Aqil Khan
Ladle & Tundish Repair & Relining Section
(Steel Making Department Pakistan Steel).
The Steel Making Department is the main consumer of
high quality refractories which directly affects the pace of production, quality
and cost per ton of steel.
Ladle Repair Section.
The
ladle repair and relining section is a vital integral unit of Steel Making
Department Pakistan Steel which is located in the aisle of Continuous Casting bay
at the back of 1300 tonne Mixer section.
After casting a heat, the empty steel ladle
from Continuous Caster is sent back to Ladle Repair Section where they are
inspected for refractory wear / mechanical fitness and then prepared for
subsequent LD-Converter’s steel tapping and
Continuous casting operation cycle.
As per project ,the working refractory lining
of Steel Casting ladles (SMD) was comprised of Fire clay bricks ( lining life
was 9-11 heats), later ( with the efforts of Pakistan Steel’s
engineers) switched over to local Tar-dolomite bricks (25-30heats) in the year 1988-89, which brought improvements and smoothness in the steel
production cycle.
As per project, the steel teeming ladles of steel making
department were provided with sleeve’s stopper and nozzle mechanism which were
inferior against resistance to aggressive slags and were unable to cope up the
teeming demand of high quality aggressive steel grades.
Later
in the year 1984-85, the steel teeming mechanism
was replaced with imported hydraulically operated Ladle slidegate mechanism
with its superior quality shaped refractories (Al2O3 content 70% or higher)
shown below.
A similar slidegate system was also proposed to replace the
stoppers and nozzles system for steel dispensing from tundish to moulds but due
to high maintenance cost it was not found economically viable for present level
of production.
Tundish Repair Section.
Tundish repair &
relining section is located adjacent to ladle repair section. The tundish is a
boat shaped refractory lined intermediate vessel (specially designed for continuous
casting machines), where liquid steel goes down from steel ladle through slide gate
and falls on its middle portion (impact pad) and exits out through bottom
metering nozzles of tundish to feed the liquid steel to water cooled copper
moulds for acquiring desired shapes ( Slabs, Blooms or Billets ).
Due to work load, steel making department has its own
trained masonry workforce which remains continuously engaged in ladles and
tundish relining work.
Tundish Refractories.
The tundish distributes liquid steel to the moulds through
metering nozzles located along its bottom thereby provides stable stream to
moulds and keep steel flow constant in continuous casting process.
As
per project, the working lining of tundishes were provided with fireclay brick work.
Every time the tundish had to be freshly relined with new bricks and then dried
and preheated for 5-6 hours before start of casting process which was extensively
labour, material and time consuming process.
Later
in the year 1985-86, it was decided to switch-over from Brick lining to Silica boards
lining which significantly reduced relining and preheating cycle time for each
tundish.
For
tonnage steel grades, where steel quality is not so critical, silica board
lining is very attractive as low cost lining system. With less labour requirement, the lining boards are quick and easy to
install.
Monoblock
Stoppers and Tundish Metering Nozzles.
Mostly the campaign life of a tundish is determined by the endurance
of refractories of stoppers & metering nozzles. Tundish metering nozzles in
combination with mono-block stopper controls the flow of stream of liquid steel
before it exits the tundish.
As per project, the sleeve stoppers and low Zirconia metering nozzles were provided for
steel casting tundishes, caused frequent tundish failures specially during
casting of low carbon Aluminium Killed steel grades.
During continuous casting the deposition of deoxidation and
reoxidation products (mainly aluminium oxide inclusions) inside metering nozzle causes clogging. The severe nozzle clogging
needs to be removed by oxygen lancing. Extensive O2 lancing leads to
failure of metering nozzle due to fast erosion and results in replacement of
tundish during casting operation. Consequently these events bloat additional
costs for tundish refurbishment and sometime termination of continuous casting
process which reduce the net casting throughput and productivity.
In order to overcome the above problems, the data of failures
of local production practice was investigated and analyzed and lastly the high Alumina
- graphite refractory monoblock stoppers of present dimensions and high
zirconia (ZrO2 %
≥ 67
) refractory
metering nozzles were brought in practice
in 1986.
The
above combinations of refractories have proved to be good and helped to stabilize present
production pace but Steel production is facing an increasing demand for
high cleanliness with high quality and high performance.
