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Fabrication, manufacturing

  Improved machinability     7 of 12
 
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:

  1. 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).
  2. 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.
  3. 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.
 

 
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