Steels_ Metallurgy and Applications, Third Edition

148 Steels: MetaUurgy and Applications

indication of a viable alternative to normalizing was published in 1958 when

Vanderb~k 7 reported that European steel producers were adopting lower than

normal finishing temperatures during rolling, in order to refine the structure and

improve mechanical properties. This practice became known as controlled rolling

but in more recent years, the term thermomechanical processing has been used

increasingly to embrace both modified hot-rolling and in-line accelerated-cooling

operations.

Outline of process

The traditional hot-rolling operation for plates is shown schematically in
Figure 2.7(a). Typically, slabs are soaked at temperatures of about 1200-1250~
and these are rolled progressively to lower plate thicknesses, often finishing at
temperatures above 10~~ In plain carbon steels, soaking at 1200-1250"C
produces a coarse austenite grain size and rolling just below that range
results in rapid recrystallization. Even at a finishing temperature of 1000*C,
recrystallization and subsequent grain growth will be relatively rapid, resulting in
the generation of a coarse austenite grain size. On cooling to room temperature,
this results in the formation of a coarse ferdte grain size and the material must
be normalized in order to refine the microstructure.
In controlled rolling, the operation is a two-stage process and, as illustrated in
Figure 2.7(b), a time delay is introduced between roughing and finishing. This
allows the finishing operations to be carded out at temperatures below the recrys-
tallization temperature, which results in the formation of fine pancaked austenite
grains and transformation to a fine-grained ferrite structure. In the 40 or so years
since its introduction, a considerable amount of research work has been carried
out world-wide on controlled rolling which has led to the development of mate-
rials with vastly improved mechanical properties compared with those obtained
in conventional, heat-treated steels. Whereas the early experiments in controlled
rolling were carded out on plain carbon steels, it soon became evident that the
process was greatly facilitated by the addition of carbide-forming elements. In
particular, it was shown that the addition of about 0.05% Nb caused a marked
retardation in recrystallization which allowed controlled rolling to be carded out
at significantly higher temperatures.

Slab reheating

The slab-reheating stage is important in that it controls the amount of micro-
alloying elements taken into solution and also the starting grain size. The
solubility curves for NbC and VN in steels of different carbon and nitrogen
contents are shown in Figure 2.3 and were referred to earlier. For most
commercial grades of steel, Sellars 8 states that complete solution of VC is
expected at the standard normalizing temperature of 920~ and VN at somewhat
higher temperatures, whereas Nb(CN), AlN and TiC require temperatures in the
range 1150-1300~ TiN is the most stable compound and little dissolution is
expected to take place at normal reheating temperatures. Whereas the presence of
fine, undissolved carbonitride particles will serve to maintain a fine austenite grain