Steels_ Metallurgy and Applications, Third Edition

Low-carbon structural steels 147

Whereas the above equations clearly identify the solid solution strengthening
effects of an element such as manganese, it must be borne in mind that manganese
also contributes to strength by other means. Thus by depressing the temperature of
transformation of austenite to ferrite, manganese causes further strengthening by:

1. Refining the ferrite grain size.


2. Refining the size of precipitation-strengthening particles, e.g. Nb(CN) and
   V4C3.

These effects are illustrated in Figure 2.6, which shows the effect of manganese
on the yield strength of a V-N steel, normalized from 900~ This figure also
indicates that free nitrogen contributes very little to the overall strength of this
particular steel in spite of having a very large strengthening coefficient. This is
due to the fact that most of the nitrogen in this particular alloy remains out of
solution as VN at a temperature of 900~ and therefore very little free nitrogen
is available for solid solution strengthening. Whereas the VN particles refine the
austenite grain size and produce a fine ferrite grain size, vanadium in solution
leads subsequently to a dispersion-strengthening effect of the order of 75 N/mm 2.

Controlled rolling/thermomechanical processing

As stated earlier, the traditional route to a fine grain size in ferrite-pearlite struc-
tural steels has been to incorporate grain-refining elements, such as aluminium,
and to normalize the materials from about 920~ after rolling. However, prior to
the introduction of continuous casting, basic carbon steel plate was made from
semi-killed (balanced) ingots and the additional costs associated with aluminium
grain refinement were very considerable, as illustrated by the cost data from the
early 1960s shown in Table 2.2.
In the late 1950s, steel users were also gaining experience with Nb-treated,
micro-alloy steels which provided substantially higher strengths than plain carbon
steel in the as-rolled condition but with a significant reduction in toughness
compared with aluminium-grain-refined steels. However, when the micro-alloy
steels were normalized to improve their impact properties, their strength advan-
tage was forfeited. There was therefore the need for an alternative route to a
fine grain size in structural steel plate which would overcome both the cost
and strength penalties associated with traditional normalizing. In fact, the first

Table 2.2

Requirement

nil (mild steel)

Si killing

Grain refinement

Normalizing

Impact testing < 0~

Extras Accumulative price

(s Price (s

• 42.62
  7.50 50.12
  3.O0 53.12
  3.00 56.12
  3.50 59.62
  ............ .......