44 Steels: Metallurgy and Applicationscarbides reduces and eventually becomes zero at about 0.02% carbon depending
on the annealing temperature. Below this carbon content, the reprecipitation
process becomes progressively more difficult since it involves a nucleation as
well as a diffusion process and the reprecipitation process may remain incomplete.
The result is a rise in strength and the steel ceases to be completely non-ageing.
Such a steel exhibits bake hardening as discussed further below.
The most important metallurgical feature of the processing of an aluminium-
killed steel to give high rm values concerns the distribution of aluminium nitride.
The main requirement is that the aluminium nitride is held in solution in the
hot band structure prior to cold reduction. It can then lead to an enhancement
of the (111) y fibre texture components and to a decrease in the deleterious
(100) texture components during annealing. At the same time a pancake grain
structure develops. It is generally agreed that the beneficial effect is caused
by the precipitation of aluminium nitride or the clustering of aluminium and
nitrogen on sub-boundary sites during slow heating to the annealing temperature.
This enables the beneficial (111) components to be strong components in the
recrystallized texture. The effect is associated with an inhibition of recovery
and subgrain growth and an increase in recrystallization temperature compared
with a similar steel treated to precipitate the aluminium nitride prior to cold
reduction. The maximum amount of inhibition of recrystaUization does not,
however, correlate with the optimum texture. The best texture, giving the highest
rm value, is obtained when there is just sufficient inhibition of recrystallization
and when the drag on grain boundary movement as a result of aluminium nitride
particles decays slowly as a result of particle coarsening. 76 The slow coarsening
of the aluminium nitride particles leads to the preferential development of the
(111) components and a coarser grain size due to restricted nucleation of other
components.
Clearly, the optimum aluminium and nitrogen contents identified in Figure 1.43
provide the optimum amount of inhibition of recrystallization which decays at
the optimum rate for normal processing conditions. It is generally agreed, as
indicated by Hutchinson, 44 that the pancake grain structure is caused by the
preferential precipitation of aluminium nitride on the prior cold-worked grain
boundaries which provide an anisotropic barrier to grain boundary movement.
The aluminium nitride must be taken into solution at the slab reheat stage in
order that it may be held in solution in the hot band structure prior to coiling
and cold rolling.
For practical purposes, the solubility of aluminium nitride in austenite is given
by: 77
log [Al][N] = -6770/T + 1.033where T is the absolute temperature.
It may be shown, therefore, that relatively high slab reheat temperatures are
required to take the aluminium and nitrogen into solution and the most commonly
used temperature is 1250"C.
Relatively low coiling temperatures below 600~ are used to avoid substantial
precipitation of aluminium nitride in the hot band, otherwise a deterioration in rm