Semi-killed steels are produced by adding what to molten steel to remove impurities?

• A. Large amounts of deoxidizers added to molten steel to remove impurities.
• B. Significant nitrogen gas injection.
• C. Small amounts of deoxidizers added to molten steel to remove impurities.
• D. No additives.

Answer: (C)

The key idea is partial deoxidation. Semi-killed steel is made by adding deoxidizers to molten steel in limited amounts, enough to remove much of the oxygen but not all of it. This partial deoxidation reduces impurities and helps control porosity during solidification, giving a balance of formability and machinability that suits certain applications. If you added large amounts of deoxidizers, you’d drive the melt toward fully killed steel with nearly complete oxygen removal and different properties, which isn’t what semi-killed steel aims for. Nitrogen injection isn’t part of the semi-killed process, and leaving the melt without additives would leave more dissolved oxygen and impurities. So small amounts of deoxidizers to remove impurities is the correct approach.