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Processes - Ferrous (Iron / Steel): Casting / Pouring / Solidification

Nearly all liquid steel is solidified into semi-finished shapes such as slabs, blooms and billets. Continuous casting now has essentially replaced ingot casting due to benefits that include increased yield, improved product quality, energy savings and reduced cost.

Steel Casting
After the steel has been refined, it is ready to be cast into ingots or continuous strips (see the diagram on the following page). In the continuous-casting process, molten steel is delivered in ladles and poured into a reservoir, or tundish, from where it is released into the mold by gravity feed. The casting machine can have either one (single-strand caster) or multiple molds (multi-strand caster). The steel cools as it passes through the mold and forms a solid shell or "skin." As the steel proceeds onto the run-out table with a series of hot-handling rollers, the center of the steel solidifies, yielding a semi-finished shape at a specified width and thickness. Depending on the type of caster used, billets, blooms, rounds, thin slabs, or thick slabs are produced. A cutting torch is used at the end of the roll line to cut the steel to the desired length.

The functions of the caster pouring system are to transfer metal from the ladle to the caster, control flow to the caster, minimize slag entrainment, minimize oxygen pickup from the pouring system, cause flotation of inclusions, and minimize heat loss. Flow is controlled both at the ladle and tundish.

Issues

1. Because of the harsh operating conditions, the life of many components is less than one heat.
2. Clogging of the caster pouring system is the single largest operational problem resulting in reduced quality and production delays.

Oxygen contamination occurs from exposure to air while the molten metal is contained in an uncovered ladle or tundish or while it is being poured from the ladle to the tundish or from the tundish to the casting mold. Nitrogen pick-up also occurs and can be detrimental to the steel chemistry.

There are several applications for heating technology in continuous casting.

  • Heating of tundish refractory lining (for drying) in order to eliminate hydrogen pickup in the molten steel and to reduce molten metal heat loss on filling the tundish.
  • Heating the submerged entry nozzle before casting to minimize possibility of metal freezing at the start of casting.
  • Heating the tundish during casting to minimize temperature variation in molten steel (this allows casting with as low a superheat as possible).
  • After solidification is complete, Oxy-fuel torches cut the continuous strand to length.

The photo on the right shows oxy-gas torches cutting continuously cast slabs to length.

Natural gas burners (whether air-fuel or Oxy-fuel) are used for both tundish and tundish nozzle heating. Energy requirements for both needs range from 0.2 to 0.5 Mcf per ton of cast steel. The tundish nozzle portion of this total is small, about 0.01 Mcf / ton and will vary depending on the preheat temperature, the number of ladles cast in a row and the tundish capacity.

Tundish heating during casting has generally been electric (either resistance, induction or plasma) for two primary reasons:

1. These systems can provide rapid heating when liquid metal temperature starts to fall.
2. Pickup of oxygen, hydrogen, or nitrogen from combustion products can be a concern for some steel grades.

Ingot casting is used for small batches of specialty steels or for end products with certain shape requirements (e.g., intermediate- and large-bar applications or high performance bar and tubing applications). Ingot casting also continues to be used by foundries and specialty steel makers to produce large cross sections or thick plates. During ingot casting, the molten steel is poured (teemed) into a series of molds and allowed to solidify to form ingots. After the molds are stripped away, the ingots are reheated to uniform temperature in any number of reheat furnaces (batch or continuous) to prepare them for rolling or otherwise forming them into products because of the reheating step.