Steels_ Metallurgy and Applications, Third Edition

Low-carbon strip steels 63

temperature for the completion of recrystallization by about 20~ and increases
the yield stress by about 20 N/ram 2, with a larger increase for vanadium-bearing
steels. 1~ As expected, higher annealing temperatures lead to lower strength for
both batch and continuous-type annealing. 1~176176
As with mild steel, temper rolling is the last stage in the process sequence
that substantially affects the properties. Usually, it is found that a higher temper
rolling reduction is needed to remove the yield point of a micro-alloyed steel
than for a mild steel.
In summary, cold-rolled and annealed micro-alloyed steels are steels with low
r values and moderate n values for their strength. They are used, therefore, for
high-strength, structural applications with modest formability requirements.

Solid solution-strengthened steels

Each type of mild steel may be given a modest increase in strength by the addi-
tion of elements that remain in solution in the final product. The elements most
commonly used are phosphorus, manganese and silicon, though boron may also
be used in IF steels. Carbon and nitrogen are the most potent solid solution-
strengthening elements but substantial quantities of these elements in solution
are not normally used because they lead to deleterious room temperature strain
ageing. Very small quantities in solution, however, are used to give bake hard-
ening (see below) but the amounts involved do not lead to a major increase in
strength in the as-supplied condition.
Solid solution-strengthened steels usually have a minimum yield stress in the
range 220-300 N/ram 2, though higher strengths may be obtained depending on
the processing (mainly annealing) facilities available. Phosphorus is the element
most commonly used for fairly low increases in strength since it is relatively
cheap and gives a higher increase in strength per unit addition than manganese
or silicon. The phosphorus addition is usually restricted to well below 0.1%,
however, to avoid problems with welding and because phosphorus may lead
to secondary cold work embrittlement of IF steels (see below). For the higher-
strength steels, the strengthening by phosphorus is often supplemented first by
manganese and then by silicon.
An advantage of solid solution-strengthened steels is that they retain the general
characteristics of the mild steel from which they are derived. Thus, they retain
good formability, including fairly high r values. A further advantage over micro-
alloyed steel is that there is less loss in strength from the hot-rolled to the
cold-rolled condition. Cold mill loads are, therefore, correspondingly lower for
a given strength in the final batch-annealed product.

Solid solution-strengthened AK steels

The following equation indicates approximately how the tensile strength of a
batch-annealed product varies with the steel composition (wt%). 95

TSBA = 270 + 441 [C] + 64[Mn] + 98[Si] + 930[P]

Another equation is as follows: tt~

TS~A = 292 + 563[C] + 678[P] + 90[Si] + 18[Mn] - 1534[S]