is a welding method that actively utilizes heat generation loss. Resistance welding is a welding method in which a large current flows while a pressure is applied to a welding material to obtain heat due to contact resistance generated at the contact surface between the materials and the resistivity of the material. Means.

1 is a view showing a configuration diagram and a welding principle of a general resistance welding apparatus.

As shown in FIG. 1, the resistance welding device is a cylinder which is connected to the upper and lower electrodes 10 and 20 and the electrodes 10 and 20 that are movable in the vertical direction and moves the electrodes 10 and 20 in the vertical direction. (Not shown) and a base (not shown) supporting the same.

 When two or more overlapping steel sheets 30 and 40 are energized while being pressed by the upper and lower electrodes 10 and 20, they are melted at the interface of the steel sheet by Joule heat to form a nugget 50. . That is, when the current is flowed by pressing the two welding materials 30 and 40 with the electrodes 10 and 20, heat is generated by the resistivity of the materials 30 and 40, which is proportional to the resistivity. The resistance between the materials 30 and 40 and the contact surfaces 2 and 6 of the electrodes 10 and 20 is greater than the resistances of the materials 30 and 40 or the electrodes 1, 3, 5 and 7 of the electrodes 10 and 20. It is larger and heat is generated mainly between the materials because the resistance of the contact surfaces between the materials 30 and 40 is relatively large compared to the resistance between the materials 30 and 40 and the contact surfaces 2 and 6 of the electrodes 10 and 20. The material is melted and joined.

In general, the steel sheet used for the vehicle body is composed of a thin outer plate and a thick inner plate, and a reinforcing material is added between the outer plate and the inner plate at the site where collision characteristics or rigidity are required.

FIG. 2 shows a structural example of a steel sheet used for a vehicle body, and is composed of an outer plate material, a reinforcing material, and an inner plate material, and has two types of interfaces: an outer plate material / reinforcing material interface and a reinforcing material / inner plate material interface. Since the resistance at the reinforcement / inner plate interface is much larger than the outer plate / reinforcement interface, melting occurs first at the reinforcement / inner plate interface of the steel sheet constituting the vehicle body, and then melting of the outer plate / reinforcement material occurs.

 However, in general, when the ratio of the thickness of the reinforcing material to the thickness of the inner plate and the thickness of the outer plate (hereinafter referred to as "thickness ratio") exceeds 4 to 5 ((surface thickness) / (reinforcement thickness + inner plate thickness)> 5) Unwelding occurs at the interface between the shell and reinforcement (JFE 枝 報 No.16, June 2007).

That is, when the thickness ratio is 5 or more, when the magnitude of the main welding current is increased, melting starts at the reinforcement / inner plate interface with a large relative contact resistance, and both the reinforcing material and the inner plate material are melted before melting occurs at the outer / reinforcing material interface. As a result, a phenomenon occurs in which the inner plate material and the electrode on the inner plate material are melted, so that the main welding current cannot be increased above the threshold value, and thus the outer plate material / reinforcement interface is not welded.

Therefore, the present invention is to solve the above problems according to the prior art, even if the tip area of the electrode in contact with the outer plate material is larger than the tip area of the electrode in contact with the inner plate material, even if the thickness ratio is 5 or more It is an object of the present invention to provide a resistance welding method in which welding between stiffeners can be made.

In the resistance welding method of a high strength steel sheet according to an aspect of the present invention, in a method of resistance welding three steel sheets having a bonding interface having a different contact resistance, the contact resistance is relatively smaller than the outer steel sheet on the side of the bonding interface having a relatively small contact resistance. Position the electrodes with a large tip area adjacent to each other, and arrange the outer steel sheet on the side of the junction interface having a relatively high contact resistance with the electrodes having a small tip area adjacent to each other, and then apply current to each of the electrodes under pressure. It is characterized in that the welding each weld interface.

As described above, the resistance welding method of the high strength steel sheet according to the present invention can reduce the time difference in which melting occurs due to the difference in the bonding resistance, and improve the welding efficiency of the relatively thin outer plate and reinforcing material contact surfaces.

Hereinafter, a resistance welding method of a high strength steel sheet according to the present invention will be described in detail with reference to the accompanying drawings.

In the resistance welding method of a high strength steel sheet according to the present invention, in the method of resistance welding three steel sheets having a bonding interface with different contact resistance, the tip area is relatively large with the outer steel sheet on the side of the bonding interface with relatively small contact resistance. Position the large electrodes to be adjacent to each other, and arrange the outer steel sheet on the side of the junction interface having a relatively large contact resistance so that the electrodes having a small tip area are adjacent to each other, and then flow a current while applying pressure to the respective electrodes. It is characterized by welding each joint interface.

3 is a diagram illustrating a configuration of welding three high strength steel sheets using electrodes having the same tip area, and FIG. 4 illustrates three sheets of high strength steel sheets using electrodes having different tip areas according to an embodiment of the present invention. Is a view showing a configuration for welding the.

In the case of welding three high-strength steel sheets having different thicknesses with the electrode 500 having the same tip area, the interface B of the reinforcing material / inner plate material having a large contact resistance is melted first when the molten current flows through the electrode. As a result, the outer sheet material / stiffener interface A having a small contact resistance is later melted.

Generally, there is no problem when melting three steel sheets having a thickness ratio of less than 5 by a resistance welding method. However, when welding three steel sheets having a thickness ratio of 5 or more, melting of the reinforcing material 200 and the inner plate material 300 may occur. Continuously, the reinforcement 200 and the inner plate 300 are all melted, but the outer plate / reinforcement interface A may not be welded, and the inner plate material (before the melting of the outer plate / reinforcement interface A) occurs. Welding of the 300 and the lower electrode 500 may occur.

On the other hand, when welding three high-strength steel sheets using two electrodes (500, 600) having different tip areas according to an embodiment of the present invention, the outer plate material 100, which is an outer steel sheet of the joint interface that the melting occurs late The contact area between the outer plate material 100 and the upper electrode 600 can be widened by increasing the tip area of the upper electrode 600 in contact with the upper electrode 600. As such, when the contact area is increased, the bonding resistance of the outer plate / reinforcement interface A can be increased, so that melting of the outer plate / reinforcement interface A can occur quickly.

The shape of the front end surface of the upper or lower electrode shown in FIG. 4 is just one example, and may be changed in various forms.

FIG. 5 is a cross-sectional view of a welded section when three high-strength steel sheets are welded using electrodes having the same tip area, and FIG. 6 is a diagram illustrating three sheets using electrodes having different tip areas according to an embodiment of the present invention. It is a figure which shows the weld cross section in the case of welding high strength steel plate.

EXAMPLE

For example, FIG. 5 and FIG. 6 show that the outer plate member having a tensile strength of 200 MPa of 0.7 mm thickness, the reinforcing member having a tensile strength of 780 MPa of 3.0 mm thickness, and the inner plate member having a tensile strength of 780 MPa of 2.0 mm thickness are bonded to each other. The cross section of the welded part when energizing with 3 kN, pressurization time 25 cycles (1 cycle = 1/60 second), main welding time 17 cycles, and main welding current of 7 mA is shown.

As shown in FIG. 5, the reinforcement / inner plate interface (B) is well welded, but the outer plate / reinforcement interface (A) is in the unbonded state 700, while the tip area of the outer plate material is relatively large. When the electrode is disposed, as shown in FIG. 6, melting occurs at the outer plate / reinforcement interface A, and thus the welded result 700 may be confirmed.

The above embodiment is a case where the thickness ratio is 7.1 (that is, the thickness ratio = outer plate thickness / (reinforcement thickness + inner plate thickness), 0.7 / (2.0 + 3.0) = 7.1), in the case of the thickness ratio of 5 or more in the prior art The problem of cosmetic contact between the plate and reinforcement interfaces is solved.

Preferably, the tip area ratio of the two electrodes is 1.4 to 2.0, and if the area is wide according to the curvature of the electrodes, the current density is increased.

Table 1 below compares the result of welding (A) using a common electrode with the result of welding (B) by the resistance welding method according to the present invention. As a result of resistance welding the outer plate material, reinforcing material and inner plate material used for the vehicle body, it can be confirmed that welding is not generated at the outer plate material / reinforcing material interface when the thickness ratio is 5 or more.

Thickness ratio 2.0 3.0 4.0 6.0 A welding welding welding Beauty B welding welding welding welding

The embodiments of the present invention are disclosed for purposes of illustration, and those skilled in the art will be able to make various modifications, changes, and additions within the spirit and scope of the present invention, and such modifications, changes, and additions. Should be considered to be within the scope of the following claims.

In addition, the drawings shown as an embodiment is reduced or simplified in order to explain the contents of the present invention, the present invention can be variously modified and changed without departing from the spirit or the scope of the present invention.

1 is a view showing a configuration diagram and a welding principle of a general resistance welding apparatus.

2 is a diagram illustrating a configuration example of a steel plate used for a vehicle body.

3 is a diagram illustrating a configuration in which three high strength steel sheets are welded using electrodes having the same tip area.

4 is a diagram illustrating a configuration of welding three high-strength steel sheets using electrodes having different tip areas according to an embodiment of the present invention.

FIG. 5 is a diagram showing a welded section in the case where three high strength steel sheets are welded using electrodes having the same tip area.

FIG. 6 is a cross-sectional view of a welded part when three high-strength steel sheets are welded using electrodes having different tip areas according to an exemplary embodiment of the present invention.

Claims (4)

In the method of resistance welding three steel sheets having a junction interface with different contact resistance, Position the outer steel sheet on the side of the bonding interface having a relatively small contact resistance adjacent to the electrode having a large tip area and adjacent the outer steel plate on the side of the bonding interface having a large contact resistance adjacent to the electrode having a relatively small tip area. And arranging, and welding the respective joining interfaces by flowing a current while applying pressure to the respective electrodes. The method of claim 1, The thickness of the outer plate material which is the outer steel plate on the side of the joint interface with relatively small contact resistance, and the thickness of the inner plate material which is the outer steel plate specific to the bonding interface with relatively high contact resistance, and the reinforcing material located between the outer plate material and the inner plate material The ratio is five or more resistance welding method. The method of claim 1, Resistive welding method characterized in that the tip area ratio of the two electrodes is 1.4 to 2.0. The method of claim 1, The three steel sheets are resistance welding method, characterized in that the tensile strength is 60kgf or more.

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