Steels_ Metallurgy and Applications, Third Edition

64 Steels: Metallurgy and Applications

An equivalent equation for a continuously annealed steel is as follows: ill

TScA = 477 + 48[Mn] + 127[Si] + 918[P] - 0.019(AT*C)

where AT~ is the annealing temperature. It is clear that the strengthening effect
from each element is similar for each type of annealing but that, as expected, the
precise effects depend on the different processing used.
The prior processing requirements for each type of steel are the same as for the
equivalent type of mild steel. Thus a low coiling temperature below 600*C must
be used for the batch-annealed product to retain nitrogen in solution following
hot rolling, in order that satisfactory r values are obtained from the final product.
For the same reason, the continuously annealed product should be processed
using a high coiling temperature to precipitate nitrogen as aluminium nitride.
Low manganese and extra-low-carbon contents should also be used if high r
values are to be obtained.
Different workers have reported different effects of phosphorus on r value, but
it is now generally agreed that an increase in strength is accompanied by a small
drop in r value.
The effects of cold reduction and annealing conditions are generally similar to
the effects for the equivalent type of mild steel. A variation in annealing temper-
ature over the practical range, however, has a greater effect for the continuously
annealed product than for the batch-annealed product. High annealing tempera-
tures up to 800"C or more should be used for the continuously annealed steel if
high r values are to be obtained.

Solid solution-strengthened IF steels

These steels are usually based on titanium, niobium or titanium plus niobium

IF ultra-low-carbon steels but with sufficient phosphorus, manganese, silicon or

boron to give the strength required. They are usually processed by continuous

annealing but may also be processed by batch annealing. They are used in the

uncoated condition when higher formability is required than can be obtained

from an aluminium-killed steel, but provide the only way of achieving a highly

formable high-strength product from a hot dip coating line (see below).

Figure 1.69 gives a plot of properties versus [Si] + [Mn] + 10[P] for niobium-

bearing ULC IF steels. These curves imply that the increase in strength from

manganese is equivalent to that from silicon and that the effect of phosphorus

is ten times as great. The drop in r value with increasing strength is, however,

greatest for manganese and least for phosphorus. Figure 1.70 gives an example

of the effect of solid solution additions to a titanium steel. In this case the effect

of silicon is greater than the effect of manganese as it is in an aluminium-killed

steel. For the titanium steel, the decrease in elongation is greatest for a phosphorus

addition and least for silicon (Figure 1.71). Although boron has a high affinity

for nitrogen, it imparts solid solution strengthening to titanium-treated IF steel

because the nitrogen present is precipitated as titanium nitride.

An important effect of phosphorus in IF steels is the greater tendency for

secondary cold work embrittlement. This is discussed further in a later section.