Steels_ Metallurgy and Applications, Third Edition

Low-carbon strip steels 45

ID =

1.5

1.0

I ....... I ......... I , , _L__

550 600 650 700

Coiling temperature (~

Figure 1.44 Variation of rm value of batch-annealed, aluminium-killed steel with coiling
temperature, aluminium content 0.036% (Whiteley and Wise 78)

2.0

~ 1.5

~E

1.0

,, 9 I ' ' ,,

820 860 900 940

Finishing temperature (~

Figure 1.45 Variation of rm value with finisl i, g temperature for batch-annealed,
aluminium-killed steel coiled at 525-565~ (Parayil and Gupta 79)

value would be obtained (Figure 1.44). Finishing temperatures must also be in
the single-phase austenite region to avoid a similar fall in rm value (Figure 1.45)
Manganese has a detrimental effect on rm value, but the magnitude depends
markedly on the carbon content (Figure 1.46). The highest rm values tend to be
provided by about 70% cold reduction and the heating rate during annealing also
has an effect. Figure 1.47, for example, shows that there is a general tendency
for the rm value to decrease with increasing heating rate but that there are a series
of optimum heating rates that provide higher rm values than the general trend,
depending on the aluminium content. These optimum heating rates also provide
maxima in grain size and coincide with minima in yield stress. The optimum
heating rate was shown to depend approximately linearly on the aluminium
content, s~ but later work showed that it depended also on the nitrogen and
manganese contents and on the cold reduction, s2 The equation below was found
to give the optimum heating rate in "C/hour where JAil, IN] and [Mn] represent
the weight percentages of these elements in solid solution and CR is the cold
reduction:

log (optimum heating rate) = 18.3 + 2.7 log [A1][N][Mn]/CR

It is well known that an increase in temperature and/or time of annealing leads
to a decrease in yield stress and an increase in elongation. Over the temperature
range up to 700~ the softening may be described by an apparent activation