In steelmaking metallurgy, which refractory nozzle material is most commonly used for continuous casting of steel (tundish/nozzle applications), owing to its excellent erosion and thermal shock resistance?

Introduction / Context:
Continuous casting of steel places extreme demands on nozzle refractories. The material must withstand high temperatures, steep thermal gradients during start–stop cycles, erosive/chemical attack by molten steel and slag, and mechanical wear. This question checks practical metallurgy knowledge of which refractory composition is typically preferred for casting nozzles in tundish and ladle systems.

Given Data / Assumptions:

• Operating temperatures are above 1500 °C with cyclic thermal loads.
• Molten steel and slag are chemically aggressive and can erode/penetrate porous refractories.
• Nozzle materials should resist erosion, chemical attack, and thermal spalling while maintaining dimensional stability.

Concept / Approach:
Zirconia (often stabilized) and zircon (ZrSiO4) based refractories exhibit high refractoriness, excellent corrosion resistance to basic/acidic slags, low wettability by molten steel, and good thermal shock resistance when engineered with suitable microstructures. These properties make zircon/zirconia the benchmark choice for continuous-casting nozzles. Alternative materials like silicon carbide offer good thermal shock but may oxidize/transform, and thoria/beryllia are expensive, scarce, or present radiological/toxicity concerns, limiting common steel plant use.

Step-by-Step Solution:
Identify key service requirements: thermal shock resistance, corrosion/erosion resistance, non-wettability.Match material properties: zircon/zirconia meet these needs better than common alternatives.Assess practicality and safety: thoria/beryllia limited by hazards and cost; SiC oxidizes and can degrade.Conclude zircon/zirconia is the standard nozzle material for continuous casting.

Verification / Alternative check:
Industry practice and vendor datasheets for tundish/slide-gate nozzles frequently specify zirconia/zircon compositions or alumina–zirconia–graphite blends, confirming the dominance of zirconia-rich materials in this application.

Why Other Options Are Wrong:
Thoria: excellent high-T properties but radiological concerns and cost restrict routine steel plant use.Carborundum (SiC): good thermal shock yet susceptible to oxidation and chemical interaction under service.Beryllia: outstanding thermal conductivity but toxic and costly; rarely used in steel nozzles.Magnesia–chrome: good basic slag resistance but not the standard for precision casting nozzles.

Common Pitfalls:
Assuming highest refractoriness alone decides selection; nozzle materials must also resist wetting/erosion and survive thermal cycling. Health, safety, and cost factors also matter.

Final Answer:
Zircon (zirconia-based) nozzles