KR102010073B1 - Spot welding method - Google Patents

Abstract

Spot welding method according to an embodiment of the present invention, overlapping a plurality of galvanized steel sheet, pressing a region where the plurality of galvanized steel sheet is overlapped with a pair of electrodes, the pair of electrodes Preheating a first current WC1 for a first time WT1 and welding a second current WC2 for a second time WT2 between the pair of electrodes; The first time WT1 may be set to a range satisfying the following equation.
[Equation]
WT1 (msec) = a * (tb + 0.05 * tc), 50 ≦ a ≦ 200, where tb is the total thickness of the base material (mm) and tc is the total thickness of the galvanized layer (um).

Description

Spot welding method {SPOT WELDING METHOD}

The present invention relates to a spot welding method. Specifically, it relates to a method of spot welding a galvanized steel sheet.

Due to the strong demand for lighter body weight and improved collision stability in the automotive industry, the applied materials have been increasingly strengthened, and in recent years, the application of steel materials with a tensile strength of 1000 MPa has been greatly increased. In addition, the application rate of galvanized steel sheet for the purpose of improving the corrosion resistance by increasing the life of the vehicle and the increase of the warranty period is increasing. Therefore, the development of galvanized ultra high strength steel is also in progress. In general, spot welding, a kind of resistance welding, is most widely applied to automobile parts assembly.

Zinc-plated ultra high strength steel is a problem of weld brittleness due to the high carbon equivalent of the material. (HSLA) There are problems that occur more easily than steel. Recently, the development of materials capable of securing high strength and elongation at the same time by increasing the residual austenite fraction, such as TRIP (TRansformation Induced Plasticity) steel, is underway. In general, it is known that the austenite phase is more sensitive to LME than the ferrite phase.

In the conventional spot welding method, as shown in FIG. 2, a single pulse welding is performed at a constant current or a multipulse welding of at least two pulses is performed.

Patent Registration 10-1744427

An object to be solved in an embodiment of the present invention is to provide a spot welding method in which no crack occurs due to liquid metal embrittlement (LME).

According to an embodiment of the present invention, a spot welding method includes overlapping a plurality of galvanized steel sheets, pressing a region in which the plurality of galvanized steel sheets overlap, with a pair of electrodes, between the pair of electrodes Preheating a first current WC1 for a first time WT1 and welding a second current WC2 for a second time WT2 between the pair of electrodes; One time WT1 may be set to a range satisfying the following equation.

[Equation]

WT1 (msec) = a * (tb + 0.05 * tc), 75≤a≤115, where tb is the total thickness of the base material (mm), tc is the total thickness of the galvanized layer (μm), a is the thickness in time The parameter to convert.

The magnitude of the first current WC1 may be greater than or equal to 0.6 times and less than or equal to 0.9 times the magnitude of the second current WC2.

And, between the preheating step and the welding step, further comprising a cooling step of not flowing electricity to the pair of electrodes, the holding time of the cooling step may be 50msec or more.

According to one embodiment of the present invention, it is possible to provide a spot welding method in which no crack occurs due to liquid metal embrittlement (LME).

1 is a schematic diagram for explaining a spot melting point method.
2 is a view for explaining a conventional spot welding method.
3 is a cross-sectional photograph of a welded metal part formed by a conventional spot welding method.
4 is a view showing three basic elements required for liquid metal embrittlement (LME) crack generation.
5 is an equilibrium diagram of Fe—Zn.
It is a figure explaining the spot welding method by this invention.

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

However, embodiments of the present invention may be modified in various other forms, and the scope of the present invention is not limited to the embodiments described below. In addition, the embodiments of the present invention are provided to more completely explain the present invention to those skilled in the art.

1 is a schematic diagram for explaining a spot melting point method.

As shown in FIG. 1, first, two steel plates 1A and 1B which are to-be-welded materials are piled up. Then, two sheets of steel sheets 1A are energized while pressing the electrodes 2A and 2B made of a copper alloy from both sides, that is, as shown in FIG. 1, with respect to the overlapping portions of the steel sheets 1A and 1B. , 1B) to form a molten metal portion. After the welding current is completed, the molten metal portion is rapidly cooled and solidified by thermal conduction by the electrodes 2A and 2B to be water cooled and thermal conduction by the steel sheets 1A and 1B itself, and the two steel sheets 1A and 1B are solidified. The nugget 3 is formed in between. By forming such a nugget 3, two steel sheets 1A and 1B are welded. The indentation 4 may be formed on the surface of the steel sheets 1A and 1B by being pressed by the electrodes 2A and 2B. The spot weld 10 formed by spot welding includes not only the nugget 3 but also the heat affected zone HAZ around the nugget 3.

3 is a cross-sectional photograph of a welded metal part formed by a conventional spot welding method.

Figure 3 shows the spot weld cross section between the galvanized TRIP steel (top) and mild steel (bottom), showing the LME cracks generated on the TRIP steel heat affected zone (HAZ) surface of the left edge of the nugget. Although it did not occur in mild steel, such cracks are easily observed in high-strength TRIP steel. These LME cracks are cracked as molten zinc penetrates along the grain boundary at the point where the temperature of the material rises during welding, and the low melting point zinc plated layer is melted and the tensile stress caused by thermal deformation due to electrode pressurization and welding thermal history. It is known to develop into. Such surface cracks are not allowed in automobile companies.

4 is a view showing three basic elements required for liquid metal embrittlement (LME) crack generation.

Three basic elements required for LME cracking are shown. LME cracking occurs only when all three factors of the substrate's sensitivity, liquid metal and tensile stress are met. In other words, in order to prevent LME cracking, it is necessary to change the condition under which only one of these three elements does not cause LME cracking. In order to prevent LME cracking in a given base material, there may be a method of changing the properties of the liquid metal or minimizing the tensile stress of the weld.

One of the ways to change the properties of the liquid metal is to increase the melting point of the liquid metal. In order to increase the melting point of the liquid metal, that is, the galvanized layer, alloy elements such as Fe, Al, and Ni may be added, but they do not satisfy the standards of automobile companies due to corrosion resistance, adhesion, and environmental problems. Therefore, there is a need for a method of increasing the melting point of a plated layer at a given zinc plating condition other than alloy plating.

5 is an equilibrium diagram of Fe—Zn.

In the case of a plated steel sheet, various alloy phases are made by the diffusion of Fe of a base material to a plating layer by heating. 5 is an equilibrium diagram of Fe—Zn, where various intermetallic compounds exist, and each compound has a higher melting point than the pure Zn phase (η). As the Fe diffusion amount increases, the melting point of the compound increases to 780 ° C, which is almost twice that of 420 ° C of pure Zn. Fe diffusion is a function of temperature and time. If the temperature is high and the heating time increases, alloying of the plating layer will proceed rapidly. However, when the entire plating steel is alloyed, problems such as deterioration of corrosion resistance and adhesion may occur. Therefore, a method of increasing the melting point of the plating layer by locally preheating only the portion to be welded will be effective. The local preheating method has a method of utilizing an additional heating device, but there is a problem that it is not easy to attach the additional heating device in the automobile assembly line.

It is a figure explaining the spot welding method by this invention.

Referring to FIG. 6, the spot welding method according to the present invention may be configured with two or more pulses in order to heat, that is, pre-heat, the plating layer before the main welding during spot welding. The spot welding method of the present invention includes a preheat pulse Weld1 for alloying the plating layer before the welding pulse Weld2. In one embodiment of the present invention, the pressing force may be kept constant during the preheating by applying the preheat pulse weld1 and the welding by applying the welding pulse Weld2. The degree of alloying of the plating layer varies depending on the time WT1 of the preheat pulse Weld1 and the current size WC1. Therefore, appropriate preheating pulse conditions need to be determined according to the total thickness tb of the base material, the total thickness tc of the plating layer, and the like.

The spot welding method of the present invention is a welding method that satisfies the following preheating conditions in welding the total thickness tb (mm) of the base material with a base material tensile strength of 800 Mpa or more and the total thickness tc (μm) of the galvanized layer. When two plated steel sheets are welded, the total thickness of the base material tb (mm) is the sum of the thicknesses of the two base materials, and the total thickness of the galvanized layer tc (μm) is the thickness of the galvanized layers plated on the two base materials. It is the sum of the thicknesses. In Formula 1 below, a is a parameter for converting thickness to time. A cooling step is further included between the base material preheating pulse Weld1 and the welding pulse Weld2, and the holding time CT of the cooling step is as follows.

WT1 (msec) = a * (tb + 0.05 * tc), 75≤a≤115 [Equation 1]

WC1 (A) = b * WC2, 0.6≤b≤0.9 [Equation 2]

CT (msec) ≥ 50

According to the present invention, the LME crack can be reduced by preheating the plating layer under preheating conditions satisfying the above Equations 1 and 2 before the main welding in the spot welding process without the addition of an additional heating device.

Example

Spot welding was carried out using DC welding machine on two layers of electro galvanized (EG) 1200Mpa high-strength steel (thickness, 1.2mm) of 18μm double-sided plating thickness, applying various preheating conditions. The results of measuring the LME cracks are shown in Table 1.

Preheat pulse Cooling time
(msec) Saw welding pulse Keep pressurized after welding pulse
(msec) Max LME Crack (μm) 1st time
(msec) First current
(kA) 2nd time
(msec) Second current
(kA) Comparative Example 1 - - - 280 6.2 200 42 Comparative Example 2 160 3.2 100 280 6.2 200 38 Comparative Example 3 320 3.2 100 280 6.2 200 36 Comparative Example 4 480 3.2 100 280 6.2 200 37 Comparative Example 5 160 4.5 100 280 6.2 200 40 Example 1 320 4.5 100 280 6.2 200 0 Example 2 480 4.5 100 280 6.2 200 0 Comparative Example 6 50 9.5 100 280 6.2 200 60 Comparative Example 7 100 9.5 100 280 6.2 200 70

Although described with reference to the above embodiments, those skilled in the art can variously modify and change the present invention without departing from the spirit and scope of the invention described in the claims below. You will understand.

1A, 1B: steel sheet, 2A, 2B: electrode, 3: nugget, 4: intent (concave), 10: spot weld, HAZ: heat affected zone

Claims (4)

Superimposing a plurality of galvanized steel sheets;
Pressing the region where the plurality of galvanized steel sheets overlap with a pair of electrodes;
Preheating a first current WC1 for a first time WT1 between the pair of electrodes; And
And welding a second current WC2 between the pair of electrodes by flowing the second current WC2 for a second time WT2.
The first welding time (WT1) is set to a range that satisfies the following equation.

[Equation]
WT1 (msec) = a * (tb + 0.05 * tc), 75≤a≤115, where tb is the total thickness of the base material (mm), tc is the total thickness of the galvanized layer (μm), a is the thickness in time The parameter to convert.
According to claim 1,
The magnitude of the first current (WC1) is 0.6 times or more and 0.9 times or less of the size of the second current (WC2).
The method of claim 2,
Between the preheating and welding,
Further comprising a cooling step of not flowing electricity to the pair of electrodes,
Spot welding method of the cooling step is 50msec or more.
According to claim 1,
The pressing force is kept constant during the preheating and welding steps.

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