KR101674772B1 - Spot welding method for ultra-high strength steel sheet - Google Patents

Abstract

TECHNICAL FIELD [0001] The present invention relates to a spot welding method of an ultra-high strength steel sheet which can be used for welding an automobile body or the like requiring high strength.

Description

TECHNICAL FIELD [0001] The present invention relates to a spot welding method for an ultrahigh strength steel plate,

TECHNICAL FIELD [0001] The present invention relates to a spot welding method of an ultra-high strength steel sheet which can be used for welding an automobile body or the like requiring high strength.

In the automobile industry, the material applied to the automobile industry has been increasingly intensified due to the continuous demand for lighter weight and improved crash stability. Recently, the application of the steel material having the tensile strength of 1000 MPa has been greatly increased.

However, when such an ultra-high strength steel is applied to automobile parts, the weldability is becoming an obstacle.

That is, in order to increase the strength of the steel, a large amount of carbon and other alloying elements are generally added, which increases the carbon equivalent of the hardenability and weakens the welded portion, There are difficulties.

In general, spot welding, which is a kind of resistance welding, is most widely applied for assembling automobile parts.

Conventionally, in the spot welding method of high strength steel, a single pulse (a) or a multi-pulse (b) welding of two or more pulses is often performed under a constant pressing force at the same welding current as in Fig. However, when such a welding method is applied, high strength carbon steel having a high carbon equivalent will have a higher brittleness of the welded portion, resulting in Interface Fracture (IF) or Partial Interface Fracture (PIF) A problem that the collision characteristic of the vehicle body is lowered may be caused.

Therefore, it is necessary to improve the welding strength of the high strength steel. To do so, it is necessary to improve the notch sensitivity of the welded portion, that is, the ductility. In order to improve the ductility of the welded part, it is necessary to lower the hardness of the welded part or reduce the solidified segregation of the sulfur (S) and phosphorous (P) components.

As a countermeasure thereto, a post-weld heat treatment method such as post-weld tempering has been proposed, but this method has a disadvantage in that the process time is long and the applicable carbon (C) content is limited. In practice, when the steel material containing 0.2% carbon (C) is welded by the above method, the ductility of the welded portion can be greatly improved. However, the steel material containing 0.2% carbon (C) Respectively.

An aspect of the present invention is to provide a spot welding method of an ultra-high strength steel sheet capable of suppressing the brittleness of a welded portion during spot welding of a high strength steel to improve the welding strength.

According to an aspect of the present invention, there is provided a method of spot welding an ultra-high strength steel sheet having a tensile strength of 1000 MPa or more,

Pressing the electrode with a first pressing force on a spot portion of the super high strength steel sheet to be welded to bring the electrode into contact with the ultra high strength steel sheet; Performing a primary energization after applying the first pressing force; Cooling the spot region by interrupting the current for a time proportional to the thickness of the super high strength steel sheet; And performing a secondary energization at a magnitude of a current proportional to a magnitude of a current of the primary energization during a time proportional to a time of blocking the current at the spot portion,

And a second pressing force is applied in a magnitude proportional to the first pressing force within -50 msec before the end of the primary energization to +50 msec after the end of the primary energization.

When resistance spot welding of an ultra-high strength steel sheet is carried out by the welding method according to the present invention, it is possible to secure a nugget with a sufficient size and to obtain an excellent weld portion without defects such as brittleness of the welded portion, .

BRIEF DESCRIPTION OF DRAWINGS FIG. 1 is a graph showing a pressing force pattern and a current application pattern in a conventional spot welding.
2 is a graph showing a pressing force pattern and a current application pattern according to an embodiment of the present invention.
FIG. 3 illustrates an example in which the shape of the notched portion is modified when the welding method according to the embodiment of the present invention is applied.
4 is a graph showing a pressing force pattern and an energizing pattern of the inventive example 5 according to the embodiment of the present invention.

The inventors of the present invention have studied in depth how to prevent the brittleness of the welded portion during spot welding of high-strength steel.

Particularly, the inventor of the present invention confirmed that it is possible to effectively prevent the stress concentration on the notch portion of the spot weld portion and to effectively prevent the brittleness of the weld portion when the segregation in the weld portion is reduced. In order to alleviate stress concentration in the notch portion, It has been found that it is advantageous to change the shape.

Therefore, the present inventors devised a pressing force pattern capable of alleviating stress concentration by deforming the notched portion shape and a conductive pattern capable of reducing the degree of segregation in the welded portion.

Hereinafter, the present invention will be described in detail with reference to the drawings.

2 is a graph showing a spot welding method of an ultra-high strength steel sheet according to the present invention.

As shown in Fig. 2, in a method of spot welding an ultrahigh strength steel sheet having a tensile strength of 1000 MPa or more according to an aspect of the present invention, an electrode is pressed to a spot portion to be welded of the ultra high strength steel sheet with a first pressing force, To the ultra-high strength steel sheet; Performing a primary energization after applying the first pressing force; Cooling the spot region by interrupting the current for a time proportional to the thickness of the super high strength steel sheet; And performing secondary energization at a magnitude of a current proportional to a magnitude of a current of the primary energizing current for a time proportional to a time at which the current is blocked at the spot portion, wherein before or after the end of the primary energizing And a second pressing force is applied in a size proportional to the first pressing force.

In the following, each of the above-mentioned steps performed sequentially will be described in more detail.

First, it is preferable to press the electrode to a spot portion of an ultra-high strength steel sheet having a tensile strength of 1000 MPa or more prepared for spot welding, and it is preferable that the pressing is performed while the electrode is in contact with the ultra high strength steel sheet.

In the present invention, the pressing force at this time is denoted by the first pressing force, and the first pressing force is preferably in the range of 2000 to 6000 N.

If the first pressing force is less than 2000 N, the spot portions may not be tightly sealed with each other, resulting in poor welding. On the other hand, if the first pressing force is more than 6000 N, the electrodes may be welded to the spot portions, I can not.

It is preferable to conduct the primary energization in the state where the electrode is pressed as described above.

At this time, it is preferable to conduct the primary energization at a current of 5.0 to 12.0 kA for 10 to 40 cycles.

, t: 피용접재 두께)의 너깃경을 확보하기 위한 것으로, 너깃경이 너무 작으면 기본적인 용접부 물성을 얻을 수 없게 된다.The primary energization of the shinker is more than a certain size

, t: the thickness of the welded material). If the nugget diameter is too small, the basic weldment properties can not be obtained.

The cycle and current magnitude of the primary energization vary depending on the type and thickness of the material, but if the above-mentioned range is satisfied, a desired ferrule can be obtained.

It is preferable to perform the step of cooling the spot of the molten spot for a predetermined time by blocking the applied current after completion of the primary energization according to the above.

However, in the present invention, it is intended to alleviate the stress concentration applied to the notch portion by raising the pressing force already applied before or after the completion of the primary energization.

More specifically, it is preferable that a pressing force is applied in a range of -50 msec to +50 msec before the end of the primary energization in proportion to the first pressing force. In the present invention, the pressing force at this time is specified as a second pressing force do.

As described above, the second pressing force is preferably proportional to the first pressing force, preferably 130 to 250% of the first pressing force.

If the magnitude of the second pressing force is less than 130% of the first pressing force magnitude, the effect of mitigating the notch stress concentration can not be sufficiently obtained. On the other hand, if the magnitude exceeds 250%, welding failure may occur due to excessive pressure.

According to the present invention, as described above, the magnitude of the pressing force before or after the completion of the primary welding is increased as compared with the first pressing force. In this case, by deforming the shape of the notch portion during spot welding, stress concentration on the notch portion is alleviated There is an effect that can be.

That is, by increasing the magnitude of the pressing force during the secondary energization, a part of the nugget portion is discharged to the notch portion, so that the shape of the notch portion can be deformed. For example, as shown in FIG. 3, the shape of the notch portion formed in the initial V-shape can be deformed into the shape of the notch portion as a part of the nugget is discharged to the notch portion by the second pressing force, It is possible to mitigate stress concentration at the notch portion by greatly reducing the magnitude of the stress.

It is preferable to perform the step of cooling the molten spot portion for a predetermined time in the state where the second pressing force is applied as described above.

At this time, the cooling time (CT) should be given an appropriate time enough for the nugget to solidify sufficiently, but if it is too long, the process time is increased and also the conditions of the subsequent secondary energization are affected. Therefore, it is preferable that the time for cooling by interrupting the current is cooled for a time (1/60 seconds) of 3 to 7 times the thickness (mm) of the ultra-high strength steel sheet so as to satisfy the following expression .

[Equation 1]

Cooling time = a x t, [3? A? 7, t = thickness of steel plate]

Equation (1) is an equation that is calculated through experiments so that the time for cooling to the proper temperature for performing the secondary energization after the primary energization can be derived in proportion to the thickness.

It is estimated that solidification segregation occurs in the nugget solidified at the cooling time (CT). Therefore, in order to decompose the solidified segregation, it is preferable to perform reheating by secondary energization. In this way, it is absolutely necessary to avoid re-melting the nugget during the secondary energization. On the other hand, it is difficult to decompose the solidification segregation if the heating is insufficient. Therefore, it is necessary to set the appropriate current amount and the energization time.

Preferably, the secondary energization is performed for a time of 1.0 to 3.0 times the current interruption time so as to satisfy the following expression (2). It has been found that it is preferable to conduct secondary energization in proportion to the time of cooling through various experiments, and it is preferable to carry out the secondary energization for 1.0 to 3.0 times of the cooling time.

&Quot; (2) "

Secondary energization time = cooling time x b, [1.0? B? 3.0]

It is preferable that the magnitude of the current at the time of the secondary energization is such that the magnitude of the current is 70 to 90% of the magnitude of the current of the primary energization so as to satisfy the following expression (3). If the secondary energization is carried out at a magnitude of less than 70% of the magnitude of the current during the primary energization, the effect of reheating is insufficient. On the other hand, if the secondary energization exceeds 90%, the welded portion is remelted, There is no improvement because it happens again.

&Quot; (3) "

The magnitude of the current of the secondary energization = c (the magnitude of the primary energizing current), [0.7? C? 0.9]

The ultrahigh strength steel material referred to in the present invention is a material suitable for automobile parts and the like, and when the spot welding method according to the present invention is applied in manufacturing parts intended to be welded, it is possible to suppress the occurrence of brittleness have.

On the other hand, the pattern for increasing the pressing force at the time when the pressing force pattern of the present invention, that is, the initial pressing force is applied and the welding has been performed for a certain period of time, is a pattern for applying the energizing pattern of the first energization- In addition, the present invention can be applied to a case where welding is performed with a single energized pattern. At this time, it is preferable that the time for increasing the pressing force is immediately before or after the completion of the single energization.

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 )

Resistance spot welding was performed under the welding conditions shown in Table 1 for ultra high strength having a tensile strength of 1000 MPa and a thickness of 1.0 mm.

After completion of the resistance spot welding in accordance with each condition, the cross talk tensile strength (CTS) and the nugget diameter were measured and are shown together in Table 1 below.

In this case, the cross talk tensile strength (CTS) was measured by the method specified in ISO 14272: 2000, and the nugget diameter was measured as the average value of the major and minor axes of the button after the tensile test.

In Table 1, the hold time means a time to hold the electrode under pressure with the electrode after completing energization. Since the cooling water flows into the electrode, the longer the holding time, the greater the cooling speed of the weld.

In the following Table 1, the time of the second pressurization refers to the end of the first energization, '0' means immediately after the end, '-' means before the end, and '+' means the time after the end.

division
1st
Pressure
(N) Primary energization Second pressurization Cooling
(msec) Secondary energization maintain
time
(msec) Nugget
diameter
(mm) CNS
(kN) time
(msec) electric current
(kA) Point
(msec) Pressing force
(N) time
(msec) electric current
(kA) Conventional Example 1 4500 200 - - - - - - 100 1.9 2.21 Conventional Example 2 4500 200 - - - - - - 100 2.6 2.45 Comparative Example 1 4500 200 8 0 4500 100 170 7.2 100 3.0 2.74 Comparative Example 2 4500 200 8 0 4500 100 170 7.2 100 3.4 2.79 Inventory 1 4500 200 8 -35 6500 100 170 7.2 100 5.1 4.17 Inventory 2 4500 200 8 -35 6500 100 170 7.2 100 5.6 4.17 Inventory 3 4500 200 8 0 6500 100 170 7.2 100 5.2 4.85 Honorable 4 4500 200 8 0 6500 100 170 7.2 100 4.5 4.41 Inventory 5 4500 200 8 +35 6500 100 170 7.2 100 5.5 4.36 Inventory 6 4500 200 8 +35 6500 100 170 7.2 100 5.5 4.31

As shown in Table 1, Examples 1 to 6 in which spot welding was performed by the method proposed by the present invention were confirmed to have not only excellent welding strength (cross tensile strength, CTS) but also large nugget diameter have.

Particularly, in the first to sixth inventions, the solidification segregation occurring at the end of the flange during secondary welding has been reused and the degree of segregation has been reduced to improve the welding strength. By applying a higher pressing force before or after completion of the primary energization, The button nugget was formed largely by relaxing stress concentration.

On the other hand, in the case of Conventional Examples 1 and 2 in which the second pressing force is not applied and the secondary energization is not performed at all, it is confirmed that the diameter of the button nugget is formed to be small as well as the welding strength.

In addition, in the case of Comparative Examples 1 and 2 in which the pressure was applied in the same manner as the first pressing force before or after the completion of the primary energization even though the secondary energization was performed as in the present invention, And the diameter of the button nugget is formed smaller than the diameter of the button.

Particularly, in the case of Conventional Examples 1 and 2, fracture occurred due to high brittleness due to occurrence of solidification segregation, and in Comparative Examples 1 and 2, the weld strength was lowered because the stress concentration concentration reduction effect of the notched portion was not obtained .

On the other hand, Fig. 3 shows the energizing and pressing force patterns of the inventive example 5.

Claims (5)

A method of spot welding an ultra-high strength steel sheet having a tensile strength of 1000 MPa or more,
Pressing the electrode with a first pressing force on a spot portion of the super high strength steel sheet to be welded to bring the electrode into contact with the ultra high strength steel sheet;
Performing a primary energization after applying the first pressing force;
Cooling the spot portion by interrupting the current for a cycle (1/60) of 3 to 7 times the thickness (mm) of the ultra-high strength steel sheet so as to satisfy the following equation (1); And
Performing a secondary energization at a magnitude of a current of 70 to 90% of the magnitude of the current of the primary energization so that the following Equation (3) is satisfied for a time proportional to the time of blocking the current at the spot portion,
Applying a second pressing force within a range of -50 msec before the end of the primary energization to +50 msec after the end of the primary energization in a size of 130 to 250% of the first pressing force,
And the notched portion shape is deformed such that a part of the nugget is discharged to the notch portion by the second pressing force.

[Equation 1]
Cooling time (cycle) = a 占 t, [3? A? 7, t = thickness of steel plate]

&Quot; (3) "
The magnitude of the current of the secondary energization = c (the magnitude of the primary energizing current), [0.7? C? 0.9]
delete delete The method according to claim 1,
Wherein the secondary energization is performed for a time of 1.0 to 3.0 times of the current interruption time so as to satisfy the following expression (2).

&Quot; (2) "
Secondary energization time = cooling time (cycle) x b, [1.0? B? 3.0] delete

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