Steels_ Metallurgy and Applications, Third Edition

Low-carbon strip steels 69

higher strength may be obtained by alloying with phosphorus, manganese or

silicon to provide solid solution strengthening. The addition of phosphorus also

increases the bake hardening by refining the grain size, but manganese leads to

a reduction in the bake hardening. Silicon increases the bake hardening since

it delays carbon precipitation, ll7 but it causes a higher yield point elongation.

Silicon is only used, therefore, when sufficient strength cannot be obtained by

the use of other elements.

A batch-annealed, bake-hardening steel with a yield stress below 150 N/ram 2

may be obtained using an ultra-low-carbon IF steel with a small titanium and

niobium addition to combine with part of the carbon but to leave sufficient excess

carbon to provide bake hardening. 117

Continuously annealed, low-carbon, aluminium-killed, bake-hardening steel
Bake-hardening, aluminium-killed steels may be produced by continuous
annealing using the method given above for the production of formable
aluminium-killed steel. The retention of carbon in solution to provide the bake
hardening is a natural consequence of the fast cooling rates that must be used
on a continuous line. The rapid cooling and overageing sections on any line are
designed to remove as much carbon as possible, and as with a batch-annealed
bake-hardening steel, the strength may be increased using suitable solid solution-
strengthening elements, such as phosphorus, manganese or silicon with similar
overall effects.
The steels need to posses fairly high r values in order to provide formability
suitable for the intended applications and this is achieved by the use of a low
nitrogen content and a high coiling temperature using the same mechanisms as
for low strength steel. An increase in annealing temperature causes a decrease
in yield point elongation after ageing, an increase in r value and a decrease in
strength, lls Other work ll9 has shown that the bake hardening tendency may be
controlled at a satisfactory level by over cooling prior to overageing and reheating
to an optimum overageing temperature.
The use of low carbon, aluminium-killed steel provides a method for producing
bake-hardening steel that is suitable for use on any continuous annealing line
which incorporates a relatively long overageing section. The method is not suit-
able for use on hot dip coating lines which do not incorporate such a section.

Ultra-low-carbon, interstitial-free, bake-hardening steels

Continuously annealed, bake-hardening steels, containing ultra-low-carbon
contents, may be produced in several ways, depending on the r values required
and on the steelmaking capability available, but most of these steels are alloyed
with varying quantities of titanium and/or niobium. With the lower additions,
some carbon is left uncombined and this solute carbon is available, therefore, to
provide the bake hardening. With the higher additions, substantially all the carbon
is combined with the titanium or niobium prior to annealing. The free carbon
needed to provide the bake hardening is obtained, therefore, by annealing at a
sufficiently high temperature such as 8500C to take carbon back into solution. The
steel must then be cooled quickly to minimize the reformation of the precipitates