Alloying elements can be added during
secondary steelmaking specifically to modify the steel
inclusion
population. These
alloys
can be added as 'lump additions', but more commonly are injected
into the steel. The choice of addition is highly dependant
on the steel type being made, its processing conditions and
final application.
One important aspect of this decision, is the amount of
additional cost which can be tolerated by the customer associated
with the treatment required to give enhanced product performance
(additions for inclusion modification are often highly expensive).
Some commonly added materials and their role in modifying the
steel inclusion population is outlined below:
Sulphur
Sulphur can be added up to ~ 0.35 % by weight, and is one
of the cheapest additions. It is used in free-cutting steels
in combination with manganese additions, to give manganese
sulphide inclusions which deform plastically during rolling
and cutting; these elongated inclusions promote chip formation
and breakup during subsequent machining. They also reduce
cutting temperatures and tool wear.
Lead
This metal is also added to free-cutting steels to improve
machinability; the lead
is deep injected into the steel (0.15 - 0.35%), requiring
fume extraction to remove toxic lead vapour. The lead forms
discrete particles in the steel structure on solidification,
often present as tails on manganese sulphide inclusions.
During machining the lead melts locally at the tool / workpiece
interface acting as a lubricant and reducing tool wear.
Tellurium
Is again deep injected in free-cutting steels to improve
machinability (~ 0.1 wt%) and is commonly co-injected with lead;
higher levels of tellurium cause cracking during hot working (it is
also a very expensive alloy).
Tellurium is very toxic and therefore requires strict precautions
to be taken during steelmaking. It is present as manganese telluride
in the solid product and has a similar effect to lead. It also acts
to modify the shape of manganese sulphide inclusions from an
elongated to globular morphology.
Bismuth
Often co-injected with lead and tellurium in free-cutting steels
(~0.1 wt%). It has a similar effect to lead but it is a considerably
more expensive addition.
Selenium
Selenium can be added up to a level of 0.15 wt% and modifies the shape of
MnS inclusions.
Calcium
Calcium is widely used in a number of different inclusion
engineering applications, such as the following:
- Calcium treatment of manganese sulphide inclusions gives
species which remain globular during rolling. This treatment
is used in steels such as pipe plate where MnS stringers
can cause planes of weakness within the steel, along which
lamellar
tearing can occur (non-isotropic properties).
- Treatment of hard, angular, abrasive
alumina
inclusions in aluminium
deoxidised steels, gives calcium aluminate inclusions
which are softer and globular at rolling temperatures,
thereby improving the material's processing characteristics.
- Some inclusions found in steel have a tendency to block
the nozzles
in continuous casting machines, resulting in casting being
terminated prematurely, lost output and increased costs.
Calcium treatment can be used to modify the inclusion
population in steels with a propensity for blockage, to
give low melting point species which will not clog the
caster nozzles.