Steels_ Metallurgy and Applications, Third Edition

50 Steels: Metallurgy and Applications

280

A

>-

200

48

C

UJ

40

o

O

o_o.o.o.o.o.o o

0 0

0 0

I I ,,^0

0.001 0.002 0.~)3 0.004

Nitrogen content (wt%)

Figure 1.51 Effect of nitrogen content on the yieM stress and elongation of subcriticaUy
annealed rimming steel (Toda et aL85)

2so

>- 200

Coiling temperature = 565~

-- ,, ~

675 ~

• _
  o 730 ~
  / 1% Tempor rolled
  ~" -0.04 wt%C
  o. 1 0. o3 o.o s o:o 7 o.oo9 o.oli
  Nitrogen content (wt%)

Figure 1.52 Effect of nitrogen content on the yield stress of aluminium-killed steel contin-
uously annealed at 760~ for different coiling temperatures (Pradhan 89)

major deleterious influence on the annealing process. Low total nitrogen contents
however, are still important if the highest formability is to be obtained.
An addition of boron is often used to improve the formability of continuously
annealed steel by using an amount that is just able to combine with the nitrogen,
the reaction taking place during hot rolling. It is not necessary, therefore, to use
a high coiling temperature as is the case if the nitrogen is to be combined with
aluminium. The deleterious effects of using a high coiling temperature (higher
yield loss and more difficult pickling) can, therefore, be avoided. The formability
of a boron-bearing, low coiling temperature steel is, however, lower than that
of a high coiling temperature, non-boron steel due to the presence of relatively
fine carbides which dissolve quickly on annealing. Boron-bearing continuously
annealed AK steels are, therefore, only suitable for use for Fe P01 applications.
It is mentioned above that high slab reheat temperatures should be used for
aluminium-killed steel for batch annealing in order that aluminium nitride is taken