Steels_ Metallurgy and Applications, Third Edition

10 Steels: Metallurgy and Applications

a series of multicomponent systems have now been developed. These programs
can be used to predict the amounts of titanium, niobium, carbon, nitrogen and
sulphur, ctc, retained in solution in equilibrium at any temperature as well as the
amounts and compositions of the precipitates.
In addition to producing precipitation reactions on transformation to ferrite,
elements such as titanium or niobium also play an important part in controlling the
recrystallization kinetics during the hot-rolling stage. Elements in solid solution
may retard recrystallization through the process of solute drag but the strain-
induced precipitation of niobium carbonitride is also very effective in retarding or
stopping recrystallization. The final effect depends on the initial austenite grain
size, the temperature, the amount of deformation given after the last previous
recrystallization and the level of alloy addition. 14 As an example, Figure 1.5
shows how the critical strain for austenite recrystaUization varies with prior
austenite grain size and rolling temperature for a steel containing 0.03% niobium.
On cooling of a recrystallized austenitc structure, ferrite grains may nucleate at
the austenite grain boundaries. With an unrecrystallized austenite, the austcnite
grain boundary area per unit volume is increased due to grain elongation. The
nucleation frequency per unit grain boundary area is higher and nucleation may
also occur on deformation bands. 15 The overall effect is that a finer ferritc
grain size is produced from a deformed unrecrystallized austcnite than from
a recrystallized austenite. Figure 1.6 shows that for a given grain boundary area
per unit volume Sv, the ferritc grain size also decreases with increasing cooling
rate. 16 These effects arc utilized in the processing of micro-alloyed steels and arc
discussed further in the section on high-strength steels.
There is a limit to the strengthening that can be achieved by a combination
of grain refinement, solid solution and precipitation effects. Transformation
strengthening is, therefore, employed to obtain values of tensile strength above

Strain

0 0.2 0.4 0.6 0.8 1.0 1.2 1.4 1.6

--1 I I I I 1!! '

1200 1150 100 1050 ~

"~ / / / / rolling

" / / /

~ o13

,... C

~

7/I I I._! 1. I ..... I

0 20 40 60 80

Critical reduction for

recrystallizatlon (%)

Figure 1.5 Effect of initial grain size on the critical rolling reduction needed for austenite
recrystallization of a 0.03 wt% Nb steel reheated to 1250" C for 20 min (Kozasu et al. 15)
(The initial grain size was varied by rolling at high temperatures.)