KR20150075572A

KR20150075572A - Spot welding method for ultra-high strength steel sheet

Info

Publication number: KR20150075572A
Application number: KR1020130163656A
Authority: KR
Inventors: 최두열; 손원근
Original assignee: 주식회사 포스코
Priority date: 2013-12-26
Filing date: 2013-12-26
Publication date: 2015-07-06

Abstract

The present invention relates to a method of spot welding an ultra-high strength steel sheet having a tensile strength of 800 MPa or more, the method comprising the steps of: performing primary energization of a spot portion to be welded of the super high strength steel sheet; 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 the magnitude of the current of the primary energizing current for a time proportional to the time of blocking the current at the spot portion, .

Description

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

TECHNICAL FIELD The present invention relates to a spon 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, due to the strong demand for light weight of the vehicle body and improvement of the collision stability, the applied materials have become stronger and stronger. Recently, the application of the steel material with the tensile strength of 1000 MPa is greatly increased. However, weldability is becoming an obstacle in applying such ultra high strength steels to automobile parts. In general, a large amount of carbon and alloying elements are added to achieve high strength. As a result, the amount of carbon equivalent, which indicates hardenability, is increased, so that the welded portion becomes weak, which is difficult to secure the performance required for the parts and the vehicle body. In general, spot welding, which is a type of resistance welding, is most widely applied to assembling automobile parts.

In the conventional high-strength steel spot welding method, as shown in Fig. 1, a multi-pulse welding with a single pulse or two or more pulses is often performed at the same welding current. However, when such a welding method is applied, high strength carbon steel having a high carbon equivalent increases the brittleness of the welded portion, resulting in an interface fracture (IF) or a partial interface fracture (PIF) A problem that the collision characteristic of the vehicle body is lowered may be caused.

In order to improve the weld strength of high-strength steels, the brittleness of welds must be reduced. Spot welding has a much faster cooling rate than other welding processes. Therefore, the spot welds of most high strength steels are composed of 100% martensite phase. However, even if it is the same martensite phase, the higher the content of the alloy element such as carbon is, the stronger the embrittlement is, and if the content of the impurity element such as S or P is high even in the welded part of the same alloy component, solidification segregation occurs and the brittleness of the martensite It becomes stronger. However, in order to secure the quality of steel, the downward trend of alloying elements is limited, and the impurity elements are limited to the reduction due to the load of the steelmaking process.

An object of the present invention is to provide a spot welding method of an ultra-high strength steel sheet capable of reducing the brittleness of a welded portion and improving welding strength.

The present invention relates to a method of spot welding an ultra-high strength steel sheet having a tensile strength of 800 MPa or more, the method comprising the steps of: performing primary energization of a spot portion to be welded of the super high strength steel sheet; 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 the magnitude of the current of the primary energizing current for a time proportional to the time at which the current is cut off in the spot region, by a spot welding method of an ultra- do.

Preferably, the time for cooling by interrupting the current is a cycle (1/60 second) of 3 to 7 times the thickness (mm) of the ultra-high strength steel sheet.

Preferably, the secondary energization is performed for 1.5 times the current interruption time.

Preferably, the secondary energization is carried out with a magnitude of a current of 70 to 90% of the magnitude of the current of the primary energization.

The conventional electric current pattern has a narrow range of the appropriate current and a low welding strength, so that there is a high risk of welding failure at the time of welding of the actual vehicle body. However, by applying the electric current pattern of the present invention and performing spot welding, It is possible to reduce the defective quality of welding due to the scattering at the time of assembling and sufficiently expand the width of the welded end cage and also to reduce the brittleness of the welded portion and to improve the welding strength.

FIG. 1 is a graph showing a current application pattern during spot welding.
2 is a graph showing a current-carrying pattern according to an embodiment of the present invention.

The present invention relates to reducing the brittleness of a high-strength steel welded portion by controlling a current-carrying pattern of spot welding.

To reduce the brittleness of the spot welded portion, recently introduced energization pattern is formed by nugget through energization by a post-tempering technique and then transforms the welded portion into a martensite phase through a sufficient cooling time. Thereafter, a tempering pattern having a lower current than that of the energization is added to reheat the welded portion to a temperature of 700 to 900 占 폚 to make the martensite phase into tempered martensite, thereby lowering the hardness. However, in this method, as the process time increases more than three times, it is not applicable to the actual process due to the problem of productivity decrease in the actual production line.

Accordingly, the present inventor devised a current-carrying pattern that reduces the degree of segregation by reusing the coagulated segregation that has occurred at the end of the flange in order to reduce the brittleness of the high-strength steel spot welded portion unlike the above-mentioned energizing pattern.

2, the spot welding method of an ultra-high strength steel sheet according to the present invention is characterized in that a primary energization (1) is performed in order to generate and grow a nugget at a spot portion to be welded of an ultra- , Cooling the spot region by interrupting the current for a time proportional to the thickness of the ultra-high strength steel sheet, cooling the spot region during a period of time proportional to the time of blocking the current at the spot region, And performing secondary energization (2) at the magnitude of the current proportional to the magnitude of the current to reheat the nugget.

It is preferable that the primary energization is performed with a current of 5.0 to 12.0 kA for 10 to 40 cycles.

The cooling time (CT) should give an appropriate time for the nugget to sufficiently coagulate, but if it is too long, the process time is increased and also the condition of the secondary energization (2) is 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 * 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). In order to decompose the solidified segregation, re-melting of the nugget in the reheating with the secondary energization (2) should be absolutely avoided. On the other hand, when the heating is not sufficient, the solidification segregation is difficult to be decomposed. Therefore, it is necessary to set the magnitude 2C of the current of the secondary energization 2 and the secondary energization time W2T.

&Quot; (2) "

Second energization time = cooling time * 1.5

&Quot; (3) "

The magnitude of the current in the secondary energization = b * (the magnitude of the primary energizing current), [0.7? B? 0.9]

It is preferable that the secondary energization time is performed for 1.5 times as long as the time for cooling by interrupting the current, as shown in Equation (2). It is preferable to conduct the secondary energization for 1.5 times the cooling time if it is found that it is preferable to conduct the secondary energization in proportion to the cooling time through various experiments.

It is preferable that the magnitude of the current of the secondary energization is in a magnitude of 70 to 90% of the magnitude of the current of the primary energization, as in Equation (3). When the secondary energization is carried out at a magnitude of less than 70% of the magnitude of the current of the primary energizing current, the effect of reheating is insufficient. When the energizing current exceeds 90%, the welding portion is remelted and the heat treatment effect disappears, There is no improvement because it happens again.

Hereinafter, the spot welding method of the ultra-high strength steel sheet of the present invention will be described in more detail with reference to the embodiments of the present invention.

[ Example ]

Resistance spot welding was performed under the welding conditions shown in Table 1 for ultra high strength having a tensile strength of 1200 Mpa and a thickness of 1.2 mm. In the following Table 1, the holding time means a holding time in the state where the electrode is pressurized after being energized. Since the cooling water flows into the electrode, the cooling rate of the welding portion becomes larger as the holding time becomes longer .

The cross-sectional tensile strength (CTS) in the following Table 1 was measured according to the method specified in ISO 14272: 2000, and the nugget diameter was measured as the mean value of the major axis and minor axis of the button after the cross test.

Primary energization Cooling
Cycle Secondary energization Retention time
(cycle) Nugget
diameter
(mm) Cross tensile strength
(kN) Energization time
(cycle) Energizing current
(kA) Energization time
(cycle) Energizing current
(kA) Comparative Example 1 20 5 0 0 0 10 3.0 3.9 Comparative Example 2 20 6 0 0 0 10 3.6 4.8 Comparative Example 3 20 7 0 0 0 10 5.4 6.6 Inventory 1 20 5 6 10 4.3 10 3.9 5.3 Inventory 2 20 6 6 10 5.1 10 4.7 6.6 Inventory 3 20 7 6 10 6.0 10 6.5 7.9

As can be seen from Table 1, Examples 1 to 3 in which the welding was conducted under the energization conditions of the present invention were compared with those in Comparative Examples 1 to 3 in which welding was performed under the conventional energization conditions, as well as welding strength (cross tensile strength, CTS) It can be confirmed that the button nugget is formed largely.

Comparative Example 3 and Inventive Example 3 were compared. Comparative Example 3 and Inventive Example 3 demonstrate that the time and current of the primary energization are the same, but Example 3 in which the secondary energization is additionally performed has a large nugget size and a high cross- The measurement can be confirmed. In the case of Example 3, the secondary energization was carried out to improve the welding strength (cross tensile strength, CTS) by reducing the degree of segregation caused by the solidification segregation occurring at the end of the jacket, and in Comparative Example 3, The brittle fracture occurred in the cruciform tensile test, and the size of the nugget was measured to be smaller than that of Inventive Example 3, and the weld strength (cross tensile strength, CTS) was also measured to be low.

While the illustrative embodiments of the present invention have been shown and described, various modifications and alternative embodiments may be made by those skilled in the art. Such variations and other embodiments will be considered and included in the appended claims, all without departing from the true spirit and scope of the invention.

1: Primary energization CT: Cooling time
2: Secondary energizing 2C: Second energizing current
2T: Second energization time

Claims

A method of spot welding an ultra high strength steel sheet having a tensile strength of 800 MPa or more,
Performing primary energization of a spot portion to be welded of the super high strength steel sheet;
Cooling the spot region by interrupting the current for a time proportional to the thickness of the super high strength steel sheet; And
And performing secondary energization at a magnitude of a current proportional to a magnitude of the current of the primary energizing current for a time proportional to the time of blocking the current in the spot region.

The spot welding method of an ultra-high strength steel sheet according to claim 1, wherein the time for cooling by interrupting the current is a period of cycle (1/60 second) of 3 to 7 times the thickness (mm) of the super high strength steel sheet.

The spot welding method of an ultra-high strength steel plate according to claim 1, wherein the secondary energization is performed for 1.5 times the current interruption time.

The spot welding method of an ultra-high strength steel plate according to claim 1, wherein the secondary energization is carried out with a current of 70 to 90% of the magnitude of the current of the primary energization.