Steels_ Metallurgy and Applications, Third Edition

Low-carbon strip steels 49

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Carbon content (wt%)

Figure 1.50 Variation of the mechanical properties of aluminium-killed steel with carbon
content, continuously annealed at 700 and 850~ (Ono et al. 87)

carbon content at the start of overageing and will be discussed in more detail
later. The lowest yield stress values and highest elongation values were obtained
with ultra-low-carbon contents below 0.002% which also corresponded with the
lowest ageing index. This was clearly due to the ultra-low total amount of carbon
present in the steel, regardless of its position in the structure.
The grain size of continuously annealed, aluminium-killed steel increases
with the coarsening of precipitates in the hot band which are mainly carbides
and aluminium nitride. This may be achieved using high coiling temperatures,
depending on the carbon content. Matsudo et al. 88 and Hutchinson 2~ showed that
higher rm values are obtained with increasing carbide size due to the slower
dissolution rate of coarse carbides during heating. This enables recrystallization
to occur in a matrix that is relatively free from interstitial carbon.
The effect of nitrogen content on the properties of a rimming steel without
an aluminium addition is illustrated in Figure 1.51, which shows an increase in
yield stress with increasing nitrogen content. There is also a decrease in rm value
and grain size with increasing nitrogen content in aluminium-killed steel and
a corresponding influence on yield stress depending on the coiling temperature
used (Figure 1.52). High coiling temperatures lead to the combining of aluminium
and nitrogen. The combined nitrogen is then no longer able to have any further