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.
Production of Rimmed Steels.
As is the case with killed steels, deoxidising agents can be added to react with the dissolved oxygen and prevent the formation of carbon monoxide bubbles. Deoxidising agent include, manganese, ferrosilicon and aluminium. In the case of aluminium, it reacts with the dissolved oxygen during solidification to form alumina (aluminium oxide, Al2O3).
Rimmed steels differ from killed steels in that the amount of deoxidising agent added is less. Killed steels are totally deoxidised, whereas rimmed steels are only partially deoxidised.
The amount of deoxidising agent added to a rimmed steel is usually quite small, so some carbon monoxide bubbles will form in the casting. The amount of porosity formed is controlled so that it is sufficient to offset the volume decrease during solidification.
Advantages of Rimmed Steels
Apart from avoiding piping shrinkage as is common with killed steels, rimmed steels also have smooth attractive surfaces on sheet products after processing.
Disadvantages of Rimmed Steels
As mentioned they contain a degree of porosity due to the formation of carbon monoxide bubbles. They are also softer than killed steels for a given composition.
Steels That are Typically Rimmed
Most steels with carbon contents lower then 0.15% are rimmed.