Deoxidation & Soundness — Additions to Low-Carbon Steels Which element is commonly added to low-carbon steels to prevent porosity by deoxidising the melt and promoting sound ingots?

Introduction / Context:
Porosity in steel cast products often results from dissolved gases and oxides formed during solidification. Deoxidation practice in steelmaking is crucial for producing sound ingots and continuous-cast products with minimal blowholes and piping defects.

Given Data / Assumptions:

• Melt treatment focuses on removing dissolved oxygen to avoid gas porosity.
• Alloying levels are typical of low-carbon steels.
• Standard deoxidisers are considered.

Concept / Approach:
Silicon is a strong deoxidiser. Added as ferrosilicon, it reacts with dissolved oxygen to form stable silica, thereby reducing gas evolution and porosity. Aluminium is another powerful deoxidiser (killed steels), but among the listed choices, silicon best represents the common practice. Manganese chiefly combines with sulphur to form MnS and prevent hot shortness; it is not the primary deoxidiser for porosity control. Sulphur and phosphorus are generally undesirable — they embrittle and do not prevent porosity.

Step-by-Step Solution:
Identify the defect: porosity from gases/oxides during solidification.Choose a strong deoxidiser: silicon (and aluminium in many practices) remove dissolved oxygen.Rule out Mn as a deoxidiser of first choice; it targets S control (hot shortness).Exclude S and P; both are impurities that worsen toughness and do not reduce porosity.

Verification / Alternative check:
Steelmaking texts list “rimming, capped, semi-killed, killed” steels; silicon and aluminium additions move steel toward semi-killed/killed conditions with improved soundness.

Why Other Options Are Wrong:
Sulphur: increases hot shortness and inclusions; not a deoxidiser.Phosphorus: raises strength but reduces toughness; not for deoxidation.Manganese: useful for sulphur control, not primarily to prevent porosity.

Common Pitfalls:
Confusing sulphur control (Mn additions) with oxygen removal (Si/Al deoxidation).

Final Answer:
Silicon