Which class of steel is fully-deoxidized and typically has a large shrinkage cavity or "pipe" at the top?

The class of steel referred to in the question is killed steel, which undergoes a deoxidation process that results in the removal of oxygen and minimizes the formation of gas bubbles within the steel during solidification. This process allows for a denser and more uniform final product.

Killed steel is characterized by its excellent mechanical properties and reduced tendency to form large shrinkage cavities or "pipes" compared to other steel classes. However, it is recognized that killed steels still may exhibit some minor surface imperfections as they cool, and it is this specific attribute that pertains to the question regarding the presence of a large shrinkage cavity at the top.

Compared to rimmed and semi-killed steels, which are intentionally left with some residual oxygen to promote certain characteristics, killed steel is fully deoxidized, making it different in terms of plasticity and soundness. Rimmed steel typically has a higher likelihood of surface defects and imperfections, while semi-killed steel can still develop some gases leading to cavities or porosity during solidification. Fully-killed steel implies an even greater level of deoxidation than killed steel but is generally a more specific term. Thus, it is the standard killed steel that most directly fits the description provided in the question.