Steels_ Metallurgy and Applications, Third Edition

Low-carbon structural steels 141

YS

(N/mm ~')

500 -

27J

ITT

• 100

400

300

200

100

~~ ITT

_ ~~ -

50

0

-50

-100

I ,, _ I. I I I I -150

0 2 4 6 8 10 12 14

Ferrlte grain size, d-l~(mm -112)

Figure 2.1 Effect of ferrite grain size on yield strength and impact properties

form a fine dispersion of AIN. These particles pin the austenite grain boundaries
at the normal heat treatment temperatures just above Ac3 (typically 850-9200C,
depending upon carbon content) and therefore result in the formation of a fine
austenite grain size. In turn, a fine austenite grain size results in the formation of
a fine ferrite grain size on cooling to room temperature.
Although the austenite grain size is of major importance, other factors also play
a part in developing a fine ferrite grain size. Thus the addition of elements such
as carbon and manganese or an increase in the cooling rate from the austenite
temperature range will lead to a refinement of the ferrite grains. In either case,
this is achieved by depressing the temperature of transformation of austenite to
ferrite. However, there is obviously a limit to the amount of strengthening that
can be obtained by this mechanism before transformation is depressed to such an
extent that it leads to the formation of bainite or martensite and the introduction
of transformation strengthening.

Solid solution strengthening

The solid solution strengthening effects of the common alloying elements are
illustrated in Figure 2.2 and work by Picketing and Gladman 4 has provided the
strengthening coefficients shown in Table 2.1 for ferrite-pearlite steels containing
up to 0.25% C and 1.5% Mn. These data illustrate the very powerful strengthening
effects of the interstitial elements, carbon and nitrogen, but it must be borne
in mind that these elements have only a very limited solid solubility in ferrite.
However, both carbon and nitrogen also have a very adverse effect on toughness.
Of the substitutional elements, phosphorus is the most potent and, as indicated
in Chapter 1, additions of up to about 0.1% P are incorporated in the higher
strength rephosphorized grades that are used in automotive body panels. However,