Steel deoxidation terminology: “Killed steels” are best described as steels that are __________.

Introduction / Context:
Deoxidation practice during steelmaking strongly influences casting soundness and segregation. Terms like “rimmed,” “killed,” and “semi-killed” describe the extent of deoxidation and the resulting solidification behaviour in ingots and continuous casting.

Given Data / Assumptions:

• “Killed steel” implies strong deoxidation with agents such as aluminium or silicon.
• We focus on dissolved oxygen behaviour at the time of solidification.
• Impurity elements (P, S) are treated separately from oxygen control.

Concept / Approach:
Killed steels are thoroughly deoxidised so that there is negligible dissolved oxygen left to evolve CO during solidification. As a result, they solidify quietly (hence “killed”), producing uniform ingots free from gas porosity characteristic of rimmed steels. While killed steels can be made by various primary routes (BOF, EAF), the term describes deoxidation state, not the furnace type. It also does not imply near-zero phosphorus and sulphur; those are controlled by refining and secondary metallurgy, not by deoxidation alone.

Step-by-Step Solution:
Identify defining feature: full deoxidation → negligible dissolved oxygen.Link effect: quiet solidification with fewer gas-related defects.Reject misconceptions tying the term to specific furnace routes or P/S levels.Choose “are free from oxygen” as the best descriptive option.

Verification / Alternative check:
Steelmaking references consistently define killed steels as fully deoxidised (Al-killed, Si-killed).

Why Other Options Are Wrong:
Minimum impurity level is vague and not definitive.

L-D process can produce any deoxidation level; it is not synonymous with “killed.”

Near-zero P/S is not guaranteed by deoxidation alone.

High gas content is the opposite of killed practice.

Common Pitfalls:
Equating “killed” with “clean of all impurities”; ignoring that it specifically addresses oxygen and boiling during solidification.

Final Answer:
are free from oxygen