/ Steel (Ferrous): Secondary Steel Making / Refining / Tundishes
steel making steps utilize transfer ladles and ladle metallurgy stations to further
refine steel that has been processed in a BOF or EAF. A term often used is to
produce "clean steel", which originally described a steel product with
a low level of oxide and sulfide inclusions. The term now also includes demands
for steels with low levels of phosphorus, hydrogen, nitrogen and even carbon.
vessels range in sophistication from simple to quite involved. This relatively
simple illustration of a transfer ladle heating station with argon stirring illustrates
how Oxy-fuel systems can be applied.
completed in a transfer ladle is desulfurization. To remove sulfur from the liquid
metal, it is necessary to use a sulfide-forming agent such as a calcium compound.
Agents typically used to desulfur steel and achieve sulfur contents below 10 ppm
are either calcium alloys like CaSi or low melting point synthetic slags containing
aluminum. Due to the high oxidation potential in the BOF, little additional desulfurization
occurs during the oxygen steel making process. When starting with a low sulfur
content in iron, an increase in sulfur can be observed in BOF operations from
scrap additions. Typically, the sulfur content after tapping is also around 0.010%
(100 ppm) and a further lowering is made via ladle refining metallurgy in ladle
to use an active slag and inert gas bubbling in the ladle also improves the cleanliness
of the steel with regard to oxide inclusions. In the recent past, the natural
gas industry sponsored trials of an oxy-gas ladle heating furnace. A video clip
showing how the trial was arranged and the benefits of an Oxy-Gas ladle heating
system is available here:
currently in operation, AGA Gas participated in the ladle heating trial and is
confident that this technology is viable for certain steel grades not overly sensitive
to hydrogen pick-up. They also state it could be applied as a low-cost holder
in instances where caster production interruptions occur (especially when vacuum
degassing is available). AGA also points out that any resulting hydrogen pick-up
can be effectively removed in the ladle refining station.
stations, also referred to as Ladle Metallurgy Furnace (LMF) station, generally
use an electric arc system to adjust (raise) the temperature of the heat. Some
molten metal reheating, after final alloy additions are made, is usually required
prior to transfer to the tundish and caster. A vacuum degassing ladle system can
also be utilized to outgas (remove hydrogen) or decarburize molten steel. The
range of refining and heating possibilities is shown below.
equipment offers a number of options and benefits including desulfurization, homogenization
of the melt, and the ability to fine-tune the chemical composition and then repeat
this chemical composition control from heat to heat. The same options allow for
adjustment of the casting temperature. If lower temperatures are necessary, it
is carried out by adding well selected scrap until the optimum temperature is
reached. Raising the temperature in the ladle refining station is usually completed
with electric arcs (that operate similar to an EAF only lower power), induction,
by an allotropic process (i.e.; by adding aluminum and oxygen to the steel bath),
or after the ladle has transferred the molten refined steel to the tundish. A
graphic illustrating the typical location of these torches is shown below. Technical
literature indicates that approximately 20 plasma torch tundish installations
review also indicates one intriguing possibility for natural gas tundish heating.
A "Zero Oxidation" system, meaning that no combustion gasses are in
contact with the liquid metal, for tundish heating schematic is illustrated here.
developed Zero Oxidation system is reported to be installed at a continuous casting
facility in Japan. The device is called an N2 jet heater and uses the principle
of a storage type heat exchanger. A set of two heaters are used, each containing
a regenerator; and it is possible to heat Nitrogen gas to 1,773°K (2,730°F).
The high-temperature N2 then will be piped to the tundish and will maintain high
furnace (and metal) temperature under non-oxidizing conditions. After the system
was applied to the tundish heater, the rate of occurrence of inclusion-related
defects in the quality of the solidified slab was reported as one-fifth to one-third
of the past levels. If the system is successful, then this natural gas option
could provide continuous caster operators a significant and credible approach
to high capital cost electric tundish heating methods.
systems are already used for tundish drying (after refractory repair or reline)
and heating before the initial heat is poured into the tundish. Examples of gas-fired
tundish heating and drying systems are shown below.