KR102010069B1 - Method for resistance spot welding of multy-layer steel sheet - Google Patents

Abstract

One aspect of the present invention is to provide a resistance point welding method for a multi-layered steel sheet which can obtain a good welded portion by minimizing the difference in resistance of joints when welding a plurality of steel sheets having different physical properties.

Description

Resistance Spot Welding of Multi-Layer Steel Sheet {METHOD FOR RESISTANCE SPOT WELDING OF MULTY-LAYER STEEL SHEET}

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a resistance spot welding method of a steel sheet, and more particularly, to a resistance spot welding method of a multilayer steel sheet which can be welded by stacking a plurality of steel sheets.

In recent years, with the trend of fuel saving due to lighter body weight and higher oil prices, the demand for high strength and light weight manufacturing technology for automobile parts is increasing. Therefore, the method of joining the plurality of plate materials having different materials or different thicknesses and strengths from the viewpoint of manufacturing automobile members or the like that meets the purpose and purpose of use is a field that requires continuous research.

Among them, as a technology for joining a plurality of plate joints, resistance spot welding (RSW) has been mainly used in the automobile industry, which has been manufactured for automobile production to this day because of easy automation and suitable for mass production process. Most commonly used in the process.

Resistance spot welding means that a large current is flowed under pressure and heat is generated by the contact resistance generated at the contact surface between the metals and the specific resistance of the metal, so that the joint is formed by the applied pressure when the metal is heated or melted. Say the method.

For example, when a large current of kA unit is applied while applying pressure to two electrodes positioned above and below a two-ply welding base material, heat is generated by the contact resistance generated at the contact surface between the electrode and the welding base material and the resistivity of the welding base material. This occurs, whereby the metal is heated and melted and then cooled and solidified to form a joint.

On the other hand, in the case of high-strength materials, an increase in the amount of alloy addition is inevitable to improve the strength. However, when the amount of the alloy addition increases, the resistance of the material increases. There is a problem that it is difficult to secure homage.

Figure 1 shows the change in the specific resistance according to the alloying amount of the base material.

As such, high strength materials have a disadvantage in that it is difficult to secure a healthy welded portion due to a difference in resistance between interfaces during welding.

Therefore, in welding welding several steel sheets, development of the welding technique which can ensure the healthy welding part by a contact part is calculated | required.

One aspect of the present invention is to provide a resistance point welding method of a multi-layered steel sheet which can obtain a good welded portion by minimizing the difference in resistance of the joints when welding the steel sheets having different physical properties in multiple layers.

The problem to be solved by the present invention is not limited to the above-mentioned problem, another task that is not mentioned will be clearly understood by those skilled in the art from the following description.

One aspect of the present invention, a) a step of contacting by pressing the electrode to the joint to be bonded after overlapping two or more high-strength steel sheet and one or more low-strength steel sheet; b) preliminary welding after contacting said electrode; c) cooling after performing the preliminary welding; And d) performing the main welding after the cooling, wherein the preliminary welding is performed at a welding current I1 of 6.8 to 7.2 kA, and the main welding is performed for a welding time T2 of 165 to 500 ms. Provides resistance point welding method of steel sheet.

According to the present invention, even if a plurality of overlapping welding of steel materials having different physical properties, there is an effect that can form a good weld by minimizing the difference in resistance of the joint.

1 is a graph showing a change in the specific resistance according to the alloying amount of the welding base material.
2 is a graph (a) showing a conventional welding current pattern and a graph (b) showing a welding current pattern according to an embodiment of the present invention.
Figure 3 shows an example of a three-ply welded joint structure in one embodiment of the present invention.

The inventors of the present invention recognizes a problem in that a good weld is not formed because uniform heat generation does not occur due to a difference in resistance at the welding part when overlapping welding steel materials having different physical properties, and can solve the problem. We have studied in depth what is possible.

As a result, optimization of the welding current pattern, that is, preliminary welding and main welding are performed step by step, and it is confirmed that a good weld can be secured even when welding the multilayer steel sheet by applying different amounts of current in each welding step. Came to complete.

On the other hand, in an effort to solve the above problems, by using an electrode having a large cross-sectional area for the material of high resistance in the multi-layer joint, and using an electrode having a small cross-sectional area for the material of low resistance, the heating difference between the interfaces is reduced. There was an attempt. However, this method has a disadvantage in that the actual process to be processed into a wide variety of electrodes for each type of material, there is a problem that you need to introduce a grinder in the case of reusing after polishing the electrode.

On the contrary, in the present invention, it is possible to use a resistance spot welding electrode which is generally used, and thus it will be economically advantageous.

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

2 is a graph (a) showing a conventional welding current pattern and a graph (b) showing a welding current pattern according to an embodiment of the present invention.

As shown in FIG. 2 (b), the resistance spot welding method of a multilayer steel sheet according to one aspect of the present invention includes a) an electrode at a joint portion which is bonded after overlapping two or more high strength steel sheets and one or more low strength steel sheets; Pressing to contact; b) preliminary welding after contacting said electrode; c) cooling after performing the preliminary welding; And d) performing main welding after the cooling.

Hereinafter, the resistance point welding method of the multilayer steel sheet of the present invention will be described in more detail step by step.

First, as a base material to be welded, two or more high-strength steel sheets and one or more low-strength steel sheets may be prepared and stacked, and the electrodes may be squeezed to the joints to be joined. Pressing the electrode can be performed while the electrode is in contact with the joint.

On the other hand, the present invention provides a method for joining and welding a plurality of steel sheets having different physical properties, two or more high-strength steel sheet with a tensile strength of 950MPa or more, one or more low-strength steel sheet with a tensile strength of 400MPa or less It can be done.

As an example, as shown in FIG. 3, welding can be performed in a structure in which two high-strength steel sheets are first stacked and then one low-strength steel sheet is stacked on at least one surface thereof.

In addition, the type of the high strength steel sheet and the low strength steel sheet is not particularly limited, but may be, for example, one or more of a cold rolled steel sheet, a hot dip steel sheet and an alloyed hot dip steel sheet. For example, the high strength steel sheet may be carbon steel, high manganese steel (Mn 14% or more).

As described above, preliminary welding can be performed by applying a current to the electrode pressed to the joint portion in which a plurality of steel sheets overlap.

The preliminary welding is a process for forming an energization path on two or more high strength steel sheets, and in the present invention, the preliminary welding may reduce the difference in resistance between the interfaces at the joint.

More specifically, the preliminary welding may be performed at a welding current I1 of 6.8 to 7.2 kA.

If the welding current is less than 6.8kA during the preliminary welding, it is difficult to sufficiently reduce the weld resistance, and it is difficult to obtain a melted portion having a desired diameter. On the other hand, if it exceeds 7.2kA, a spatter, called a flying phenomenon, is generated, which causes a problem of deterioration in the quality of the welded part.

In one aspect of the present invention, the welding time T1 at the time of preliminary welding with the above-described welding current I1 can be performed at a time shorter than or equal to the welding time T2 at the time of the subsequent welding.

That is, the welding time at the time of the said preliminary welding can be performed within the range which satisfy | fills following formula (1), respectively.

Formula (1)

T2 × 0.5 ≤ T1 ≤ T2

(In formula (1), T1 means welding time in preliminary welding, T2 means welding time in main welding, and the unit is ms.)

According to the conditions described above, it is possible to ensure that the melt portion diameter D1 of the welded portion formed at the portion where two or more high strength steel sheets abut upon preliminary welding satisfies the following formula (2).

If the diameter D1 of the molten portion is 2√t or less, the weld strength of the weld portion obtained by completing the subsequent main welding cannot be ensured excellently. The upper limit of the diameter of the melting part D1 is not particularly limited, but may be, for example, less than 4√t.

Formula (2)

2√t <D1

(T in the formula (2) means the thickness of the high strength steel sheet.)

As described above, after the preliminary welding is completed, it is preferable to block the applied current to cool the molten joint by the preliminary welding.

This is a step performed to more effectively lower the interface resistance at the joint during subsequent main welding. That is, there is a limit to lowering the interfacial resistance when the energization is continuously performed.

The cooling may be performed for 10 ms to 3000 ms, and if the cooling time is less than 10 ms, the effect by cooling cannot be sufficiently obtained. On the other hand, if the cooling time exceeds 3000 ms, the joint is solidified and the tempering effect may appear.

Therefore, it is preferable to perform main welding after cooling for sufficient time as mentioned above.

The main welding may be performed for 165 to 500 ms time at a welding current of 6.4 to 8.2 kA in order to obtain a weld portion having excellent strength while further lowering the interfacial resistance.

In this welding, if the welding current is less than 6.4kA or the welding time is less than 165ms, it is difficult to form a sufficient weld, whereas if the welding current exceeds 8.2kA, it is difficult to obtain a good quality weld due to the spatter generation. On the other hand, when the welding time exceeds 500ms, the wear of the welding electrode is increased, so the electrode needs to be replaced frequently, which leads to a problem in that productivity is reduced.

The weld formed by the welding current pattern proposed by the present invention has good quality, and in particular, the cross tensile strength can be secured to 2.3 kN or more.

Hereinafter, the present invention will be described in more detail with reference to Examples. However, it is necessary to note that the following examples are only for illustrating the present invention in more detail, and are not intended to limit the scope of the present invention. This is because the scope of the present invention is determined by the matters described in the claims and the matters reasonably inferred therefrom.

( Example )

After preparing two high strength steel sheets having a tensile strength of 1470 MPa and a thickness of 1.4 mm, a low strength steel sheet having a tensile strength of 270 MPa and a thickness of 0.7 mm was prepared and overlapped with the structure of FIG.

At this time, resistance spot welding was performed under the conditions shown in Table 1 below, and a dome type tip diameter 6 mm was used as the electrode.

After completion of the resistance point welding, the weld portion formed after the presence or absence, the button diameter and the cross tensile strength of the thinnest plate after fracture was evaluated, and the results are shown in Table 2 below.

At this time, there is no spatter generation, the reference button diameter satisfies 5√t or more (where t represents the thickness (mm) of the thinnest steel sheet), and made the standard of the present invention for welds having a cross tensile strength of 2.3 kN or more. .

Condition Pressing force
(kN) T1
(ms) I1
(kA) Cooling time
(ms) T2
(ms) I2
(kA) Formula (1)
Satisfaction One 3 333 4.8 - - - - 2 3 333 5.2 - - - - 3 3 333 5.6 - - - - 4 3 333 6.0 - - - - 5 3 333 6.4 - - - - 6 3 333 6.8 - - - - 7 3 333 7.2 - - - - 8 3 333 7.4 - - - - 9 3 133 6.8 34 167 5.2 ○ 10 3 133 6.8 34 167 5.6 ○ 11 3 133 6.8 34 167 6.0 ○ 12 3 133 6.8 34 167 6.4 ○ 13 3 133 6.8 34 167 6.8 ○ 14 3 133 6.8 34 167 7.2 ○ 15 3 133 6.8 34 167 7.6 ○ 16 3 133 6.8 34 167 8.0 ○ 17 3 133 6.8 34 167 8.2 ○ 18 3 133 6.8 34 167 8.4 ○ 19 3 133 7.4 34 167 6.8 ×

Condition Spatter generation Break button diameter
(mm) Cross tensile strength
(kN) Weldment size (mm) Judgment division One × 1.35 0.81 - × Comparative Example 1 2 × 1.69 0.81 - × Comparative Example 2 3 × 1.97 1.54 - × Comparative Example 3 4 × 2.83 2.1 - × Comparative Example 4 5 × 3.10 2.59 - × Comparative Example 5 6 × 3.43 2.79 - × Comparative Example 6 7 ○ 4.24 2.54 - × Comparative Example 7 8 ○ 4.03 3.14 - × Comparative Example 8 9 × 1.45 1.3 2.49 × Comparative Example 9 10 × 1.69 0.91 2.48 × Comparative Example 10 11 × 4.12 2.09 2.50 × Comparative Example 11 12 × 4.34 2.32 2.42 ○ Inventive Example 1 13 × 5.03 2.55 2.49 ○ Inventive Example 2 14 × 5.54 3.09 2.48 ○ Inventive Example 3 15 × 5.93 3.04 2.45 ○ Inventive Example 4 16 × 5.94 3.14 2.49 ○ Inventive Example 5 17 × 6.01 3.19 2.47 ○ Inventive Example 6 18 ○ 6.18 3.14 2.41 × Comparative Example 12 19 ○ 4.56 2.38 2.48 × Comparative Example 13

(The welded part diameter in Table 2 shows the measurement of the melted part size (diameter) of the welded part immediately after the preliminary welding.)

In Table 1, the conditions 1 to 8 are the resistance spot welding by the conventional welding pattern method. Among these, the welded portions of Comparative Examples 1 to 6 formed under the conditions 1 to 6 did not generate spatter, but did not satisfy the reference button diameter. In addition, the welds of Comparative Examples 7 and 8 formed under the conditions 7 and 8 satisfied the reference button diameter, but it was difficult to secure a good weld due to spatter generation.

On the other hand, it can be seen that conditions 9 to 18 were performed by resistance spot welding by the welding pattern method proposed in the present invention.

However, Comparative Examples 9 to 11 in which the amount of welding current is too small in the main welding are inferior to the cross tensile strength, and Comparative Example 12 in which the amount of welding current is too large in the main welding and Comparative Example 13 in which the amount of welding current is too large in the preliminary welding are spatters. It occurred and it was difficult to secure a good weld.

On the other hand, Inventive Examples 1 to 6, in which welding was performed under the conditions proposed by the present invention, could secure a button diameter without generating a stepper, and also showed excellent cross tensile strength.

This is due to the formation of a predetermined conduction path through preliminary welding, which can generate more heat input than the conventional one without spatter generation.

Claims (7)

a) pressing two or more high-strength steel sheets and one or more low-strength steel sheets and then contacting them by pressing an electrode to a joint to be joined;
b) preliminary welding after contacting said electrode;
c) cooling after performing the preliminary welding; And
d) performing main welding after said cooling,
The preliminary welding is performed with a welding current (I1) of 6.8 to 7.2 kA and a welding time (T1) satisfying the following formula (1), and the main welding is performed for a welding time (T2) of 165 to 500 ms. Resistance point welding method.

Formula (1)
T2 × 0.5 ≤ T1 ≤ T2
(In formula (1), T1 means welding time in preliminary welding, T2 means welding time in main welding, and the unit is ms.)
delete The method of claim 1,
The cooling method is a resistance point welding method of a multilayer steel sheet is to be performed for a time of 10 ~ 3000ms.
The method of claim 1,
The welded portion formed after the main welding resistance point welding method of a multi-layered steel sheet having a cross tensile strength of 2.3kN or more.
The method of claim 1,
A resistance point welding method of a multilayer steel sheet, in which a molten portion diameter (D1) of a weld portion formed after the preliminary welding satisfies the following formula (2).

Formula (2)
2√t <D1
(T in the formula (2) means the thickness of the high strength steel sheet.)
The method of claim 1,
The high strength steel sheet is a steel sheet having a tensile strength of 950 MPa or more, and the low strength steel sheet is a steel sheet having a tensile strength of 400 MPa or less.
The method of claim 1,
The high strength steel sheet and the low strength steel sheet is a resistance point welding method of a multi-layer steel sheet is one or more of cold rolled steel sheet, hot-dip galvanized steel sheet and alloyed hot-dip steel sheet.

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