Silicon Killed Steel is the Steel that has been deoxidized (calmed) using Silicon instead of just Aluminium.
The Reaction: Silicon reacts with oxygen to form silica, preventing gas bubbles.
The "Sandelin Effect": In galvanizing, silicon content is critical. If silicon is in the "reactive range" (0.04% - 0.12%), the zinc coating grows uncontrollably thick and brittle. Manufacturers must carefully control silicon levels to ensure a smooth galvanized finish.