During the steel making process, oxygen may become dissolved in the liquid metal. During solidification, the dissolved oxygen can combine with carbon to form carbon monoxide bubbles. The carbon is added to the steel as an alloying element.
The carbon monoxide bubbles are often trapped in the casting and can act as initiation points for failure.
How Killed Steels are Produced and Their Advantages
Formation of the carbon monoxide bubbles can be eliminated through the addition of deoxidising agents such as aluminium, ferrosilicon and manganese. In the case of aluminium, the dissolved oxygen reacts with it to form aluminium oxide (Alumina, Al2O3). The formation of alumina not only prevents the formation of bubbles or porosity, but the tiny particles or inclusions also pin grain boundaries during heat treatment processes, preventing grain growth. Completely deoxidised steel are known as “killed steels”.
They have a more uniform analysis and are relatively free from ageing. For a given carbon and manganese content, killed steels are usually harder then rimmed steels.
The disadvantage of using killed steels is they often display deep pipe shrinkage.
Steels That are Typically Killed
Steel that are generally killed include:
• Steels with carbon contents greater then 0.25%
• All forging grades of steel
• Structural steels with carbon content between 0.15 to 0.25%
• Some special steel in the lower carbon ranges