Steels_ Metallurgy and Applications, Third Edition

Low-carbon strip steels 47

of time on a continuous line by changing the steel chemistry, prior processing

and annealing temperature.

The general problem of matching the properties was resolved into three compo-

nents:

1. Obtaining a suitably large grain size.


2. Obtaining a suitable orientation texture.


3. Minimizing the carbon and nitrogen in interstitial solid solution at the end of
   the process.

As with batch annealing, the steel must undergo recrystaUization and grain growth
during the annealing cycle. Carbon is inevitably taken into solution unless it is
already combined as a stable carbide. It must, therefore, be substantially reprecip,
itated during cooling in order that the steel undergoes only relatively slow room
temperature strain ageing to ensure that stretcher strain markings are avoided on
subsequent pressing.
The grain size problem was solved by using a number of scavenging reactions
involving mainly aluminium, nitrogen, carbon, manganese and sulphur to purify
the ferrite matrix and by using a suitably high annealing temperature, s4 The
texture problem was solved by the same means as the grain size problem and
the carbon in solution problem was solved mainly by the incorporation of a
'so-called' overageing section into the cooling part of the annealing cycle. This
usually consists of holding the steel at temperatures in the range 350-450"C for
up to a few minutes or allowing the steel to cool slowly from such temperatures
as illustrated previously in Figure 1.2.
Early work by Toda et al. s5 showed that controlling the manganese in relation
to the sulphur content had a marked effect on grain size and hence properties.
They defined a parameter denoted K by the equation:

K = [Mn] - 55/32 IS] - 55/16101

where the symbols in square brackets represent the weight percentages of these
elements in the steel. They observed that for rimming steel, there was a peak in
grain size, elongation and rm value when the k value was in the range 0-0.15%,
as illustrated in Figure 1.48. In addition, there was a corresponding minimum in
yield stress. Further work confirmed that a similar type of relationship applied to
aluminium-ldlled steel, provided the term involving oxygen was removed since
no oxygen would be available in such a steel to combine with manganese.
The effect of manganese was also dependent on the carbon content, as iUus-
trated in Figure 1.49 which also shows that the detrimental effect of manganese
was greatly reduced when the carbon in the steel was present as coarse carbides,
arising from the use of a high coiling temperature such as 750"C.
The effect of carbon content is further illustrated in Figure 1.50, which shows
that there is a minimum in yield stress at about 0.01-0.02% carbon which corre-
sponds with a maximum in elongation. The maximum in yield stress below
0.01% carbon coincided with a maximum in ageing index and was clearly asso-
ciated with additional carbon being retained in interstitial solid solution. The
reason is that the tendency for reprecipitation during overageing depends on the