통전 패턴을 이용한 스폿 용접 연구

Abstract

최근 자동차 산업에서는 차체 경량화 및 충돌 안정성 향상에 대한 요구가 증가되어 왔다. 이에 따라 소재의 고강도화가 진행되며, 고강도의 강재가 적용되는 사례가 늘어나고 있다. 고강도강의 사용에 따라 고강도강 스폿 용접이 일반적으로 적용되고 있으며, 고강도강의 스폿 용접 방법으로 단일 펄스 또는 2펄스 이상의 다펄스 용접이 실시되고 있다. 그러나 고강도강의 경우 탄소당량이 높아 취성이 커져 계면 파괴가 발생하고, 용접강도가 낮아지게 되어 충돌특성이 저하되는 등의 문제가 있었다. 이러한 문제를 해결하기 위해 타용접에 비해 냉각속도가 매우 빠른 스폿 용접의 경우 100% 마르텐사이트 상으로 구성되어 있는 점을 이용하여 후열처리(post-tempering) 기법을 사용하여 마르텐사이트상을 템퍼드 마르텐사이트상으로 전이함으로서 용접부의 취성을 줄이는 방법들이 연구되어 왔다. 그러나, 후열처리 기법의 경우 경도를 낮추는데는 용이하나 공정시간이 3배 이상 증가하게 되어, 비용과 시간이 증가하는 결과를 초래했다. 따라서, 본 연구는 후열처리 기법의 단점인 공정시간이 증가하는 문제를 해결하고 용접부에 충분한 강도를 증가시킬 수 있는 용접 방법을 찾고자 한다..| In recent years, the weight of automobile bodies has been reduced to improve fuel economy and to reduce CO2 emission. Steel is used as the material of most parts of an automobile because there is a good balance between the cost, formability, and strength of steel.

Welding is necessary in assembling an automobile body, and the quality of a weld is directly linked to the reliability of an automobile body. Among welding methods used to assemble an automobile body, resistance spot welding has advantages of low operation cost and high degree of flexibility in welding positions. Welding is performed at several thousand spots in each automobile body. Therefore, resistance spot welding has an important function in ensuring reliability of an automobile body.

It is known that the tensile shear strength of a spot welded joint tends to increase as the tensile strength of a steel sheet increases. However, the cross tension strength may increase only negligibly as the tensile strength of a steel sheet increases, or may decrease on the contrary. I

In order to ensure the strength of a welded joint manufactured by resistance spot welding of high strength steel sheets, the welding method may be improved by increasing the number of welds or by increasing the nugget diameter. Therefore, in order to ensure the strength of a welded joint with a nugget diameter that is equal to or smaller than those of existing methods, in-process postheating treatment methods, in which is performed after main current for forming a nugget, have been developed. In particular, a method to temper martensite is a method in which a weld is temporarily cooled and then reheated. With this method, a nugget is temporarily solidified, transformed to hard martensite, and then reheated, and thereby a nugget and a heat affected zone are softened. As a result, the toughness of the nugget is increased and the stress concentration in the vicinity of the weld is reduced, and thereby increase in the strength of the joint can be realized. A large number of studies have been made on this method.

However, existing in-process tempering treatment methods and pulsation welding methods have many problems. The available range of weld current is narrow, because the weld current is selected so as to enable sufficient resistance heating with a current lower than or equal to the main weld current. Therefore, these methods are greatly influenced by slight changes in weld current and weld time. As a result, these methods have a problem in that they do not have sufficiently wide conditions that allows them to be implemented and stably carried out in a manufacturing site, in which various disturbances occur (for example, a large decrease in weld current that exceeds 50% of the main weld current may occur). In addition, in order to effectively generate heat by using an weld current that is lower than or equal to the main weld current, a sufficient weld time is required, and therefore the total weld time (defined as the time from when the first welding is started to when the last welding is finished), including a cooling time, may be increased.

An object of this paper is to provide a method of resistance spot welding of a sheet set including a high strength steel sheet with which the aforementioned problems are solved and a high joint strength can be achieved even for the total weld time not to increase.; In recent years, the weight of automobile bodies has been reduced to improve fuel economy and to reduce CO2 emission. Steel is used as the material of most parts of an automobile because there is a good balance between the cost, formability, and strength of steel.

Welding is necessary in assembling an automobile body, and the quality of a weld is directly linked to the reliability of an automobile body. Among welding methods used to assemble an automobile body, resistance spot welding has advantages of low operation cost and high degree of flexibility in welding positions. Welding is performed at several thousand spots in each automobile body. Therefore, resistance spot welding has an important function in ensuring reliability of an automobile body.

It is known that the tensile shear strength of a spot welded joint tends to increase as the tensile strength of a steel sheet increases. However, the cross tension strength may increase only negligibly as the tensile strength of a steel sheet increases, or may decrease on the contrary. I

In order to ensure the strength of a welded joint manufactured by resistance spot welding of high strength steel sheets, the welding method may be improved by increasing the number of welds or by increasing the nugget diameter. Therefore, in order to ensure the strength of a welded joint with a nugget diameter that is equal to or smaller than those of existing methods, in-process postheating treatment methods, in which is performed after main current for forming a nugget, have been developed. In particular, a method to temper martensite is a method in which a weld is temporarily cooled and then reheated. With this method, a nugget is temporarily solidified, transformed to hard martensite, and then reheated, and thereby a nugget and a heat affected zone are softened. As a result, the toughness of the nugget is increased and the stress concentration in the vicinity of the weld is reduced, and thereby increase in the strength of the joint can be realized. A large number of studies have been made on this method.

However, existing in-process tempering treatment methods and pulsation welding methods have many problems. The available range of weld current is narrow, because the weld current is selected so as to enable sufficient resistance heating with a current lower than or equal to the main weld current. Therefore, these methods are greatly influenced by slight changes in weld current and weld time. As a result, these methods have a problem in that they do not have sufficiently wide conditions that allows them to be implemented and stably carried out in a manufacturing site, in which various disturbances occur (for example, a large decrease in weld current that exceeds 50% of the main weld current may occur). In addition, in order to effectively generate heat by using an weld current that is lower than or equal to the main weld current, a sufficient weld time is required, and therefore the total weld time (defined as the time from when the first welding is started to when the last welding is finished), including a cooling time, may be increased.

An object of this paper is to provide a method of resistance spot welding of a sheet set including a high strength steel sheet with which the aforementioned problems are solved and a high joint strength can be achieved even for the total weld time not to increase.