Friday, October 4, 2013

Ladle & Tundish Repair Section Steel Making Dept. Pakistan Steel.

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 ).

 After completion of refractory life campaign, the tundishes are de-skulled and prepared  for relining work.

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 (ZrO% ≥  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.

So  future demand of higher quality steel grades with increase production rate will be needing to switch over to further superior quality refractories which may be mostly fulfilled by exploiting local resources and upgrading the manufacturing capabilities of the Refractories production plant at Pakistan Steel.

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