Steels_ Metallurgy and Applications, Third Edition

66 Steels: Metallurgy and Applications

~10 c

._o

_o c 6

._c

I1)

a

o

o/Si

eJ i ,. .i , , i.._ i

20 40 6'0 80 100

Increase in tensile strength (MPa)

Figure 1.71 Deterioration of elongation on strengthening by solid solution elements

(After Katoh et al. l~)

Figure 1.72
a tensile test

D~h~~it lily (BH)

/ 11 ........... Ageing

(~ 170"C- 20 min

I

Prestrain

2%

Strain

The measurement of bake hardening (BH) and work hardening (WH) using

Bake-hardening steels

The essence of a bake-hardening steel is that it has the ability to increase its
yield stress significantly during a paint-stoving process after forming by means
of a type of strain-ageing process. In the as-delivered condition the steel has the
formability, therefore, of a relatively low-strength steel but the performance in
service of a steel with a much higher yield stress. Paint stoving is often carried
out using a heat treatment at 170~ for about 20 minutes and bake hardening is
most useful in the part of any pressing which is subject to a low degree of work
hardening. A standard index to quantify bake hardening is used, therefore, which
involves straining a sample by 2% in a standard tensile test, baking the sample at
170"C for 20 minutes and then continuing the tensile test. The difference between
the lower yield stress after baking and the 2% flow stress before baking is taken
as the bake-hardening index, as indicated in Figure 1.72.