Which steel type is fully deoxidized and shows no gas evolution?

• A. Capped steels
• B. Rimmed steels
• C. Semi-killed steels
• D. Killed steels

Answer: (D)

In steelmaking, removing oxygen from the melt is crucial to prevent gas forming during solidification. When a melt is fully deoxidized, dissolved gases don’t come out as bubbles as it cools, so the metal solidifies with a clean, uniform structure. This is what happens with killed steels: they are deoxidized with strong agents (like silicon or aluminum) to bind oxygen and eliminate free oxygen in the melt. Because the oxygen and other gases are effectively removed, there’s no gas evolution as the steel solidifies, yielding a compact, homogeneous material.

Rimmed and semi-killed steels aren’t fully deoxidized, so gases can evolve during solidification, causing porosity or a nonuniform structure. Capped steels involve a different casting practice and don’t achieve the same level of deoxidation, so they can also show gas-related issues. Thus, the fully deoxidized, gas-free option is the killed steel.