Steels_ Metallurgy and Applications, Third Edition

280 Steels: Metallurgy and Applications

of about 850~ prior to oil quenching. Tempering at 550-650~ then produces

tensile strengths in the range 800-1100 N/mm 2. However, in the mid-1970s,

German manufacturers demonstrated that these strength levels could be produced

in a micro-alloy, medium-carbon steel (49 MnVS3) after air cooling from the

forging operation, thus eliminating the need for heat treatment. Since that time,

major effort has been devoted to the development of micro-alloy forging steels in

Europe and Japan and these steels have gradually been introduced as substitutes

for quenched and tempered steels in some automotive components.

Metallurgical considerations

As indicated in earlier chapters, niobium, titanium and vanadium are used as
micro-alloying elements in low-carbon steels, although high soaking tempera-
tures must be employed in order to achieve substantial solution of Nb(CN), TiC
and TiN. However, vanadium has a high solubility in austenite, regardless of
the carbon content, and is therefore the most suitable micro-alloying element
for medium-carbon steels. On cooling from the solution treatment temperature,
vanadium carbonitride precipitates in both the proeutectoid ferrite and the ferrite
lamellae of the pearlite. The physical metallurgy of these steels has been reviewed
by Gladman. 46
As illustrated in Figure 3.43, the tensile properties of these grades increase
progressively with vanadium content and, depending upon the levels of strength
required, vanadium contents in the range 0.05-0.2% are employed. The level
of precipitation strengthening is also influenced by the nitrogen content and

Figure 3.43
Mn steel

1000

800

E 600

v

" 400

if)

200

0

0

25

A

El ,--

2o "~ N

e-

_15 W

, I I. 10

0.05 0.10 0.15

Vanadium (%)

Effect of vanadium on the tensile properties of air-cooled 0.45% C 0.9%