joints as the strength increased. JFE Steel developed the
“Pulse SpotTM” welding technology to satisfy stable
joint properties by controlling the segregation and
strength distribution of welded joints6). As a feature of
this technology, a short-time, high-voltage pulsed current
is applied after the main current. In conventional
welding, a current pattern that utilizes the tempering
effect of the high strength part by applying a temper current
has long been used. In comparison with that
method, the developed method makes it possible to
obtain more stable weld properties in a short time. A
typical Pulse SpotTM welding current pattern is shown in
Fig. 2. Recently, in addition to new welding processes,
JFE Steel has also explored an approach to improvement
of joint properties by studying the controlling factors for
their strength characteristics based on a fracture mechanics
analysis of resistance spot welds of high tensile
strength materials7).
In addition to development of unique resistance spot
welding technologies for high tensile strength materials,
JFE Steel is also engaged in technical development for
improvement of welding efficiency. In particular, in
order to perform resistance spot welding in the automobile
assembly process, it is necessary to access the welds
from both sides of the steel sheets by applying robots to
welding, but for this, it was necessary to manage an
access route and space to insert the welding guns from
both sides. Therefore, JFE Steel developed a single-side
spot resistance welding technology which enables welding
by access from only one side. Although series type
single-side spot welding methods by two electrodes have
also been studied for many years, it was difficult to
manage a stable current flow; instead JFE Steel developed
the indirect type single-side spot welding method
to solve this problem8). In this method, it is necessary to
optimize the position of the grounding electrode depending
on the component, but in addition to this, the electrode
force and current are also controlled during welding,
as illustrated in Fig. 3. Taking advantage of this
feature, this is a welding technology with excellent
robustness which makes it possible to suppress instabil