Steels_ Metallurgy and Applications, Third Edition

Low-carbon strip steels 51

into solution and retained in solution in the hot band structure by means of a low
coiling temperature. The ideal requirements for continuous annealing are the exact
opposite. The use of a low slab reheat temperature leaves undissolved aluminium
nitride particles present in the structure prior to hot rolling and these particles
act as nucleation sites for precipitation after hot coiling. There is, therefore, a
progressive decrease in yield stress with decreasing slab reheat temperature, as
illustrated in Figure 1.53.
Finishing in the single-phase austenite region is preferred for continuous
annealing if good formability is required, since lower rm values are obtained when
lower finishing temperatures are used. A practice has been developed, however,
as mentioned previously, which utilizes hot rolling in the ferrite region. This will
be discussed further at the end of this section.
The overall effect of coiling temperature on the properties of two aluminium-
killed steels rolled in the austenite region is illustrated in Figure 1.54. The effect
is clearly greater for the higher carbon steel than for the lower carbon steel.
Figure 1.55 shows that for a 0.044% carbon steel, the rm value increases
progressively with cold reduction up to about 80%, whereas the increase
continues to at least 90% for the 0.018% carbon steel. This behaviour is similar
to that of an IF steel but is in contrast to the behaviour of an aluminium-ldlled
steel, processed by batch annealing. Clearly a very high cold reduction should
be used for continuously annealed steels if the highest formability is required.
Each part of the annealing cycle needs to be carefully designed if the most
favourable formability is required. A suitable grain size and texture is obtained,
as mentioned previously by the appropriate scavenging effects, but this must
be combined with a suitable annealing temperature. As shown in Figure 1.56,
an annealing temperature close to 850~ gives a higher rm value and a lower
yield stress than annealing at 700~ but the cooling part of the cycle is equally
important. It is used to reprecipitate the carbon that is inevitably taken into
solution, down to a sufficiently low level to give an adequately low level of
room temperature strain ageing.

200

~. 19o

Z~^2 180

"o |

170

160

.... z + i z - i r z 1

o

goo o

OcP:O 0 O000(P , 0 O0^0

: -

. o Co~ j
i +"~ ,,, l,,- t ,,I ,, I .... '
1120 1140 1160 1180 1200 1220 1240
Slab reheat temperature (~

29

28

27

L26

2s ~.

24

23

Figure 1.53 Variation of YS with the reheating furnace temperature for continuously

annealed, extra-low-carbon, aluminium-killed steel (After Prum et al. 9~