Abstract
In view of fuel efficiency and driving stability, JFE Steel has newly developed
three kinds of high strength steel tubes useful for weight reduction of automotive suspension
parts. These tubes have excellent formability and practical properties for suspension parts by
means of high purity ladle metallurgy, high precision controlled hot-rolling and online post
annealing for weld seam. The steel tube for quench type torsion beams is the most suitable
when particularly high static strength and fatigue endurance are required. The tube has both
quench hardenability and excellent press and hydro formability. The amount of carbon and
carbon equivalent is limited to the lowest level enough to secure fatigue strength and
hardenability, and ferrite amount is assured by the high precision controlled cooling in hot-rolling
process. Such adjustment of chemical composition also improves the impact
characteristics and the hydrogen embrittlement resistance after quenching through the
limitation of maximum hardness and the fine precipitation of carbide in the martensite lath
structure. Developed TS 780MPa grade steel tube for automotive suspension parts consists of
ferrite and bainite microstructure, which prevents strain concentration to the soft phase
compared with ferrite and martensite dual-phase microstructure. This relatively
homogeneous microstructure allows the tube to be bent with a radius of less than 2D (D: outer
diameter of the tube). It also contributes to excellent fatigue endurance, toughness, and
paintability along with lower silicon contents. TS 780MPa glade "HISTORY" tube has also
been developed for the parts that require higher formability, especially bendability; its
elongation and the r-value are as high as 35% or more and 1.2 or more, respectively. The
excellent elongation and r-value are due to extremely fine microstructure and specific crystal
orientation, which are obtained in the high-temperature tube stretch-reducing process.
Practical properties such as toughness and paintability are also secured by low carbon content
and homogeneous and fine microstructure.
Keywords: Weight reduction, tube torsion beam, tube suspension arm, formability,
fatigue endurance