Definition - What does Capped Steel mean?
Capped steel is a type of steel formed by limiting gas generation in its granular structure, therefore making it an intermediate steel between rimmed and killed steel types. The capping operation reduces the number of gas voids inside the ingot. It has a more uniform composition than rimmed steel.
Corrosionpedia explains Capped Steel
Capped steel is mechanically or chemically restricted in terms of gas production when the ingot is cast. It is formed by introducing the cast into a bottle-top mold and covering the mold neck with a fitted cap. This ensures a pressure buildup inside the confined molten metal.