Abstract:
The need to reduce weight while maintaining good mechanical properties
in materials used in the automotive industry has over the years seen an
increased exploitation of various steels to meet this new demand. In line
with this development, the ferritic stainless steel family has seen a wide
application in this industry, with the AISI 436 type increasingly being used
for automotive trims and mufflers for exhaust systems, as well as a
significant part of this steel’s application being for the manufacture of
wheel nuts and wheel nut caps in trucks, mainly through the deep drawing
process. However, there have been reports of some poor surface
roughening of this material during deep drawing, with tearing and/or
cracking also reported in some instances. This has been suspected to
possibly be associated with some local differences in localized mechanical
properties between grains and grain clusters of the rolled and annealed
material.
In order to investigate the poor surface roughness exhibited by AISI 436
ferritic stainless steel (FSS) during deep-drawing, Lankford values (Rmean
and Δr), grain size, and microtextures of various sheet samples from
this steel were studied. The chemical composition range for the samples
was 0.013–0.017% C, 17–17.4% Cr, 0.9–1% Mo, and 0.4–0.5% Nb. The
steels were subjected to various hot and cold rolling processing routes i.e.
involving industrial direct rolling (DR) or intermediate annealing rolling
(IR), and the drawability and final surface qualities of the steels were
compared. It was found that the DR route gave an average R-mean and Δr
value of 1.9 and -1.4 respectively, while the IR route yielded an average Rmean
and Δr value of 1.6 and 0.52 respectively. The high Δr value for the DR processing route had a substantial adverse effect on the drawability. IR
samples exhibited a smoother surface finish on visual inspection, while
clear flow lines were visible on the DR samples, despite the fact that DR is
the preferred industrial processing route due to the reduced production
costs it offers. This observation was also confirmed through SEM
examinations. The difference in the surface quality was attributed to
microtexture. However, the mechanism responsible for this difference still
needs to be identified.
