KR20190076756A - Method for resistance spot welding of multy-layer steel sheet - Google Patents

Abstract

According to an aspect of the present invention, provided is a method for resistance spot welding of a multi-layer steel sheet, which can obtain an excellent welding part by minimizing resistance difference of a connection part when steel sheets having different properties of matter are welded by being stacked in multiple layers.

Description

[0001] METHOD FOR RESISTANCE SPOT WELDING OF MULTI-LAYER STEEL SHEET [0002]

BACKGROUND OF THE INVENTION 1. Field of the Invention [0002] The present invention relates to a resistance spot welding method for a steel sheet, and more particularly, to a resistance point welding method for a multi-layer steel sheet capable of performing welding by overlapping a plurality of steel sheets.

Recently, there has been a growing demand for high-strength and lightweight manufacturing technology for automobile parts and the like due to the weight reduction of the vehicle body and the fuel saving trend due to high oil prices. Therefore, a method of joining a plurality of plates having different materials or different thicknesses and strengths from each other in view of manufacturing automobile parts and the like that meet the purpose and purpose is an area that needs continuous research.

Among them, Resistance Spot Welding (RSW) has been mainly used in the automobile industry as a technique for joining joints of a plurality of plate materials. Because of its ease of automation and suitable for mass production processes, Which is the most widely used process.

Resistance spot welding is a technique in which a large current is flowed under pressure to obtain heat due to the contact resistance and the intrinsic resistance of the metal on the contact surfaces of the metals so that the bonding is carried out by the applied pressure when the metal is heated or melted .

For example, when a large current of kA is applied in a state where pressure is applied to two electrodes located above and below a two-layer welding base material, contact resistance generated at the contact surface between the electrode and the welding base material, Whereby the metal is heated and melted, and then the metal is cooled and solidified to be bonded.

On the other hand, in the case of a high-strength material, an increase in the amount of the alloy is inevitably required to increase the strength. However, when the amount of the alloy is increased, the resistance of the material increases, There is a problem in that it becomes difficult to secure respect.

Fig. 1 shows a change in resistivity according to the addition amount of the alloy of the base metal.

As described above, high strength materials have a disadvantage in that it is difficult to secure a sound welded portion due to the resistance difference between the interfaces at the time of welding.

Therefore, development of a welding technique capable of securing a sound welded portion at the contact portion when welding a plurality of steel sheets by overlapping is required.

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

The problems to be solved by the present invention are not limited to the above-mentioned problems, and other problems not mentioned can be clearly understood by those skilled in the art from the following description.

According to an aspect of the present invention, there is provided a method of manufacturing a steel sheet, comprising the steps of: a) pressing and contacting an electrode to a joint where two or more high-strength steel sheets and one or more low- b) performing preliminary welding after contacting the electrodes; c) performing the preliminary welding and cooling; And d) performing the preliminary welding after the cooling, wherein the preliminary welding is performed with a welding current (I1) of 6.8 to 7.2 kA and the main welding is performed during a welding time (T2) of 165 to 500 ms. A method of resistance point welding of a steel sheet is provided.

According to the present invention, it is possible to form a good welded portion by minimizing the resistance difference in the joints even if the steel material having different physical properties is welded in multiple layers.

1 is a graph showing a change in specific resistance according to an addition amount of an alloy of a 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.
3 shows an example of a three-ply welded joint structure in an embodiment of the present invention.

The inventors of the present invention have recognized and solved the problem that a good weld can not be formed due to the fact that uniform heat generation can not be caused due to the difference in resistance in the joint portion at the time of welding when the steel material having different physical properties is welded in multiple layers The study of depth and depth was studied.

As a result, it is confirmed that welding can be ensured even when the multi-layered steel plate is welded, because the optimization of the welding current pattern, that is, the preliminary welding and the main welding, is performed step by step and the amount of current is differently applied at each welding step. .

On the other hand, as an effort to solve the above-mentioned problem, it has been attempted to reduce an exothermic difference between interfaces by using an electrode having a large cross-sectional area on a material having a high resistance in a multilayered joint and an electrode having a small cross- There was an attempt. However, this method has a disadvantage in that it must be processed into a very wide variety of electrodes for each material in an actual process, and there is a problem that a polisher is additionally introduced when the electrode is used after being polished and reused.

On the contrary, in the present invention, a resistance spot welding electrode which is generally used can be used, which is economically advantageous.

Hereinafter, a resistance spot welding method of a multilayer steel plate according to an aspect of the present invention will be described in detail with reference to the 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), a method of resistance spot welding of a multi-layered steel sheet according to one aspect of the present invention comprises the steps of a) welding two or more high strength steel sheets and one or more low strength steel sheets, So as to contact the surface of the substrate; b) performing preliminary welding after contacting the electrodes; c) performing the preliminary welding and cooling; And d) performing the main welding after cooling.

Hereinafter, the resistance spot 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 the electrodes may be squeezed on the joints which are superimposed and joined. The pressing of the electrode can be performed in a state in which the electrode is in contact with the joint.

It is another object of the present invention to provide a method for joining a plurality of steel plates having different physical properties to each other and welding the same to a steel plate having two or more high strength steel plates each having a tensile strength of 950 MPa or more and one or more low strength steel plates having a tensile strength of 400 MPa or less Can be overlapped.

As an example, as shown in Fig. 3, two high-strength steel sheets may be firstly overlapped, and then welding may be performed with a structure in which one low-strength steel sheet is stacked on at least one side thereof.

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

The preliminary welding can be performed by applying a current to the pressed electrode in the joint where the multiple steel plates are overlapped as described above.

The preliminary welding is a step for forming an energization pass on two or more high-strength steel sheets. In the present invention, the effect of reducing the resistance difference between the interfaces at the joint portion can be obtained through such preliminary welding.

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

If the welding current is less than 6.8 kA in the preliminary welding, it is difficult to sufficiently reduce the resistance of the welded portion, and it becomes difficult to obtain a molten portion having a desired diameter. On the other hand, if it exceeds 7.2 kA, there is a problem that the quality of the welded portion is deteriorated due to occurrence of a spatter called a spalling phenomenon.

In one aspect of the present invention, the welding time (T1) in the preliminary welding with the welding current (I1) described above can be made shorter than or equal to the welding time (T2) in the subsequent welding.

That is, the welding time in the preliminary welding can be performed within a range satisfying the following formula (1), respectively.

Equation (1)

T2 占 0.5? T1? T2

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

According to the above-described conditions, it is possible to ensure that the diameter D1 of the molten part of the weld formed at the portion where two or more high-strength steel plates meet during preliminary welding satisfies the following formula (2).

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

Equation (2)

2? T <D1

(In the formula (2), t denotes the thickness of the high-strength steel sheet.)

After completion of the preliminary welding in accordance with the above description, it is preferable to interrupt the applied current to cool the molten joint by preliminary welding.

This is a process for further lowering the interfacial resistance at the subsequent welded joint at the time of welding. That is, there is a limit in lowering the interface resistance when the energization is continuously performed.

The cooling can be performed for 10 ms to 3000 ms. If the cooling time is less than 10 ms, the cooling effect can not be sufficiently obtained. On the other hand, if the cooling time exceeds 3000 ms, the joining part may solidify and the tempering effect may occur.

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

The main welding can be performed at a welding current of 6.4 to 8.2 kA for a period of 165 to 500 ms in order to obtain a welded portion having an excellent strength while further reducing the interfacial resistance.

If the welding current is less than 6.4 kA or the welding time is shorter than 165 ms, it is difficult to form sufficient welds. On the other hand, if the welding current exceeds 8.2 kA, it is difficult to obtain good quality welds due to the occurrence of spatter. On the other hand, when the welding time exceeds 500 ms, the wear of the welding electrode is increased and the electrode needs to be frequently replaced, thereby deteriorating productivity.

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

Hereinafter, the present invention will be described more specifically by way of examples. It should be noted, however, that the following examples are intended to illustrate the invention in more detail and not to limit the scope of the invention. The scope of the present invention is determined by the matters set forth in the claims and the matters reasonably inferred therefrom.

( Example )

Strength steel sheet having a tensile strength of 1470 MPa and a thickness of 1.4 mm was prepared and then a low-strength steel sheet having a tensile strength of 270 MPa and a thickness of 0.7 mm was prepared and laminated to the structure shown in Fig.

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

After the resistance spot welding was completed, the presence or absence of spalling on the welded portion formed, the thickness of the thinnest plate material after the fracture and the cross tensile strength were evaluated, and the results are shown in Table 2 below.

At this time, the standard of the present invention was applied to a welded portion having a cross-sectional tensile strength of 2.3 kN or more, satisfying the reference button radius of 5? T or more (where t denotes the thickness (mm) of the thinnest steel sheet) .

Condition Pressing force
(kN) T1
(ms) I1
(kA) Cooling time
(ms) T2
(ms) I2
(kA) Equation (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 occurrence Thrust button
(mm) Cross tensile strength
(kN) Welded part 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 ○ Inventory 1 13 × 5.03 2.55 2.49 ○ Inventory 2 14 × 5.54 3.09 2.48 ○ Inventory 3 15 × 5.93 3.04 2.45 ○ Honorable 4 16 × 5.94 3.14 2.49 ○ Inventory 5 17 × 6.01 3.19 2.47 ○ Inventory 6 18 ○ 6.18 3.14 2.41 × Comparative Example 12 19 ○ 4.56 2.38 2.48 × Comparative Example 13

(In Table 2, the diameter of the weld is the measured value of the welded part size (diameter) immediately after preliminary welding.)

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

On the other hand, conditions 9 to 18 show that resistance spot welding is performed by the welding pattern method proposed in the present invention.

However, in Comparative Examples 9 to 11 in which the amount of welding current is too small, the cross-tensile strength is low. In Comparative Example 12 in which the amount of welding current is too large during the main welding and Comparative Example 13 in which the amount of welding current is too large during preliminary welding, And it was difficult to secure a good welded portion.

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

This is because a predetermined electric conduction path is formed through preliminary welding, so that it is possible to generate more heat than before without spattering during welding.

Claims (7)

a) pressing and contacting an electrode to a joint where two or more high-strength steel sheets and one or more low-strength steel sheets are overlapped and then joined;
b) performing preliminary welding after contacting the electrodes;
c) performing the preliminary welding and cooling; And
d) performing the main welding after cooling,
Wherein the preliminary welding is performed with a welding current (I1) of 6.8 to 7.2 kA and the main welding is performed during a welding time (T2) of 165 to 500 ms.
The method according to claim 1,
Wherein the welding time (T1) in the preliminary welding satisfies the following formula (1).

Equation (1)
T2 占 0.5? T1? T2
(In the equation (1), I2 means the welding current during welding, and T2 in the formula (2) means the welding time during welding.)
The method according to claim 1,
And the cooling is performed for a time of 10 to 3000 ms.
The method according to claim 1,
Wherein the welded portion formed after the main welding has a cross tensile strength of 2.3 kN or more.
The method according to claim 1,
Wherein the molten part diameter (D1) of the weld formed after the preliminary welding satisfies the following formula (2).

Equation (2)
2? T <D1
(In the formula (2), t denotes the thickness of the high-strength steel sheet.)
The method according to 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 according to claim 1,
Wherein the high-strength steel sheet and the low-strength steel sheet are at least one of a cold-rolled steel sheet, a hot-dip galvanized steel sheet and an alloyed hot-dip galvanized steel sheet.

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