| JFE Steel has
developed and commercialized from TS780 to
TS1470MPa grade cold-rolled ultra high-strength
steel sheets with excellent press-formability,
spot-weldability, and stability of mechanical
properties and dramatically improve crashworthiness
and fuel economy when applied in automobile
bodies/reinforcement parts and seat frame
parts. In response to strict formability requirements,
the company conducted basic and detailed studies
of changes in the microstructure and propagation
of microscopic cracks during forming, and
realized remarkable improvements in elongation
and stretch-flange formability in comparison
with conventional steels by adopting a low
carbon equivalent design. In particular, as
expanded application of TS980MPa grade steel
sheets is expected in the future, JFE Steel
has developed a lineup of three types of TS980MPa
grade steels with different balances of mechanical
properties corresponding to the material application
and joining method. These products have already
earned an excellent reputation with users.
This paper introduces two newly developed
TS980MPa grade cold rolled steel sheets, one
with excellent stretch-flange formability
and the other for mechanical joining. Surface
cracking in mechanical joining, which is strongly
related to local elongation, has been suppressed
by achieving a single martensite phase. |
